Comparative ultrastructure and expression of L‐selectin on bovine αβ and γδ T cells

Compared to most αβ T cells, bovine γδ T cells express two to five times the level of L‐selectin and have a much higher capacity to roll on monolayers of endothelial cells, platelets, and leukocytes in assays done under physiological flow. To gain additional insight into the basis for these differences, scanning (SEM) and transmission (TEM) electron microscopy were used to compare the cellular ultrastructure of bovine γδ and αβ T cells and to study the expression of L‐selectin on these cells. It is interesting that γδ T cells had more than twice as many microvilli and other surface projections on a per cell basis as αβ T cells. This was not due to the γδ T cell being larger; after adhesion and fixation procedures used for the EM studies, γδ T cells averaged 3.9 ± 0.01 μm in diameter, whereas αβ T cells were slightly larger (5.7 ± 0.01 μm in diameter). As previously shown for human neutrophils and lymphocytes, L‐selectin was preferentially localized to the tips of the microvilli. In contrast, WC1, a lineage‐specific antigen on γδ T cells, was localized on the plasmalemma between the microvilli. Our findings suggest that more effective rolling of γδ T cells in various in vitro flow assays may be due to the greater number of microvilli on γδ T cells leading to a higher number of contact sites during adhesion events. In addition, this physical parameter may explain the increased level of L‐selectin expression on γδ versus αβ T cells because L‐selectin is clustered at the tips of microvilli. J. Leukoc. Biol . 64: 104–107; 1998.