TOLERIZATION OF ANTI-GAL-ALPHA-1-3GAL NATURAL ANTIBODY-FORMING B-CELLS BY INDUCTION OF MIXED CHIMERISM

Xenotransplantation could overcome the severe shortage of allogeneic o rgans, a major factor limiting organ transplantation. Unfortunately, t ransplantation of organs from pigs, the most suitable potential donor species, results in hyperacute rejection in primate recipients, due to the presence of anti-Gal alpha 1-3Gal (Gal) natural antibodies (NAbs) in their sera. We evaluated the ability to tolerize anti-Gal NAb-prod ucing B cells in alpha 1,3-galactosyltransferase knockout (GalT KO) mi ce using bone marrow transplantation (BMT) from GalT(+/+) wild-type (W T) mice. Lasting mixed chimerism was achieved in KO mice by cotranspla ntation of GalT KO and WT marrow after lethal irradiation. The levels of anti-Gal NAb in sera of mixed chimeras were reduced markedly 2 wk a fter BMT, and became undetectable at later time points. Immunization w ith Gal(+/+) xenogeneic cells filed to stimulate anti-Gal antibody pro duction in mixed chimeras, whereas the production of non-Gal-specific antixenoantigen antibodies was stimulated. An absence of anti-Gal-prod ucing B cells was demonstrated by enzyme-linked immunospot assays in m ixed KO+WT-->KO chimeras. Thus, mixed chimerism efficiently induces an ti-Gal-specific a cell tolerance in addition to T cell tolerance, prov iding a single approach to overcoming both the humoral and the cellula r immune barriers to discordant xenotransplantation.