Expression of PD-1 by CD4 and CD8 T cells infiltrating into tumors. Cryopreserved tumor digests from patients with metastatic melanoma (Pt) were thawed and immediately stained with CD3, CD8, and PD-1 mAbs. (A) The dot plots were gated on CD3⁺ lymphocytes. Quadrants were set based on isotype control mAbs. The percentages represent the fraction of PD-1⁺ T cells in CD8 T cells (top right) or CD4 T cells (top left). CD3⁺ CD8⁻ T cells were considered CD4 T cells throughout the study. (B) The percentage of PD-1⁺ CD4 T cells per total CD4 T-cell population and PD-1⁺ CD8 T cells per total CD8 T-cell population in tumor digests from 28 patients with metastatic melanoma is shown. P value was calculated using the paired t test.