EFFECTS OF DEXAMETHASONE ON BOVINE CIRCULATING T-LYMPHOCYTE POPULATIONS

In cattle, gamma delta T cells represent a higher proportion of circul ating T cells than in humans, Bovine gamma delta T cells can be recogn ized by expression of gamma delta T cell receptor (gamma delta TCR) de terminants or by a 215/230-kDa surface antigen (WC1), WC1 is expressed on 90% or more of circulating bovine gamma delta T cells, The effects of dexamethasone on this and other subsets (CD3, CD2, CD4, CD8) of pe ripheral blood T lymphocytes were determined by flow cytometric analys is, Twelve 15-month old bulls were injected with dexamethasone (0.04 m g/kg/day) for 3 consecutive days and four bulls were untreated control , Blood samples were collected daily for 3 days before dexamethasone i njections and for an additional 7 days starting on the third injection day, Data were recorded as percent positive cells and as mean fluores cent intensity (MFI) of positive cells, Initially, CD3+ cells represen ted 65-73% of all peripheral blood mononuclear cells (PBMC). Dexametha sone reduced CD3+ cells to 30%, and these recovered to 50% positive ce lls by 9 days after the fast dexamethasone injection, Loss of CD3+ cel ls was not due to reductions in alpha beta T cells because dexamethaso ne did not influence the percent CD2+, CD4+, or CD8+ cells, However, p ercent WC1+ cells rapidly declined from a baseline of 26.4% of PBMC to < 60% by the final injection. During injections, the MFT of WC1 incre ased, The MFI of WC1 returned to control values 7 days after the last injection of dexamethasone, but the percent gamma delta T cells recove red to only 14% WC1+ PBMC by the final day of the study, During its ma ximum effects on WC1, dexamethasone also caused a profound decrease of L-selectin MFI on remaining PBMC (mostly alpha beta T cells and monoc yte/macrophages), In a second trial, two-color analyses determined tha t dexamethasone did not increase apoptosis in WC1+ cells and did not r educe L-selectin MFI on either CD2+ or WC1+ cells. The cumulative resu lts suggested that dexamethasone promoted gamma delta T cell migration out of peripheral blood via an L-selectin-independent mechanism and t hat dexamethasone did not alter ap T cell migration kinetics.