CD4 and graft versus host disease: Given that MLL-AF9 cells express MHC-II after transfer and are recognized by both CD4+ and CD8+ T cells in vitro and in vivo (20, 21), we used this model to assess the relative contributions of GrA produced by all T cells (i.e., CD4+ and CD8+ T cells) versus that produced only by CD4+ T cells in both the GVHD and GVL response.