The distribution of mononuclear cells isolated from the bovine mammary gland during the non-lactating (dry) period was examined using monoclonal antibodies against leukocyte cell surface antigens, cellular light scattering properties, and the presence of nonspecific esterase. Most of the mononuclear cells isolated during the dry period were lymphocytes. T cells predominated until about 1 week prior to parturition. During the week prior to calving, the percentage of B cells increased until it approximated T cells. The ratio of CD4 : CD8 cells was 2-3 : 1 for mammary gland T cells. This was similar to the ratio found in peripheral blood. At dry-off, about 12% of mammary mononuclear cells were macrophages. The macrophage percentage increased (to about 30%) at mid-dry and remained at this levels until parturition. PMN's were isolated with the mononuclear cells during the first 2 weeks dry and the week prior to calving. Three methods were used to identify mammary macrophages. Esterase staining (as an enzymatic method), forward angle/90° light scatter (based on size and internal complexity), and MHC class II/forward angle light scatter (based on size and surface makers) were compared. Each method yielded similar specificity for macrophage identification. Non-adherent cell fractions, obtained by passage of the cells over Sephadex G-10 columns, were enriched in CD4 positive T cells, somewhat depleted of B cells, and depleted of macrophages and PMN's. Cells eluted from G-10 columns, with lidocaine, were mostly lymphocytes, but reflected the cells loaded onto the column.
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