Transretinal migration of astrocytes and brain/spinal cord-like cells arising from subretinally transplanted human retinal organoids

Subretinal transplantation of human retinal organoid-derived cells can potentially restore vision lost in photoreceptor dystrophies. Prior studies analyzed the maturation, integration, and function of transplanted organoid cells in the subretinal region, but detailed molecular analysis of transplanted photoreceptor and of cells that migrate to other retinal layers has not been conducted. Here, we characterized migratory and non-migratory organoid-derived cells following subretinal transplantation into a dystrophic host. Retinal organoids derived from Crx-tdTomato+ H9 human embryonic stem cells (hESC) (aged D134) were transplanted into immunodeficient Rd1/NS mice. Graft-derived cells using single-cell RNA sequencing and histological analysis were analyzed 4.5 months later. Age-matched human retinal organoids maintained in vitro served as controls. Graft-derived cells migrated to all retinal layers, and underwent long-distance tangential migration. Transcriptomic analysis of transplanted cells identified two cell types not found in cultured controls – PAX2-positive retinal astrocytes, and ARX/NKX2-2/HOXC8-positive brain/spinal cord-like neural precursors. Some migrating cells were proliferative, but overall less proliferation was observed in the transplants than the cultured organoids. Transplanted rod and cone photoreceptors remained in the subretinal space, and were more mature than age-matched photoreceptors in cultured organoids. This study shows that extrinsic signals present in the degenerative subretinal space promote maturation of photoreceptors in transplanted retinal organoids, while also inducing formation of migratory cell populations that are not normally derived from retinal progenitors, either in cultured organoids or in vivo. This has important implications for the design and safety of cell-based therapies for treating photoreceptor dystrophies. Overall design: Human retinal organoids transplanted into the rd1 mouse retina or age matched cultured organoids were dissociated into a single cell suspension and run on the 10x Genomics Chromium Single Cell System platform using ver3.1 chemistry. Libraries were sequenced on Illumina NextSeq 500 and processed through the Cell Ranger pipeline using the GRCh38 reference genome.