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Platelet‐induced neutrophil activation: platelet‐expressed fibrinogen induces the oxidative burst in neutrophils by an interaction with CD11C/CD18

Summary. Mutual contacts and platelet‐expressed fibrinogen seem to be required for the stimulation of neutrophils by activated platelets. The β2‐integrins CDllb/CD18 and CDllc/CD18 are potential receptors for fibrinogen on neutrophils. In order to investigate whether binding of fibrinogen to these integrins is involved, monoclonal antibodies (MoAbs) and Gly‐Pro‐Arg‐Pro (GPRP) peptide that inhibits fibrinogen binding to CDllc/CD18 were checked for their effects on the interaction of activated platelets and neutrophils. The luminol‐amplified chemilumi‐nescence (CL) as a measure for the oxidative burst of neutrophils was recorded simultaneously to the platelet aggregation in mixed cell suspensions. The adhesion of platelets and neutrophils was determined microscopically. The thromboxane A2 mimetic U46619 was used as a potent platelet agonist but that does not stimulate neutrophils. Stimulation of the platelets with U46619‐induced platelet aggregation and a strong CL of neutrophils. The platelet‐induced activation of neutrophils required added fibrinogen which fibronectin or thrombospondin could not substitute for. Cytochalasin D (Cyto D) that blocks actin polymerization totally abrogated the platelet‐induced CI of neutrophils. The MoAb OKM1 against CDllb, which blocks fibrinogen binding to CDllb/CD18 as well as the MoAbs I0T16 and IOT18 directed against CDlla and CD18, respectively, had no effect. In contrast, the MoAb LeuM5 which inhibits the binding of fibrinogen to CDllc/CD18 revealed a strong inhibition. Furthermore, GPRP peptide which CDllc/CD18 recognizes on the Aa‐chain of fibrinogen also strongly inhibited the platelet‐induced CL of neutrophils, whereas control peptides such as Gly‐His‐Arg‐Pro (GHRP) or Gly‐Pro‐Gly‐Gly (GPGG) had no effect. In contrast to the platelet‐induced CL of neutrophils, Cyto D, MoAb against CD11c and GPRP peptide did not inhibit the CL induced by FMLP and PAF in pure neutrophil suspensions. They also did not affect U46619‐induced platelet aggregation. The adhesion of platelets and neutrophils was neither dependent on added fibrinogen nor inhibited by Cyto D, MoAb against CD1 lc and the GPRP‐peptide. Therefore fibrinogen and actin polymerization seem not to be required for the adhesion of neutrophils to platelets. However, the activation of neutrophils depends on the interaction of CDllc/CD18 with the Aa‐chain of platelet‐expressed fibrinogen and the contractile system of neutrophils.