Associations between function and composition of blood mononuclear leukocyte populations from Holstein bulls treated with dexamethasone.

To characterize further the effects of corticosteriod-induced stress on the immune system of dairy cattle, functional and phenotypic characteristics of populations of blood mononuclear leukocytes from control and treated (0.04 mg dexamethasone/kg per d for 3 consecutive d) Holstein bulls were evaluated concurrently. In vivo administration of dexamethasone caused a > or = 97% reduction in in vitro secretion of interferon-gamma by pokeweed mitogen-stimulated mononuclear leukocytes by d 2 after the first treatment. In vitro secretion of immunoglobulin M was reduced by > 50% on d 2 and 3 after the first treatment, but returned to normal concentrations sooner than did interferon-gamma secretion. Concurrent with changes in the secretion of these proteins were changes in the mean fluorescence intensities of major histocompatibility class I and II antigens and the WC1 antigen and in the proportion of B cells, CD3 T cells, gamma delta T cells, and cells in the leukocyte population expressing major histocompatibility class II antigens. Examination of the relationships between protein secretion in vitro and the composition of the blood mononuclear leukocyte population indicated that secretion was associated positively with the proportion of CD3 T cells (primarily the gamma delta T-cell subset) and the expression of major histocompatibility class I and II molecules and associated negatively with the proportion of cells expressing major histocompatibility class II antigens. Overall, these results suggest that corticosteroid-mediated stress in dairy cattle impairs secretion of proteins that are critical to normal cellular and humoral immune responses, an effect that is strongly linked with changes in the composition of the circulating mononuclear leukocyte population.