Contribution of primary and secondary capture to leukocyte accumulation in vivo

During inflammation, selectin and integrin adhesion molecules promote leukocyte capture, rolling, and arrest on activated endothelium. In addition to interacting with endothelial cells (primary capture), leukocytes can also interact with adherent leukocytes through an L-selectin-dependent mechanism (secondary capture). We used intravital microscopy of TNF-α-treated mouse cremaster muscles to investigate the potential role of primary and secondary capture in leukocyte accumulation in vivo. In TNF-α-treated venules (diameters of 23-108 μm), leukocyte accumulation occurred in clusters (19-50 μm in length; 8-44 μm in width) that were almost entirely composed of slowly rolling leukocytes. The majority of leukocytes entered the cluster by rolling along the endothelium. Of these, 17% passed through the cluster without a reduction in rolling velocity while others experienced a sharp reduction in rolling velocity in the absence (59%) or presence (15%) of other leukocytes within one cell diameter. Cluster formation was significantly reduced in E-selectin-deficient mice where slow leukocyte rolling is abolished. Only 1% of leukocytes were observed to be recruited into the cluster from the free flow through secondary capture by slowly rolling leukocytes, and cluster formation was normal in L-selectin-deficient mice. Thus, primary capture by the endothelium and not secondary capture by adherent leukocytes appears to dominate leukocyte accumulation in vivo.