PD-L1−/− recipients of PD-1−/− T cells also exhibited a reduction in the severity of GVHD when compared to WT recipients, indicating that the interaction of recipient PD-L1 with CD80 expressed by donor CD4+ T cells in the absence of PD-1 increases their expansion, survival and pathogenesis (25). This evidence concerns the gene PDCD1 and graft versus host disease.