Photoreceptor (PR) cell replacement therapy by cell transplantation represents a promising therapeutic approach. Preclinical animal studies are encouraging, but many fundamental questions still remain to provide a solid basis for effective clinical translation. A key question is whether effective cell replacement therapies can be developed for inherited retinal dystrophies and macular degenerative diseases, i.e. which patients and pathology stages are optimal targets. Another key challenge, although evidence support successful integration of mouse and human PRs and the restoration of some visual function in small animal disease models, the number of integrating cells is still low. The mechanisms regulating and limiting cell transplant integration are still unknown, and might differ between animals and human. Data indicate that the properties of the host retina and the cell transplant both might determine if transplants may enter, structurally/functionally integrate and survive in the retina. Animal studies suggest that retinal degeneration, e.g. scars and glial pathology, might be a physical barrier for cell transplants. Thus, what defines and how can we stimulate intrinsic cell transplant properties for high integration competence, and a permissive host retina environment? To help overcoming such gaps and to facilitate clinical translation, a human experimental system might be useful. Human retinal organoid (HRO) technology enables the generation of 3D tissues that offer new perspectives for fundamental research and translational medicine. We established an efficient and robust hiPSC–derived HRO system, and develop with it an inducible complex retinal pathology model with cone and rod PR degeneration and glial pathologies. From this model, we identified novel pathomechanisms, that led to new hypotheses how to use latter to optimize transplant integration. Using our HRO system and pathology model, we established a preclinical human cell replacement therapy research platform in the first funding period, showing that PR transplants integrate into HRO with late-stage pathology in high numbers. Here, we will use this system for mechanistic studies of cell transplantation: to manipulate host HRO and transplants, and to optimize PR structural and functional integration. Our specific aims are: (1) Determine if PR transplant integration is pathology phenotype- and stage-dependent. We established several novel pathology models with different phenotypes in HROs. (2) Investigate if defined PR properties provide competence to actively invade the host retina, or whether PRs are taken up by the host retina. (3) Utilize the function of defined retinal pathomechanisms to stimulate PRs transplant integration. Together, the proposed project aims to perform hypothesis-driven, reproducible, quantifiable and mechanistic studies in a preclinical human setting, that might advance PR transplantation towards clinical translation.