Direct Evidence That the MHC Class Ib Antigen Qa-2 Regulates the Fetal Immune Response to Prenatal Allotransplantation.

The failure to achieve durable engraftment following prenatal transplantation in immunologically normal human fetal recipients calls for a closer examination of the fetal immune response to allotransplantation. Previous studies in mice suggest that the fetal innate immune system functions as a critical barrier to allogeneic engraftment mediated by recognition of MHC class Ib antigens. We hypothesized that Qa-2 (the putative murine homolog for HLA-G) might play an essential role in the modulation of fetal immune response to prenatally transplanted allogeneic cells. To address this hypothesis, we utilized B6.K1 mice as a donor strain. B6.K1 mice are Qa-2 deficient and are congenic with wild-type B6.Ly5.2 mice. Light density mononuclear cells (LDMCs) were harvested from the livers of 14 dpc fetal B6.K1 or B6.Ly5.2 mice and transplanted into age-matched allogeneic Balb/c fetal recipients at a dose of 10⁵ cells per fetus. Following delivery, peripheral blood chimerism was assessed serially in the recipients. Survival to weaning was similar between the groups without evidence of GVHD. At 3 weeks of age, recipients of B6.K1 cells demonstrated significantly lower peripheral blood chimerism levels than recipients of B6.Ly5.2 control cells. By 6 months of age, nearly all of the recipients of B6.K1 cells had lost their chimerism. Conversely, the chimerism levels in recipients of B6.Ly5.2 control cells remained stable suggesting that donor Qa-2 expression was essential for allograft survival. To assess the competitive capacity of the B6.K1 donor cells in the absence of immunologic disparity, B6.K1 or B6.Ly5.2 fetal liver LDMCs were transplanted into congenic B6.Ly5.1 hosts at the same cell dose per fetus. This resulted in stable long-term engraftment of the B6.K1 cells in all recipients. Chimerism levels were identical to those recipients who received B6.Ly5.2 control cells, confirming that the engraftment disparities observed in the allogeneic recipients resulted from immunologic rejection. To assess the resilience of this apparent Qa-2-dependent innate immune barrier, the allogeneic transplantation experiments were then repeated at a ten-fold higher donor cell dose (10⁶ cells/fetus). Early chimerism levels remained significantly lower in allogeneic recipients of Qa-2 deficient cells compared to controls. However, recipients of B6.K1 cells maintained their engraftment for more than 6 months indicating that the Qa-2-dependent fetal immune barrier may be overcome with higher levels of circulating antigen. From these experiments we conclude:

1. Host allorecognition of the class Ib antigen Qa-2 is crucial for durable engraftment following in utero transplantation;

2. The failed engraftment of Qa-2 deficient hematopoietic cells does not result from a defective competitive engraftment capacity;

3. Qa-2 dependent fetal immune rejection may be diminished by higher levels of early chimerism.

These experiments provide direct evidence for the critical role of MHC class Ib antigens in regulation of the fetal immune response to allotransplantation. Additionally, the demonstration of reliable engraftment following transplantation of higher cell doses provides a translationally relevant approach to enhance the clinical success of prenatal transplantation in immunologically normal hosts.