MHC antigen expression and cellular response in spontaneous and induced rejection of intracerebral neural xenografts in neonatal rats.

Fetal mouse retinae transplanted to the mesencephalon of neonatal rats generally survive for prolonged periods of time without immune suppression suggesting that such grafts enjoy a degree of immunological privilege. A small, but consistent percentage of these transplants, however, ultimately undergo spontaneous rejection. In addition, rejection can be induced by (1) systemically sensitizing the host to the donor antigens by placing a mouse skin graft or (2) producing a local degenerative process adjacent to the graft by removing the host eye contralateral to the side of the retinal transplant. To elucidate the immunological events that underly spontaneous and induced rejection in this system, we examined the distribution of lymphocytes, astrocytes, microglia, and cells expressing major histocompatibility complex (MHC) antigens in unrejected grafts, in transplants showing spontaneous rejection, and in grafts undergoing induced rejection. In unrejected grafts, increased astrocytic and microglial staining was seen around the photoreceptor layer of the graft and at the graft-host interface, but no lymphocytes and only occasional cells expressing MHC antigens were detected. In contrast, spontaneously rejecting grafts showed widespread MHC, lymphocytic, astrocytic, and microglial immunoreactivity that extended well beyond the limits of the transplant into the surrounding host brain. Skin graft-induced rejection produced a temporally consistent, comparatively localized enhancement of astrocytic, microglial and MHC immunoreactivity and infiltration of lymphocytes. Four to five days after skin grafting, before neural graft rejection was detectable histologically, MHC immunoreactivity was demonstrated within the transplant coinciding with the presence of small numbers of lymphocytes and an increase in microglial staining. By 8 days, grafts had undergone profound necrosis. Intense astrocytosis, microglial staining, MHC immunoreactivity, and perivascular lymphocytic cuffing were present within the graft and at the graft-host interface. With longer survival times, several of these changes were also detected within the visual pathways, suggesting that the regions to which the graft projected were also involved, though in a delayed fashion. After eye removal, the temporal pattern of rejection was more protracted and considerably less uniform than that seen after skin grafting. At 7 days, prominent microglial, astrocytic, and MHC immunoreactivity was seen in the area of distribution of the host optic axons within the superior colliculus and to a lesser extent around the graft itself, however, no infiltration of lymphocytes was detected. With longer survival times, an increasing percentage of grafts showed signs of overt rejection with perivascular cuffing by lymphocytes; however, even at 21 days, a small number of grafts remained free of lymphocytic infiltration, despite the presence of intense MHC, astrocytic, and microglial staining. We conclude that the different rejection models studied may involve fundamentally different triggers of the host immune system, but that in each case MHC expression may be the precedent step to graft rejection.