Mammary intra-epithelial lymphocytes promote lactogenesis and offspring fitness

Breastfeeding is an obligatory requirement of mammalian survival. This fundamental process is associated with the adaptation of maternal physiology including the transformation of the mammary gland into a milk-secreting organ. How maternal immunity contributes to 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 during pregnancy is associated with thymic involution and a paradoxical enrichment in intraepithelial lymphocyte (IEL) precursors that no longer migrate to the gut, but instead preferentially accumulate within the mammary gland. Within this compartment, IEL precursors differentiate into T-bet-expressing unconventional CD8?? lymphocytes with a cytotoxic signature which accumulate within the mammary epithelium in an IL-15-dependent manner. Mammary intra-epithelial lymphocytes control milk production by favoring differentiation and maturation of both contractile and milk-secreting cells, thereby promoting offspring fitness. Altogether, this work uncovers a previously unappreciated contribution of the maternal adaptive immune system in organismal remodeling during pregnancy that is associated with mammary gland development and function. Overall design: Human breastmilk samples from three different donors 3, 10 and 11 months post-partum.