ANTI-GAL-ALPHA-1-3GAL ANTIBODY-RESPONSE TO PORCINE BONE-MARROW IN UNMODIFIED BABOONS AND BABOONS CONDITIONED FOR TOLERANCE INDUCTION

Background Mixed lymphohematopoietic chimerism can provide an effectiv e means of inducing longterm immunological tolerance and has been docu mented in a monkey allograft model, A conditioning regimen including n onmyeloablative or myeloablative irradiation and splenectomy has been used to induce chimerism in a pig-to-primate transplantation model. Si nce the presence of anti-Gal alpha 1-3Gal (alpha Gal) natural antibodi es leads to the hyperacute rejection of pig organs transplanted into p rimates, extracorporeal immunoaffinity adsorption (EIA) of anti-alpha Gal antibodies is also included in the regimen. The effect of the tole rance induction protocol on the anti-alpha Gal antibody response has b een assessed. Methods. Anti-alpha Gal antibody was measured after the EIA of plasma through an alpha Gal immunoaffinity column in baseline s tudies involving two unmodified baboons, one splenectomized baboon, an d one baboon that received a challenge with porcine bone marrow (BM), and in three groups of baboons (n=2 in each group) that received diffe rent conditioning regimens for tolerance induction. Group 1 received a nonmyeloablative conditioning regimen without porcine BM transplantat ion, Group 2 received nonmyeloablative conditioning with pig BM transp lantation and pig cytokine therapy. Group 3 received myeloablative con ditioning, an autologous BM transplant (with BM depleted of CD2(+) or CD2(+)/CD20(+) cells), and pig BM transplantation, Results. In the bas eline studies, a single EIA of anti-alpha Gal antibodies in an unmodif ied animal initially depleted anti-alpha Gal antibody, followed by a m ild rebound. Nonmyeloablative conditioning (group 1) in the absence of pig cell exposure reduced the rate of anti-alpha Gal antibody return. Pig BM cells markedly stimulated anti-alpha Gal antibody production i n an unmodified baboon (alpha Gal IgM and IgG levels increased 40- and 220-fold, respectively). This response was significantly reduced (to an only 2- to 5.5-fold increase of IgM and IgG) in baboons undergoing nonmyeloablative conditioning (group 2). A myeloablative conditioning regimen (group 3) prevented the antibody response to pig BM, with the reduction in response being greater in the baboon that received autolo gous BM depleted of both CD2(+) and CD20(+) cells. No new antibody dir ected against pig non-alpha Gal antigens was detected ill any baboon d uring the 1 month follow-up period. Conclusions. (i) EIA of anti-alpha Gal antibody in unmodified baboons results in a transient depletion f ollowed by a mild rebound of antibody; (ii) exposure to pig BM cells r esults in a substantial increase in anti-alpha Gal antibody production ; (iii) a nonmyeloablative conditioning regimen reduces the rate of an tibody return and (iv) markedly reduces the response to pig BM cells; (v) the anti-alpha Gal response is completely suppressed by a myeloabl ative regimen if CD2(+) and CD20(+) cells are eliminated from the auto logous BM inoculum. Furthermore, (vi) challenge with pig BM cells appe ars to stimulate only an anti-alpha Gal antibody response without the development of other (non-alpha Gal) anti-pig antibodies. We conclude that regimens used for T-cell tolerance induction can be beneficial in reducing the anti-alpha Gal antibody response to porcine BM.