Traugh, N.; Trepicchio, C.; Rauner, G.; Parrish, M.; Kuperwasser, C.

Mesenchymal-epithelial interactions are well-established for organogenesis, but the role of immune cells in these processes remains less explored. Organoids offer powerful models for studying tissue development, especially where in vivo models are limited. In this study, we established an immune-epithelial co-culture model using an advanced 3D hydrogel-based breast organoid model that enables real-time investigation of human breast organogenesis. We employed live imaging and cell tracking techniques to visualize the induction process and the formation of an immune-mediated niche. Incorporation of immune cells into the organoid structure demonstrates their dual role as inducers of epithelial proliferation and as a persistent component of the stem cell niche. In addition, co-culturing epithelial cells with tissue-resident immune cells from healthy donors, particularly those enriched in double negative T cells, significantly accelerates organoid development and increases organoid formation through TGF-{beta} signaling. These findings highlight a previously unappreciated role for immune-epithelial crosstalk in early tissue development and provide insights into the dynamic nature of stem cell niches.