PDCD1LG2 and graft versus host disease: As PD-L1 KO allo-HSCT recipients had severe acute GvHD and died within 10 days after transplant (S3 Fig), we sacrificed WT B6, PD-L1 KO and PD-L2 KO allo-HSCT recipients on day 8 after transplant for phenotypic analysis of splenocytes.