L-selectin-mediated leukocyte adhesion in vivo: microvillous distribution determines tethering efficiency, but not rolling velocity.

Adhesion receptors that are known to initiate contact (tethering) between blood-borne leukocytes and their endothelial counterreceptors are frequently concentrated on the microvilli of leukocytes. Other adhesion molecules are displayed either randomly or preferentially on the planar cell body. To determine whether ultrastructural distribution plays a role during tethering in vivo, we used pre-B cell transfectants expressing L- or E-selectin ectodomains linked to transmembrane/intracellular domains that mediated different surface distribution patterns. We analyzed the frequency and velocity of transfectant rolling in high endothelial venules of peripheral lymph nodes using an intravital microscopy model. Ectodomains on microvilli conferred a higher efficiency at initiating rolling than random distribution which, in turn, was more efficient than preferential expression on the cell body. The role of microvillous presentation was less accentuated in venules below 20 micrometers in diameter than in larger venules. In the narrow venules, tethering of cells with cell body expression may have been aided by forced margination through collision with erythrocytes. L-selectin transfected cells rolled 10-fold faster than E-selectin transfectants. Interestingly, rolling velocity histograms of cell lines expressing equivalent copy numbers of the same ectodomain were always similar, irrespective of the topographic distribution. Our data indicate that the distribution of adhesion receptors has a dramatic impact on contact initiation between leukocytes and endothelial cells, but does not play a role once rolling has been established.

Negative regulation of T cell homing by CD43.

We report that the cell surface mucin CD43 acts as an anti-adhesin on T lymphocytes. CD43-deficient murine lymphocytes homed significantly more frequently to secondary lymphoid organs than wild-type cells. Intravital microscopy of peripheral lymph node venules revealed that CD43-deficient lymphocytes were twice as likely to tether, roll, and stick than wild-type cells. This effect was due to CD43 interference with the homing receptor, L-selectin, and was most pronounced in venules with low L-selectin ligand density. In vitro, CD43-deficient cells tethered to L-selectin ligands more efficiently and rolled more slowly than wild-type lymphocytes. Thus, CD43 exerts a negative regulatory effect on T cell trafficking by counterbalancing L-selectin-mediated adhesion.