The role of bystander activation among CD8αα T cells in controlling Mycobacterium tuberculosisinfection

Abstract

Abstract Classical cytotoxic T cells have a CD8 coreceptor with an αβ heterodimer and recognize peptide antigens via major histocompatibility (MHC) molecules. CD8 T cells with an αα homodimer structure are nonclassical and can recognize non-MHC molecules independently of antigen. CD8αα T cells have a higher propensity to undergo T cell receptor (TCR) independent bystander activation via pro-inflammatory cytokines. Bystander activation of CD8 T cells has shown a protective effect during Mycobacterium tuberculosis (M.tb) infection, but the distinct roles of CD8αα and CD8αβ T cells have not been elucidated. To begin assessing this, we compared the levels of CD8αα and CD8αβ T cell proliferation in response to TCR dependent stimulation using anti-CD3/CD28 beads. We observed that CD8αα T cells had lower levels of proliferation compared to CD8αβ T cells after three days of stimulation. After seven days, a population of CD8αα T cells did not proliferate while nearly all CD8αβ T cells showed high proliferative capacity. We next stimulated healthy peripheral blood mononuclear cells with IL-12, IL-15, and IL-18 to induce bystander activation and found 52.8% of CD8αα T cells secreted granzyme B compared to 25.1% of CD8αβ T cells. Interestingly, CD8αα T cells also had higher levels of CD69 expression than CD8αβ T cells (62.6% and 38.2%, respectively). Finally, we present plans for defining the role of bystander activated CD8αα and CD8αβ T cells during M.tb infection using tonsil organoids. These data reveal that CD8αα T cells have a distinct bimodal response to TCR mediated activation as well as an increased cytotoxic and activated phenotype after bystander activation. We further establish a foundation for future bystander activation studies in M.tb infection. Supported by grants from the NIH (R01 AI146072)