Apoptotic vesicles crossprime CD8 T cells and protect against tuberculosis

Immune-complex mimics as a molecular platform for adjuvant-free vaccine delivery

Protein-based vaccine development faces the difficult challenge of finding robust yet non-toxic adjuvants suitable for humans. Here, using a molecular engineering approach, we have developed a molecular platform for generating self-adjuvanting immunogens that do not depend on exogenous adjuvants for induction of immune responses. These are based on the concept of Immune Complex Mimics (ICM), structures that are formed between an oligomeric antigen and a monoclonal antibody (mAb) to that antigen. In this way, the roles of antigens and antibodies within the structure of immune complexes are reversed, so that a single monoclonal antibody, rather than polyclonal sera or expensive mAb cocktails can be used. We tested this approach in the context of Mycobacterium tuberculosis (MTB) infection by linking the highly immunogenic and potentially protective Ag85B with the oligomeric Acr (alpha crystallin, HspX) antigen. When combined with an anti-Acr monoclonal antibody, the fusion protein formed ICM which bound to C1q component of the complement system and were readily taken up by antigen-presenting cells in vitro. ICM induced a strong Th1/Th2 mixed type antibody response, which was comparable to cholera toxin adjuvanted antigen, but only moderate levels of T cell proliferation and IFN-γ secretion. Unfortunately, the systemic administration of ICM did not confer statistically significant protection against intranasal MTB challenge, although a small BCG-boosting effect was observed. We conclude that ICM are capable of inducing strong humoral responses to incorporated antigens and may be a suitable vaccination approach for pathogens other than MTB, where antibody-based immunity may play a more protective role.

Clonal expansions of CD8+ T cells with IL-10 secreting capacity occur during chronic Mycobacterium tuberculosis infection

The exact role of CD8⁺ T cells during Mycobacterium tuberculosis (Mtb) infection has been heavily debated, yet it is generally accepted that CD8⁺ T cells contribute to protection against Mtb. In this study, however, we show that the Mtb-susceptible CBA/J mouse strain accumulates large numbers of CD8⁺ T cells in the lung as infection progresses, and that these cells display a dysfunctional and immunosuppressive phenotype (PD-1⁺, Tim-3⁺, CD122⁺). CD8⁺ T cell expansions from the lungs of Mtb-infected CBA/J mice were also capable of secreting the immunosuppressive cytokine interleukin-10 (IL-10), although in vivo CD8⁺ T cell depletion did not significantly alter Mtb burden. Further analysis revealed that pulmonary CD8⁺ T cells from Mtb-infected CBA/J mice were clonally expanded, preferentially expressing T cell receptor (TcR) Vβ chain 8 (8.2, 8.3) or Vβ 14. Although Vβ8⁺ CD8⁺ T cells were responsible for the majority of IL-10 production, in vivo depletion of Vβ8⁺ did not significantly change the outcome of Mtb infection, which we hypothesize was a consequence of their dual IL-10/IFN-γ secreting profiles. Our data demonstrate that IL-10-secreting CD8⁺ T cells can arise during chronic Mtb infection, although the significance of this T cell population in tuberculosis pathogenesis remains unclear.

Revisiting the Rich's formula: an update about granulomas in human tuberculosis

The formula proposed by Rich in 1951 explained the formation in a tuberculous lesion in a period that was unknown cellular functions, cytokines and other immunological aspects involved in granuloma formation by tuberculosis; its components are assembled conceptually to explain the pathogenic mechanisms involved in the granulomatous lesion in tuberculosis. In this manuscript, we report an update of Rich's formula based on the new and old concepts about pathogenic mechanisms involved in the granulomatous lesion in tuberculosis. Current knowledge allows us to conclude that the balance between the characteristics of the bacillus and host protective response is necessary to indicate the outcome of pathogenesis, infection or active disease and the necrosis degree of the tuberculosis lesion.

Within the Enemy's Camp: contribution of the granuloma to the dissemination, persistence and transmission of Mycobacterium tuberculosis

Pulmonary tuberculosis, caused by Mycobacterium tuberculosis (M.tb) represents a leading global health concern, with 8.7 million newly emerging cases, and 1.4 million reported deaths annually. Despite an estimated one third of the world's population being infected, relatively few infected individuals ever develop active clinical disease. The ability of the host to remain latently infected while preventing disease is thought to be due to the generation of a robust type 1 immune response in the lung, capable of controlling, but not clearing, M.tb. A key feature of the type 1 immune response to M.tb is the formation of immune cellular aggregates termed granuloma. The granuloma structure has long been considered a hallmark of host's protective response toward M.tb. Historically, a correlative relationship between granuloma formation/maintenance and bacterial control has been seen in models where disrupted granuloma formation or structure was found to be fatal. Despite this established relationship much about the granuloma's role in M.tb immunity remains unknown. Recent publications suggest that the granuloma actually aids the persistence of M.tb and that the development of a necrotic granuloma is essential to person-to-person transmission. Our group and others have recently demonstrated that enclosed within the granuloma is a population of immunologically altered antigen-presenting cells and T lymphocyte populations. Of note, the ability of these populations to produce type 1 cytokines such as interferon-gamma, and bactericidal products including nitric oxide, are significantly reduced, while remaining competent to produce high levels immunosuppressive interleukin-10. These observations indicate that although the chronic granuloma represents a highly unique environment, it is more similar to that of a tumor than an active site of bacterial control. In this review we will explore what is known about this unique environment and its contribution to the persistence of M.tb.

"With a little help from my friends": efferocytosis as an antimicrobial mechanism

The uptake of apoptotic cells by phagocytes is defined as efferocytosis. In this issue of Cell Host & Microbe, Martin et al. (2012) and Yang et al. (2012) report that macrophage- and neutrophil-mediated efferocytosis of apoptotic cells containing mycobacteria is an innate antibacterial effector mechanism.

Dying to live: how the death modality of the infected macrophage affects immunity to tuberculosis

Virulent Mycobacterium tuberculosis (Mtb) inhibits apoptosis and triggers necrosis of host macrophages to evade innate delay in the initiation of adaptive immunity. Necrosis is a mechanism used by bacteria to exit macrophage, evade the host defenses, and disseminate while apoptosis is associated with diminished pathogen viability. We have recently demonstrated that eicosanoids regulate cell death program of either human or murine macrophages infected with Mtb. We have defined prostaglandin E2 (PGE2) as a pro-apoptotic host lipid mediator which protects against necrosis. In contrast, lipoxin A4 (LXA4) is a pro-necrotic lipid mediator which suppresses PGE2 synthesis, resulting in mitochondrial damage and inhibition of plasma membrane repair mechanisms; this ultimately leads to the induction of necrosis. Thus, the balance between PGE2 and LXA4 determines whether Mtb-infected macrophages undergo apoptosis or necrosis and this balance determines the outcome of infection.

Characterizing virulence-specific perturbations in the mitochondrial function of macrophages infected with Mycobacterium tuberculosis

To probe how the pathogen Mycobacterium tuberculosis controls host cellular death pathways, we compared mitochondrial responses in human macrophages infected either with the avirulent mycobacterial strain H37Ra, or its virulent counterpart H37Rv. Following H37Ra infection, induction of the apoptotic response was foreshadowed by the early suppression of stress-induced mitochondrial activity. In contrast, mitochondria in H37Rv-infected cells displayed robust activity with increased membrane potential and ATP synthesis. An examination of the mitochondrial proteome revealed that attenuation of mitochondrial function was also coupled with the vigorous activation of bactericidal mechanisms in H37Ra-infected cells. In contrast, augmentation of mitochondrial activity by H37Rv enabled manipulation of host cellular mechanisms to inhibit apoptosis on the one hand, while ensuring fortification against anti-microbial pathways on the other. These results thus provide novel insights into the molecular interplay that facilitates adaptation of virulent mycobacteria within the hostile intracellular milieu of the host macrophage.

Antigen-specific CD8(+) T cells and protective immunity to tuberculosis

The continuing HIV/AIDS epidemic and the spread of multi-drug resistant Mycobacterium tuberculosis has led to the perpetuation of the worldwide tuberculosis epidemic. While M. bovis BCG is widely used as a vaccine, it lacks efficacy in preventing pulmonary tuberculosis in adults [1]. To combat this ongoing scourge, vaccine development for tuberculosis is a global priority. Most infected individuals develop long-lived protective immunity, which controls and contains M. tuberculosis in a T cell-dependent manner. An effective T cells response determines whether the infection resolves or develops into clinically evident disease. Consequently, there is great interest in determining which T cells subsets mediate anti-mycobacterial immunity, delineating their effector functions, and evaluating whether vaccination can elicit these T cells subsets and induce protective immunity. CD4(+) T cells are critical for resistance to M. tuberculosis in both humans and rodent models. CD4(+) T cells are required to control the initial infection as well as to prevent recrudescence in both humans and mice [2]. While it is generally accepted that class II MHC-restricted CD4(+) T cells are essential for immunity to tuberculosis, M. tuberculosis infection elicits CD8(+) T cells responses in both people and in experimental animals. CD8(+) T cells are also recruited to the lung during M. tuberculosis infection and are found in the granulomas of infected people. Thus, how CD8(+) T cells contribute to overall immunity to tuberculosis and whether antigens recognized by CD8(+) T cells would enhance the efficacy of vaccine strategies continue to be important questions.

Vaccination against tuberculosis: how can we better BCG?

Tuberculosis remains one of the most significant human diseases of the developing world, accounting for 3800 worldwide deaths per day. Although we currently have a vaccine for tuberculosis, BCG, this is insufficient at protecting from adult pulmonary tuberculosis in the parts of the world where a good vaccine is most needed. This has prompted the search for new vaccination strategies that can protect better than BCG, or can boost BCG-induced immunity. We discuss these subjects in line with what is known of the immune responses to BCG and Mycobacterium tuberculosis - the etiological agent of the disease, as well as the particular difficulties facing development of new vaccines against tuberculosis. A greater understanding of the factors constituting optimal protection against Mycobacterium tuberculosis infection, as well as which pathogenic factors facilitate active disease, will accelerate the delivery of safe vaccines able to restrict active tuberculosis and thus impede contagion.

Microparticles from mycobacteria-infected macrophages promote inflammation and cellular migration

Mycobacterium tuberculosis infection is characterized by a strong inflammatory response whereby a few infected macrophages within the granuloma induce sustained cellular accumulation. The mechanisms coordinating this response are poorly characterized. We hypothesized that microparticles (MPs), which are submicron, plasma membrane-derived vesicles released by cells under both physiological and pathological conditions, are involved in this process. Aerosol infection of mice with M. tuberculosis increased CD45(+) MPs in the blood after 4 wk of infection, and in vitro infection of human and murine macrophages with mycobacteria enhanced MP release. MPs derived from mycobacteria-infected macrophages were proinflammatory, and when injected into uninfected mice they induced significant neutrophil, macrophage, and dendritic cell recruitment to the injection site. When incubated with naive macrophages, these MPs enhanced proinflammatory cytokine and chemokine release, and they aided in the disruption of the integrity of a respiratory epithelial cell monolayer, providing a mechanism for the egress of cells to the site of M. tuberculosis infection in the lung. In addition, MPs colocalized with the endocytic recycling marker Rab11a within macrophages, and this association increased when the MPs were isolated from mycobacteria-infected cells. M. tuberculosis-derived MPs also carried mycobacterial Ag and were able to activate M. tuberculosis-specific CD4(+) T cells in vivo and in vitro in a dendritic cell-dependent manner. Collectively, these data identify an unrecognized role for MPs in host response against M. tuberculosis by promoting inflammation, intercellular communication, and cell migration.

The role of dendritic cells in Mycobacterium tuberculosis infection

The immune response against Mycobacterium tuberculosis is multifactorial, involving a network of innate and adaptive immune responses. Characterization of the immune response, a clear understanding of the dynamics and interplay of different arms of the immune response are critical to allow the development of better tools for combating tuberculosis. Dendritic cells (DCs) are one of the key cells in bridging innate and adaptive immune response through their significant role in capturing, processing and presenting antigens. The outcome of interaction of M. tuberculosis with DCs is not fully understood and the available reports are contradictory were some findings reported that DCs strengthen the cellular immune response against mycobacterium infection whereas others reported M. tuberculosis impairs the function of DCs were infected DCs are poor stimulators of M. tuberculosis Ag-specific CD4 T cells. Other studies showed that the outcome depends on M. tuberculosis strain type and type of receptor on DCs during recognition. In this review I shall highlight the recent findings in the outcome of interaction of Mycobacterium tuberculosis with DCs.

The balance of apoptotic and necrotic cell death in Mycobacterium tuberculosis infected macrophages is not dependent on bacterial virulence

Background

An important mechanism of Mycobacterium tuberculosis pathogenesis is the ability to control cell death pathways in infected macrophages: apoptotic cell death is bactericidal, whereas necrotic cell death may facilitate bacterial dissemination and transmission.

Methods

We examine M.tuberculosis control of spontaneous and chemically induced macrophage cell death using automated confocal fluorescence microscopy, image analysis, flow cytometry, plate-reader based vitality assays, and M.tuberculosis strains including H37Rv, and isogenic virulent and avirulent strains of the Beijing lineage isolate GC1237.

Results

We show that bacterial virulence influences the dynamics of caspase activation and the total level of cytotoxicity. We show that the powerful ability of M.tuberculosis to inhibit exogenously stimulated apoptosis is abrogated by loss of virulence. However, loss of virulence did not influence the balance of macrophage apoptosis and necrosis – both virulent and avirulent isogenic strains of GC1237 induced predominantly necrotic cell death compared to H37Rv which induced a higher relative level of apoptosis.

Conclusions

This reveals that macrophage necrosis and apoptosis are independently regulated during M. tuberculosis infection of macrophages. Virulence affects the level of host cell death and ability to inhibit apoptosis but other strain-specific characteristics influence the ultimate mode of host cell death and alter the balance of apoptosis and necrosis.

The recombinant tuberculosis vaccine rBCG ΔureC::hly(+) induces apoptotic vesicles for improved priming of CD4(+) and CD8(+) T cells

BACKGROUND

The recombinant BCG ΔureC::hly(+) (rBCG) vaccine candidate is more efficient than parental BCG (pBCG) against tuberculosis (TB) in preclinical models. Evidence exists for superior CD4 and CD8 T cell stimulation. Although the responsible immune mechanisms are incompletely understood, crosspriming of CD8 T cells has been proposed as a major mechanism underlying better protection of rBCG over pBCG. The present study investigates the role of apoptotic vesicles from pBCG- and rBCG-infected macrophages in crosspriming.

METHODS

Apoptotic vesicles were isolated from pBCG- and rBCG-infected mouse macrophages. The priming potential of the isolated vesicles was evaluated in terms of dendritic cell activation and specific T cell stimulation.

RESULTS

Apoptotic vesicles from both pBCG- and rBCG-infected macrophages activated dendritic cells but to a different degree. Overall, rBCG-infected apoptotic vesicles induced more profound CD4 and CD8 T cell responses as compared to pBCG.

CONCLUSIONS

These data support the notion that the improved vaccine efficacy of rBCG rests on enhanced crosspriming as a consequence of stronger apoptosis.

Biomarkers of latent TB infection

For the last 100 years, the tuberculin skin test (TST) has been the only diagnostic tool available for latent TB infection (LTBI) and no biomarker per se is available to diagnose the presence of LTBI. With the introduction of M. tuberculosis-specific IFN-gamma release assays (IGRAs), a new area of in vitro immunodiagnostic tests for LTBI based on biomarker readout has become a reality. In this review, we discuss existing evidence on the clinical usefulness of IGRAs and the indefinite number of potential new biomarkers that can be used to improve diagnosis of latent TB infection. We also present early data suggesting that the monocyte-derived chemokine inducible protein-10 may be useful as a novel biomarker for the immunodiagnosis of latent TB infection.

Differential induction of macrophage cell death by antigens of a clustered and a non-clustered multidrug-resistant Mycobacterium tuberculosis strain from Haarlem family

Some multidrug-resistant (MDR) Mycobacterium tuberculosis (Mtb) genotypes are the cause of large outbreaks, including strain M identified in Argentina. In contrast, its kin strain 410 has only caused a single case to date. Cell wall antigens from Mtb were associated with the modulation of macrophage (MΦ) cell death, and the ability to inhibit of MΦ apoptosis is considered a virulence mechanism. In this study, the ability these two clinical isolates with divergent epidemiology to induce MΦ cell death was evaluated using whole inactivated bacteria. We showed that gamma-irradiated (I-) strains induced MΦ necrosis, the strongest inducer being I-410. Cell death biased towards apoptosis with the heat-killed (hk) strains, both hk-MDR strains being poorer inducers of MΦ apoptosis than was H37Rv. These effects were partly due to their ability to induce anti-apoptotic mechanisms which were not related to the lack of tumor necrosis factor alpha induction or a compensatory effect of interleukin-10. The most noticeable difference between strain M and strain 410 was the ability shown by hk-M to interfere with apoptosis induced by hk-H37Rv. Thus, heat-stable and heat-labile antigens from these epidemiologically divergent Mtb strains differ in their ability to manipulate MΦ death.

Mycobacterium tuberculosis inhibits neutrophil apoptosis, leading to delayed activation of naive CD4 T cells

Mycobacterium tuberculosis promotes its replication by inhibiting the apoptosis of infected macrophages. A proapoptotic M. tuberculosis mutant lacking nuoG, a subunit of the type I NADH dehydrogenase complex, exhibits attenuated growth in vivo, indicating that this virulence mechanism is essential. We show that M. tuberculosis also suppresses neutrophil apoptosis. Compared to wild-type, the nuoG mutant spread to a larger number of lung phagocytic cells. Consistent with the shorter lifespan of infected neutrophils, infection with the nuoG mutant resulted in fewer bacteria per infected neutrophil, accelerated bacterial acquisition by dendritic cells, earlier trafficking of these dendritic cells to lymph nodes, and faster CD4 T cell priming. Neutrophil depletion abrogated accelerated CD4 T cell priming by the nuoG mutant, suggesting that inhibiting neutrophil apoptosis delays adaptive immunity in tuberculosis. Thus, pathogen modulation of apoptosis is beneficial at multiple levels, and enhancing phagocyte apoptosis promotes CD4 as well as CD8 T cell responses.

Vaccines against tuberculosis: where are we and where do we need to go?

In this review we discuss recent progress in the development, testing, and clinical evaluation of new vaccines against tuberculosis (TB). Over the last 20 years, tremendous progress has been made in TB vaccine research and development: from a pipeline virtually empty of new TB candidate vaccines in the early 1990s, to an era in which a dozen novel TB vaccine candidates have been and are being evaluated in human clinical trials. In addition, innovative approaches are being pursued to further improve existing vaccines, as well as discover new ones. Thus, there is good reason for optimism in the field of TB vaccines that it will be possible to develop better vaccines than BCG, which is still the only vaccine available against TB.

Tuberculosis vaccine development: strength lies in tenacity

The past decade has witnessed a tremendous increase in the development of novel vaccines against tuberculosis (TB). In mice, each of these vaccine candidates stimulates an immune response that reduces the bacillary load, reflecting control but not sterilization of infection. Yet, the immune mechanisms underlying vaccine efficacy are only partially understood. In parallel to clinical assessment of current candidates, the next generation of vaccine candidates still needs to be developed. This requires basic research on how to induce the most efficacious immune response. Equally important is the dissection of immune responses in patients, latently infected healthy individuals, and participants of clinical vaccine trials. Amalgamation of this information will foster the way towards more efficacious vaccination strategies that not only prevent disease, but prevent or abolish infection.

YopJ-induced caspase-1 activation in Yersinia-infected macrophages: independent of apoptosis, linked to necrosis, dispensable for innate host defense

Yersinia outer protein J (YopJ) is a type III secretion system (T3SS) effector of pathogenic Yersinia (Yersinia pestis, Yersinia enterocolitica and Yersinia pseudotuberculosis) that is secreted into host cells. YopJ inhibits survival response pathways in macrophages, causing cell death. Allelic variation of YopJ is responsible for differential cytotoxicity in Yersinia strains. YopJ isoforms in Y. enterocolitica O:8 (YopP) and Y. pestis KIM (YopJ^KIM) strains have high cytotoxic activity. In addition, YopJ^KIM-induced macrophage death is associated with caspase-1 activation and interleukin-1β (IL-1β secretion. Here, the mechanism of YopJ^KIM-induced cell death, caspase-1 activation, and IL-1β secretion in primary murine macrophages was examined. Caspase-3/7 activity was low and the caspase-3 substrate poly (ADP-ribose) polymerase (PARP) was not cleaved in Y. pestis KIM5-infected macrophages. In addition, cytotoxicity and IL-1β secretion were not reduced in the presence of a caspase-8 inhibitor, or in B-cell lymphoma 2 (Bcl-2)-associated X protein (Bax)/Bcl-2 homologous antagonist/killer (Bak) knockout macrophages, showing that YopJ^KIM-mediated cell death and caspase-1 activation occur independent of mitochondrial-directed apoptosis. KIM5-infected macrophages released high mobility group protein B1 (HMGB1), a marker of necrosis, and microscopic analysis revealed that necrotic cells contained active caspase-1, indicating that caspase-1 activation is associated with necrosis. Inhibitor studies showed that receptor interacting protein 1 (RIP1) kinase and reactive oxygen species (ROS) were not required for cytotoxicity or IL-β release in KIM5-infected macrophages. IL-1β secretion was reduced in the presence of cathepsin B inhibitors, suggesting that activation of caspase-1 requires cathepsin B activity. Ectopically-expressed YopP caused higher cytotoxicity and secretion of IL-1β in Y. pseudotuberculosis-infected macrophages than YopJ^KIM. Wild-type and congenic caspase 1 knockout C57BL/6 mice were equally susceptible to lethal infection with Y. pseudotuberculosis ectopically expressing YopP. These data suggest that YopJ-induced caspase-1 activation in Yersinia-infected macrophages is a downstream consequence of necrotic cell death and is dispensable for innate host resistance to a strain with enhanced cytotoxicity.

The ambiguity in immunology

In the present article, we discuss the various ambiguous aspects of the immune system that render this complex biological network so highly flexible and able to defend the host from different external invaders. This ambiguity stems mainly from the property of the immune system to be both protective and harmful. Immunity cannot be fully protective without producing a certain degree of damage (immunopathology) to the host. The balance between protection and tissue damage is, therefore, critical for the establishment of immune homeostasis and protection. In this review, we will consider as ambiguous, various immunological tactics including: (a) the opposing functions driving immune responses, immune-regulation, and contra-regulation, as well as (b) the phenomenon of chronic immune activation as a result of a continuous cross-presentation of apoptotic T cells by dendritic cells. All these plans participate principally to maintain a state of chronic low-level inflammation during persisting infections, and ultimately to favor the species survival.

CD8(+) T cells in leishmania infections: friends or foes?

Host protection against several intracellular pathogens requires the induction of CD8⁺ T cell responses. CD8⁺ T cells are potent effector cells that can produce high amounts of pro-inflammatory cytokines and kill infected target cells efficiently. However, a protective role for CD8⁺ T cells during Leishmania infections is still controversial and largely depends on the infection model. In this review, we discuss the role of CD8⁺ T cells during various types of Leishmania infections, following vaccination, and as potential immunotherapeutic targets.

Adaptive immunity to fungi

Only a handful of the more than 100,000 fungal species on our planet cause disease in humans, yet the number of life-threatening fungal infections in patients has recently skyrocketed as a result of advances in medical care that often suppress immunity intensely. This emerging crisis has created pressing needs to clarify immune defense mechanisms against fungi, with the ultimate goal of therapeutic applications. Herein, we describe recent insights in understanding the mammalian immune defenses deployed against pathogenic fungi. The review focuses on adaptive immune responses to the major medically important fungi and emphasizes how dendritic cells and subsets in various anatomic compartments respond to fungi, recognize their molecular patterns, and signal responses that nurture and shape the differentiation of T cell subsets and B cells. Also emphasized is how the latter deploy effector and regulatory mechanisms that eliminate these nasty invaders while also constraining collateral damage to vital tissue.

Cell death in the maintenance and abrogation of tolerance: the five Ws of dying cells

The mammalian immune system continually faces death in the form of its own dead and dying cells that arise during normal tissue turnover, infections, cellular damage, and cancer. Complex decisions must then be made that will permit a protective response to pathogens, while at the same time destroying tumors but not attacking vital systems of the host that could lead to autoimmunity. By using an investigative technique termed the five Ws (who, what, when, where, and why), we will examine how the immune system responds to antigens generated via cell death. This analysis will give us a better understanding of the molecular differences fundamental to tolerogenic or immunogenic cell death, the cells that sense and react to the dead cells, and the consequences of these fundamental elements on the maintenance or abrogation of tolerance.

Optimization of inhaled therapies for tuberculosis: the role of macrophages and dendritic cells

Inhaled therapies in the form of drugs or vaccines for tuberculosis treatment were reported about a decade ago. Experts around the world met to discuss the scientific progress in inhaled therapies at the international symposium "Optimization of inhaled Tuberculosis therapies and implications for host-pathogen interactions" held in New Delhi, India on November 3-5, 2009. The meeting was organized by the Central Drug Research Institute (CDRI) Lucknow, India. The lung is the main route for infection with Mycobacterium tuberculosis bacilli and the primary site of reactivation of latent disease. The only available vaccine BCG is relatively ineffective at preventing tuberculosis disease and current therapy requires prolonged treatment with drugs which results in low patient compliance. Consequently, there is a need to design new vaccines and therapies for this disease. Recently there has been increased interest in the development of inhaled formulations to deliver anti-mycobacterial drugs and vaccines directly to the lung and many of these therapies are designed to target lung macrophages and dendritic cells. However, the development of effective inhaled therapies requires an understanding of the unique function and immunosuppressive environment of the lung which is driven, in part, by alveolar macrophages and dendritic cells. In this review, we will discuss the role of alveolar macrophages and dendritic cells in the host immune response to M. tuberculosis infection and the ways in which inhaled therapies might enhance the anti-microbial response of phagocytes and boost pulmonary immunity.

Mitogen-activated protein kinases mediate the production of B-cell lymphoma 2 protein by Mycobacterium tuberculosis in monocytes

Changes in the levels of antiapoptotic protein B-cell lymphoma 2 (Bcl-2) protein has been reported in murine and human tuberculosis. We investigated the role of mitogen-activated protein kinase pathways in the production of Bcl-2 protein in THP-1 human monocytes infected with Mycobacterium tuberculosis H37Rv and H37Ra. Analysis of phosphorylation profiles of mitogen-activated protein kinase kinase-1, extracellular-signal regulated kinase 1/2, mitogen-activated protein kinase kinase 3/6, and p38 mitogen-activated protein kinase; B-cell lymphoma 2 kinetics; and tumor necrosis factor-α (TNF-α) secretion levels showed variation between the two strains. Mycobacterium tuberculosis H37Rv induced higher Bcl-2 and lower TNF-α levels, whereas H37Ra the reverse. The strains also differed in their usage of CD14 and human leukocyte antigen-DR receptors in mediating extracellular-signal regulated kinase 1/2 and p38 mitogen-activated protein kinase activation. Mycobacterium tuberculosis H37Rv- and H37Ra-induced Bcl-2 production was reduced by specific inhibitors of mitogen-activated protein kinase kinase-1 (PD98059) and p38 (SB203580), but increased by nuclear factor κB (NF-κB) inhibitor (BAY 11-7082). TNF-α production by both strains was reduced in the presence of specific inhibitors of mitogen-activated protein kinase kinase-1 (PD98059), p38 (SB203580), and NF-κB (BAY 11-7082). Furthermore, inhibition of NF-κB was accompanied by an increase in strain-induced extracellular-signal regulated kinase 1/2 phosphorylation. Collectively, these results indicate for the first time that the production of Bcl-2 and TNF-α by M. tuberculosis H37Rv/H37Ra-infected THP-1 human monocytes is mediated through mitogen-activated protein kinases and NF-κB.

Mycobacterium tuberculosis infection induces non-apoptotic cell death of human dendritic cells

Background

Dendritic cells (DCs) connect innate and adaptive immunity, and are necessary for an efficient CD4⁺ and CD8⁺ T cell response after infection with Mycobacterium tuberculosis (Mtb). We previously described the macrophage cell death response to Mtb infection. To investigate the effect of Mtb infection on human DC viability, we infected these phagocytes with different strains of Mtb and assessed viability, as well as DNA fragmentation and caspase activity. In parallel studies, we assessed the impact of infection on DC maturation, cytokine production and bacillary survival.

Results

Infection of DCs with live Mtb (H37Ra or H37Rv) led to cell death. This cell death proceeded in a caspase-independent manner, and without nuclear fragmentation. In fact, substrate assays demonstrated that Mtb H37Ra-induced cell death progressed without the activation of the executioner caspases, 3/7. Although the death pathway was triggered after infection, the DCs successfully underwent maturation and produced a host-protective cytokine profile. Finally, dying infected DCs were permissive for Mtb H37Ra growth.

Conclusions

Human DCs undergo cell death after infection with live Mtb, in a manner that does not involve executioner caspases, and results in no mycobactericidal effect. Nonetheless, the DC maturation and cytokine profile observed suggests that the infected cells can still contribute to TB immunity.

Immunization with apoptotic phagocytes containing Histoplasma capsulatum activates functional CD8(+) T cells to protect against histoplasmosis

We have previously revealed the protective role of CD8(+) T cells in host defense against Histoplasma capsulatum in animals with CD4(+) T cell deficiency and demonstrated that sensitized CD8(+) T cells are restimulated in vitro by dendritic cells that have ingested apoptotic macrophage-associated Histoplasma antigen. Here we show that immunization with apoptotic phagocytes containing heat-killed Histoplasma efficiently activated functional CD8(+) T cells whose contribution was equal to that of CD4(+) T cells in protection against Histoplasma challenge. Inhibition of macrophage apoptosis due to inducible nitric oxide synthase (iNOS) deficiency or by caspase inhibitor treatment dampened the CD8(+) T cell but not the CD4(+) T cell response to pulmonary Histoplasma infection. In mice subcutaneously immunized with viable Histoplasma yeasts whose CD8(+) T cells are protective against Histoplasma challenge, there was heavy granulocyte and macrophage infiltration and the infiltrating cells became apoptotic. In mice subcutaneously immunized with carboxyfluorescein diacetate succinimidyl ester (CFSE)-labeled apoptotic macrophages containing heat-killed Histoplasma, the CFSE-labeled macrophage material was found to localize within dendritic cells in the draining lymph node. Moreover, depleting dendritic cells in immunized CD11c-DTR mice significantly reduced CD8(+) T cell activation. Taken together, our results revealed that phagocyte apoptosis in the Histoplasma-infected host is associated with CD8(+) T cell activation and that immunization with apoptotic phagocytes containing heat-killed Histoplasma efficiently evokes a protective CD8(+) T cell response. These results suggest that employing apoptotic phagocytes as antigen donor cells is a viable approach for the development of efficacious vaccines to elicit strong CD8(+) T cell as well as CD4(+) T cell responses to Histoplasma infection.

Polymer-mediated DNA vaccine delivery via bystander cells requires a proper balance between transfection efficiency and cytotoxicity

Direct targeting of dendritic cells is an ideal goal for DNA vaccine delivery in order to stimulate both arms of the immune system. However, dendritic cells are often difficult to transfect using nonviral polyplexes. Here we show that transfecting bystander cells such as fibroblasts with PEI/DNA complexes leads to efficient cross-presentation of a model antigen by dendritic cells and subsequent activation of antigen-specific CD8(+) T cells. Maturation of dendritic cells is also stimulated after co-culture with transfected fibroblasts. Such outcomes depend on a proper balance between transfection efficiency and polyplex-induced cytotoxicity in the fibroblasts. In fact, substantial cytotoxicity is desirable and even necessary for cross-presentation and cross-priming of T cells. This study illustrates a new pathway of polymer-based DNA vaccine delivery via bystander cells without direct targeting of antigen-presenting cells and highlights the importance of exploiting polymer-induced cytotoxicity for the benefit of immune activation.

Functional capacity of Mycobacterium tuberculosis-specific T cell responses in humans is associated with mycobacterial load

High Ag load in chronic viral infections has been associated with impairment of Ag-specific T cell responses; however, the relationship between Ag load in chronic Mycobacterium tuberculosis infection and functional capacity of M. tuberculosis-specific T cells in humans is not clear. We compared M. tuberculosis-specific T cell-associated cytokine production and proliferative capacity in peripheral blood from adults with progressively higher mycobacterial loads-that is, persons with latent M. tuberculosis infection (LTBI), with smear-negative pulmonary tuberculosis (TB), and smear-positive TB. Patients with smear-positive TB had decreased polyfunctional IFN-γ(+)IL-2(+)TNF-α(+) and IL-2-producing specific CD4 T cells and increased TNF-α single-positive cells, when compared with smear-negative TB and LTBI. TB patients also had increased frequencies of M. tuberculosis-specific CD8 T cells, compared with LTBI. M. tuberculosis-specific CD4 and CD8 T cell proliferative capacity was profoundly impaired in individuals with smear-positive TB, and correlated positively with ex vivo IFN-γ(+)IL-2(+)TNF-α(+) CD4 T cells, and inversely with TNF-α single-positive CD4 T cells. During 6 mo of anti-TB treatment, specific IFN-γ(+)IL-2(+)TNF-α(+) CD4 and CD8 T cells increased, whereas TNF-α and IFN-γ single-positive T cells decreased. These results suggest progressive impairment of M. tuberculosis-specific T cell responses with increasing mycobacterial load and recovery of responses during therapy. Furthermore, these data provide a link between specific cytokine-producing subsets and functional capacity of M. tuberculosis-specific T cells, and between the presence of specific CD8 T cells ex vivo and active TB disease. These data have potentially significant applications for the diagnosis of TB and for the identification of T cell correlates of TB disease progression.

A proteomic view of mycobacteria

Tuberculosis, the disease caused by Mycobacterium tuberculosis, remains a relevant public health issue. This is due mostly to the coepidemiology with HIV/AIDS, the appearance of multidrug-resistant strains globally, and failure of BCG (bacillus Calmette-Guerin) vaccination to confer complete protection. This bacterium was one of the first to have its genome sequenced, yet over a decade after the release of the genomic information, the characterization of its phylogenetic tree and of different strain variants inside this species revealed that much is still needed to be done for a full understanding of the M. tuberculosis genome and proteome. Current methods using LC-MS/MS and hybrid high-resolution mass spectrometers can identify 2400-2800 proteins of the 4000 predicted genes in M. tuberculosis. In this article, we review relevant details of this bacterium's pathology and immunology, describing articles where proteomics helped the community to tackle some of the organism biology, from understanding strain diversity, cellular structure composition, immunogenicity, and host-pathogen interactions. Finally, we will discuss the challenges yet to be fulfilled in order to better characterize M. tuberculosis by proteomics.

Mycobacterium tuberculosis and the intimate discourse of a chronic infection

Mycobacterium tuberculosis is an extremely successful pathogen that demonstrates the capacity to modulate its host both at the cellular and tissue levels. At the cellular level, the bacterium enters its host macrophage and arrests phagosome maturation, thus avoiding many of the microbicidal responses associated with this phagocyte. Nonetheless, the intracellular environment places certain demands on the pathogen, which, in response, senses the environmental shifts and upregulates specific metabolic programs to allow access to nutrients, minimize the consequences of stress, and sustain infection. Despite its intracellular niche, Mycobacterium tuberculosis demonstrates a marked capacity to modulate the tissues surrounding infected cells through the release of potent, bioactive cell wall constituents. These cell wall lipids are released from the host cell by an exocytic process and induce physiological changes in neighboring phagocytes, which drives formation of a granuloma. This tissue response leads to the generation and accumulation of caseous debris and the progression of the human tuberculosis granuloma. Completion of the life cycle of tuberculosis requires damaging the host to release infectious bacteria into the airways to spread the infection. This damage reflects the pathogen's ability to subvert the host's innate and acquired immune responses to its own nefarious ends.

Interferon gamma activated macrophages kill mycobacteria by nitric oxide induced apoptosis

Mycobacterium tuberculosis is an intracellular pathogen of macrophages and escapes the macrophages' bactericidal effectors by interfering with phagosome-lysosome fusion. IFN-γ activation renders the macrophages capable of killing intracellular mycobacteria by overcoming the phagosome maturation block, nutrient deprivation and exposure to microbicidal effectors including nitric oxide (NO). While the importance about NO for the control of mycobacterial infection in murine macrophages is well documented, the underlying mechanism has not been revealed yet. In this study we show that IFN-γ induced apoptosis in mycobacteria-infected macrophages, which was strictly dependent on NO. Subsequently, NO-mediated apoptosis resulted in the killing of intracellular mycobacteria independent of autophagy. In fact, killing of mycobacteria was susceptible to the autophagy inhibitor 3-methyladenine (3-MA). However, 3-MA also suppressed NO production, which is an important off-target effect to be considered in autophagy studies using 3-MA. Inhibition of caspase 3/7 activation, as well as NO production, abolished apoptosis and elimination of mycobacteria by IFN-γ activated macrophages. In line with the finding that drug-induced apoptosis kills intracellular mycobacteria in the absence of NO, we identified NO-mediated apoptosis as a new defense mechanism of activated macrophages against M. tuberculosis.

Tuberculosis vaccines in clinical trials

Effective prophylactic and/or therapeutic vaccination is a key strategy for controlling the global TB epidemic. The partial effectiveness of the existing TB vaccine, bacille Calmette-Guérin (BCG), suggests effective vaccination is possible and highlights the need for an improved vaccination strategy. Clinical trials are evaluating both modifications to the existing BCG immunization methods and also novel TB vaccines, designed to replace or boost BCG. Candidate vaccines in clinical development include live mycobacterial vaccines designed to replace BCG, subunit vaccines designed to boost BCG and therapeutic vaccines designed as an adjunct to chemotherapy. There is a great need for validated animal models, identification of immunological biomarkers of protection and field sites with the capacity for large-scale efficacy testing in order to develop and license a novel TB vaccine or regimen.

Apoptosis is an innate defense function of macrophages against Mycobacterium tuberculosis

Two different forms of death are commonly observed when Mycobacterium tuberculosis (Mtb)-infected macrophages die: (i) necrosis, a death modality defined by cell lysis and (ii) apoptosis, a form of death that maintains an intact plasma membrane. Necrosis is a mechanism used by bacteria to exit the macrophage, evade host defenses, and spread. In contrast, apoptosis of infected macrophages is associated with diminished pathogen viability. Apoptosis occurs when tumor necrosis factor activates the extrinsic death domain pathway, leading to caspase-8 activation. In addition, mitochondrial outer membrane permeabilization leading to activation of the intrinsic apoptotic pathway is required. Both pathways lead to caspase-3 activation, which results in apoptosis. We have recently demonstrated that during mycobacterial infection, cell death is regulated by the eicosanoids, prostaglandin E(2) (proapoptotic) and lipoxin (LX)A(4) (pronecrotic). Although PGE(2) protects against necrosis, virulent Mtb induces LXA(4) and inhibits PGE(2) production. Under such conditions, mitochondrial inner membrane damage leads to macrophage necrosis. Thus, virulent Mtb subverts eicosanoid regulation of cell death to foil innate defense mechanisms of the macrophage.

Exosomes released from M. tuberculosis infected cells can suppress IFN-γ mediated activation of naïve macrophages

Background

Macrophages infected with Mycobacterium tuberculosis (M.tb) are known to be refractory to IFN-γ stimulation. Previous studies have shown that M.tb express components such as the 19-kDa lipoprotein and peptidoglycan that can bind to macrophage receptors including the Toll-like receptor 2 resulting in the loss in IFN-γresponsiveness. However, it is unclear whether this effect is limited to infected macrophages. We have previously shown that M.tb-infected macrophages release exosomes which are 30–100 nm membrane bound vesicles of endosomal origin that function in intercellular communication. These exosomes contain mycobacterial components including the 19-kDa lipoprotein and therefore we hypothesized that macrophages exposed to exosomes may show limited response to IFN-γ stimulation.

Methodology/Principal Findings

Exosomes were isolated from resting as well as M.tb-infected RAW264.7 macrophages. Mouse bone marrow-derived macrophages (BMMØ) were treated with exosomes +/− IFN-γ. Cells were harvested and analyzed for suppression of IFN-γ responsive genes by flow cytometry and real time PCR. We found that exosomes derived from M.tb H37Rv-infected but not from uninfected macrophages inhibited IFN-γ induced MHC class II and CD64 expression on BMMØ. This inhibition was only partially dependent on the presence of lipoproteins but completely dependent on TLR2 and MyD88. The exosomes isolated from infected cells did not inhibit STAT1 Tyrosine phosphorylation but down-regulated IFN-γ induced expression of the class II major histocompatibity complex transactivator; a key regulator of class II MHC expression. Microarray studies showed that subsets of genes induced by IFN-γ were inhibited by exosomes from H37Rv-infeced cells including genes involved in antigen presentation. Moreover, this set of genes partially overlapped with the IFN-γ-induced genes inhibited by H37Rv infection.

Conclusions

Our study suggests that exosomes, as carriers of M.tb pathogen associated molecular patterns (PAMPs), may provide a mechanism by which M.tb may exert its suppression of a host immune response beyond the infected cell.

Lipids, apoptosis, and cross-presentation: links in the chain of host defense against Mycobacterium tuberculosis

Eicosanoids regulate whether human and murine macrophages infected with Mycobacterium tuberculosis die by apoptosis or necrosis. The death modality is important since apoptosis is associated with diminished pathogen viability and should be viewed as a form of innate immunity. Apoptotic vesicles derived from infected macrophages are also an important source of bacterial antigens that can be acquired by dendritic cells to prime antigen-specific T cells. This review integrates in vitro and in vivo data on how apoptosis of infected macrophages is linked to development of T cell immunity against M. tuberculosis.

Mycobacterium tuberculosis lipoarabinomannan enhances LPS-induced TNF-α production and inhibits NO secretion by engaging scavenger receptors

Lipoarabinomannan capped with terminal oligomannosides (ManLAM) is a component of mycobacteria cell wall enabling Mycobacterium tuberculosis to infect macrophages. We found that short treatment (3.5h) of macrophage-like J774 cells and thioglycollate-elicited peritoneal murine macrophages with ManLAM and its deacylated form enhanced LPS-stimulated release of tumor necrosis factor-α (TNF-α). In contrast, prolong incubation of J774 cells with ManLAM (16h) led to inhibition of LPS-stimulated TNF-α production. LPS-triggered secretion of nitric oxide (NO) was suppressed by ManLAM and its deacylated form. Effects of ManLAM and its deacylated derivative were mimicked by dextran sulfate, a general ligand of scavenger receptors. The enhancement of LPS-induced TNF-α production by dextran sulfate was partially reversed by an antibody neutralizing scavenger receptor SR-PSOX/CXCL16 while the stimulatory activity of deacylated ManLAM was reversed by an antibody neutralizing class B scavenger receptor CD36. Our data suggest that CD36 mediates the activity of ManLAM and its deacylated form leading to TNF-α release in LPS-stimulated J774 cells and peritoneal murine macrophages, while NO production is modulated by unknown scavenger receptors.

Co-administration of IL-1+IL-6+TNF-α with Mycobacterium tuberculosis infected macrophages vaccine induces better protective T cell memory than BCG

BCG has been administered globally for more than 75 years, yet tuberculosis (TB) continues to kill more than 2 million people annually. Further, BCG protects childhood TB but is quite inefficient in adults. This indicates that BCG fails to induce long-term protection. Hence there is a need to explore alternative vaccination strategies that can stimulate enduring T cell memory response. Dendritic cell based vaccination has attained extensive popularity following their success in various malignancies. In our previous study, we have established a novel and unique vaccination strategy against Mycobacterium tuberculosis (M. tb) and Salmonella typhimurium by utilizing infected macrophages (IM). In short-term experiments (30 days), substantial degree of protection was observed. However, remarkable difference was not observed in long-term studies (240 days) due to failure of the vaccine to generate long-lasting memory T cells. Hence, in the present study we employed T cell memory augmenting cytokines IL-1+IL-6+TNF-α and IL-7+IL-15 for the induction of the enhancement of long-term protection by the vaccine. We co-administered the M. tb infected macrophages vaccine with IL-1+IL-6+TNF-α (IM-1.6.α) and IL-7+IL-15 (IM-7.15). The mice were then rested for a reasonably large period (240 days) to study the bona fide T cell memory response before exposing them to aerosolized M. tb. IM-1.6.α but not IM-7.15 significantly improved memory T cell response against M. tb, as evidenced by recall responses of memory T cells, expansion of both central as well as effector memory CD4 and CD8 T cell pools, elicitation of mainly Th1 memory response, reduction in the mycobacterial load and alleviated lung pathology. Importantly, the protection induced by IM-1.6.α was significantly better than BCG. Thus, this study demonstrates that not only antigen-pulsed DCs can be successfully employed as vaccines against cancer and infectious diseases but also macrophages infected with M. tb can be utilized with great efficacy especially in protection against TB.

Pathogenesis, immunology, and diagnosis of latent Mycobacterium tuberculosis infection

Phagocytosis of tubercle bacilli by antigen-presenting cells in human lung alveoli initiates a complex infection process by Mycobacterium tuberculosis and a potentially protective immune response by the host. M. tuberculosis has devoted a large part of its genome towards functions that allow it to successfully establish latent or progressive infection in the majority of infected individuals. The failure of immune-mediated clearance is due to multiple strategies adopted by M. tuberculosis that blunt the microbicidal mechanisms of infected immune cells and formation of distinct granulomatous lesions that differ in their ability to support or suppress the persistence of viable M. tuberculosis. In this paper, current understanding of various immune processes that lead to the establishment of latent M. tuberculosis infection, bacterial spreading, persistence, reactivation, and waning or elimination of latent infection as well as new diagnostic approaches being used for identification of latently infected individuals for possible control of tuberculosis epidemic are described.

Novel developments in the epidemic of human immunodeficiency virus and tuberculosis coinfection

Tuberculosis (TB) disease remains one of the highest causes of mortality in HIV-infected individuals, and HIV-TB coinfection continues to grow at alarming rates, especially in sub-Saharan Africa. Surprisingly, a number of important areas regarding coinfection remain unclear. For example, increased risk of TB disease begins early in the course of HIV infection; however, the mechanism by which HIV increases this risk is not well understood. In addition, there is lack of consensus on the optimal way to diagnose latent TB infection and to manage active disease in those who are HIV infected. Furthermore, effective point-of-care testing for TB disease remains elusive. This review discusses key areas in the epidemiology, pathogenesis, diagnosis, and management of active and latent TB in those infected with HIV, focusing attention on issues related to high- and low-burden areas. Particular emphasis is placed on controversial areas where there are gaps in knowledge and on future directions of study.

New approaches in the diagnosis and treatment of latent tuberculosis infection

With nearly 9 million new active disease cases and 2 million deaths occurring worldwide every year, tuberculosis continues to remain a major public health problem. Exposure to Mycobacterium tuberculosis leads to active disease in only ~10% people. An effective immune response in remaining individuals stops M. tuberculosis multiplication. However, the pathogen is completely eradicated in ~10% people while others only succeed in containment of infection as some bacilli escape killing and remain in non-replicating (dormant) state (latent tuberculosis infection) in old lesions. The dormant bacilli can resuscitate and cause active disease if a disruption of immune response occurs. Nearly one-third of world population is latently infected with M. tuberculosis and 5%-10% of infected individuals will develop active disease during their life time. However, the risk of developing active disease is greatly increased (5%-15% every year and ~50% over lifetime) by human immunodeficiency virus-coinfection. While active transmission is a significant contributor of active disease cases in high tuberculosis burden countries, most active disease cases in low tuberculosis incidence countries arise from this pool of latently infected individuals. A positive tuberculin skin test or a more recent and specific interferon-gamma release assay in a person without overt signs of active disease indicates latent tuberculosis infection. Two commercial interferon-gamma release assays, QFT-G-IT and T-SPOT.TB have been developed. The standard treatment for latent tuberculosis infection is daily therapy with isoniazid for nine months. Other options include therapy with rifampicin for 4 months or isoniazid + rifampicin for 3 months or rifampicin + pyrazinamide for 2 months or isoniazid + rifapentine for 3 months. Identification of latently infected individuals and their treatment has lowered tuberculosis incidence in rich, advanced countries. Similar approaches also hold great promise for other countries with low-intermediate rates of tuberculosis incidence.

Future vaccination strategies against tuberculosis: thinking outside the box

With almost a dozen vaccine candidates in clinical trials, tuberculosis (TB) research and development is finally reaping the first fruits of its labors. Vaccine candidates in clinical trials may prevent TB disease reactivation by efficiently containing the pathogen Mycobacterium tuberculosis (Mtb). Future research should target vaccines that achieve sterile eradication of Mtb or even prevent stable infection. These are ambitious goals that can be reached only by highly cooperative engagement of basic immunologists, vaccinologists, and clinical researchers--or in other words, by translation from basic immunology to vaccine research and development, as well as reverse translation of insights from clinical trials back to hypothesis-driven research in the basic laboratory. Here, we review current and future strategies toward the rational design of novel vaccines against TB, as well as the progress made thus far, and the hurdles that need to be overcome in the near and distant future.

CD8 cells of patients with diffuse cutaneous leishmaniasis display functional exhaustion: the latter is reversed, in vitro, by TLR2 agonists

Leishmania mexicana (Lm) causes localized (LCL) and diffuse (DCL) cutaneous leishmaniasis. DCL patients have a poor cellular immune response leading to chronicity. It has been proposed that CD8 T lymphocytes (CD8) play a crucial role in infection clearance, although the role of CD8 cytotoxicity in disease control has not been elucidated. Lesions of DCL patients have been shown to harbor low numbers of CD8, as compared to patients with LCL, and leishmanicidal treatment restores CD8 numbers. The marked response of CD8 towards Leishmania parasites led us to analyze possible functional differences between CD8 from patients with LCL and DCL. We compared IFNγ production, antigen-specific proliferation, and cytotoxicity of CD8 purified from PBMC against autologous macrophages (MO) infected with Leishmania mexicana (MOi). Additionally, we analyzed tissue biopsies from both groups of patients for evidence of cytotoxicity associated with apoptotic cells in the lesions. We found that CD8 cell of DCL patients exhibited low cytotoxicity, low antigen-specific proliferation and low IFNγ production when stimulated with MOi, as compared to LCL patients. Additionally, DCL patients had significantly less TUNEL⁺ cells in their lesions. These characteristics are similar to cellular “exhaustion” described in chronic infections. We intended to restore the functional capacity of CD8 cells of DCL patients by preincubating them with TLR2 agonists: Lm lipophosphoglycan (LPG) or Pam3Cys. Cytotoxicity against MOi, antigen-specific proliferation and IFNγ production were restored with both stimuli, whereas PD-1 (a molecule associated with cellular exhaustion) expression, was reduced. Our work suggests that CD8 response is associated with control of Lm infection in LCL patients and that chronic infection in DCL patients leads to a state of CD8 functional exhaustion, which could facilitate disease spread. This is the first report that shows the presence of functionally exhausted CD8 T lymphocytes in DCL patients and, additionally, that pre-stimulation with TLR2 ligands can restore the effector mechanisms of CD8 T lymphocytes from DCL patients against Leishmania mexicana-infected macrophages.

Leishmania mexicana causes localized and diffuse cutaneous leishmaniasis. Whereas the former is a benign form the disease, diffuse cutaneous leishmaniasis is a chronic disfiguring disease, for which no cure is available, and the immune cells of these patients respond poorly to the parasite. It has been proposed that the elimination of Leishmania-infected cells by CD8 T cells is crucial for disease control. We compared the functional characteristics of CD8 T cells from patients with localized and diffuse cutaneous leishmaniasis. We found that CD8 T cells from patients with diffuse cutaneous leishmaniasis were functionally exhausted, as compared to patients with the benign form of the disease. We were able to restore functional capacity of these cells by culturing them with molecules that stimulate TLR2. This is the first report showing that stimulation of the TLR2 can restore effector mechanisms in functionally exhausted CD8 cells from patients with diffuse cutaneous leishmaniasis. This finding will help design novel treatment schemes for patients infected with the parasite Leishmania mexicana who have the progressive, incurable form of diffuse cutaneous leishmaniasis.

CD36 deficiency attenuates experimental mycobacterial infection

Background

Members of the CD36 scavenger receptor family have been implicated as sensors of microbial products that mediate phagocytosis and inflammation in response to a broad range of pathogens. We investigated the role of CD36 in host response to mycobacterial infection.

Methods

Experimental Mycobacterium bovis Bacillus Calmette-Guérin (BCG) infection in Cd36^+/+ and Cd36^-/- mice, and in vitro co-cultivation of M. tuberculosis, BCG and M. marinum with Cd36^+/+ and Cd36^-/-murine macrophages.

Results

Using an in vivo model of BCG infection in Cd36^+/+ and Cd36^-/- mice, we found that mycobacterial burden in liver and spleen is reduced (83% lower peak splenic colony forming units, p < 0.001), as well as the density of granulomas, and circulating tumor necrosis factor (TNF) levels in Cd36^-/- animals. Intracellular growth of all three mycobacterial species was reduced in Cd36^-/- relative to wild type Cd36^+/+ macrophages in vitro. This difference was not attributable to alterations in mycobacterial uptake, macrophage viability, rate of macrophage apoptosis, production of reactive oxygen and/or nitrogen species, TNF or interleukin-10. Using an in vitro model designed to recapitulate cellular events implicated in mycobacterial infection and dissemination in vivo (i.e., phagocytosis of apoptotic macrophages containing mycobacteria), we demonstrated reduced recovery of viable mycobacteria within Cd36^-/- macrophages.

Conclusions

Together, these data indicate that CD36 deficiency confers resistance to mycobacterial infection. This observation is best explained by reduced intracellular survival of mycobacteria in the Cd36^-/- macrophage and a role for CD36 in the cellular events involved in granuloma formation that promote early bacterial expansion and dissemination.