REGULATION OF L-SELECTIN AND CD18 ON BOVINE NEUTROPHILS BY GLUCOCORTICOIDS - EFFECTS OF CORTISOL AND DEXAMETHASONE

The responsiveness of bovine neutrophil L-selectin and CD18 to in vivo glucocorticoid administration was characterized by flow cytometric an alysis. Blood was sampled intensively from dairy cows treated for 3 da ys with placebo, cortisol, or dexamethasone. Immunostaining was perfor med on whole blood (100 mu l) that was left unstimulated or was stimul ated with platelet-activating factor (PAF; 1 mu g/ml blood) prior to i ncubation with fluorescein isothiocyanate-conjugated monoclonal antibo dies against L-selectin and CD18. Results were expressed as the percen tage of positive-staining cells and as mean fluorescence intensity (MF I) of those cells. Total leukocyte count and leukocyte differentials w ere also performed on all blood samples. Dexamethasone caused nearly c omplete down-regulation of L-selectin (P <.01) on the surface of gated cells and reduced to half the MFI of CD18 (P <.01). Compared with val ues for the placebo group, dexamethasone began to cause L-selectin dow nregulation within 8 h after the first injection and these effects per sisted until 48 h after the third injection. This was correlated in ti me with an acute reduction in the proportion of cells that stained pos itive for L-selectin (from 98% before dexamethasone injections to a lo w of 17% by 40 h after the first injection). Dexamethasone also caused leukocytosis and neutrophilia during this time interval. In contrast, CD18 down-regulation was delayed until 16 h after the second dexameth asone injection and persisted for roughly 8 days. However, at no time during the experiment did dexamethasone influence the proportion of ga ted cells staining positive for CD18 (always 100%). Effects of cortiso l were generally similar in pattern to those of dexamethasone but were more subtle and more readily detected when PAF was added to blood pri or to immunostaining. These results strongly suggest that one mechanis m of the anti-inflammatory action of glucocorticoids is to induce dram atic down-regulation of L-selectin and CD18 adhesion molecules on bloo d neutrophils.