Immune privilege of allogeneic neuroretinal transplants in the subconjunctival space

The extent of site and tissue-associated immune privilege is of great interest in transplantation experiments involving the CNS. In the present paper we have explored neuroretinal immune privilege by transplantation to a non-immune privileged site.

Fetal and adult full-thickness rabbit neuroretinal grafts were placed in the subconjunctival space of immunocompetent rabbit hosts. Morphological examination was performed after 2-31 days (fetal grafts, n = 46), and after 8 days (adult grafts, n = 4).

Hematoxylin and eosin-stained sections and immunohistochemistry directed against microtubule-associated protein 2 (MAP2) revealed surviving grafts containing retinal neurons in the majority of eyes with fetal grafts. In all specimens, a mild inflammatory reaction was evident as seen with major histocompatibility complex class II (MHC-II) labeling. Short-term grafts survived well and displayed lamination and rosette formation whereas older grafts appeared more disorganized and were more often rejected. Müller cell fibers labeled with glial fibrillary acidic protein (GFAP) were present in grafts from 15 days and onwards. Adult grafts were destroyed after 8 days.

Allogeneic fetal full-thickness neuroretinal transplants can survive for several weeks in a non-immune privileged environment in which adult grafts are rapidly rejected. Fetal grafts gradually shrink, lose their architecture and go through a glial transformation accompanied by low-grade inflammation. The rabbit neuroretina thus appears to enjoy partial immune privilege, the extent of which depends on the development state of the tissue. The characterization of neuroretinal immune privilege will hopefully influence future clinical trials of retinal transplantation.