0041-1337/02/7301-1/0 TRANSPLANTATION Vol. 73, 1–2, No. 1, January 15, 2002 Copyright © 2002 by Lippincott Williams & Wilkins, Inc. Printed in U.S.A. Transplantation  ANALYSIS & COMMENTARY Pig Hematopoietic Cell Chimerism in Baboons Conditioned with a Nonmyeloablative Regimen and CD154 Blockade. Transplantation 2002: 73: 12. L. Bu¨ hler, M. Awwad, S. Treter, Q. Chang, M. Basker, I. P. J. Alwayn, K. Teranishi, T. Ericsson, K. Moran, D. Harper, B. Kurilla-Mahon, C. A. Huang, R. Sackstein, M. Sykes, M. E. White-Scharf, D. H. Sachs, J. D. Down, and D. K. C. Cooper XENOTRANSPLANTATION SO CLOSE, BUT YET SO FAR Research in xenogeneic organ transplantation has made enormous progress since the initial studies. In this issue of Transplantation, Bu¨ hler and colleagues present a report, in which they are able to extend the con- cept of tolerance induction via the in- duction of a mixed hematopoietic chi- merism from the allogeneic to the discordant pig-to-primate xenogeneic combination. For the first time, they are able to show that a conditioning regimen, which initially has been de- veloped to and shown to be powerful enough to induce donor-specific allo- geneic tolerance in different small and large animal models (1), as well as in humans, can be transferred into the discordant pig-to-primate xenoge- neic combination. Over the years, the group has consequently developed and adapted their original regimen. They now show, that costimulatory blockade of the CD 40 ligand on re- cipient T cells with a monoclonal an- tibody (2), added to the basic regimen of central, deletional tolerance induc- tion leads to a substantial macro- scopic, multilineage chimerism in the peripheral blood of recipient baboons over a period of more than 4 weeks. The basic, nonmyeloablative protocol adopted from allogeneic tolerance in- duction studies consists of recipient splenectomy and irradiation, adsorp- tion of anti-Gal 1–3Gal antibodies in recipients using Gal 1–3Gal col- umns, immunosuppression, supple- mentation of porcine hematopoietic growth factors in recipients and the transplantation of porcine hemato- poietic stem cells (3). Although macroscopic chimerism diminishes after 5 and disappears af- ter a maximum of 33 days, microchi- merism could be detected in the pe- riphery of the respective animals for a period of more than 100 days. In addition porcine colony-forming ac- tivity in recipients for up to 33 days was shown. Undoubtedly, it is some- what euphemistic to interpret these data as “engraftment” of the porcine hematopoietic stem cells, as the au- thors have done, however, the persis- tence of hematopoietic stem cells in recipient baboons after transplanta- tion of mobilized porcine peripheral blood stem cells for more than 4 weeks suggests that such engraft- ment may or should indeed be possi- ble. A more profane interpretation of the data would simply suggest that the authors have developed an excel- lent immunosuppressive protocol, which allows short- term survival of their, supposedly tolerizing inoculum and that the disappearance of macro- chimerism is simply the result of an ongoing rejection of the stem cell graft. Indeed, it has been shown previously, that porcine grafts can survive for more than 4 weeks in non- human primates with immunosup- pressive regimens similar to the basic protocol used by the authors. Fur- thermore, Sykes has shown in a pre- vious article (4), that even in the con- cordant, rat-to-mouse combination, the donor hematopoietic stem cells enjoy a distinct and critical disadvan- tage due to incompatibility of hema- topoietic cytokines (5) and other, yet unidentified factors that lead to the ultimate demise of the graft and sub- sequent loss of chimerism. Therefore, it seems highly unlikely, that perma- nent engraftment, which is needed for the induction of stable chimerism and tolerance, can be ensued simply by supplementing a cocktail of three porcine hematopoietic growth factors. In the run toward long-term engraft- ment, many unidentified physiologi- cal and molecular hurdles may ap- pear in the future. However, there is no reason why any of these factors should prevent successful engraft- ment of porcine hematopoietic stem cells in humans or non-human primates. An additional interesting observa- tion of the authors is that costimula- tory blockade, shown here for the first time in xenogeneic organ trans- plantation, can prevent humoral sen- sitization of recipient baboons under- going xenogeneic cell or organ transplantation. Because naturally occurring antibodies against Gal 1–3Gal sugar moieties, present on all porcine cells, are responsible for hyperacute rejection of porcine or- gans in humans, and the humoral arm of the primate immune system may play an important role in me- dium and long-term graft rejection also, prevention of sensitization may be another significant immunomodu- latory step in promoting porcine graft survival. In this exciting report, the authors show for the first time that engraft- ment of porcine hematopoietic cells in a discordant xenogeneic environment may indeed be used to induce toler- ance, especially, if one now takes into account as has previously been shown, that xenogeneic tolerance can be induced by hematopoietic chimer- ism in the concordant rat-to-mouse animal model and that B cell toler- ance toward Gal 1–3Gal sugar moieties can be induced by the same method. In conclusion, these data suggest that the concept of donor-specific, xe- nogeneic central tolerance induction via hematopoietic chimerism is not only a theoretical academic exercise, but could be practically applied to clinical xenotransplantation. Their data demonstrate how far re- search in xenogeneic organ trans- plantation has progressed and how close to clinical application not only xenotransplantation, but the even more exciting concept of tolerance in- 1