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Induction of β1integrin (CD49/CD29) expression in polymorphonuclear leukocytes (PMN) has been shown to be associated with transendothelial migration recently. Yet, β1integrin expression is relatively insensitive to cell activation with soluble agonists, such as N‐formyl‐methionyl‐leucyl‐phenylalanine (fMLP). We hypothesized that β2integrins (CD11/CD18), critically involved in PMN adhesion and extravasation, may play a role in regulating β1integrin expression in PMN. Antibody cross‐linking of CD18, mimicking adhesion‐dependent engagement of β2integrins, resulted in rapid, tyrosine kinase‐dependent upregulation of β1integrins. This response was potentiated by simultaneous chemoattractant (fMLP) stimulation of PMN. Moreover, upregulation of β1integrins evoked by CD18 cross‐linking was found to support adhesion of fMLP‐stimulated PMN to matrix proteins and also was critical for the ability of PMN to migrate in collagen gels in response to a gradient of fMLP. Taken together, these data demonstrate that engagement of β2integrins in human PMN induces β1integrin expression in these cells of significance for their migration in the extravascular tissue. Thus, β2integrins may serve the function to regulate PMN locomotion in extravascular tissue via receptor crosstalk with β1integrins.

Cell adhesion molecules are critically involved in the multistep process of leukocyte recruitment in inflammation. The specific receptors used by polymorphonuclear leukocytes (PMN) for locomotion in extravascular tissue have as yet not been identified. By means of immunofluorescence flow cytometry and laser scanning confocal microscopy, this study demonstrated that surface expression of the α2β1(VLA-2) integrin, though absent on blood PMN, is induced in extravasated PMN collected from human skin blister chambers, and rat PMN accumulated in the peritoneal cavity after chemotactic stimulation. Intravital time-lapse videomicroscopy was used to investigate chemoattractant-induced PMN locomotion in the rat mesentery in vivo. Local administration of function-blocking monoclonal antibody or peptide recognizing the α2β1integrin reduced PMN migration velocity in the extravascular tissue by 73% ± 3% and 70% ± 10%, respectively ( means ± SD). The distance f-met-leu-phe peptide (fMLP)-stimulated human PMN migrated in a collagen gel in vitro was markedly reduced by treatment with anti-α2mAbs or peptide, whereas no effect was observed with antibodies or peptides recognizing the α4β1or α5β1integrins. Further evidence for a critical role of expression of α2β1integrin in PMN locomotion in extravascular tissue was obtained in the mouse air pouch model of acute inflammation where chemoattractant-induced PMN recruitment was substantially inhibited by local anti-α2mAb treatment. Thus, expression of α2β1integrin on extravasated PMN has been identified and a novel role of this receptor in regulating the extravascular phase of leukocyte trafficking in inflammation has been formulated.

Leukotriene B4 (LTB4), a metabolite of arachidonic acid, is known to be a potent chemotactic and chemokinetic substance. We have used the hamster cheek pouch microcirculation model to study the effect of LTB4 on vascular permeability and the involvement of neutrophil granulocytes in this response. Intravascular fluorescein-labeled dextran (mol wt 150,000) was used as a tracer of macromolecular permeability. Topical application of LTB4 (150 nM-5 µM) to the hamster cheek pouch resulted in an immediate increase in adhering leukocytes in postcapillary venules and later larger venules. Leukocyte accumulation was reversible, but continued longer the higher the dose of LTB4 used. Subsequently, a dose-dependent increase in vascular permeability was seen at postcapillary and larger venules, with a maximum 10–20 min after application; the maximum occurred later the higher the dose of LTB4. Depletion of neutrophil granulocytes by pretreatment of the animals with antineutrophil serum obtained from immunized rabbits significantly decreased the permeability response to LTB4, whereas the response to histamine was unaffected. These results suggest that neutrophil granulocytes play a role in LTB4-mediated permeability increase. LTB4 may be of importance both for the leukocyte accumulation and for the edema formation seen in inflammatory reactions.