Expression of surface antigens on blood and mammary leukocytes in lactating and dry ewes

Flow cytometry was used to study the expression of CD4, CD8, CD14, CD18, WC1, MHCII, B-cell and L-selectin on CD45+ leukocytes in ovine blood and mammary secretions from lactating and dry ewes. The proportion of CD4+ lymphocytes was significantly lower and the proportion of CD8+ lymphocytes significantly higher in mammary secretions than in blood. This suggests that a selective migration to or retention of CD8+ cells in favour of CD4+ cells occurs in the mammary gland. L-selectin staining intensity on lymphocytes and neutrophils was lower in mammary secretions than in blood. In contrast, CD18 staining intensity was higher on leukocytes in milk, but not in dry secretions, than on corresponding cells in blood. These changes in adhesion molecules are in accord with shedding of L-selectin and up-regulation of CD18 during transendothelial migration. Expression of leukocyte antigens differed with lactational status mainly in mammary secretions. In milk, the staining intensity of CD18+ lymphocytes and neutrophils was higher than in dry secretions and more milk neutrophils expressed L-selectin. In contrast, greater numbers of IL-2R+ and MHCII+ lymphocytes and CD14+ leukocytes were observed in dry secretions. An increased proportion of these subpopulations could lead to increased immunological responsiveness of the udder during the dry period.