The relative importance of T cell subsets in immunity and immunopathology of airborne Mycobacterium tuberculosis infection in mice

Vaccine-elicited memory CD4+ T cell expansion is impaired in the lungs during tuberculosis

Immunological memory is the key biological process that makes vaccines possible. Although tuberculosis vaccines elicit protective immunity in animals, few provide durable protection. To understand why protection is transient, we evaluated the ability of memory CD4⁺ T cells to expand, differentiate, and control Mycobacterium tuberculosis. Both naïve and memory CD4⁺ T cells initially proliferated exponentially, and the accumulation of memory T cells in the lung correlated with early bacterial control. However, later during infection, memory CD4⁺ T cell proliferation was curtailed and no protection was observed. We show that memory CD4⁺ T cells are first activated in the LN and their recruitment to the lung attenuates bacterial growth. However, their interaction with Mtb-infected macrophages does not promote continued proliferation. We conclude that a lack of sustained expansion by memory-derived T cells in the lung limits the durability of their protection, linking their slower expansion with transient protection in vaccinated mice.

Vaccines elicit pathogen-specific memory T cells whose early and potent activation upon infection should provide long-lasting control of bacterial growth. Although many experimental vaccines generate memory CD4⁺ T cells and can control the growth of Mycobacterium tuberculosis (Mtb) early during infection, none reliably provide protection from pulmonary tuberculosis (TB) that is durable. Although the etiology of the clinical failure of memory T cells is not well understood, few studies monitor memory T cell fate and function throughout chronic infection. Using both clonal and polyclonal models of Mtb-specific memory CD4⁺ T cell function during TB, we show that the expansion of memory-derived T cell responses is impaired in the lungs, compared with the primary (naïve) CD4 response. Despite expressing a protective effector phenotype, and reducing bacterial growth early after Mtb challenge, we further show that memory CD4⁺ T cells do not proliferate in response to Mtb-infected macrophages. Their impaired expansion corresponded with waning protection in vaccinated mice later during infection. We propose that both the induction of memory T cell proliferation by infected macrophages, and the durability of vaccine-elicited T cell responses during TB should serve as preclinical vaccine benchmarks.

Th1 cytokines, true functional signatures for protective immunity against TB?

The lack of an effective preventative vaccine against tuberculosis (TB) presents a great challenge to TB control. Since it takes an extremely long time to accurately determine the protective efficacy of TB vaccines, there is a great need to identify the surrogate signatures of protection to facilitate vaccine development. Unfortunately, antigen-specific Th1 cytokines that are currently used to evaluate the protective efficacy of the TB vaccine, do not align with the protection and failure of TB vaccine candidates in clinical trials. In this review, we discuss the limitation of current Th1 cytokines as surrogates of protection and address the potential elements that should be considered to finalize the true functional signatures of protective immunity against TB.

Role of Granulocyte-Macrophage Colony-Stimulating Factor Production by T Cells during Mycobacterium tuberculosis Infection

Mice deficient for granulocyte-macrophage colony-stimulating factor (GM-CSF^−/−) are highly susceptible to infection with Mycobacterium tuberculosis, and clinical data have shown that anti-GM-CSF neutralizing antibodies can lead to increased susceptibility to tuberculosis in otherwise healthy people. GM-CSF activates human and murine macrophages to inhibit intracellular M. tuberculosis growth. We have previously shown that GM-CSF produced by iNKT cells inhibits growth of M. tuberculosis. However, the more general role of T cell-derived GM-CSF during infection has not been defined and how GM-CSF activates macrophages to inhibit bacterial growth is unknown. Here we demonstrate that, in addition to nonconventional T cells, conventional T cells also produce GM-CSF during M. tuberculosis infection. Early during infection, nonconventional iNKT cells and γδ T cells are the main source of GM-CSF, a role subsequently assumed by conventional CD4⁺ T cells as the infection progresses. M. tuberculosis-specific T cells producing GM-CSF are also detected in the peripheral blood of infected people. Under conditions where nonhematopoietic production of GM-CSF is deficient, T cell production of GM-CSF is protective and required for control of M. tuberculosis infection. However, GM-CSF is not required for T cell-mediated protection in settings where GM-CSF is produced by other cell types. Finally, using an in vitro macrophage infection model, we demonstrate that GM-CSF inhibition of M. tuberculosis growth requires the expression of peroxisome proliferator-activated receptor gamma (PPARγ). Thus, we identified GM-CSF production as a novel T cell effector function. These findings suggest that a strategy augmenting T cell production of GM-CSF could enhance host resistance against M. tuberculosis.

Mycobacterium tuberculosis is the bacterium that causes tuberculosis, the leading cause of death by any infection worldwide. T cells are critical components of the immune response to Mycobacterium tuberculosis. While gamma interferon (IFN-γ) is a key effector function of T cells during infection, a failed phase IIb clinical trial and other studies have revealed that IFN-γ production alone is not sufficient to control M. tuberculosis. In this study, we demonstrate that CD4⁺, CD8⁺, and nonconventional T cells produce GM-CSF during Mycobacterium tuberculosis infection in mice and in the peripheral blood of infected humans. Under conditions where other sources of GM-CSF are absent, T cell production of GM-CSF is protective and is required for control of infection. GM-CSF activation of macrophages to limit bacterial growth requires host expression of the transcription factor PPARγ. The identification of GM-CSF production as a T cell effector function may inform future host-directed therapy or vaccine designs.

CISH controls bacterial burden early after infection with Mycobacterium tuberculosis in mice

CISH gene has been associated with increased susceptibility to human tuberculosis. We found that cish^-/- mice had higher M. tuberculosis load in spleens and lungs up to 2.5 weeks after infection but not later compared to controls. Cish mRNA levels were increased in lungs at early and late time points after M. tuberculosis infection. In relation, the titers of inos and tnf mRNA in lungs were reduced early after infection of cish^-/- mice. The transfer of cish^-/- and control T cells conferred rag1^-/- mice similar protection to infection with M. tuberculosis. Macrophages showed increased cish mRNA levels after M. tuberculosis infection in vitro. However, mycobacterial uptake and growth in cish^-/- and control macrophages was similar. Thus, we here show that CISH mediates control of M. tuberculosis in mice early after infection via regulation of innate immune mechanisms.

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.

Striking the right immunological balance prevents progression of tuberculosis

INTRODUCTION

Tuberculosis (TB) caused by infection with Mycobacterium tuberculosis (Mtb) is a major burden for human health worldwide. Current standard treatments for TB require prolonged administration of antimycobacterial drugs leading to exaggerated inflammation and tissue damage. This can result in the reactivation of latent TB culminating in TB progression. Thus, there is an unmet need to develop therapies that would shorten the duration of anti-TB treatment and to induce optimal protective immune responses to control the spread of mycobacterial infection with minimal lung pathology.

FINDINGS

Granulomata is the hallmark structure formed by the organized accumulation of immune cells including macrophages, natural killer cells, dendritic cells, neutrophils, T cells, and B cells to the site of Mtb infection. It safeguards the host by containing Mtb in latent form. However, granulomata can undergo caseation and contribute to the reactivation of latent TB, if the immune responses developed to fight mycobacterial infection are not properly controlled. Thus, an optimal balance between innate and adaptive immune cells might play a vital role in containing mycobacteria in latent form for prolonged periods and prevent the spread of Mtb infection from one individual to another.

CONCLUSION

Optimal and well-regulated immune responses against Mycobacterium tuberculosis may help to prevent the reactivation of latent TB. Moreover, therapies targeting balanced immune responses could help to improve treatment outcomes among latently infected TB patients and thereby limit the dissemination of mycobacterial infection.

The Immune Interaction between HIV-1 Infection and Mycobacterium tuberculosis

The modulation of tuberculosis (TB)-induced immunopathology caused by human immunodeficiency virus (HIV)-1 coinfection remains incompletely understood but underlies the change seen in the natural history, presentation, and prognosis of TB in such patients. The deleterious combination of these two pathogens has been dubbed a "deadly syndemic," with each favoring the replication of the other and thereby contributing to accelerated disease morbidity and mortality. HIV-1 is the best-recognized risk factor for the development of active TB and accounts for 13% of cases globally. The advent of combination antiretroviral therapy (ART) has considerably mitigated this risk. Rapid roll-out of ART globally and the recent recommendation by the World Health Organization (WHO) to initiate ART for everyone living with HIV at any CD4 cell count should lead to further reductions in HIV-1-associated TB incidence because susceptibility to TB is inversely proportional to CD4 count. However, it is important to note that even after successful ART, patients with HIV-1 are still at increased risk for TB. Indeed, in settings of high TB incidence, the occurrence of TB often remains the first presentation of, and thereby the entry into, HIV care. As advantageous as ART-induced immune recovery is, it may also give rise to immunopathology, especially in the lower-CD4-count strata in the form of the immune reconstitution inflammatory syndrome. TB-immune reconstitution inflammatory syndrome will continue to impact the HIV-TB syndemic.

BCG vaccine powder-laden and dissolvable microneedle arrays for lesion-free vaccination

Live attenuated Bacille Calmette-Guerin (BCG) bacillus is the only licensed vaccine for tuberculosis prevention worldwide to date. It must be delivered intradermally to be effective, which causes severe skin inflammation and sometimes, permanent scars. To minimize the side effects, we developed a novel microneedle array (MNA) that could deliver live attenuated freeze-dried BCG powder into the epidermis in a painless, lesion-free, and self-applicable fashion. The MNA was fabricated with biocompatible and dissolvable hyaluronic acid with a deep cave formed in the basal portion of each microneedle, into which BCG powder could be packaged directly. Viability of BCG vaccine packaged in the caves and the mechanical strength of the powder-laden MNA did not alter significantly before and after more than two months of storage at room temperature. Following insertion of the MNA into the skin, individual microneedle shafts melted away by interstitial fluid from the epidermis and upper dermis, exposing the powder to epidermal tissues. The powder sucked interstitial fluid, dissolved slowly, and diffused into the epidermis in a day against the interstitial fluid influx. Vaccination with BCG-MNA caused no overt skin irritation, in marked contrast to intradermal vaccination that provoked severe inflammation and bruise. While causing little skin irritation, vaccination efficacy of BCG-MNAs was comparable to that of intradermal immunization whether it was evaluated by humoral or cellular immunity. This powder-laden and dissolvable MNA represents a novel technology to sufficiently deliver live attenuated vaccine powders into the skin.

The introduction of mesenchymal stromal cells induces different immunological responses in the lungs of healthy and M. tuberculosis infected mice

Mesenchymal stromal cells (MSC) have strong immunomodulatory properties and therefore can be used to control inflammation and tissue damage. It was suggested recently that MSC injections can be used to treat multi-drug resistant tuberculosis (TB). However, MSC trafficking and immunomodulatory effects of MSC injections during Mycobacterium tuberculosis (Mtb) infection have not been studied. To address this issue we have analyzed MSC distribution in tissues and local immunological effects of MSC injections in Mtb infected and uninfected mice. After intravenous injection, MSC accumulated preferentially in the lungs where they were located as cell aggregates in the alveolar walls. Immunological analysis of MSC effects included detection of activated, IFN-γ and IL-4 producing CD4⁺ lymphocytes, the frequency analysis of dendritic cells (CD11c⁺F4/80) and macrophages (CD11c^-F4/80⁺) located in the lungs, the expression of IA/IE and CD11b molecules by these cells, and evaluation of 23 cytokines/chemokines in lung lysates. In the lungs of uninfected mice, MSC transfer markedly increased the percentage of IFN-γ⁺ CD4⁺ lymphocytes and dendritic cells, elevated levels of IA/IE expression by dendritic cells and macrophages, augmented local production of type 2 cytokines (IL-4, IL-5, IL-10) and chemokines (CCL2, CCL3, CCL4, CCL5, CXCL1), and downregulated type 1 and hematopoietic cytokines (IL-12p70, IFN-γ, IL-3, IL-6, GM-CSF). Compared to uninfected mice, Mtb infected mice had statistically higher “background” frequency of activated CD69⁺ and IFN-γ⁺ CD4⁺ lymphocytes and dendritic cells, and higher levels of cytokines in the lungs. The injections of MSC to Mtb infected mice did not show statistically significant effects on CD4⁺ lymphocytes, dendritic cells and macrophages, only slightly shifted cytokine profile, and did not change pathogen load or slow down TB progression. Lung section analysis showed that in Mtb infected mice, MSC could not be found in the proximity of the inflammatory foci. Thus, in healthy recipients, MSC administration dramatically changed T-cell function and cytokine/chemokine milieu in the lungs, most likely, due to capillary blockade. But, during Mtb infection, i.e., in the highly-inflammatory conditions, MSC did not affect T-cell function and the level of inflammation. The findings emphasize the importance of the evaluation of MSC effects locally at the site of their predominant post-injection localization and question MSC usefulness as anti-TB treatment.

Construction of human MASP-2-CCP1/2SP, CCP2SP, SP plasmid DNA nanolipoplexes and the effects on tuberculosis in BCG-infected mice

The lectin pathway, one of the complement cascade systems, provides the primary line of defense against invading pathogens. The serine protease of MASP-2 plays an essential role in complement activation of the lectin pathway. The C-terminal segment of MASP-2 is comprised of the CCP1-CCP2-SP domains, and is the crucial catalytic segment. However, what is the effect of CCP1-CCP2-SP domains in controlling chronic infection is unknown. In order to evaluate the potential impact of CCP1-CCP2-SP domains on tuberculosis, we constructed the human MASP-2 CCP1/2SP, CCP2SP and SP recombinant plasmids, and delivered these plasmids by DNA-DOTAP:cholesterol cationic nanolipoplexes to BCG-infected mice. After 21 days post DNA-DOTAP:chol nanolipoplexes application, we analyzed bacteria loads of pulmonary, pathology of granuloma, lymphocyte subpopulations. The C3a, C4a and MASP-2 levels in serum were measured with enzyme-linked immunosorbent assays. Compared to the control group that received GFP DNA-DOTAP:chol nanolipoplexes, MASP-2 CCP1/2SP DNA-DOTAP:chol nanolipoplexes treated group showed significantly enlarged pulmonary granulomas lesion (P < 0.05) and did not reduce bacteria loads in the lung tissue (P < 0.05). Furthermore, the levels of C3a in serum were decreased (P < 0.05), the number and percentage of PD1⁺ and Tim3⁺ cells subgroups were increased in BCG-infected mice after treated with MASP-2 CCP1/2SP DNA-DOTAP:chol nanolipoplexes (P < 0.05). But, there was no statistical difference in the serum C4a and MASP-2 level among DNA nanolipoplexes treated groups (P > 0.05). These findings provided experimental evidence that MASP-2 CCP1/2SP DNA nanolipoplexes shown the negative efficacy in controlling Mycobacterium tuberculosis infection, and displayed a potential role of down-regulating T-cell-mediated immunity in tuberculosis.

The Goldilocks model of immune symbiosis with Mycobacteria and Candida colonizers

Mycobacteria and Candida species include significant human pathogens that can cause localized or disseminated infections. Although these organisms may appear to have little in common, several shared pathways of immune recognition and response are important for both control and infection-related pathology. In this article, we compare and contrast the innate and adaptive components of the immune system that pertain to these infections in humans and animal models. We also explore a relatively new concept in the mycobacterial field: biological commensalism. Similar to the well-established model of Candida infection, Mycobacteria species colonize their human hosts in equilibrium with the immune response. Perturbations in the immune response permit the progression to pathologic disease at the expense of the host. Understanding the immune factors required to maintain commensalism may aid with the development of diagnostic and treatment strategies for both categories of pathogens.

Adoptive Transfer of Phosphoantigen-Specific γδ T Cell Subset Attenuates Mycobacterium tuberculosis Infection in Nonhuman Primates

The dominant Vγ2Vδ2 T cell subset recognizes phosphoantigen and exists only in humans and nonhuman primates. Despite the discovery of γδ T cells >30 y ago, a proof-of-concept study has not been done to prove the principle that the Vγ2Vδ2 T cell subset is protective against Mycobacterium tuberculosis and other infections. In this study, we used an adoptive cell-transfer strategy to define the protective role of Vγ2Vδ2 T cells in a primate tuberculosis (TB) model. Vγ2Vδ2 T cells for adoptive transfer displayed central/effector memory and mounted effector functions, including the production of anti-M. tuberculosis cytokines and inhibition of intracellular mycobacteria. They also expressed CXCR3/CCR5/LFA-1 trafficking/tissue-resident phenotypes and consistently trafficked to the airway, where they remained detectable from 6 h through 7 d after adoptive transfer. Interestingly, the test group of macaques receiving transfer of Vγ2Vδ2 T cells at weeks 1 and 3 after high-dose (500 CFU) M. tuberculosis infection exhibited significantly lower levels of M. tuberculosis infection burdens in lung lobes and extrapulmonary organs than did the control groups receiving PBLs or saline. Consistently, adoptive transfer of Vγ2Vδ2 T cells attenuated TB pathology and contained lesions primarily in the infection site of the right caudal lung lobe, with no or reduced TB dissemination to other lobes, spleen, or liver/kidney; in contrast, the controls showed widespread TB dissemination. The proof-of-concept finding supports the view that the dominant Vγ2Vδ2 T cell subset may be included in the rational design of a TB vaccine or host-directed therapy.

MHC Ib molecule Qa-1 presents Mycobacterium tuberculosis peptide antigens to CD8+ T cells and contributes to protection against infection

A number of nonclassical MHC Ib molecules recognizing distinct microbial antigens have been implicated in the immune response to Mycobacterium tuberculosis (Mtb). HLA-E has been identified to present numerous Mtb peptides to CD8⁺ T cells, with multiple HLA-E-restricted cytotoxic T lymphocyte (CTL) and regulatory T cell lines isolated from patients with active and latent tuberculosis (TB). In other disease models, HLA-E and its mouse homolog Qa-1 can act as antigen presenting molecules as well as regulators of the immune response. However, it is unclear what precise role(s) HLA-E/Qa-1 play in the immune response to Mtb. In this study, we found that murine Qa-1 can bind and present Mtb peptide antigens to CD8⁺ T effector cells during aerosol Mtb infection. Further, mice lacking Qa-1 (Qa-1^-/-) were more susceptible to high-dose Mtb infection compared to wild-type controls, with higher bacterial burdens and increased mortality. The increased susceptibility of Qa-1^-/- mice was associated with dysregulated T cells that were more activated and produced higher levels of pro-inflammatory cytokines. T cells from Qa-1^-/- mice also had increased expression of inhibitory and apoptosis-associated cell surface markers such as CD94/NKG2A, KLRG1, PD-1, Fas-L, and CTLA-4. As such, they were more prone to cell death and had decreased capacity in promoting the killing of Mtb in infected macrophages. Lastly, comparing the immune responses of Qa-1 mutant knock-in mice deficient in either Qa-1-restricted CD8⁺ T_regs (Qa-1 D227K) or the inhibitory Qa-1-CD94/NKG2A interaction (Qa-1 R72A) with Qa-1^-/- and wild-type controls indicated that both of these Qa-1-mediated mechanisms were involved in suppression of the immune response in Mtb infection. Our findings reveal that Qa-1 participates in the immune response to Mtb infection by presenting peptide antigens as well as regulating immune responses, resulting in more effective anti-Mtb immunity.

The disease tuberculosis (TB) is caused by the microbe Mycobacterium tuberculosis (Mtb), and remains a major public health concern. More research is needed to understand the diverse immune responses against Mtb to develop better vaccines. Mouse Qa-1 and its human counterpart HLA-E are nonclassical MHC I molecules that can activate or inhibit immune responses in a variety of diseases. However, their role during the immune response to Mtb remains unknown. We found that Qa-1 can present Mtb peptides to activate CD8⁺ T effector cells during aerosol Mtb infection. Further, Mtb-infected mice that lacked Qa-1 (Qa-1^-/-) had higher numbers of bacteria and died more often than infected mice that expressed Qa-1 (Qa-1^+/+). The lack of Qa-1 results in over-activation of the immune response upon infection, which is less efficient in controlling Mtb. Using mice expressing different mutant forms of Qa-1, we showed that Qa-1 can regulate immune responses against Mtb through the interaction with inhibitory CD94/NKG2A receptors as well as the activation of regulatory CD8⁺ T cells. We believe our study sheds light on the diverse mechanisms at play in generating protective immune responses against Mtb and will inform future mouse and human studies.

Transcriptional networks are associated with resistance to Mycobacterium tuberculosis infection

RATIONALE

Understanding mechanisms of resistance to M. tuberculosis (M.tb) infection in humans could identify novel therapeutic strategies as it has for other infectious diseases, such as HIV.

OBJECTIVES

To compare the early transcriptional response of M.tb-infected monocytes between Ugandan household contacts of tuberculosis patients who demonstrate clinical resistance to M.tb infection (cases) and matched controls with latent tuberculosis infection.

METHODS

Cases (n = 10) and controls (n = 18) were selected from a long-term household contact study in which cases did not convert their tuberculin skin test (TST) or develop tuberculosis over two years of follow up. We obtained genome-wide transcriptional profiles of M.tb-infected peripheral blood monocytes and used Gene Set Enrichment Analysis and interaction networks to identify cellular processes associated with resistance to clinical M.tb infection.

MEASUREMENTS AND MAIN RESULTS

We discovered gene sets associated with histone deacetylases that were differentially expressed when comparing resistant and susceptible subjects. We used small molecule inhibitors to demonstrate that histone deacetylase function is important for the pro-inflammatory response to in-vitro M.tb infection in human monocytes.

CONCLUSIONS

Monocytes from individuals who appear to resist clinical M.tb infection differentially activate pathways controlled by histone deacetylase in response to in-vitro M.tb infection when compared to those who are susceptible and develop latent tuberculosis. These data identify a potential cellular mechanism underlying the clinical phenomenon of resistance to M.tb infection despite known exposure to an infectious contact.

Biosafety Level 3 setup for multiphoton microscopy in vivo

Multiphoton microscopy has revealed important insights into cellular behavior in vivo. However, its application in infectious settings often encounters technical, safety and regulatory limitations that prevent its wider use with highly virulent human pathogens. Herein, we present a method that renders multiphoton microscopy in vivo compatible with biosafety level 3 regulations and present an example of its application and potential to visualize a Mycobacterium tuberculosis infection of the mouse lung.

Intracellular growth of Mycobacterium tuberculosis after macrophage cell death leads to serial killing of host cells

A hallmark of pulmonary tuberculosis is the formation of macrophage-rich granulomas. These may restrict Mycobacterium tuberculosis (Mtb) growth, or progress to central necrosis and cavitation, facilitating pathogen growth. To determine factors leading to Mtb proliferation and host cell death, we used live cell imaging to track Mtb infection outcomes in individual primary human macrophages. Internalization of Mtb aggregates caused macrophage death, and phagocytosis of large aggregates was more cytotoxic than multiple small aggregates containing similar numbers of bacilli. Macrophage death did not result in clearance of Mtb. Rather, it led to accelerated intracellular Mtb growth regardless of prior activation or macrophage type. In contrast, bacillary replication was controlled in live phagocytes. Mtb grew as a clump in dead cells, and macrophages which internalized dead infected cells were very likely to die themselves, leading to a cell death cascade. This demonstrates how pathogen virulence can be achieved through numbers and aggregation states.

DOI:http://dx.doi.org/10.7554/eLife.22028.001

Every year, around two million people worldwide die from tuberculosis, a disease caused by the bacterium Mycobacterium tuberculosis (Mtb). The bacteria generally infect the lungs. In response, the immune system forms structures called granulomas that attempt to control and isolate the infecting pathogens.

Granulomas consist of immune cells known as macrophages, which engulf the M. tuberculosis bacteria and isolate them in a cellular compartment where the bacteria either cannot grow or are killed. However, if a large number of macrophages in a granuloma die, the granuloma’s core liquefies and the structure is coughed up into the airways, from where M. tuberculosis bacteria are transmitted to other people. But how do the bacteria manage to cause the extensive death of the cells that are supposed to control the infection?

By imaging M. tuberculosis in human macrophages using time-lapse microscopy, Mahamed et al. reveal that the bacteria break down macrophage control by serially killing macrophages. M. tuberculosis cells first clump together and ‘gang up’ on a macrophage, which engulfs the clump and dies because the bacteria overwhelm it. This does not kill the bacteria, and they rapidly grow inside the dead macrophage. The dead cell is then cleaned up by another macrophage. However, the increasing number of bacteria inside the dead macrophage means that the new macrophage is even more likely to die than the first one. Hence, the bacteria use dead macrophages as fuel to grow on and as bait to attract the next immune cell.

Overall, Mahamed et al. show that once a clump of M. tuberculosis initiates death of a single macrophage, it may lead to serial killing of other macrophages and a loss of control over the infection. An important next step will be to understand how the initial clump of bacteria is allowed to form.

DOI:http://dx.doi.org/10.7554/eLife.22028.002

A polymorphism in human MR1 is associated with mRNA expression and susceptibility to tuberculosis

The MR1 antigen-presenting system is conserved among mammals and enables T cells to recognize small molecules produced by bacterial pathogens, including Mycobacterium tuberculosis (M.tb). However, it is not known whether MR1-mediated antigen presentation is important for protective immunity against mycobacterial disease. We hypothesized that genetic control of MR1 expression correlates with clinical outcomes of tuberculosis infection. We performed an MR1 candidate gene association study and identified an intronic single-nucleotide polymorphism (rs1052632) that was significantly associated with susceptibility to tuberculosis in a discovery and validation cohort of Vietnamese adults with tuberculosis. Stratification by site of disease revealed that rs1052632 genotype GG was strongly associated with the development of meningeal tuberculosis (odds ratio=2.99; 95% confidence interval (CI) 1.64-5.43; P=0.00006). Among patients with meningeal disease, absence of the G allele was associated with an increased risk of death (hazard ratio=3.86; 95% CI 1.49-9.98; P=0.005). Variant annotation tools using public databases indicate that rs1052632 is strongly associated with MR1 gene expression in lymphoblastoid cells (P=0.004) and is located within a transcriptional enhancer in epithelial keratinocytes. These data support a role for MR1 in the pathogenesis of human tuberculosis by revealing that rs1052632 is associated with MR1 gene expression and susceptibility to tuberculosis in Vietnam.

Antigen-Specific IFN-γ Responses Correlate with the Activity of M. tuberculosis Infection but Are Not Associated with the Severity of Tuberculosis Disease

IFN-γ is a key cytokine in antituberculosis (TB) defense. However, how the levels of its secretion affect M. tuberculosis (Mtb) infection is not clear. We have analyzed associations between IFN-γ responses measured in QuantiFERON®-TB Gold In-tube (QFT) assay, TB disease severity, and Mtb infection activity. TB severity was evaluated based on the results of radiological, microbiological, and clinical examinations. Antigen-driven IFN-γ secretion did not correlate with TB severity. Mitogen-induced IFN-γ secretion correlated inversely with the form of pulmonary pathology and the area of affected pulmonary tissue; the levels of spontaneous IFN-γ secretion correlated with patients' age (r = 0.395, p = 0.001). Mtb infection activity was evaluated based on radiological data of lung tissue infiltration, destruction, dissemination or calcification, and condensation. The rate of positive QFT results and the levels of antigen-driven IFN-γ secretion increased in a row: patients with residual TB lesions < patients with low TB activity < patients with high TB activity. Thus, antigen-driven IFN-γ secretion and QFT results did not associate with TB severity but associated with the infection activity. The results suggest that quantitative parameters of IFN-γ secretion play a minor role in determining the course of TB disease but mirror the activity of the infectious process.

IL-21 signaling is essential for optimal host resistance against Mycobacterium tuberculosis infection

IL-21 is produced predominantly by activated CD4⁺ T cells and has pleiotropic effects on immunity via the IL-21 receptor (IL-21R), a member of the common gamma chain (γ_c) cytokine receptor family. We show that IL-21 signaling plays a crucial role in T cell responses during Mycobacterium tuberculosis infection by augmenting CD8⁺ T cell priming, promoting T cell accumulation in the lungs, and enhancing T cell cytokine production. In the absence of IL-21 signaling, more CD4⁺ and CD8⁺ T cells in chronically infected mice express the T cell inhibitory molecules PD-1 and TIM-3. We correlate these immune alterations with increased susceptibility of IL-21R^−/− mice, which have increased lung bacterial burden and earlier mortality compared to WT mice. Finally, to causally link the immune defects with host susceptibility, we use an adoptive transfer model to show that IL-21R^−/− T cells transfer less protection than WT T cells. These results prove that IL-21 signaling has an intrinsic role in promoting the protective capacity of T cells. Thus, the net effect of IL-21 signaling is to enhance host resistance to M. tuberculosis. These data position IL-21 as a candidate biomarker of resistance to tuberculosis.

Granulomas and Inflammation: Host-Directed Therapies for Tuberculosis

Tuberculosis (TB) remains a leading global health problem that is aggravated by emergence of drug-resistant strains, which account for increasing number of treatment-refractory cases. Thus, eradication of this disease will strongly require better therapeutic strategies. Identification of host factors promoting disease progression may accelerate discovery of adjunct host-directed therapies (HDTs) that will boost current treatment protocols. HDTs focus on potentiating key components of host anti-mycobacterial effector mechanisms, and limiting inflammation and pathological damage in the lung. Granulomas represent a pathological hallmark of TB. They are comprised of impressive arrangement of immune cells that serve to contain the invading pathogen. However, granulomas can also undergo changes, developing caseums and cavities that facilitate bacterial spread and disease progression. Here, we review current concepts on the role of granulomas in pathogenesis and protective immunity against TB, drawing from recent clinical studies in humans and animal models. We also discuss therapeutic potential of inflammatory pathways that drive granuloma progression, with a focus on new and existing drugs that will likely improve TB treatment outcomes.

Cytokines and Chemokines in Mycobacterium tuberculosis Infection

Chemokines and cytokines are critical for initiating and coordinating the organized and sequential recruitment and activation of cells into Mycobacterium tuberculosis-infected lungs. Correct mononuclear cellular recruitment and localization are essential to ensure control of bacterial growth without the development of diffuse and damaging granulocytic inflammation. An important block to our understanding of TB pathogenesis lies in dissecting the critical aspects of the cytokine/chemokine interplay in light of the conditional role these molecules play throughout infection and disease development. Much of the data highlighted in this review appears at first glance to be contradictory, but it is the balance between the cytokines and chemokines that is critical, and the "goldilocks" (not too much and not too little) phenomenon is paramount in any discussion of the role of these molecules in TB. Determination of how the key chemokines/cytokines and their receptors are balanced and how the loss of that balance can promote disease is vital to understanding TB pathogenesis and to identifying novel therapies for effective eradication of this disease.

Suppression of autophagy and antigen presentation by Mycobacterium tuberculosis PE_PGRS47

Suppression of major histocompatibility complex (MHC) class II antigen presentation is believed to be among the major mechanisms used by Mycobacterium tuberculosis to escape protective host immune responses. Through a genome-wide screen for the genetic loci of M. tuberculosis that inhibit MHC class II-restricted antigen presentation by mycobacteria-infected dendritic cells, we identified the PE_PGRS47 protein as one of the responsible factors. Targeted disruption of the PE_PGRS47 (Rv2741) gene led to attenuated growth of M. tuberculosis in vitro and in vivo, and a PE_PGRS47 mutant showed enhanced MHC class II-restricted antigen presentation during in vivo infection of mice. Analysis of the effects of deletion or over-expression of PE_PGRS47 implicated this protein in the inhibition of autophagy in infected host phagocytes. Our findings identify PE_PGRS47 as a functionally relevant, non-redundant bacterial factor in the modulation of innate and adaptive immunity by M. tuberculosis, suggesting strategies for improving antigen presentation and the generation of protective immunity during vaccination or infection.

Suppression of autophagy and antigen presentation by Mycobacterium tuberculosis PE_PGRS47

Suppression of major histocompatibility complex (MHC) class II antigen presentation is believed to be among the major mechanisms used by Mycobacterium tuberculosis to escape protective host immune responses. Through a genome-wide screen for the genetic loci of M. tuberculosis that inhibit MHC class II-restricted antigen presentation by mycobacteria-infected dendritic cells, we identified the PE_PGRS47 protein as one of the responsible factors. Targeted disruption of the PE_PGRS47 (Rv2741) gene led to attenuated growth of M. tuberculosis in vitro and in vivo, and a PE_PGRS47 mutant showed enhanced MHC class II-restricted antigen presentation during in vivo infection of mice. Analysis of the effects of deletion or over-expression of PE_PGRS47 implicated this protein in the inhibition of autophagy in infected host phagocytes. Our findings identify PE_PGRS47 as a functionally relevant, non-redundant bacterial factor in the modulation of innate and adaptive immunity by M. tuberculosis, suggesting strategies for improving antigen presentation and the generation of protective immunity during vaccination or infection.

The CD4(+) T cell methylome contributes to a distinct CD4(+) T cell transcriptional signature in Mycobacterium bovis-infected cattle

We hypothesised that epigenetic regulation of CD4⁺ T lymphocytes contributes to a shift toward a dysfunctional T cell phenotype which may impact on their ability to clear mycobacterial infection. Combined RNA-seq transcriptomic profiling and Reduced Representation Bisulfite Sequencing identified 193 significantly differentially expressed genes and 760 differentially methylated regions (DMRs), between CD4⁺ T cells from M. bovis infected and healthy cattle. 196 DMRs were located within 10 kb of annotated genes, including GATA3 and RORC, both of which encode transcription factors that promote T_H2 and T_H17 T helper cell subsets respectively. Gene-specific DNA methylation and gene expression levels for the TNFRSF4 and Interferon-γ genes were significantly negatively correlated suggesting a regulatory relationship. Pathway analysis of DMRs identified enrichment of genes involved in the anti-proliferative TGF-β signaling pathway and TGFB1 expression was significantly increased in peripheral blood leukocytes from TB-infected cattle. This first analysis of the bovine CD4⁺ T cell methylome suggests that DNA methylation directly contributes to a distinct gene expression signature in CD4⁺ T cells from cattle infected with M. bovis. Specific methylation changes proximal to key inflammatory gene loci may be critical to the emergence of a non-protective CD4⁺ T cell response during mycobacterial infection in cattle.

Understanding the pathophysiology of the human TB lung granuloma using in vitro granuloma models

Tuberculosis remains a major human health threat that infects one in three individuals worldwide. Infection with Mycobacterium tuberculosis is a standoff between host and bacteria in the formation of a granuloma. This review will introduce a variety of bacterial and host factors that impact individual granuloma fates. The authors describe advances in the development of in vitro granuloma models, current evidence surrounding infection and granuloma development, and the applicability of existing in vitro models in the study of human disease. In vitro models of infection help improve our understanding of pathophysiology and allow for the discovery of other potential models of study.

Tuberculosis Therapy Modifies the Cytokine Profile, Maturation State, and Expression of Inhibitory Molecules on Mycobacterium tuberculosis-Specific CD4+ T-Cells

Background

Little is known about the expression of inhibitory molecules cytotoxic T-lymphocyte antigen-4 (CTLA-4) and programmed-death-1 (PD-1) on Mycobacterium tuberculosis (Mtb)-specific CD4 T-cells and how their expression is impacted by TB treatment.

Methods

Cryopreserved PBMCs from HIV-TB co-infected and TB mono-infected patients with untreated and treated tuberculosis (TB) disease were stimulated for six hours with PPD and stained. Using polychromatic flow cytometry, we characterized the differentiation state, cytokine profile, and inhibitory molecule expression on PPD-specific CD4 T-cells.

Results

In our HIV-TB co-infected cohort, TB treatment increased the proportion of PPD-specific CD4 T-cells co-producing IFN-γ⁺IL-2⁺TNF-α⁺ and IFN-γ⁺IL-2⁺ (p = 0.0004 and p = 0.0002, respectively) while decreasing the proportion of PPD-specific CD4 T-cells co-producing IFN-γ⁺MIP1-β⁺TNF-α⁺ and IFN-γ⁺MIP1-β⁺. The proportion of PPD-specific CD4 T-cells expressing an effector memory phenotype decreased (63.6% vs 51.6%, p = 0.0015) while the proportion expressing a central memory phenotype increased (7.8% vs. 21.7%, p = 0.001) following TB treatment. TB treatment reduced the proportion of PPD-specific CD4 T-cells expressing CTLA-4 (72.4% vs. 44.3%, p = 0.0005) and PD-1 (34.5% vs. 29.2%, p = 0.03). Similar trends were noted in our TB mono-infected cohort.

Conclusion

TB treatment alters the functional profile of Mtb-specific CD4 T-cells reflecting shifts towards a less differentiated maturational profile and decreases PD-1 and CTLA-4 expression. These could serve as markers of reduced mycobacterial burden. Further study is warranted.

A novel multivalent tuberculosis vaccine confers protection in a mouse model of tuberculosis

Mycobacterium tuberculosis infects one third of the world's population. Due to variable efficacy of the Bacille Calmette Guerin (BCG) vaccine, development of novel TB vaccines remains a priority. Here, we demonstrate the protective efficacy of a novel multivalent DNA vaccine, which contains 15 synthetic antigens targeting the Mtb ESX secretion system.

Nonclassical MHC Ib-restricted CD8+ T Cells Recognize Mycobacterium tuberculosis-Derived Protein Antigens and Contribute to Protection Against Infection

MHC Ib-restricted CD8+ T cells have been implicated in host defense against Mycobacterium tuberculosis (Mtb) infection. However, the relative contribution of various MHC Ib-restricted T cell populations to anti-mycobacterial immunity remains elusive. In this study, we used mice that lack MHC Ia (Kb-/-Db-/-), MHC Ia/H2-M3 (Kb-/-Db-/-M3-/-), or β2m (β2m-/-) to study the role of M3-restricted and other MHC Ib-restricted T cells in immunity against Mtb. Unlike their dominant role in Listeria infection, we found that M3-restricted CD8+ T cells only represented a small proportion of the CD8+ T cells responding to Mtb infection. Non-M3, MHC Ib-restricted CD8+ T cells expanded preferentially in the lungs of Mtb-infected Kb-/-Db-/-M3-/- mice, exhibited polyfunctional capacities and conferred protection against Mtb. These MHC Ib-restricted CD8+ T cells recognized several Mtb-derived protein antigens at a higher frequency than MHC Ia-restricted CD8+ T cells. The presentation of Mtb antigens to MHC Ib-restricted CD8+ T cells was mostly β2m-dependent but TAP-independent. Interestingly, a large proportion of Mtb-specific MHC Ib-restricted CD8+ T cells in Kb-/-Db-/-M3-/- mice were Qa-2-restricted while no considerable numbers of MR1 or CD1-restricted Mtb-specific CD8+ T cells were detected. Our findings indicate that nonclassical CD8+ T cells other than the known M3, CD1, and MR1-restricted CD8+ T cells contribute to host immune responses against Mtb infection. Targeting these MHC Ib-restricted CD8+ T cells would facilitate the design of better Mtb vaccines with broader coverage across MHC haplotypes due to the limited polymorphism of MHC class Ib molecules.

Insufficient Generation of Mycobactericidal Mediators and Inadequate Level of Phagosomal Maturation Are Related with Susceptibility to Virulent Mycobacterium tuberculosis Infection in Mouse Macrophages

Tuberculosis is caused by Mycobacterium tuberculosis infection, and it remains major life-threatening infectious diseases worldwide. Although, M. tuberculosis has infected one-third of the present human population, only 5–10% of immunocompetent individuals are genetically susceptible to tuberculosis. All inbred strains of mice are susceptible to tuberculosis; however, some mouse strains are much more susceptible than others. In a previous report, we showed that Th1-mediated immunity was not responsible for the differential susceptibility between mouse models. To examine whether these susceptibility differences between inbred mouse strains are due to the insufficient production of effector molecules in the early stage of innate immunity, we investigated mycobacteriostatic function of bone marrow-derived macrophages (BMDMs) in resistant (BALB/c and C57BL/6) and susceptible strains (DBA/2) that were infected with virulent M. tuberculosis (H37Rv) or attenuated M. tuberculosis (H37Ra). The growth rate of virulent M. tuberculosis in infected cells was significantly higher in DBA/2 BMDMs, whereas the growth of the attenuated strain was similar in the three inbred mouse BMDM strains. In addition, the death rate of M. tuberculosis-infected cells increased with the infectious dose when DBA/2 BMDMs were infected with H37Rv. The intracellular reactive oxygen species level was lower in DBA/2 BMDMs that were infected with virulent M. tuberculosis at an early post-infection time point. The expression levels of phagosomal maturation markers, including early endosomal antigen-1 (EEA1) and lysosome-associated membrane protein-1 (LAMP-1), were significantly decreased in DBA/2 BMDM that were infected with virulent M. tuberculosis, whereas IFNγ-treatment restored the phagosomal maturation activity. The nitric oxide (NO) production levels were also significantly lower in DBA/2 BMDMs that were infected with virulent H37Rv at late post-infection points; however, this was not observed with the attenuated H37Ra strain. Furthermore, IFNγ-treatment rescued the low NO production level and insufficient M. tuberculosis growth control of DBA/2 BMDMs to the same level as of both resistant strains. The secreted TNF-α and IL-10 level were not significantly different between strains. Therefore, our findings suggest that DBA/2 BMDMs may have defects in the phagosomal maturation process and in inflammatory mediator production, as they showed innate immune defects when infected with the virulent, but not attenuated M. tuberculosis strain.

ESX-1 exploits type I IFN-signalling to promote a regulatory macrophage phenotype refractory to IFNγ-mediated autophagy and growth restriction of intracellular mycobacteria

The ability of macrophages to eradicate intracellular pathogens is normally greatly enhanced by IFNγ, a cytokine produced mainly after onset of adaptive immunity. However, adaptive immunity is unable to provide sterilizing immunity against mycobacteria, suggesting that mycobacteria have evolved virulence strategies to inhibit the bactericidal effect of IFNγ-signalling in macrophages. Still, the host-pathogen interactions and cellular mechanisms responsible for this feature have remained elusive. We demonstrate that the ESX-1 type VII secretion systems of Mycobacterium tuberculosis and Mycobacterium  marinum exploit type I IFN-signalling to promote an IL-12(low) /IL-10(high) regulatory macrophage phenotype characterized by secretion of IL-10, IL-27 and IL-6. This mechanism had no impact on intracellular growth in the absence of IFNγ but suppressed IFNγ-mediated autophagy and growth restriction, indicating that the regulatory phenotype extends to function. The IFNγ-refractory phenotype was partly mediated by IL-27-signalling, establishing functional relevance for this downstream cytokine. These findings identify a novel macrophage-modulating function for the ESX-1 secretion system that may contribute to suppress the efficacy of adaptive immunity and provide mechanistic insight into the antagonistic cross talk between type I IFNs and IFNγ in mycobacterial infection.

The Secreted Protein Rv1860 of Mycobacterium tuberculosis Stimulates Human Polyfunctional CD8+ T Cells

We previously reported that Rv1860 protein from Mycobacterium tuberculosis stimulated CD4(+)and CD8(+)T cells secreting gamma interferon (IFN-γ) in healthy purified protein derivative (PPD)-positive individuals and protected guinea pigs immunized with a DNA vaccine and a recombinant poxvirus expressing Rv1860 from a challenge with virulent M. tuberculosis We now show Rv1860-specific polyfunctional T (PFT) cell responses in the blood of healthy latently M. tuberculosis-infected individuals dominated by CD8(+) T cells, using a panel of 32 overlapping peptides spanning the length of Rv1860. Multiple subsets of CD8(+) PFT cells were significantly more numerous in healthy latently infected volunteers (HV) than in tuberculosis (TB) patients (PAT). The responses of peripheral blood mononuclear cells (PBMC) from PAT to the peptides of Rv1860 were dominated by tumor necrosis factor alpha (TNF-α) and interleukin-10 (IL-10) secretions, the former coming predominantly from non-T cell sources. Notably, the pattern of the T cell response to Rv1860 was distinctly different from those of the widely studied M. tuberculosis antigens ESAT-6, CFP-10, Ag85A, and Ag85B, which elicited CD4(+) T cell-dominated responses as previously reported in other cohorts. We further identified a peptide spanning amino acids 21 to 39 of the Rv1860 protein with the potential to distinguish latent TB infection from disease due to its ability to stimulate differential cytokine signatures in HV and PAT. We suggest that a TB vaccine carrying these and other CD8(+) T-cell-stimulating antigens has the potential to prevent progression of latent M. tuberculosis infection to TB disease.

Control of T cell antigen reactivity via programmed TCR downregulation

The T cell receptor (TCR) is unique in that its affinity for ligand is unknown prior to encounter and can vary by orders of magnitude. How the immune system regulates individual T cells that display highly different reactivity to antigen remains unclear. Here we identified that activated CD4⁺ T cells, at the peak of clonal expansion, persistently downregulate TCR expression in proportion to the strength of initial antigen recognition. This programmed response increases the threshold for cytokine production and recall proliferation in a clone-specific manner, ultimately excluding clones with the highest antigen reactivities. Thus, programmed TCR downregulation represents a negative feedback mechanism to constrain T cell effector function with a suitable time delay, thereby allowing pathogen control while avoiding excess inflammatory damage.

Macrophage takeover and the host-bacilli interplay during tuberculosis

Macrophages are key type of antigen-presenting cells that arbitrate the first line of defense against various intracellular pathogens. Tuberculosis, both pulmonary and extrapulmonary, is an infectious disease of global concern caused by Mycobacterium tuberculosis. The bacillus is a highly successful pathogen and has acquired various strategies to downregulate critical innate-effector immune responses of macrophages, such as phagosome-lysosome fusion, autophagy, induction of cytokines, generation of reactive oxygen and nitrogen species and antigen presentation. In addition, the bacilli also subvert acquired immunity. In this review, we aim to provide an overview of different antimycobacterial immune functions of macrophage and the strategies adopted by the bacilli to manipulate these functions to favor its survival and replication inside the host.

Evasion of Innate and Adaptive Immunity by Mycobacterium tuberculosis

Through thousands of years of reciprocal coevolution, Mycobacterium tuberculosis has become one of humanity's most successful pathogens, acquiring the ability to establish latent or progressive infection and persist even in the presence of a fully functioning immune system. The ability of M. tuberculosis to avoid immune-mediated clearance is likely to reflect a highly evolved and coordinated program of immune evasion strategies that interfere with both innate and adaptive immunity. These include the manipulation of their phagosomal environment within host macrophages, the selective avoidance or engagement of pattern recognition receptors, modulation of host cytokine production, and the manipulation of antigen presentation to prevent or alter the quality of T-cell responses. In this article we review an extensive array of published studies that have begun to unravel the sophisticated program of specific mechanisms that enable M. tuberculosis and other pathogenic mycobacteria to persist and replicate in the face of considerable immunological pressure from their hosts. Unraveling the mechanisms by which M. tuberculosis evades or modulates host immune function is likely to be of major importance for the development of more effective new vaccines and targeted immunotherapy against tuberculosis.

Multiple Inflammatory Cytokines Converge To Regulate CD8+ T Cell Expansion and Function during Tuberculosis

The differentiation of effector CD8(+) T cells is a dynamically regulated process that varies during different infections and is influenced by the inflammatory milieu of the host. In this study, we define three signals regulating CD8(+) T cell responses during tuberculosis by focusing on cytokines known to affect disease outcome: IL-12, type I IFN, and IL-27. Using mixed bone marrow chimeras, we compared wild-type and cytokine receptor knockout CD8(+) T cells within the same mouse following aerosol infection with Mycobacterium tuberculosis. Four weeks postinfection, IL-12, type 1 IFN, and IL-27 were all required for efficient CD8(+) T cell expansion in the lungs. We next determined if these cytokines directly promote CD8(+) T cell priming or are required only for expansion in the lungs. Using retrogenic CD8(+) T cells specific for the M. tuberculosis Ag TB10.4 (EsxH), we observed that IL-12 is the dominant cytokine driving both CD8(+) T cell priming in the lymph node and expansion in the lungs; however, type I IFN and IL-27 have nonredundant roles supporting pulmonary CD8(+) T cell expansion. Thus, IL-12 is a major signal promoting priming in the lymph node, but a multitude of inflammatory signals converge in the lung to promote continued expansion. Furthermore, these cytokines regulate the differentiation and function of CD8(+) T cells during tuberculosis. These data demonstrate distinct and overlapping roles for each of the cytokines examined and underscore the complexity of CD8(+) T cell regulation during tuberculosis.

A Higher Activation Threshold of Memory CD8+ T Cells Has a Fitness Cost That Is Modified by TCR Affinity during Tuberculosis

T cell vaccines against Mycobacterium tuberculosis (Mtb) and other pathogens are based on the principle that memory T cells rapidly generate effector responses upon challenge, leading to pathogen clearance. Despite eliciting a robust memory CD8⁺ T cell response to the immunodominant Mtb antigen TB10.4 (EsxH), we find the increased frequency of TB10.4-specific CD8⁺ T cells conferred by vaccination to be short-lived after Mtb challenge. To compare memory and naïve CD8⁺ T cell function during their response to Mtb, we track their expansions using TB10.4-specific retrogenic CD8⁺ T cells. We find that the primary (naïve) response outnumbers the secondary (memory) response during Mtb challenge, an effect moderated by increased TCR affinity. To determine whether the expansion of polyclonal memory T cells is restrained following Mtb challenge, we used TCRβ deep sequencing to track TB10.4-specific CD8⁺ T cells after vaccination and subsequent challenge in intact mice. Successful memory T cells, defined by their clonal expansion after Mtb challenge, express similar CDR3β sequences suggesting TCR selection by antigen. Thus, both TCR-dependent and -independent factors affect the fitness of memory CD8⁺ responses. The impaired expansion of the majority of memory T cell clonotypes may explain why some TB vaccines have not provided better protection.

CD8⁺ T cells are important for enforcing latency of tuberculosis, and for Mtb control in patients with HIV and low CD4 counts. While vaccines that primarily elicit CD4⁺ T cell responses have had difficulty preventing active pulmonary TB, a TB vaccine that elicits a potent memory CD8⁺ T cells is a logical alternative strategy. Memory T cells are thought to respond more rapidly than the primary (naïve) response. However, by directly comparing naïve and memory TCR retrogenic CD8⁺ T cells specific for the TB10.4 antigen during infection, we observe memory-derived T cells to be less fit than naïve-derived T cells. We relate the reduced fitness of memory CD8⁺ T cells to their lower sensitivity to antigen and show that fitness can be improved by increasing TCR affinity. Using a novel method for tracking CD8⁺ T cells elicited by vaccination during the response to Mtb aerosol challenge in intact mice, we observe the robust expansion of a new primary response as well as clonal selection of the secondary response, likely driven by TCR affinity. We propose that generating memory T cells with high affinities should be a goal of vaccination against TB.

Infection with Mycobacterium tuberculosis induces the Warburg effect in mouse lungs

To elucidate the little-known bioenergetic pathways of host immune cells in tuberculosis, a granulomatous disease caused by the intracellular pathogen Mycobacterium tuberculosis, we characterized infected murine lung tissue by transcriptomic profiling and confocal imaging. Transcriptomic analysis revealed changes of host energy metabolism during the course of infection that are characterized by upregulation of key glycolytic enzymes and transporters for glucose uptake, and downregulation of enzymes participating in the tricarboxylic acid cycle and oxidative phosphorylation. Consistent with elevated glycolysis, we also observed upregulation of a transporter for lactate secretion and a V type H(+) -ATPase involved in cytosolic pH homeostasis. Transcription profiling results were corroborated by immunofluorescence microscopy showing increased expression of key glycolytic enzymes in macrophages and T cells in granulomatous lesions. Moreover, we found increased mRNA and protein levels in macrophages and T cells of hypoxia inducible factor 1 alpha (HIF-1α), the regulatory subunit of HIF-1, a master transcriptional regulator. Thus, our findings suggest that immune cells predominantly utilize aerobic glycolysis in response to M. tuberculosis infection. This bioenergetic shift is similar to the Warburg effect, the metabolic signature of cancer cells. Finding immunometabolic changes during M. tuberculosis infection opens the way to new strategies for immunotherapy against tuberculosis.

T cells and adaptive immunity to Mycobacterium tuberculosis in humans

The adaptive immune response mediated by T cells is critical for control of Mycobacterium tuberculosis (M. tuberculosis) infection in humans. However, the M. tuberculosis antigens and host T-cell responses that are required for an effective adaptive immune response to M. tuberculosis infection are yet to be defined. Here, we review recent findings on CD4(+) and CD8(+) T-cell responses to M. tuberculosis infection and examine the roles of distinct M. tuberculosis-specific T-cell subsets in control of de novo and latent M. tuberculosis infection, and in the evolution of T-cell immunity to M. tuberculosis in response to tuberculosis treatment. In addition, we discuss recent studies that elucidate aspects of M. tuberculosis-specific adaptive immunity during human immunodeficiency virus co-infection and summarize recent findings from vaccine trials that provide insight into effective adaptive immune responses to M. tuberculosis infection.

Immunoevasion and immunosuppression of the macrophage by Mycobacterium tuberculosis

By virtue of their position at the crossroads between the innate and adaptive immune response, macrophages play an essential role in the control of bacterial infections. Paradoxically, macrophages serve as the natural habitat to Mycobacterium tuberculosis (Mtb). Mtb subverts the macrophage's mechanisms of intracellular killing and antigen presentation, leading ultimately to the development of tuberculosis (TB) disease. Here, we describe mechanisms of Mtb uptake by the macrophage and address key macrophage functions that are targeted by Mtb-specific effector molecules enabling this pathogen to circumvent host immune response. The macrophage functions described in this review include fusion between phagosomes and lysosomes, production of reactive oxygen and nitrogen species, antigen presentation and major histocompatibility complex class II expression and trafficking, as well as autophagy and apoptosis. All these are Mtb-targeted key cellular pathways, normally working in concert in the macrophage to recognize, respond, and activate 'proper' immune responses. We further analyze and discuss major molecular interactions between Mtb virulence factors and key macrophage proteins and provide implications for vaccine and drug development.

Immunology studies in non-human primate models of tuberculosis

Non-human primates, primarily macaques, have been used to study tuberculosis for decades. However, in the last 15 years, this model has been refined substantially to allow careful investigations of the immune response and host-pathogen interactions in Mycobacterium tuberculosis infection. Low-dose challenge with fully virulent strains in cynomolgus macaques result in the full clinical spectrum seen in humans, including latent and active infection. Reagents from humans are usually cross-reactive with macaques, further facilitating the use of this model system to study tuberculosis. Finally, macaques develop the spectrum of granuloma types seen in humans, providing a unique opportunity to investigate bacterial and host factors at the local (lung and lymph node) level. Here, we review the past decade of immunology and pathology studies in macaque models of tuberculosis.

Suboptimal Antigen Presentation Contributes to Virulence of Mycobacterium tuberculosis In Vivo

Mycobacterium tuberculosis commonly causes persistent or chronic infection, despite the development of Ag-specific CD4 T cell responses. We hypothesized that M. tuberculosis evades elimination by CD4 T cell responses by manipulating MHC class II Ag presentation and CD4 T cell activation and tested this hypothesis by comparing activation of Ag85B-specific CD4 T cell responses to M. tuberculosis and M. bovis bacillus Calmette-Guérin (BCG) Pasteur in vivo and in vitro. We found that, although M. tuberculosis persists in lungs of immunocompetent mice, M. bovis BCG is cleared, and clearance is T cell dependent. We further discovered that M. tuberculosis-infected macrophages and dendritic cells activate Ag85B-specific CD4 T cells less efficiently and less effectively than do BCG-infected cells, in vivo and in vitro, despite higher production and secretion of Ag85B by M. tuberculosis. During BCG infection, activation of Ag85B-specific CD4 T cells requires fewer infected dendritic cells and fewer Ag-producing bacteria than during M. tuberculosis infection. When dendritic cells containing equivalent numbers of M. tuberculosis or BCG were transferred to mice, BCG-infected cells activated proliferation of more Ag85B-specific CD4 T cells than did M. tuberculosis-infected cells. Differences in Ag85B-specific CD4 T cell activation were attributable to differential Ag presentation rather than differential expression of costimulatory or inhibitory molecules. These data indicate that suboptimal Ag presentation contributes to persistent infection and that limiting Ag presentation is a virulence property of M. tuberculosis.

Association of Genetic Polymorphisms of IFNGR1 with the Risk of Pulmonary Tuberculosis in Zahedan, Southeast Iran

Aim. The present study was undertaken to find out the possible association between interferon-gamma (IFN-γ) receptor 1 (IFNGR1) gene polymorphisms and risk of pulmonary tuberculosis (PTB) in a sample of Iranian population. Methods. Polymorphisms of IFNGR1 rs1327474 (−611 A/G), rs11914 (+189 T/G), rs7749390 (+95 C/T), and rs137854905 (27-bp ins/del) were determined in 173 PTB patients and 164 healthy subjects. Results. Our findings showed that rs11914 TG genotypes decreased the risk of PTB in comparison with TT (OR = 0.36, 95% CI = 0.21–0.62, and p = 0.0002). The rs11914 G allele decreased the risk of PTB compared with T allele (OR = 0.41, 95% CI = 0.25–0.68, and p = 0.0006). IFNGR1 rs7749390 CT genotype decreased the risk of PTB in comparison with CC genotype (OR = 0.55, 95% CI = 0.32–0.95, and p = 0.038). No significant association was found between IFNGR1 rs1327474 A/G polymorphism and risk/protective of PTB. The rs137854905 (27-bp I/D) variant was not polymorphic in our population. Conclusion. Our findings showed that IFNGR1 rs11914 and rs7749390 variants decreased the risk of PTB susceptibility in our population.

Th1 and Th17 Cells in Tuberculosis: Protection, Pathology, and Biomarkers

The outcome of Mycobacterium tuberculosis (Mtb) infection ranges from a complete pathogen clearance through asymptomatic latent infection (LTBI) to active tuberculosis (TB) disease. It is now understood that LTBI and active TB represent a continuous spectrum of states with different degrees of pathogen “activity,” host pathology, and immune reactivity. Therefore, it is important to differentiate LTBI and active TB and identify active TB stages. CD4⁺ T cells play critical role during Mtb infection by mediating protection, contributing to inflammation, and regulating immune response. Th1 and Th17 cells are the main effector CD4⁺ T cells during TB. Th1 cells have been shown to contribute to TB protection by secreting IFN-γ and activating antimycobacterial action in macrophages. Th17 induce neutrophilic inflammation, mediate tissue damage, and thus have been implicated in TB pathology. In recent years new findings have accumulated that alter our view on the role of Th1 and Th17 cells during Mtb infection. This review discusses these new results and how they can be implemented for TB diagnosis and monitoring.

Mycobacterium-Infected Dendritic Cells Disseminate Granulomatous Inflammation

The disappearance and reformation of granulomas during tuberculosis has been described using PET/CT/X-ray in both human clinical settings and animal models, but the mechanisms of granuloma reformation during active disease remains unclear. Granulomas can recruit inflammatory dendritic cells (iDCs) that can regulate local T-cell responses and can carry bacteria into the lymph nodes, which is crucial for generating systemic T-cell responses against mycobacteria. Here, we report that a subset of mycobacterium-infected iDCs are associated with bacteria-specific T-cells in infected tissue, outside the granuloma, and that this results in the formation of new and/or larger multi-focal lesions. Mycobacterium-infected iDCs express less CCR7 and migrate less efficiently compared to the non-infected iDCs, which may support T-cell capture in granulomatous tissue. Capture may reduce antigen availability in the lymph node, thereby decreasing systemic priming, resulting in a possible regulatory loop between systemic T-cell responses and granuloma reformation. T-cell/infected iDCs clusters outside the granuloma can be detected during the acute and chronic phase of BCG and Mtb infection. Our studies suggest a direct role for inflammatory dendritic cells in the dissemination of granulomatous inflammation.

T Cell Responses against Mycobacterial Lipids and Proteins Are Poorly Correlated in South African Adolescents

Human T cells are activated by both peptide and nonpeptide Ags produced by Mycobacterium tuberculosis. T cells recognize cell wall lipids bound to CD1 molecules, but effector functions of CD1-reactive T cells have not been systematically assessed in M. tuberculosis-infected humans. It is also not known how these features correlate with T cell responses to secreted protein Ags. We developed a flow cytometric assay to profile CD1-restricted T cells ex vivo and assessed T cell responses to five cell wall lipid Ags in a cross-sectional study of 19 M. tuberculosis-infected and 22 M. tuberculosis-uninfected South African adolescents. We analyzed six T cell functions using a recently developed computational approach for flow cytometry data in high dimensions. We compared these data with T cell responses to five protein Ags in the same cohort. We show that CD1b-restricted T cells producing antimycobacterial cytokines IFN-γ and TNF-α are detectable ex vivo in CD4(+), CD8(+), and CD4(-)CD8(-) T cell subsets. Glucose monomycolate was immunodominant among lipid Ags tested, and polyfunctional CD4 T cells specific for this lipid simultaneously expressed CD40L, IFN-γ, IL-2, and TNF-α. Lipid-reactive CD4(+) T cells were detectable at frequencies of 0.001-0.01%, and this did not differ by M. tuberculosis infection status. Finally, CD4 T cell responses to lipids were poorly correlated with CD4 T cell responses to proteins (Spearman rank correlation -0.01; p = 0.95). These results highlight the functional diversity of CD1-restricted T cells circulating in peripheral blood as well as the complementary nature of T cell responses to mycobacterial lipids and proteins. Our approach enables further population-based studies of lipid-specific T cell responses during natural infection and vaccination.

Complexity and Controversies over the Cytokine Profiles of T Helper Cell Subpopulations in Tuberculosis

Tuberculosis (TB) is a contagious infectious disease caused by the TB-causing bacillus Mycobacterium tuberculosis and is considered a public health problem with enormous social impact. Disease progression is determined mainly by the balance between the microorganism and the host defense systems. Although the immune system controls the infection, this control does not necessarily lead to sterilization. Over recent decades, the patterns of CD4+ T cell responses have been studied with a goal of complete understanding of the immunological mechanisms involved in the maintenance of latent or active tuberculosis infection and of the clinical cure after treatment. Conflicting results have been suggested over the years, particularly in studies comparing experimental models and human disease. In recent years, in addition to Th1, Th2, and Th17 profiles, new standards of cellular immune responses, such as Th9, Th22, and IFN-γ-IL-10 double-producing Th cells, discussed here, have also been described. Additionally, many new roles and cellular sources have been described for IL-10, demonstrating a critical role for this cytokine as regulatory, rather than merely pathogenic cytokine, involved in the establishment of chronic latent infection, in the clinical cure after treatment and in keeping antibacillary effector mechanisms active to prevent immune-mediated damage.

Correlates of Vaccine-Induced Protection against Mycobacterium tuberculosis Revealed in Comparative Analyses of Lymphocyte Populations

A critical hindrance to the development of a novel vaccine against Mycobacterium tuberculosis is a lack of understanding of protective correlates of immunity and of host factors involved in a successful adaptive immune response. Studies from our group and others have used a mouse-based in vitro model system to assess correlates of protection. Here, using this coculture system and a panel of whole-cell vaccines with varied efficacy, we developed a comprehensive approach to understand correlates of protection. We compared the gene and protein expression profiles of vaccine-generated immune peripheral blood lymphocytes (PBLs) to the profiles found in immune splenocytes. PBLs not only represent a clinically relevant cell population, but comparing the expression in these populations gave insight into compartmentally specific mechanisms of protection. Additionally, we performed a direct comparison of host responses induced when immune cells were cocultured with either the vaccine strain Mycobacterium bovis BCG or virulent M. tuberculosis. These comparisons revealed host-specific and bacterium-specific factors involved in protection against virulent M. tuberculosis. Most significantly, we identified a set of 13 core molecules induced in the most protective vaccines under all of the conditions tested. Further validation of this panel of mediators as a predictor of vaccine efficacy will facilitate vaccine development, and determining how each promotes adaptive immunity will advance our understanding of antimycobacterial immune responses.

Cell-to-cell transfer of M. tuberculosis antigens optimizes CD4 T cell priming

During Mycobacterium tuberculosis and other respiratory infections, optimal T cell activation requires pathogen transport from the lung to a local draining lymph node (LN). However, the infected inflammatory monocyte-derived dendritic cells (DCs) that transport M. tuberculosis to the local lymph node are relatively inefficient at activating CD4 T cells, possibly due to bacterial inhibition of antigen presentation. We found that infected migratory DCs release M. tuberculosis antigens as soluble, unprocessed proteins for uptake and presentation by uninfected resident lymph node DCs. This transfer of bacterial proteins from migratory to local DCs results in optimal priming of antigen-specific CD4 T cells, which are essential in controlling tuberculosis. Additionally, this mechanism does not involve transfer of the whole bacterium and is distinct from apoptosis or exosome shedding. These findings reveal a mechanism that bypasses pathogen inhibition of antigen presentation by infected cells and generates CD4 T cell responses that control the infection.