key: cord-0804753-mj61ni3x
authors: Sandhaus, Shayna; Swick, Andrew G.
title: Specialized proresolving mediators in infection and lung injury
date: 2020-11-28
journal: Biofactors
DOI: 10.1002/biof.1691
sha: 426751cb8e35e42b5d76c350e157f246f68ebe79
doc_id: 804753
cord_uid: mj61ni3x

Specialized proresolving mediators (SPMs) are endogenous lipid metabolites of long‐chain polyunsaturated fatty acids that are involved in promoting the resolution of inflammation. Many disease conditions characterized by excessive inflammation have impaired or altered SPM biosynthesis, which may lead to chronic, unresolved inflammation. Exogenous administration of SPMs in infectious conditions has been shown to be effective at improving infection clearance and survival in preclinical models. SPMs have also shown tremendous promise in the context of inflammatory lung conditions, such as acute respiratory distress syndrome and chronic obstructive pulmonary disease, mostly in preclinical settings. To date, SPMs have not been studied in the context of the novel Coronavirus, severe acute respiratory syndrome Coronavirus‐2 (SARS‐CoV‐2), however their preclinical efficacy in combatting infections and improving acute respiratory distress suggest they may be a valuable resource in the fight against Coronavirus disease‐19 (COVID‐19). Overall, while the research on SPMs is still evolving, they may offer a novel therapeutic option for inflammatory conditions.

Upon the detection of an invading pathogen or injury, the body initiates the process of acute inflammation as a response. Neutrophils are rapidly recruited to the site of infection to aid in pathogen clearance and host tissue response. This phase of immune response, called "initiation", is coordinated by many proinflammatory Abbreviations: 15-HETE, 15-hydroxyeicosatetraenoic acid; 17(S)-HDHA, 17(S)-hydroxydocosahexaenoic acid; 18(R)-HEPE, 18(R)hydroxyeicosapentaenoic acid; AA, arachidonic acid; ALX/FPR2, lipoxin A4 receptor/formyl peptide receptor 2; ARDS, acute respiratory distress syndrome; AT, aspirin triggered; CF, cystic fibrosis; CLP, cecal ligation and puncture; COPD, chronic obstructive pulmonary disease; DHA, docosahexaenoic acid; DRV1/GPR32, resolvin D1 receptor/G protein-coupled receptor 32; EPA, eicosapentaenoic acid; ERV1/CMKLR1, E series resolvin receptor/chemokine-like receptor 1; HSV, herpes simplex virus; IL, interleukin; LCPUFA, long-chain polyunsaturated fatty acids; LOX, lipoxygenase; LTB4, leukotriene B4; LXA4, lipoxin A4; MaR1, maresin 1; MERS-CoV, middle eastern respiratory syndrome Coronavirus; MyD88, myeloid differentiation primary response 88; NF-κB, nuclear factor κB; NK cells, natural killer cells; NPD1, neuroprotectin D1; PD1, protectin D1; PDX1, protectin DX1; PLA2G2D, phospholipase A2 group IID; RSV, respiratory syncytial virus; RvD1-RvD6, resolvins D1-D6; RvE1, resolvin E1; RvE2, resolvin E2; SAA, serum amyloid A; SARS-CoV-2, severe acute respiratory syndrome Coronavirus-2; SPM, specialized proresolving mediator; Th, T helper cells; TLR4, toll-like receptor 4; TNF-α, tumor necrosis factor α. mediators, including leukotrienes, prostaglandins, bradykinin, cytokines, and others. In order to return infected or inflamed tissue to homeostasis, several processes must be completed. These processes include stopping the recruitment of granulocytes to the site; inducing macrophage phagocytosis to clear apoptotic immune cells, pathogens, and other debris; and restoring and repairing any damaged tissue. 1, 2 For much of history, the second phase of inflammation, called "resolution", was believed to be a passive process. We are now aware that resolution of inflammation is an active process. A class of proresolving mediators that are biosynthesized during the acute inflammatory response was discovered, called specialized proresolving mediators (SPMs). SPMs are lipid mediators that actively help in the resolution of inflammation. SPMs are involved in all resolution phase processes and are therefore essential to return any tissue to healthy working order. 3, 4 SPMs help to stop the recruitment of granulocytes, bring in macrophages to clear apoptotic neutrophils and debris, and restore injured tissue homeostasis. 5 Without sufficient SPMs, acute inflammation can become chronic. It is now well established that chronic inflammation is an underlying factor in many diseases, from neurodegeneration to cardiovascular disease. 6, 7 SPMs are enzymatically derived from long-chain polyunsaturated fatty acids (LCPUFA) like arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). SPMs are generally classified as lipoxins (from AA), E-series resolvins (from EPA), and D-series resolvins, protectins, and maresins (from DHA) ( Figure 1 ).

Lipoxins are synthesized from AA via one of several lipoxygenase (LOX) enzymes, including 5-, 12-, and 15-LOX, derived from leukocytes, platelets, and monocytes, respectively. Some pathways of 5-LOX, as well as COX-2, are involved in converting AA to the proinflammatory mediators, leukotrienes, and prostaglandins. 8 The stereochemical structure of the individual types of lipoxins is dependent on the distinct pathway utilized in their biosynthesis.

The omega-3 EPA is converted to 18(R)-hydroxyEPA (18 (R)-HEPE) via CYP450 enzymes. 18(R)-HEPE is further transformed to resolvins RvE1 and RvE2 via the 5-LOX enzyme. Additionally, EPA can be converted into the biologically active S enantiomer, 18(S)-RvE1 by acetylated COX enzymes. 8 

The omega-3 DHA is converted to D-series resolvins (RvD1-RvD6), protectins, and maresins via 12-and 15-LOX enzymes. DHA is acted on by 15-LOX and then peroxidase to form 17(S)-hydroxyDHA (17(S)-HDHA), which is further converted by 5-LOX to form the resolvins. The intermediate between DHA and 17(S)-HDHA can also be converted to protectin D1 by hydrolase enzymes in the mucosal lining of the respiratory tract. 8 In addition, DHA can be converted to maresins via 12-LOX action. 5 As mentioned, there are various isomers depending on which of several potential pathways is utilized; for example, the "aspirin-triggered" D-series resolvin isomers are synthesized when COX-2 enzymes are nitrosylated or acetylated. 9 SPMs have distinct and overlapping roles in the immune response and tissue regeneration; the individual roles of the various isomers are outside the scope of this review but are elucidated in other review articles such as the ones cited here. 4, 10 In general, when resolution of an inflammatory response is required, SPMs act on several different immune and tissue-resident cells. The different SPMs interact with cognate receptors, such as the lipoxin A4 (LXA4) receptor ALX/FPR2 (a G protein-coupled receptor) and the RvE1 receptor chemokine-like receptor 1 (ERV1/CMKLR1), at nanomolar concentrations. 5 ALX/FPR2 is expressed on multiple immune cells, including eosinophils, neutrophils, T cells, monocytes, and others. This receptor can also interact with other SPMs, such as RvD1, 15-epi-LXA4, and 17-epi-RvD1. 3 The named resolvin D1 receptor, DRV1/GPR32, is also expressed in various immune cells such as neutrophils, monocytes, and lymphocytes. 3 Interestingly, some SPMs also appear to interact with proinflammatory receptors as partial agonists, such as RvE1's interaction with the receptor BLT1 to enhance neutrophil apoptosis. 11 This ability to act as partial agonists may offer an additional mechanism by which SPMs modulate proinflammatory signaling. 3 SPMs act on these various receptors to inhibit neutrophil infiltration and promote their clearance after apoptosis. SPMs augment macrophage functions to promote phagocytosis of apoptotic neutrophils, microorganisms, and debris. Other immune cells, such as eosinophils, natural killer (NK) cells, innate lymphoid cells, and lymphocytes are also affected by SPMs. 5 For example, DHA-derived SPMs have been shown to be essential for modulating T helper cells (Th1 and Th17) recruitment and response, reducing their number and cytokine production to resolve chronic inflammation. 12, 13 T helper lymphocytes are critically involved in ongoing inflammatory conditions. 14 In addition, SPMs also help to resolve inflammation by reducing levels of proinflammatory mediators and inducing the production of antiinflammatory mediators, as well as reducing levels of reactive oxygen and nitrogen species. 7,15 These SPMmediated effects help to resolve the inflammation and regenerate any damaged tissue so that the body can return to homeostasis. Under healthy conditions, this process effectively limits inflammatory responses. However, under conditions of severe infection, chronic inflammatory conditions, or other comorbidities, this process can be impaired and leads to chronic, unresolved inflammation.

In the context of infection, SPMs are involved in resolving the acute response to pathogen invasion. Under conditions where SPM biosynthesis is stunted or insufficient, such as in obese individuals or severe infections, or when there are defects in SPM receptor expression, immune responses can be stunted as well. 5 The SPM shortage and/or other defects in the resolution pathway can contribute to unresolved inflammation as well as impaired antibody production. 16, 17 It can be hypothesized that exogenous administration of SPMs could help in resolution of unresolved inflammation. Thus far, this hypothesis has been tested mostly in preclinical settings

The 2019 novel Coronavirus disease (COVID-19) is a pandemic viral infection that usually causes fever and respiratory symptoms such as coughing and difficulty breathing. Severe cases may progress to acute respiratory distress and other life-threatening conditions. Patients with COVID-19 tend to have higher leukocyte numbers and increased levels of proinflammatory cytokines. 18 Additionally, obese individuals are at a much higher risk of severe COVID-19 infection than nonobese individuals. It has been demonstrated that obese versus nonobese individuals have markedly reduced levels SPMs, and it has been suggested that this deficit may be a reason behind the more severe course of COVID-19 infection in obesity. 19 SPMs have the potential to aid in addressing common problems of bacterial and viral infections, such as antibiotic resistance, vaccine response, and more. They also have the ability to block leukocyte recruitment and resolve inflammation. SPMs have also been studied for their therapeutic role in lung conditions, like chronic obstructive pulmonary disease (COPD) and acute respiratory distress syndrome (ARDS). As some of the more distressing aspects of COVID-19 are pneumonia-like complications, acute respiratory distress, and excessive inflammatory response, 18 it is possible that SPMs may have a potential therapeutic role to play. [20] [21] [22] Therefore, this review will discuss the research surrounding SPMs and their role in infection with an emphasis on inflammatory lung conditions.

SPMs have been studied in the context of many kinds of infections, including bacterial infections. It is clear that SPMs play a very important role in helping resolve inflammation caused by bacterial infection. The proper balance between proresolution mediators and proinflammatory mediators appears to be important in the body's response to infection or insult. During certain active bacterial infections, coadministration of SPMs has been shown to lower the doses of antibiotics necessary to clear infections, as well as aiding in the healing process even when administered alone. 23 SPMs therefore have great potential to help address the growing issue of antibiotic resistance. It is essential to fully elucidate the roles SPMs play in bacterial infections and understand the delicate balance between proinflammatory and resolving mediators in the context of treating bacterial infections.

One area of bacterial infections that has been studied fairly extensively with SPMs is periodontitis. 24, 25 Exogenous administration of SPMs has been shown in animal studies to dramatically aid in healing periapical lesions and reducing inflammation. [26] [27] [28] [29] Periodontitis is generally caused by bacterial overgrowth, often Porphyromonas gingivalis, leading to biofilm formation and mucosal inflammation. An excessive immune response is believed to drive disease progression; and unresolved periodontitis and its resultant chronic inflammation can increase the risk for several diseases like atherosclerosis and Alzheimer's disease. 30 It is likely that periodontitis' excessive inflammation is the reason SPMs have been so successful in this area.

Multiple studies indicate that SPMs play a significant role in healing periodontitis: in rabbits that overexpressed 15-lipoxygenase, an enzyme that participates in the synthesis of lipoxins and protectins, bone loss and local inflammation from periodontitis were markedly reduced. External application of the lipoxin LXA4 significantly reduced bone loss and leukocyte infiltration as well. 26 In another rabbit model of periodontitis, the resolvin RvE1 was used alone to completely heal the tissue and bone. In addition to the local effect, RvE1 also exerted systemic reduction in inflammation as shown with reduced Creactive protein levels. 27 The resolvin RvD2 also showed dramatic healing effects in rats with periodontitis, inducing calcification around the root and healing bone lesions. 29 In human periodontal ligament cells, LXA4 resolved inflammation through the TLR4/MyD88/NF-κB signaling pathway. 31 Patients suffering from localized aggressive periodontitis were shown to have increased leukotriene (proinflammatory) levels and significantly lower SPM levels, including LXA4, 15-hydroxyeicosatetraenoic acid (15-HETE), and 14-HDHA, than asymptomatic controls. The macrophages taken from the periodontitis patients also had impaired phagocytosis activity, which RvE1 rescued. These results indicate that the resolution phase is likely impaired in periodontitis patients, which allows the persistence of inflammation and disease progression. 16 In another study, periodontitis patients were shown to have lower LXA4 levels and higher protectin D1 (PD1) and maresin 1 (MaR1) than healthy controls. 32 These results indicate that a particular balance of SPMs may be necessary to overcome periodontal disease. Additionally, LXA4 levels were shown to be correlated with periodontal parameters and metabolic syndrome, suggesting an association between periodontitis and metabolic syndrome, as well as a protective role for LXA4 in these inflammatory conditions. 33 

Sepsis is a global health concern with exceptionally high mortality rates. Unfortunately, the clinical management of sepsis is still less than optimal. Sepsis is caused by an imbalanced host immune response, with excessive inflammation and an inadequate resolution of the inflammation. 34, 35 SPMs have therefore become an area of interest in the management of sepsis. 36 While antiinflammatory therapies could potentially harm septic patients, improving the resolution phase of inflammation appears to allow an adequate immune response and improve the clearance of the bacterial pathogen. 5 LXA4 in combination with antibiotics has been shown to be more effective than antibiotics alone in improving systemic inflammation and survival in mice with sepsis caused by Escherichia coli. 37 Even LXA4 alone was effective at reducing systemic inflammation and bacterial load as well as improving survival in a cecal ligation and puncture (CLP) mouse model of sepsis. The proposed mechanism was that LXA4 reduced the production of proinflammatory mediators via an NF-κBdependent pathway and enhanced macrophage recruitment. 38 MaR1 has also been shown to improve survival in septic mice, improving organ function and reducing serum bacterial burden. The proposed mechanism also partly involved suppressing NF-κB. 39 Another study on MaR1 in sepsis validated these results, adding that the protective effects of MaR1 could also be attributed to improving the metabolic dysfunction caused by sepsis and ameliorating mitochondrial damage. 40 The resolvin RvD2 and protectin DX (PDX1) were each also shown to improve survival in septic mice by increasing macrophage activity. 41, 42 RvD1 was shown to protect mice from cardiac injury due to sepsis. 43 Other SPMs have improved survival in animal models as well. 44 In human sepsis patients, SPM levels correlate with survival and may potentially be used as biomarkers to stratify patient risk. 45, 46 While SPMs have shown great promise in improving sepsis outcomes, one study suggested that a careful, welltimed balance of proinflammatory and proresolving mediators is necessary to ensure a proper initiation and resolution to the body's immune response to insult. This was demonstrated in a mouse model of pneumosepsis induced with Klebsiella pneumoniae; when the mice were treated with LXA4 in early stages of sepsis, the infection worsened. When the mice were treated with LXA4 in later stages, it reduced the excessive inflammation and improved survival. 47 It is worth noting that one of the earliest measurable changes in an inflammatory response is a sharp increase in LXA4 that quickly wanes. LXA4 is again increased during the resolution phase. 48 The other studies discussed in this section have all shown beneficial effects of administering SPMs in early stages of sepsis and none reported adverse outcomes. It is possible that the different methods of inducing sepsis (CLP vs. K. pneumoniae) play a role in the different outcomes, however further study is needed. It bears emphasizing that the vast majority of sepsis studies have not indicated any cause for concern.

When discussing immune responses to infection, it is crucial to emphasize that both inadequate and excessive inflammation are detrimental. In the case of infections with Mycobacterium tuberculosis, a slow-growing mycobacterium that can often cause latent infections, the Th1-mediated immune response (proinflammatory) is essential. The Th2-mediated response that is responsible for limiting inflammation appears to be almost inconsequential during tuberculosis infection. The Th1 response is significantly impacted by the balance between leukotriene B4 (LTB4, a proinflammatory mediator) and lipoxin A4. 49 If either side of the immune response is faulty or excessive, the host may be adversely impacted. 50 5-lipoxygenase (5-LOX), an enzyme involved in lipoxin and leukotriene production, was shown to play a significant role in tuberculosis infections in mice. During the acute response to infection (up to 10 days after being exposed), LXA4 and LTB4 levels were sharply increased. After the initial acute response, only LXA4 continued being synthesized in high amounts (the chronic stage of infection). In mice deficient in 5-LOX (5-lo −/− ), survival rates were increased following infection with M. tuberculosis. The deficient mice also had decreased bacterial burdens in their lungs compared to WT mice after 21 and 42 days, indicating that negative regulation of the Th1 response could be detrimental in tuberculosis infections. 49 In humans, variants of the genes that encode for 5-LOX and LTA4 hydrolase (which can alter levels of lipoxins and leukotrienes) modulate the individual's susceptibility to tuberculosis infection, underscoring the importance of this balance in the tuberculosis immune response. 51, 52 In patients with newly-diagnosed active tuberculosis infections, higher levels of D-series resolvins (RvD1 and RvD2) were detected compared to asymptomatic and presumably uninfected controls, indicating that these resolvins are involved in the acute phase of immune response to recently activated tuberculosis. 53 Levels of SPMs in the cerebrospinal fluids of patients with a more advanced form of the disease, tuberculosis meningitis, were associated with disease severity and mortalitythose with severe disease had increased inflammatory eicosanoids and decreased SPMs, while those with higher levels of SPMs tended to survive through the 80 day study. The specific SPMs detected were 5-LO-derived, including RvT2 and RvT4, indicating these SPMs may be important for resolving the chronic stage of severe tuberculosis infection. 54 It could be possible that if SPMs were to be exogenously administered in the case of a tuberculosis infection, the timing would be important so as not to affect the initial Th1 immune response. Exogenous administration of several SPMs, including RvD1 and MaR1, reduced tuberculosis-induced TNF-α production and tuberculosis growth in infected human macrophages, indicating that they may potentially have therapeutic effects in tuberculosis-induced inflammation. 55 However, this area of research still requires clarification and the effects of exogenous SPM administration has not been tested in humans or animal models of tuberculosis. Further study is necessary to clarify the role SPMs play in different stages of tuberculosis infection.

SPMs have also been studied in the context of other bacterial infections, such as bacterial pneumonia (which will be discussed further in the sections below on lung conditions) and others. When comparing susceptible and resistant mouse models of bacterial keratitis induced by Pseudomonas aeruginosa, it was found that the 5/12/15-LOX pathways were essential for resistance. Imbalances in the lipoxygenase enzymatic pathways contributed to susceptibility, as demonstrated by lower levels of 15-HETE and 17-hydroxydocosahexaenoic acid (17-HDHA). 56 In infant mice with enteropathogenic Citrobacter rodentium infection, which was used as a model for enteropathogenic E. coli in humans, postinfection treatment with the resolvins RvD1 and RvD5 significantly reduced bacterial loads and rescued the infant mice from death. Even more strikingly, the treatment protected the mice from reinfection. 57 

As with bacterial infections, SPMs have been studied in the context of viral infections, although perhaps not as extensively. Nevertheless, SPMs have shown promise in improving viral outcomes in several preclinical models.

SPMs may play a vital role in the response to influenza infection. In a study with mice, the highly virulent H5N1 virus was shown to cause dysregulation in the inflammatory response: the virus caused changes in the expression of genes responsible for biogenesis and signaling of lipoxins. 58 Protectins were shown to have protective effects against influenza in mice. PD1 levels were suppressed during severe infection and correlated with the pathogenicity of the H5N1 viruses. PD1 also inhibited viral replication by targeting the export of viral RNA. Prophylactic treatment with PD1 improved survival from severe infection. Importantly, exogenous treatment with PD1 improved lung function even at stages of infection where current antiviral drugs by themselves are no longer effective. Treating mice with an antiviral and PD1 2 days postinfection completely rescued mice from death. 59, 60 Some researchers have pointed out that there is confusion surrounding PD1 in the literature, and the active lipid mediator may in fact be PDX, an isomer with a different route of biosynthesis. 61 It is possible that either PD1, PDX, or both are involved in blocking influenza.

Under conditions of obesity, immune responses may be blunted. In humans as in mice, obesity was associated with impaired B cell secretion and stimulation. When the obese mice were provided DHA in their diet, they had improved antibody titers in response to influenza infection. Importantly, the DHA did not directly target B cells; rather, DHA administration led to an increase in the SPMs 14-HDHA, 17-HDHA, and PDX. These SPMs helped boost B cell activity and increased antibody titers. 62 A mouse model of influenza vaccination demonstrated that coadministration of the SPM 17-HDHA with the vaccine led to better outcomes: the mice given concurrent 17-HDHA had increased antibody titers and enhanced protection against live H1N1 infection, indicating that SPMs may serve as potential flu vaccine adjuvants. 63 An in vitro study provided further evidence for SPMs' potential role as flu vaccine adjuvants; LXB4 was shown to boost memory B cell activation, which could help provide enhanced antibody-mediated immune response. 64 

To date, only one study has directly examined the relationship between proresolving mediators and Coronaviruses, specifically severe acute respiratory syndrome-Coronavirus (SARS-CoV). In the study, the age-related increase in disease severity was explored in mice and determined to be at least partly dependent on the increased expression of phospholipase A2 (PLA2) group IID (PLA2G2D) in the aged mouse lungs. As mice age, the enzyme PLA2G2D expression increases. PLA2G2D is involved in the biosynthesis of SPMs, as it hydrolyzes phospholipids to release the LCPUFA that then undergo further metabolism to form SPMs. Under normal aging conditions, this helps address the increased inflammation due to aging. However, in this study, the increased PLA2G2D levels (and the corresponding increase in free AA, EPA, and DHA) in the lungs led to worse outcomes from SARS-CoV infection. In middleaged mice that lacked the expression of PLA2G2D (Pla2g2d −/− ), survival was increased to more than 80%, while all the WT mice died. The authors conclude that higher basal levels of proresolving mediators in the lungs might be detrimental in the context of a lung infection, as it may dampen the initial immune response. 65 This appears to contradict to the numerous studies that have shown SPMs have many beneficial effects in infections, and particularly lung diseases, and to not be immunosuppressive. The higher levels of AA measured in the mouse lungs could also have led to higher leukotriene LTB4 levels, which may help explain the results. Unfortunately, the effect of exogenous administration of SPMs in SARS and other Coronavirus infections has not been tested yet and thus no firm conclusion can be reached.

Some authors posit that LCPUFA and their SPM derivatives may be effective against Coronaviruses like SARS-CoV, MERS-CoV, and SARS-CoV-2 (the novel Coronavirus that causes COVID-19), as LCPUFA have shown inhibitory effects against enveloped viruses in vitro. 66, 67 In addition, obese individuals have been shown to have SPM deficits, which may be at least partly responsible for their more severe course of COVID-19 illness due to dysfunctional resolution of inflammation. 19 Further study is necessary before any conclusions can be drawn in this regard.

SPMs have been studied with other viral infections as well. In mice with herpes simplex virus (HSV)-induced keratitis, topical administration of a prodrug form of neuroprotectin D1 (NPD1) helped reduce the severity of the keratitis by decreasing levels of proinflammatory mediators such as IL-6 and several matrix metalloproteinases (MMPs), as well as dampening the recruitment of neutrophils to the cornea. 68 Similar results were seen in another study with mice with HSV-induced keratitis; treatment with RvE1 controlled corneal lesion severity. RvE1 appeared to work similarly to NPD1, lowering neutrophil and Th1 recruitment to the cornea, increasing antiinflammatory IL-10 levels, and inhibiting the production of proinflammatory mediators like IL-6. 12

Respiratory syncytial virus (RSV) primarily targets airway epithelial cells and lung macrophages, inducing a strong inflammatory response. In 5-lo −/− mice (mice lacking the 5-LOX enzyme), RSV infection was worsened, and they had increased lung pathology from elevated COX-2. Although this example is with a viral infection, it is interesting to note that this is the opposite effect that was seen in the tuberculosis mouse model, where 5-lo −/− mice had improved outcomes. Treating macrophages lacking 5-LOX with LXA4 and RvE1 partially counteracted the COX-2 pathology, and treating RSV-infected mice with a COX-2 inhibitor enhanced lung 5-LOX expression, suggesting that there was improved resolution of inflammation. 69 

While SPMs have not been studied extensively in other viral conditions, there are preliminary data that indicate they do play a role in regulating the immune response to other viral infections. 70 Kaposi's sarcoma (KS) is a cancer that forms in the linings of blood vessels and is caused by Kaposi's sarcoma-associated herpesvirus (KSHV). In a KS cell line, treatment with LXA4 or its epimer reduced levels of proinflammatory COX-2, PGE2, LTB4, and IL-6 and IL-8 cytokines. Treatment also increased levels of the anti-inflammatory IL-10 as well as reducing angiogenic factors. 71 These results suggest that lipoxins may serve as a valuable therapeutic for KS. 72 In vitro studies indicate that SPMs appear to be involved in direct response to infection with other viruses, such as pneumonia virus of mouse and mouse adenovirus, 73 as well as HIV, 74 as viral infection increased the production of lipoxins, however these results have not been replicated or expounded upon in further studies to the best of our knowledge.

While SPMs clearly play an important role in resolving viral infections, the research is still in early phases and needs to be further elucidated.

The role of SPMs in lung and airway diseases has been extensively studied in preclinical models. Conditions like ARDS, COPD, cystic fibrosis (CF), asthma, as well as respiratory illnesses due to infections as were touched on in previous sections, have all been explored. 3 SPMs have shown tremendous promise in many of these conditions, which is important as the standard of care in many cases is insufficient to improve disease outcomes. [75] [76] [77] 4.1 | Acute respiratory distress syndrome ARDS is characterized by excessive inflammation in the lungs that causes life-threatening respiratory failure. There are limited treatment options. ARDS can be caused by sterile injuries like burns, or by pathogens including bacteria and viruses. During ARDS, there is increased permeability of the alveolar endothelial and epithelial barriers, leading to edema and fluid buildup in the alveoli and impaired gas exchange. At times, excessive neutrophils penetrate the lungs and cause tissue injury. 3 SPMs have been shown to be important mediators in resolving preclinical models of lung injury.

One mechanism by which SPMs may aid in the resolution of ARDS is by regulating alveolar fluid clearance. 78 Under healthy conditions, ion transport across alveolar epithelial cells is responsible for creating the osmotic gradient necessary for fluid clearance and maintaining homeostasis. Under ARDS, the damaged barriers do not allow for effective fluid clearance. Additionally, excessive proinflammatory mediators like TNF-α, and IL-1β are present in pulmonary edema fluids and if left unchecked can promote alveolar injury. SPMs appear to be closely involved in keeping the excessive inflammation in check and allow for alveolar fluid clearance. In a rat model of acute lung injury, treatment with LXA4 stimulated alveolar fluid clearance and resolved pulmonary edema via activation of alveolar epithelial sodium channels. 79 In another rat model of acute lung injury, LXA4 helped to heal lung tissue by inhibiting IL-6 and TNF-α as well as improving alveolar fluid clearance. 80 RvD1 and MaR1 have also both been shown to improve fluid clearance in animal models of acute lung injury, at least partly via improving alveolar epithelial sodium channels as well as the PI3k signaling pathway that leads to Na,K-ATPase activation. 81, 82 In mice with lipopolysaccharide (LPS)induced acute lung injury, RvD1 treatment helped to protect the mice and improve lung pathology. The treated mice also had reduced TNF-α and neutrophil infiltration into the lungs. 83 Additionally, PDX improved lung histopathology, reduced lung inflammation, and mitigated pulmonary edema in a mouse model of acute lung injury. 84 While further studies, and particularly clinical studies, are necessary, the preclinical data suggest that SPMs could offer a new therapeutic strategy to decrease the severity and duration of ARDS by improving alveolar fluid clearance and decreasing excessive inflammation.

COPD is a condition that arises from prolonged exposure to smoke or environmental toxins. Patients with COPD often have chronic lung inflammation and are susceptible to infection. 85 The standard treatment, inhaled corticosteroids with bronchodilators, is not particularly effective in many cases. COPD patients are specifically susceptible to persistent infection because of impaired macrophage ability to clear pathogens. During infection, COPD patients may exhibit an excessive neutrophil response that does more harm than good. Additionally, COPD patients exhibit altered fatty acid metabolism from airway epithelial remodeling, leading to lower levels of some SPMs. 86, 87 SPMs have therefore been proposed as treatment options to dampen the excessive neutrophil response as well as address the imbalance between proinflammatory and proresolution mediators in COPD. 85 Serum amyloid A (SAA) is an apolipoprotein that is sharply increased during severe COPD infections. It is involved in recruiting neutrophils to the lungs, which under COPD becomes excessive. SAA and the proresolving mediator LXA4 both compete to bind to the G-protein coupled receptor ALX/FPR2, and so they oppose each other's actions. 88 LXA4 blocks the SAAmediated recruitment of neutrophils to the lungs to help resolve the associated damage. 89 In studies with patient lung tissue samples, proresolving and metabolic pathways were disrupted. This indicates that exogenous administration of SPMs may help prevent the chronic inflammation and emphysema that arises from COPD. 90 Furthermore, in mice exposed to long-term cigarette smoke, RvD1 treatment reduced the development of emphysema and inflammation. 90 RvD1 treatment in mice exposed to cigarette smoke also reduced inflammation in the lungs caused by neutrophil infiltration as well as increasing levels of the anti-inflammatory IL-10. 91 Similar results were seen with the aspirin triggered form of RvD1 (AT-RvD1) in mice with cigarette-induced emphysema. 92 In mice exposed to inhaled organic dust, MaR1 reduced neutrophil infiltration, IL-6, and TNF-α. 93 

Respiratory tract infections are caused by a multitude of viral and bacterial pathogens, such as Coronaviruses, hantaviruses, influenza, Staphylococcus aureus, and Streptococcus pneumoniae. Several infections also cause the host to become susceptible to a secondary coinfection. As mentioned in the sections on bacterial and viral infections, SPMs play an important role in resolving the inflammation caused by infection.

A mouse model of pneumonia triggered by bacterial infection demonstrated that RvE1, when administered before the injury, decreased neutrophil accumulation by more than 50% and helped clear the bacteria. Additionally, levels of proinflammatory cytokines were reduced and survival improved. 94 RvE1 was also shown to resolve acute lung inflammation in mice caused by bacterial infection by improving the phagocytical clearance of apoptotic neutrophils. 95 Other SPMs such as 15-epi-LXA4 and AT-RvD1 have also shown effectiveness at resolving acute lung inflammation from bacteria and improving survival. 96, 97 Importantly, AT-RvD1 was shown to be effective at reducing the severity of pneumonia in a coinfection mouse model of S. pneumoniae and influenza A. The coinfected mice had increased circulating and lung levels of SAA as well as neutrophilic inflammation. The mice were then treated with AT-RvD1 at the acute phase of infection and subsequently experienced a reduction in pneumococcal load and improvement in the severity of pneumonia. 98 

SPMs have also been studied in other conditions that affect the lungs, such as CF, asthma, and allergies. [99] [100] [101] In CF, the lungs generally have persistent mucus secretions that can obstruct airways and cause susceptibility to infection. Chronic lung inflammation is common. 102 LXA4 levels were found to be suppressed in CF patient sputum, while those that had detectable levels of RvE1 also tended to have better lung function than those without detectable levels. 103 There is preclinical evidence that SPM treatment may help improve lung inflammation in CF and lead to less severe disease. 99, 104 In a mouse model of CF, RvD1 treatment reduced P. aeruginosa burden in the lungs and improved disease scores. 101 Some clinical evidence indicates that boosting SPMs in CF patients may have some benefit as well; use of Acebilustat, a LTA4 hydrolase inhibitor that can limit leukotriene production and increase lipoxins, reduced neutrophil concentrations in patient sputum. 105, 106 Asthma in humans is associated with reduced levels of SPMs, specifically LXA4 and PD1, when compared to healthy controls. 3, 107, 108 The first human trial with SPMs was conducted in asthmatic patients, where LXA4 inhalation was shown to shift the response curve to a bronchial inhalation challenger, LTC4. 109 Many other preclinical studies have demonstrated beneficial effects of SPMs on asthma and allergic inflammation. 3, 5 Taken as a whole, the existing data on SPMs in lung conditions indicate that they are a highly promising category of new therapeutics that may help to limit excessive inflammation and improve disease pathology.

SPMs have become an increasingly popular area of research because of their widespread potential therapeutic value. As we have become aware of the dangers of unresolved inflammation in many disease states, the best ways to counter inflammation have been thoroughly explored. From anti-inflammatory therapies to SPMs, quenching chronic inflammation has become an area of public health concern. The worry, however, that quenching inflammation could dampen the body's (helpful) immune response has been raised. SPMs are attractive options, as they are widely considered to not be immunosuppressive like many anti-inflammatories and may expedite microbial clearance and activate the resolution of inflammation. 10 The overwhelming majority of preclinical models have shown treatment of inflammatory conditions with exogenous SPMs effectively promotes resolution of inflammation, thereby improving disease pathology, and enhancing survival. A few preclinical studies suggest that successful treatment with SPMs may depend on timing of SPM administration or basal SPM levels. It is possible that under certain conditions, allowing the body to launch an immune response before administering SPMs to resolve it would be advisable, although further research is necessary. In conditions that are characterized by established chronic inflammation and/or faulty resolution pathways, such as infections, ARDS, and COPD, SPM administration has been shown to be helpful in preclinical models.

While SPMs have not yet been directly studied in the context of COVID-19, it is reasonable to suggest that they could be a valuable therapeutic option. One of the strongest risk factors of severe COVID-19 illness, obesity, is linked to decreased SPM levels. In addition, many of the distressing aspects of COVID-19, including the pneumonia-like presentation, ARDS, and excessive inflammation and leukocyte number, have been shown to be improved in preclinical studies with SPMs. SPMs may be tested as adjuvants to a new vaccine or coadministered with antiviral drugs to hopefully improve patient outcome.

Taken as a whole, the research on SPMs as therapeutics is still fairly new. Preclinical models indicate that there is reason to be optimistic that SPMs can become useful tools to improve outcomes in pathogenic infections, lung diseases, and other inflammatory conditions.

The authors would like to acknowledge Dr. Gerard Bannenberg and Dr. Jana Barlic-Dicen for their critical review and suggestions, and Dr. Cristina Matthewman for helping design the graphical abstract.

Shayna Sandhaus and Andrew G. Swick are paid employees of Life Extension Inc.

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

ORCID Shayna Sandhaus https://orcid.org/0000-0002-9314-9781

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How to cite this article: Sandhaus S, Swick AG. Specialized proresolving mediators in infection and lung injury