Temporal pattern of host responses against intrastriatal grafts of syngeneic, allogeneic or xenogeneic embryonic neuronal tissue in rats.

The host response to immunologically incompatible intrastriatal neural grafts was studied using immunohistochemical techniques. Dissociated ventral mesencephalic tissue from embryonic donors of either syngeneic, allogeneic or xenogeneic (mouse) origin was stereotaxically implanted into adult rats. The brains were analysed 4 days, 2 weeks or 6 weeks after grafting with antibodies against the following antigenic structures: major histocompatibility complex (MHC) class I antigens; MHC class II antigens; complement receptor (CR) 3 (marker for microglia and macrophages); helper T-lymphocyte antigen-cluster of differentiation (CD) 4; cytotoxic T-lymphocyte antigen-CD8; tyrosine hydroxylase (TH) (marker for transplanted dopaminergic neurons). The number of surviving TH-positive cells was not different at the various time points in either the syngeneic or allogeneic groups, whereas the xenogeneic cells were all rejected by 6 weeks. The host reactions were similar in character in the syngeneic and allogeneic groups. At 4 days after implantation, there were increased levels of expression of MHC class I and II antigens. In and around the grafts, there were cellular infiltrates consisting of activated microglia, macrophages, CD4- and CD8-positive lymphocytes. At 6 weeks, MHC expression was reduced and the cellular infiltrates had subsided with only low numbers of activated microglia cells and CD8-positive lymphocytes remaining. In the xenogeneic group, at 4 days, some grafts contained cavities, possibly reflecting acute rejection. At later stages, the xenografts were heavily infiltrated by macrophages, activated microglial cells and T-lymphocytes, and at 6 weeks all the xenografts were rejected. Taken together, the results suggest that there is an inflammation caused by the implantation process which leads to an accumulation of host defence cells. This, in turn, leads to increased MHC expression in and around the grafts. In syngeneic grafts, these reactions are short lasting and weak; for allografts slightly more pronounced and longer lasting than syngeneic grafts, but not sufficient to cause rejection. For xenografts, the reactions are more intense and lead to transplant rejection. Thus, a strong sustained inflammatory response may be an important determinator for the failure of histoincompatible neural grafts. It can be speculated that a short-term anti-inflammatory treatment of graft recipients may be a sufficient immunosuppressive regimen to allow long-term graft survival.