Transplanted human photoreceptors differentially survive, incorporate, and mature in mildly and severely degenerated mouse retinae

Photoreceptor transplantation is considered a disease-agnostic therapeutic strategy for retinal degenerative diseases with highly heterogenous genetic, molecular, and cellular pathologies. While integration of human photoreceptors enriched from stem cell-derived retinal organoids was noted in previous preclinical studies, the potential influence of retinal degeneration severity on transplantation efficiency has not been systematically assessed. Here, we employed mice presenting mild or severe retinal degeneration as recipients for human induced pluripotent stem cell-derived photoreceptors. Donor cells formed multi-cellular clusters that structurally integrated from 3 weeks post-transplantation (wpt) in mildly degenerated retinas, closely interacting with host Muller glia, resulting in proper maturation characterized by inner/outer segment and synapse formation by 26 wpt. In contrast, in severely degenerated hosts, donor photoreceptors remained mainly singularized and scattered in the subretinal space, showing limited structural integration or signs of maturation. Differential maturation of donor cells in mild vs. severe hosts was confirmed by single-cell RNA-sequencing analysis. However, transplantation at the beginning of the degeneration process of the severe model allowed structural integration and maturation of donor photoreceptors, despite complete loss of endogenous photoreceptors over time. The study thus shows that survival, integration, and maturation of donor photoreceptors depend on the degenerative retinal microenvironment shaping significantly transplantation efficiency.