High-dose porcine hematopoietic cell transplantation combined with CD40 ligand blockade in baboons prevents an induced anti-pig humoral response

Background. In pig-to-primate organ transplantation, hyperacute rejection c an be prevented, but the organ is rejected within days by acute vascular re jection, in which induced high-affinity anti-Gal alpha 1-3Gal (alpha Gal) I gG and possibly antibodies directed against new porcine (non-alpha Gal) ant igenic determinants are considered to play a major role. We have explored t he role of an anti-CD40L monoclonal antibody in modifying the humoral respo nse to porcine hematopoietic cells in baboons pretreated with a nonmyeloabl ative regimen. Methods, Porcine peripheral blood mobilized progenitor cells obtained by le ukapheresis from both major histocompatibility complex-inbred miniature swi ne (n=7) and human decay-accelerating factor pigs (n=3) were transplanted i nto baboons. Group 1 baboons (n=3) underwent whole body (300 cGy) and thymi c (700 cGy) irradiation, T cell depletion with ATG, complement depletion wi th cobra venom factor, short courses of cyclosporine, mycophenolate mofetil , porcine hematopoietic growth factors, and anti-alpha Gal antibody depleti on by immunoadsorption before transplantation of high doses (2-4 x 10(10)/c ells/kg) of peripheral blood mobilized progenitor cells. In group 2 (n=5), cyclosporine was replaced by eight doses of anti-CD40L monoclonal antibodie s over 14 days. The group 3 baboons (n=2) received the group 1 regimen plus 2 doses of anti-CD40L monoclonal antibodies (on days 0 and 2), Results, In group 1, sensitization to alpha Gal (with increases in IgM and IgG; of 3- to 6-fold and 100-fold, respectively) and the development of ant ibodies to new non-alpha Gal porcine antigens occurred within 20 days. In g roup 2, no sensitization to alpha Gal or non-alpha Gal determinants was see n, but alpha Gal-reactive antibodies did return to their pre- peripheral bl ood mobilized progenitor cells transplant levels. In group 3, attenuated se nsitization to alpha Gal antigens was seen after cessation of cyclosporine and mycophenolate mofetil therapy at 30 days (IgM 4-fold, IgG: 8-30-fold), but no antibodies developed against new porcine determinants. In no baboon did anti-CD40L monoclonal antibodies prevent sensitization to its own murin e antigene. Conclusions. We believe these studies are the first to consistently demonst rate prevention of a secondary humoral response after cell or organ transpl antation in a pig-to-primate model. The development of sensitization to the murine elements of the anti-CD40L monoclonal antibodies suggests that nonr esponsiveness to cell membrane-bound antigen (e.g., alpha Gal) is a specifi c phenomenon and not a general manifestation of immunological unresponsiven ess. T cell costimulatory blockade may facilitate induction of mixed hemato poietic chimerism and, consequently, of tolerance to pig organs and tissues .