MHC expression in fragment and full-thickness allogeneic embryonic retinal transplants

The study was carried out to evaluate the expression of major histocompatibility complex (MHC) molecules in retinal transplants with different tissue integrity.

Twelve adult rabbits received an allogeneic subretinal neuroretinal transplant, in the form of either fragmented embryonic cells or a complete full-thickness embryonic retina. A controlled transvitreal approach was used for both transplantation types. The grafts were examined histologically after 31 or 49 days with hematoxylin and eosin staining and immunohistochemical analysis of MHC class I and class II expression.

All five fragment transplants developed into rosettes. Two of them displayed MHC class I-labeled cells, and four MHC class II-labeled cells. The cells were concentrated on the scleral side of the graft, and there was also a marked increase of labeled cells in the choroid. MHC labeling was often associated with defects in the retinal pigment epithelium. Six of the seven full-thickness grafts displayed a laminated morphology with well-developed retinal layers. The seventh consisted of rosettes. None of these grafts displayed MHC class I- or class II-labeled cells.

The findings suggest that host immune response against fragmented and intact neuroretinal grafts is different, indicating tissue integrity as one factor affecting graft-host immune interactions. The absence of immune response in full-thickness grafts is encouraging and important in the struggle to find therapies for retinal degenerative disease.