SummarySwine leucocyte antigen (SLA) class II molecules on porcine (p) cells play a crucial role in xenotransplantation as activators of recipient human CD4 + T cells. A human dominant-negative mutant class II transactivator (CIITA-DN) transgene under a CAG promoter with an endotheliumspecific Tie2 enhancer was constructed. CIITA-DN transgenic pigs were produced by nuclear transfer/embryo transfer. CIITA-DN pig cells were evaluated for expression of SLA class II with/without activation, and the human CD4 + T-cell response to cells from CIITA-DN and wild-type (WT) pigs was compared. Lymphocyte subset numbers and T-cell function in CIITA-DN pigs were compared with those in WT pigs. The expression of SLA class II on antigen-presenting cells from CIITA-DN pigs was significantly reduced (40-50% reduction compared with WT; P < 0Á01), and was completely suppressed on aortic endothelial cells (AECs) even after activation (100% suppression; P < 0Á01). The human CD4 + T-cell response to CIITA-DN pAECs was significantly weaker than to WT pAECs (60-80% suppression; P < 0Á01). Although there was a significantly lower frequency of CD4 + cells in the PBMCs from CIITA-DN (20%) than from WT (30%) pigs (P < 0Á01), T-cell proliferation was similar, suggesting no significant immunological compromise. Organs and cells from CIITA-DN pigs should be partially protected from the human cellular immune response.