CD28 signaling strength regulates the cell fate of TCF-1+ PD-1+ CD8 T cells

Abstract

Abstract During chronic infections and cancer, CD8 T cells that recognize pathogens/tumors can persist in a dysfunctional state characterized by high expression of the inhibitory receptor Programmed Cell Death (PD)-1. T cell factor (TCF)-1+PD-1+ progenitor exhausted cells (Tpex) maintain the antigen-specific T cell pool by self-renewal and differentiation into effector-like TCF-1negPD-1+CD8 T cells. Previous studies described that among PD-1+ CD8 T cells, only Tpex respond to PD-1 blockade therapy. However, the molecular mechanisms that support Tpex self-renewal or differentiation are not understood. Tpex have high CD28 expression and we demonstrated that CD28 costimulation is required for response to PD-1 targeted therapy. To better understand the role of CD28 on Tpex, we evaluated how reduction or abrogation of CD28 signaling impacted virus-specific CD8 T cells in mice infected with lymphocytic choriomeningitis virus (LCMV). During established chronic infection, deletion of both Cd28 alleles resulted in reduction of Tpex and TCF-1neg subsets. In contrast, when CD28 signaling was reduced (deletion of one allele), TCF-1negPD-1+ CD8 T cells decreased, but the number of Tpex was not affected. RNA sequencing and metabolic assays revealed that sustained CD28 signaling during persistent antigen stimulation is required to maintain mitochondrial fitness of Tpex. Our work supports the novel hypothesis that CD28 signaling strength modulates Tpex fate decision through metabolic regulation. Our data also suggest continuous CD28 signaling is required for long-term maintenance of antigen-specific PD-1+ CD8 T cells. These findings have important implications for checkpoint therapy in cancer.