High antigen levels do not preclude B-cell tolerance induction to alpha 1,3-Gal via mixed chimerism

Background:Studies of bone marrow transplantation (BMT) from wild-type mice or rats to alpha 1,3-galactosyltransferase (GalT) knockout mice have demonstrated that induction of mixed chimerism tolerizes not only T cells, but also natural antibody-producing B cells, even across xenogeneic barriers. Given that rodent cells express lower levels of the alpha Gal epitope than the more clinically relevant porcine species, the consequences of exposure to cells expressing high levels of alpha Gal on the ability to induce B-cell tolerance are unknown. Methods:The effects on chimerism and anti- alpha Gal B-cell tolerance of an i.p. injection of 10 super(9) porcine RBC were evaluated in GalT knockout mice receiving wild-type allogeneic BMT after non-myeloablative conditioning with T-cell depleting monoclonal antibodies, thymic irradiation, and low-dose total body irradiation. Results:Achievement of mixed chimerism and tolerance of anti- alpha Gal-producing B cells was not affected by exposure to high-density alpha Gal at the time of BMT. The absence of induced anti- alpha Gal or anti-pig antibody responses in conditioned control mice suggested that the B-cell xeno-response to pig is T-cell-dependent. Conclusion:High alpha Gal density on pig cells might not preclude the ability to achieve tolerance of pre-existing alpha Gal-reactive human B cells via induction of mixed chimerism. This strategy has the potential to induce B-cell tolerance to non- alpha Gal epitopes, against which natural antibodies have been found in the sera of healthy humans.