TOLERANCE TO DISCORDANT XENOGRAFTS .1. SHARING OF HUMAN NATURAL ANTIBODY DETERMINANTS ON MINIATURE SWINE BONE-MARROW CELLS AND ENDOTHELIAL-CELLS

Xenotransplantation could potentially overcome the organ shortage that currently limits the field of transplantation. Because of their breed ing characteristics as well as their size and physiologic similarities to humans, we have chosen miniature swine as possible xenograft donor s, and are currently attempting to develop a means of using mixed xeno geneic chimerism as an approach to tolerance induction in swine-to-pri mate species combinations. One major barrier to organ grafting from pi g to man is the presence in human serum of preformed natural antibodie s (NAb) reacting with antigens expressed on porcine endothelial cells and causing hyperacute rejection. Previous experiments performed in ou r laboratory have shown that both humoral and cellular tolerance can b e induced in a concordant xenogeneic species combination (rat --> mous e) using donor bone marrow infusion following conditioning with a nonm yeloablative regimen. Induction of chimerism in these animals was asso ciated with a marked reduction in the level of IgM natural antibodies that recognize rat bone marrow cells. A similar approach could also le ad to humoral and cellular tolerance induction in the swine --> human species combination, permitting transplantation of vascularized organs from the swine donor. To determine the potential of bone marrow trans plantation to induce a state of ''natural antibody tolerance,'' it was essential to determine whether or not all human NAb target antigens e xpressed on swine EC are also expressed on cells derived from swine bo ne marrow. We have addressed this question by evaluating the ability o f various swine bone marrow-derived cells to absorb human IgM and IgG NAb that bind to swine EC. Our results demonstrate that swine bone mar row cells and their progeny can absorb almost all IgM NAb that bind to swine EC, as detected by flow cytometric and ELISA assays. Specificit y of absorption was demonstrated, as total serum IgM levels declined o nly minimally after absorption on swine BMC and to an extent comparabl e to that observed following absorption with human cells, which did no t deplete swine EC-binding NAb. Human IgG binding to swine EC was also completely absorbed by swine BMC. These results suggest that a state of ''NAb tolerance'' could be induced by successful swine marrow engra ftment in man. Furthermore, swine PBL, platelets, and EC were able to absorb most IgM NAb that bound to swine BMC, suggesting that absorptio n using antigen from any of these tissues might facilitate marrow engr aftment, and hence tolerance induction, in this species combination.