The α₄-integrin supports leukocyte rolling and adhesion in chronically inflamed postcapillary venules in vivo

A role for the α₄-integrin (α₄β₁ or α₄β₇), has been implicated in the recruitment of peripheral blood mononuclear cells (PBMCs) to sites of inflammation. However, the adhesive interactions (i.e., tethering, rolling, and adhesion) mediated by the α₄-integrin have not been characterized in vivo. The objective of this study was to establish a model wherein postcapillary venules were chronically inflamed, and then use intravital microscopy to identify the adhesive interactions mediated by the α₄- integrin in vivo. Between 4 and 20 d after immunization with Mycobacterium butyricum, animals developed a systemic vasculitis characterized by large increases in the numbers of rolling and adhering leukocytes within mesenteric venules. The selectins could only account for ~50% of the leukocyte rolling whereas the remaining cells rolled exclusively via the α₄-integrin. Anti- α₄ therapy also eliminated the increase in leukocyte adhesion observed in this model, whereas selectin therapies and an anti-CD18 (β₂-integrin) monoclonal antibody (mAb) did not reduce adhesion. A serum against polymorphonuclear leukocytes (PMNs) was used to confirm that a significant proportion of rolling cells, and most of the adhering cells were PBMCs. Sequential treatment with anti-PMN serum and the anti-α₄ mAb demonstrated that α₄-dependent rolling was distinct from PMN rolling populations. Initial leukocyte tethering via the α₄-integrin could not be demonstrated in this model, whereas L-selectin did support leukocyte tethering. These data suggest that the α₄-integrin can mediate both rolling and adhesion in the multistep recruitment of PMBCs in vivo, and these interactions occur independently of the selectins and β₂-integrins.