We have previously described a form of xenograft rejection. mediated by natural killer (NK) cells, occurring in pig-to-primate organ transplants beyond the period of antibody-mediated hyperacute rejection. In this study. two distinct NK activation pathways were identified as mechanisms of pig aortic endotheliual cell (PAEC) lysis by human NK cells. Using an antibody-dependent cellular cytotoxicity (ADCC) assay, a progressive increase in human NK lysis of PAEC was observed following incubation with human IgG at increasing serum titer. In the absence of IgG, a second mechanism of PAEC lysis by human NK cells was observed following activation with IL-2. IL-2 activation of human NK cells increased lysis of PAEC by over 3-fold compared with ADCC. These results indicate that IL-2 activation of human NK cells induces significantly higher levels of lytic activity than does conventional ADCC involving IgG and FcRIII. We next investigated the role of MHC class I molecules in the regulation of NK lysis following IL-2 activation. PAEC expression of SLA class I molecules was increased by up to 75% by treatment with human TN Fa. Following treatment with TNFa at I u/ml, IL-2 activated human NK lysis of PAEC was inhibited at every effector:target (E:T) ratio tested. Maximal effect occurred at an E:T ratio of 10: I, with TN Fa inhibiting specific lysis by 59% (p

Keywords: Xenotransplantation, NKcells, Major Histocompatibility Complex, pig