Immunodominant Tuberculosis CD8 Antigens Preferentially Restricted by HLA-B

CD8+ T cells are essential for host defense to intracellular bacterial pathogens such as Mycobacterium tuberculosis (Mtb), Salmonella species, and Listeria monocytogenes, yet the repertoire and dominance pattern of human CD8 antigens for these pathogens remains poorly characterized. Tuberculosis (TB), the disease caused by Mtb infection, remains one of the leading causes of infectious morbidity and mortality worldwide and is the most frequent opportunistic infection in individuals with HIV/AIDS. Therefore, we undertook this study to define immunodominant CD8 Mtb antigens. First, using IFN-γ ELISPOT and synthetic peptide arrays as a source of antigen, we measured ex vivo frequencies of CD8+ T cells recognizing known immunodominant CD4+ T cell antigens in persons with latent tuberculosis infection. In addition, limiting dilution was used to generate panels of Mtb-specific T cell clones. Using the peptide arrays, we identified the antigenic specificity of the majority of T cell clones, defining several new epitopes. In all cases, peptide representing the minimal epitope bound to the major histocompatibility complex (MHC)-restricting allele with high affinity, and in all but one case the restricting allele was an HLA-B allele. Furthermore, individuals from whom the T cell clone was isolated harbored high ex vivo frequency CD8+ T cell responses specific for the epitope, and in individuals tested, the epitope represented the single immunodominant response within the CD8 antigen. We conclude that Mtb-specific CD8+ T cells are found in high frequency in infected individuals and are restricted predominantly by HLA-B alleles, and that synthetic peptide arrays can be used to define epitope specificities without prior bias as to MHC binding affinity. These findings provide an improved understanding of immunodominance in humans and may contribute to a development of an effective TB vaccine and improved immunodiagnostics

Human Innate Mycobacterium tuberculosis–Reactive αβTCR+ Thymocytes

The control of Mycobacterium tuberculosis (Mtb) infection is heavily dependent on the adaptive Th1 cellular immune response. Paradoxically, optimal priming of the Th1 response requires activation of priming dendritic cells with Th1 cytokine IFN-γ. At present, the innate cellular mechanisms required for the generation of an optimal Th1 T cell response remain poorly characterized. We hypothesized that innate Mtb-reactive T cells provide an early source of IFN-γ to fully activate Mtb-exposed dendritic cells. Here, we report the identification of a novel population of Mtb-reactive CD4− αβTCR+ innate thymocytes. These cells are present at high frequencies, respond to Mtb-infected cells by producing IFN-γ directly ex vivo, and display characteristics of effector memory T cells. This novel innate population of Mtb-reactive T cells will drive further investigation into the role of these cells in the containment of Mtb following infectious exposure. Furthermore, this is the first demonstration of a human innate pathogen-specific αβTCR+ T cell and is likely to inspire further investigation into innate T cells recognizing other important human pathogens

The Mycobacterium tuberculosis Phagosome Is a HLA-I Processing Competent Organelle

Mycobacterium tuberculosis (Mtb) resides in a long-lived phagosomal compartment that resists maturation. The manner by which Mtb antigens are processed and presented on MHC Class I molecules is poorly understood. Using human dendritic cells and IFN-γ release by CD8+ T cell clones, we examined the processing and presentation pathway for two Mtb–derived antigens, each presented by a distinct HLA-I allele (HLA-Ia versus HLA-Ib). Presentation of both antigens is blocked by the retrotranslocation inhibitor exotoxin A. Inhibitor studies demonstrate that, after reaching the cytosol, both antigens require proteasomal degradation and TAP transport, but differ in the requirement for ER–golgi egress and new protein synthesis. Specifically, presentation by HLA-B8 but not HLA-E requires newly synthesized HLA-I and transport through the ER–golgi. Phenotypic analysis of the Mtb phagosome by flow organellometry revealed the presence of Class I and loading accessory molecules, including TAP and PDI. Furthermore, loaded HLA-I:peptide complexes are present within the Mtb phagosome, with a pronounced bias towards HLA-E:peptide complexes. In addition, protein analysis also reveals that HLA-E is enriched within the Mtb phagosome compared to HLA-A2. Together, these data suggest that the phagosome, through acquisition of ER–localized machinery and as a site of HLA-I loading, plays a vital role in the presentation of Mtb–derived antigens, similar to that described for presentation of latex bead-associated antigens. This is, to our knowledge, the first description of this presentation pathway for an intracellular pathogen. Moreover, these data suggest that HLA-E may play a unique role in the presentation of phagosomal antigens

Human Neonatal Dendritic Cells Are Competent in MHC Class I Antigen Processing and Presentation

Neonates are clearly more susceptible to severe disease following infection with a variety of pathogens than are adults. However, the causes for this are unclear and are often attributed to immunological immaturity. While several aspects of immunity differ between adults and neonates, the capacity of dendritic cells in neonates to process and present antigen to CD8+ T cells remains to be addressed. We used human CD8+ T cell clones to compare the ability of neonatal and adult monocyte-derived dendritic cells to present or process and present antigen using the MHC class I pathway. Specifically, we assessed the ability of dendritic cells to present antigenic peptide, present an HLA-E–restricted antigen, process and present an MHC class I-restricted antigen through the classical MHC class I pathway, and cross present cell-associated antigen via MHC class I. We found no defect in neonatal dendritic cells to perform any of these processing and presentation functions and conclude that the MHC class I antigen processing and presentation pathway is functional in neonatal dendritic cells and hence may not account for the diminished control of pathogens

The evolving research agenda for paediatric tuberculosis infection

There are unique challenges facing the diagnosis and management of tuberculosis infection in children. Following exposure to an infectious tuberculosis case and subsequent infection, children frequently progress to tuberculosis disease more rapidly than adults. Increasingly, investigators recognize the concept of sub clinical disease, an entity referring to early asymptomatic disease. Our understanding of the pathogenesis of tuberculosis in children remains limited but could be improved through animal models, laboratory studies evaluating the responses of blood or respiratory samples to mycobacteria in vitro, as well as evaluating immune responses in children exposed to tuberculosis. Identifying children with sub-clinical disease, at high risk of progression to clinically apparent disease, through biomarker discover, would mean that treatment could be targeted to those most likely to benefit. These studies could be embedded in large observational or interventional cohorts. The optimization and discovery of novel treatments for tuberculosis infection in children need to account for mechanisms of action of tuberculosis drugs as well as child-specific factors including pharmacokinetics and appropriate formulations. In this article we present the result of discussions at a large international meeting and the series of research priorities that were developed

Human IL-10 Producing T Cells Specific for Mycobacterium tuberculosis.

IL−10 producing Mtb−specific CD4+ T cells can be detected in pulmonary TB patients with persistent anergy. Aim of the study was to define the spectrum of ex vivo frequencies of IL−10 producing Mtb−specific CD4+ and CD8+ T cells in adults. Peripheral blood mononuclear cells were collected from uninfected adults and subjects with latent tuberculosis infection or active tuberculosis. Monocyte−derived dendritic cells (DC) were infected overnight with Mtb (MOI=50:1) and incubated with different concentrations of positively selected autologous CD4+ and CD8+ T cells in an IL−10 ELISPOT assay. In all subjects we detected additional IL−10 producing cells with the addition of T cells to Mtb−infected DC, compared to Mtb−infected DC alone. We next focused on CD8+ T cells and asked if they represent the additional IL−10 producing cells. Autologous DC were left uninfected or infected with Mtb (MOI=20:1). After overnight incubation, positively selected CD8+ T cells were added and incubated overnight. Then, CD8+ T cells were positively selected from these cultures using magnetic beads, and RNA was isolated and subjected to RT−PCR. Relative quantitation of IL−10 RNA showed that CD8+ T cells were induced to produce IL−10 in response to Mtb−infected DC, suggesting that T cells are a source of the augmented IL−10 production previously seen in co−cultures of Mtb−infected DC with T cells. We next sought to isolate IL−10 producing Mtb−specific CD8+ T cells using a limiting dilution T cell cloning approach. T cells from wells exhibiting growth were analyzed by ELISPOT for their production of IFN−g and/or IL−10 in response to Mtb−infected autologous DC. In all donors, IFN−g producing CD8+ T cells were most frequently isolated. Most donors also had IL−10 producing CD8+ T cells, the majority of which also produced IFN−g. Finally, we confirmed that IL−10 has the potential to inhibit IFN−g CD4+ T cell responses to Mtb antigens