Breast milk leukocytes may play a role in protecting the infant from pathogens. The dynamics and the role of lymphocytes in human cytomegalovirus (HCMV)-seropositive mothers shedding HCMV into breast milk during the first months postpartum (p.p.) are mostly unclear.
Breast milk cells were analyzed by Pappenheim panoptic and alpha-naphthyl acetate esterase staining as well as by imaging and polychromatic flow cytometry to simultaneously establish their morphological and phenotypic properties. The latter were characterized in HCMV-seropositive and seronegative mothers´ breast milk cells at different time points p.p.
Panoptic staining of breast milk cells revealed the presence of monocytes/macrophages, granulocytes and lymphocytes. Imaging flow cytometry data combining phenotypic and morphological analysis identified NKT-like cells, NK cells, epithelial cells, T cells and monocytes/macrophages. HCMV-seropositive but not -seronegative mothers had significantly higher T cell frequencies in mature milk.
The presence of lymphocyte subsets in breast milk may be more influenced by the HCMV-seropositivity of the mother than previously recognized.