INDUCTION OF TOLERANCE IN NONDEFECTIVE MICE AFTER IN-UTERO TRANSPLANTATION OF MAJOR HISTOCOMPATIBILITY COMPLEX-MISMATCHED FETAL HEMATOPOIETIC STEM-CELLS

Significant morbidity and mortality are associated with the conditioni ng therapy needed for postnatal bone marrow transplantation (BMT) for inherited diseases. This could be eliminated with hematopoietic stem c ell (HSC) transplantation in utero, when the immunoincompetence of the fetus permits engraftment without the need for immunosuppressive ther apy. We have established an in utero (day 11 to day 13) model of HSC t ransplantation in nondefective, allogeneic major histocompatibility co mplex (MHC)-mismatched mice. Donor cells were from pooled fetal livers of C57BL/6 (H2(b), GPI-1b) mice. Engraftment was tested by quantitati ve polymerase chain reaction (PCR) for the Y chromosome in female reci pients (with 0.0001% sensitivity). Eight percent (3 of 36) of allogene ic mismatched (Balb-c, H-2(d)) recipients and 25% (3 of 12) of congeni c (C57B1/6, GPI-1a) recipients showed durable engraftment (male donor cells detected beyond 20 weeks of age) based on analysis of peripheral blood leukocytes (P > .08). When spleen and liver were analyzed, 51% (17 of 33) of allogeneic recipients and 50% (6 of 12) of congenic reci pients showed durable engraftment (P > .3). The percent donor cells th at durably engrafted varied from as low as 0.0001% in spleen and river to as high as 0.6% in peripheral blood. Postnatal boosting with a sin gle dose of allogeneic MHC-mismatched donor cells in a tolerant, engra fted mouse resulted in a significant increase in donor cells in the pe ripheral blood from 0.2% pre-boost to 5% 6 months after the boost. The re was no evidence of engraftment in nontolerant mice after the postna tal boost with a similar dose of donor cells. Twenty-two allogeneic re cipients were evaluated for donor skin graft acceptance at 6 to 12 mon ths of age. Three mice with engraftment in blood and/or tissue permane ntly accepted donor skin grafts, one of them with donor cells detectab le only in the liver. Six additional mice that showed prolonged skin g raft acceptance had no evidence of durable engraftment in the blood bu t were engrafted in the liver and/or spleen. The degree of engraftment in tolerant mice was low (less than or equal to 0.1% donor cells). We conclude that at an early gestational age in nondefective mice (1) hi gh rates of durable engraftment are achievable, although the degree of engraftment is usually low (less than 1%); (2) the percent of donor c ells in the peripheral blood may be increased by a postnatal boost of donor cells in tolerant animals without conditioning therapy; (3) MHC appears to have little influence on engraftment efficiency at an early gestational age; (4) a Very small number of circulating donor cells i n the blood or the tissues is sufficient for the induction and mainten ance of tolerance, and (5) the presence of donor cells in the circulat ing blood is not necessary for prolonged skin graft acceptance or main tenance of permanent skin graft acceptance. (C) 1995 by The American S ociety of Hematology.