XENOGENEIC HUMAN ANTI-PIG CYTOTOXICITY MEDIATED BY ACTIVATED NATURAL-KILLER-CELLS

Even if hyperacute rejection, which is mediated by human natural antib odies (nAb) and complement, could be prevented, xenoreactive human ant i-pig cellular responses may lead to delayed and/or chronic xenograft rejection. Among the cell populations participating in such rejection, Mt cells have been proposed as an important component. In this study we report the in vitro cytotoxic activity of natural killer (NK) cells obtained from healthy human donors against porcine target cells. Fres hly isolated peripheral blood mononuclear cells (PBMC) and purified NK cells (CD16+/CD56+, CD3-, CD20-, CD33-) exhibited little or no cytoto xic activity when tested on porcine phytohemagglutinin (PHA)-stimulate d lymphoblasts or bone marrow- or aortic-derived endothelial cell line s in the presence of serum-free medium. Killing was considerably highe r in the presence of human decomplemented plasma, containing xenoreact ive nAb, or purified Gal(alpha 1,3)Gal-reactive antibodies, suggesting that antibody dependent cell-mediated cytotoxicity (ADCC) mediated by NK cells is an important mechanism involved in xenogeneic cytotoxicit y. After incubation of human PBMC for 6 days in the presence of irradi ated xenogeneic porcine or allogeneic stimulator cells, or in the pres ence of exogenous interleukin 2 (IL-2), the cytotoxic activity of the bulk cultures as well as that of isolated NK cells (separated from sti mulated bulk cultures) against xenogeneic targets increased considerab ly, and corresponded to an increased NK-specific lysis of K562 target cells. Cell surface staining and flow cytometry showed that CD16+/CD56 +, CD3- NK cells composed ca. 25% of short-term (6 days) xenogeneic, a llogeneic, or IL-2 stimulated bulk cultures. In summary, these data su ggest that, in contrast to allogeneic cell-mediated killing, xenogenei c human anti-porcine cytotoxicity includes an important contribution f rom NK cells.