The exhausted CD4+CXCR5+ T cells involve the pathogenesis of human tuberculosis disease

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 modulating T cells and NK cells response to Mycobacterium tuberculosis infection

Many subversive mechanisms promote the occurrence and development of chronic infectious diseases and cancer, among which the down-regulated expression of immune-activating receptors and the enhanced expression of immune-inhibitory receptors accelerate the occurrence and progression of the disease. Recently, the use of immune checkpoint inhibitors has shown remarkable efficacy in the treatment of tumors in multiple organs. However, the expression of immune checkpoint molecules on natural killer (NK) cells by Mycobacterium tuberculosis (Mtb) infection and its impact on NK cell effector functions have been poorly studied. In this review, we focus on what is currently known about the expression of various immune checkpoints in NK cells following Mtb infection and how it alters NK cell-mediated host cytotoxicity and cytokine secretion. Unraveling the function of NK cells after the infection of host cells by Mtb is crucial for a comprehensive understanding of the innate immune mechanism of NK cells involved in tuberculosis and the evaluation of the efficacy of immunotherapies using immune checkpoint inhibitors to treat tuberculosis. In view of some similarities in the immune characteristics of T cells and NK cells, we reviewed the molecular mechanism of the interaction between T cells and Mtb, which can help us to further understand and explore the specific interaction mechanism between NK cells and Mtb.

STAT3 regulation of Mtb-specific T cell function in active pulmonary tuberculosis patients

BACKGROUND

Tuberculosis (TB) remains one of the most serious infectious diseases in the world. Our aim was to investigate the regulatory role of STAT3 and pSTAT3 in the regulation of T cell immunophenotype and cell function.

METHODS

Twenty-five active pulmonary tuberculosis (APTB) patients, 18 latent tuberculosis infection (LTBI) patients, and 20 healthy controls (HCs) enrolled in this study. T cell phenotype and expression of STAT3 and pSTAT3 were detected by flow cytometry.

RESULTS

Compared with HCs, the expression of pSTAT3 in CD4⁺ T and CD8⁺ T cells in peripheral blood of APTB patients was increased, and the expression was higher in pleural effusion. Multifunctional T cells that simultaneously secrete IFN-γ, TNF-α and IL-17A have higher pSTAT3 expression levels. Mtb-specific T cells from APTB patients had a higher cell frequency of the STAT3⁺ pSTAT3⁺ phenotype and a reduced cell frequency of the STAT3⁺ pSTAT3^- phenotype compared with LTBI patients. Mtb-specific T cells with STAT3⁺ pSTAT3⁺ phenotype had higher expression of PD-1 and PD-L1, while cells with STAT3⁺ pSTAT3^- phenotype had higher expression of Bcl-2.

CONCLUSIONS

STAT3 and pSTAT3 in T cells of APTB patients feature in the process of anti-apoptosis and cytokine secretion. At the same time, the higher pSTAT3 may be related to the degree of cell functional exhaustion. The pSTAT3 level of T cells is related to the infection status and may indicate the clinical activity of the disease, which provides a new idea for the clinical identification and treatment of active pulmonary tuberculosis.

Immunological hyporesponsiveness in tuberculosis: The role of mycobacterial glycolipids

Glycolipids constitute a major part of the cell envelope of Mycobacterium tuberculosis (Mtb). They are potent immunomodulatory molecules recognized by several immune receptors like pattern recognition receptors such as TLR2, DC-SIGN and Dectin-2 on antigen-presenting cells and by T cell receptors on T lymphocytes. The Mtb glycolipids lipoarabinomannan (LAM) and its biosynthetic relatives, phosphatidylinositol mannosides (PIMs) and lipomannan (LM), as well as other Mtb glycolipids, such as phenolic glycolipids and sulfoglycolipids have the ability to modulate the immune response, stimulating or inhibiting a pro-inflammatory response. We explore here the downmodulating effect of Mtb glycolipids. A great proportion of the studies used in vitro approaches although in vivo infection with Mtb might also lead to a dampening of myeloid cell and T cell responses to Mtb glycolipids. This dampened response has been explored ex vivo with immune cells from peripheral blood from Mtb-infected individuals and in mouse models of infection. In addition to the dampening of the immune response caused by Mtb glycolipids, we discuss the hyporesponse to Mtb glycolipids caused by prolonged Mtb infection and/or exposure to Mtb antigens. Hyporesponse to LAM has been observed in myeloid cells from individuals with active and latent tuberculosis (TB). For some myeloid subsets, this effect is stronger in latent versus active TB. Since the immune response in individuals with latent TB represents a more protective profile compared to the one in patients with active TB, this suggests that downmodulation of myeloid cell functions by Mtb glycolipids may be beneficial for the host and protect against active TB disease. The mechanisms of this downmodulation, including tolerance through epigenetic modifications, are only partly explored.

Opposing effects of T cell receptor signal strength on CD4 T cells responding to acute versus chronic viral infection

A hallmark of adaptive immunity is CD4 T cells’ ability to differentiate into specialized effectors. A long-standing question is whether T cell receptor (TCR) signal strength can dominantly instruct the development of Th1 and T follicular helper (Tfh) cells across distinct infectious contexts. We characterized the differentiation of murine CD4 TCR transgenic T cells responding to altered peptide ligand lymphocytic choriomeningitis viruses (LCMV) derived from acute and chronic parental strains. We found that TCR signal strength exerts opposite and hierarchical effects on the balance of Th1 and Tfh cells responding to acute versus persistent infection. TCR signal strength correlates positively with Th1 generation during acute but negatively during chronic infection. Weakly activated T cells express lower levels of markers associated with chronic T cell stimulation and may resist functional inactivation. We anticipate that the panel of recombinant viruses described herein will be valuable for investigating a wide range of CD4 T cell responses.

March of Mycobacterium: miRNAs intercept host cell CD40 signalling

The disease tuberculosis is fatal if untreated. It is caused by the acid-fast bacilli Mycobacterium tuberculosis. Mycobacterium resides and replicates within the alveolar macrophages, causing inflammation and granuloma, wherein macrophage-T cell interactions enhance the inflammation-causing pulmonary caseous lesions. The first interactions between Mycobacterium and the receptors on macrophages decide the fate of Mycobacterium because of phagolysosomal impairments and the expression of several miRNAs, which may regulate CD40 expression on macrophages. While the altered phagolysosomal functions impede antigen presentation to the T cell-expressed antigen receptor, the interactions between the macrophage-expressed CD40 and the T cell-expressed CD40-ligand (CD40L or CD154) provide signals to T cells and Mycobacterium-infected macrophages. These two functions significantly influence the resolution or persistence of Mycobacterium infection. CD40 controls T-cell polarisation and host-protective immunity by eliciting interleukin-12p40, nitric oxide, reactive oxygen species and IFN-γ production. Indeed, CD40-deficient mice succumb to low-dose aerosol infection with Mycobacterium because of deficient interleukin (IL)-12 production leading to impaired IFN-γ-secreting T-cell response. In contrast, despite generating fewer granulomas, the CD40L-deficient mice developed anti-mycobacterial T-cell responses to the levels observed in the wild-type mice. These host-protective responses are significantly subdued by the Mycobacterium-infected macrophage produced TGF-β and IL-10, which promote pro-mycobacterial T-cell responses. The CD40-CD40L-induced counteractive immune responses against Mycobacterium thus present a conundrum that we explain here with a reconciliatory hypothesis. Experimental validation of the hypothesis will provide a rationale for designing anti-tubercular immunotherapy.

Depression and recovery of IL-17A secretion in mitogen responses in patients with active tuberculosis-a prospective observational study

BACKGROUND/PURPOSE

T-helper cell 17 (Th17) is a distinct subset of CD4+ T lymphocytes that is important in the pathogenesis of Mycobacterium tuberculosis infection. This study aims to investigate the characteristics of interleukin (IL)-17A and Th17-related cytokines after stimulation with phytohemagglutinin in patients with active tuberculosis (TB).

METHODS

This prospective cohort study enrolled patients with culture-confirmed active TB. QuantiFERON-TB Gold In-Tube (QFT-GIT) assay was performed upon TB diagnosis and at 2 months after TB treatment. Their non-TB-specific secretion of IL-17A and Th17-related cytokines were measured in supernatants of mitogen tubes in QFT-GIT and compared to those of active TB contacts with or without latent TB infection. We analyzed the association between IL-17A secretions and TB presentation and treatment outcomes.

RESULTS

A total of 108 patients with TB and 64 non-TB cases were enrolled. The secretion of IL-17A, IL-21, IL-23, and IL-6 were lower in active TB patients upon TB diagnosis. In active TB patients, lower IL-17A secretions were associated with higher grades of sputum smear. In the multivariate analysis, lower IL-17A secretions served as an independent factor associated with 2-month culture non-conversion (odds ratio 23.04, 95% confidence interval [CI] 1.69-84.78) and on-treatment mortality (hazard ratio 28.54, 95% CI 1.30-99.25). The levels of IL-23, and IL-6 significantly increased after 2 months of anti-TB treatment.

CONCLUSION

The non-TB-specific IL-17A secretions were lower in active TB patients upon TB diagnosis and associated with higher disease severity and worse treatment outcomes. Trend of recovery of the depressed Th17-related cytokines was noted after effective anti-TB treatment.

Monitoring Anti-tuberculosis Treatment Response Using Analysis of Whole Blood Mycobacterium tuberculosis Specific T Cell Activation and Functional Markers

Background

Blood-based biomarkers have been proposed as an alternative to current sputum-based treatment monitoring methods in active tuberculosis (ATB). The aim of this study was to validate previously described phenotypic, activation, and cytokine markers of treatment response in a West African cohort.

Methods

Whole blood immune responses to Mycobacterium tuberculosis ESAT-6/CFP-10 (EC) and purified protein derivative (PPD) were measured in twenty adults at baseline and after 2 months of standard TB treatment. Patients were classified as fast or slow responders based on a negative or positive sputum culture result at 2 months, respectively. Cellular expression of activation markers (CD38, HLA-DR), memory markers (CD27), and functional intracellular cytokine and proliferation (IFN-γ, Ki-67, TNF-α) markers were measured using multi-color flow cytometry.

Results

There was a significant increase in the proportion of CD4⁺CD27⁺ cells expressing CD38 and HLA-DR following EC stimulation at 2 months compared to baseline (p = 0.0328 and p = 0.0400, respectively). Following PPD stimulation, slow treatment responders had a significantly higher proportion of CD8⁺CD27^–IFN-γ⁺ (p = 0.0105) and CD4⁺CD27⁺HLA-DR⁺CD38⁺ (p = 0.0077) T cells than fast responders at baseline. Receiver operating curve analysis of these subsets resulted in 80% sensitivity and 70 and 100% specificity, respectively (AUC of 0.82, p = 0.0156 and 0.84, p = 0.0102).

Conclusion

Our pilot data show reductions in expression of T cell activation markers were seen with treatment, but this was not associated with fast or slow sputum conversion at 2 months. However, baseline proportions of activated T cell subsets are potentially predictive of the subsequent speed of response to treatment.

Innate lymphoid cell and adaptive immune cell cross-talk: A talk meant not to forget

Innate lymphoid cells (ILCs) are a relatively new class of innate immune cells with phenotypical characters of lymphocytes but genotypically or functionally behave as typical innate immune cells. They have been classically divided into 3 groups (group 1 ILCs or ILC1s, group 2 ILCs or ILC2s, and group 3 ILCs or ILC3s). They serve as the first line of defense against invading pathogens and allergens at mucosal surfaces. The adaptive immune response works effectively in association with innate immunity as innate immune cells serve as APCs to directly stimulate the adaptive immune cells (various sets of T and B cells). Additionally, innate immune cells also secrete various effector molecules, including cytokines or chemokines impacting the function, differentiation, proliferation, and reprogramming among adaptive immune cells to maintain immune homeostasis. Only superantigens do not require their processing by innate immune cells as they are recognized directly by T cells and B cells. Thus, a major emphasis of the current article is to describe the cross-talk between different ILCs and adaptive immune cells during different conditions varying from normal physiological situations to different infectious diseases to allergic asthma.