Gamma delta T cell responses associated with the development of tuberculosis in health care workers

It Takes a Village: The Multifaceted Immune Response to Mycobacterium tuberculosis Infection and Vaccine-Induced Immunity

Despite co-evolving with humans for centuries and being intensely studied for decades, the immune correlates of protection against Mycobacterium tuberculosis (Mtb) have yet to be fully defined. This lapse in understanding is a major lag in the pipeline for evaluating and advancing efficacious vaccine candidates. While CD4+ T helper 1 (TH1) pro-inflammatory responses have a significant role in controlling Mtb infection, the historically narrow focus on this cell population may have eclipsed the characterization of other requisite arms of the immune system. Over the last decade, the tuberculosis (TB) research community has intentionally and intensely increased the breadth of investigation of other immune players. Here, we review mechanistic preclinical studies as well as clinical anecdotes that suggest the degree to which different cell types, such as NK cells, CD8+ T cells, γ δ T cells, and B cells, influence infection or disease prevention. Additionally, we categorically outline the observed role each major cell type plays in vaccine-induced immunity, including Mycobacterium bovis bacillus Calmette-Guérin (BCG). Novel vaccine candidates advancing through either the preclinical or clinical pipeline leverage different platforms (e.g., protein + adjuvant, vector-based, nucleic acid-based) to purposefully elicit complex immune responses, and we review those design rationales and results to date. The better we as a community understand the essential composition, magnitude, timing, and trafficking of immune responses against Mtb, the closer we are to reducing the severe disease burden and toll on human health inflicted by TB globally.

Effects of BCG vaccination on donor unrestricted T cells in two prospective cohort studies

BACKGROUND

Non-protein antigen classes can be presented to T cells by near-monomorphic antigen-presenting molecules such as CD1, MR1, and butyrophilin 3A1. Such T cells, referred to as donor unrestricted T (DURT) cells, typically express stereotypic T cell receptors. The near-unrestricted nature of DURT cell antigen recognition is of particular interest for vaccine development, and we sought to define the roles of DURT cells, including MR1-restricted MAIT cells, CD1b-restricted glucose monomycolate (GMM)-specific T cells, CD1d-restricted NKT cells, and γδ T cells, in vaccination against Mycobacterium tuberculosis.

METHODS

We compared and characterized DURT cells following primary bacille Calmette-Guerin (BCG) vaccination in a cohort of vaccinated and unvaccinated infants, as well as before and after BCG-revaccination in adults.

FINDINGS

BCG (re)vaccination did not modulate peripheral blood frequencies, T cell activation or memory profiles of MAIT cells, CD1b-restricted GMM-specific and germline-encoded mycolyl-reactive (GEM) cells or CD1d-restricted NKT cells. By contrast, primary BCG vaccination was associated with increased frequencies of γδ T cells as well as a novel subset of CD26⁺CD161⁺TRAV1-2^- IFN-γ-expressing CD4⁺ T cells in infants.

INTERPRETATION

Our findings, that most DURT cell populations were not modulated by BCG, do not preclude a role of BCG in modulating other qualitative aspects of DURT cells. More studies are required to understand the full potential of DURT cells in new TB vaccine strategies.

FUNDING

Aeras, the National Institutes of Health, and the Bill and Melinda Gates Foundation.

Precision medicine in the clinical management of respiratory tract infections including multidrug-resistant tuberculosis: learning from innovations in immuno-oncology

PURPOSE OF REVIEW

In the light of poor management outcomes of antibiotic-resistant respiratory tract infection (RTI)-associated sepsis syndrome and multidrug-resistant tuberculosis (MDR-TB), new management interventions based on host-directed therapies (HDTs) are warranted to improve morbidity, mortality and long-term functional outcomes. We review developments in potential HDTs based on precision cancer therapy concepts applicable to RTIs including MDR-TB.

RECENT FINDINGS

Immune reactivity, tissue destruction and repair processes identified during studies of cancer immunotherapy share common pathogenetic mechanisms with RTI-associated sepsis syndrome and MDR-TB. T-cell receptors (TCRs) and chimeric antigen receptors targeting pathogen-specific or host-derived mutated molecules (major histocompatibility class-dependent/ major histocompatibility class-independent) can be engineered for recognition by TCR γδ and natural killer (NK) cells. T-cell subsets and, more recently, NK cells are shown to be host-protective. These cells can also be activated by immune checkpoint inhibitor (ICI) or derived from allogeneic sources and serve as potential for improving clinical outcomes in RTIs and MDR-TB.

SUMMARY

Recent developments of immunotherapy in cancer reveal common pathways in immune reactivity, tissue destruction and repair. RTIs-related sepsis syndrome exhibits mixed immune reactions, making cytokine or ICI therapy guided by robust biomarker analyses, viable treatment options.

Immunological mechanisms by which concomitant helminth infections predispose to the development of human tuberculosis

Helminthic infections afflict over 1.5 billion people worldwide, while Mycobacterium tuberculosis infects one third of the world's population, resulting in 2 million deaths per year. Although tuberculosis and helminthic infections coexist in many parts of the world, and it has been demonstrated that the T-helper 2 and T-regulatory cell responses elicited by helminths can affect the ability of the host to control mycobacterial infection, it is still unclear whether helminth infections in fact affect tuberculosis disease. In this review article, current progress in the knowledge about the immunomodulation induced by helminths to diminish the protective immune responses to bacille Calmette-Guerin vaccination is reviewed, and the knowledge about the types of immune responses modulated by helminths and the consequences for tuberculosis are summarized. In addition, recent data supporting the significant reduction of both M. tuberculosis antigen-specific Toll-like receptor (TLR) 2 and TLR9 expression, and pro-inflammatory cytokine responses to TLR2 and TLR9 ligands in individuals with M. tuberculosis and helminth co-infection were discussed. This examination will allow to improve understanding of the immune responses to mycobacterial infection and also be of great relevance in combating human tuberculosis.

Modulation of cell death by M. tuberculosis as a strategy for pathogen survival

It has been clearly demonstrated that in vitro, virulent M. tuberculosis can favor necrosis over apoptosis in infected macrophages, and this has been suggested as a mechanism for evading the host immune response. We recently reported that an effect consistent with this hypothesis could be observed in cells from the blood of TB patients, and in this paper, we review what is known about evasion strategies employed by M. tuberculosis and in particular consider the possible interaction of the apoptosis-inhibiting effects of M. tuberculosis infection with another factor (IL-4) whose expression is thought to play a role in the failure to control M. tuberculosis infection. It has been noted that IL-4 may exacerbate TNF-α-induced pathology, though the mechanism remains unexplained. Since pathology in TB typically involves inflammatory aggregates around infected cells, where TNF-α plays an important role, we predicted that IL-4 would inhibit the ability of cells to remove M. tuberculosis by apoptosis of infected cells, through the extrinsic pathway, which is activated by TNF-α. Infection of human monocytic cells with mycobacteria in vitro, in the presence of IL-4, appears to promote necrosis over apoptosis in infected cells—a finding consistent with its suggested role as a factor in pathology during M. tuberculosis infection.

Human type 1 and 17 responses in latent tuberculosis are modulated by coincident filarial infection through cytotoxic T lymphocyte antigen-4 and programmed death-1

Mycobacterium tuberculosis and filarial coinfection is highly prevalent, and the presence of a tissue-invasive helminth may modulate the predominant type 1 T helper (Th1; interferon [IFN]-gamma-mediated) response needed to control M. tuberculosis infection. By analyzing the cellular responses to mycobacterial antigens in patients who had latent tuberculosis with or without filarial infection, we were able to demonstrate that filarial infection coincident with M. tuberculosis infection significantly diminishes M. tuberculosis-specific Th1 (interleukin [IL]-12 and IFN-gamma) and type 17 T helper (Th17; IL-23 and IL-17) responses related to increased expression of cytotoxic T lymphocyte antigen (CTLA)-4 and programmed death (PD)-1. Blockade of CTLA-4 restored production of both IFN-gamma and IL-17, whereas PD-1 blockade restored IFN-gamma production only. Thus, coincident filarial infection exerted a profound inhibitory effect on protective mycobacteria-specific Th1 and Th17 responses in latent tuberculosis, suggesting a mechanism by which concomitant filarial (and other systemic helminth) infections predispose to the development of active tuberculosis in humans.

Post-exposure vaccination against Mycobacterium tuberculosis

Enhancing immunity to tuberculosis in animal models after exposure to the infection has proved difficult. In this study we used a newly described flow cytometric technique to monitor changes in cell populations accumulating in the lungs of guinea pigs challenged by low-dose aerosol infection with Mycobacterium tuberculosis and vaccinated 10 days later. On day 40 after infection the fusion protein F36 and a pool of Ag85A and ESAT6 vaccines had significant effects on the bacterial load, showed increased expression of the activation marker CD45+ on CD4+ T cells, and reduced numbers of heterophils. Lung pathology and pathology scores were marginally improved in animals given these vaccines, but lymph node pathology was not influenced. Despite early effects no changes in long-term survival were seen. These results suggest that a single post-exposure vaccination can initially slow the disease process. However, this effect is transient, but this could be of use in an multidrug resistant/extremely drug resistant outbreak situation because it could potentially slow the infection long enough to complete drug susceptibility testing and initiate effective chemotherapy.

Subpopulations of bovine WC1(+) gammadelta T cells rather than CD4(+)CD25(high) Foxp3(+) T cells act as immune regulatory cells ex vivo

Regulatory T cells (Treg) are regarded essential components for maintenance of immune homeostasis. Especially CD4(+)CD25(high) T cells are considered to be important regulators of immune reactivity. In humans and rodents these natural Treg are characterized by their anergic nature, defined as a non-proliferative state, suppressive function and expression of Foxp3. In this study the potential functional role of flowcytometry-sorted bovine white blood cell populations, including CD4(+)CD25(high) T cells and gammadelta T cell subpopulations, as distinct ex vivo regulatory cells was assessed in co-culture suppression assays. Our findings revealed that despite the existence of a distinct bovine CD4(+)CD25(high) T cell population, which showed Foxp3 transcription/expression, natural regulatory activity did not reside in this cell population. In bovine co-culture suppression assays these cells were neither anergic nor suppressive. Subsequently, the following cell populations were tested functionally for regulatory activity: CD4(+)CD25(low) T cells, WC1(+), WC1.1(+) and WC1.2(+) gammadelta T cells, NK cells, CD8(+) T cells and CD14(+) monocytes. Only the WC1.1(+) and WC1.2(+) gammadelta T cells and CD14(+) monocytes proved to act as regulatory cells in cattle, which was supported by the fact that these regulatory cells showed IL-10 transcription/expression. In conclusion, our data provide first evidence that cattle CD4(+)CD25(high)Foxp3(+) and CD4(+)CD25(low) T cells do not function as Treg ex vivo. The bovine Treg function appears to reside in the gammadelta T cell population, more precisely in the WC1.1(+) and the WC1.2(+) subpopulation, major populations present in blood of cattle in contrast to non-ruminant species.

CXCR5 identifies a subset of Vgamma9Vdelta2 T cells which secrete IL-4 and IL-10 and help B cells for antibody production

Vgamma9Vdelta2 T lymphocytes recognize nonpeptidic Ags and mount effector functions in cellular immune responses against microorganisms and tumors, but little is known about their role in Ab-mediated immune responses. We show here that expression of CXCR5 identifies a unique subset of Vgamma9Vdelta2 T cells which express the costimulatory molecules ICOS and CD40L, secrete IL-2, IL-4, and IL-10 and help B cells for Ab production. These properties portray CXCR5+ Vgamma9Vdelta2 T cells as a distinct memory T cell subset with B cell helper function.

Helminthes could influence the outcome of vaccines against TB in the tropics

Helminthes, infections widespread in the tropics, are known to elicit a wide range of immunomodulation characterized by dominant Th2 type immune responses, chronic immune activation as well as up-regulated regulatory T cell activity. Such a wide range of immunomodulation caused by helminthes may have an impact on the host's ability to cope with subsequent infections and/or may affect the efficacy of vaccination. Indeed studies conducted in humans living in helminth-endemic areas and in animal models showed that helminth infection makes the host more permissive to mycobacterial infections and less able to benefit from vaccination. These observations have fundamental practical consequences if confirmed by large and appropriately controlled clinical studies. Eradication of worms could offer an affordable, simple and novel means to reduce the burden of the tuberculosis problem that at the moment seems to be getting out of control in sub-Saharan Africa. This information would also be of great relevance in the design of vaccines against diseases of major public health importance, including malaria and HIV/AIDS.

Human Vgamma9Vdelta2 T cells: promising new leads for immunotherapy of infections and tumors

Vgamma9Vdelta2 T cells, a major human peripheral gammadelta T-cell subset, react in vitro against a wide array of microbial agents and tumor cells. This broad reactivity pattern is conferred by non-peptidic phosphorylated isoprenoid pathway metabolites, referred to as phosphoantigens, which are able to specifically activate this gammadelta T-cell subset in a T-cell receptor dependent fashion. Recent studies provide new insights into the mode of action of phosphoantigens on Vgamma9Vdelta2 T cells and might explain how their recognition can allow detection of infected or altered self by the immune system. The broad antimicrobial and antitumoral reactivity of Vgamma9Vdelta2 T cells, their ability to produce inflammatory cytokines involved in protective immunity against intracellular pathogens and tumors, and their strong cytolytic and bactericidal activities suggest a direct involvement in immune control of cancers and infections. These observations have recently aided development of novel immunotherapeutic approaches aimed at Vgamma9Vdelta2 T-cell activation, which have already yielded encouraging results.

Vaccines for tuberculosis: novel concepts and recent progress

Three-quarters of a century after the introduction of Mycobacterium bovis BCG, the first tuberculosis vaccine, new vaccines for tuberculosis are finally entering clinical trials. This breakthrough is based not only on advances in proteomics and genomics which have made the construction of new vaccines possible, but also on a greatly expanded knowledge of the immunology of tuberculosis. Here we review our current understanding of how Mycobacterium tuberculosis subverts or survives the host's immune response to cause disease and why the current vaccination strategy, which relies on BCG, is only partially successful in countering the pathogen. This provides a background for describing the new generation of vaccines designed to supplement or replace the current vaccine and the different approaches they take to stimulate immunity against M. tuberculosis.