Tolerance to cardiac allografts induced in utero with fetal liver cells

Background: Induction of immunological tolerance in utero has potential application in pediatric cardiac transplantation. We describe an inexpensive, reproducible, and well-characterized model of allogeneic tolerance induction in utero using donor strain fetal liver cells. Methods and Results: Each of 9 Lewis (LEW, RT1 ¹) rat fetuses in one litter (group 1) and 10 LEW fetuses in another litter (group 2) were inoculated intraperitoneally at 17 to 18 days of gestation with 1 x 10 ⁶ ACI (RT1 ^a) rat fetal liver cells. Ten LEW fetuses in a third litter inoculated with PBS (group 3) and 10 LEW noninoculated fetuses in a fourth litter (group 4) served as controls. The LEW rats were brought to term, and groups 1, 3, and 4 were grafted with neonatal ACI skin within 24 hours of birth and with heterotopic ACI hearts at 8 to 10 weeks of age; group 2 rats received only an ACI heart graft at 8 to 10 weeks. Skin and cardiac grafts were monitored by daily visual inspection and palpation, respectively. Peripheral blood lymphocyte (PBL) populations in all LEW recipients were analyzed with flow cytometry. All LEW fetuses survived to term and developed normally. The ACI skin and cardiac allografts on 3 of the 9 LEW rats in group I are viable to date (skin, > 170 days; cardiac, > 100 days). The remaining 6 recipients of this group and all animals in groups 2, 3, and 4 rejected their skin and cardiac grafts by postgrafting day 7. Significant PBL chimerism (1.57%) was observed in only 1 tolerant rat. Conclusions: We describe allogeneic tolerance induction in utero to both rat skin and cardiac tissue with donor-strain fetal liver cells. Compared with previous studies using adult splenocytes as the tolerogen, we achieved a higher frequency of tolerance with a markedly lower cell inoculum and no abortions. Interestingly, both donor-strain fetal liver cells and neonatal skin grafts were required to maintain tolerance into adulthood. Immunocompetence sufficient to reject allografts was noted in neonates, and PBL chimerism was not prominent in tolerant recipients.