Antigen-specific profiling identifies T-bet melanoma-specific CD8 T cells associated with response to neoadjuvant PD-1 blockade.

Despite widespread immune profiling in cancer immunotherapy, the antigen-specific responses that drive clinical outcomes remain poorly defined. In a prospective neoadjuvant trial (NCT04013854) of a single-dose anti-PD-1 (nivolumab) in stage III melanoma, we performed antigen-specific profiling of melanoma and viral-specific CD8 T cells across blood, tumor, and lymph node compartments. Using combinatorial tetramers, we detected melanoma-specific CD8 T cells in 72% of HLA-A1, -A2, and -A3 patients. These cells displayed distinct phenotypes shaped by tissue and antigen context. Tumor-infiltrating T-bet intermediate exhausted CD8 T cells were strongly associated with pathologic response, while CD39 terminal exhausted cells marked non-response. T-bet and CD39 expression also stratified responses in uninvolved lymph nodes, suggesting early divergence of therapeutic immune trajectories. Longitudinal profiling revealed that circulating melanoma-specific CD8 T cell dynamics was antigen-dependent and associated with clinical outcomes. Our findings highlight the clinical value of antigen-specific profiling and identify mechanistic correlates of anti-PD-1 efficacy.