Immunological reactions to neural grafts in the central nervous system

Immunological rejection is a lasting, although highly variable, threat to allo- and xenogeneic neural tissue grafted to the CNS of rodents, monkeys and man. One major determinant for rejection of intracerebral CNS grafts appears to be induction of major histocompatibility complex (MHC) antigens on the donor CNS cells. We have previously examined the cellular immune response against neural mouse xenografts undergoing rejection in the adult rat brain. In this study we focus on the astro- and microglial reactions within and around the graft, and the potential of individual host rat and donor mouse brain cells to express MHC antigens. Previous light microscopical observations of expression of rat MHC antigen class I by endothelial cells, microglial cells, and invading leukocytes were extended to the ultrastructural level and found to include a few astrocytes. Rat and mouse MHC antigen class II was only detected on leukocytes and activated microglial cells. The findings imply that within grafts of brain or spinal cord tissue donor astrocytes, microglial cells and endothelial cells can be induced to act as target cells for class I specific host T cytotoxic cells, while only (graft and host) microglial cells can be induced to express MHC antigen class II and present antigen to sensitized (and possibly also resting) host T helper cells.