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

Abstract

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.

Figure 1

Forced mammary gland involution in mice. Hematoxylin and Eosin stained sections taken from the mammary glands of mifce that have undergone forced involution. On day 1, residual milk components (arrow) can be seen within the alveolar lumens which still resemble the lactating gland. Apoptotic cells can also be seen shed into the alveolar lumens (arrowhead). By day 5, the alveolar structures have collapsed and adipocytes appear as a more prominent component of the gland. By day 10, there is scant evidence of the mammary gland epithelium and the gland is primarily made up of fat, resembling the mature virgin gland. (Bar=0.2 mm).

Figure 2

Mammary gland epithelial cell-mediated apoptotic cell clearance in vivo. Late apoptotic cells (asterisk) can be seen in the alveolar lumen. An apoptotic cell can be seen within the cytoplasm of a viable epithelial cell (arrowhead) with a rim of clear-appearing extracellular fluid surrounding the apoptotic cell (the spacious phagosome).

Figure 3

Distribution of immune cells in the mammary gland during involution. Immunolocalization of cells from the immune system using an anti-CD45 antibody on day 4 of forced involution. Note that immune cells can be found scattered throughout the interstitium as well as adjacent to alveolar lumens (arrowhead) and mammary gland ducts (arrow).

Figure 4

Schematic diagram of mammary gland remodeling during involution. a Stage 1 of forced involution in mice (initial 48 h) is characterized by programmed cell death of epithelial cells and infiltration of the gland with neutrophils. At this stage the alveolar luminal structure of lactation remains intact, and viable epithelial cells are the predominant nonprofessional phagocytes responsible for removing apoptotic cells and milk fat globules. Note the relative paucity of adipocytes. b Stage 2 involution is characterized by massive epithelial cell apoptosis and alveolar collapse with infiltration of the gland by macrophages and lymphocytes. Macrophages likely mediate a proportion of apoptotic cell and milk fat globule phagocytosis at this time. Evidence of ingested apoptotic cells and milk fat globules within the cytoplasm of the remaining viable epithelial cells supports their ongoing participation in engulfment at this stage. The role of B-lymphocytes and plasma cells that infiltrate the gland is unclear. As Stage 2 involution progresses, adipocytes differentiate and repopulate the mammary gland stroma and with the loss of epithelial cells become the predominant cell type.