Directed differentiation protocols have recently been developed to produce stem cell-derived α (SC-α) cells as a potential component of a complete cell-based therapy for T1D, to complement the more widely studied stem cell-derived β (SC-β) cells. Differentiation protocols for SC-β cells produce off-target cell populations implicated in the development of outgrowths in SC-β cell grafts, but outgrowths from SC-α cells have not been explored. This study identifies that engrafted SC-α cells generate outgrowths of similar composition to SC-β cell outgrowths. Both cell types share outgrowth-driving populations marked by SOX9, CDX2, or SOX2. Single-cell RNA sequencing was used to reveal an enrichment in cell-adhesion signaling events in outgrowth-driving populations. Small-molecule inhibition of the Notch pathway was insufficient to disrupt all three outgrowth-driving populations. A comprehensive disruption of cell-adhesion signaling via single-cell dispersion and reaggregation is found to reduce the outgrowth propensity in engrafted SC-α and SC-β cells. Together, these results suggest that disrupting residual progenitor cells with SC-α and SC-β cell clusters can enhance the safety profile of these cell therapy products for T1D therapy.

Stem cell-derived β (SC-β) cells are a promising therapy for type 1 diabetes (T1D), but their long-term efficacy is limited by immune-mediated graft rejection. Platelet-derived exosome product (PEP) has emerged as a novel immunomodulatory agent, although its effect on SC-β cell xenograft rejection remains unclear. Here, we evaluated PEP in stimulated human peripheral blood mononuclear cells (PBMCs) and in an immunocompetent mouse model of SC-β cell transplantation under the kidney capsule. Immune-related gene expression was assessed by quantitative PCR, immune cell infiltration by immunofluorescence, and graft function by circulating human insulin. In vitro, PEP reduced NK-cell-associated gene expression, including NK1.1, EOMES, and KLRK1, and increased IL-10 expression. In vivo, untreated SC-β cell xenografts showed progressive immune infiltration and loss of detectable human insulin by day 14, whereas PEP-treated grafts retained detectable insulin through day 14 across co-transplantation, pretreatment, and systemic administration strategies. PEP co-transplantation delayed, but did not prevent, xenograft rejection, with graft loss observed by day 21. This delay was associated with reduced NK1.1+ cell infiltration and lower expression of selected inflammatory and rejection-associated markers, including NK1.1, Nos2, and Nlrp3. These findings suggest that PEP modulates graft-associated immune responses and may serve as an adjunct immunomodulatory strategy for stem cell-based therapies for type 1 diabetes. Further studies are needed to evaluate sustained graft durability, safety, and translational efficacy.