key: cord-1022579-7wc4o8nv
authors: Morgan, Jeffrey; Muskat, Kaylin; Tippalagama, Rashmi; Sette, Alessandro; Burel, Julie; Lindestam Arlehamn, Cecilia S.
title: Classical CD4 T cells as the cornerstone of antimycobacterial immunity
date: 2021-03-09
journal: Immunol Rev
DOI: 10.1111/imr.12963
sha: 154e83f686a98abbf30446e1475555aa71059c83
doc_id: 1022579
cord_uid: 7wc4o8nv

Tuberculosis is a significant health problem without an effective vaccine to combat it. A thorough understanding of the immune response and correlates of protection is needed to develop a more efficient vaccine. The immune response against Mycobacterium tuberculosis (Mtb) is complex and involves all aspects of the immune system, however, the optimal protective, non‐pathogenic T cell response against Mtb is still elusive. This review will focus on discussing CD4 T cell immunity against mycobacteria and its importance in Mtb infection with a primary focus on human studies. We will in particular discuss the large heterogeneity of immune cell subsets that have been revealed by recent immunological investigations at an unprecedented level of detail. These studies have identified specific classical CD4 T cell subsets important for immune responses against Mtb in various states of infection. We further discuss the functional attributes that have been linked to the various subsets such as upregulation of activation markers and cytokine production. Another important topic to be considered is the antigenic targets of Mtb‐specific immune responses, and how antigen reactivity is influenced by both disease state and environmental exposure(s). These are key points for both vaccines and immune diagnostics development. Ultimately, these factors are holistically considered in the definition and investigations of what are the correlates on protection and resolution of disease.

annually and ~ 10 million new infections are reported each year. 2 In 2019, tuberculosis was the ninth leading cause of death worldwide and the leading cause from a single infectious agent, ranking above HIV/ AIDS. 2 The severity of this situation is compounded by the fact that many cases in low-income and middle-income countries go undiagnosed and thus untreated. Additionally, with the COVID-19 pandemic in 2020/21, tuberculosis management has been neglected; patients have discontinued their treatment due to lockdowns, new cases are not visiting clinics despite symptoms, and co-infection with SARS-CoV-2 may lead to increased mortality. 3 Thus, it is expected that the number of tuberculosis cases will rise further in the coming years.

Mtb infections are traditionally classified into active TB (ATB) infection or a quiescent/latent state (LTBI). ATB is typically defined as the presence of symptoms and/or Mtb smear/culture positivity.

However, it is now well accepted that Mtb infection should be seen as a continuous spectrum with high heterogeneity and no clear segregation between the LTBI and ATB group. [4] [5] [6] The distinction between ATB and LTBI is often made based on presence of symptoms and culture positivity for simplicity in clinical and research settings. Typically, LTBI will have a positive tuberculin skin test (TST) and/or Interferon Gamma Release assay (IGRA), but this may also be true for individuals who have eliminated their infection. Therefore, the IGRA + group contains a spectrum of individuals from those who have cleared their infection to individuals with subclinical TB disease, and not all within this group have the same likelihood of developing active disease. 4 Moreover, the TST and IGRA tests have limited usefulness in areas with high TB burden and TB endemic areas. LTBI individuals with a more recent infection, or with presence of co-morbidity factors such as HIV, or diabetes are at higher risk of developing active disease. In addition to comorbidities and time since infection, severity of ATB is also dependent on additional factors, such as the infecting Mtb strain and how far the infection has progressed. In this review, we compare Mtb-specific classical CD4 T cell immune responses in LTBI (usually defined as IGRA + ) vs ATB (individuals with symptoms) as this is commonplace in the scientific literature. However, we realize that improved diagnostics and molecular tools to allow more granularity on the disease spectrum will allow more in-depth studies in the future and the characterization of immune correlates of protection that can more closely cover this spectrum.

The majority of infected individuals control the pathogen by mounting a successful, long-lived, and protective immune response, leading to either elimination of the bacteria or a persistent latent infection which is not associated with significant clinical symptoms.

However, approximately 10% of latently infected individuals eventually develop active disease. 7, 8 The risk of developing ATB is higher in individuals that are immunocompromised (due to age, corticosteroid use, malnutrition, and HIV infection). The lengthy treatment is expensive and requires a combination of multiple antibiotics.

In many parts of the world, access to these drugs is limited and compliance with the drug regime is often poor, thus favoring the de- The vaccination of children with M. bovis BCG results in a 60%-80% decrease in the incidence of active TB. However, in most developed countries BCG vaccination is not recommended due to the relatively low incidence of disease and variable effectiveness in preventing first time pulmonary TB in adults, a large fraction of active disease cases. 2 A new vaccine against TB is required, preferably targeting adolescents and adults who represent the vast majority of new cases 1, 2, 9 and are responsible for spreading Mtb infection.

Ideally, a vaccine should be protective irrespective of Mtb infection status, that is, both in individuals with and without evidence of latent infection, and prevent progression to active disease, reinfection, and reactivation. A recent prevention of infection trial using BCG and a subunit TB vaccine candidate provided encouraging results showing reduced rates of sustained QuantiFERON conversions. 10 As a complimentary approach to developing a new vaccine, advanced diagnostic tools could theoretically identify individuals at high risk of developing active TB disease through systematic screening.

The high-risk individuals could then be treated before they become infectious, which would also contribute to a reduction of TB cases.

Immunodiagnostic tests for TB rely on the detection of an immune response against mycobacterial antigens, either by delayed hypersensitivity reaction (Tuberculin skin test; TST), or by detection of IFNγ following in vitro stimulation (IGRA). A TST can produce a false-positive result due to prior BCG vaccination, and may produce a false-negative result due to other factors such as immunosuppression or malnutrition. 11 In summary, new vaccines and immunodiagnostics could provide a quantum leap in the fight against TB. However, to accomplish these goals a precise understanding of the characteristics of immune responses and their impact on disease progression and susceptibility is required. The rest of this review will focus on these issues.

Human T cell responses to Mtb involve classically restricted CD4 and CD8 αβ T cells, 12, 13 and non-classically restricted T cells such as NKT (CD1), MAIT (MR1) and γδ T cells. [14] [15] [16] Depletion of CD4 T cells demonstrated that while CD8 + T cells and other immune cells also play a protective role against Mtb, they alone cannot compensate for the lack of a dominant CD4 T cell response. 17 The importance of CD4 T cells in the defense against Mtb is also supported by the fact that patients with HIV infection (which leads to reduced CD4 T cell counts) are more susceptible to primary Mtb infection, reinfection, and reactivation. 18 CD4 T cells primarily act by secreting a variety of cytokines that attract other immune cells to the site of infection and initiate the differentiation of different CD4 T cell subsets capable of performing effector functions. The ability of T cells to recognize Mtb-infected antigen presenting cells is a key step in containing the infection.

Srivastava et al was able to demonstrate using mouse models that direct recognition of Mtb-infected cells by CD4 T cells is required for control of the infection. 19 IFNγ + production by CD4 T cells is commonly associated with control of Mtb infection. 8, [20] [21] [22] [23] The essential role of IFNγin the protective immunity to mycobacteria is made apparent in individuals with genetic defects in the IFNγ receptor, who have an increased susceptibility to infection with mycobacteria. 24 However, several reports demonstrated other anti-tuberculosis CD4 T cell effector functions not accounted for by IFNγ production. The likelihood of developing ATB does not correlate with either the amount of produced IFNγ or the pattern of co-production with other cytokines. 25 Accordingly, the focus of our review is on classical CD4 T cells, since they represent a major component of the T cell response against Mtb. We consider both IFNγ production, as well as other effector and phenotypic functions associated withCD4T cells and control of Mtb infection ( Figure 1 , Table 1 ).

Flow cytometry is by far the most commonly used technique to interrogate the phenotype of Mtb-specific CD4 T cells. There are several strategies to identify Mtb-specific CD4 T cells from bulk CD4 T cells.

The most common strategy is to stimulate cells in vitro with Mtbderived reagents: either whole preparations (eg, Mtb lysate, PPD), or peptides (such as the ones used for the IGRA assay targeted against the two proteins ESAT-6 and CFP10, or "megapools", see below). The advantage of peptides is that they are defined synthetic reagents with little variation across batches and thus generate results highly consistent across experiments. They can also be selected based on their MHC binding to target either CD4 or CD8 T cells. 26, 27 However, they are usually limited to only a few proteins or epitopes of interest. To overcome this limitation, our group has designed a peptide pool that combine 300 primarily MHC class II restricted epitopes that represent more than 80 Mtb proteins (see below).

Regardless of the stimuli used, surrogate markers of antigenspecificity are needed to identify the cells with antigen-specific reactivity following stimulation. For Mtb-specific T cells, the most commonly used measurement is IFNγ. However, as discussed more below, not all Mtb-specific CD4 T cells express IFNγ. Many more cytokines are produced such as IL-2, TNF, and IP-10, as well as other cellular changes occurring in response to Mtb stimuli. To overcome the hurdle of having to specifically select which cytokines to measure, there are assays that measure the expression of surface proteins that are specifically induced upon T cell activation. 28 These surface proteins, typically called activation-induced markers, encompasses TNF family receptors OX40, CD137, and CD154, as well as CD69 and PD-L1.

Another strategy to characterize the phenotype of antigenspecific T cells is to use multimeric staining reagents (eg, MHC tetramers), which require precise knowledge of the epitopes' HLA restriction and HLA expression of the subjects. Multimers captures all T cells capable of binding a given epitope:MHC combination, thus only work in subjects that express the specific MHC allele, and recognize the specific epitope. 29 Therefore, multimers are ideally suited for in-depth characterization of a small representative set of F I G U R E 1 Summary of characteristics of Mtb-specific classical CD4 T cells for differentiation of disease stages and potential correlates of protection. Mtb-specific classical CD4 T cells express different characteristics depending on the disease stage of the individual. There is a higher frequency of Tcm, Th1* and CD153 in LTBI. ATB has higher frequency of Tem and Tscm, HLA-DR expression and increased differentiation. Th1* and CD153 are important for control of Mtb infection. Other cytokines and chemokines include: IL-2, IL-10, IL-17, TNFα, and CXCL9/10/11/12/13 TA B L E 1 Differential CD4 T cell phenotypes observed in major stages of Mtb infection (LTBI, ATB, and severe ATB), and following BCG vaccination 

It is often possible to assess T cell responses directly ex vivo by using pools of different epitopes or peptides, so that the overall frequency of responding cells is enhanced. [30] [31] [32] [33] [34] This approach is particularly key to analyze small sample volumes. This "megapool" approach is based on large numbers of peptides pooled and formulated using sequential lyophilization. 35 Specifically, for detection of Mtb-specific responses we have described and validated a comprehensive megapool of 300 Mtb epitopes representing more than 80 Mtb proteins, 29 derived from a proteome-wide screen for epitopes and antigens rec- Humans ↑ frequency of triple producing T cells compared to LTBI 57, 120, 122, 142, 143, [196] [197] [198] Note: Generally, a marker is included under a specific stage of Mtb infection if it has been described as increased in that stage relative to the other disease stages. Differentially expressed markers that may serve as potential correlate of protection are italicized in red; Dx markers that can distinguish LTBI from ATB are italicized in blue.

infection in humans. 39, 43 This approach has also been useful in providing evidence for mutations that are detrimental to host immunity against Mtb and other mycobacteria. 45, 47, 48 

As mentioned above, understanding the complexity of CD4 T cells responses to Mtb and their functional attributes is key to developing correlates of protection and informing the design and testing of vaccines and immunodiagnostics. In this and the following sections, we address these issues.

CD4 T cells can be divided into naïve and memory popula- In LTBI, Mtb-specific CD4 T cells have been shown to predominantly express the CCR7 + CD45RA − Tcm phenotype, 52 similarly to BCG-specific CD4 T cells after BCG vaccination in newborns. 53 In

Tem phenotype. 52, 54, 55 These cells might also represent effector T cells (Teff) since Teff are expected to downregulate CCR7. 56 Individuals with ATB have a higher proportion of effector memory cells, likely with less tissue homing capacity but higher effector functions, compared to latently infected individuals. 57, 58 Whereas the vast majority of Mtb-specific CD4 T cells falls into the memory compartment, a small but not negligible fraction of Mtb-specific CD4 T cells have a naive CD45RA + CCR7 + CD27 + phenotype. 36, 53, 54 These cells, initially named naive-like cells, were subsequently identified as stem cell memory T cells, or Tscm. Tscm are a subset of long-lived memory CD4 T cells that can hold specificity to multiple pathogenic or self-derived antigens in humans and hold enhanced ability for self-renewal and multipotency. 59 Transcriptomic analysis of tetramer sorted cells showed that Mtb-specific Tscm cells have a transcriptomic profile highly similar to bulk Tscm but also share phenotypic and functional properties with both central memory and effector T cells. 60 Based on these results, it was suggested that Mtb-specific Tscm might therefore represent a less differentiated subset of Mtb-specific T cells.

primary Mtb infection 60 and BCG vaccination, 61 and their blood frequency is increased in ATB compared to LTBI. 55 The function of antigen-specific Tscm in the context of TB remain unclear. Adoptive transfer of Mtb-specific memory T cells with a naive-like phenotype in mice showed a higher degree of protection compared to Mtbspecific Tem transfer, 55 suggesting they might hold an important protective role in TB.

Taken together, these data suggest a heterogeneity within Mtbspecific memory CD4 T cell subsets that varies depending on an individual's position on the spectrum of infection. This is important to consider in progression studies and vaccine efficacy trials.

Different 

Mtb infection is Th1* (also called Th1 co-expressing CCR6, Th17.1, Th1Th17, Th17/Th1, and Th1/Th17 cells), which form their own distinct population of CXCR3 + CCR6 + CCR4 − cells co-expressing the transcription factors Tbet and RORC. 62

Our work and others have shown that Th1* contain the majority of Mtb-specific T cells in IGRA + individuals. 36, 62, 66 We have also found that mycobacteria-specific (including non-tuberculous mycobacteria; NTM) epitopes are also recognized by Th1* cells, in both

Mtb-infected and uninfected individuals. 67 The difference between polyclonal and Mtb-specific stimuli indicated that it is possible to identify cellular heterogeneity within the overall Th1* subset. Applying bulk transcriptomics on sorted memory CD4 T cells in IGRA + individuals, we revealed 74 differentially expressed genes that were able to distinguish IGRA + individuals and

Mtb-uninfected controls. 42 In this study, we further refined the Mtbreactivity was restricted to the GPA33 − CD62L − compartment within Th1*. 42 CD62L had also previously been found to be downregulated in Mtb-specific CD4T cells. 74 A recent study that used single-cell RNA sequencing to define cellular responses associated with control of Mtb infection (ie, bacterial killing in granulomas), identified higher proportions of CD4

T cells expressing a "hybrid Th1/Th17 immune response", 75 thus further strengthening the hypothesis that Th1* have a role in containment of Mtb infection. This may be mediated in part by their expression of CCR6, which mediates cell homing to inflamed tissues, 76 and thus allows peripheral localization. Tissue resident memory cells are also known to co-express CXCR3 and CCR6. 77 In ATB, several studies also reported that Mtb reactivity within circulating CD4 T cells maps to the Th1* subset. 70, 78 However, another report studying Mtb-tetramer + CD4 T cells showed a more diverse expression of chemokine receptors, with downregulation of CXCR3 in ATB compared to LTBI. 68 A caveat in using chemokine receptors for phenotyping Mtbspecific CD4 T cells reside in the fact that most assays identifying

Mtb-specific CD4 T cells rely on in vitro stimulation, which impacts the surface expression of chemokine receptors. 68, 70 For such studies, the use of tetramers for identifying Mtb-specific CD4 T cells is thus preferred over in vitro stimulation. 66, 68 Alternatively, pre-sorting of CD4 T cell subsets based on chemokine receptors followed by antigen-specific in vitro stimulation can be utilized. 42 The CD27 − CD127 − PD1 + phenotype is typically associated with more differentiated T cells (compared to CD27 + CD127 + PD1 − cells), including effector T cells. 56 Thus, in active disease, Mtb-specific CD4

T cells bear a phenotype that reflect enrichment for highly differentiated effector T cells compared to latent infection.

In addition to their differentiation phenotype, another major distinction between Mtb-specific CD4 T cells in ATB vs LTBI lies in the expression of activation markers. In ATB, HLA-DR, CD38 and

Ki67 are strongly upregulated in Mtb-specific CD4 T cells compared to LTBI. 68, 84, 88, 89 Upregulation of these three markers in antigenspecific T cells was also reported during acute infection with HIV, EBV, or CMV compared to chronic infection. 90, 91 In particular,

ising sensitivity and specificity to discriminate between ATB and LTBI, 88, 92 and it was also proposed as a potential prognostic marker for progression to active disease. 93 41, 96 and was associated with lower bacterial load in humans. 39 Recently it was shown that the phenotypic profile of Mtb-specific CD4 T cells, using HLA-DR, CD27, and CD153, can be used to assess severity of TB disease and monitor treatment. 43 Overall, these studies provide strong evidence that the measurement of activation markers is a powerful tool for capturing CD4 T cells that are important to the control of Mtb infection. 101, 102 Hence, parameters other than IFNγ alone, may be key for future TB diagnostics. This is also relevant for individuals who are highly exposed to Mtb (household contacts of patients with TB) but consistently test negative for both TST and IGRA, that is, so called "resistors". 103 Early within the immune response mounted by CD4 T cells, rapid short-lived IL-2 secretion is genetically controlled and is key in signaling proliferation and differentiation of other immune cells. 104 IL-2 is important for extracellular killing of mycobacteria, as well as granuloma formation. IL-2 has been found to be an accurate indicator for differentiating between ATB and LTBI. 105 Particularly, IL-2 in combination with other cytokines makes for useful ratios that accurately distinguish ATBI and LTBI. 106 Specific ratios of IL-2 to IFNγ in longterm stimulation are suggestive of LTBI. 107 Besides being a strong indicator of ATB, IL-2 has also been shown to induce Foxp3 + Treg growth and response, without impairment of macaque anti-TB immunity. 108 CD4 + CD25 + Foxp3 + Treg cells may be essential to a healthy TB response in humans 109 

One frequently studied cytokine in Mtb infections is IL-17, which works synergistically and cross-regulatorily with IFNγ. 112 Stimulation with IL-23 triggers Th17 cells to produce and secrete IL-21, IL-22, and IL-17, the latter of which plays a multifaceted role in TB. 113 IL-17 has been shown to induce non-hematopoietic cells to produce CXCL13, an important chemokine required for localization that will be discussed later. 114 In addition to IL-17, other cytokines that are members of the tumor necrosis factor family also correlate with ATB. As with inflammatory cytokines, TNFα works synergistically with IFNγ inMtb infection and may be more frequently produced during an active Mtbinfection. 120 In an analysis of multiple cytokines after Mtb-antigen stimulation it was found that only TNFα was not significantly more abundant in LTBI. 121 Furthermore, increased single-positive TNFα Mtb-specific CD4 T cells were also found to be highly predictive of active TB. 122 TNFα is primarily produced by macrophages, but can also be secreted by CD4 T cells, and plays a managerial role. TNFα activates macrophages via an autocrine/paracrine mechanism, recruits lymphocytes and monocytes to the infection cite via chemokine signaling, restricts mycobacterial growth in granulomas and promotes tissue inflammation and apoptosis, and promotes DC maturation via TNFR1 and DC survival via TNFR2. [123] [124] [125] [126] Furthermore, it may also be involved in controlling Treg responses. 127 Due to the complex role of TNFα in the immune response against Mtb, it is no surprise individuals who receive TNFα-neutralizing medication also have an increased likelihood of developing active TB. 128 

There is also evidence for Mtb-specific IL-10 production in humans with active TB, where IL-10 mediates inhibition of antigen presentation to T cells, and therefore mediates a decreased ability to clear infection contributing to TB pathogenesis. 129 Furthermore, IL-10 is produced after BCG vaccination, and is responsible for a subsequent reduction of Mtb-specific Th immune responses. 130, 131 IL-10 has also been shown to be elevated in serum from active pulmonary TB patients. 132 Chemokines are significant for their role in signaling the location of infection. For example, Th17 cells are known to express CXCL9, CXCL10, and CXCL11 upon Mtb challenge, which in turn recruit IFNγ producing CD4 T cells to the lung. 133 Interferon gamma-induced protein 10 (CXCL10 or IP-10) in particular has been shown to be at least as effective as IFNγin diagnostic assays for Mtb infection. 134, 135 When testing for IP-10 levels, the QuantiFERON-TB Gold Plus (QFT-Plus) test revealed that in both individuals with LTBI and ATB IP-10 levels

were elevated compared to uninfected subjects, and IP-10 was found to be partially increased in IGRA + individuals. 136 IP-10 is a promising biomarker for ATB 137,138 but there is still controversy surrounding IP-10's ability to accurately distinguish between ATB and LTBI. 139 Lastly, polyfunctional CD4 T cells are also scrutinized for their role within Mtb infection. 140 In this review, polyfunctional CD4 T cells are defined as dual-or triple-producing CD4 T cells that secrete proinflammatory cytokines in response to Mtb. 141 Pertaining to Mtb infection, higher frequency of triple producing T cells (IFNγ, IL-2, and TNFα) was reported during ATB as compared to LTBI; and IL-2/IFNγ are increased during LTBI. 142 During treatment of TB, triple producers are increased in numbers, 143 and after completion of treatment and the clearance of TB, polyfunctional producers decrease. 144 148) . In addition, some proteins have been described and referred to as "resuscitation antigens". 157, 158 These are small bacterial proteins that promote proliferation of dormant mycobacteria and are therefore believed to be involved in the reactivation of Mtb. 159 However, these antigens have not been studied in the context of being preferentially expressed or recognized by a certain stage of Mtb infection.

Additionally, a proteome-wide screen for Mtb-reactivity in disease stages other than healthy IGRA + , such as individuals with active TB or BCG vaccinated individuals, has not yet been performed.

It would address an important gap in knowledge; as to date most investigations have targeted either the antigens known to be recognized in LTBI, or specific antigen subgroups selected on the basis of a particular hypothesis.

A proteome-wide screen would be of interest for both diag- 

Mtb-specific reactivity can also be influenced by exposure to nontuberculous mycobacteria (NTM) and other environmental microbes. Significant reactivity exists in Mtb uninfected individuals that is directed against epitopes conserved among bacteria in the mycobacteria genus, a factor to be considered in diagnostic applications, but also potentially offering an avenue to boost general and widespread reactivity. 40, 67 Non-tuberculous mycobacteria vary in their ability and the extent to which they cause clinically significant symptoms or disease in humans. 161 They also vary in other factors such as in vitro growth characteristics and ecological niche, living and multiplying in a variety of human and environmental reservoirs. [161] [162] [163] The majority of NTM are present ubiquitously in the environment including soil, seawater, treated/untreated freshwater and a variety of organic and inorganic surfaces. [161] [162] [163] Exposure to NTM may result in colonization, infection, and/or pathology that is detectable in the skin or respiratory and gastrointestinal tracts of healthy humans. 161, 164 Mtb-uninfected individuals who have not received BCG are capable of responding to Mtb-derived antigens. 67 NTM are capable of inducing cross-reactive T-cell responses to Mtb-derived epitopes. [165] [166] [167] [168] Higher baseline positive responses to PPD in Mtbuninfected adults compared to children might reflect the increased likelihood of NTM exposure with age. 169 In our studies we found that control subjects, who were both TB-negative and not immunized with BCG, also responded, albeit to a lower extent to Mtb-derived T cell epitopes. 36 This led to a follow up study focusing on Mtb/NTM cross-reactivity at the level of the specific epitopes. 67 This analysis revealed that their reactivity was likely due to previous NTM exposure since the epitopes they recognize were also conserved in NTM species. 67 

It is possible that T cell epitopes conserved between Mtb, NTM, and BCG that elicit cross-reactive responses offer protection in the form of heterologous immunity or, to the complete contrary, act deleteriously by preventing the institution of BCG-induced protective responses, creating diagnostic challenges, 163, 170 and/or confounding evaluation of investigative vaccination strategies. 171 It is hypothesized that exposure to environmental mycobacteria contributes to variable BCG efficacy, 172, 173 with increasing NTM exposure negatively correlated with efficacy. Several intriguing hypotheses have been proposed to explain how environmental NTM exposure may or may not provide protection against Mtb and contribute to variable BCG efficacy. The "masking" and "blocking"

hypotheses propose very different mechanisms to explain how NTM cross-reactivity contributes to this variability (recently reviewed in 174).

In addition to being influenced by NTM, we have found evidence that Mtb-specific epitope reactivity is influenced by the microbiome. 40 We observed differential recognition of Mtbderived epitopes that was associated with the time period when individuals with active TB undergo treatment. These "treatment sensitive" epitopes are more conserved in the microbiome than "persistent" epitopes. Thus, the strong antibiotic regimen against TB results in the loss of reactivity against a subset of Mtb epitopes, broadly conserved across the microbiome. The influence of epitope conservation in the microbiome in active TB using longitudinal samples and subject-specific microbiome sequences remains to be determined. Table 1 ), as this cell subset alone is capable of mediating potent anti-mycobacterial immunity. 17 

The long-held paradigm, based largely on animal, but also human studies, is that BCG mediates its protective effects via secretion of IFNγ by Th1 polarized CD4 T cells. 17, 178, 180 Thus, IFNγ is the gold standard biomarker by which to assess protection provided by BCG or other candidate TB vaccines. 181 Defective IFNγ signaling, such as in individuals who develop neutralizing antibodies against IFNγ, 182 increases susceptibility to mycobacterial infection and disease. 45, 47, 48 In addition, in the MVA85A vaccine efficacy trial, while boosting with MVA85A did not improve protection upon primary inoculation with BCG, low BCG-specific IFNγ production by PBMCs from BCG-vaccinated infants associated with increased risk of developing TB disease over the next three years of life. 183 Despite evidence that IFNγ is needed for host resistance to

Mtb, the correlation between IFNγ and protection against TB is notoriously inconsistent between studies. 120, 184, 185 In contrast to the MVA85A efficacy trial, the only other infant CoP study using vaccine

infants and protection against culture-positive TB two years after vaccination. 25 Moreover, IFNγ positively correlated with symptoms of active pulmonary disease such as fever and weight loss in Mtbinfected individuals. 186 Thus, it is critical to look beyond Th1 at other T cell subpopulations-such as polyfunctional T cells producing multiple cytokines, subsets expressing specific activation markers, and memory subsets at different stages of differentiation and thus equipped with different tissue homing and effector capabilities, for their suitability as CoPs.

As discussed above, CD4 T cells can be further characterized based on their ability to produce multiple cytokines. Multifunctional/polyfunctional cells had first been associated with protection in other infectious diseases, namely Leishmania 187 and HIV, particularly when antigen load is low. 188, 189 Moreover, studies in mice found an association between IFNγ/TNFα/IL-2 triple-producing or IFNγ/IL-2 doubleproducing T cells and protection against TB. [190] [191] [192] [193] Intravenous administration of BCG induced polyfunctional CD4 T cells dually

producing IFNγ and TNFα that were associated with reduced disease pathology in NHPs. 194 However, the relationship between polyfunctionality and protection against human Mtb infection is less clear. 17 In support of a protective role, some studies report increased polyfunctional T cells in patients with LTBI compared to ATB. Moreover, reduced polyfunctional T cell responses in patients with active disease could be recovered with antibiotic treatment for TB. 122, 143, 191, 195 However, on the opposite spectrum, others report an association between increased polyfunctional T cell responses and ATB. 57, 120, 122, 143, [196] [197] [198] Adding to the controversy, the CoP infant 

A CDC report from 2004, followed up on healthcare workers in a TB moderate to high incidence are in Taiwan. 212 The screening led to the discovery of 60 suspected ATB cases amongst the healthcare workers. Investigations into the origin of the cluster revealed that an elderly patient was admitted for 12 weeks to the general ward before he was diagnosed with ATB. Between 1998 and 2002, all specialized TB hospitals in Taiwan were closed due to the SARS outbreak and as a result more cases were being managed in a general hospital setting, increasing the nosocomial transmission of Mtb. 212 This meant that Mtb infections were either overlooked or misdiagnosed during the outbreak.

Similarly, during the current COVID-19 pandemic, medical and human resources were being re-directed to the care of COVID-19

patients, while the care of most other diseases was left on the backburner. TB patients were suddenly confronted with the lack of access to diagnosis and treatment facilities. They were also less likely to leave the safety of their homes to obtain necessary treatment. The Hinduja Hospital, a tertiary care hospital in India, observed a drop of 85% out-patient visits in April 2020, following the lockdown. 213 The indirect effects of COVID-19 are not always obvious. For India, the lockdown may result in an around an additional 40 000 TB cases annually for the next five years, and a 5.7%

increase in TB deaths. 213 TB has been around far longer than COVID-19 and the pandemic has caused TB to be left on the sideline, while the world focuses their attention and resources on resolving the current crisis. However, the surge in interest towards COVID-19 can also be beneficial toward developing newer and better diagnostics and therapeutics against TB. For instance, many studies have found that COVID-19 disease and severity correlate with an increased frequency in circulating HLA-DR + CD4 T cells. [214] [215] [216] As discussed previously in this review, HLA-DR expression on Mtb-specific CD4 T cells has also been repeatedly associated with ATB compared to LTBI y. 68, 84, 88, 89 Thus, there seem to be similarities in the immune cell subsets and immune pathways that correlates with protection and/or disease severity against SARS-CoV-2 and

Mtb. Understanding the complex relationship between these two pathogens and elucidating the molecular mechanisms behind susceptibility and disease severity in single and co-infections will be fundamental to the development of preventive and treatment strategies for Mtb and Mtb/SARS-CoV-2 infections.

Pathogen-specific T cell immunity is a key host mechanism to control There is a large heterogeneity between different studies aiming to identify which immune parameters will be of most use for correlates of protection. The need to have more control for technical variability is crucial, which would allow for comparison between studies. A limitation in comparing results between different studies is the variability in the definition of the different study cohorts included, in particularly for LTBI where there is often considerable heterogeneity. There is also a need for further research defining and characterizing Mtb-specific T cell responses in cohorts representing the entire spectrum of infection, as well as ages and co-morbidities.

The so far elusive CoPs are likely not one single immune marker, but instead a combination of secreted and expressed functional molecules acting together.

In addition, it is important to consider the diversity between populations, and the environment they live in, as well as the complex host-pathogen interactions between humans and Mtb, as well as other environmental and commensal bacteria.

Systems biology approaches that combines several molecular levels of information (proteome, genome, transcriptome) and environment (such as microbiomes) and other large-scale endeavors have been and will continue to be very successful in identifying and characterizing the immune response against Mtb.

This work was supported by the National Institutes of Health grant number 75N93019C00067.

The authors declare no conflict of interest.

All authors listed have made substantial, direct and intellectual contribution to the work, and approved it for publication.

Data discussed were all retrieved from published literature as specified in the reference list.

Cecilia S. Lindestam Arlehamn https://orcid. org/0000-0001-7302-8002

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Th1 versus Th2 T cell polarization by whole-cell and acellular childhood pertussis vaccines persists upon re-immunization in adolescence and adulthood

Lack of allergy to timothy grass pollen is not a passive phenomenon but associated with the allergen-specific modulation of immune reactivity

Definition of human epitopes recognized in tetanus toxoid and development of an assay strategy to detect Ex vivo Tetanus CD4+ T cell responses

Global assessment of dengue virus-specific CD4(+) T cell responses in dengue-endemic areas

Human CD8+ T-cell responses against the 4 dengue virus serotypes are associated with distinct patterns of protein targets

Automatic generation of validated specific epitope sets

Memory T cells in latent Mycobacterium tuberculosis infection are directed against three antigenic islands and largely contained in a CXCR3+CCR6+ Th1 subset

Identifying specificity groups in the T cell receptor repertoire

Limited pulmonary mucosal-associated invariant T Cell accumulation and activation during Mycobacterium tuberculosis infection in Rhesus Macaques

Mycobacterium tuberculosis-specific CD4 T cells expressing CD153 inversely associate with bacterial load and disease severity in human tuberculosis

Differential recognition of Mycobacterium tuberculosis-specific epitopes as a function of tuberculosis disease history

Host resistance to pulmonary Mycobacterium tuberculosis infection requires CD153 expression

Transcriptomic analysis of CD4(+) T cells reveals novel immune signatures of latent tuberculosis

Disease extent and antitubercular treatment response correlates with Mycobacterium tuberculosis-specific CD4 T-cell phenotype regardless of HIV-1 status

Limited recognition of Mycobacterium tuberculosis-infected macrophages by polyclonal CD4 and CD8 T cells from the lungs of infected mice

Disruption of an antimycobacterial circuit between dendritic and helper T cells in human SPPL2a deficiency

Antigen availability shapes T cell differentiation and function during tuberculosis

Impairment of immunity to Candida and Mycobacterium in humans with bi-allelic RORC mutations

Human IFNgamma immunity to mycobacteria is governed by both IL-12 and IL-23

Transient immune activation in BCG-vaccinated infant Rhesus Macaques is not sufficient to influence oral simian immunodeficiency virus infection

The TBspecific CD4(+) T cell immune repertoire in both cynomolgus and rhesus macaques largely overlap with humans

Two subsets of memory T lymphocytes with distinct homing potentials and effector functions

T-cell immunophenotyping distinguishes active from latent tuberculosis

Longitudinal changes in CD4(+) T-cell memory responses induced by BCG vaccination of newborns

CD4 and CD8 T-cell responses to mycobacterial antigens in African children

Human CD4 T-cells with a naive phenotype produce multiple cytokines during mycobacterium tuberculosis infection and correlate with active disease

The who's who of T-cell differentiation: human memory T-cell subsets

IFNgamma/TNFalpha specific-cells and effector memory phenotype associate with active tuberculosis

Polyfunctional Mycobacterium tuberculosis-specific effector memory CD4+ T cells at sites of pleural TB

A human memory T cell subset with stem cell-like properties

functional, antigenspecific stem cell memory (TSCM) CD4(+) T cells are induced by human Mycobacterium tuberculosis infection

A subset of mycobacteria-specific CD4(+) IFN-gamma(+) T Cell expressing naive phenotype confers protection against tuberculosis infection in the lung

Surface phenotype and antigenic specificity of human interleukin 17-producing T helper memory cells

Complexity and controversies over the cytokine profiles of T helper cell subpopulations in tuberculosis

Regulatory T cells in Mycobacterium tuberculosis infection

Compartmentalization of immune responses in human tuberculosis: few CD8+ effector T cells but elevated levels of FoxP3+ regulatory t cells in the granulomatous lesions

transcriptional profile of tuberculosis antigen-specific T cells reveals novel multifunctional features

Immunological consequences of intragenus conservation of Mycobacterium tuberculosis T-cell epitopes

Characterization of Mycobacterium tuberculosis-specific cells using MHC class II tetramers reveals phenotypic differences related to HIV infection and tuberculosis disease

BCG revaccination boosts adaptive polyfunctional Th1/Th17 and innate effectors in IGRA+ and IGRA-Indian adults

Th17, and Th1Th17 lymphocytes during tuberculosis: Th1 lymphocytes predominate and appear as low-differentiated CXCR3(+)CCR6(+) cells in the blood and highly differentiated CXCR3(+/-)CCR6(-) Cells in the lungs

Pulmonary Mycobacterium tuberculosis control associates with CXCR3-and CCR6-expressing antigen-specific Th1 and Th17 cell recruitment

The rate of CD4 T cell entry into the lungs during Mycobacterium tuberculosis infection is determined by partial and opposing effects of multiple chemokine receptors

CXCR3 directs antigen-specific effector CD4+ T cell migration to the lung during parainfluenza virus infection

Mycobacterium tuberculosis culture filtrate protein 10-specific effector/memory CD4(+) and CD8(+) T cells in tubercular pleural fluid, with biased usage of T cell receptor Vbeta chains

Single-cell profiling of tuberculosis lung granulomas reveals functional lymphocyte signatures of bacterial control. bioRxiv

Multiple chemokine receptors, including CCR6 and CXCR3, regulate antigen-induced T cell homing to the human asthmatic airway

Resident memory T cells (T(RM)) are abundant in human lung: diversity, function, and antigen specificity

Active tuberculosis is characterized by highly differentiated effector memory Th1 cells

Loss of receptor on tuberculinreactive T-cells marks active pulmonary tuberculosis

Reduced CD27 expression on antigen-specific CD4+ T cells correlates with persistent active tuberculosis

Monitoring CD27 expression to evaluate Mycobacterium tuberculosis activity in HIV-1 infected individuals in vivo

Mtb-specific CD27low CD4 T cells as markers of lung tissue destruction during pulmonary tuberculosis in humans

Assessment of CD27 expression as a tool for active and latent tuberculosis diagnosis

Phenotypic changes on Mycobacterium tuberculosis-specific CD4 T cells as surrogate markers for tuberculosis treatment efficacy

Study of CD27 and CCR4 markers on specific CD4(+) T-cells as immune tools for active and latent tuberculosis management

Assessment of the novel T-cell activation marker-tuberculosis assay for diagnosis of active tuberculosis in children: a prospective proof-of-concept study

PD-1 expression on Mycobacterium tuberculosis-specific CD4 T Cells is associated with bacterial load in human tuberculosis

Biomarkers on patient T cells diagnose active tuberculosis and monitor treatment response

Circulating HLA-DR+CD4+ effector memory T cells resistant to CCR5 and PD-L1 mediated suppression compromise regulatory T cell function in tuberculosis

Direct visualization of antigenspecific CD8+ T cells during the primary immune response to Epstein-Barr virus In vivo

Memory CD8+ T cells vary in differentiation phenotype in different persistent virus infections

Analysis of the phenotype of Mycobacterium tuberculosis-specific CD4+ T cells to discriminate latent from active tuberculosis in HIV-uninfected and HIV-infected individuals

T-cell activation is an immune correlate of risk in BCG vaccinated infants

A subset of circulating blood mycobacteria-specific CD4 T cells can predict the time to Mycobacterium tuberculosis sputum culture conversion

Mycobacterium tuberculosis-specific T cell activation identifies individuals at high risk of tuberculosis disease. medRxiv

A novel role of CD30L/CD30 signaling by T-T cell interaction in Th1 response against mycobacterial infection

The TAM-TB assay-a promising TB immune-diagnostic test with a potential for treatment monitoring

Monitoring antituberculosis treatment response using analysis of whole blood Mycobacterium tuberculosis specific T cell activation and functional markers

The utility of an interferon gamma release assay for diagnosis of latent tuberculosis infection and disease in children: a systematic review and meta-analysis

Serial interferon-gamma release assay in children with latent tuberculosis infection and children with tuberculosis

The end of the binary era: revisiting the spectrum of tuberculosis

The spectrum of latent tuberculosis: rethinking the biology and intervention strategies

IFN-gamma-independent immune markers of Mycobacterium tuberculosis exposure

IL-2 secretion by CD4+ T cells in vivo is rapid, transient, and influenced by TCRspecific competition

Is interleukin-2 an optimal marker for diagnosing tuberculosis infection? A systematic review and metaanalysis

Combined analysis of IFNgamma, IL-2, IL-5, IL-10, IL-1RA and MCP-1 in QFT supernatant is useful for distinguishing active tuberculosis from latent infection

Potential role for Mycobacterium tuberculosis specific IL-2 and IFN-gamma responses in discriminating between latent infection and active disease after long-term stimulation

IL-2 simultaneously expands Foxp3+ T regulatory and T effector cells and confers resistance to severe tuberculosis (TB): implicative Treg-T effector cooperation in immunity to TB

The role of foxp3-expressing regulatory T cells and T helpers in immunopathogenesis of multidrug resistant pulmonary tuberculosis

Conditioning of naive CD4(+) T cells for enhanced peripheral Foxp3 induction by nonspecific bystander inflammation

Application of IL-2 therapy to target T regulatory cell function

IL-23 and IL-17 in tuberculosis

Th1 and Th17 cells in tuberculosis: protection, pathology, and biomarkers

Unexpected role for IL-17 in protective immunity against hypervirulent Mycobacterium tuberculosis HN878 infection

Interleukin-17 limits hypoxia-inducible factor 1alpha and development of hypoxic granulomas during tuberculosis

CD4 T cell-intrinsic role for the T helper 17 signature cytokine IL-17: effector resistance to immune suppression

AM IL-17 and Th17 cells in tuberculosis

Outbreaks of mycobacterium tuberculosis MDR strains induce high IL-17 T-cell response in patients with MDR tuberculosis that is closely associated with high antigen load

IL-17 and IFN-γ expression in lymphocytes from patients with active tuberculosis correlates with the severity of the disease

Pattern and diversity of cytokine production differentiates between Mycobacterium tuberculosis infection and disease

Cellmediated immune responses to in vivo-expressed and stagespecific Mycobacterium tuberculosis antigens in latent and active tuberculosis across different age groups

Dominant TNF-alpha+ Mycobacterium tuberculosis-specific CD4+ T cell responses discriminate between latent infection and active disease

TNF regulates chemokine induction essential for cell recruitment, granuloma formation, and clearance of mycobacterial infection

Multiscale computational modeling reveals a critical role for TNF-alpha receptor 1 dynamics in tuberculosis granuloma formation

Immunological control of tuberculosis: role of tumour necrosis factor and more

Dendritic cell maturation and survival are differentially regulated by TNFR1 and TNFR2

The phenotypic and functional consequences of tumour necrosis factor receptor type 2 expression on CD4(+) FoxP3(+) regulatory T cells

Tuberculosis associated with infliximab, a tumor necrosis factor α-neutralizing agent

The role of IL-10 in immune regulation during M. tuberculosis infection

IL-10 dampens the Th1 and Tc activation through modulating DC functions in BCG vaccination

Blockade of IL-10 signaling during bacillus Calmette-Guerin vaccination enhances and sustains Th1, Th17, and innate lymphoid IFN-gamma and IL-17 responses and increases protection to Mycobacterium tuberculosis infection

Serum concentrations of cytokines in patients with active tuberculosis (TB) and after treatment

IL-23 and IL-17 in the establishment of protective pulmonary CD4+ T cell responses after vaccination and during Mycobacterium tuberculosis challenge

IP-10 release assays in the diagnosis of tuberculosis infection: current status and future directions

Use of lateral flow assays to determine IP-10 and CCL4 levels in pleural effusions and whole blood for TB diagnosis

Evaluation of IP-10 in Quantiferon-Plus as biomarker for the diagnosis of latent tuberculosis infection

Diagnostic accuracy of interferongamma-induced protein 10 for differentiating active tuberculosis from latent tuberculosis: a meta-analysis

Diagnostic performance of a seven-marker serum protein biosignature for the diagnosis of active TB disease in African primary healthcare clinic attendees with signs and symptoms suggestive of TB

IFN-[gamma], but not IP-10, MCP-2 or IL-2 response to RD1 selected peptides associates to active tuberculosis

Polyfunctional CD4(+) T cells as targets for tuberculosis vaccination

Quest for correlates of protection against tuberculosis

Multifunctional CD4(+) T cells correlate with active Mycobacterium tuberculosis infection

Functional capacity of Mycobacterium tuberculosis-specific T cell responses in Humans is associated with mycobacterial load

Expanded polyfunctional T cell response to mycobacterial antigens in TB disease and contraction post-treatment

A comparative analysis of polyfunctional T cells and secreted cytokines induced by Bacille Calmette-Guerin immunisation in children and adults

Polyfunctional CD4 T-cells correlate with in vitro mycobacterial growth inhibition following Mycobacterium bovis BCG-vaccination of infants

A side-by-side comparison of T cell reactivity to fifty-nine Mycobacterium tuberculosis antigens in diverse populations from five continents

A systematic review on novel Mycobacterium tuberculosis antigens and their discriminatory potential for the diagnosis of latent and active tuberculosis

Human T-cell responses to 25 novel antigens encoded by genes of the dormancy regulon of Mycobacterium tuberculosis

Human T-and B-cell reactivity to the 16 kDa α-crystallin protein of Mycobacterium tuberculosis

Nonreplicating persistence of Mycobacterium tuberculosis

Two sensor kinases contribute to the hypoxic response of Mycobacterium tuberculosis

Regulation of the Mycobacterium tuberculosis hypoxic response gene encoding alpha -crystallin

Recognition of stagespecific mycobacterial antigens differentiates between acute and latent infections with Mycobacterium tuberculosis

T-Cell recognition of the HspX protein of Mycobacterium tuberculosis correlates with latent M. tuberculosis infection but not with M. bovis BCG vaccination

Immunogenicity of eight dormancy regulon-encoded proteins of Mycobacterium tuberculosis in DNA-vaccinated and tuberculosis-infected mice

Identification of human T-cell responses to Mycobacterium tuberculosis resuscitation promoting factors in long-term latently infected individuals

Identification of T-cell antigens specific for latent Mycobacterium tuberculosis infection

Bacterial dormancy and culturability: the role of autocrine growth factors

Immunization with Mycobacterium tuberculosis-specific antigens bypasses T cell differentiation from prior bacillus Calmette-Guerin vaccination and improves protection in mice

Practice guidelines for clinical microbiology laboratories: mycobacteria

Phylogeny of the genus Mycobacterium: many doubts, few certainties

An official ATS/ IDSA statement: diagnosis, treatment, and prevention of nontuberculous mycobacterial diseases

Nontuberculous mycobacteria in respiratory infections: advances in diagnosis and identification

Variation in protection by BCG: implications of and for heterologous immunity

Geographic latitude and the efficacy of bacillus Calmette-Guerin vaccine

Immunogenicity of various mycobacteria and the corresponding levels of cross-protection developed between species

Adoptive protection of the Mycobacterium tuberculosis-infected lung. Dissociation between cells that passively transfer protective immunity and those that transfer delayed-type hypersensitivity to tuberculin

Modified vaccinia Ankaraexpressing Ag85A, a novel tuberculosis vaccine, is safe in adolescents and children, and induces polyfunctional CD4+ T cells

Environmental sources of nontuberculous mycobacteria

Function and potentials of M. tuberculosis epitopes

Protection by BCG vaccine against tuberculosis: a systematic review of randomized controlled trials

Nontuberculous Mycobacteria and heterologous immunity to tuberculosis

Lindestam Arlehamn CS. Is mapping the BCG vaccine-induced immune responses the key to improving the efficacy against tuberculosis?

Nomenclature for immune correlates of protection after vaccination

Correlates of protective immunity to Mycobacterium tuberculosis in humans

Systems approaches to correlates of protection and progression to TB disease

Current approaches toward identifying a correlate of immune protection from tuberculosis

Rational approach to selection and clinical development of TB vaccine candidates

Defining features of protective CD4 T cell responses to Mycobacterium tuberculosis

Is interferon-gamma the right marker for bacille Calmette-Guérin-induced immune protection? The missing link in our understanding of tuberculosis immunology

Anticytokine autoantibody-associated immunodeficiency

Safety and efficacy of MVA85A, a new tuberculosis vaccine, in infants previously vaccinated with BCG: a randomised, placebo-controlled phase 2b trial

Poor correlation between BCG vaccination-induced T cell responses and protection against tuberculosis

Correlation of ESAT-6-specific gamma interferon production with pathology in cattle following Mycobacterium bovis BCG vaccination against experimental bovine tuberculosis

Levels of interferon-gamma and interleukin-2 receptor-alpha for bronchoalveolar lavage fluid and serum were correlated with clinical grade and treatment of pulmonary tuberculosis

Multifunctional TH1 cells define a correlate of vaccine-mediated protection against Leishmania major

Tuberculosis: latency and reactivation

Superior control of HIV-1 replication by CD8+ T cells is reflected by their avidity, polyfunctionality, and clonal turnover

high-level cytokine-producing Th1 cells in the lung, but not spleen, correlate with protection against Mycobacterium tuberculosis aerosol challenge in mice

Protection and polyfunctional T cells induced by Ag85B-TB10.4/IC31 against Mycobacterium tuberculosis is highly dependent on the antigen dose

Tuberculosis subunit vaccination provides long-term protective immunity characterized by multifunctional CD4 memory T cells

Vaccine-induced antituberculosis protective immunity in mice correlates with the magnitude and quality of multifunctional CD4 T cells

Alternative BCG delivery strategies improve protection against Mycobacterium tuberculosis in non-human primates: protection associated with mycobacterial antigen-specific CD4 effector memory T-cell populations

Functional profile of CD4+ and CD8+ T cells in latently infected individuals and patients with active TB

Multifunctional CD4 T cell responses in patients with active tuberculosis

Mycobacterium tuberculosis-specific CD4+, IFN[gamma]+, and TNF[alpha]+ multifunctional memory T cells coexpress GM-CSF

Multifunctional CD4+T cells correlate with active Mycobacterium tuberculosis infection

Initiation and regulation of T-cell responses in tuberculosis

Distinct functions of antigen-specific CD4 T cells during murine Mycobacterium tuberculosis infection

Andersen P Control of chronic mycobacterium tuberculosis infection by CD4 KLRG1-IL-2-secreting central memory cells

Protective CD4 T cells targeting cryptic epitopes of Mycobacterium tuberculosis resist infection-driven terminal differentiation

Intravascular staining for discrimination of vascular and tissue leukocytes

Cutting edge: control of Mycobacterium tuberculosis infection by a subset of lung parenchymahoming CD4 T cells

Prevention of tuberculosis in macaques after intravenous BCG immunization

Active or latent tuberculosis increases susceptibility to COVID-19 and disease severity

Active tuberculosis, sequelae and COVID-19 co-infection: first cohort of 49 cases

Clinical characteristics of COVID-19 and active tuberculosis co-infection in an Italian reference hospital

Tuberculosis, COVID-19 and migrants: preliminary analysis of deaths occurring in 69 patients from two cohorts

Previous and active tuberculosis increases risk of death and prolongs recovery in patients with COVID-19

HIV and risk of COVID-19 death: a population cohort study from the Western Cape Province

Nosocomial Transmission of Mycobacterium tuberculosis found through screening for severe acute respiratory syndrome-Taipei

COVID-19 -Tuberculosis interactions: when dark forces collide

Deep immune profiling of COVID-19 patients reveals distinct immunotypes with therapeutic implications

Comprehensive mapping of immune perturbations associated with severe COVID-19

Imbalance of regulatory and cytotoxic SARS-CoV-2-reactive CD4(+) T cells in COVID-19