Developmental relationships of four exhausted CD8 T cell subsets reveals underlying transcriptional and epigenetic control mechanisms

Abstract Exhausted CD8 T cells (TEX) are essential during chronic viral infections and cancer. Two TEX subpopulations including a progenitor and a more terminally exhausted subset cooperate to maintain an active immune response during antigen persistence. However, non-overlapping delineations of these populations have suggested a more complex developmental biology. Here, using the LCMV mouse model of chronic viral infection, we identify four distinct TEX subsets based on Ly108 (Slamf6) and CD69 expression revealing a novel stepwise developmental framework. We reveal the transcriptional and epigenetic control mechanisms and associated biological changes underlying each TEX subset transition. Two TCF1+ progenitors were identified along with a novel TCF1-intermediate subset that re-engaged some aspects of effector biology. This subset depended on T-bet and was re-invigorated upon PD-L1 blockade. Ultimately, Tox coordinated loss of T-bet and differentiation into a fourth, terminally exhausted subset. These data define a new developmental hierarchy of TEX and reveal distinct biological properties with direct relevance to immunotherapy. Defining the control mechanisms of this TEX subset hierarchy provides novel opportunities to manipulate TEX biology for clinical goals.