MHC expression in syngeneic and allogeneic retinal cell transplants in therat

Background: The major histocompatibility complexes, MHC class I and II, are found only sparsely or not at all in the retina. Since the eye is immunopr ivileged, we decided to investigate how the MHC class I and Il antigens wer e influenced by a retinal transplant and whether this could be correlated t o rejection of the transplant, Methods: Fetal neural retinas of Sprague-Daw ley (SD) rats were implanted in the subretinal space of adult Lewis and SD rats. After 5 weeks the retinas and the transplants were evaluated with ant ibodies against MHC class I and II antigens as well as microglia. Results: In the syngeneic transplants no upregulation of MHC class I antigen was see n and no MHC class II-positive cells could be detected. In the allogeneic t ransplants, on the other hand, there was marked upregulation of MHC class I antigen. Numerous MHC class II antigen-positive cells were seen in the sub retinal transplant but also in the host retina. Conclusion: Allogeneic reti nal transplants seem to grow and thrive just as well as syngeneic transplan ts, but in the former there is considerable upregulation of MHC expression. Our interpretation of these results is that the allogeneic transplants are recognized as nonself, but that there is also something that modifies this reaction of the immune system at this level, preventing the rejection that would normally ensue.