PD-1 blockade-unresponsive human tumor-infiltrating CD8+ T cells are marked by loss of CD28 expression and rescued by IL-15

Blockade of programmed death-1 (PD-1) reinvigorates exhausted CD8+ T cells, resulting in tumor regression in cancer patients. Recently, reinvigoration of exhausted CD8+ T cells following PD-1 blockade was shown to be CD28-dependent in mouse models. Herein, we examined the role of CD28 in anti-PD-1 antibody-induced human T cell reinvigoration using tumor-infiltrating CD8+ T cells (CD8+ TILs) obtained from non-small-cell lung cancer patients. Single-cell analysis demonstrated a distinct expression pattern of CD28 between mouse and human CD8+ TILs. Furthermore, we found that human CD28+CD8+ but not CD28-CD8+ TILs responded to PD-1 blockade irrespective of B7/CD28 blockade, indicating that CD28 costimulation in human CD8+ TILs is dispensable for PD-1 blockade-induced reinvigoration and that loss of CD28 expression serves as a marker of anti-PD-1 antibody-unresponsive CD8+ TILs. Transcriptionally and phenotypically, PD-1 blockade-unresponsive human CD28-PD-1+CD8+ TILs exhibited characteristics of terminally exhausted CD8+ T cells with low TCF1 expression. Notably, CD28-PD-1+CD8+ TILs had preserved machinery to respond to IL-15, and IL-15 treatment enhanced the proliferation of CD28-PD-1+CD8+ TILs as well as CD28+PD-1+CD8+ TILs. Taken together, these results show that loss of CD28 expression is a marker of PD-1 blockade-unresponsive human CD8+ TILs with a TCF1- signature and provide mechanistic insights into combining IL-15 with anti-PD-1 antibodies.