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

Abstract

Meconium aspiration syndrome is a serious condition of the newborn characterized by pulmonary inflammation with substantial neutrophil infiltration. We recently showed that meconium is a potent activator of complement. The aim of the present study was to investigate a possible role for complement in meconiuminduced neutrophil activation. Meconium was incubated in human whole blood anticoagulated with lepirudin, a specific thrombin inhibitor that does not affect complement activation. Complement activation was detected by measuring the terminal complement complex. Neutrophil oxidative burst and changes in CD11b and L-selectin expression were measured by flow cytometry. Complement was inhibited using the MAb 166-32 and 137-26, which block factor D and neutralize C5a, respectively. Meconium markedly activated the neutrophils, as revealed by up-regulation of CD11b, accentuation of L-selectin shedding, and induction of oxidative burst. Complement inhibition using the anti-factor D antibody completely (95-100%) blocked meconium-induced changes in CD11b and L-selectin expression, whereas oxidative burst was reduced by 60 -70%. The anti-C5a antibody inhibited the neutrophil activation to the same extent as anti-factor D. The data suggest that complement activation is largely responsible for the neutrophil inflammatory responses induced by meconium in vitro and that C5a is a key mediator of this response. Abbreviations ARDS, acute respiratory distress syndrome AU, arbitrary units DHR, dihydrorhodamine LPS, lipopolysaccharide MAS, meconium aspiration syndrome MFI, median fluorescence intensity PE, phycoerythrin PMA, phorbol-12-␤-myristat-13-␣-acetate TCC, terminal SC5b-9 complement complex Meconium aspiration syndrome (MAS) may cause a profound lung inflammation similar to acute respiratory distress syndrome (ARDS). A substantial increase in neutrophils in the airways; increased levels of inflammatory cytokines, eicosanoids, superoxide anions, and endothelin-1; expression of inducible nitric oxide synthase and cyclooxygenase-2; and activation of nuclear factor-B have been described in MAS (1-8). A profound inflammatory response and the release of leukocyte cytotoxic products have an important role in initiation and propagation of lung injury in ARDS, in which neutrophils and their cytotoxic products play a crucial role (9).The complement system is a key mediator of inflammation. Complement-mediated activation of neutrophils and monocytes leads to cytokine production; release of lipid mediators and histamine; oxidative burst; and altered expression of adhesion molecules, including increased expression of CD11b and shedding, revealed as decreased expression of CD62L. We showed recently that meconium is a potent activator of the alternative complement pathway in vitro (10). To study the interaction between all participating inflammatory systems, we used an in vitro human whole-blood model that is based on the use of lepirudin, a thrombin-specific anticoagulant that does not interfere with complement activation (11). We hypot...

Meconium Aspiration Syndrome: Possible Pathophysiological Mechanisms and Future Potential Therapies

“…C5a is placed at the top of the list of potentially harmful inflammatory mediators. Previous studies from our group demonstrated that meconium activates neutrophils through C5a in vitro [24]. We later documented that meconium has a potential to activate the initial complement pathways on a broader basis [18].…”

Candidate inhibitors of porcine complement

“…The role of complement activation has been shown to play an important part in the pathogenesis of acute inflammatory conditions like I/R-injury (91), meconium aspiration syndrome (MAS) (92), trauma (93) and sepsis (94). In addition complement is involved in chronic inflammatory diseases like glomerulonephritis and rheumatoid arthritis (95)(96)(97).…”

Complement activation in thrombotic thrombocytopenic purpura

“…Interestingly, infections and pregnancy may also induce activation of the complement system. Soluble complement activation related mediators, including anaphylatoxins and the terminal complement complex SC5b-9, may directly activate endothelial cells [11,12], neutrophils [13,14] and platelets [15,16]. Indeed, plasma of TTP patients was shown to induce apoptosis of endothelial cells and platelets [17][18][19][20] and activation of neutrophils and monocytes [21], leading to the formation of platelet-leukocyte Table 3 Correlation between complement activation product concentrations in the samples of patients with acute TTP, before the initiation of PEX-series (n = 13) complexes [22].…”