Developing mammary terminal duct lobular units have a dynamic mucosal and stromal immune microenvironment

The human breast and ovine mammary gland undergo a striking degree of postnatal development, leading to formation of terminal duct lobular units (TDLUs). In this study we interrogated aspects of sheep TDLU growth to increase understanding of ovine mammogenesis and as a model for the study of breast development. Mammary epithelial proliferation is significantly higher in lambs less than two months old than in peri-pubertal animals. Ki67 expression is polarized to the leading edge of the developing TDLUs. Intraepithelial ductal macrophages exhibit striking periodicity and significantly increased density in lambs approaching puberty. Stromal macrophages are more abundant centrally than peripherally. The developing ovine mammary gland is infiltrated by intraepithelial and stromal T lymphocytes that are significantly more numerous in older lambs. In the stroma, hotspots of Ki67 expression colocalize with large aggregates of lymphocytes and macrophages. Multifocally these aggregates exhibit distinct organization consistent with tertiary lymphoid structures. The lamb mammary gland thus exhibits a dynamic mucosal and stromal immune microenvironment and, as such, constitutes a valuable model system that provides new insights into postnatal breast development. Summary statement Development of terminal duct lobular units in the sheep mammary gland involves distinct growth phases and macrophage and lymphocyte fluxes. Tertiary lymphoid structures are present subjacent to the mucosal epithelium. ### Competing Interest Statement The authors have declared no competing interest.