Effects of dexamethasone* on bovine circulating T lymphocyte populations

In cattle, γδ T cells represent a higher proportion of circulating T cells than in humans. Bovine γδ T cells can be recognized by expression of γδ T cell receptor (γδ TCR) determinants or by a 215/230‐kDa surface antigen (WC1). WC1 is expressed on 90% or more of circulating bovine γδ 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 injection 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 mononuclear 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 αβ 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 γδ 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 αβ 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+ or WC1 + cells. The cumulative results suggested that dexamethasone promoted γδ T cell migration out of peripheral blood via an L‐selectin‐independent mechanism and that dexamethasone did not alter αβ T cell migration kinetics.