Bovine γδ T cells express high levels of functional peripheral lymph node homing receptor (L-selectin)

Lymphocyte migration from the blood into specific tissues Is directed by their expression of adhesion molecules referred to as homing receptors. The homing receptor L-selectln, for example, directs the migration of lymphocytes into peripheral lymph nodes (PLN). Since bovine γδ T cells, a major lymphocyte subset in peripheral blood (25-50%), represent only a minor subset in PLN, we examined whether these cells lack expression or function of L-selectin. We found that bovine γδ T cells expressed L-selectln at levels higher (2- to 5-fold) than αβ T cells and B cells. Furthermore, γδ T cells accumulated along the vascular wall of venules that support lymphocyte extravasation into PLN (MECA-79⁺ venules) in vivo and bound mouse PLN high endothelial cell venules in an In vivo binding assay. In contrast to this primary adhesive event, we directly demonstrate that γδ T cells in vivo do not appreciably extravasate from the blood into the parenchyma of lymph nodes. Since the lack of functional L-selectln expression could not account for the inability of γδ T cells to enter PLN, we tested for other differences between γδ T cells and PLN homing lymphocytes related to the processes following primary adhesion; for instance, the down-regulation of L-selectin expression following short-term activation and the expression of accessory adhesion molecules necessary for transendothellal migration. We found that γδ and αβ T cells demonstrate differential down-regulation of L-selectin after PMA activation. Kinetic analysis revealed that, at all time points after PMA treatment, L-selectin expression remained significantly higher on γδ T cells and was down-regulated at a slower rate compared with αβ T cells. However, the expression levels of CD44 and CD18 on γδ and αβ T cells were found to be equivalent. This study Is the first to demonstrate for lymphocytes that the expression of L-selectln alone does not predict a PLN homing capacity. Our results suggest that the γδ T cells' reduced ability to enter PLN may be due to inefficient down-regulation of L-selectln compared with non-γδ lymphocytes, thus potentially disrupting the dynamics of the extravasation event.