ICOS and graft versus host disease: Induction of GVHD involves the priming of donor T cells by antigen presenting cell (APC) co-stimulation through CD28-CD80/86 and ICOS interactions; GVHD is inhibited by CTLA-4 and programmed death (PD)/programmed death ligand 1 (PD-L1) interactions [9–11].