THE IMPORTANCE OF NONIMMUNE FACTORS IN RECONSTITUTION BY DISCORDANT XENOGENEIC HEMATOPOIETIC-CELLS

Bone marrow transplantation has been shown to induce donor-specific to lerance in rodent models. This approach could potentially be applied t o xenotransplantation across discordant species barriers. To evaluate host factors resisting hematopoietic cell engraftment, we have develop ed two model systems utilizing the combination of swine into severe co mbined immunodeficient (SCID) mice. SCID mice lack functional B and T lymphocytes, and can therefore be used to evaluate nonimmune factors r esisting marrow engraftment, and for adoptive transfer studies to test the role of immune cells and antibodies. First we transplanted swine bone marrow cells into SCID mice conditioned with whole-body irradiati on (4 Gy). For nine weeks following the intravenous administration of 10(8) swine bone marrow cells, up to 3.8% of peripheral blood leukocyt es were of swine origin, as determined by flow cytometry (FCM). These cells were all of the myeloid lineage. Swine IgG was also detectable i n the serum for up to 14 weeks. The bone marrow of the reconstituted m ice contained low percentages of swine myeloid cells, and swine myeloi d progenitors could be detected for up to 20 weeks after bone marrow t ransplantation. In a second model, we grafted thymus and liver tissue from 45-69-day-old swine fetuses under the kidney capsule of 4 Gy-irra diated SCID mice. A suspension containing 10(8) swine fetal liver cell s (FLC) was also administered i.p. Long-term repopulation with swine T cells was observed, with up to 1.5% swine T cells detected in the WBC , peritoneum, and spleen for at least 5.5 months postgrafting. These T cells expressed either CD4 or CD8, whereas up to 17.6% of cells in th e thymic grafts expressed both CD4 and CD8. The i.p. FLC suspension wa s required for optimal long-term graft maintenance. Our studies show t hat (1) low level myeloid and B lymphocyte reconstitution can be achie ved by transferring adult swine BMC to irradiated SCID recipients; (2) swine myeloid progenitors were detectable long-term in BMC of these m ice, suggesting that stem cell engraftment was achieved; and (3) T cel l reconstitution of SCID mice by swine progenitors requires cotranspla ntation of a swine stromal environment, as is provided by fetal swine thymus/liver grafts. We conclude that nonimmune factors such as those provided by species-specific stromal environments are important for re constitution of some lineages by discordant hematopoietic stem cells.