RNA innate immunity constitutes a barrier for interspecies chimerism.

Creating interspecies chimeras with human pluripotent stem cells (hPSCs) offers a promising strategy for modeling human development and generating donor organs; however, poor human cell integration remains a major barrier. Most existing efforts to improve human chimerism focus on genetically modifying donor hPSCs, while altering the host embryo remains largely unexplored. Using an interspecies PSC competition model, we discovered that RNA innate immunity in "winner" mouse cells drives the competitive elimination of hPSCs. Disrupting RNA-sensing pathways reduced the competitiveness and viability of mouse PSCs, and mouse embryos lacking Mavs-a key gene in RNA innate immunity-led to markedly improved human cell survival and chimerism. We also found that contact-dependent horizontal RNA transfer likely underlies this immune activation. Overall, our study uncovers a previously unrecognized role for RNA innate immunity in cell competition and demonstrates that targeting host immune pathways represents a powerful approach to improve human chimerism in animals.