Human embryonic retinal cell transplants in athymic immunodeficient rat hosts.

This study investigates the possibility to use the athymic "nude" rat as a host for the transplantation of human embryonic retinal cells without immunosuppression. The long-term development of such transplants is compared with results from our earlier study that used immunosuppressed rats, and showed transplant immunoreactivity for S-antigen. Several additional cell markers have been included: rhodopsin, rod (alpha-transducin, neuron-specific enolase (NSE), synaptophysin (SYN), cone-specific opsins, vimentin, cellular retinaldehyde binding protein (CRALBP), glial fibrillary acidic protein (GFAP), rat major histocompatibility antigen class II (MHC-II) and a rat macrophage marker (Ox-42). Human retinal cells (9-13 wk postconception) were transplanted to the eyes of 28 athymic rats. Host rats were kept in microisolator cages for up to 48 wk after surgery. Host immune response and the development of the transplants were studied using histology, immunohistochemistry and electron microscopy. When using retinas of donors 9-11 wk postconception, transplants grew to 2-3 mm in diameter with many rosettes, in 31 of 35 eyes. Transplants derived from donors 12-13 wk postconception did not survive as well (8 out of 11 eyes), were smaller and less organized. All transplants fused well with the host retina, better than corresponding transplants to immunosuppressed rat hosts. Most transplants appeared to be healthy, even after long survival times, and only occasionally were MHC-II positive macrophages observed in transplants or host retinas. All retinal layers were observed, except for an inner limiting membrane on the vitreous surface. The oldest transplants (34-57 wk total age = donor age + time after surgery) exhibited well developed photoreceptors, rods and cones, with inner and outer segments. SYN-staining showed the development of inner and outer plexiform layers. Although many cones stained for SYN and NSE, few were immunoreactive for red-green or blue opsin. Most rods became immunoreactive for S-antigen and rhodopsin. Transplant Müller cells stained for vimentin and CRALBP. Immunoreactivity for GFAP developed slowly and was not completely expressed in all transplant Müller cells until 44 wk total age. Nude rats offer an excellent model for the study of human retinal xenografts without the negative effects of immunosuppression. Compared to immunosuppressed rats, transplantation to nude rats gives consistent results and superior long-term survival of hosts and transplants.