Effects of dexamethasone on bovine circulating T lymphocyte populations

In cattle, gamma delta T cells represent a higher proportion of circulating T cells than in humans. Bovine gamma delta T cells can be recognized by expression of gamma delta cell receptor (gamma delta TCR) determinants 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 peripheral blood T lymphocytes were determined by flow cytometric analysis. Twelve 15-month old bulls were injected with dexamethasone (0.04 mg/kg/day) for 3 consecutive days and four bulls were untreated controls. Blood samples were collected daily for 3 days before dexamethasone injections and for an additional 7 days starting on the third day. Data were recorded as percent positive cells and as mean fluorescent intensity (MFI) of positive cells. Initially, CD3+ cells represented 65-73% of all peripheral blood clear cells (PBMC). Dexamethasone reduced CD3+/- cells (PBMC). Dexamethasone reduced CD3+ cells to 30% and these recovered to 50% positive cells by 9 days after the last dexamethasone injection. Loss of CD3+ cells was not due to reductions in alpha beta T cells because dexamethasone did not influence the percent CD2+, CD4+, or CD8+ cells. However, percent WC1+ cells rapidly declined from a baseline of 26.4% of PBMC to < 6% by the final injection. During injections, the MFI of WC1 increased. The MFI of WC1 returned to control values 7 days after the last injection of dexamethasone, but the percent gamma delta T cells recovered to only 14% WC1+ PBMC by the final day of the study. During its maximum effects on WC1, dexamethasone also caused a profound decrease of L-selectin MFI on remaining PBMC (mostly alpha beta T cells and monocyte/macrophages). In a second trial, two-color analyses determined that dexamethasone did not increase apoptosis in WC1+ cells and did not reduce L-selectin MFI on either CD2+ of WC1+ cells. The cumulative results suggested that dexamethasone promoted gamma delta T cell migration out of peripheral blood via an L-selectin-independent mechanism and that dexamethasone did not alter alpha beta T cell migration kinetics.