A rodent model of in utero chimeric tolerance induction

In utero tolerance induction has potential application in pediatric heart transplantation. Immunotolerance appears to be more easily induced in the fetus before full immunocompetence is established; however, the mechanisms behind this phenomenon are still undefined.

One hundred thirty Lewis (RT1l) rat fetuses from 10 litters were inoculated intraperitoneally at 18 days gestation with 1 x 10(7) ACI (RT1a) rat fetal liver cells. Fifty of the 100 viable neonates successfully brought to term were grafted with neonatal ACI skin within 24 hours of birth and heterotopic ACI hearts at 8 to 10 weeks of age (group 1A); the remaining 50 neonates only received heterotopic ACI heart grafts at 8 to 10 weeks (group 1B). Control groups consisted of 50 Lewis fetuses (five litters) inoculated in utero with phosphate-buffered saline solution (group 2) and 50 Lewis fetuses (five litters) that received no inoculum (group 3); all of these surviving progeny received both neonatal ACI skin and adult ACI cardiac allografts. Skin and cardiac grafts were monitored by daily visual inspection and palpation, respectively. Limiting dilution analysis was performed among all groups to assess precursor cytotoxic lymphocyte frequencies. Likewise, peripheral blood lymphocyte and splenocyte populations were analyzed with flow cytometry to detect allogeneic chimerism.

Abortion rates among groups 1, 2, and 3 were 23% (30/130 abortions), 10% (5/50 abortions), and 6% (3/50 abortions), respectively. Tolerance to both ACI skin and cardiac allografts was induced in 14 of the 50 group 1A Lewis recipients (28%). Tolerance was not achieved in any of the recipients in groups 1B, 2, or 3. Limiting dilution analysis among all groups revealed a marked reduction of precursor cytotoxic T-lymphocytes in tolerant allograft recipients compared with recipients in the other groups. Flow cytometry detected significant splenocyte chimerism among tolerant rats; significant peripheral blood chimerism was not noted.

We describe allogeneic tolerance induction in utero to both rat skin and heart tissue by use of donor-strain fetal liver cells. Compared with previous studies with adult splenocytes as the tolerogen, we achieved a higher frequency of tolerance with a markedly lower cell inoculum and lower abortion rate. Allogeneic chimerism was noted in the tolerant recipients, suggesting hematopoietic stem cell engraftment. Cytotoxic T-lymphocyte precursor frequencies were markedly depressed in tolerant animals. Interestingly, both donor-strain fetal liver cells and neonatal skin grafts were required to induce tolerance. These data suggest a period of hematopoietic "education" during and shortly after hematopoietic stem cell engraftment.