Cutting Edge: Regulatory T cells prevent efficient clearance of Mycobacterium tuberculosis

Improving regulatory T cell-based therapy: insights into post-translational modification regulation

Regulatory T (Treg) cells are pivotal for maintaining immune homeostasis and play essential roles in various diseases, such as autoimmune diseases, graft-versus-host disease (GVHD), tumors, and infectious diseases. Treg cells exert suppressive function via distinct mechanisms, including inhibitory cytokines, granzyme or perforin-mediated cytolysis, metabolic disruption, and suppression of dendritic cells. Forkhead Box P3 (FOXP3), the characteristic transcription factor, is essential for Treg cell function and plasticity. Cumulative evidence has demonstrated that FOXP3 activity and Treg cell function are modulated by a variety of post-translational modifications (PTMs), including ubiquitination, acetylation, phosphorylation, methylation, glycosylation, poly(ADP-ribosyl)ation, and uncharacterized modifications. This review describes Treg cell suppressive mechanisms and summarizes the current evidence on PTM regulation of FOXP3 and Treg cell function. Understanding the regulatory role of PTMs in Treg cell plasticity and function will be helpful in designing therapeutic strategies for autoimmune diseases, GVHD, tumors, and infectious diseases.

Distinct characteristics of BTLA/HVEM axis expression on Tregs and its impact on the expansion and attributes of Tregs in patients with active pulmonary tuberculosis

Introduction

Pulmonary tuberculosis (PTB) remains one of the deadliest infectious diseases. Understanding PTB immunity is of potential value for exploring immunotherapy for treating chemotherapy-resistant PTB. CD4+CD25+Foxp3+ regulatory T cells (Tregs) are key players that impair immune responses to Mycobacteria tuberculosis (MTB). Currently, the intrinsic factors governing Treg expansion and influencing the immunosuppressive attributes of Tregs in PTB patients are far from clear.

Methods

Here, we employed flow cytometry to determine the frequency of Tregs and the expression of B and T lymphocyte attenuator (BTLA) and its ligand, herpesvirus entry mediator (HVEM), on Tregs in patients with active PTB. Furthermore, the expression of conventional T cells and of programmed death-ligand 1 (PD-L1) and programmed death-1 (PD-1) on Tregs in patients with active PTB was determined. We then examined the characteristics of BTLA/HVEM expression and its correlation with Treg frequency and PD-L1 and PD-1 expression on Tregs in PTB patients.

Results

The frequency of Tregs was increased in PTB patients and it had a relevance to PTB progression. Intriguingly, the axis of cosignal molecules, BTLA and HVEM, were both downregulated on the Tregs of PTB patients compared with healthy controls (HCs), which was the opposite of their upregulation on conventional T cells. Unexpectedly, their expression levels were positively correlated with the frequency of Tregs, respectively. These seemingly contradictory results may be interpreted as follows: the downregulation of BTLA and HVEM may alleviate BTLA/HVEM cis-interaction-mediated coinhibitory signals pressing on naïve Tregs, helping their activation, while the BTLA/HVEM axis on effector Tregs induces a costimulatory signal, promoting their expansion. Certainly, the mechanism underlying such complex effects remains to be explored. Additionally, PD-L1 and PD-1, regarded as two of the markers characterizing the immunosuppressive attributes and differentiation potential of Tregs, were upregulated on the Tregs of PTB patients. Further analysis revealed that the expression levels of BTLA and HVEM were positively correlated with the frequency of PD-1+Tregs and PD-L1+Tregs, respectively.

Conclusion

Our study illuminated distinct characteristics of BTLA/HVEM axis expression on Tregs and uncovered its impact on the expansion and attributes of Tregs in patients with active PTB. Therefore, blockade of the BTLA/HVEM axis may be a promising potential pathway to reduce Treg expansion for the improvement of anti-MTB immune responses.

Effect of Live and Fragmented Saccharomyces cerevisiae in the Feed of Pigs Challenged with Mycoplasma hyopneumoniae

Currently, the responsible use of antimicrobials in pigs has allowed the continuous development of alternatives to these antimicrobials. In this study, we describe the impact of treatments with two probiotics, one based on live Saccharomyces cerevisiae (S. cerevisiae) and another based on fragmented S. cerevisiae (beta-glucans), that were administered to piglets at birth and at prechallenge with Mycoplasma hyopneumoniae. Thirty-two pigs were divided into four groups of eight animals each. The animals had free access to water and food. The groups were as follows: Group A, untreated negative control; Group B, inoculated by nebulization with M. hyopneumoniae positive control; Group C, first treated with disintegrated S. cerevisiae (disintegrated Sc) and inoculated by nebulization with M. hyopneumoniae; and Group D, treated with live S. cerevisiae yeast (live Sc) and inoculated by nebulization with M. hyopneumoniae. In a previous study, we found that on Days 1 and 21 of blood sampling, nine proinflammatory cytokines were secreted, and an increase in their secretion occurred for only five of them: TNF-α, INF-α, INF-γ, IL-10, and IL-12 p40. The results of the clinical evolution, the degree of pneumonic lesions, and the productive parameters of treated Groups C and D suggest that S. cerevisiae has an immunomodulatory effect in chronic proliferative M. hyopneumoniae pneumonia characterized by delayed hypersensitivity, which depends on the alteration or modulation of the respiratory immune response. The data presented in this study showed that S. cerevisiae contributed to the innate resistance of infected pigs.

Mechanisms underlying immunosuppression by regulatory cells

Regulatory cells, such as regulatory T cells (Tregs), regulatory B cells (Bregs), and myeloid-derived suppressor cells (MDSCs), play a crucial role in preserving immune tolerance and controlling immune responses during infections to prevent excessive immune activation. However, pathogens have developed strategies to hijack these regulatory cells to decrease the overall effectiveness of the immune response and persist within the host. Consequently, therapeutic targeting of these immunosuppressive mechanisms during infection can reinvigorate the immune response and improve the infection outcome. The suppressive mechanisms of regulatory cells are not only numerous but also redundant, reflecting the complexity of the regulatory network in modulating the immune responses. The context of the immune response, such as the type of pathogen or tissue involved, further influences the regulatory mechanisms involved. Examples of these immunosuppressive mechanisms include the production of inhibitory cytokines such as interleukin 10 (IL-10) and transforming growth factor beta (TGF-β) that inhibit the production of pro-inflammatory cytokines and dampen the activation and proliferation of effector T cells. In addition, regulatory cells utilize inhibitory receptors like cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and programmed cell death protein 1 (PD-1) to engage with their respective effector cells, thereby suppressing their function. An alternative approach involves the modulation of metabolic reprogramming in effector immune cells to limit their activation and proliferation. In this review, we provide an overview of the major mechanisms mediating the immunosuppressive effect of the different regulatory cell subsets in the context of infection.

Circular RNAs in tuberculosis: From mechanism of action to potential diagnostic biomarker

Tuberculosis (TB), caused by Mycobacterium tuberculosis (M. tuberculosis), continues to be a major global health concern. Understanding the molecular intricacies of TB pathogenesis is crucial for developing effective diagnostic and therapeutic approaches. Circular RNAs (circRNAs), a class of single-stranded RNA molecules characterized by covalently closed loops, have recently emerged as potential diagnostic biomarkers in various diseases. CircRNAs have been demonstrated to modulate the host's immunological responses against TB, specifically by reducing monocyte apoptosis, augmenting autophagy, and facilitating macrophage polarization. This review comprehensively explores the roles and mechanisms of circRNAs in TB pathogenesis. We also discuss the growing body of evidence supporting their utility as promising diagnostic biomarkers for TB. By bridging the gap between fundamental circRNA biology and TB diagnostics, this review offers insights into the exciting potential of circRNAs in combatting this infectious disease.

Immunotherapy and biomarkers in patients with lung cancer with tuberculosis: Recent advances and future Directions

Lung cancer (LC) and tuberculosis (TB) are two major global public health problems, and the incidence of LC-TB is currently on the rise. Therefore effective clinical interventions are crucial for LC-TB. The aim of this review is to provide up-to-date information on the immunological profile and therapeutic biomarkers in patients with LC-TB. We discuss the immune mechanisms involved, including the immune checkpoints that play an important role in the treatment of patients with LC-TB. In addition, we explore the susceptibility of patients with LC to TB and summarise the latest research on LC-TB. Finally, we discuss future prospects in this field, including the identification of potential targets for immune intervention. In conclusion, this review provides important insights into the complex relationship between LC and TB and highlights new advances in the detection and treatment of both diseases.

Withaferin A Protects against Primary and Recurrent Tuberculosis by Modulating Mycobacterium-Specific Host Immune Responses

The fate of Mycobacterium tuberculosis infection is governed by immune signaling pathways that can either eliminate the pathogen or result in tuberculosis (TB). Anti-TB therapy (ATT) is extensive and is efficacious only against active, drug-sensitive strains of M. tuberculosis. Due to severe side effects, ATT often causes impairment of host immunity, making it imperative to use novel immunotherapeutics for better clinical outcomes. In this study, we have explored the immunomodulatory potential of withaferin A (WA) as an immunotherapeutic against TB. Here, we demonstrate that WA can constrain intracellular drug-sensitive and -resistant strains of M. tuberculosis by augmenting host immune responses. We also established the potential of WA treatment in conjunction with isoniazid. We show that WA directs the host macrophages toward defensive M1 polarization and enhances T_H1 and T_H17 immune responses against M. tuberculosis infection. The reduced bacterial burden upon T cell adoptive transfer further corroborated the augmented T cell responses. Interestingly, WA stimulated the generation of T cell memory populations by instigating STAT signaling, thereby reducing the rate of TB recurrence due to reactivation and reinfection. We substantiate the prospects of WA as a potent adjunct immunomodulator that enriches protective memory cells by prompting STAT signaling and improves host defense against M. tuberculosis. IMPORTANCE Despite being extensive, conventional antituberculosis therapy (ATT) is barely proficient in providing sterile immunity to tuberculosis (TB). Failure to constrain the escalating global TB burden due to the emergence of drug-resistant bacterial strains and immune dampening effects of ATT necessitates adjunct immunotherapeutics for better clinical outcomes. We evaluated the prospects of withaferin A (WA), an active constituent of Withania somnifera, as an adjunct immunomodulator against diverse M. tuberculosis strains. WA efficiently restricts the progression of TB by stimulating antimycobacterial host responses, protective immune signaling, and activation of diverse immune cell populations. Protective effects of WA can be attributed to the enrichment of memory T cells by induction of STAT signaling, thereby enhancing resistance to reinfections and reactivation of disease. We ascertained the immunotherapeutic potential of WA in boosting host immune responses against M. tuberculosis.

Reinventing the human tuberculosis (TB) granuloma: Learning from the cancer field

Tuberculosis (TB) remains one of the deadliest infectious diseases in the world and every 20 seconds a person dies from TB. An important attribute of human TB is induction of a granulomatous inflammation that creates a dynamic range of local microenvironments in infected organs, where the immune responses may be considerably different compared to the systemic circulation. New and improved technologies for in situ quantification and multimodal imaging of mRNA transcripts and protein expression at the single-cell level have enabled significantly improved insights into the local TB granuloma microenvironment. Here, we review the most recent data on regulation of immunity in the TB granuloma with an enhanced focus on selected in situ studies that enable spatial mapping of immune cell phenotypes and functions. We take advantage of the conceptual framework of the cancer-immunity cycle to speculate how local T cell responses may be enhanced in the granuloma microenvironment at the site of Mycobacterium tuberculosis infection. This includes an exploratory definition of "hot", immune-inflamed, and "cold", immune-excluded TB granulomas that does not refer to the level of bacterial replication or metabolic activity, but to the relative infiltration of T cells into the infected lesions. Finally, we reflect on the current knowledge and controversy related to reactivation of active TB in cancer patients treated with immune checkpoint inhibitors such as PD-1/PD-L1 and CTLA-4. An understanding of the underlying mechanisms involved in the induction and maintenance or disruption of immunoregulation in the TB granuloma microenvironment may provide new avenues for host-directed therapies that can support standard antibiotic treatment of persistent TB disease.

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.

The 1, 2-ethylenediamine SQ109 protects against tuberculosis by promoting M1 macrophage polarization through the p38 MAPK pathway

Directly Observed Treatment Short-course (DOTs), is an effective and widely recommended treatment for tuberculosis (TB). The antibiotics used in DOTs, are immunotoxic and impair effector T cells, increasing the risk of re-infections and reactivation. Multiple reports suggest that addition of immune-modulators along with antibiotics improves the effectiveness of TB treatment. Therefore, drugs with both antimicrobial and immunomodulatory properties are desirable. N¹-(Adamantan-2-yl)-N²-[(2E)-3,7-dimethylocta-2,6-dien-1-yl]ethane-1,2-diamine (SQ109) is an asymmetric diamine derivative of adamantane, that targets Mycobacterial membrane protein Large 3 (MmpL3). SQ109 dissipates the transmembrane electrochemical proton-gradient necessary for cell-wall biosynthesis and bacterial activity. Here, we examined the effects of SQ109 on host-immune responses using a murine TB model. Our results suggest the pro-inflammatory nature of SQ109, which instigates M1-macrophage polarization and induces protective pro-inflammatory cytokines through the p38-MAPK pathway. SQ109 also promotes Th1 and Th17-immune responses that inhibit the bacillary burden in a murine model of TB. These findings put forth SQ109 as a potential-adjunct to TB antibiotic therapy.

The adamantine derivative SQ109 induces protective pro-inflammatory cytokines and promotes Th1 and Th17-immune responses that inhibit bacterial burden in a tuberculosis mouse model.

Evaluation of the effect of T regulatory cell depletion and donor BCG vaccination on Mycobacterium tuberculosis H37Ra infection using an in vitro model of human PBMC infection

This study evaluated the effect of T regulatory cells (Treg cells) and the impact of BCG vaccination history of donors using an in vitro model of Mycobacterium tuberculosis H37Ra infection of peripheral blood mononuclear cells (PBMCs). PBMCs from donors with or without prior BCG vaccination were depleted of Treg cells (PBMCs-Tregs) or not depleted with Treg cells (PBMCs + Tregs) were infected up to 8 days with Mtb H37Ra. Cell aggregates were smaller in PBMCs-Tregs compared to PBMCs + Tregs at day 8 post-infection. Mtb CFUs were higher in the PBMCs-Tregs compared to PBMCs + Tregs at days 3, 5 and 8. The levels of IL-17, IFN-γ (at days 3 and 5), and TNF-α and IL-6 (at day 3) were lower in PBMCs-Tregs compared to PBMCs + Tregs. In contrast, the levels of IL-10 and IL-4 cytokines were higher at day 3 in PBMCs-Tregs compared to PBMCs + Tregs. BCG vaccination status of donors had no impact on the mycobacterial culture, level of cytokines and immune cell populations. This study shows that depletion of Tregs in human PBMCs infected with Mtb H37Ra in vitro leads to a shift from a Th1 to a Th2 cytokine rich environment that supports the survival of Mtb in this model.

Increase of CD4+CD25highFoxP3+ cells impairs in vitro human microbicidal activity against Mycobacterium tuberculosis during latent and acute pulmonary tuberculosis

Background

Regulatory T cells (Tregs) play a critical role during Mycobacterium tuberculosis (Mtb) infection, modulating host responses while neutralizing excessive inflammation. However, their impact on regulating host protective immunity is not completely understood. Here, we demonstrate that Treg cells abrogate the in vitro microbicidal activity against Mtb.

Methods

We evaluated the in vitro microbicidal activity of peripheral blood mononuclear cells (PBMCs) from patients with active tuberculosis (TB), individuals with latent tuberculosis infection (LTBI, TST+/IGRA+) and healthy control (HC, TST-/IGRA-) volunteers. PBMCs, depleted or not of CD4⁺CD25⁺ T-cells, were analyzed to determine frequency and influence on microbicidal activity during in vitro Mtb infection with four clinical isolates (S1, S5, R3, and R6) and one reference strain (H37Rv).

Results

The frequency of CD4⁺CD25^highFoxP3⁺ cells were significantly higher in Mtb infected whole blood cultures from both TB patients and LTBI individuals when compared to HC. Data from CD4⁺CD25⁺ T-cells depletion demonstrate that increase of CD4⁺CD25^highFoxP3⁺ is associated with an impairment of Th-1 responses and a diminished in vitro microbicidal activity of LTBI and TB groups.

Conclusions

Tregs restrict host anti-mycobacterial immunity during active disease and latent infection and thereby may contribute to both disease progression and pathogen persistence.

Our immune system has an enormous capacity of recognizing and responding to foreign antigens and, likewise, presents an extremely efficient mechanism of controlling these responses. Here, we investigated how a specific cell type with regulatory abilities can interfere in the immunological response against tuberculosis bacillus. For this, we used blood samples from individuals sensitized with the bacillus and patients with active pulmonary tuberculosis to understand how these cells act and their impact on the host/parasite relationship in the development of the disease. We could observe the negative impact that such regulatory cells cause during the immune response against Mycobacterium tuberculosis, decreasing the control/elimination of the bacillus in asymptomatic individuals and patients with tuberculosis. We also observed a recovery in the immune response when Treg cells were removed during in vitro challenge, restoring the capacity of Mtb clearance. Thus, these regulatory cells, when present, may represent a possible facilitator of the asymptomatic permanence of the bacillus, or even of the development of the disease itself. These data allowed us to see latency and tuberculosis from a new angle and thus postulate new approaches to fight tuberculosis.

Anti-tuberculosis chemotherapy alters TNFR2 expression on CD4+ lymphocytes in both drug-sensitive and -resistant tuberculosis: however, only drug-resistant tuberculosis maintains a pro-inflammatory profile after a long time

Background

Tuberculosis (TB) is an infectious disease. During TB, regulatory T cells (Treg) are related to poor prognosis. However, information about conventional and unconventional Treg (cTreg and uTreg, respectively) is limited. The tumour necrosis factor (TNF) and its receptors (TNFR1 and TNFR2) are necessary for mycobacterial infection, and TNFR2 signalling is required to maintain Treg.

Methods

A blood sample of drug-susceptible (DS-TB) and drug-resistant tuberculosis (DR-TB) patients was obtained before (basal) and after 2 and 6 months of anti-TB therapy. Expression of TNF, TNFR1, and TNFR2 (transmembrane form, tm) on cTreg, uTreg, activated CD4+ (actCD4+), and CD4+ CD25− (CD4+) T cell subpopulations were evaluated. The main objective was to identify immunological changes associated with sensitive/resistant Mtb strains and with the use of anti-TB therapy.

Results

We found that after 6 months of anti-TB therapy, both DS- and DR-TB patients have decreased the frequency of cTreg tmTNF+, CD4+ tmTNFR1+ and CD4+ tmTNFR2+. Nevertheless, after 6 months of therapy, only DR-TB patients decreased the frequency of actCD4+ tmTNF+ and actCD4+ tmTNFR2+, exhibited a systemic inflammatory status (high levels of TNF, IFN-γ and IL-12), and their purified CD4+ T cells showed that TNF and TNFR2 are up-regulated at the transcriptional level. Moreover, DS- and DR-TB down-regulated TNFR1 and other proteins associated with Treg (FOXP3 and TGFβ1) in response to the anti-TB therapy.

Conclusion

These results partially explain the differences in the immune response of DS-TB vs DR-TB. The frequency of actCD4+ tmTNFR2+ cells and inflammatory status should be considered in the follow-up of therapy in DR-TB patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10020-021-00320-4.

Identification of Potentially Therapeutic Immunogenic Peptides From Paracoccidioides lutzii Species

Paracoccidioidomycosis (PCM) is an endemic mycosis in Latin America caused by the thermodimorphic fungi of the genus Paracoccidioides spp. Paracoccidioides lutzii (PL) is one of the 5 species that constitute the Paracoccidioides genus. PL expresses low amounts of glycoprotein (Gp) 43 (PLGp43) and PLGp43 displays few epitopes in common with the P. brasiliensis (PB) immunodominant antigen PBGp43, which is commonly used for serological diagnosis of PCM. This difference in structure between the glycoproteins markedly reduces the efficiency of serological diagnosis in patients infected with PL. We previously demonstrated that peptide 10 (P10) from the PBGp43 induces protective immune responses in in vitro and in vivo models of PB PCM. Since, P10 has proven to be a promising therapeutic to combat PB, we sought to identify peptides in PL that could similarly be applied for the treatment of PCM. PL yeast cell proteins were isolated from PL: dendritic cell co-cultures and subjected to immunoproteomics. This approach identified 18 PL peptides that demonstrated in silico predictions for immunogenicity. Eight of the most promising peptides were synthesized and applied to lymphocytes obtained from peptide-immunized or PL-infected mice as well as to in vitro cultures with peptides or dendritic cells pulsed the peptides. The peptides LBR5, LBR6 and LBR8 efficiently promoted CD4⁺ and CD8⁺ T cell proliferation and dendritic cells pulsed with LBR1, LBR3, LBR7 or LBR8 stimulated CD4⁺ T cell proliferation. We observed increases of IFN-γ in the supernatants from primed T cells for the conditions with peptides without or with dendritic cells, although IL-2 levels only increased in response to LBR8. These novel immunogenic peptides derived from PL will be employed to develop new peptide vaccine approaches and the proteins from which they are derived can be used to develop new diagnostic assays for PL and possibly other Paracoccidioides spp. These findings identify and characterize new peptides with a promising therapeutic profile for future against this important neglected systemic mycosis.

PMN-MDSC Frequency Discriminates Active Versus Latent Tuberculosis and Could Play a Role in Counteracting the Immune-Mediated Lung Damage in Active Disease

Tuberculosis (TB), due to Mycobacterium tuberculosis infection, is still the principal cause of death caused by a single infectious agent. The balance between the bacillus and host defense mechanisms reflects the different manifestations of the pathology. Factors defining this variety are unclear and likely involve both mycobacterial and immunological components. Myeloid derived suppressor cells (MDSC) have been shown to be expanded during TB, but their role in human TB pathogenesis is not clear. We evaluated the frequency of circulating MDSC by flow-cytometry in 19 patients with active TB, 18 with latent TB infection (LTBI), and 12 healthy donors (HD) as control. Moreover, we investigated the capacity of MDSC to modulate the mycobactericidal activity of monocytes. The association between MDSC level and TB chest X-ray severity score was analyzed. We observed that, unlike active TB, polymorphonuclear (PMN)-MDSC are not expanded in LTBI patients, and, by performing a receiver operating characteristic (ROC) curve analysis, we found that PMN-MDSC frequency supported the discrimination between active disease and LTBI. Interestingly, we observed an association between PMN-MDSC levels and the severity of TB disease evaluated by chest X-ray. Specifically, PMN-MDSC frequency was higher in those classified with a low/mild severity score compared to those classified with a high severity score. Moreover, PMN-MDSC can impact mycobacterial growth by inducing ROS production in Bacillus Calmette et Guerin (BCG)-infected monocytes. This effect was lost when tested with M. tuberculosis (MTB), In conclusion, our data indicate that the elevated frequency of PMN-MDSC in IGRA-positive individuals is associated with active TB. Our findings also pointed out a beneficial role of PMN-MDSC during human active TB, most likely associated with the limitation of inflammation-induced tissue damage.

Perspectives and Advances in the Understanding of Tuberculosis

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), remains a leading cause of death due to infection in humans. To more effectively combat this pandemic, many aspects of TB control must be developed, including better point of care diagnostics, shorter and safer drug regimens, and a protective vaccine. To address all these areas of need, better understanding of the pathogen, host responses, and clinical manifestations of the disease is required. Recently, the application of cutting-edge technologies to the study of Mtb pathogenesis has resulted in significant advances in basic biology, vaccine development, and antibiotic discovery. This leaves us in an exciting era of Mtb research in which our understanding of this deadly infection is improving at a faster rate than ever, and renews hope in our fight to end TB. In this review, we reflect on what is known regarding Mtb pathogenesis, highlighting recent breakthroughs that will provide leverage for the next leaps forward in the field.

Pathology of TB/COVID-19 Co-Infection: The phantom menace

Tuberculosis (TB) and coronavirus disease 2019 (COVID-19) are currently the two main causes of death among infectious diseases. There is an increasing number of studies trying to elucidate the interactions between Mycobacterium tuberculosis and SARS-CoV-2. Some of the first case reports point to a worsening of respiratory symptoms in co-infected TB/COVID-19 individuals. However, data from the cohort studies has shown some conflicting results. This study proposes to conduct a systematic review on the current literature on TB/COVID-19 co-infection cohorts, evaluating clinical and epidemiological data, focusing on its implications to the immune system. From an immunological perspective, the TB/COVID-19 co-infection has the potential to converge in a "perfect storm". The disorders induced by each pathogen to the immunomodulation tend to induce an unbalanced inflammatory response, which can promote the progression and worsening of both diseases. Understanding the nature of the interactions between M. tuberculosis and SARS-CoV-2 will be crucial for the development of therapeutic strategies against co-infection.

Interleukin-33 signaling exacerbates experimental infectious colitis by enhancing gut permeability and inhibiting protective Th17 immunity

A wide range of microbial pathogens is capable of entering the gastrointestinal tract, causing infectious diarrhea and colitis. A finely tuned balance between different cytokines is necessary to eradicate the microbial threat and to avoid infection complications. The current study identified IL-33 as a critical regulator of the immune response to the enteric pathogen Citrobacter rodentium. We observed that deficiency of the IL-33 signaling pathway attenuates bacterial-induced colitis. Conversely, boosting this pathway strongly aggravates the inflammatory response and makes the mice prone to systemic infection. Mechanistically, IL-33 mediates its detrimental effect by enhancing gut permeability and by limiting the induction of protective T helper 17 cells at the site of infection, thus impairing host defense mechanisms against the enteric pathogen. Importantly, IL-33-treated infected mice supplemented with IL-17A are able to resist the otherwise strong systemic spreading of the pathogen. These findings reveal a novel IL-33/IL-17A crosstalk that controls the pathogenesis of Citrobacter rodentium-driven infectious colitis. Manipulating the dynamics of cytokines may offer new therapeutic strategies to treat specific intestinal infections.

Relationship between expression mRNA gene Treg, Treg, CD4+, and CD8+ protein levels with TST in tuberculosis children: A nested case-control

BACKGROUND

The ability of Mycobacterium tuberculosis to survive intracellularly, provides a cellular adaptive immune response played by specific T cells to defend against tuberculosis. The adaptive immune response to Bacillus of Calmette and Guerin (BCG) immunization is responded to by B cells, T Follicular B helper, T regulatory, restriction CD1, CD8+, CD4+, Th1, Th2, and Th17. BCG immunization can cause a tuberculin test reaction to being positive. The tuberculin test is a method for diagnosing TB infection and for screening individuals for latent infection and assessing the rate of TB infection in a given population.

METHODS

a nested case-control survey was conducted on patients with a diagnosis of TB and parents 0-18 years of age from 3 hospitals in Indonesia during September-November 2019 with a total sample of 69 people undergoing clinical examinations, supporting and diagnosing subjects, blood sampling 1-2 cc for examination mRNA gene Treg, Treg, CD 4+, and CD 8+, then centrifuged at 3000 rpm for 10 min to support blood cells and serum.

RESULTS

There was a significant relationship between expression of mRNA gene Treg with TST (p = 0,000), Treg with TST (p = 0,000), and CD4+ with TST (p = 0,000). Meanwhile, CD8 + was not significantly associated with TST (p = 0.118).

CONCLUSIONS

It is necessary to check the expression of mRNA gene Treg, Treg, CD4+, and CD8+ with more samples to find the mean value that shows the protective value of further TB.

Tuberculosis vaccination needs to avoid 'decoy' immune reactions

Current search for a new effective vaccine against tuberculosis involves selected antigens, vectors and adjuvants. These are being evaluated usually by their booster inoculation following priming with Bacillus Calmette-Guerin. The purpose of this article is to point out, that despite being attenuated of virulence, priming with BCG may still involve immune mechanisms, which are not favourable for protection against active disease. It is postulated, that the responsible 'decoy' constituents selected during the evolution of pathogenic tubercle bacilli may be involved in the evasion from bactericidal host resistance and stimulate immune responses of a cytokine phenotype, which lead to the transition from latent closed granulomas to reactivation with infectious lung cavities. The decoy mechanisms appear as favourable for most infected subjects but leading in a minority of cases to pathology which can effectively transmit the infection. It is proposed that construction and development of new vaccine candidates could benefit from avoiding decoy-type immune mechanisms.

Brucella abortus and Pregnancy in Mice: Impact of Chronic Infection on Fertility and the Role of Regulatory T Cells in Tissue Colonization

Stealthy intracellular bacterial pathogens are known to establish persistent and sometimes lifelong infections. Some of these pathogens also have a tropism for the reproductive system, thereby increasing the risk of reproductive disease and infertility. To date, the pathogenic mechanism involved remains poorly understood. Here, we demonstrate that Brucella abortus, a notorious reproductive pathogen, has the ability to infect the nonpregnant uterus, sustain infection, and induce inflammatory changes during both acute and chronic stages of infection. In addition, we demonstrated that chronically infected mice had a significantly reduced number of pregnancies compared to naive controls. To investigate the immunologic mechanism responsible for uterine tropism, we explored the role of regulatory T cells (Tregs) in the pathogenesis of Brucella abortus infection. We show that highly suppressive CD4+FOXP3+TNFR2+ Tregs contribute to the persistence of Brucella abortus infection and that inactivation of Tregs with tumor necrosis factor receptor II (TNFR2) antagonistic antibody protected mice by significantly reducing bacterial burden both systemically and within reproductive tissues. These findings support a critical role of Tregs in the pathogenesis of persistence induced by intracellular bacterial pathogens, including B. abortus Results from this study indicate that adverse reproductive outcomes can occur as sequelae of chronic infection in nonpregnant animals and that fine-tuning Treg activity may provide novel immunotherapeutic and prevention strategies against intracellular bacterial infections such as brucellosis.

Emerging patterns of regulatory T cell function in tuberculosis

Tuberculosis (TB) is one of the top 10 causes of mortality worldwide from a single infectious agent and has significant implications for global health. A major hurdle in the development of effective TB vaccines and therapies is the absence of defined immune‐correlates of protection. In this context, the role of regulatory T cells (T_reg), which are essential for maintaining immune homeostasis, is even less understood. This review aims to address this knowledge gap by providing an overview of the emerging patterns of T_reg function in TB. Increasing evidence from studies, both in animal models of infection and TB patients, points to the fact the role of T_regs in TB is dependent on disease stage. While T_regs might expand and delay the appearance of protective responses in the early stages of infection, their role in the chronic phase perhaps is to counter‐regulate excessive inflammation. New data highlight that this important homeostatic role of T_regs in the chronic phase of TB may be compromised by the expansion of activated human leucocyte antigen D‐related (HLA‐DR)⁺CD4⁺ suppression‐resistant effector T cells. This review provides a comprehensive and critical analysis of the key features of T_reg cells in TB; highlights the importance of a balanced immune response as being important in TB and discusses the importance of probing not just T_reg frequency but also qualitative aspects of T_reg function as part of a comprehensive search for novel TB treatments.

MHC-II alleles shape the CDR3 repertoires of conventional and regulatory naïve CD4+ T cells

T cell maturation and activation depend upon T cell receptor (TCR) interactions with a wide variety of antigenic peptides displayed in a given major histocompatibility complex (MHC) context. Complementarity-determining region 3 (CDR3) is the most variable part of the TCRα and -β chains, which govern interactions with peptide-MHC complexes. However, it remains unclear how the CDR3 landscape is shaped by individual MHC context during thymic selection of naïve T cells. We established two mouse strains carrying distinct allelic variants of H2-A and analyzed thymic and peripheral production and TCR repertoires of naïve conventional CD4⁺ T (T_conv) and naïve regulatory CD4⁺ T (T_reg) cells. Compared with tuberculosis-resistant C57BL/6 (H2-A^b) mice, the tuberculosis-susceptible H2-A^j mice had fewer CD4⁺ T cells of both subsets in the thymus. In the periphery, this deficiency was only apparent for T_conv and was compensated for by peripheral reconstitution for T_reg We show that H2-A^j favors selection of a narrower and more convergent repertoire with more hydrophobic and strongly interacting amino acid residues in the middle of CDR3α and CDR3β, suggesting more stringent selection against a narrower peptide-MHC-II context. H2-A^j and H2-A^b mice have prominent reciprocal differences in CDR3α and CDR3β features, probably reflecting distinct modes of TCR fitting to MHC-II variants. These data reveal the mechanics and extent of how MHC-II shapes the naïve CD4⁺ T cell CDR3 landscape, which essentially defines adaptive response to infections and self-antigens.

BCG vaccination reduces the mortality of Mycobacterium tuberculosis-infected type 2 diabetes mellitus mice

Diabetes is a significant risk factor for the development of active tuberculosis. In this study, we used a mouse model of type 2 diabetes mellitus (T2DM) to determine the effect of prior Bacillus Calmette-Guérin (BCG) vaccination on immune responses to Mycobacterium tuberculosis (Mtb) infection. We found that, at 6-7 months after Mtb infection, 90% of the Mtb-infected T2DM mice died, whereas only 50% of BCG-vaccinated T2DM-Mtb-infected mice died. Moreover, 40% of the PBS-treated uninfected T2DM mice and 30% of the uninfected BCG-vaccinated T2DM mice died, whereas all uninfected and infected nondiabetic mice survived. BCG vaccination was less effective in reducing the lung bacterial burden of Mtb-infected T2DM mice compared with Mtb-infected nondiabetic mice. BCG vaccination significantly reduced lung inflammation in Mtb-infected T2DM mice compared with that of unvaccinated T2DM mice infected with Mtb. Furthermore, reduced mortality of BCG-vaccinated Mtb-infected T2DM mice is associated with expansion of IL-13-producing CXCR3+ Tregs in the lungs of Mtb-infected T2DM mice. Recombinant IL-13 and Tregs from BCG-vaccinated Mtb-infected T2DM mice converted proinflammatory M1 macrophages to antiinflammatory M2 macrophages. Our findings suggest a potentially novel role for BCG in preventing excess inflammation and mortality in T2DM mice infected with Mtb.

Regulatory T cells in paracoccidioidomycosis

This review addresses the role of regulatory T cells (Tregs), which are essential for maintaining peripheral tolerance and controlling pathogen immunity, in the host response against Paracoccidioides brasiliensis, a primary fungal pathogen. A brief introduction on the general features of Treg cells summarizes their main functions, subpopulations, mechanisms of suppression and plasticity. The main aspects of immunity in the diverse forms of the P. brasiliensis infection are presented, as are the few extant studies on the relevance of Treg cells in the control of severity of the human disease. Finally, the influence of Toll-like receptors, Dectin-1, NOD-like receptor P3 (NLRP3), Myeloid differentiation factor-88 (MyD88), as well as the enzyme indoleamine 2,3 dioxygenase (IDO) on the expansion and function of Treg cells in a murine model of pulmonary paracoccidioidomycosis (PCM) is also discussed. It is demonstrated that some of these components are involved in the negative control of Treg cell expansion, whereas others positively trigger the proliferation and activity of these cells. Finally, the studies here summarized highlight the dual role of Treg cells in PCM, which can be protective by controlling excessive immunity and tissue pathology but also deleterious by inhibiting the anti-fungal immunity necessary to control fungal growth and dissemination.

Regulatory T Cells Subvert Mycobacterial Containment in Patients Failing Extensively Drug-Resistant Tuberculosis Treatment

RATIONALE

The advent of extensively drug-resistant (XDR) tuberculosis (TB) and totally drug-resistant TB, with limited or no treatment options, has facilitated renewed interest in host-directed immunotherapy, particularly for therapeutically destitute patients. However, the selection and utility of such approaches depend on understanding the host immune response in XDR-TB, which hitherto remains unexplored.

OBJECTIVES

To determine the host immunological profile in patients with XDR-TB, compared with drug-sensitive TB (DS-TB), using peripheral blood and explanted lung tissue.

METHODS

Blood and explanted lung tissue were obtained from patients with XDR-TB (n = 31), DS-TB (n = 20), and presumed latent TB infection (n = 20). T-cell phenotype (T-helper cell type 1 [Th1]/Th2/Th17/regulatory T cells [Tregs]) was evaluated in all patient groups, and Treg function assessed in XDR-TB nonresponders by coculturing PPD-preprimed effector T cells with H37Rv-infected monocyte-derived macrophages, with or without autologous Tregs. Mycobacterial containment was evaluated by counting colony-forming units.

MEASUREMENTS AND MAIN RESULTS

Patients failing XDR-TB treatment had an altered immunophenotype characterized by a substantial increase in the frequency (median; interquartile range) of CD4⁺CD25⁺FoxP3⁺ Tregs (11.5%; 5.9-15.2%) compared with DS-TB (3.4%; 1.6-5.73%; P < 0.001) and presumed latent TB infection (1.8%; 1.2-2.3%; P < 0.001), which was unrelated to disease duration. Tregs isolated from patients with XDR-TB suppressed T-cell proliferation (up to 90%) and subverted containment of H37Rv-infected monocyte-derived macrophages (by 30%; P = 0.03) by impairing effector T-cell function through a mechanism independent of direct cell-to-cell contact, IL-10, TGF (transforming growth factor)-β, and CTLA-4 (cytotoxic T-lymphocyte-associated protein 4).

CONCLUSIONS

Collectively, these data suggest that Tregs may be contributing to immune dysfunction, and bacterial persistence, in patients with XDR-TB. The relevant cellular pathways may serve as potential targets for immunotherapeutic intervention.

Immune Response to Mucosal Brucella Infection

Brucellosis is one of the most prevalent bacterial zoonosis of worldwide distribution. The disease is caused by Brucella spp., facultative intracellular pathogens. Brucellosis in animals results in abortion of fetuses, while in humans, it frequently manifests flu-like symptoms and a typical undulant fever, being osteoarthritis a common complication of the chronic infection. The two most common ways to acquire the infection in humans are through the ingestion of contaminated dairy products or by inhalation of contaminated aerosols. Brucella spp. enter the body mainly through the gastrointestinal and respiratory mucosa; however, most studies of immune response to Brucella spp. are performed analyzing models of systemic immunity. It is necessary to better understand the mucosal immune response induced by Brucella infection since this is the main entry site for the bacterium. In this review, some virulence factors and the mechanisms needed for pathogen invasion and persistence are discussed. Furthermore, some aspects of local immune responses induced during Brucella infection will be reviewed. With this knowledge, better vaccines can be designed focused on inducing protective mucosal immune response.

Teleost contributions to the understanding of mycobacterial diseases

Few pathogens have shaped human medicine as the mycobacteria. From understanding biological phenomena driving disease spread, to mechanisms of host-pathogen interactions and antibiotic resistance, the Mycobacterium genus continues to challenge and offer insights into the basis of health and disease. Teleost fish models of mycobacterial infections have progressed significantly over the past three decades, now supplying a range of unique tools and new opportunities to define the strategies employed by these Gram-positive bacteria to overcome host defenses, as well as those host antimicrobial pathways that can be used to limit its growth and spread. Herein, we take a comparative perspective and provide an update on the contributions of teleost models to our understanding of mycobacterial diseases.

The phytochemical bergenin as an adjunct immunotherapy for tuberculosis in mice

The widespread availability and use of modern synthetic therapeutic agents have led to a massive decline in ethnomedical therapies. However, these synthetic agents often possess toxicity leading to various adverse effects. For instance, anti-tubercular treatment (ATT) is toxic, lengthy, and severely impairs host immunity, resulting in posttreatment vulnerability to reinfection and reactivation of tuberculosis (TB). Incomplete ATT enhances the risk for the generation of multidrug- or extensively drug-resistant (MDR or XDR, respectively) variants of Mycobacterium tuberculosis (M. tb), the TB-causing microbe. Therefore, a new therapeutic approach that minimizes these risks is urgently needed to combat this deadly disease and prevent future TB epidemics. Previously, we have shown that the phytochemical bergenin induces T helper 1 (Th1)- and Th17 cell-based protective immune responses and potently inhibits mycobacterial growth in a murine model of M. tb infection, suggesting bergenin as a potential adjunct agent to TB therapy. Here, we combined ATT therapy with bergenin and found that this combination reduces immune impairment and the length of treatment in mice. We observed that co-treatment with the anti-TB drug isoniazid and bergenin produces additive effects and significantly reduces bacterial loads compared with isoniazid treatment alone. The bergenin co-treatment also reduced isoniazid-induced immune impairment; promoted long-lasting, antigen-specific central memory T cell responses; and acted as a self-propelled vaccine. Of note, bergenin treatment significantly reduced the bacterial burden of a multidrug-resistant TB strain. These observations suggest that bergenin is a potent immunomodulatory agent that could be further explored as a potential adjunct to TB therapy.

Spatial and temporal localization of immune transcripts defines hallmarks and diversity in the tuberculosis granuloma

Granulomas are the pathological hallmark of tuberculosis (TB) and the niche where bacilli can grow and disseminate or the immunological microenvironment in which host cells interact to prevent bacterial dissemination. Here we show 34 immune transcripts align to the morphology of lung sections from Mycobacterium tuberculosis-infected mice at cellular resolution. Colocalizing transcript networks at <10 μm in C57BL/6 mouse granulomas increase complexity with time after infection. B-cell clusters develop late after infection. Transcripts from activated macrophages are enriched at subcellular distances from M. tuberculosis. Encapsulated C3HeB/FeJ granulomas show necrotic centers with transcripts associated with immunosuppression (Foxp3, Il10), whereas those in the granuloma rims associate with activated T cells and macrophages. We see highly diverse networks with common interactors in similar lesions. Different immune landscapes of M. tuberculosis granulomas depending on the time after infection, the histopathological features of the lesion, and the proximity to bacteria are here defined.

CCR4-dependent reduction in the number and suppressor function of CD4+Foxp3+ cells augments IFN-γ-mediated pulmonary inflammation and aggravates tuberculosis pathogenesis

Chronic pulmonary inflammation marked predominantly by CD4⁺IFN-γ⁺ cells is the hallmark of tuberculosis pathogenesis in immunocompetent adults, who are substantially affected by this disease. Moreover, CD4⁺Foxp3⁺ cell-mediated suppression contributes to infection susceptibility. We addressed the role of CD4⁺Foxp3⁺ cells in tuberculosis pathogenesis, because this aspect has not been addressed during chronic infection. We targeted CCR4, which induces the influx of CD4⁺Foxp3⁺ cells into the lungs. CCR4^−/− mice exhibited a lower frequency of CD4⁺Foxp3⁺ cells at 15, 30, and 70 days of infection than their wild-type counterparts. However, only at 70 days of infection was an exacerbated IFN-γ-mediated immune response associated with apparent tuberculosis pathogenesis and susceptibility. In addition, CCR4^−/− mice exhibited a decrease in the suppressor function of CD4⁺Foxp3⁺ cells. Adoptive transfer of Foxp3⁺ cells into infected CCR4^−/− mice restored pulmonary inflammation and bacterial load to levels observed in wild-type mice. Our findings suggest that CD4⁺Foxp3⁺ cells play a time-dependent role in tuberculosis and highlight that CCR4 plays a critical role in the balance of IFN-γ-mediated inflammation by regulating the influx and function of CD4⁺Foxp3⁺ cells. Our findings are translationally relevant, as CD4⁺Foxp3⁺ cells or CCR4 could be a target for immunotherapy, considering the heterogeneity of tuberculosis in immunocompetent adults.

Principles of Immunotherapy: Implications for Treatment Strategies in Cancer and Infectious Diseases

The advances in cancer biology and pathogenesis during the past two decades, have resulted in immunotherapeutic strategies that have revolutionized the treatment of malignancies, from relatively non-selective toxic agents to specific, mechanism-based therapies. Despite extensive global efforts, infectious diseases remain a leading cause of morbidity and mortality worldwide, necessitating novel, innovative therapeutics that address the current challenges of increasing antimicrobial resistance. Similar to cancer pathogenesis, infectious pathogens successfully fashion a hospitable environment within the host and modulate host metabolic functions to support their nutritional requirements, while suppressing host defenses by altering regulatory mechanisms. These parallels, and the advances made in targeted therapy in cancer, may inform the rational development of therapeutic interventions for infectious diseases. Although "immunotherapy" is habitually associated with the treatment of cancer, this review accentuates the evolving role of key targeted immune interventions that are approved, as well as those in development, for various cancers and infectious diseases. The general features of adoptive therapies, those that enhance T cell effector function, and ligand-based therapies, that neutralize or eliminate diseased cells, are discussed in the context of specific diseases that, to date, lack appropriate remedial treatment; cancer, HIV, TB, and drug-resistant bacterial and fungal infections. The remarkable diversity and versatility that distinguishes immunotherapy is emphasized, consequently establishing this approach within the armory of curative therapeutics, applicable across the disease spectrum.

Current approaches toward identifying a correlate of immune protection from tuberculosis

INTRODUCTION

Mycobacterium tuberculosis kills more people than any other pathogen. Vaccination is the most cost-effective control measure for any infectious disease. Development of an effective vaccine against tuberculosis is hindered by the uncertain predictive value of preclinical animal models, incomplete understanding of protective immunity and lack of validated immune correlates of protection (COP).

AREAS COVERED

Here we review what is known about protective immunity against M.tb, the preclinical and clinical cohorts that can be utilized to identify COP, and COP that have been identified to date.

EXPERT COMMENTARY

The identification of COP would allow the rational design and development of vaccine candidates which can then be optimized and prioritized based on the induction of these immune responses. Once validated in field efficacy trials, such COP could potentially facilitate the development and licensure of vaccines, in combination with human efficacy data. The identification and validation of COP would represent a very significant advance in TB vaccine development. Every opportunity to collect samples and cohorts on which to cross-validate pre-existing COP and identify novel COP should be exploited. Furthermore, global cooperation and collaboration on such samples will ensure that the utility of such precious samples is fully exploited.

DosR proteins of Mycobacterium tuberculosis upregulate effector T cells and down regulate T regulatory cells in TB patients and their healthy contacts

It is well established that the current problem of tuberculosis (TB) can be combated by overcoming the drawbacks of the currently available BCG vaccine. This would involve incorporation of antigens that can control TB at all stages including the dormant phase which is generally ignored. Hence, DosR regulon proteins, which are expressed in latent infection, could prove to be very good vaccine candidates as they can possibly target the silent but most predominant form of TB infection. In the present study, the immune response to two DosR proteins Rv2627 and Rv2628 has been studied in PBMCs derived from normal individuals, TB patients and healthy contacts of TB patients. It was found that these antigens were capable of stimulating a strong IFN-γ+ T cell response along with accentuation of memory T cells and other protective cytokines such as IL-2 and IL-17. At the same time these proteins decreased the frequencies of immune-suppressor regulatory T cells in in vitro stimulation of PBMC from both patients and their contacts. Considering all these facts together, we suggest Rv2627 and Rv2628 to be one of the extremely promising candidates for incorporation into a post exposure subunit vaccine against TB.

Depletion of regulatory T cells in ongoing paracoccidioidomycosis rescues protective Th1/Th17 immunity and prevents fatal disease outcome

In human paracoccidioidomycosis (PCM), a primary fungal infection typically diagnosed when the disease is already established, regulatory T cells (Treg) cells are associated with disease severity. Experimental studies in pulmonary PCM confirmed the detrimental role of these cells, but in most studies, Tregs were depleted prior to or early during infection. These facts led us to study the effects of Treg cell depletion using a model of ongoing PCM. Therefore, Treg cell depletion was achieved by treatment of transgenic C57BL/6DTR/eGFP (DEREG) mice with diphtheria toxin (DT) after 3 weeks of intratracheal infection with 1 × 106 Paracoccidioides brasiliensis yeasts. At weeks 6 and 10 post-infection, DT-treated DEREG mice showed a reduced number of Treg cells associated with decreased fungal burdens in the lungs, liver and spleen, reduced tissue pathology and mortality. Additionally, an increased influx of activated CD4+ and CD8+ T cells into the lungs and elevated production of Th1/Th17 cytokines was observed in DT-treated mice. Altogether, our data demonstrate for the first time that Treg cell depletion in ongoing PCM rescues infected hosts from progressive and potentially fatal PCM; furthermore, our data indicate that controlling Treg cells could be explored as a novel immunotherapeutic procedure.

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.

Regulatory T-cell therapy in liver transplantation

Modern immunosuppression drug regimens have produced excellent short-term survival after liver transplantation but it is generally accepted that the side effects of these medications remain a significant contributing factor for less satisfactory long term outcomes. The liver has unique tolerogenic properties as evidenced by the higher rates of operational tolerance seen in liver transplant recipients compared to other solid organ transplants, and therefore, liver transplantation offers an attractive setting in which to study tolerizing therapies. CD4⁺ CD25⁺ FOXP3⁺ regulatory T cells (Tregs) are crucial for maintenance of self-tolerance and prevention of autoimmune disease and are therefore an appealing potential candidate for use as a tolerizing cell therapy. In this review, we summarize the evidence from drug withdrawal trials of spontaneous operational tolerance in liver transplantation, the unique immunology of the hepatic microenvironment, the evidence for the use of CD4⁺ CD25⁺ FOXP3⁺ regulatory T cells as a tolerance inducing therapy in liver transplantation and the challenges in producing clinical grade Treg cell products.

The Immunoregulation of Th17 in Host against Intracellular Bacterial Infection

T helper 17 cells (Th17) constitute a distinct subset of helper T cells with a unique transcriptional profile (STAT3, RORγ, and RORα), cytokine production pattern (IL17 family), and requirement of specific cytokines for their differentiation (TGF-β, IL6, IL21, and IL23). Recent studies involving experimental animals and humans have shown that Th17/IL17 plays a crucial role in host defense against a variety of pathogens, including bacteria and viruses. The underlying mechanisms by which Th17 performs include dendritic cell (DC) regulation, neutrophil recruitment, Th1 modulation, and T regulatory cell (Treg) balance. In recent years, researchers have generated an accumulating wealth of evidence on the role of Th17/IL17 in protective immunity to intracellular bacterial pathogens, such as Mycobacterium tuberculosis and Chlamydia trachomatis, which are one of the most important pathogens that inflict significant socioeconomic burden across the globe. In this article, we reviewed the current literature on the functions and mechanisms by which Th17/IL17 responds to intracellular bacterial infections. A better understanding of Th17/IL17 immunity to pathogens would be crucial for developing effective prophylactics and therapeutics.

Reinforcing the Functionality of Mononuclear Phagocyte System to Control Tuberculosis

The mononuclear phagocyte system (MPS) constitutes dendritic cells, monocytes, and macrophages. This system contributes to various functions that are essential for maintaining homeostasis, activation of innate immunity, and bridging it with the adaptive immunity. Consequently, MPS is highly important in bolstering immunity against the pathogens. However, MPS is the frontline cells in destroying Mycobacterium tuberculosis (Mtb), yet the bacterium prefers to reside in the hostile environment of macrophages. Therefore, it may be very interesting to study the struggle between Mtb and MPS to understand the outcome of the disease. In an event when MPS predominates Mtb, the host remains protected. By contrast, the situation becomes devastating when the pathogen tames and tunes the host MPS, which ultimately culminates into tuberculosis (TB). Hence, it becomes extremely crucial to reinvigorate MPS functionality to overwhelm Mtb and eliminate it. In this article, we discuss the strategies to bolster the function of MPS by exploiting the molecules associated with the innate immunity and highlight the mechanisms involved to overcome the Mtb-induced suppression of host immunity. In future, such approaches may provide an insight to develop immunotherapeutics to treat TB.

Altered expression of antigen-specific memory and regulatory T-cell subsets differentiate latent and active tuberculosis

Although one-third of the world population is infected with Mycobacterium tuberculosis, only 5-10% of the infected individuals will develop active tuberculosis (TB) disease and the rest will remain infected with no symptoms, known as latent TB infection (LTBI). Identifying biomarkers that differentiate latent and active TB disease enables effective TB control, as early detection, treatment of active TB and preventive treatment of individuals with LTBI are crucial steps involved in TB control. Here, we have evaluated the frequency of antigen-specific memory and regulatory T (Treg) cells in 15 healthy household contacts (HHC) and 15 pulmonary TB patients (PTB) to identify biomarkers for differential diagnosis of LTBI and active TB. Among all the antigens tested in the present study, early secretory antigenic target-6 (ESAT-6) -specific CD4⁺ and CD8⁺ central memory (Tcm) cells showed 93% positivity in HHC and 20% positivity in PTB. The novel test antigens Rv0753c and Rv0009 both displayed 80% and 20% positivity in HHC and PTB, respectively. In contrast to Tcm cells, effector memory T (Tem) cells showed a higher response in PTB than HHC; both ESAT-6 and Rv0009 showed similar positivity of 80% in PTB and 33% in HHC. PTB patients have a higher proportion of circulating antigen-reactive Treg cells (CD4⁺  CD25⁺  FoxP3⁺ ) than LTBI. Rv2204c-specific Treg cells showed maximum positivity of 73% in PTB and 20% in HHC. Collectively, our data conclude that ESAT-6-specific Tcm cells and Rv2204c-specific Treg cells might be useful biomarkers to discriminate LTBI from active TB.

Adenoviral vector-mediated GM-CSF gene transfer improves anti-mycobacterial immunity in mice - role of regulatory T cells

Granulocyte macrophage-colony stimulating factor (GM-CSF) is a hematopoietic growth factor involved in differentiation, survival and activation of myeloid and non-myeloid cells with important implications for lung antibacterial immunity. Here we examined the effect of pulmonary adenoviral vector-mediated delivery of GM-CSF (AdGM-CSF) on anti-mycobacterial immunity in M. bovis BCG infected mice. Exposure of M. bovis BCG infected mice to AdGM-CSF either applied on 6h, or 6h and 7days post-infection substantially increased alveolar recruitment of iNOS and IL-12 expressing macrophages, and significantly increased accumulation of IFNγ^pos T cells and particularly regulatory T cells (Tregs). This was accompanied by significantly reduced mycobacterial loads in the lungs of mice. Importantly, diphtheria toxin-induced depletion of Tregs did not influence mycobacterial loads, but accentuated immunopathology in AdGM-CSF-exposed mice infected with M. bovis BCG. Together, the data demonstrate that AdGM-CSF therapy improves lung protective immunity against M. bovis BCG infection in mice independent of co-recruited Tregs, which however critically contribute to limit lung immunopathology in BCG-infected mice. These data may be relevant to the development of immunomodulatory strategies to limit immunopathology-based lung injury in tuberculosis in humans.

Mycobacterium tuberculosis infection modulates adipose tissue biology

Mycobacterium tuberculosis (Mtb) primarily resides in the lung but can also persist in extrapulmonary sites. Macrophages are considered the prime cellular habitat in all tissues. Here we demonstrate that Mtb resides inside adipocytes of fat tissue where it expresses stress-related genes. Moreover, perigonadal fat of Mtb-infected mice disseminated the infection when transferred to uninfected animals. Adipose tissue harbors leukocytes in addition to adipocytes and other cell types and we observed that Mtb infection induces changes in adipose tissue biology depending on stage of infection. Mice infected via aerosol showed infiltration of inducible nitric oxide synthase (iNOS) or arginase 1 (Arg1)-negative F4/80⁺ cells, despite recruitment of CD3⁺, CD4⁺ and CD8⁺ T cells. Gene expression analysis of adipose tissue of aerosol Mtb-infected mice provided evidence for upregulated expression of genes associated with T cells and NK cells at 28 days post-infection. Strikingly, IFN-γ-producing NK cells and Mtb-specific CD8⁺ T cells were identified in perigonadal fat, specifically CD8⁺CD44^-CD69⁺ and CD8⁺CD44^-CD103⁺ subpopulations. Gene expression analysis of these cells revealed that they expressed IFN-γ and the lectin-like receptor Klrg1 and down-regulated CD27 and CD62L, consistent with an effector phenotype of Mtb-specific CD8⁺ T cells. Sorted NK cells expressed higher abundance of Klrg1 upon infection, as well. Our results reveal the ability of Mtb to persist in adipose tissue in a stressed state, and that NK cells and Mtb-specific CD8⁺ T cells infiltrate infected adipose tissue where they produce IFN-γ and assume an effector phenotype. We conclude that adipose tissue is a potential niche for Mtb and that due to infection CD8⁺ T cells and NK cells are attracted to this tissue.

In 2015, tuberculosis (TB) affected 10.4 million individuals causing 1.8 million deaths per year. Yet, a much larger group– 2 billion people–harbors latent TB infection (LTBI) without clinical symptoms, but at lifelong risk of reactivation. The physiological niches of Mycobacterium tuberculosis (Mtb) persistence remain incompletely defined and both pulmonary and extrapulmonary sites have been proposed. Adipose tissue constitutes 15–25% of total body mass and is an active production site for hormones and inflammatory mediators. The increasing prevalence of obesity, has led to greater incidence of type 2 diabetes. These patients suffer from three times higher risk of developing TB, pointing to a potential link between adipose tissue and TB pathogenesis. In individuals with LTBI, Mtb survives in a stressed, non-replicating state with low metabolic activity and resting macrophages serve as preferred habitat and become effectors after appropriate stimulation. Here we demonstrate that Mtb can infect and persist within adipocytes where it upregulates stress-related genes. In vivo, relative proportions of leukocyte subsets infiltrating adipose tissue varied under different conditions of infection. During natural aerosol Mtb infection, distinct leukocyte subsets, including mononuclear phagocytes, Mtb-specific CD8⁺ T cells and NK cells infiltrated adipose tissue and became activated. Thus, our study shows that adipose tissue is not only a potential reservoir for this pathogen but also undergoes significant alteration during TB infection.

Suppressor Cell-Depleting Immunotherapy With Denileukin Diftitox is an Effective Host-Directed Therapy for Tuberculosis

Host-directed therapies that augment host immune effector mechanisms may serve as important adjunctive therapies for tuberculosis treatment. We evaluated the activity of denileukin diftitox in an acute mouse model of tuberculosis (TB) infection and analyzed the cellular composition and bacterial burden in lungs and spleens. These in vivo studies show that denileukin diftitox potentiates standard TB treatment in the mouse model, an effect which may be due to depletion of T-regulatory and myeloid-derived suppressor cells during TB infection. Our results indicate that denileukin diftitox and other suppressor cell-depleting therapies may be useful adjunctive, host-directed therapies for TB.

Mycobacterium bovis BCG and New Vaccines for the Prevention of Tuberculosis

Tuberculosis infects millions of people worldwide and remains a leading global killer despite widespread neonatal administration of the tuberculosis vaccine, bacillus Calmette-Guérin (BCG). BCG has clear and sustained efficacy, but after 10 years, its efficacy appears to wane, at least in some populations. Fortunately, there are many new tuberculosis vaccines in development today, some in advanced stages of clinical trial testing. Here we review the epidemiological need for tuberculosis vaccination, including evolving standards for administration to at risk individuals in developing countries. We also examine proven sources of immune protection from tuberculosis, which to date have exclusively involved natural or vaccine exposure to whole cell mycobacteria. After summarizing evidence for the use and efficacy of BCG, we detail the most promising new candidate vaccines against tuberculosis. The global need for a new tuberculosis vaccine is acute and huge, but clinical trials to be completed in the coming few years are likely either to identify a new tuberculosis vaccine or to substantially reframe how we understand immune protection from this historical scourge.

Arsenic exposure impels CD4 commitment in thymus and suppress T cell cytokine secretion by increasing regulatory T cells

Arsenic is globally infamous for inducing immunosuppression associated with prevalence of opportunistic infection in exposed population, although the mechanism remains elusive. In this study, we investigate the effect of arsenic exposure on thymocyte lineage commitment and the involvement of regulatory T cells (T_reg) in arsenic-induced immunosuppression. Male Balb/c mice were exposed to 0.038, 0.38 and 3.8 ppm sodium arsenite for 7, 15 and 30 days through oral gavage. Arsenic exposure promoted CD4 lineage commitment in a dose dependent manner supported by the expression of ThPOK in thymus. Arsenic also increased splenic CD4⁺ T cells and promoted their differentiation into T_reg cells. In parallel, arsenic exposure induced immunosuppression characterized by low cytokine secretion from splenocytes and increased susceptibility to Mycobacterium fortuitum (M. fortuitum) infection. Therefore, we linked arsenic-induced rise in T_reg cells with suppressed T_h1 and T_h2 related cytokines, which has been reversed by inhibition of T_reg cells in-vivo using wortmannin. Other parameters like body weight, kidney and liver function, histoanatomy of thymus and spleen as well as thymocyte and splenocytes viability were unaltered by arsenic exposure. Taken together our findings indicated that environmentally relevant dose of arsenic enhanced differentiation of T_reg cells which in turn induce immunosuppression in experimental animals.

Regulation of Immunity to Tuberculosis

Immunity against Mycobacterium tuberculosis requires a balance between adaptive immune responses to constrain bacterial replication and the prevention of potentially damaging immune activation. Regulatory T (Treg) cells express the transcription factor Foxp3+ and constitute an essential counterbalance of inflammatory Th1 responses and are required to maintain immune homeostasis. The first reports describing the presence of Foxp3-expressing CD4+ Treg cells in tuberculosis (TB) emerged in 2006. Different Treg cell subsets, most likely specialized for different tissues and microenvironments, have been shown to expand in both human TB and animal models of TB. Recently, additional functional roles for Treg cells have been demonstrated during different stages and spectrums of TB disease. Foxp3+ regulatory cells can quickly expand during early infection and impede the onset of cellular immunity and persist during chronic TB infection. Increased frequencies of Treg cells have been associated with a detrimental outcome of active TB, and may be dependent on the M. tuberculosis strain, animal model, local environment, and the stage of infection. Some investigations also suggest that Treg cells are required together with effector T cell responses to obtain reduced pathology and sterilizing immunity. In this review, we will first provide an overview of the regulatory cells and mechanisms that control immune homeostasis. Then, we will review what is known about the phenotype and function of Treg cells from studies in human TB and experimental animal models of TB. We will discuss the potential role of Treg cells in the progression of TB disease and the relevance of this knowledge for future efforts to prevent, modulate, and treat TB.

T Cell-Derived IL-10 Impairs Host Resistance to Mycobacterium tuberculosis Infection

Tuberculosis (TB), caused by Mycobacterium tuberculosis infection, is a leading cause of mortality and morbidity, causing ∼1.5 million deaths annually. CD4⁺ T cells and several cytokines, such as the Th1 cytokine IFN-γ, are critical in the control of this infection. Conversely, the immunosuppressive cytokine IL-10 has been shown to dampen Th1 cell responses to M. tuberculosis infection impairing bacterial clearance. However, the critical cellular source of IL-10 during M. tuberculosis infection is still unknown. Using IL-10 reporter mice, we show in this article that during the first 14 d of M. tuberculosis infection, the predominant cells expressing IL-10 in the lung were Ly6C⁺ monocytes. However, after day 21 postinfection, IL-10–expressing T cells were also highly represented. Notably, mice deficient in T cell–derived IL-10, but not mice deficient in monocyte-derived IL-10, showed a significant reduction in lung bacterial loads during chronic M. tuberculosis infection compared with fully IL-10–competent mice, indicating a major role for T cell–derived IL-10 in TB susceptibility. IL-10–expressing cells were detected among both CD4⁺ and CD8⁺ T cells, expressed high levels of CD44 and Tbet, and were able to coproduce IFN-γ and IL-10 upon ex vivo stimulation. Furthermore, during M. tuberculosis infection, Il10 expression in CD4⁺ T cells was partially regulated by both IL-27 and type I IFN signaling. Together, our data reveal that, despite the multiple immune sources of IL-10 during M. tuberculosis infection, activated effector T cells are the major source accounting for IL-10–induced TB susceptibility.

A review of computational and mathematical modeling contributions to our understanding of Mycobacterium tuberculosis within-host infection and treatment

Tuberculosis (TB) is an ancient and deadly disease characterized by complex host-pathogen dynamics playing out over multiple time and length scales and physiological compartments. Computational modeling can be used to integrate various types of experimental data and suggest new hypotheses, mechanisms, and therapeutic approaches to TB. Here, we offer a first-time comprehensive review of work on within-host TB models that describe the immune response of the host to infection, including the formation of lung granulomas. The models include systems of ordinary and partial differential equations and agent-based models as well as hybrid and multi-scale models that are combinations of these. Many aspects of M. tuberculosis infection, including host dynamics in the lung (typical site of infection for TB), granuloma formation, roles of cytokine and chemokine dynamics, and bacterial nutrient availability have been explored. Finally, we survey applications of these within-host models to TB therapy and prevention and suggest future directions to impact this global disease.