A Reinterpretation of Cell Transplantation: GFP Transfer From Donor to Host Photoreceptors

The utilization of fluorescent reporter transgenes to discriminate donor versus host cells has been a mainstay of photoreceptor transplantation research, the assumption being that the presence of reporter(+) cells in outer nuclear layer (ONL) of transplant recipients represents the integration of donor photoreceptors. We previously reported that GFP(+) cells in the ONL of cone-GFP transplanted retinas exhibited rod-like characteristics, raising the possibility that GFP signal in recipient tissue may not be a consequence of donor cell integration. To investigate the basis for this mismatch, we performed a series of transplantations using multiple transgenic donor and recipient models, and assessed cell identity using nuclear architecture, immunocytochemistry, and DNA prelabeling. Our results indicate that GFP(+) cells in the ONL fail to exhibit hallmark elements of donor cells, including nuclear hetero/euchromatin architecture. Furthermore, GFP signal does not appear to be a consequence of classic donor/host cell fusion or transfating post-transplant, but is most likely due to material exchange between donor and host photoreceptors. This transfer can be mediated by rods and cones, is bidirectional between donor and host cells, requires viable photoreceptors, occurs preferentially at sites of outer limiting membrane disruption and can be detected in second-order retinal neurons and Muller glia. Collectively, these data warrant re-evaluation of the use of lineage tracing fluorescent reporters in transplantation studies involving the retina and other CNS tissues. Furthermore, the reinterpretation of previous functional rescue data, based on material exchange, rather than cell integration, may offer a novel approach to vision rescue.