Using an established model of sub-acute GvHD (B6 → CB6F1) in which allo-HSCT recipients experienced 30% mortality and 20% weight loss by 100 days post transplant (S1 Fig), we first investigated the kinetics of PD-1, CTLA-4, PD-L1 and foxp3 expression on donor and host cells in hematolymphoid organs that are a target for GvHD. Here, CTLA4 is linked to graft versus host disease.