Maternal intra-epithelial lymphocytes support lactogenesis and offspring fitness

Breastfeeding is an obligatory requirement of mammalian survival. This fundamental process is associated with the remodeling of maternal physiology including the transformation of mammary gland into milk-secreting organ. While adaptive immunity has been associated with the broad control of host physiology, how maternal immunity contributes to organismal remodeling during pregnancy including mammary gland remodeling and function remains largely unknown. Here, we show that maternal adaptive immunity plays a critical role in shaping lactogenesis. Specifically, physiological adaptation to pregnancy is associated with thymic involution and paradoxical enrichment in intraepithelial lymphocyte precursors that no longer migrate to the gut but preferentially accumulate within the mammary gland. Within this compartment the precursors develop as T-bet expressing lymphocytes accumulating within the mammary epithelium in an IL-15 dependent manner. Mammary intra-epithelial lymphocytes control milk production by favoring differentiation of both contractile and milk-secreting cells, thereby affecting offspring fitness. Altogether, this work uncovers a previously unappreciated contribution of the maternal adaptive immune system in organismal remodeling during pregnancy associated with mammary gland development and function.