Does the transplantation process modify the immunogenicity of fetal adrenal grafts in rat?

The concept that fetal tissue transplants enjoy an immunologic privilege grounds on the primary immaturity of major histocompatibility complex (MHC) expression. However, experiences in human organ transplantation reveal that the immunogenicity of any graft could be modified by external factors such as ischemia. Consequently, the question arises, whether the process of transplantation modifies the immunogenicity of fetal grafts. In a syngeneic rat model (Lewis), fetal adrenal glands were transplanted into the greater omentum of adult hosts. After harvesting the grafts sequentially, the immunogenicity was evaluated by analyzing the expression and distribution of the MHC classes I and II and were compared with untreated organs of equivalent age. The untreated fetal adrenal gland depicted little immunogenicity. However, compared with age-matched untreated control organs, at 2 wk after transplantation, the grafts demonstrated an increased expression of MHC I and II, upregulated throughout the entire adrenal cortex. No signs of MHC-mediated rejection were found. The upregulation of MHC persisted until the eighth week after transplantation. At 3 months after transplantation the expression of MHC I and II returned to the normal pattern of untreated controls. As this study used a purely syngeneic model, the immunologic changes observed could not be induced by a graft vs. host incompatibility, instead they were caused by experimental factors. The expressions of MHC class I and II was increased at 2 wk, but these proteins did not induce a T-cell mediated rejection or cellular infiltration. In conclusion, these findings question the concept of an immunologic privilege of fetal tissue transplants. Instead, experimental factors may modify the tissue's primary immaturity of its MHC. Further investigations must evaluate, whether the increase in MHC expression will have an impact on the rejection of fetal adrenal grafts in allogeneic hosts.