CD8A and graft versus host disease: In addition to expansion, alloreactive T cells differentiate into multiple helper T cell (CD4+) and effector T cell (CD8+) subtypes, all of which are involved in allogeneic T cell responses with Th1 and Th17 cells thought to promote GVHD, whereas Th2 and Treg cells limit GVHD (1, 7, 8).