Regulation of CD8+ T cell immunity to tuberculosis

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Project Summary. The bacterium Mycobacterium tuberculosis (Mtb) causes more human deaths than any other pathogen. Mtb elicits strong CD8 T cell responses in people and CD8 T cells make an important contribution to protection against virulent Mtb in animal models. Human CD8 T cells kill intracellular Mtb, CD8 depletion in non-human primates leads to severe disseminated TB, and recombinant BCG designed to elicit better CD8 T cell responses is effective in mice and clinical trials have started. CD8 T cells also appear to enforce latency!in both mice and in people. Despite the vigorous immune response, Mtb evades clearance by adapting to its host, an idea originally based on its ability to survive in the phagosome and avoid antibody immunity. We are now learning that Mtb may also avoid T cell immunity. An unexpected finding from the genomic analysis of Mtb is that the genes encoding T cell epitopes are hyper-conserved, which has been interpreted to mean that the host T cell response benefits the survival of Mtb, possibly by creating sufficient inflammation to promote transmission. How does one reconcile T cells as an instrument of TB control, with Mtb benefiting from T cell responses? Our recent data suggests that not all T cell antigens are alike. We hypothesize that TB10 is a decoy antigen. We use the term ‘decoy’ to describe its ability to elicit a strong CD8 T cell response that poorly recognizes Mtb-infected macrophages. In the context of immune evasion, decoy antigens induce T cells that provoke inflammation, but as they fail to recognize infected cells, do not control Mtb infection. Their immunodominance impairs T cell responses to other antigens, which might be more efficiently presented by Mtb-infected cells and could be targets of protective immunity. Aim 1 will test the “decoy” hypothesis, based on the idea that if a decoy antigen is removed, the residual T cell response will be more effective. We will also will determine the cellular and molecular mechanisms for how Mtb avoids sampling of it antigens by MHC I. Although Mtb evades CD8 T cell immunity, ultimately, CD8 promote control of Mtb. The hypothesis of Aim 2 is that a ‘protective’ function of CD4 T cells is to help CD8 T cells differentiate and express functions that mediate protection against TB. In the absence of CD4 help, we predict that CD8s become dysfunctional and confer suboptimal protection. Our new preliminary data shows that ‘helped’ CD8 T cells expand, acquire effector function, and mediate host protection better than ‘helpless’ CD8 T cells. Here we expect to establish that helped CD8s mediate greater protection than helpless CD8s during primary and secondary (i.e., memory) responses against Mtb infection. We will define the molecular differences between helped and helpless CD8 T cells that mediate better protection; and identify the CD4 T cell factors that mediate help during TB. My lab has developed an innovative and productive research program that seeks to understand how CD8 T cells restrict bacterial growth and how Mtb defeats CD8 immunity. Our goal is to elicit protective CD8s and recruit them into beneficial responses as part of an effort to guide vaccine development. !