CTLA4 and graft versus host disease: Engraftment in the complete absence of cytoreductive conditioning also has been possible in mice receiving repetitive infusions of donor BM and anti-CD40L mAb (26) or a very high number of donor BM cells (termed “megadose”) in the context of anti-CD40L mAb and CTLA4-Ig (27); notably, anti-CD40L mAb and CTLA4-Ig are also effective in inhibiting donor anti-host T cell responses culminating in GVHD (25, 32).