CTLA4 and graft versus host disease: We show herein that, despite the abundant expression of inducible PD-1 and CTLA-4 by donor CD4+ and CD8+ T cells, and inducible PD-L1 expression by both host and donor cells in lethally irradiated allo-HSCT recipients, expression of these molecules failed to prevent the development of severe GvHD in allo-HSCT recipients.