Tumor antigen–specific CD8 T cells express PD-1 in tumors. Tumor digests from patients were thawed and immediately stained with anti-CD3, anti-CD8, and anti–PD-1 mAb and MART-1 tetramer. (A) Three representative patients are shown. The dot plots were gated on CD3+ CD8+ lymphocytes. The numbers represent the percentage of CD3+ CD8+ T cells for each quadrant, and the percentage values represent the fraction of PD-1+ T cells in MART-1 tetramer+ CD8 T cells (top right) and in MART-1 tetramer− CD8 T cells (bottom left). (B) The overall frequency of PD-1+ T cells in MART-1 tetramer− (MART-1−) and MART-1 tetramer+ (MART-1+) tumor-infiltrating CD8 T cells is shown for 6 patients. P value was calculated using the paired t test.