PDCD1 and graft versus host disease: We first found that most CD8+ T cells in the spleen of mice with acute GvHD at 7 days post-transplantation (dpt) were composed of donor T cells (Supplementary Fig. 1) and ~90% of CD8+ T cells highly expressed PD-1, reflecting TCR-mediated activation of alloreactive T cells (Fig. 1D).