PDCD1LG2 and graft versus host disease: To investigate the mechanism by which PD-L1 expression in host tissue reduces GvHD severity [17], we determined the numbers and phenotype of CD45.1+ (donor spleen-derived) CD4+ and CD8+ T cells in the spleens of B6 allo-HSCT recipients that were PD-L1 KO, PD-L2 KO or WT.