Bovine Milk Lymphocytes Display the Phenotype of Memory T Cells and Are Predominantly CD8+

The phenotype of bovine milk lymphocytes was investigated and compared to peripheral blood lymphocytes using monoclonal antibodies specific for bovine leukocyte differentiation antigens and flow cytometry. T Lymphocytes traffic selectively into bovine milk while B lymphocytes represent a minor population in milk by comparison to peripheral blood. The vast majority of T cells in milk express αβ T cell receptors and are predominantly CD8+. T cells in milk express twofold higher levels of CD2 and fivefold lower levels of CD45R, characteristics associated with memory T cells. Grouping of cows by lactational stage and analysis of lymphocyte subpopulation percentages indicated that CD4+ T cells are present in relatively low numbers in milk of cows in the first 50 days of lactation and have a significant tendency to increase in number as lactation progresses.