key: cord-336201-fl606l3b
authors: Daryabor, Gholamreza; Atashzar, Mohamad Reza; Kabelitz, Dieter; Meri, Seppo; Kalantar, Kurosh
title: The Effects of Type 2 Diabetes Mellitus on Organ Metabolism and the Immune System
date: 2020-07-22
journal: Front Immunol
DOI: 10.3389/fimmu.2020.01582
sha: 
doc_id: 336201
cord_uid: fl606l3b

Metabolic abnormalities such as dyslipidemia, hyperinsulinemia, or insulin resistance and obesity play key roles in the induction and progression of type 2 diabetes mellitus (T2DM). The field of immunometabolism implies a bidirectional link between the immune system and metabolism, in which inflammation plays an essential role in the promotion of metabolic abnormalities (e.g., obesity and T2DM), and metabolic factors, in turn, regulate immune cell functions. Obesity as the main inducer of a systemic low-level inflammation is a main susceptibility factor for T2DM. Obesity-related immune cell infiltration, inflammation, and increased oxidative stress promote metabolic impairments in the insulin-sensitive tissues and finally, insulin resistance, organ failure, and premature aging occur. Hyperglycemia and the subsequent inflammation are the main causes of micro- and macroangiopathies in the circulatory system. They also promote the gut microbiota dysbiosis, increased intestinal permeability, and fatty liver disease. The impaired immune system together with metabolic imbalance also increases the susceptibility of patients to several pathogenic agents such as the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Thus, the need for a proper immunization protocol among such patients is granted. The focus of the current review is to explore metabolic and immunological abnormalities affecting several organs of T2DM patients and explain the mechanisms, whereby diabetic patients become more susceptible to infectious diseases.

The metabolic syndrome is defined by the presence of metabolic abnormalities such as obesity, dyslipidemia, insulin resistance, and subsequent hyperinsulinemia in an individual (1) . Dyslipidemia, the main characteristic of metabolic syndrome, is defined by decreased serum levels of high-density lipoproteins (HDLs) but increased levels of cholesterol, free fatty acids (FFAs), triglycerides (TG), VLDL, small dense LDL (sdLDL), and oxidized LDL (ox-LDL) ( Table 1 ) (2) . Individuals with the metabolic syndrome are much more likely to develop type 2 diabetes mellitus (T2DM), cardiovascular diseases (CVDs), and fatty liver disease (2) (3) (4) . T2DM, the most common form of diabetes (∼90%), is characterized by a systemic inflammatory disease accompanied by insulin resistance (IR) or decreased metabolic response to insulin in several tissues, including the adipose tissue, liver, and skeletal muscle, as well as by reduced insulin synthesis by pancreatic beta cells (4, 5) . Studies on immunometabolism have indicated that the metabolic states and immunological processes are inherently interconnected (6) . In this scenario, metabolites derived from the host or microbiota regulate immunological responses during health and disease (6) . Accordingly, in obese individuals, expanded adipose tissue at different locations, by initiating and perpetuating the inflammation, induces a chronic low-level inflammatory state that promotes IR (4). Every organ system in human body can be affected by diabetes, but the extent of organ involvement depends largely on the severity and duration of the disease (Figure 1 and Table 1 ). During the progression of diabetes, hyperglycemia promotes mitochondrial dysfunction and induces the formation of reactive oxygen species (ROS) that cause oxidative stress in several tissues such as blood vessels and pancreatic beta cells (7) (8) (9) . Accumulating damage to the mitochondria, as well as several macromolecules, including proteins, lipids, and nucleic acids by ROS promotes the process of aging (10) . As a result, pancreatic β cells that require functional mitochondria to maintain insulin synthesis fail to generate high enough levels of insulin (11, 12) . In the absence of compensatory mechanisms, stress-responsive intracellular signaling molecules are activated and cellular damage occurs. Elevated intracellular levels of ROS and subsequent oxidative stress play an important role in the pro-atherosclerotic consequences of diabetes and the development vascular complications (9, 13) . Moreover, the non-enzymatic covalent attachment of glucose and its toxic derivatives [e.g., glyoxal, methylglyoxal (MGO), and 3deoxyglucosone] to the biological macromolecules such as nucleic acids, lipids, and proteins leads to the formation of advanced glycation end products (AGEs) (14, 15) . Accumulated AGEs block the insulin signaling pathway and promote inflammation (16, 17) . In addition, the attachment of AGEs to their receptors [e.g., CD36, galectin-3, scavenger receptors types I (SR-A1), and II (SR-A2)] on the surfaces of immune cells in the circulation and tissues activates the expression of pro-inflammatory cytokines and increases free radical generation (18) . Furthermore, due to the chronic exposure of cells to high glucose levels in untreated T2DM patients, glucose toxicity might occur in several organs. This will FIGURE 1 | Effects of T2DM on body organs. T2DM is an inflammatory state that affects circulatory system, gastrointestinal tract, pancreatic beta cells, liver, and skeletal muscles and makes them dysfunctional. NFALD, non-alcoholic fatty liver disease; NASH, non-alcoholic steatohepatitis; ER, endoplasmic reticulum. eventually lead to nephropathy, cardiomyopathy, neuropathy, and retinopathy.

Gut microbiome dysbiosis is another important factor that can facilitate the induction and progression of metabolic diseases such as T2DM (19) . The gut microbiome dysbiosis, by altering the barrier functions of intestine and the host metabolic status, promotes the insulin resistance in diabetic patients (19) . Diabetes also impairs the immune system and increases the susceptibility of patients to serious and prolonged infections (20) . This is likely to be the case with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), as well (21, 22) . In the current paper we will review recent research to explore the impairment of body organs in T2DM patients and explain how diabetic patients become more susceptible to certain infectious diseases.

Vascular homeostasis is an important function of the endothelium. Under homeostatic conditions, the ECs maintain the integrity of blood vessels, modulate blood flow, deliver nutrients to the underlying tissues, regulate fibrinolysis and coagulation, control platelet adherence and patrol the trafficking of leukocytes (Figure 2A ) (23) . Normal ECs also internalize high-density lipoproteins (HDLs) and its main protein part apolipoprotein A-I (apoA-I) in a receptor-mediated manner to activate endothelial cell nitric oxide (eNOS) synthase and promote anti-inflammatory and antiapoptotic mechanisms ( Figure 2B ) (24) . HDL receptors on the surfaces of ECs include: the ATP-binding cassette (ABC) transporters A1 and G1, the scavenger receptor (SR)-B1 and the ecto-F1-ATPase (24) .

According to the epidemiological studies, diabetes mellitus is considered as one of the main risk factors for CVD (Figure 1 ) (25) . From the beginning of T2DM, the functions of ECs are impaired, which is the main cause of disease-related side-effects (26) . ECs can initiate and perpetuate the inflammatory milieu during the pathogenesis of diabetes. Due to the negative impacts of hyperglycemia and subsequent oxidative stress, CVDs are more common among diabetic patients (27) . It has been observed that incubation of human aortal endothelial cells (HAECs) with a medium containing high glucose concentrations (HG, 20 mM) increases the intracellular levels of MGO and glycated proteins that in turn activate the unfolded protein response (UPR) and trigger inflammatory and prothrombotic pathways (28) . Glycated apoA-I, which is formed during hyperglycemia, modifies its structure, decreases its lipid-binding ability, prevents cholesterol efflux from macrophages and impairs its anti-inflammatory function (29, 30) . Vaisar et al. have shown that HDLs from diabetic patients have a reduced capacity to trigger eNOS production and suppress tumor necrosis factor-α (TNF-α)mediated inflammatory responses within ECs (31) .

Diseases such as T2DM that induce high levels of vascular injury are accompanied by an elevated number of circulating endothelial cells (CECs) (32) . T2DM-related risk factors such as dyslipidemia, hyperglycemia, and hyperinsulinemia as well as other conditions (e.g., inadequate physical activity, smoking, and high blood pressure) facilitate the formation of atherosclerotic plaques/lesions (33) . Dyslipidemia, due to the elevated flux of FFA from insulin-resistant tissues and spillover from entry (C) Blood vessels in T2DM patients. During the progression of the disease, red blood cells become glycated, while activated ECs synthesize elevated levels of adhesion molecules and chemokines that facilitate monocytes recruitment, adhesion, and transmigration across the endothelium toward the subendothelial region. Monocytes are then differentiated into macrophages and eventually, by excess lipid uptake, generate foam cells. Subsequently, further immune cell infiltration into the atherosclerotic lesion occurs, where their inflammatory cytokines promote platelet activation, EC apoptosis, and increased generation of ROS and Ox-LDL. (D) interactions between oxLDL and its receptor aggravate ROS generation, NF-κB activation and inflammation. EC, endothelial cell; RBC, red blood cell; PLT, platelet; HDL, high-density lipoprotein; Ox-LDL, Oxidized low-density lipoprotein; ROS, reactive oxygen species; eNOS, endothelial nitric oxide synthase; NO, Nitric oxide; LOX-1, lectin-type oxidized LDL receptor 1.

into adipocytes, is considered as an important risk factor for developing CVD among diabetic patients. This is because dyslipidemia promotes inflammation, endothelial dysfunction, and platelet hyperactivation (34, 35) . During the progression of atherosclerosis, lipids, immune cells, and extracellular matrix accumulate in the arterial intima or subendothelial regions ( Figure 2C ) (33) . Advanced plaques can impede blood flow and cause tissue ischemia or might become disrupted and generate a thrombus that stops the blood flow of important organs. Vascular complications of diabetes engage either tiny or large blood vessels (micro-and macroangiopathy, respectively). Microangiopathies, which can be seen in the kidneys, vasa nervorum and eye tissues, cause nephropathy, neuropathy, and retinopathy.

Macroangiopathies, by inducing atherosclerosis in the coronary, carotid, and peripheral arteries, increase the risk of myocardial infarction (MI), stroke and peripheral artery disease (PAD). Macrovascular complications due to EC dysfunction are considered as an important cause of mortality and morbidity among diabetic patients (36) . Oxidative stress has an essential role in the induction of vascular complications during the course of diabetes (8) . EC dysfunction (e.g., delayed replication, dysregulated cell cycling, and apoptosis), as well as enhanced ox-LDL formation are some consequences of oxidative stress. It has been well-established that sdLDL and ox-LDL have an enhanced atherogenic ability and are more useful biomarkers than total LDL for predicting CVD (37, 38) . sdLDL particles have a smaller size than other LDL particles. Thus, sdLDL particles are more easily oxidized, and their atherogenic potential is enhanced. During oxidative stress, levels of ox-LDL increase by the excess action of reactive oxygen species (ROS) (13) . Subsequently, ox-LDL interaction with scavenger receptors, including CD36, SR-A1/CD204, SR-B1, and lectin-like ox-LDL receptor-1 (LOX-1) on the surface of ECs activates the NADPH oxidase that in turn increases the expression of ROS and activates the transcription factor NF-αB (39) . Afterwards, the expression of LOX-1, adhesion molecules (e.g., selectins and integrins) and the secretion of pro-inflammatory cytokines and chemokines are increased, while NO synthesis is decreased in ECs ( Figure 2D ) (39) (40) (41) . EC-derived chemokines bind to their cognate receptors on the surfaces of monocytes and recruit them toward the inflamed endothelium. Following this, selectin-based rolling and integrin-based attachment of monocytes to the ECs cause their migration toward the subendothelial region, where they develop into lipid-laden macrophages or foam cells later on (42) .

The scavenger receptor LOX-1 plays an important role in the uptake of ox-LDL during atherogenesis. It is strongly expressed on the surfaces of ECs, but has an inducible pattern of expression on the surface of macrophages and smooth muscle cells (43) . The accelerated uptake of ox-LDL by macrophages accounts for their transformation into foam cells, the initial hallmark of atherosclerosis (41, 43) .

Besides, diabetes leads to both quantitative and qualitative defects in circulating angiogenic progenitor cells (CAPCs) that take part in the repair of injured endothelium (44) . It has been shown that humans or mice with decreased numbers of CD31 + CD34 + CD133 + CD45 dim Sca-1 + Flk-1 + CAPCs have an increased prevalence of T2DM, elevated HbA1c levels and aggravated CVD risk scores (44, 45) .

In diabetic patients, despite elevated serum levels of proangiogenic molecules, like angiopoietin-1/2, EPO, and VEGF-A, angiogenesis is impaired. This is mainly due to the decreased expression levels of VEGFR2 and CXCR4 on the surfaces of CAPCs, which makes them unresponsive to the angiogenic factors (44, 46) . It has also been shown that circulating proangiogenic granulocytes composed of eosinophils and neutrophils are also impaired in diabetic patients (47) . Besides, elevated levels of AGEs in T2DM cause EC dysfunction and vascular inflammation (48) . Ren et al. have shown that incubation of human coronary artery endothelial cells (HCAECs) with AGEs causes decreased expression (at both mRNA and protein levels) and enzymatic activity of eNOS, increased levels of ROS, diminished mitochondrial membrane potential and declined activity of catalase and superoxide dismutase in treated cells (49) . Another study by Lan et al. has shown that AGEs in the pancreas decrease EC viability and induce their apoptosis in an NFκB signaling-related manner (50) . However, apigenin (4 ′ ,5,7trihydroxyflavone) can protect ECs against oxidative stress and subsequent inflammatory reactions mediated by AGEs (51) . Apigenin binds to methylglyoxal (MGO) and forms a complex that inhibits AGE formation. Chettab et al. have shown that the expression of ICAM-1 as well as the production of IL-8, are significantly increased in HUVECs cultured in HG medium compared to cells cultured in normal glucose (NG, 5.5 mM) conditions (52). Bammert et al. found out that incubation of HUVECs with HG media promotes the generation of endothelial microparticles (EMPs) that, when added to normally cultured HUVECs, downregulate the expression of anti-apoptotic microRNA miR-Let7a, but enhance the synthesis of active caspase-3 and cause cell apoptosis (53) . Several microRNAs, including miR-21, miR-26a, miR-30, miR-92a, miR-126, miR-139, miR-199a, miR-222, and miR-let7d, regulate vascular homeostasis. It has been shown that the expressions of miR-26a and miR-126 are significantly reduced in circulating MPs isolated from diabetic patients compared with normal individuals. This could be involved in making diabetic individuals more susceptible to coronary heart disease (54) . Moreover, HG media upregulate the expression of NADPH oxidase that will induce the generation of ROS. This leads to subsequent apoptosis of the HUVECs through a ROS-dependent caspase-3 pathway (55).

Su et al. have demonstrated that argirein medication, by inactivating NADPH oxidase, can prevent endothelial cell apoptosis in a rat model of T2DM and hence attenuate vascular dysfunction (56) . HG further increases the permeability of the HUVECs in a protein kinase C (PKC)-dependent manner (57, 58) . Hassanpour et al. showed that incubation of endothelial progenitor cells with the serum of T2DM patients inhibits their migration toward bFGF, increases their expression of VEGFR-2, but reduces their expression of VEGFR-1 and induces their apoptosis (59) . However, humanin (HN), a mitochondriumderived peptide, is cytoprotective against apoptosis during pathological conditions, such as diabetes mellitus (60) . It has been demonstrated that simultaneous incubation of H9C2 cells, a line of rat cardiac myoblasts, with H 2 O 2 and HN decreases the intracellular levels of ROS, preserve mitochondrial function/structure and decline cellular apoptosis (61) . Wang et al. have indicated that the treatment of HUVECs with HN before their incubation with HG medium increases the expression of eNOS, while decreasing the expression of endothelin 1 (ET-1), VCAM-1, TNF-α, IL-1β, and E-selectin in a krüppellike factor 2 (KLF2)-dependent manner. Such changes in the expression of integrins prevent the attachment of monocytes to HUVECs (62) . Accordingly, HN might be used to prevent the development of hyperglycemia-associated EC dysfunction in T2DM.

EC activation and expression of adhesion molecules also facilitate activation and adhesion of platelets. This will increase the risk of thrombosis and promote the development of thrombotic angiopathy, typical for diabetic patients. Platelets are tiny anucleated cellular fragments generated from megakaryocytes in the bone marrow. They circulate in the blood for ∼5-9 days and play essential roles in hemostasis and in controlling vascular integrity (63) . Circulating inactive platelets move in the proximity of vessel walls (Figure 2A ) and rapidly get activated in response to vascular injury. At the end of their life, platelets are cleared from circulation with the action of the liver and spleen-resident macrophages. Platelets have an essential role in the initiation and progression of inflammation. Platelet hyperactivation that occurs during inflammatory states (e.g., T2DM) facilitates the pathogenesis of CVDs ( Figure 2C ) (64, 65) . It has been shown that elevated levels of resistin, an adipokine, in diabetic patients enhances oxidative stress, promotes endothelial dysfunction and facilitates platelet activation (66) . Activated platelets with an increased mean volume [mean platelet volume (MPV)] secrete microparticles (MPs) and soluble adhesion molecules (e.g., sP-selectin and sCD40L) that in turn activate endothelial and immune cells (67) (68) (69) . Higher levels of platelet-derived MPs, which correlate positively with fasting blood sugar and glycated hemoglobin, have been shown in newly diagnosed T2DM patients compared to healthy individuals (70) .

In T2DM patients thrombotic microangiopathies can lead to the development of CVDs (71) . Platelets in the patients adhere to ECs and aggregate more rapidly than in healthy individuals thereby increasing the risk of thrombosis. In a mouse model of T2DM, Zhu et al. have shown that AGEs interact with CD36, a member of the type 2 scavenger receptor family, on the surfaces of murine platelets to activate them and induce a prothrombotic state (72) . Elevated levels of the P2Y12 receptor on the surface of platelets in T2DM expose diabetic patients to a prothrombotic condition. This receptor has an essential role in platelet activation (73) . Zhou et al. have shown that long non-coding RNA (lncRNA) metallothionein 1 pseudogene 3 (MT1P3), which is markedly upregulated in megakaryocytes of T2DM patients, enhances the expression of p2y12 receptor in platelets (74) . They indicated that this is due to the inhibitory action of MT1P3 on miR-126.

Virtually all parts of the human digestive system, including the gastrointestinal tract, pancreas, and the liver are affected by diabetes.

The GIT is populated with a myriad of microorganisms, including principally bacteria but also archaea, viruses, fungi, and protozoans that dynamically influence the health status and homeostasis of the host. The physiological functions of the GIT resident microbes improve gut integrity, protect against microbial pathogens and regulate immune responses (75) . Mucosal barriers, such as intestinal epithelial cells (IECs) and the mucus layer, spatially isolate the host immune system and gut microbiota to prevent unnecessary immune activation and intestinal inflammation. They also facilitate the uptake of nutrients through receptors and transporters. However, hyperglycemia, in a GLUT2-dependent manner, can influence the mucus and alter the integrity of adherence and tight junctions between intestinal epithelial cells of diabetic mice. This will enhance the permeability of the intestinal barrier leading to so called "leaky gut." Subsequently, hyperglycemia may facilitate the dispersal of an enteric infection into a systemic infection (Figure 1 ) (76) . Interestingly, the reversal of hyperglycemia, conditional deletion of GLUT2 from the IECs and inhibition of glucose metabolism will fix the barrier dysfunction and prevent the spread of bacteria (76). Xu et al. have shown that Faecalibacterium prausnitzii, one of the most frequent commensal bacteria in normal individuals with essential roles in gut homeostasis, generates anti-inflammatory molecules that enhance the expression of tight junctions and improve intestinal integrity during diabetes (77) . However, in some cases, gut microbiota dysbiosis or altered microbial composition of the intestines could induce T2DM and lead to its progression (78) .

Of interest, the widely used antidiabetic drug metformin can improve barrier integrity and restore the healthy microbiota composition of the gut in diabetic patients (79) . The intestinal commensal bacterium Akkermansia muciniphila can also act as a sentinel to reduce microbial translocation across the gut and prevent the subsequent inflammation in patients with T2DM (80) . Hyperglycemia can further decrease the intracellular levels of glutathione (GSH) but increase iNOS activity and NO production in the IECs (81). Zhao et al. have found out that hyperglycemia in a PKCα-dependent manner inhibits the ubiquitination, internalization and degradation of the divalent metal transporter 1 (DMT1) present on the microvillar membranes of IECs. Subsequently, intestinal iron uptake is enhanced and accumulated iron ions aggravate diabetes-related complications and increase mortality (82, 83) .

The pancreas consists of the exocrine and endocrine compartments. The endocrine part is made of different cell types, including α, β, δ, and ε cells that secrete glucagon, insulin, somatostatin, and ghrelin hormones, respectively. These cells are aggregated into specialized structures called islets of Langerhans, which play an important role in controlling blood glucose levels through the secretion of insulin and glucagon. In T2DM, despite normal levels of β-cell replication and islet formation, β-cell apoptosis is increased so that the number of cells declines by ∼50% (Figure 1 ) (84) . During the progression of T2DM, the insulin-resistant state forces β-cells to compensate for the lack of insulin by elevating its synthesis to restore the normal blood glucose level. However, in severe diabetic patients, β-cell exhaustion, and subsequent persistent hyperglycemia occur (7) . Furthermore, chronic elevated serum levels of free fatty acids, seen in obesity and T2DM, induce lipotoxicity in beta-cells and suppress their insulin secretion ability (85) . To alleviate chronic inflammation, overcome insulin resistance (IR) and to prevent β-cell apoptosis, stem cells or stem cell derivatives such as insulin-producing cells (IPCs) and exosomes have been suggested (86) (87) (88) (89) . Their effects are believed to be mainly due to their anti-inflammatory activities.

Secretagogin (SCGN) is predominantly expressed by pancreatic β-cells protecting their normal functions. SCGN also acts as an insulin binding protein to make it more stable, avoid its aggregation, improve its functions and enhance its secretion (90, 91) . In T2DM patients, due to the islet cell dysfunction and endoplasmic reticulum (ER) stress, serum levels of SCGN are elevated reflecting stress and dysfunctional islet cells (92) . Moreover, in patients with T2DM, islet amyloid polypeptide (IAPP or amylin), a peptide hormone and one of the main secretory products of pancreatic β-cells, tends to deposit in the islets of Langerhans, form insoluble fibrils and impair secretory functions of β-cells (93) . IAPP is costored with insulin in the secretory granules of pancreatic β cells. In steady-state conditions it regulates food intake, insulin secretion, and glucose metabolism (94). Ribeiro et al. have noted that pancreatic extracellular vesicles (EVs) from healthy individuals, but not from T2DM patients, directly bind to IAPPs and prevent amyloid formation within the pancreatic islets (95) . The authors showed that the altered protein-lipid composition of the EVs is the main reason for this discrepancy (95) . However, Chatterjee et al. have shown that β-cells from T2DM patients have a dysfunctional proteasome complex that fails to degrade pancreatic IAPP, whereby amyloid formation is induced (96) . Furthermore, in T2DM patients, lipids accelerate the formation of fibrillary IAPP, which aggravates islet cell damage (97) .

Dhar et al. have demonstrated that chronic use of MGO in Sprague-Dawley rats increases the expression of NF-αB, MGO-derived AGEs and their receptors in pancreatic β cells. MGO can also induce apoptosis of islet β cells, increase fasting plasma glucose levels and impair glucose tolerance (98) . In T2DM patients the plasma level of MGO directly correlates with fasting blood sugar and HbA1c levels (99) . Bo et al. further showed that MGO in a dose-based manner impairs insulin secretion of pancreatic β-cell lines MIN6 and INS-1 through increased generation of ROS and by induction of mitochondrial dysfunction (100). Robertson et al. have found out that elevated levels of ROS in pancreatic β-cells inhibit the pancreas duodenum homeobox-1 (PDX-1) transcription factor that is needed for insulin synthesis (7) . It has been shown that chronic use of MGO in animals could induce T2DM, while simultaneous use of alagebrium, which breaks AGE compounds, attenuates the disease (98) . It has also been reported that during the course of diabetes dedifferentiation and conversion of β-cells into αand δ-"like" cells occurs (101) . In conclusion, the pancreatic β cell function is progressively reduced during the progression of T2DM.

The liver is by far the most important metabolic organ with essential roles in regulating homeostasis and mediating glucose and lipid metabolism. Metabolic activities of the tissue are precisely controlled by the actions of metabolic substrates, including free fatty acids (FFAs) and hormones (102) . T2DM patients usually suffer from a chronic liver condition called non-alcoholic fatty liver disease (NAFLD). It is characterized by steatosis that means ectopic fat storage in hepatocytes and subsequent insulin resistance (Figure 1 ) (103) . Lipid accumulation in hepatocytes leads to impaired biogenesis of miR-206 that facilitates insulin signaling and prevents lipogenesis (104) . Several factors such as obesity, increased serum levels of fatty acids, and insulin resistance can increase the risk of fatty liver disease. P2Y2 receptor, through the induction of the c-Jun N-terminal kinase (JNK) and prevention of insulin signaling, can promote insulin resistance in hepatocytes in T2DM (105) . In some cases, NAFLD may progress into an aggressive form of inflammatory fatty liver disease called non-alcoholic steatohepatitis (NASH), which might cause liver cirrhosis and organ failure (106). Dang et al. have indicated that exosomes released from the adipose tissues of obese mice due to the smaller miR-141-3p content can promote insulin resistance in the murine hepatocyte cell line AML12 (alpha mouse liver 12) (107) . The adipokine visfatin that is released from the adipose tissue of obese individuals has also been shown to activate the pro-inflammatory STAT3 signaling pathway and NF-κB in the human liver cell line HepG2 and promote their insulin resistant state (108) . Nevertheless, the hepatocyte growth factor (HGF) can alleviate the insulin resistance of hepatocytes and control their triglyceride and cholesterol contents (109) .

Skeletal muscle (SM) is the main tissue that releases glucose after insulin stimulation. Hence, insulin resistance in SM has a pivotal role in the metabolic dysregulation of T2DM. Insulin resistance in SM is the primary defect of T2DM that facilitates the progression of fatty liver disease, deposition of fat in the liver (Figure 1 ) (110) . Skeletal muscle from diabetic patients expresses less genes related to insulin signaling and metabolic pathways, but more apoptosis and immune-related genes (111) . This inflammatory milieu is mainly due to the proinflammatory actions of obesity-related adipose tissue mediators, which are released into the circulation and promote inflammation within the SM (4). Furthermore, obesity causes intermyocellular and perimuscular adipose tissue expansion that acts like adipose tissue depots to enhance SM inflammation (112) . It has been shown that human skeletal muscle cells (hSMC), isolated from diabetic patients, after a 24-h culture generate significantly more TNF-α, IL-6, IL-8, IL-15, monocyte chemotactic protein (MCP)-1, Growth-Related Oncogene (GRO)-α, and follistatin compared to non-diabetic individuals (113) . This altered secretion of myokines (e.g., cytokines secreted by SMs) is an intrinsic feature of SM during the progression of T2DM. In SM, GLUT-4, which is quickly translocated to the cell surface, facilitates glucose uptake in response to insulin hormone as well as muscle contraction. Pinto-Junior et al. have shown that the use of AGE-albumin in rats increases the expression of the inflammatory molecule NF-κB1 within the SM. NF-κB1 binds to the promoter of the GLUT-4 gene and suppresses its expression (at both mRNA and protein levels) (114) . Accordingly, GLUT-4 levels on the surfaces of SM decrease and subsequently, whole-body IR develops.

The immune system is generally classified into two main arms, innate and adaptive (or acquired) immunity. Adaptive immunity is mediated by B cells, which produce antibodies and T cells, which are classified into CD4 + helper cells and cytotoxic CD8 + cells. A considerable literature has discussed the dysfunctional immune responses in diabetic patients ( Table 2 ) (115) (116) (117) (118) (119) (120) . Abnormal immune cell activation and subsequent inflammatory environment has an essential role in the progression of T2DM (121) . In this regard, chronic inflammation due mainly to the activation of the myeloid cell lineage (e.g., macrophages and neutrophils), is directly related to the induction of IR (4, 122). 

Their numbers are elevated, are larger and more granular, express diminished levels of antioxidant genes but elevated levels of pro-apoptotic and pro-inflammatory genes.

Complement system Attachment of C-type lectin proteins to mannose residues is decreased, lectin pathway is impaired, CD59 activity is reduced, MAC deposition in vascular walls is increased.

Dendritic cells (DCs) Their numbers and activity are reduced.

Their cholesterol efflux is decreased, generate foam cells, have dysfunctional efferocytosis.

Are activated, constitutively release NETs, produce high levels of MPO, ROS, and calprotectin (S100A8/A9), are more susceptible to apoptosis, their migration, phagocytosis and microbial killing are impaired.

NK cells Their numbers are increased but are usually dysfunctional, express high levels of GLUT4 but decreased levels of NKG2D and NKp46, have reduced degranulation capacity, are more susceptible to apoptosis.

NKT cells Their numbers are increased, produce high levels of IFN-γ, IL-4, and IL-17, express high levels of NKp30, NKG2D, and NKp44 but low levels of NKG2A and 158b.

Innate lymphoid cells (ILCs) ILC1s are increased and produce high levels of IFN-γ.

Humoral immunity (B cells) Germinal centers are reduced, Ab production and isotype switching is defective, Abs become glycated, Abs fail to activate complement. functions of osteoclasts, which are bone-resident innate immune cells (126) . This may affect bone structure and delay bone healing. Defects in the innate, as well as adaptive immunity, are supposed to be the main cause of diabetic individuals' susceptibility to infections (127) . Furthermore, some microorganisms, especially bacteria, in hyperglycemic conditions are better nourished and become more virulent, while also having a better milieu to cause infections.

Complement System

The complement system is a first-line defense mechanism against invading microorganisms. It acts via different but interconnected classical, alternative, and lectin pathways (128) . Ilyas et al. have shown that under high glucose conditions, the attachment of Ctype lectin proteins to high-mannose containing glycoproteins is substantially decreased in a dose-dependent manner. These carbohydrate-binding proteins include mannose-binding lectin (MBL), surfactant protein D (SP-D), dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN, CD209), and DC-SIGN-related (DC-SIGNR) protein (129) . Reduced binding of MBL in the presence of high levels of sugar causes a significant reduction in the lectin pathway activity, but does not influence classical or alternative pathway activity (129) . Nevertheless, Barkai et al. did not find significant differences in the function of classical or MBL pathways between T2DM and healthy individuals (130) . However, significantly decreased activity of ficolin-3-mediated lectin and alternative pathways, as well as decreased levels of C4d and soluble complement C5b-9 (sC5b-9) were seen in diabetic patients with Escherichia coli-mediated urinary tract infections (130) . This may be linked to a reduced ability of diabetics to protect themselves against bacterial infections. The lipopolysaccharides of certain Gram-negative bacteria, like Salmonella serotype O6,7 as well as the cell walls of fungi, are rich in mannose. Possibly, because of this, in addition to additional provision of nutrients, an increased prevalence of fungal infections is seen in T2DM patients (131, 132) . Patel et al. found a significantly higher prevalence of oral candida carriage in diabetic patients compared to healthy controls (131) . They found that Candida albicans was the most commonly isolated species followed by C. tropicalis, but uncommon species such as C. lusitaniae and C. lipolytica were also isolated (131) . Another study by Jhugroo et al. showed that C. albicans is the predominant yeast isolated from oral mucosal lesions of diabetic patients, followed by. C. tropicalis and C. krusei 

Dendritic cells (DCs) are a heterogeneous population of specialized and professional antigen-presenting cells (APCs) that create a crucial link between the innate and adaptive immune responses (136, 137) . Some studies have shown that the numbers of DCs are reduced in both type 1 and 2 diabetes (138, 139). Seifarth et al. have found that T2DM patients with poor metabolic control have decreased numbers of both myeloid and plasmacytoid DCs compared with healthy controls. This could make them more susceptible to opportunistic infections (139) . In the case of good blood glucose control, the reduction in DC numbers was less prominent but still significant, especially for myeloid DC1 (mDC1) cells (139) . Another study by Blank et al. demonstrated that women with T2DM and poor glycemic control (HbA1c ≥ 7%) have fewer numbers of circulating plasmacytoid DCs (pDCs) compared to diabetic women with good glycemic control (HbA1c < 7%) or to healthy women (140). Montani et al. have recently shown that hyperglycemic medium and hyperglycemic sera derived from T2DM patients prevent the maturation of monocytes into effective DCs and their activation in vitro (141) . Interestingly, quercetin, a flavonoid with antiinflammatory and antioxidant characteristics, prevented such effects (141) .

Macrophages are important immune cells that play critical roles through all stages of the pathogenesis of T2DMrelated atherosclerosis (41). Swirski et al. have shown a significantly elevated number of pro-inflammatory monocytes in the circulation of ApoE −/− mice, an animal model of atherosclerosis, compared to control mice (142) . Modifications of the lipoproteins in the arterial walls of diabetic individuals make them pro-inflammatory and activate the overlying endothelium. In response, monocytes are recruited into the subendothelial region, differentiate into macrophages and internalize the accumulated lipoproteins. Finally, cholesterol-laden foam cells are generated. They promote inflammation and progression of the disease through the synthesis and secretion of cytokines, chemokines, ROS, and matrix metalloproteinases (MMPs) (Figure 2C ) (42) . Foam cells lose their migratory potential, die by apoptosis and generate a necrotic core within the atherosclerotic plaque (143) .

It has been demonstrated that the use of mesenchymal stem cells in ApoE −/− mice reduces the numbers of monocytes/macrophages at the site of inflammation, decreases lipid deposition and diminishes plaque size (144). Ma et al. have studied the effects of long-term hyperglycemia in diabetic mice and found out that compared to non-diabetic control mice, the numbers of F4/80 + macrophages isolated from spleen (SPMs), as well as from peritoneal exudates (PEMs) of diabetic mice are significantly decreased (145) . Subsequently, Sun et al. showed that stimulation of PEMs from diabetic mice in vitro with IFN-γ and lipopolysaccharide (LPS) significantly decreased the expression of intercellular adhesion molecule 1 (ICAM-1 or CD54), CD86, TNF-α, and IL-6, while it increased the production of nitric oxide (NO) (146) . They further showed that stimulation of PEMs isolated from diabetic mice with IL-4 caused an enhanced arginase activity (146) . Kousathana et al. have demonstrated that circulating monocytes isolated from diabetic patients produce higher levels IL-6, while having an impaired activation of the NLRP3 inflammasome and subsequently reduced IL-1β production (147) . However, they showed that proper glycemic control would restore such modifications. Poor inflammatory responses in circulating monocytes, as well as in macrophages, are responsible for elevated susceptibility to infections and their severity in patients with T2DM.

Macrophages play a critical role in tissue repair. Early in wound healing, they are pro-inflammatory to clear pathogens and debris but later, they resolve inflammation and promote tissue repair. In pathological conditions, failure to transform from pro-inflammatory to the anti-inflammatory proliferative phase can cause chronic inflammation in the affected tissue (148).

have shown that an impaired wound healing process in animals with T2DM is due to high levels of NLRP3 inflammasome activity, which promotes the generation of IL-1β and IL-18 in macrophages (149, 150) . Efficient skin wound healing process is mediated by the up-regulation of the peroxisome proliferatoractivated receptor (PPAR)-γ in macrophages that convert their pro-inflammatory phenotype into healing-related. PPARγ suppresses cytokine production by macrophages and hence is upregulated in inflamed tissue-resident macrophages. However, in T2DM, PPARγ expression is down-regulated in skin-resident macrophages that enhance the activity of NLRP-3 inflammasome and cause chronic inflammation. Using myeloid-specific PPARγ −/− mice, it has been shown that the absence of PPAR-γ in macrophages is sufficient to delay the healing process and extend tissue inflammation (150) .

In T2DM patients, chronic hyperglycemia and hyperlipidemia trigger the secretion of a damage-associated S100A8 molecule (calgranulin A) from pancreatic islets that in turn increase macrophage infiltration (151) . Westwell-Roper et al. have shown that IAPP aggregates in T2DM patients polarize islet-resident macrophages toward the M1-like F4/80 + CD11b + CD11c + phenotype that produces pro-inflammatory cytokines, including TNF-α, IL-1β, and IL-6. Furthermore, M1 cells promote islet inflammation, cause β-cell malfunction and apoptosis (152) . In T2DM, excess phagocytosis of apoptotic β-cells by macrophages induces their lysosomal permeabilization, generation of ROS, inflammasome activation, and pro-inflammatory cytokines secretion (153) . Collectively, these observations reveal that the functions and plasticity of macrophages are compromised during the progression of T2DM.

Neutrophils are the most prevalent circulating leukocytes and one of the main components of innate immunity. They are recruited to the sites of infection through chemotaxis following complement activation, most importantly by C5a. Activated neutrophils bind via their surface receptors to induced ligands on the surfaces of inflamed endothelial cells to migrate to tissues. There they phagocytose and kill invading microbes with lysosomal enzymes, antimicrobial peptides and by the generation of ROS (154). Neutrophils from patients with T2DM, but not from healthy individuals, are activated and produce elevated levels of ROS. So, it could increase the risk of random organ injury (155) . In diabetic patients, the plasma levels of homocysteine are elevated, which is mainly due to its impaired clearance rate (156) . This will induce neutrophils to constitutively release neutrophil extracellular traps (NETs) that can cause vascular damage and delays in wound healing (157, 158) . It has been shown that the circulating level of hydrogen sulfide (H 2 S) is significantly reduced in fasting blood of patients with T2DM compared with healthy individuals as well as in streptozotocin-induced diabetic rats compared with controls (159) . H 2 S is produced from cysteine by the action of several enzymes. It acts as a regulator of cell signaling and homeostasis (160) .

It is essential to maintain balanced levels of antioxidants and protect tissues from oxidative stress (160) . The use of H 2 S or the endogenous L-cysteine in vitro blocks the production of IL-8 and monocyte chemoattractant protein-1 (MCP-1) in the human U937 monocyte cell line incubated in high-glucose medium (159) . Yang et al. have shown that H 2 S treatment decreases NETosis and enhances the healing process of diabetic wounds by preventing ROS-dependent ERK1/2 and p38 activation (161) . It has been shown that the levels of NET components, including histones, elastase and proteinase-3, are elevated in the sera from patients with diabetic foot ulcers (162) . Wang et al. have recently indicated that HG dramatically enhances NADPH oxidasedependent NET generation in diabetic rats and humans. It was proposed that this could have a role in the induction of diabetic retinopathy (163) . Indeed, patients with T2DM have elevated plasma levels of MGO, which can induce the production of proinflammatory cytokines like TNF-α, IL-6, and IL-8 by neutrophils and make them more susceptible to apoptosis (99) .

Myeloperoxidase (MPO), which is abundantly produced by neutrophils, but only to a small extent by monocytes and macrophages, might be useful as an early biomarker of inflammation in diabetic individuals (164) . Binding of MPO to endothelial cells increases its half-life. Thereby, more proinflammatory oxidant hypochloric acid (HClO) is generated that extends the damage to blood vessels (165) . In T2DM patients, neutrophil activities, including migration, phagocytosis and microbial killing are impaired. This makes diabetic individuals more susceptible to infections (166) . It has been welldocumented that neutrophils isolated in animal models of T2DM have an impaired TLR4 signaling pathway. This is reflected as a diminished cytokine and chemokine production, possibly as a consequence of reduced phosphorylation of NFκB and IκBα (167) . The half-life of these neutrophils as well as their in vivo migration and myeloperoxidase activity are decreased.

During hyperglycemia, neutrophils produce calprotectin (S100A8/A9), which interacts with the receptor for advanced glycation end products (RAGE) on the surface of hepatic Kupffer cells and promotes the synthesis of IL-6 (168). Subsequently, IL-6 stimulates hepatocytes to increase the generation of thrombopoietin that in turn attaches to its receptor on the surfaces of bone marrow precursor cells and megakaryocytes to enhance their proliferation and expansion. This results in reticulated thrombocytosis, which means elevated megakaryocyte activity and thrombopoiesis. Interestingly, diabetes-related thrombocytosis and subsequent atherothrombosis can be reduced by lowering blood glucose, depleting Kupffer cells or neutrophils or by preventing the binding of S100A8/A9 to RAGE using paquinimod (168) . Thom et al. have shown that the incubation of human and murine neutrophils with HG medium would cause their cytoskeletal and membrane instability. This will induce the generation of 0.1 to 1 µm diameter microparticles and activate the NLRP3 inflammasome (169) . Microparticles, which are potently pro-inflammatory, are found in the circulation of healthy individuals, but their generation is increased during cell activation in several diseases, including T2DM and cardiovascular diseases (170, 171) . Furthermore, serum levels of soluble FasL (sFasL) are increased in patients with T2DM thereby activating neutrophils and aggravating the inflammatory milieu (172, 173) . The proinflammatory roles of sFasL are mediated through increased amounts or activity of NFκB, IL-1β, caspase-1, CD11b/CD18, and ROS (173) . Caspase-1 activation prevents the sFasL-dependent apoptosis of neutrophils and inhibits their expression of Fas and caspase-3 (173) . Accordingly, hyperglycemia disturbs the normal functions of neutrophils and increases the susceptibility to infections by pathogenic microorganisms. The expression level of NKG2D is negatively correlated with HbA1c levels implying that chronic hyperglycemia would cause NK cell dysfunction (176) . Also, hyperglycemia increases the expression of unfolded protein response (UPR) genes in NK cells and induces their apoptosis (176) .

NKT cells express simultaneously markers of both T cells (TCR and CD3) and NK cells [CD16, CD56, CD314 (NKG2D), and CD337 (NKp30)]. NKT cell subsets produce a broad range of cytokines, including GM-CSF, IFN-γ, TNF-α, IL-2, IL-4, IL-5, IL-9, IL-10, IL-13, IL-17, and IL-21 (178) . They recognize lipids and glycolipids presented by CD1d molecules. Phoksawat et al. have shown that the frequency of CD3 + CD4 + CD28 null CD56 + NKG2D hi NKT cells, which produce high levels of IL-17, are increased in diabetic patients and their numbers are directly correlated with HbA1c levels (179, 180) . Lv et al. have recently shown that the numbers of CD3 + CD56 + NKT cells are higher in diabetic patients compared to healthy individuals (181) . They further showed that such cells are mostly CD4 + , produce elevated levels of IFN-γ and IL-4 and express high levels of NKp30, NKG2D, and NKp44 but low levels of inhibitory receptors NKG2A and 158b (181) . The co-culture of these cells with HUVECs significantly decreased their proliferation and migration abilities that were mainly IL-4dependent (181) . Taken together these studies show that diabetic individuals appear to have elevated levels of inflammationpromoting NKT cells.

ILCs are critical effectors of innate immunity that produce both regulatory and pro-inflammatory cytokines to promote tissue repair, immunity, and inflammation (182) . Mature ILCs lack the TCRs. Based on their cell surface markers, cytokine production as well as expression of transcription factors the ILCs are classified into types 1, 2, and 3 (183) . These correspond to the different types of CD4+ T helper cells: Th1, Th2, and Th17, respectively. IFN-γ is the cytokine signature of ILC1s, while type 2 cytokines (e.g., IL-5 and IL-13) are mainly produced by ILC2s and the main product of ILC3s are IL-17 and IL-22. Regarding transcription factors, T-bet is mainly expressed by ILC1s, GATA3 and RORα are mostly expressed by ILC2s and RORγt is predominantly expressed by ILC3 (183) .

In T2DM, the numbers of circulating as well as adipose tissue-resident ILC1s are increased compared with normal individuals (184, 185) . The frequency of circulating ILC1s is positively correlated with fasting plasma glucose (FPG), HbA1c, homeostasis model assessment for insulin resistance (HOMA-IR), serum-free fatty acids (FFAs) and adipose tissue insulin resistance index (Adipo-IR) (184, 185) . It has also been shown that patients with increased numbers of ILC1 have an elevated risk of developing T2DM (184) . A study by Wang et al. indicated that adipose tissue-resident ILC1s, via the production of IFN-γ, promote tissue fibrosis and induce diabetes in obese individuals (185) . Liu et al. have demonstrated that the numbers of ILC2s as well as serum cytokine levels of IL-4, IL-5, and IL-13 are significantly elevated in diabetic kidney disease patients and have a positive correlation with disease severity (186) . They further demonstrated that ILC2s, through the TGF-β1 signaling pathway, are involved in renal fibrosis seen in diabetic kidney disease (184) . However, Galle-Treger et al. indicated that the engagement of the glucocorticoid-induced tumor necrosis factor receptor (GITR/or TNFRSF18) on the surface of activated ILC2s promotes their secretion of IL-5 and IL-13, ameliorates glucose homeostasis, protects against the onset of and improves established insulin resistance (187) . The protective role of ILC2s during acute metabolic stress has also been well-documented by Dalmas et al. (188) .

Humoral Immunity (B Cells) Elevated levels of blood glucose generate covalent sugar adducts with several proteins through non-enzymatic glycation. This can impair humoral immunity in many ways, e.g., by modifying the structure and functions of immunoglobulins (Igs) (189) (190) (191) (192) (193) (194) . Such modifications in the structure of Igs can be determined using matrix-assisted laser desorption ionization (MALDI) mass spectrometry (119, 191) . The molecular mass of Igs in diabetic patients is higher than in normal subjects (189) . This can lead to reduced efficiency of vaccines that stimulate humoral immunity in these patients. It has been shown that immunization with influenza (flu) vaccines in diabetic patients induces normal or even elevated levels of flu-specific antibodies compared with normal individuals (195) (196) (197) (198) . However, the ability of the dysfunctional glycated antibodies to neutralize viruses is impaired, which will increase the susceptibility to infections. Farnsworth et al. have shown that in T2DM, class switch defects in the assembly of antibody genes are also present (199) . In a model system, mice with T2DM have decreased amounts of specific anti-Staphylococcus aureus antibodies (total as well as IgG), which will increase the risk of infection and morbidity of diabetic mice. However, the levels of IgM were elevated, but inefficient in protecting against infection, possibly because of their inability to directly promote phagocytosis. In another study, Farnsworth et al. have demonstrated that defects in humoral immunity, as shown by decreased levels of total IgG and anti-Staphylococcus aureus antibody, aggravate foot infections in a murine model of T2DM (200) . This was due to a reduced germinal center induction and decreased numbers of T and Blymphocytes within the germinal centers. This causes failures in antibody generation and class-switch recombination (200) . Mathews et al. have shown that the protective levels of antibodies against Streptococcus pneumoniae surface protein A are lower in diabetic patients compared to non-diabetic individuals. These antibodies also have a reduced potential to trigger complement activation on the surface of pneumococci, whereby phagocytosis of the bacteria becomes compromised (201) . They showed that hyperglycemia reduces both the antibody titers as well as the ability to deposit complement on the bacteria. The abovementioned changes in the ability to protect against S. aureus and S. pneumoniae are important, because these bacteria belong to the most common infection-causing pathogens in diabetic patients. Another major group is constituted by Gram-negative bacteria that commonly cause e.g., urinary tract infections.

Many studies have shown that T-cell functions are impaired in individuals with T2DM (202) (203) (204) (205) . Elevated levels of activated CD4 + CD278 + T helper cells, cytotoxic T-cells, and Th17 cells have been observed in obese diabetic patients compared to nonobese ones (205, 206) . Nevertheless, PBMCs isolated from obese diabetic patients produced smaller amounts of IL-2, IL-6, and TNF-α after stimulation with phytohemagglutinin (PHA) (205) . Martinez et al. indicated that diabetic patients have reduced pathogen-specific memory Th17 responses as well as decreased numbers of CD4+ T cells in response to stimulation with Streptococcus pneumoniae (206) . Th17 cells are critical for the recruitment of neutrophils to the infection site and improve the phagocytosis of invading bacteria and yeast (207) . Moura et al. have shown that diabetic patients, particularly those with foot ulcers, have reduced levels of naive T-cells, but an elevated number of effector T cells and a reduction in the TCR-Vβ repertoire diversity (204) . The observed changes are mainly due to an abnormal amount of inflammatory cytokines (e.g., IFN-γ and TNF-α) produced during infection and to subsequent robust stimulation of T-cells. Leung et al. have reported that ischemic tissues of T2DM patients contain elevated numbers of TNF-α and IFN-γ producing Th1 cells but diminished numbers of regulatory T cells (Tregs), which suppress angiogenesis and decrease vascular density (208) .

The high rate of infectious diseases in T2DM patients might also be linked to a reduction in the mitochondrial DNA function that causes downstream lymphocyte dysfunction and subsequently increased susceptibility to infection (209) (210) (211) (212) . In support, we have recently shown that the numbers of IFN-γ producing cells against cytomegalovirus (CMV), Epstein-Barr virus (EBV), and influenza virus are fewer in T2DM patients compared to normal controls (202) . Kumar et al. have also investigated the functions of CD8 + T cells and NK cells in the whole blood of T2DM patients infected with Mycobacterium tuberculosis (M.tb). Compared to controls, the patients exhibited a reduction in cytokine production (IFN-γ, IL-2, IL-17A/F, and TNF-α) and decreased expression of cytotoxic molecules (perforin, granzyme B, and CD107a) (203, 213) . These studies conclude that the functions of both CD4 + and CD8 + T-cell are defective in T2DM patients.

T2DM is usually associated with an elevated risk of asymptomatic bacteriuria, urinary tract infections (UTIs), pyelonephritis and non-sexually transmitted genital infections, such as balanitis and vulvovaginal infections (213) (214) (215) . The incidence of infections with a complicated course is significantly higher in diabetic patients compared to healthy controls ( Table 3) . It seems that it is principally defects in the innate immune responses of diabetic individuals that are responsible for the increased susceptibility and prevalence of infections (4, 225 (199, 201) CD8 + Tcells Mycobacterium tuberculosis (203, 213) susceptible to the causative pathogen of Lyme disease, Borrelia burgdorferi (216) . The disease is mainly due to the ability of the bacteria to escape complement opsonization and attack, which leads to an impaired uptake and killing of bacteria by neutrophils (227) . Neutrophil dysfunction also increases the susceptibility of diabetic animals to Staphylococcus aureus (217) (233) . During the progression of T2DM in human subjects, the basal phenotype of macrophages is altered so their capacity to control Mycobacterium tuberculosis is diminished (222) . Martinez et al. have indicated that alveolar macrophages isolated from diabetic mice express decreased levels of macrophage receptor with collagenous structure (MARCO) and CD14 that are engaged in the recognition of trehalose 6,6'-dimycolate, a bacterial cell wall component (223) . Diabetes increases the severity of tuberculosis (TB) and enhances the risk of progression to the active form in latent infections (234, 235) . Diabetic TB patients have elevated frequencies of Th1 and Th17 cells as well as increased serum levels of inflammatory cytokines, including IFN-γ, TNF-α, IL-1β, IL-2, IL-6, IL-17A, and IL-18 but decreased levels of IL-22 compared to non-diabetic TB patients. This can contribute to dysfunctional immune responses and poor immune control of a TB infection (236) . A positive correlation between the serum levels of IFN-γ, TNF-α, IL-2, and IL-17A with Hb-A1c levels was also observed. This indicates an association between impaired control of diabetes and the proinflammatory milieu. Tripathi et al. have demonstrated that serum levels of IL-22 were significantly decreased in TB-infected T2DM mice and humans compared to non-diabetic TB-infected mice and humans (224) . They revealed that the treatment of TB-infected diabetic mice with recombinant IL-22 or ILC3s (cellular source of IL-22) increased the survival of mice, prevented the accumulation of neutrophils near alveoli, diminished the generation of neutrophil elastase 2 (ELA2) and prevented epithelial cell damage (224) . Tan et al. have shown that B. pseudomallei and M. tuberculosisinfected PBMCs of diabetic patients fail to produce IL-12. This leads to a decreased IFN-γ production, poor bacterial killing and elevated intracellular bacterial loads (237) . An impaired IL-12 production is mainly due to decreased intracellular glutathione (GSH) concentrations within the infected cells of diabetic individuals (237) . Such a combination of an inflammatory microenvironment and dysfunctional immune responses enhances the bacterial load and can subsequently amplify lung injury and fibrosis in diabetic TB patients. Chellan et al. have further shown that infections caused by Enterococcus faecalis, Staphylococcus aureus, and Pseudomonas aeruginosa are more prevalent in the wounds of diabetic patients (238) . T2DM patients are more susceptible to UTIs caused by antibioticresistant Escherichia coli, Proteus spp., Klebsiella spp., coagulasenegative staphylococci, Enterobacter spp., and enterococci (215, 239) . Diabetic patients are also more susceptible to Helicobacter pylori (H. pylori) infections (240).

Cui et al. have recently reported that T2DM patients have an increased risk of infection with Kaposi's sarcoma-associated herpesvirus (KSHV or HHV-8) (241) . They further showed that the viral load and antibody titers are positively correlated with blood glucose levels (241) . Diabetic patients also have been shown to have an increased risk of infection with the severe acute respiratory syndrome coronavirus (SARS-CoV) ( (248) . The influenza virus that usually causes self-limiting infections can induce severe forms of the disease in diabetic patients (249, 250) . Following the 2009 H1N1 influenza pandemic, diabetic individuals suffered from more severe infections compared to non-diabetic people (251, 252) . Diabetic patients have also a higher prevalence of chronic cytomegalovirus (CMV), Herpes simplex virus (especially HSV-1), and varicellazoster virus infections (253) (254) (255) . Accordingly, it seems that the immune response against viruses is impaired in diabetics, and these patients need more care during viral infections.

Coronavirus virions are enveloped positive-strand RNA spherical viruses with a diameter of ∼125 nm characterized by spike proteins projecting from their surface and with an unusual large RNA genome (256) . The spike (S) protein of the virus binds to its receptor on the surface of cells by which intracellular proteases are induced (257) (258) (259) . Subsequently, the S protein priming and cleavage occurs that allow viral fusion to the plasma membrane and entrance of viral genome into the cells (259) . SARS-CoV and SARS-CoV-2 use angiotensin-converting enzyme 2 (ACE2) as their receptor while MERS-CoV uses dipeptidyl peptidase-4 (DPP4) to enter the cells (260, 261) . ACE2 is strongly expressed in blood vessels, pancreas, intestine, brain, lungs, heart, and testis (262) .

Interestingly, nasal epithelial cells, especially goblet, and ciliated cells express the highest levels of ACE2 and the intracellular protease transmembrane serine protease 2 (TMPRSS2) that facilitates the entrance of the SARS-COV-2 (263) . Furthermore, the expression of ACE2 is significantly up-regulated in diabetic patients and those treated with ACE inhibitors (264) . Coronaviruses cause respiratory, enteric and central nervous system (CNS) diseases in various animal species except rats and mice (264) . Most coronavirus infections are mild, but major outbreaks of deadly pneumonia have been caused by SARS-CoV, MERS-CoV, and SARS-CoV-2 in 2002, 2014, and 2019-2020, respectively (265) .

On March 11, 2020, The World Health Organization (WHO) announced the pandemic of SARS-CoV-2, the etiologic agent of coronavirus disease-19 (COVID-19) (265) . The novel coronavirus pandemic, which has emanated from Wuhan, China, promotes symptoms similar to those caused by the SARS-CoV outbreak in 2002. The viral pandemic, which has put the world on alert, has caused over 7.9 × 10 6 confirmed human cases and at least 43 × 10 4 deaths throughout the world (https://www.worldometers.info/coronavirus/) by June 14, 2020. Most of the infected people experience only mild to moderate respiratory disease and recover soon without the need for special treatment. However, aged individuals and those with health problems, including diabetes, obesity, cardiovascular disease (CVD), hypertension, immune deficiency, and chronic respiratory disease are more likely to develop serious illness (https://www.who.int/health-topics/coronavirus#tab= tab_1). Patients death is mainly due to the acute respiratory distress syndrome, disseminated intravascular coagulation, hemorrhage, coagulopathy, acute organ (e.g., kidney, heart, liver) injury, multi-organ failure, and secondary bacterial infections (266) . Elevated levels of adipose-tissue derived adipokines, interferon, and TNF-α in diabetic patients may impair immune-responses against SARS-COV-2 (267, 268) . It has been shown that diabetic patients have impaired clearance of SARS-CoV-2 from their circulation (269) . Accordingly, diabetic patients due to the diminished viral clearance, impaired T cell function, and accompanied cardiovascular disease are more susceptible to the coronaviruses infection and subsequent cytokine release syndrome (CRS) (270, 271) . In support, elevated levels of IL-1β, IL-2, IL-6, IL-7, IL-8, IL-10, IFN-γ, interferon gamma-induced protein 10 (IP-10), granulocyte colony-stimulating factor (G-CSF), macrophage inflammatory protein 1α (MIP1α), serum ferritin, fibrinogen, plasminogen, C-reactive protein (CRP), and D-dimer have been observed in patients with COVID-19 (266, 269, 272, 273) . COVID-19 patients, especially those requiring intensive care unit (ICU) have decreased total lymphocytes (lymphopenia), T cells (both CD4+ and CD8+), B cells, and NK cells (274, 275) . It should be noted that most of the surviving T cells in such patients have an exhausted phenotype (274) . Consequently, disease severity is mainly because of the host immune response to viral infection.

Current evidence about the relationship between pathophysiological mechanisms of diabetes and COVID-19 are limited and further research is still needed.

Patients with T2DM have an elevated risk of infection with Plasmodium falciparum (276) , Toxoplasma gondii (277), Opisthorchis viverrini (278), Strongyloides stercoralis (279), Cryptosporidium parvum (280), Blastocystis hominis (281), Ascaris lumbricoides (280, 282, 283) , and Giardia lamblia (283) . Interestingly, diabetic patients who were treated with metformin had less P. falciparum infections compared to untreated patients (276) . Omaña-Molina et al. have shown that in a mouse model of T2DM the animals have an increased susceptibility to granulomatous amoebic encephalitis (GAE) caused by trophozoites of Acanthamoeba culbertsoni (284) . The possible reasons for the increased risk of diabetics for parasitic infections are metabolic abnormalities and immune dysregulation.

Chellan et al. have shown a higher prevalence of fungal infections in the wounds of diabetic patients (238) . The prevalence correlated with the levels of HbA1c. The most widely observed fungal isolates were C. albicans, Candida parapsilosis, C. tropicalis, Trichosporon asahii, and Aspergillus species. Some of them were resistant to antifungal medications (238) . Al Mubarak et al. have also demonstrated that diabetic patients with periodontitis are more susceptible to infection with C. albicans, C. dubliniensis, C. tropicalis, and C. glabrata (285) . The incidence of candidiasis was significantly increased in patients over the age of 40 with HbA1c > 9 (285). It has also been shown that diabetic patients are more susceptible to UTIs caused by C. albicans (239) .

Hyperglycemia impairs the normal functions of the circulatory system, gastrointestinal tract, pancreatic beta cells, liver as well as of skeletal muscles to boost systemic insulin resistance. A hyperglycemic environment also leads to immune cells dysfunction. It increases intestinal permeability, which subsequently enhances the risk of infections in T2DM patients. Accordingly, further research is still needed to find missing links between impaired physiological/immunological mechanisms and increased susceptibility to infections in T2DM patients. The information would be important for better therapy and the design of much more effective vaccination strategies in diabetic patients.

GD and KK conceived the study and wrote the manuscript. GD contributed to the final revision of the manuscript. MA participated in preparing the first draft. DK and SM were involved in the final revision of the manuscript. All authors contributed to the article and approved the submitted version. 

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Platelet activity and hypercoagulation in type 2 diabetes

Increased circulating resistin is associated with insulin resistance, oxidative stress and platelet activation in type 2 diabetes mellitus

Increased levels of soluble adhesion molecules in type 2 (non-insulin dependent) diabetes mellitus are independent of glycaemic control

Molecular mechanisms underpinning microparticle-mediated cellular injury in cardiovascular complications associated with diabetes

Association between mean platelet volume in the pathogenesis of type 2 diabetes mellitus and diabetic macrovascular complications in Japanese patients

Increased erythrocyte-and platelet-derived microvesicles in newly diagnosed type 2 diabetes mellitus

Type 2 diabetes and cardiovascular disease: have all risk factors the same strength?

Advanced glycation end products induce a prothrombotic phenotype in mice via interaction with platelet CD36

Central role of the P2Y12 receptor in platelet activation

Long non-coding RNA metallothionein 1 pseudogene 3 promotes p2y12 expression by sponging miR-126 to activate platelet in diabetic animal model

Introduction to the human gut microbiota

Hyperglycemia drives intestinal barrier dysfunction and risk for enteric infection

Faecalibacterium prausnitzii-derived microbial anti-inflammatory molecule regulates intestinal integrity in diabetes mellitus mice via modulating tight junction protein expression

Considering gut microbiota in treatment of type 2 diabetes mellitus

Metformin effect on gut microbiota: insights for HIV-related inflammation

The bacterium Akkermansia muciniphila: a sentinel for gut permeability and its relevance to HIV-related inflammation

High glucose decreases intracellular glutathione concentrations and upregulates inducible nitric oxide synthase gene expression in intestinal epithelial cells

The role of iron in diabetes and its complications

Hyperglycemia promotes microvillus membrane expression of DMT1 in intestinal epithelial cells in a PKCalpha-dependent manner

Betacell deficit and increased beta-cell apoptosis in humans with type 2 diabetes

Fatty acid-induced lipotoxicity in pancreatic beta-cells during development of type 2 diabetes

Mesenchymal stem cell therapy in type 2 diabetes mellitus

Establishment of insulin-producing cells from human embryonic stem cells underhypoxic condition for cell based therapy

Human mesenchymal stem cell derived exosomes alleviate type 2 diabetes mellitus by reversing peripheral insulin resistance and relieving β-cell destruction

A simple method for the generation of insulin producing cells from bone marrow mesenchymal stem cells

Secretagogin affects insulin secretion in pancreatic beta-cells by regulating actin dynamics and focal adhesion

Secretagogin regulates insulin signaling by direct insulin binding. iScience

Secretagogin is increased in plasma from type 2 diabetes patients and potentially reflects stress and islet dysfunction

Islet amyloid polypeptide, islet amyloid, and diabetes mellitus

Human IAPP amyloidogenic properties and pancreatic betacell death

Extracellular vesicles from human pancreatic islets suppress human islet amyloid polypeptide amyloid formation

Functional proteasome complex is required for turnover of islet amyloid polypeptide in pancreatic beta-cells

Lipid accelerating the fibril of islet amyloid polypeptide aggravated the pancreatic islet injury in vitro and in vivo

Chronic methylglyoxal infusion by minipump causes pancreatic beta-cell dysfunction and induces type 2 diabetes in Sprague-Dawley rats

Proinflammatory and proapoptotic effects of methylglyoxal on neutrophils from patients with type 2 diabetes mellitus

Methylglyoxal impairs insulin secretion of pancreatic β-cells through increased production of ROS and mitochondrial dysfunction mediated by upregulation of UCP2 and MAPKs

Evidence of beta-cell dedifferentiation in human type 2 diabetes

Direct effects of thyroid hormones on hepatic lipid metabolism

Prevalence and associated factors of non-alcoholic fatty liver disease in patients with type-2 diabetes mellitus

MicroRNA-206 prevents hepatosteatosis and hyperglycemia by facilitating insulin signaling and impairing lipogenesis

Evidence for P2Y2 receptor facilitation of hyperglycemiainduced insulin resistance in human hepatocytes

Exosomal transfer of obesity adipose tissue for decreased miR-141-3p mediate insulin resistance of hepatocytes

Visfatin induces inflammation and insulin resistance via the NFκb and STAT3 signaling pathways in hepatocytes

Hepatocyte growth factor alleviates hepatic insulin resistance and lipid accumulation in high-fat diet-fed mice

Pathogenesis of type 2 diabetes: tracing the reverse route from cure to cause

Transcriptional profiles of type 2 diabetes in human skeletal muscle reveal insulin resistance, metabolic defects, apoptosis, and molecular signatures of immune activation in response to infections

Intermuscular and perimuscular fat expansion in obesity correlates with skeletal muscle T cell and macrophage infiltration and insulin resistance

Altered myokine secretion is an intrinsic property of skeletal muscle in type 2 diabetes

Advanced glycation end products-induced insulin resistance involves repression of skeletal muscle GLUT4 expression

Chemotaxis of polymorphonuclear leukocytes from patients with diabetes mellitus

Impaired leucocyte functions in diabetic patients

Infections in patients with diabetes mellitus

Impaired immune responses in streptozotocin-induced type I diabetes in mice. Involvement of high glucose

Infections in patients with diabetes mellitus: a review of pathogenesis

Differential effect of hyperglycaemia on the immune response in an experimental model of diabetes in BALB/cByJ and C57Bl/6J mice: participation of oxidative stress

Type 2 diabetes as an inflammatory disease

Pyrin and hematopoietic interferon-inducible nuclear protein domain proteins: innate immune sensors for cytosolic and nuclear DNA

Glycemic reduction alters white blood cell counts and inflammatory gene expression in diabetes

Methylglyoxal disturbs the expression of antioxidant, apoptotic and glycation responsive genes and triggers programmed cell death in human leukocytes

Effect of high glucose on cytokine production by human peripheral blood immune cells and type I interferon signaling in monocytes: Implications for the role of hyperglycemia in the diabetes inflammatory process and host defense against infection

Osteoclasts in bone regeneration under type 2 diabetes mellitus

Host susceptibility factors to bacterial infections in type 2 diabetes

Complement: a key system for immune surveillance and homeostasis

High glucose disrupts oligosaccharide recognition function via competitive inhibition: a potential mechanism for immune dysregulation in diabetes mellitus

Decreased ficolin-3-mediated complement lectin pathway activation and alternative pathway amplification during bacterial infections in patients with type 2 diabetes mellitus

Oral candidal speciation, virulence and antifungal susceptibility in type 2 diabetes mellitus

Characterization of oral mucosa lesions and prevalence of yeasts in diabetic patients: a comparative study

Identification of C1q as a binding protein for advanced glycation end products

Glycation inactivation of the complement regulatory protein CD59: a possible role in the pathogenesis of the vascular complications of human diabetes

Complement activation in patients with diabetic nephropathy

Mechanisms regulating dendritic cell specification and development

Reduced frequency of peripheral plasmacytoid dendritic cells in type 1 diabetes

Reduced frequency of peripheral dendritic cells in type 2 diabetes

Circulating dendritic cell number and intracellular TNF-alpha production in women with type 2 diabetes

High glucose and hyperglycemic sera from type 2 diabetic patients impair DC differentiation by inducing ROS and activating Wnt/β-catenin and p38 MAPK

Ly-6Chi monocytes dominate hypercholesterolemia-associated monocytosis and give rise to macrophages in atheromata

Macrophage apoptosis and necrotic core development in atherosclerosis: a rapidly advancing field with clinical relevance to imaging and therapy

Human gingiva-derived mesenchymal stem cells modulate monocytes/macrophages and alleviate atherosclerosis

Diabetesinduced alteration of F4/80+ macrophages: a study in mice with streptozotocin-induced diabetes for a long term

The phenotype and functional alterations of macrophages in mice with hyperglycemia for long term

Defective production of interleukin-1 beta in patients with type 2 diabetes mellitus: Restoration by proper glycemic control

Macrophage dysfunction impairs resolution of inflammation in the wounds of diabetic mice

Sustained inflammasome activity in macrophages impairs wound healing in type 2 diabetic humans and mice

Macrophage PPARgamma and impaired wound healing in type 2 diabetes

Signaling between pancreatic beta cells and macrophages via S100 calciumbinding protein A8 exacerbates beta-cell apoptosis and islet inflammation

Resident macrophages mediate islet amyloid polypeptide-induced islet IL-1beta production and beta-cell dysfunction

Apoptotic beta-cells induce macrophage reprogramming under diabetic conditions

Preliminary study on overproduction of reactive oxygen species by neutrophils in diabetes mellitus

Effects of insulin on methionine and homocysteine kinetics in type 2 diabetes with nephropathy

Diabetes primes neutrophils to undergo NETosis, which impairs wound healing

Elevated homocysteine levels in type 2 diabetes induce constitutive neutrophil extracellular traps

Low levels of hydrogen sulfide in the blood of diabetes patients and streptozotocintreated rats causes vascular inflammation?

Antioxidant and cell-signaling functions of hydrogen sulfide in the central nervous system

Hydrogen sulfide primes diabetic wound to close through inhibition of NETosis

NETosis delays diabetic wound healing in mice and humans

Hyperglycemia induces neutrophil extracellular traps formation through an nadph oxidasedependent pathway in diabetic retinopathy

Myeloperoxidase is associated with insulin resistance and inflammation in overweight subjects with first-degree relatives with type 2 diabetes mellitus

Leukocyte-derived myeloperoxidase amplifies high-glucose-induced endothelial dysfunction through interaction with high-glucose-stimulated, vascular non-leukocyte-derived reactive oxygen species

Neutrophil function and metabolism in individuals with diabetes mellitus

Obesity and type 2 diabetes mellitus induce lipopolysaccharide tolerance in rat neutrophils

Neutrophil-derived S100 calcium-binding proteins A8/A9 promote reticulated thrombocytosis and atherogenesis in diabetes

Neutrophil microparticle production and inflammasome activation by hyperglycemia due to cytoskeletal instability

Microparticles and type 2 diabetes

Microparticles as potential biomarkers of cardiovascular disease

Increased levels of soluble Fas in serum from diabetic patients with neuropathy

sFasL-mediated induction of neutrophil activation in patients with type 2 diabetes mellitus

The dysfunction of NK cells in patients with type 2 diabetes and colon cancer

NK cell count and glucotransporter 4 (GLUT4) expression in subjects with type 2 diabetes and colon cancer

Natural killer cell function, an important target for infection and tumor protection, is impaired in type 2 diabetes

Oxidative stress mediates a reduced expression of the activating receptor NKG2D in NK cells from end-stage renal disease patients

Diverse cytokine production by NKT cell subsets and identification of an IL-17-producing CD4-NK1.1-NKT cell population

Aberrant NKG2D expression with IL-17 production of CD4+ T subsets in patients with type 2 diabetes

IL-17 production by NKG2D-expressing CD56+ T cells in type 2 diabetes

Role of natural killer T (NKT) cells in type II diabetes-induced vascular injuries

The biology of innate lymphoid cells

Innate lymphoid cells-a proposal for uniform nomenclature

Type 1 innate lymphoid cells are associated with type 2 diabetes

Adipose group 1 innate lymphoid cells promote adipose tissue fibrosis and diabetes in obesity

Group 2 innate lymphoid cells participate in renal fibrosis in diabetic kidney disease partly via TGF-β1 signal pathway

Costimulation of type-2 innate lymphoid cells by GITR promotes effector function and ameliorates type 2 diabetes

Interleukin-33-activated islet-resident innate lymphoid cells promote insulin secretion through myeloid cell retinoic acid production

Evidence for an increased glycation of IgG in diabetic patients

Glycation of monoclonal antibodies impairs their ability to bind antigen

Non-enzymatic glycation of IgG: an in vivo study

Glycation of human IgG induces structural alterations leading to changes in its interaction with anti-IgG

Reactive immunization suppresses advanced glycation and mitigates diabetic nephropathy

The immune response to influenza vaccination in diabetic patients

Humoral immune response and delayed type hypersensitivity to influenza vaccine in patients with diabetes mellitus

Cytotoxic T-cell response to influenza A subunit vaccine in patients with type 1 diabetes mellitus

The antibody response to influenza vaccination is not impaired in type 2 diabetics

A humoral immune defect distinguishes the response to Staphylococcus aureus infections in mice with obesity and type 2 diabetes from that in mice with type 1 diabetes

Exacerbated Staphylococcus aureus foot infections in obese/diabetic mice are associated with impaired germinal center reactions, Ig class switching, and humoral immunity

Impaired function of antibodies to pneumococcal surface protein A but not to capsular polysaccharide in Mexican American adults with type 2 diabetes mellitus

Islet amyloid polypeptide is not a target antigen for CD8+ T-cells in type 2 diabetes

Type 2 diabetes mellitus is associated with altered CD8(+) T and natural killer cell function in pulmonary tuberculosis

Impaired T-cell differentiation in diabetic foot ulceration

Individuals with obesity and type 2 diabetes have additional immune dysfunction compared with obese individuals who are metabolically healthy

Impaired CD4+ and T-helper 17 cell memory response to Streptococcus pneumoniae is associated with elevated glucose and percent glycated hemoglobin A1c in Mexican Americans with type 2 diabetes mellitus

Protection against Streptococcus pneumoniae serotype 1 acute infection shows a signature of Th17-and IFN-gamma-mediated immunity

Regulatory T cells promote apelin-mediated sprouting angiogenesis in type 2 diabetes

Mitochondrial hyperpolarization: a checkpoint of T-cell life, death and autoimmunity

Genetic factors related to mitochondrial function and risk of diabetes mellitus

The role of mitochondria in the pathogenesis of type 2 diabetes

Role and clinical significance of lymphocyte mitochondrial dysfunction in type 2 diabetes mellitus

Acute pyelonephritis in diabetes mellitus: Single center experience

Vulvovaginal candidiasis and its related factors in diabetic women

Urinary tract infections in patients with type 2 diabetes mellitus: review of prevalence, diagnosis, and management

Hyperglycemia impairs neutrophil-mediated bacterial clearance in mice infected with the lyme disease pathogen

Insulin treatment directly restores neutrophil phagocytosis and bactericidal activity in diabetic mice and thereby improves surgical site Staphylococcus aureus infection

Impaired phagocytosis of capsular serotypes K1 or K2 Klebsiella pneumoniae in type 2 diabetes mellitus patients with poor glycemic control

Human polymorphonuclear neutrophil responses to Burkholderia pseudomallei in healthy and diabetic subjects

Neutrophil extracellular traps exhibit antibacterial activity against Burkholderia pseudomallei and are influenced by bacterial and host factors

A critical role for neutrophils in resistance to experimental infection with Burkholderia pseudomallei

Type-2 diabetes alters the basal phenotype of human macrophages and diminishes their capacity to respond, internalise, and control Mycobacterium tuberculosis

Impaired recognition of Mycobacterium tuberculosis by alveolar macrophages from diabetic mice

IL-22 produced by type 3 innate lymphoid cells (ILC3s) reduces the mortality of type 2 diabetes mellitus (T2DM) mice infected with Mycobacterium tuberculosis

Diabetic complications and dysregulated innate immunity

Culture characterization of the skin microbiome in type 2 diabetes mellitus: a focus on the role of innate immunity

Complement evasion by Borrelia burgdorferi: serumresistant strains promote C3b inactivation

Metformin reduces airway glucose permeability and hyperglycaemiainduced Staphylococcus aureus load independently of effects on blood glucose

Impaired early cytokine responses at the site of infection in a murine model of type 2 diabetes and melioidosis comorbidity

Programmed death ligand 1 on Burkholderia pseudomallei-infected human polymorphonuclear neutrophils impairs T cell functions

Diabetes alters immune response patterns to acute melioidosis in humans

Diabetes exacerbates infection via hyperinflammation by signaling through TLR4 and RAGE

TB-diabetes co-morbidity in Ghana: the importance of Mycobacterium africanum infection

Diabetes and immunity to tuberculosis

Type 2 diabetes mellitus coincident with pulmonary tuberculosis is associated with heightened systemic type 1, type 17, and other proinflammatory cytokines

Glutathione deficiency in type 2 diabetes impairs cytokine responses and control of intracellular bacteria

Spectrum and prevalence of fungi infecting deep tissues of lowerlimb wounds in patients with type 2 diabetes

Common uropathogens and their antibiotic susceptibility pattern among diabetic patients

Helicobacter pylori infection is associated with type 2 diabetes, not type 1 diabetes: an updated meta-analysis

Kaposi's sarcoma associated herpesvirus seropositivity is associated with type 2 diabetes mellitus: a case-control study in Xinjiang, China

Plasma glucose levels and diabetes are independent predictors for mortality and morbidity in patients with SARS

Risk factors for primary middle east respiratory syndrome coronavirus illness in humans, Saudi Arabia

Prevalence of hepatitis c virus infection in type 2 diabetic patients at a tertiary care hospital

Prevalence and genotype distribution of hepatitis C virus infection among patients with type 2 diabetes mellitus

Prevalence of hepatitis B and hepatitis C among diabetes mellitus type 2 individuals

Impaired virus clearance, compromised immune response and increased mortality in type 2 diabetic mice infected with West Nile virus

Seroprevalence and risk factors associated with HBV and HCV infection among subjects with type 2 diabetes from South India

One health multiple challenges: the interspecies transmission of influenza A virus. One Health

Influenza virus and glycemic variability in diabetes: a killer combination? Front Microbiol

Diabetes and the severity of pandemic influenza A (H1N1) infection

Mortality of 2009 pandemic influenza A (H1N1) in Germany

Association of type 2 diabetes mellitus and seroprevalence for cytomegalovirus

An Association of herpes simplex virus type 1 infection with type 2 diabetes

Increased risk of Herpes Zoster in diabetic patients comorbid with coronary artery disease and microvascular disorders: a population-based study in Taiwan

Human coronavirus: host-pathogen interaction

Middle East respiratory syndrome coronavirus infection mediated by the transmembrane serine protease TMPRSS2

The spike glycoprotein of the new coronavirus 2019-nCoV contains a furinlike cleavage site absent in CoV of the same clade

SARS-CoV-2 cell entry depends on ACE2 and TMPRSS2 and is blocked by a clinically proven protease inhibitor

Identification of a receptor-binding domain in the s protein of the novel human coronavirus middle east respiratory syndrome coronavirus as an essential target for vaccine development

Covid-19, Coronavirus, SARS-CoV-2 and the small bowel

Covid-19 infection and mortality: a physiologist's perspective enlightening clinical features and plausible interventional strategies

SARS-CoV-2 entry genes are most highly expressed in nasal goblet and ciliated cells within human airways

Receptor recognition by the novel coronavirus from wuhan: an analysis based on decadelong structural studies of SARS coronavirus

The proximal origin of SARS-CoV-2

Elevated plasmin(ogen) as a common risk factor for COVID-19 susceptibility

IFN-γ/TNFα synergism as the final effector in autoimmune diabetes: a key role for STAT1/IFN regulatory factor-1 pathway in pancreatic β cell death

Clinical characteristics of 28 patients with diabetes and covid-19 in wuhan, china

COVID-19 pandemic, coronaviruses, and diabetes mellitus

Diabetes in COVID-19: Prevalence, pathophysiology, prognosis and practical considerations

Diabetes is a risk factor for the progression and prognosis of COVID-19

The trinity of COVID-19: immunity, inflammation and intervention

Reduction and functional exhaustion of t cells in patients with coronavirus disease 2019 (COVID-19)

Characteristics of peripheral lymphocyte subset alteration in COVID-19 pneumonia

Type 2 diabetes mellitus and increased risk for malaria infection

Toxoplasma gondii infection in diabetes mellitus patients in china: seroprevalence, risk factors, and case-control studies

Association between helminth infections and diabetes mellitus in adults from the Lao People's Democratic Republic: a cross-sectional study

Is there an association between positive Strongyloides stercoralis serology and diabetes mellitus?

Intestinal parasitosis and associated factors among diabetic patients attending Arba Minch Hospital, Southern Ethiopia

Intestinal parasitic infections in patients with diabetes mellitus: a case-control study

Intestinal parasitic infections among diabetes mellitus patients

Host-parasite interactions in individuals with type 1 and 2 diabetes result in higher frequency of ascaris lumbricoides and giardia lamblia in type 2 diabetic individuals

Type 2 diabetes mellitus BALB/c mice are more susceptible to granulomatous amoebic encephalitis: immunohistochemical study

The prevalence of oral Candida infections in periodontitis patients with type 2 diabetes mellitus

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.Copyright © 2020 Daryabor, Atashzar, Kabelitz, Meri and Kalantar. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.