CMV-specific T cells recognizing virus in the context of MHC-I molecules secrete inflammatory cytokines, resulting in upregulation of MHC-II molecules both on the potential colonic targets of alloreactive DP-mismatched CD4+ T cells and on the CMV-specific T cells themselves. Recognition of these mismatched MHC-II molecules results in eradication of recipient-derived CMV-specific immunity and induction of local tissue damage (GVHD), with or without recruitment of further immune effector populations. IFN, interferon. Professional illustration by Paulette Dennis.