Dynamics of antimicrobial proteins' expression and their bactericidal activity in mouse milk

Mother's milk is considered as "complete edible immune system." It contains macro- and micronutrients required to maintain infant growth and provides an excellent source for innate and adaptive immune proteins that not only protects infants from enteropathogens but also aid in the initial colonization of gut microbiota. In this study, we analyzed the milk of C57BL/6J dams and found significant changes in the composition of antimicrobial and immune proteins throughout the lactation period. Innate immune proteins, serum amyloid A, soluble CD14, and notably lipocalin-2 were detected in milk at high quantities. These proteins were substantially reduced in the milk from MyD88-deficient dams. Further, adaptive immune proteins, specifically IgA and IgG, exhibit a distinct shift during postpartum lactation stages. While IgG is the dominant immunoglobulin in milk at day 5 postpartum, by day 15 its levels were surpassed by IgA whose levels increased over time. The administration of TLR4 ligand LPS to WT dams significantly increased the aforementioned milk innate and adaptive proteins. Surprisingly, the milk from WT dams suppressed E. coli growth more effectively than milk collected from LPS-treated mice; such suppression, however, was completely lost upon boiling. Intriguingly, IgA, but not Lcn2, serves as a predominant factor in inhibiting E. coli proliferation, suggesting the critical role of IgA in regulating microbial colonization in the neonatal gut. Collectively, our findings provide insight into the dynamics of various immune proteins present in breast milk and highlight their pivotal roles in determining neonatal immune responses and microbial colonization at early stage.