Roles of the innate immune system in mammary gland remodeling during involution

Mammary gland involution is a period of intensive tissue remodeling. Over the course of a relatively brief period, a large proportion of the mammary gland epithelium undergoes programmed cell death and is removed by phagocytes. In addition, the gland is cleared of residual milk fat globules as well as milk and adipocytes become the predominant cell type. The role of the immune system in this process has not been clearly defined. Professional phagocytes derived from the immune system can participate in the clearance of apoptotic and autophagic cells, the removal of residual milk components, and the prevention of mastitis during mammary gland involution. However, many of these functions can also be performed by non-professional phagocytes (e.g. mammary epithelial cells). This review will discuss the evidence that supports a role for innate immune cells in mammary gland remodeling during involution.