The complement system is pivotal in host defense but also contributes to tissue injury in several diseases. The assembly of C3 convertases (C4b2a and C3bBb) is a prerequisite for complement activation. The convertases catalyze C3b deposition on activator surfaces. Here we describe the identification of staphylococcal complement inhibitor, an excreted 9.8-kilodalton protein that blocks human complement by specific interaction with C4b2a and C3bBb. Staphylococcal complement inhibitor bound and stabilized C3 convertases, interfering with additional C3b deposition through the classical, lectin and alternative complement pathways. This led to a substantial decrease in phagocytosis and killing of Staphylococcus aureus by human neutrophils. As a highly active and small soluble protein that acts exclusively on surfaces, staphylococcal complement inhibitor may represent a promising anti-inflammatory molecule.

The complement system is an important immune mechanism mediating both recognition and elimination of foreign bodies. The lectin pathway is one pathway of three by which the complement system is activated. The characteristic protease of this pathway is Mannan-binding lectin (MBL)-associated serine protease 2 (MASP2), which cleaves complement proteins C2 and C4. We present a novel and alternative role of MASP2 in the innate immune system. We have shown that MASP2 is capable of promoting fibrinogen turnover by cleavage of prothrombin, generating thrombin. By using a truncated active form of MASP2 as well as full-length MASP2 in complex with MBL, we have shown that the thrombin generated is active and can cleave both factor XIII and fibrinogen, forming cross-linked fibrin. To explore the biological significance of these findings we showed that fibrin was covalently bound on a bacterial surface to which MBL/MASP2 complexes were bound. These findings suggest that, as has been proposed for invertebrates, limited clotting may contribute to the innate immune response.

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Mannose- or mannan-binding lectin (MBL) is a member of the collectin protein family, which includes lung surfactant proteins SP-A and SP-D. Each member consists of similar or identical polypeptide chains with a region of collagen-like sequence followed by a C-type lectin domain. The polypeptides associate in threes to form a subunit containing a collagen-like helix, with three clustered lectin domains. These subunits associate into larger structures, usually with 12-18 polypeptides. The collectins bind to patterns of neutral sugars on surfaces (e.g. of micro-organisms) and mediate effector functions associated with killing/phagocytosis. MBL is the only collectin which activates complement. It resembles in quaternary structure the complement protein C1q, which recognizes targets via charge clusters. Binding of MBL to a surface activates MBL-associated serine proteases (MASPs) attached to MBL, and MASP-2 activates complement proteins C4 and C2. The MASPs are homologous to the C1q-associated proteases, C1r and C1s. MBL therefore activates complement by a mechanism very similar to C1q, and engages the opsonic activity of complement to clear micro-organisms. The serum concentration of MBL is very variable in humans. The variability is largely associated with mutations leading to amino acid substitutions in the collagen-like region which decrease MBL assembly and stability. Many studies demonstrate that MBL deficiency is associated with susceptibility to a range of infectious and inflammatory diseases.

SummaryMannan-binding lectin (MBL) binds microorganisms via interactions with glycans on the target surface. Bound MBL subsequently activates MBLassociated serine protease proenzymes (MASPs). A role for MBL in hepatitis C virus (HCV) infection had been indicated by previous studies examining MBL levels and polymorphisms in relation to disease progression and response to treatment. We undertook this study to investigate a possible relationship between disease progression and functional MBL/MASP-1 complex activity. A functional assay for MBL/MASP-1 complex activity was employed to examine serum samples from patients with chronic HCV infection, non-HCV liver disease and healthy controls. Intrapatient consistency of MBL/MASP-1 complex activity levels was assessed in sequential samples from a subgroup of patients. Median values of MBL/MASP-1 complex activity were higher in sera from patients with liver disease compared with healthy controls. MBL/MASP-1 complex activity levels correlate with severity of fibrosis after adjusting for confounding factors (P = 0·003). MBL/MASP-1 complex activity was associated more significantly with fibrosis than was MBL concentration. The potential role of MBL/MASP-1 complex activity in disease progression is worthy of further study to investigate possible mechanistic links.

The lectin pathway of complement is activated by multimolecular complexes that recognize and bind to microbial polysaccharides. These complexes comprise a multimeric carbohydrate recognition subunit (either mannan-binding lectin (MBL) or a ficolin), three MBL-associated serine proteases (MASP-1, -2, and -3), and MAp19 (a truncated product of the MASP-2 gene). In this study we report the cloning of chicken MASP-2, MASP-3, and MAp19 and the organization of their genes and those for chicken MBL and a novel ficolin. Mammals usually possess two MBL genes and two or three ficolin genes, but chickens have only one of each, both of which represent the undiversified ancestors of the mammalian genes. The primary structure of chicken MASP-2 is 54% identical with those of the human and mouse MASP-2, and the organization of its gene is the same as in mammals. MASP-3 is even more conserved; chicken MASP-3 shares ∼75% of its residues with human and Xenopus MASP-3. It is more widely expressed than other lectin pathway components, suggesting a possible function of MASP-3 different from those of the other components. In mammals, MASP-1 and MASP-3 are alternatively spliced products of a single structural gene. We demonstrate the absence of MASP-1 in birds, possibly caused by the loss of MASP-1-specific exons during phylogeny. Despite the lack of MASP-1-like enzymatic activity in sera of chicken and other birds, avian lectin pathway complexes efficiently activate C4.

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