The relative role that NK cells play in hyperacute and acute rejection of xenografts is unknown. This study is focused on the initial interactions between blood-borne NK cells and the vascular endothelium of the xenograft. The general objective of this project is to asses potential role of human NK cells and activated NK cells in acute vascular rejection of porcine xenografts in human recipients. Human NK cells and cytokine-activated NK cells may show significant recognition of porcine endothelial cells since the porcine cells lack expression of human MHC class I antigens. In Aim 1, the interaction of porcine endothelial cells with freshly isolated peripheral blood human NK cells or IL-2-activated NK cells will be examined by quantitative binding assays and standard 51Cr-release cytotoxicity assays. Additional consequences of these cell- cell interactions will be evaluated by assessing endothelial cell activation and the stimulation of both NK cell and endothelial cell cytokine synthesis and secretion. Natural antibodies and complement activation products may further enhance these interactions. In Aim 2, the effect of natural IgG and IgM antibodies on these interactions will be evaluated using the same battery of assays. Further, the mechanism by which antibody mediated enhancement occurs will be determined. In Aim 3, the effect of C5a-activation of NK cells and endothelial cells will be assessed as will the effect of opsonizing the endothelial cells with iC3b. In Aim 4, porcine target cells will be transfected with selected human MHC class I genes in an attempt to reduce the extent of human NK cell recognition. The findings from the proposed studies should relate to the clinical role of NK cells in xenograft rejection, mechanisms and consequences of NK cell activation, and the interplay of NK cells with other effector substances of natural and specific immunity.