Interactions between the Contact System, Neutrophils and Fibrinogen

Summary

Since plasma kallikrein activates human neutrophils, and in plasma prekallikrein (PK) circulates complexed with high molecular weight kininogen (HK), we determined whether HK could mediate kallikrein’s association with neutrophils. Human neutrophils were found to possess surface-membrane binding sites for HK but no internalization was detected at 37°C. ¹²⁵I-HK binding to neutrophils was dependent upon Zn⁺⁺, specific, saturable and reversible with a Kd of 9–18 nM and 40,000–70,000 sites per cell. Furthermore, HK found in neutrophils (240 ng/10⁷ neutrophils) also served as a cofactor for HNE secretion since neutrophils deficient in HK have reduced HNE secretion when stimulated in plasma deficient in HK or with purified kallikrein. Thus, neutrophil surface HK may serve as a receptor for kallikrein. Fibrinogen inhibited ¹²⁵I-fibrinogen bound specifically and reversibly to human neutrophils. Zn⁺⁺ (50 µM) was required for binding of ¹²⁵I-fibrinogen to neutrophils and the addition of Ca⁺⁺ (2 mM) increased the binding 2-fold. Excess HK completely inhibited binding of and displaced labeled fibringogen as well as unlabeled fibrinogen. Binding of¹²⁵I-fibrinogen was saturable with an apparent Kd of 170 nM and 140,000 sites/neutrophil. The binding of ¹²⁵I-fibrinogen to neutrophils was not inhibited by the peptide RGDS derived from the α-chain of fibrinogen, nor by the monoclonal antibodies (MAB) 10E5 to the platelet glycoprotein IIb/IIIa heterodimer. Fibrinogen binding was inhibited by a γ-chain peptide CYGHHLGGAKQAGDV and by MAB OKM1 but was not inhibited by OKM10, a MAB to a different domain of the adhesion glycoprotein Mac-1 (CR3). HK binding to neutrophils was not inhibited by OKM1. These observations were consistent with a further finding that fibrinogen is a noncompetitive inhibitor of ¹²⁵1-HK binding to neutrophils. These studies indicate that fibrinogen specifically binds to an integrin receptor (Mac-1) on the neutrophil surface through the carboxy terminal of the γ-chain and that HK inhibits this interaction.