BN rats do not reject F344 brain allografts even after systemic sensitization

Embryonic brain tissue allografts under many circumstances survive transplantation into the brain. It is generally believed that such grafts will not survive if the host animal is systemically sensitized, by skin grafting or other means, to major histocompatibility complex (MHC) antigens of the donor animal. We have found that F344 brain grafts survive in BN hosts even when the host is systemically sensitized to F344 tissue. Embryonic cerebral neocortex from F344 donors was transplanted into BN host rats (n = 95). Subsequently, the host rats were systemically sensitized with donor skin (n = 25), brain tissue (n = 41), or spleen cells (n = 6) and compared with a control group of rats consisting of allografts with no sensitization or sham procedures (n = 23). Rejection of the transplants in BN rat hosts was not provoked by any of the sensitization methods tested. Minor immunological responses that did not result in rejection were, however, present in many host animals. We did not observe infiltration of W3/13+ T cells and OX8+ cytotoxic lymphocytes in any of the groups. Nevertheless, substantial infiltrations of OX6+ antigen-presenting cells and W3/25+ helper T cells were present. There was also an extensive enhancement of MHC class I immunoreactivity in parts of the grafted tissue developing within the third ventricle, but not for the same type of graft in the lateral ventricle. This increase of MHC class I expression was not accompanied by infiltration of cytotoxic T cells. Our findings thus suggest that neural graft rejection depends on general genetic susceptibility to immune reactions, particularly experimental allergic encephalomyelitis and not only on disparity between donor and host antigens encoded by the MHC. Moreover, enhancement of MHC class I and class II expression within transplanted tissue does not predict graft rejection.