The characteristic profiles of PD-1 and PD-L1 expressions and dynamic changes during treatment in active tuberculosis

Tuberculosis and T cells: Impact of T cell diversity in tuberculosis infection

Tuberculosis is a global threat and is still a leading cause of death due to an infectious agent. The infection is spread through inhalation of M. tb containing aerosol droplets. Bacteria after reaching the lung alveoli are engulfed by alveolar macrophages, leading to an immune response. Then, pro-inflammatory cytokines are released by these macrophages, recruiting other antigen-presenting cells like dendritic cells. These cells phagocytose the bacteria and present mycobacterial antigens to naïve T cells. After activation by DCs, T cells differentiate into various T cells subsets, viz. CD4+, CD8+, Th17, Treg, Tfh cells and others display enormous diversification in their characteristics and functions. This review comprises a comprehensive literature on conventional and unconventional T cells, highlighting the polyfunctional T cells as well, their role in controlling TB infection, and their implications in the spectrum of TB infection. While some subsets such as CD4+ T cells are extensively studied, some T cell subsets such as gamma delta T cells and Tfh cells remain poorly understood in the pathophysiology of tuberculosis, despite having significant potential implications. The goal of TB eradication can be assisted by development of better vaccines against TB, which can effectively induce a robust and long-term T cells memory. The same has been discussed in the latter part of this review. BCG being the standalone commercialised TB vaccine so far has its limitations. Strategies for the enhancement of BCG along with novel studies in vaccine development, has also been discussed in great detail. Lastly, T cells display a complex interplay of an adaptive immune response against TB, with activation and enhancement of the innate immune responses. Therefore, it is critical to fully understand the role of various T cells subsets in pathophysiology of tuberculosis to provide better therapeutic inventions and improve patient care.

Immune checkpoint inhibitors in infectious disease

Following success in cancer immunotherapy, immune checkpoint blockade is emerging as an exciting potential treatment for some infectious diseases, specifically two chronic viral infections, HIV and hepatitis B. Here, we will discuss the function of immune checkpoints, their role in infectious disease pathology, and the ability of immune checkpoint blockade to reinvigorate the immune response. We focus on blockade of programmed cell death 1 (PD-1) to induce durable immune-mediated control of HIV, given that anti-PD-1 can restore function to exhausted HIV-specific T cells and also reverse HIV latency, a long-lived form of viral infection. We highlight several key studies and future directions of research in relation to anti-PD-1 and HIV persistence from our group, including the impact of immune checkpoint blockade on the establishment (AIDS, 2018, 32, 1491), maintenance (PLoS Pathog, 2016, 12, e1005761; J Infect Dis, 2017, 215, 911; Cell Rep Med, 2022, 3, 100766) and reversal of HIV latency (Nat Commun, 2019, 10, 814; J Immunol, 2020, 204, 1242), enhancement of HIV-specific T cell function (J Immunol, 2022, 208, 54; iScience, 2023, 26, 108165), and investigating the effects of anti-PD-1 and anti-CTLA-4 in vivo in people with HIV on ART with cancer (Sci Transl Med, 2022, 14, eabl3836; AIDS, 2021, 35, 1631; Clin Infect Dis, 2021, 73, e1973). Our future work will focus on the impact of anti-PD-1 in vivo in people with HIV on ART without cancer and potential combinations of anti-PD-1 with other interventions, including therapeutic vaccines or antibodies and less toxic immune checkpoint blockers.

Mycobacterium tuberculosis antigen 85B modifies BCG-induced antituberculosis immunity and favors pathogen survival

Tuberculosis is one of the deadliest infectious diseases worldwide. Mycobacterium tuberculosis has developed strategies not only to evade host immunity but also to manipulate it for its survival. We investigated whether Mycobacterium tuberculosis exploited the immunogenicity of Ag85B, one of its major secretory proteins, to redirect host antituberculosis immunity to its advantage. We found that administration of Ag85B protein to mice vaccinated with Bacillus Calmette-Guérin impaired the protection elicited by vaccination, causing a more severe infection when mice were challenged with Mycobacterium tuberculosis. Ag85B administration reduced Bacillus Calmette-Guérin-induced CD4 T-cell activation and IFN-γ, CCL-4, and IL-22 production in response to Mycobacterium tuberculosis-infected cells. On the other hand, it promoted robust Ag85B-responsive IFN-γ-producing CD4 T cells, expansion of a subset of IFN-γ/IL-10-producing CD4+FOXP3+Treg cells, differential activation of IL-17/IL-22 responses, and activation of regulatory and exhaustion pathways, including programmed death ligand 1 expression on macrophages. All this resulted in impaired intracellular Mycobacterium tuberculosis growth control by systemic immunity, both before and after the Mycobacterium tuberculosis challenge. Interestingly, Mycobacterium tuberculosis infection itself generated Ag85B-reactive inflammatory immune cells incapable of clearing Mycobacterium tuberculosis in both unvaccinated and Bacillus Calmette-Guérin-vaccinated mice. Our data suggest that Mycobacterium tuberculosis can exploit the strong immunogenicity of Ag85B to promote its own survival and spread. Since Ag85B is normally secreted by replicating bacteria and is commonly found in the lungs of the Mycobacterium tuberculosis-infected host, our findings may advance the understanding on the mechanisms of Mycobacterium tuberculosis pathogenesis and immune evasion.

Association of tuberculosis risk with genetic polymorphisms of the immune checkpoint genes PDCD1, CTLA-4, and TIM3

The immune checkpoint proteins were reported to involve to host resistance to Mycobacteria tuberculosis (Mtb). Here, we evaluated 11 single nucleotide polymorphisms (SNPs) in PDCD1, CTLA4, and HAVCR2 genes between participants with and without TB infection. Genomic DNA isolated from 285 patients with TB and 270 controls without TB infection were used to perform the genotyping assay. Odds ratios were used to characterize the association of 11 SNPs with TB risk. In this study, the various genotypes of the 11 SNPs did not differ significantly in frequency between the non-TB and TB groups. When patients were stratified by sex, however, men differed significantly from women in genotype frequencies at HAVCR2 rs13170556. Odds ratios indicated that rs2227982, rs13170556, rs231775, and rs231779 were sex-specifically associated with TB risk. In addition, the combinations of rs2227982/rs13170556 GA/TC in men and the A-C-C haplotype of rs231775-rs231777-rs231779 in women were significantly associated with TB risk. Our results indicate that rs2227982 in PDCD1 and rs13170556 in HAVCR2 are associated with increased TB susceptibility in men and that the CTLA4 haplotype appears protective against TB in women.

New Insights into Biomarkers for Evaluating Therapy Efficacy in Pulmonary Tuberculosis: A Narrative Review

Evaluating therapy efficacy is crucial for patients with tuberculosis (TB), especially those with drug-resistant tuberculosis (DR-TB). The World Health Organization currently recommends sputum smear and culture as the standard methods for evaluating pulmonary tuberculosis (PTB) therapy efficacy. However, these approaches have limitations including low sensitivity, lengthy culture periods, and susceptibility to contamination. There is an urgent need for dependable biomarkers to evaluate therapy efficacy in patients with PTB. Numerous new biomarkers of Mycobacterium tuberculosis (MTB) and the host have been used in recent studies to evaluate PTB therapy efficacy. A systematic review and update of these biomarkers can facilitate the discovery of novel biomarkers and assessment models, as well as provide a solid scientific basis for alternative indicators of evaluating therapy efficacy. In this review we summarize the recent advancements and limitations of biomarkers used to monitor therapy efficacy, highlighting the importance of utilizing a combination of biomarkers. Although some biomarkers have potential in evaluating the efficacy of therapy in patients with PTB, they also have some limitations. Further research, validation, and optimization are required to identify the most reliable and effective alternative biomarkers and apply them to clinical practice.

PD-1/PD-L1 blockade is a potent adjuvant in treatment of Staphylococcus aureus osteomyelitis in mice

There is no effective therapy for implant-associated Staphylococcus aureus osteomyelitis, a devastating complication after orthopedic surgery. An immune-suppressive profile with up-regulated programmed cell death 1/programmed death ligand 1 (PD-1/PD-L1) was identified based on our transcriptional data (GEO: GSE166522) from a mouse model of S. aureus osteomyelitis. PD-1/PD-L1 expression was up-regulated mainly in F4/80+ macrophages surrounding the abscess in S. aureus-infected bone. Mechanistically, PD-1/PD-L1 activated mitophagy to suppress production of mitochondrial reactive oxygen species (ROS), suppressing the bactericidal function of macrophages. Using neutralizing antibodies for PD-L1 or PD-1, or knockout of PD-L1 adjuvant to gentamicin markedly reduced mitophagy in bone marrow F4/80+ cells, enhanced bacterial clearance in bone tissue and implants, and reduced bone destruction in mice. PD-1/PD-L1 expression was also increased in the bone marrow from individuals with S. aureus osteomyelitis. These findings uncover a so far unknown function of PD-1/PD-L1-mediated mitophagy in suppressing the bactericidal function of bone marrow macrophages.

Comparative evaluation of interleukin-10, transforming growth factor-β, and interleukin-17 in gastrointestinal tuberculosis and crohn's disease

Background

Gastrointestinal tuberculosis (GITB) and Crohn's disease (CD) are close mimickers and difficult to discriminate. Recent work has focused on the immunological differences between GITB and CD based on cytokines related to T-regulatory cells and Th17 cells. In the present cross-sectional study, suspected cases of GITB or CD underwent extensive clinical, radiological, endoscopic, histological, and microbiological assessment. The diagnosis was based on standard criteria and response to antitubercular therapy endoscopically.

Methods

Interleukin (IL)-10, transforming growth factor-β (TGF-β), and IL-17 were measured and compared between GITB and CD along with other parameters. Fisher's exact test and Mann-Whitney U test were used as per the data type.

Results

Of the 27 patients, 11 had CD, 9 had GITB, and 7 had other conditions. Chronic diarrhea, involvement of left and long segments of the colon, and aphthous ulcers were significantly more frequent in CD; however, transverse ulcers were in GITB. IL-10 was reduced in both GITB (median-interquartile range [IQR] 9.54 [3.65-24.04]) and CD (median-IQR 13.28 [6.91-22.50]) compared to control (median-IQR 26.72 [10.34-35.43]). TGF-β showed little variation, but IL-17 was below the detection limit in most cases. None of these cytokines were significantly different between CD and GITB. The sensitivity and specificity of multiplex Mycobacterium tuberculosis-polymerase chain reaction were 44.44% and 100%, respectively.

Conclusion

Serum cytokine profiling (IL-10, IL-17, and TGF-β) could not significantly differentiate GITB and CD. Moreover, extensive molecular, transcriptomic, chemokines, and cytokine analyses may shed light on these aspects.

Characterization of multiple soluble immune checkpoints in individuals with different Mycobacterium tuberculosis infection status and dynamic changes during anti-tuberculosis treatment

BACKGROUND

Immune checkpoints are crucial for the maintenance of subtle balance between self-tolerance and effector immune responses, but the role of soluble immune checkpoints (sICs) in Mycobacterium tuberculosis (M. tb) infection remains unknown. We assessed the levels of multiple sICs in individuals with distinct M. tb infection status, and their dynamic changes during anti-tuberculosis treatment.

METHODS

We enrolled 24 patients with pulmonary tuberculosis, among which 10 patients were diagnosed with tuberculous pleurisy (TBP), 10 individuals with latent tuberculosis infection (LTBI), and 10 healthy volunteers from Wuxi Fifth People's Hospital and Huashan Hospital between February 2019 and May 2021. Plasma concentrations of thirteen sICs were measured at enrollment and during anti-tuberculosis treatment using luminex-based multiplex assay. sICs levels in tuberculous pleural effusion (TPE) and their relations to laboratory test markers of TPE were also assessed in TBP patients.

RESULTS

The circulating levels of sPD-1, sPD-L1, sCTLA-4, sBTLA, sGITR, sIDO, sCD28, sCD27 and s4-1BB were upregulated in tuberculosis patients than in healthy controls. A lower sPD-L1 level was found in LTBI individuals than in tuberculosis patients. In TBP patients, the levels of sPD-1, sPD-L2, sCD28, sCD80, sCD27, sTIM-3, sLAG-3, sBTLA, s4-1BB and sIDO increased significantly in TPE than in plasma. In TPE, sBTLA and sLAG-3 correlated positively with the adenosine deaminase level. sIDO and sCD80 correlated positively with the lactate dehydrogenase level and the percentage of lymphocytes in TPE, respectively. Meanwhile, sCD27 correlated negatively with the specific gravity and protein level in TPE. In tuberculosis patients, the circulating levels of sBTLA and sPD-L1 gradually declined during anti-tuberculosis treatment.

CONCLUSIONS

We characterized the changing balance of sICs in M. tb infection. And our results revealed the relations of sICs to laboratory test markers and treatment responses in tuberculosis patients, indicating that certain sICs may serve as potential biomarkers for disease surveillance and prognosis of tuberculosis.

THE INTESTINAL COMMENSAL, Bacteroides fragilis, MODULATES HOST RESPONSES TO VIRAL INFECTION AND THERAPY: LESSONS FOR EXPLORATION DURING Mycobacterium tuberculosis INFECTION

The gut microbiota has emerged as a critical player in host health. Bacteroides fragilis is a prominent member of the gut microbiota within the phyla Bacteroidetes. This commensal bacterium produces unique capsular polysaccharides processed by antigen-presenting cells and activates CD4⁺ T cells to secrete inflammatory cytokines. Indeed, due to their immunomodulatory functions, B. fragilis and its capsular polysaccharide-A (PSA) are arguably the most explored single commensal microbiota/symbiotic factor. B. fragilis/PSA has been shown to protect against colitis, encephalomyelitis, colorectal cancer, pulmonary inflammation, and asthma. Here, we review (1) recent data on the immunomodulatory role of B. fragilis/PSA during viral infections and therapy, (2) B. fragilis PSA's dual ability to mediate pro-and anti-inflammatory processes, and the potential for exploring this unique characteristic during intracellular bacterial infections such as with Mycobacterium tuberculosis (3) discuss the protective roles of single commensal-derived probiotic species including B. fragilis in lung inflammation and respiratory infections that may provide essential cues for possible exploration of microbiota based/augmented therapies in tuberculosis (TB). Available data on the relationship between B. fragilis/PSA, the immune system, and disease suggest clinical relevance for developing B. fragilis into a next-generation probiotic or, possibly, the engineering of PSA into a potent carbohydrate-based vaccine.

The Role of PD-1 in Acute and Chronic Infection

PD-1 as an immune checkpoint molecule down-regulates T cell activity during immune responses in order to prevent autoimmune tissue damage. In chronic infections or tumors, lasting antigen-exposure leads to permanent PD-1 expression that can limit immune-mediated clearance of pathogens or degenerated cells. Blocking PD-1 can enhance T cell function; in cancer treatment PD-1 blockade is already used as a successful therapy. However, the role of PD-1 expression and blocking in the context of acute and chronic infections is less defined. Building on its success in cancer therapy leads to the hypothesis that blocking PD-1 in infectious diseases is also beneficial in acute or chronic infections. This review will focus on the role of PD-1 expression in acute and chronic infections with virus, bacteria, and parasites, with a particular focus on recent studies regarding PD-1 blockade in infectious diseases.

BATF Potentially Mediates Negative Regulation of PD-1/PD-Ls Pathway on T Cell Functions in Mycobacterium tuberculosis Infection

Background: Previously, we have found that blockade of PD-1/PD-Ls pathway could enhance CD4⁺ T cells-mediated protective immunity in patients with active tuberculosis (ATB). However, the mechanism of PD-1/PD-Ls pathway involved in negative regulation of anti-TB immunity has been still unclear. Recently, the study of human immunodeficiency virus (HIV) infection demonstrated that PD-1 could induce the expression of basic leucine zipper ATF-like transcription factor (BATF) to inhibit CD8⁺ T cell function. While the mechanism of immune regulation of BATF in Mycobacterium tuberculosis (M. tb) infection has not yet been elucidated.

Methods: We enrolled 104 participants including ATB patients (n = 66), latent tuberculosis infection (LTBI) (n = 16) and healthy control (HC) (n = 22). The expressions of BATF in peripheral blood CD4⁺ and CD8⁺ T cells from enrolled subjects were determined using flow cytometry. Intervention with PD-1/PD-Ls pathway was performed by using blocking antibodies or human PD-L1 fusion protein. Silencing BATF in peripheral blood mononuclear cells (PBMCs) by electroporation with siRNA. Real-time quantitative PCR, CFSE dilution assay and enzyme linked immunosorbent assay (ELISA) were employed to test T cell functions after BATF knockdown.

Results: The percentages of BATF⁺CD4⁺ (P = 0.0003 and P < 0.0001, respectively) and BATF⁺CD8⁺ (P = 0.0003 and P = 0.0003, respectively) cells were significantly increased in ATB patients compared with LTBI and HC. BATF-expressing PD-1⁺ T cells in CD4⁺ and CD8⁺ T cells were much higher in ATB group than those in LTBI group (P = 0.0426 and 0.0104, respectively) and HC group (P = 0.0133 and 0.0340, respectively). There was a positive correlation between BATF expression and PD-1 expression in ATB patients (for CD4⁺ T cells, r = 0.6761, P = 0.0158; for CD8⁺ T cells, r = 0.6104, P = 0.0350). BATF knockdown could enhance IL-2 and IFN-γ secretions (P = 0.0485 and 0.0473, respectively) and CD4⁺ T cells proliferation (P = 0.0041) in vitro.

Conclusions: In the context of tuberculosis, BATF mediates negative regulation of PD-1/PD-Ls pathway on T cell functions. BATF knockdown can improve cytokine secretion and cells proliferation in vitro.

Inhibition of the PD-1/PD-L1 signaling pathway enhances innate immune response of alveolar macrophages to mycobacterium tuberculosis in mice

BACKGROUND

Mycobacterium tuberculosis (TB) is a pathogen that consequently leads to TB infection, which remains a significant global health concern. Programmed death 1 (PD-1)/programmed death-ligand 1 (PD-L1) signaling pathway is critical for terminating immune responses. The present study aimed to elucidate the regulatory role of the PD-1/PD-L1 signaling pathway in alveolar macrophages against Mycobacterium TB in mice.

METHODS

Specific pathogen free mice were initially prepared for Mycobacterium TB model establishment. The alveolar macrophages of the successfully modeled rats were evaluated to determine the levels of PD-1, PD-L1, AKT, mTOR, TNF-α, NF-κB, IL-2, IL-4, IL-6, IL-10, IL-17, IL-17A, and IFN-γ. The surface makers of macrophages (CD11c, CD16, CD86, CD163, CD206, CX3CR-1 and CSF-1R), level of ROS, apoptosis and cell cycle, were all assessed.

RESULTS

Elevated levels of PD-1 and PD-L1, decreased levels of AKT and mTOR, along with elevated levels of TNF-α, NF-κB, IL-17, IL-2, IL-6, IL-17A and IFN-γ were identified in the alveolar macrophages infected with Mycobacterium TB, while an opposite trend was observed when PD-1/PD-L1 signaling pathway was inhibited. Additionally, elevated protein levels of CD11c, CD16 and CD86, as well as an increased rate of positive ROS and cell apoptosis, levels of Bax, and a diminished percentage of alveolar macrophages at the S and G2/M stages were detected in the event of Mycobacterium TB infection. A contrasting trend to the aforementioned findings was detected when the PD-1/PD-L1 signaling pathway was inhibited.

CONCLUSION

Taken together, these results suggested that inhibition of the PD-1/PD-L1 signaling pathway enhanced the innate immune response of alveolar macrophages to Mycobacterium TB in mice.

Reactivation of tuberculosis in cancer patients following administration of immune checkpoint inhibitors: current evidence and clinical practice recommendations

Immune checkpoint inhibitors (ICBs) have revolutionized cancer treatment producing remarkable and durable responses for a range of malignancies. However, the additional modulation of immune response by ICBs may rarely cause immune-related infectious complications, including re-activation of latent tuberculosis infection (LTBC) with detrimental effects on those patients’ outcome. Here, we present two “real-world” melanoma cases that were treated in our department with blockade of PD-1/PD-L1 and developed active Mycobacterium tuberculosis (MTB) during immunotherapy. In view of these cases, we review the literature for ICB-associated MTB reactivation and discuss our considerations about the possible interactions of immunotherapy and the underlying co-existent mycobacterial infection. Based on the current evidence from preclinical findings prior to this experience, we raise questions regarding cancer patients who are at higher risk for developing MTB infection, whether ICB-treated patients should be considered immunocompromised, and how they should be managed for latent and/or active tuberculosis. Aside from the well-established clinical benefit of immunotherapy, the blockade of PD-1/PD-L1 axis may concurrently disrupt the immune control of specific opportunistic infections such as tuberculosis that should be carefully and expectantly managed in order to avoid compromising the outcome of cancer treatment and the affected patient’s survival.

PD-1 modulating Mycobacterium tuberculosis-specific polarized effector memory T cells response in tuberculosis pleurisy

Host-pathogen interactions in tuberculosis (TB) should be studied at the disease sites because Mycobacterium tuberculosis (M.tb) is predominantly contained in local tissue lesions. T-cell immune responses are required to mount anti-mycobacterial immunity. However, T-cell immune responses modulated by programmed cell death protein 1 (PD-1) during tuberculosis pleurisy (TBP) remains poorly understood. We selected the pleural fluid mononuclear cells (PFMCs) from TBP and PBMCs from healthy donors (HD), and characterized PD-1-expresing T-cell phenotypes and functions. Here, we found that the PFMCs exhibited increases in numbers of PD-1-expressing CD4⁺ and CD8⁺ T cells, which preferentially displayed polarized effector memory phenotypes. The M.tb-specific Ag stimulation increased CD4⁺ PD-1⁺ and CD8⁺ PD-1⁺ T cells, which is in direct correlation with IFN-γ production and PD-L1⁺ APCs in PFMCs of these individuals. Moreover, blockage of PD-1/PD-L1 pathway enhanced the percentage of IFN-γ⁺ T cells, demonstrating that the PD-1/PD-L1 pathway played a negative regulation in T cell effector functions. Furthermore, CD4⁺ PD-1⁺ and CD8⁺ PD-1⁺ T-cell subsets showed greater memory phenotype, activation, and effector functions for producing Th1 cytokines than PD-1^- counterparts. Thus, these PD-1⁺ T cells were not exhausted but appear to be central to maintaining Ag-specific effector. IL-12, a key immunoregulatory cytokine, enhanced the expression of PD-1 and restored a strong IFN-γ response through selectively inducing the phosphorylation of STAT4 in CD4⁺ PD-1⁺ T-bet⁺ and CD8⁺ PD-1⁺ T-bet⁺ T cells. This study therefore uncovered a previously unknown mechanism for T-cell immune responses regulated by PD-1, and may have implications for potential immune intervention in TBP.

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

Chronic T cell activation is a hallmark of pulmonary tuberculosis (PTB). The mechanisms underpinning this important phenomenon are however, poorly elucidated, though known to rely on control of T effector cells (Teff) by regulatory T cells (Treg). Our studies show that circulating natural Treg cells in adults with PTB preserve their suppressive potential but Teff cells from such subjects are resistant to Treg-mediated suppression. We found this to be due to expansion of an activated Teff subset identified by Human Leukocyte Antigen (HLA)-DR expression. Sensitivity to suppression was restored to control levels by depletion of this subset. Comparative transcriptome analysis of Teff cells that contain HLA-DR⁺ cells versus the fraction depleted of this population identified putative resistance mechanisms linked to IFNG, IL17A, IL22, PD-L1 and β-chemokines CCL3L3, CCL4 expression. Antibody blocking experiments confirmed HLA-DR⁺ Teff cells, but not the fraction depleted of HLA-DR⁺ effectors, to be resistant to Treg suppression mediated via CCR5 and PD-L1 associated pathways. In the presence of HLA-DR⁺ Teff cells, activation of NFκB downstream of CCR5 and PD-L1 was perturbed. In addition, HLA-DR⁺ Teff cells expressed significantly higher levels of Th1/Th17 cytokines that may regulate Treg function through a reciprocal counter-balancing relationship. Taken together, our study provides novel insight on how activated HLA-DR⁺CD4⁺ T cells may contribute to disease associated inflammation by compromising Treg-mediated suppression in PTB.

An important marker of progression to PTB following Mycobacterium tuberculosis (Mtb) infection in humans is elevated frequencies of HLA-DR⁺CD4⁺ T cells, reflecting chronic T cell activation. However, the mechanisms by which activated HLA-DR⁺CD4⁺ T cells contribute to disease process is not known. We show that CD25^- HLA-DR⁺CD4⁺ memory Teff from PTB patients are resistant to suppression mediated by Treg cells. An unbiased transcriptome analysis identified several key pathways that contribute to this resistance. Specifically, presence of HLA-DR⁺CD4⁺ T cells renders the effector population resistant to CCR5 and PD-L1 mediated suppression by Treg cells. In addition, the HLA-DR⁺CD4⁺ memory Teff cells express elevated levels of Th1/Th17 cytokines known to counter-regulate and dampen Treg suppression. These findings provide fresh insight to disease process in TB and identify HLA-DR⁺ Teff resistant to Treg suppression as a potential functional marker of disease.

The use of immunotherapy for the treatment of tuberculosis

INTRODUCTION

Tuberculosis (TB) is the first cause of mortality by a single infectious agent in the world, causing more than one million deaths worldwide as reported by the World Health Organization (WHO). For the optimal control of TB infection, a protective immune response that limits bacterial spread without causing damage to the host is essential. Although most healthy individuals are capable of generating protective responses, patients who suffer pulmonary TB commonly present a defective immune function. Areas covered: We intend to highlight the potential of novel immunotherapeutic strategies that enhance and promote effective immune responses. The following methodology was undertaken for establishing a literature search: the authors used PubMed to search for 'Pulmonary Tuberculosis' and keywords that denoted the novel immunotherapeutic strategies discussed in length in the text including antibodies, antimicrobial peptides, cell therapy, cytokines and gene therapy. Expert commentary: The current therapeutic regimens for this disease are complex and involve the prolonged use of multiple antibiotics with diverse side effects that lead to therapeutic failure and bacterial resistance. The standard appliance of immunotherapy and its deployment to vulnerable populations will require coordinated work and may serve as a powerful tool to combat the ensuing threat of TB.

The crucial roles of Th17-related cytokines/signal pathways in M. tuberculosis infection

Interleukin-17 (IL-17), IL-21, IL-22 and IL-23 can be grouped as T helper 17 (Th17)-related cytokines because they are either produced by Th17/Th22 cells or involved in their development. Here, we review Th17-related cytokines/Th17-like cells, networks/signals and their roles in immune responses or immunity against Mycobacterium tuberculosis (Mtb) infection. Published studies suggest that Th17-related cytokine pathways may be manipulated by Mtb microorganisms for their survival benefits in primary tuberculosis (TB). In addition, there is evidence that immune responses of the signal transducer and activator of transcription 3 (STAT3) signal pathway and Th17-like T-cell subsets are dysregulated or destroyed in patients with TB. Furthermore, Mtb infection can impact upstream cytokines in the STAT3 pathway of Th17-like responses. Based on these findings, we discuss the need for future studies and the rationale for targeting Th17-related cytokines/signals as a potential adjunctive treatment.

Immune checkpoints and their inhibition in cancer and infectious diseases

The development of chronic infections and cancer is facilitated by a variety of immune subversion mechanisms, such as the production of anti-inflammatory cytokines, induction of regulatory T (Treg) cells, and expression of immune checkpoint molecules, including CTLA-4 and PD-1. CTLA-4, expressed on T cells, interacts with CD80/CD86, thereby limiting T-cell activation and leading to anergy. PD-1 is predominantly expressed on T cells and its interaction with PD-L1 and PD-L2 expressed on antigen-presenting cells (APCs) and tumors sends a negative signal to T cells, which can lead to T-cell exhaustion. Given their role in suppressing effector T-cell responses, immune checkpoints are being targeted for the treatment of cancer. Indeed, antibodies binding to CTLA-4, PD-1, or PD-L1 have shown remarkable efficacy, especially in combination therapies, for a number of cancers and have been licensed for the treatment of melanoma, nonsmall cell lung cancer, and renal and bladder cancers. Moreover, immune checkpoint inhibitors have been shown to enhance ex vivo effector T-cell responses from patients with chronic viral, bacterial, or parasitic infection, including HIV, tuberculosis, and malaria. Although the data from clinical trials in infectious diseases are still sparse, these inhibitors have great potential for treating chronic infections, especially when combined with therapeutic vaccines.