Inhibition of C5a-induced inflammation with preserved C5b-9-mediated bactericidal activity in a human whole blood model of meningococcal sepsis

Abstract

The complement system plays an important role in the initial defense against Neisseria meningitidis. In contrast, uncontrolled activation in meningococcal sepsis contributes to the development of tissue damage and shock. In a novel human whole blood model of meningococcal sepsis, we studied the effect of complement inhibition on inflammation and bacterial killing. Monoclonal antibodies (mAbs) blocking lectin and alternative pathways inhibited complement activation by N meningitidis and oxidative burst induced in granulocytes and monocytes. Oxidative burst was critically dependent on CD11b/CD18 (CR3) expression but not on Fc␥-receptors. Specific inhibition of C5a using mAb 137-26 binding the C5a moiety of C5 before cleavage prohibited CR3 up-regulation, phagocytosis, and oxidative burst but had no effect on C5b-9 (TCC) formation, lysis, and bacterial killing. An mAb-blocking cleavage of C5, preventing C5a and TCC formation, showed the same effect on CR3, phagocytosis, and oxidative burst as the anti-C5a mAb but additionally inhibited TCC formation, lysis, and bacterial killing, consistent with a C5b-9-dependent killing mechanism. In conclusion, the anti-C5a mAb 137-26 inhibits the potentially harmful effects of N meningitidis-induced C5a formation while preserving complementmediated bacterial killing. We suggest that this may be an attractive approach for the treatment of meningococcal sepsis. IntroductionThe gram-negative bacterium Neisseria meningitidis is an important human pathogen worldwide. It can cause meningitis or fulminant meningococcal sepsis (FMS); the latter is an overwhelming and often lethal condition that may lead to death within 24 hours. The pathogenic mechanism leading to FMS is a breakdown of homeostasis by a massive activation of diverse inflammatory systems such as the cytokine network and plasma cascades such as the complement, coagulation, fibrinolytic, and kinin/kallikreinin systems. 1,2 The importance of the complement system in meningococcal disease is emphasized by several observations. Complement deficiencies are defined risk factors for meningococcal infections, indicating that complement is crucial in the initial defense against this bacterium. 3-5 On the other hand, during FMS, disease severity, tissue damage, and outcome are closely related to the degree of complement activation. 6,7 Thus, with respect to the pathogenesis of meningococcal disease, the complement system has been aptly named a double-edged sword. 8 Complement is activated on the surfaces of meningococci by one or more of the 3 initial complement-activating pathwaysclassical, lectin, and alternative. After the activation of complement factor-3 (C3) by any of these 3 pathways, a C5 convertase (C4b2a3b through the classical and lectin pathways or C3bBbC3b through the alternative pathway) is formed, and the pivotal C5 molecule is cleaved into C5a and C5b. C5b is the initial molecule in the formation of the terminal C5b-9 complement complex (TCC). Membrane-associated TCC, also designated the C5b-9 membrane attack comple...

Complement C5a Is a Key Mediator of Meconium-Induced Neutrophil Activation

“…In fact, TCC formation was slightly enhanced in the presence of anti-C5a. This observation may seem surprising at first glance but is consistent with previous data obtained when using this antibody to inhibit bacteria-induced C5a effects (50). The increased TCC formation cannot be explained by immune-complex formation because we excluded this possibility by experiments performed when this antibody was characterized (12).…”

Complement component 5 contributes to poor disease outcome in humans and mice with pneumococcal meningitis

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Properdin binding to complement activating surfaces depends on initial C3b deposition

“…Heat-inactivated bacteria were used for safety reasons. Heat inactivation causes only minor alterations to the bacterial membrane, and the biological activity is preserved (54).…”