Written by: Jyoti Madhusoodanan, Ph.D. | Issue # 116 | 2023
- Mammary glands in lactating mice contain distinct, short-lived macrophages.
- Lactation-induced macrophages (LiMACs) appear to play roles in responding to microbial infection.
- Human milk contains similar immune cells that may have related functions.
- LiMACs comprise up to 10 percent of cells in human milk.
Immune cells known as macrophages in the mammary glands of mice perform important functions such as supporting the formation of mammary ducts during puberty and helping tissues return to the pre-pregnancy state after lactation.
But mammary immune cells may also play a part in supporting infants’ immunity. At birth, babies’ immature immune systems rely on additional protection conferred by the antibodies in human milk. But human milk also carries immune cells that increase in number during infections, and less is known about their significance.
In a new study, Melanie Greter, an immunologist at the University of Zurich and her colleagues, analyzed the macrophages present in mouse mammary glands and found one distinct subset that was present only during lactation .
The team began by comparing the immune cells present in the mammary glands of lactating mice and mice that had never been pregnant. They found that two groups of tissue-resident macrophages previously reported to be in mammary glands were present  in lactating and non-lactating mice at constant levels throughout the first three weeks postpartum. But lactating animals also had a third, unique population of macrophages. By the fourth day after giving birth, these immune cells proliferated to become the majority of all myeloid cells and grew tenfold in number within the first twelve days postpartum. The proportion of these cells, named lactation-induced macrophages (liMACs), in mammary tissue decreased three weeks after birth, and they were rarely found in non-lactating animals.
LiMACs were largely present only in mammary tissue in areas of milk production. Single-cell RNA sequencing revealed that the genes expressed by these cells did not change significantly over the course of lactation. The cells “seemed to arise uniquely in the period between late pregnancy and early lactation,” the authors wrote in their publication
Considering the rapid increase in liMACs, the researchers tested whether they were a result of cells proliferating in the mammary tissue or if they were being formed by monocytes, another kind of immune cell that circulates through the body. The team labeled monocytes in mice and found that liMACs were distinct from the progeny of labeled monocytes.
Since immune cells shape the development of mammary ducts during puberty, the authors tested whether liMACs helped remodel the mammary glands during lactation. They found no difference in milk production between mice with these immune cells and those that were treated with an antibody that blocked the formation of liMACs. The researchers also found that liMACs did not alter the antibodies or other proteins present in milk.
To understand whether the maternal microbiome had an effect on liMACs, the authors compared conventional mice to germ-free mice that they exposed to four bacterial species. LiMACs were significantly more abundant in conventionally raised mice, suggesting that the microbiome influenced their formation.
LiMACs expressed several genes related to inflammation and response to a common microbial antigen named LPS, suggesting that they play a role in protecting animals from infections. The researchers also isolated liMACs and exposed them to different microbial proteins, including some from Escherichia coli and Staphylococcus aureus, two bacterial species commonly linked to mastitis. The team found the liMACs were phagocytic and responded to microbial antigens. To elucidate this function, the researchers tested liMACs in a mouse model of mastitis, where they injected animals with LPS to trigger severe inflammation. When mice were treated with a liMAC-depleting antibody prior to the LPS injection, harmful inflammation was significantly reduced, suggesting that these cells play a part in early inflammatory responses.
The researchers found cells similar to liMACs in human milk samples collected from a milk bank. Similar to cell proportions in mice, macrophages comprised 1–10% of cells in human milk. Collectively, these data suggest that macrophages in human milk may play a protective role in mastitis and other infections. “These observations indicate that liMacs might participate in immune surveillance to protect the mammary gland from invading pathogens and infection,” the authors wrote in their publication. “These data will open new avenues for investigating the functions of macrophages for mother and infant during the lactation period, in health and disease.”
 Cansever D, Petrova E, Krishnarajah S, Mussak C, Welsh CA, Mildenberger W, Mulder K, Kreiner V, Roussel E, Stifter SA, Andreadou M. Zwicky P, Jurado NP, Rehrauer H, Tan G, Liu Z, Blériot C, Ronchi F, Macpherson AJ, Ginhoux F, Natalucci G, Becher B, Greter M. Lactation-associated macrophages exist in murine mammary tissue and human milk. Nature Immunology. 2023 Jun 19:1-2.
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