Peripheral blood lymphocyte phenotypes following a single dose of anti–programmed death-1 monoclonal antibody (MDX-1106; 10 mg/kg) on day 1. (A) Percentages of circulating CD3, CD4, and CD8 cells declined steadily (two-sided P < .001), while CD19 and CD56 percentages rose reciprocally (two-sided P = .01; mixed model test for trends) over the 85-day observation period. (B) Expression of the activation markers CD25, CD45RO, and HLA-DR on circulating CD4+ and CD8+ T cells did not change significantly after treatment (Wilcoxon signed rank test and test for trends).