Lactation Stage-Dependent Changes of Lymphocyte Subpopulations in Mammary Secretions: Inversion of CD4+/CD8+ T Cell Ratios at Parturition

PROBLEM: Determination of lactation stage‐dependent changes in levels of lymphocyte subpopulations in milk. METHOD: Flow cytometric assay was used to identify and assay lymphocyte subpopulations in bovine milk at different stages of lactation. RESULTS: Lymphocyte subpopulations in mammary secretions of dairy cows change during the lactation cycle. In involuting glands (dry gland), ∼ 80–90% of lymphocytes were CD2+ T cells. The proportion of CD2+ T cells, however, decreased to ∼ 50% at the colostral stage and fluctuated between 50 to 60% in normal (mature) milk. Throughout the lactation stages, less than 5% were B cells as identified by the monoclonal antibodies against CD21 and MHC class II antigens. Subset analysis showed, however, that the proportion of CD5+ T cells decreased from 90% in involuting gland secretions to 75% in colostrum (peripartum stage), and to ∼ 40–50% in the normal (mature) milk. CD4+ T cells constituted between 45 to 55% of lymphocytes in the dry gland secretion but decreased drastically at parturition and maintained at the level below 20% throughout normal lactation. In contrast, the proportion of CD8+ T cells in the dry gland secretion was low, between 30 to 40%, but increased steadily, in an inversely‐related manner with that of CD4+ T cells, to ∼ 40–50% at parturition and maintained at ∼ 30–40% during the normal lactation stage thereafter. Two‐color immunofluorescence study revealed further that practically all of the CD8+ cells in dry gland secretions were CD2+, and approximately 40% of them were CD5—. Throughout the lactation cycle, WC1+γδ T cells comprised only 2 to 5% of lymphocytes in mammary secretions. CONCLUSIONS: T lymphocyte subpopulations change dynamically during stages of the lactation cycle. The selective migration of T lymphocyte subpopulations to and from the mammary gland, and their functional roles in the immune competence and regulation of the dam and sucklings remain to be elucidated.