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Abstract. /$2 integrins are involved in the adhesion of leukocytes to other cells and surfaces. Although adhesion is required for cell locomotion, little is known regarding the way/32 integrin-receptors affect the actin network in leukocytes. In the present study filamentous actin (F-actin) levels in non-adherent human neutrophils have been measured by phalloidin staining after antibody cross-linking of/$2 integrins. Antibody engagement of/$2 integrins resulted in a rapid and sustained (146 and 131% after 30 and 300 s, respectively) increase in the neutrophil F-actin content. This is in contrast to stimulation with N-formyl-l-methionyl-lleucyl-/-phenylalanine (fMLP), which causes a prompt and pronounced but rapidly declining rise in F-actin (214 and 127% after 15 and 300 s, respectively). Priming neutrophils with 1 nM PMA, a low concentration that did not influence the F-actin content per se, increased the magnitude of the/$2 integrin-induced response but had no effect on the kinetics (199% after 30 s and 169% after 300 s). Removal of extracellular Ca 2÷ only marginally affected the/$~ integrin-induced F-actin response for cells that were pretreated with PMA whereas the response for nonprimed cells was reduced by half. This suggests that even though extracellular Ca 2÷ has a modulatory effect it is not an absolute requirement for/$2 integrin-induced actin polymerization. /$2 integrin engagement did not affect the resting cellular level of cAMP arguing against a role of cAMP in/$2 integrin-induced actin assembly. The lack of a cAMP signal might instead explain, at least in part, the prolonged F-actin response triggered by/$2 integrins, since addition of cAMP and 1-isobutylmethylxanthine (IBMX) caused a prompt reversal of the/32 integrin-induced F-actin elevation in electropermeabilized neutrophils. Engagement of/32 integrins, as previously shown for activation of the chemotactic peptide receptor, resulted in a significant formation of PtdInsP3. The capacity of/$2 integrins and chemotactic peptide receptors to induce phosphatidylinositol trisphosphate (PtdlnsP3) formation correlated with their ability to induce actin polymerization.

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