Infection‐Specific Responses of Breastmilk Leukocytes

Breastmilk leukocytes comprise a small population of mature human milk when both the mother and infant are healthy, providing active immunity and immunocompetence in the infant and protecting the mammary gland from infection. In this study, we examined leukocyte‐subset responses to different infections of the breastfeeding dyad in mature breastmilk. Breastmilk leukocyte content was measured by flow cytometry using the pan‐immune cell marker CD45, and leukocyte subtypes were examined by co‐staining for CD14, CD3, CD4, CD8, CD56, CD19, and CD15. When both the mother and infant were healthy, breastmilk contained 0‐2% of leykocytes. However, when either the mother or the infant had an infection, breastmilk leukocyte content increased to up to 94% of total cells, with the greatest increase observed in cases of mastitis. During the latter, the majority of immune cells were activated T‐cells that responded to viral peptides if stimulated in vitro , with corresponding proliferation and cytokine secretion. Interestingly, leukocyte content returned to baseline levels (蠄2%) upon recovery from the infection. Similar responses were observed for immunoglobulins and lactoferrin. Different leukocyte subset profiles were recorded for different infections, with T‐cells differing the most between infections (P<0.004). These results demonstrate that breastmilk leukcyte content is dynamic, responding to the needs of the infant and reflecting the health status of the lactating breast. This is a new tool that can be potentially used to facilitate rapid diagnosis and treatment of conditions of the lactating breast.