CD200 and esophageal cancer: We first tested expressions of a set of immune checkpoints, including CD200, PD-1, CD137L, CD273, CD274, CTLA-4, TIM-3, CD137, CD158, CD160, B-and T-lymphocyte attenuator (BTLA), CD28, CD27, TIGIT and CD278, in circulating CD4+ and CD8+ T cells that were derived from normal donors (n = 10) or esophageal cancer patients (n = 10, Figure 1).