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 percent (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.