Microchimerism does not induce tolerance and sustains immunity after in utero transplantation

Background. To date, over 40 in utero transplants have been performed in humans; the only successes were documented in the treatment of severe combined immunodeficiency syndromes. Hemoglobinopathies and metabolic disorders are candidate diseases for this approach; however, when applied clinically, the results have been discouraging. To address the role of the fetal immune system in the outcome of in utero transplantation, we have developed a murine model of in utero transplantation in immunologically intact murine recipients and have studied chimerism and tolerance/immunity to allogeneic donor cells through the lives of the animals. Methods. We have performed experiments in which purified murine sca-1(+)/lin(-) cells and c-kit(+)/lin(-) cells of C57BL/6 (H2(b)) mice were injected into Balb/c (H2(d)) fetal recipients at early gestational ages. Chimerism was tested by highly sensitive semiquantitative polymerase chain reaction assay and tolerance/immunity to donor cells was studied by in vivo (skin grafts, responses to postnatal boosts) and in vitro (mixed lymphocyte culture, cytotoxicity, and cytokine release) assays. Results. One hundred pet-cent (10/10) of mice transplanted with c-kit(+) cells and 44% (4/9) of mice transplanted with sca(+) cells showed circulating donor cells within the first 6 months of life (P=0.031). Mice in the sca(+) group rejected donor skin grafts at a mean time of 9.1+/-0.2 days, whereas mice in the c-kit(+) group rejected donor skin grafts at a mean time of 15.1+/-0.7 days P=0.001). The difference between the transplanted groups and non-transplanted controls was also significant (P<0.05). All mice transplanted with sca(+)/lin(-) cells showed greater response to donor cells than to third-party cells at all effector to target ratios (P=0.002). Differences in response to donor alloantigen between sca(+) and c-kit(+) groups were significant P=0.003). Cytokine quantification demonstrated higher TH1 than TH2 cytokine release in all groups, and the response to donor cells was higher in the sca(+) compared with c-kit(+) mice (P=0.031). Conclusion, These results demonstrate a low level of chimerism and tolerance in mice transplanted in utero with sca(+/)lin(-) and c-kit(+)/lin(-) cells. The possibility of active in utero immunization to donor cells is supported by accelerated skin graft rejection in mice transplanted with sca(+) cells and enhanced in vitro immune responses in mice with persistent microchimerism.