Luminal hormone-responsive cells tune the regenerative remodeling of mammary glands in large mammals.

The remodeling of mammary glands during pregnancy is essential for initiating lactation. In dairy animals, the overlap of pregnancy and mammary involution triggers a unique process, regenerative remodeling, which is critical for extending lactation duration and enhancing milk production. Unlike the complete regression of lobuloalveolar structures during involution, the regenerative remodeling preserves alveolar structures and promotes rapid mammary gland renewal. However, the cellular and molecular mechanisms underlying such process remain elusive. Here, taking dairy goats (Capra hircus) as a ruminant model, we identified four luminal cell populations through single-cell RNA-sequencing and found a significant reduction in luminal hormone-responsive (LumHR) cells and an increase in luminal secretory precursors (LumSecP) during regenerative remodeling. A reduction of LumHR cells during regenerative remodeling is essential for promoting the accumulation of LumSecP. Goat mammary organoids and in vivo genetic ablation assays suggested that LumHR cells function as a crucial switch for the differentiation of LumSecP to LumSec cells through the prolactin receptor pathway. Furthermore, high levels of IRF1 inhibited while downregulation of IRF1 stimulated the proliferation of LumHR cells. We showed that IRF1 regulated the dynamics of LumHR cells through hormonal signaling targets, including ESRRB. Our findings identified a key cell type responsible for the dynamics of luminal lineages during regenerative remodeling in large mammals and highlighted the potential for accelerating tissue regeneration through targeted modulation of lineage stage-specific regulators.