T Cells home to the thymus and control infection

The thymus is a target of multiple pathogens. How the immune system responds to thymic infection is largely unknown. Despite being considered an immune-privileged organ, we detect a mycobacteria-specific T cell response in the thymus following dissemination of Mycobacterium avium or Mycobacterium tuberculosis. This response includes proinflammatory cytokine production by mycobacteria-specific CD4(+) and CD8(+) T cells, which stimulates infected cells and controls bacterial growth in the thymus. Importantly, the responding T cells are mature peripheral T cells that recirculate back to the thymus. The recruitment of these cells is associated with an increased expression of Th1 chemokines and an enrichment of CXCR3(+) mycobacteria-specific T cells in the thymus. Finally, we demonstrate it is the mature T cells that home to the thymus that most efficiently control mycobacterial infection. Although the presence of mature T cells in the thymus has been recognized for some time, to our knowledge, these data are the first to show that T cell recirculation from the periphery to the thymus is a mechanism that allows the immune system to respond to thymic infection. Maintaining a functional thymic environment is essential to maintain T cell differentiation and prevent the emergence of central tolerance to the invading pathogens.This work was supported by Portuguese Foundation for Science and Technology Grant PTDC/SAU-MII/101663/2008 and individual fellowships to C.N., C.N.-A., B.C.-R., S.R., and P.B.-S. S.M.B. was supported by National Institutes of Health Grant R01 AI067731. The Small Animal Biocontainment Suite was supported in part by Center for AIDS Research Grant P30 AI 060354

Physiological Induction of Regulatory Qa-1-Restricted CD8+ T Cells Triggered by Endogenous CD4+ T Cell Responses

T cell-dependent autoimmune diseases are characterized by the expansion of T cell clones that recognize immunodominant epitopes on the target antigen. As a consequence, for a given autoimmune disorder, pathogenic T cell clones express T cell receptors with a limited number of variable regions that define antigenic specificity. Qa-1, a MHC class I-like molecule, presents peptides from the variable region of TCRs to Qa-1-restricted CD8+ T cells. The induction of Vß-specific CD8+ T cells has been harnessed in an immunotherapeutic strategy known as the “T cell vaccination” (TCV) that comprises the injection of activated and attenuated CD4+ T cell clones so as to induce protective CD8+ T cells. We hypothesized that Qa-1-restricted CD8+ regulatory T cells could also constitute a physiologic regulatory arm of lymphocyte responses upon expansion of endogenous CD4+ T cells, in the absence of deliberate exogenous T cell vaccination. We immunized mice with two types of antigenic challenges in order to sequentially expand antigen-specific endogenous CD4+ T cells with distinct antigenic specificities but characterized by a common Vß chain in their TCR. The first immunization was performed with a non-self antigen while the second challenge was performed with a myelin-derived peptide known to drive experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis. We show that regulatory Vß-specific Qa-1-restricted CD8+ T cells induced during the first endogenous CD4+ T cell responses are able to control the expansion of subsequently mobilized pathogenic autoreactive CD4+ T cells. In conclusion, apart from the immunotherapeutic TCV, Qa-1-restricted specialized CD8+ regulatory T cells can also be induced during endogenous CD4+ T cell responses. At variance with other regulatory T cell subsets, the action of these Qa-1-restricted T cells seems to be restricted to the immediate re-activation of CD4+ T cells

Polyclonal mucosa-associated invariant T cells have unique innate functions in bacterial infection

Mucosa-associated invariant T (MAIT) cells are a unique population of αβ T cells in mammals that reside preferentially in mucosal tissues and express an invariant Vα paired with limited Vβ T-cell receptor (TCR) chains. Furthermore, MAIT cell development is dependent upon the expression of the evolutionarily conserved major histocompatibility complex (MHC) class Ib molecule MR1. Using in vitro assays, recent studies have shown that mouse and human MAIT cells are activated by antigen-presenting cells (APCs) infected with diverse microbes, including numerous bacterial strains and yeasts, but not viral pathogens. However, whether MAIT cells play an important, and perhaps unique, role in controlling microbial infection has remained unclear. To probe MAIT cell function, we show here that purified polyclonal MAIT cells potently inhibit intracellular bacterial growth of Mycobacterium bovis BCG in macrophages (MΦ) in coculture assays, and this inhibitory activity was dependent upon MAIT cell selection by MR1, secretion of gamma interferon (IFN-γ), and an innate interleukin 12 (IL-12) signal from infected MΦ. Surprisingly, however, the cognate recognition of MR1 by MAIT cells on the infected MΦ was found to play only a minor role in MAIT cell effector function. We also report that MAIT cell-deficient mice had higher bacterial loads at early times after infection compared to wild-type (WT) mice, demonstrating that MAIT cells play a unique role among innate lymphocytes in protective immunity against bacterial infection

Ag85-focused T-cell immune response controls Mycobacterium avium chronic infection

CD4+ T cells are essential players for the control of mycobacterial infections. Several mycobacterial antigens have been identified for eliciting a relevant CD4+ T cell mediated-immune response, and numerous studies explored this issue in the context of Mycobacterium tuberculosis infection. Antigen 85 (Ag85), a highly conserved protein across Mycobacterium species, is secreted at the early phase of M. tuberculosis infection leading to the proliferation of Ag85-specific CD4+ T cells. However, in the context of Mycobacterium avium infection, little is known about the expression of this antigen and the elicited immune response. In the current work, we investigated if a T cell receptor (TCR) repertoire mostly, but not exclusively, directed at Ag85 is sufficient to mount a protective immune response against M. avium. We show that P25 mice, whose majority of T cells express a transgenic TCR specific for Ag85, control M. avium infection at the same level as wild type (WT) mice up to 20 weeks post-infection (wpi). During M. avium infection, Ag85 antigen is easily detected in the liver of 20 wpi mice by immunohistochemistry. In spite of the propensity of P25 CD4+ T cells to produce higher amounts of interferon-gamma (IFNγ) upon ex vivo stimulation, no differences in serum IFNγ levels are detected in P25 compared to WT mice, nor enhanced immunopathology is detected in P25 mice. These results indicate that a T cell response dominated by Ag85-specific T cells is appropriate to control M. avium infection with no signs of immunopathology.This work was developed under the scope of the project NORTE-01-0145-FEDER-000013, supported by the Northern Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (FEDER). Fellowships from the Portuguese Foundation for Science and Technoloy (FCT) were attributed to BCR (SFRH/BD/80352/2011; QREN-POPH through the Fundo Social Europeu (FSE) and national funds from MEC] and to CN (SFRH/BPD/112001/2015; POPH through FSE and national funds from MCTES). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript