Interaction between complement subcomponent C1q and bacterial lipopolysaccharides

Innate immune recognition and inflammation in Neisseria meningitidis infection

Neisseria meningitidis (Nme) can cause meningitis and sepsis, diseases which are characterised by an overwhelming inflammatory response. Inflammation is triggered by host pattern recognition receptors (PRRs) which are activated by pathogen-associated molecular patterns (PAMPs). Nme contains multiple PAMPs including lipooligosaccharide, peptidoglycan, proteins and metabolites. Various classes of PRRs including Toll-like receptors, NOD-like receptors, C-type lectins, scavenger receptors, pentraxins and others are expressed by the host to respond to any given microbe. While Toll-like receptors and NOD-like receptors are pivotal in triggering inflammation, other PRRs act as modulators of inflammation or aid in functional antimicrobial responses such as phagocytosis or complement activation. This review aims to give an overview of the various Nme PAMPs reported to date, the PRRs they activate and their implications during the inflammatory response to infection.

Factor H as a regulator of the classical pathway activation

C1q, the first subcomponent of the classical pathway, is a charge pattern recognition molecule that binds a diverse range of self, non-self and altered self ligands, leading to pro-inflammatory complement activation. Although complement is required for tissue homeostasis as well as defence against pathogens, exaggerated complement activation can be damaging to the tissue. Therefore, a fine balance between complement activation and inhibition is necessary. We have recently found that factor H, a polyanion recognition molecule and soluble regulator of alternative pathway activation in blood and on cell surfaces, can directly compete with C1q in binding to anionic phospholipids (cardiolipin), lipid A and Escherichia coli (three known activators of the classical pathway) and acts as a direct down regulator of the complement classical pathway. This ability of factor H to dampen classical pathway activation is distinct from its role as an alternative pathway down-regulator. Thus, by directly competing for specific C1q ligands (exogenous as well as endogenous), factor H is likely to be involved in fine-tuning and balancing the C1q-driven inflammatory processes in autoimmunity and infection. However, in the case of apoptotic cells, C1q-mediated enhancement of uptake/adhesion of the apoptotic cells by monocytes was reduced by factor H. Thus, factor H may be important in controlling the inflammation, which might arise from C1q deposition on apoptotic cells.

Interactions of complement proteins C1q and factor H with lipid A and Escherichia coli: further evidence that factor H regulates the classical complement pathway

Proteins of the complement system are known to interact with many charged substances. We recently characterized binding of C1q and factor H to immobilized and liposomal anionic phospholipids. Factor H inhibited C1q binding to anionic phospholipids, suggesting a role for factor H in regulating activation of the complement classical pathway by anionic phospholipids. To extend this finding, we examined interactions of C1q and factor H with lipid A, a well-characterized activator of the classical pathway. We report that C1q and factor H both bind to immobilized lipid A, lipid A liposomes and intact Escherichia coli TG1. Factor H competes with C1q for binding to these targets. Furthermore, increasing the factor H: C1q molar ratio in serum diminished C4b fixation, indicating that factor H diminishes classical pathway activation. The recombinant forms of the Cterminal, globular heads of C1q A, B and C chains bound to lipid A and E. coli in a manner qualitatively similar to native C1q, confirming that C1q interacts with these targets via its globular head region. These observations reinforce our proposal that factor H has an additional complement regulatory role of down-regulating classical pathway activation in response to certain targets. This is distinct from its role as an alternative pathway down-regulator. We suggest that under physiological conditions, factor H may serve as a downregulator of bacterially-driven inflammatory responses, thereby fine-tuning and balancing the inflammatory response in infections with Gram-negative bacteria.

Target pattern recognition by complement proteins of the classical and alternative pathways

The complement system is a major component of the innate defence of animals against invading microorganisms, and is also essential for the recognition and clearance of damaged or structurally-altered host cells or macromolecules. The system is activated by three different pathways, each of which responds, using different recognition molecules, to a very wide range of activators. The recognition protein of the complement classical pathway, C1q is described in detail here, with comparisons to the alternative pathway.

Interaction of the globular domain of human C1q with Salmonella typhimurium lipopolysaccharide

Gram-negative bacteria can bind complement protein C1q in an antibody-independent manner and activate classical pathway via their lipopolysaccharides (LPS). Earlier studies have implicated the collagen-like region of human C1q in binding LPS. In recent years, a number of C1q target molecules, previously considered to interact with collagen-like region of C1q, have been shown to bind via the globular domain (gC1q). Here we report, using recombinant forms of the globular head regions of C1q A, B and C chains, that LPS derived from Salmonella typhimurium interact specifically with the B-chain of the gC1q domain in a calcium-dependent manner. LPS and IgG-binding sites on the gC1q domain appear to be overlapping and this interaction can be inhibited by a synthetic C1q inhibitor, suggesting common interacting mechanisms.

Soluble CD14 discriminates slight structural differences between lipid as that lead to distinct host cell activation

Soluble CD14 (sCD14) in serum is known to sensitize host cells to LPS. In the present study, the contributions of sCD14 and LPS-binding protein to a lipid A moiety from LPS preparations of periodontopathogenic Fusobacterium nucleatum sp. nucleatum were compared with that of Escherichia coli-type synthetic lipid A (compound 506). F. nucleatum lipid A was identified to be a hexa-acylated fatty acid composed of tetradecanoate (C(14)) and hexadecanoate (C(16)), similar to dodecanoate (C(12)) and C(14) in compound 506. The two lipid A specimens exhibited nearly the same reactivity in Limulus amoebocyte lysate assays, though F. nucleatum lipid A showed a weaker lethal toxicity. Both lipid A specimens showed nearly the same activities toward host cells in the absence of FBS, though compound 506 exhibited much stronger activity in the presence of FBS, sCD14, or sCD14 together with LPS-binding protein. Furthermore, native PAGE/Western immunoblot assays demonstrated that F. nucleatum lipid A had a weaker binding to sCD14 as compared with compound 506. These results suggest that sCD14 is able to discriminate the slight structural differences between these lipid As, which causes their distinct host cell activation activities.

Human adiponectin binds to bacterial lipopolysaccharide

Adiponectin has anti-inflammatory and anti-atherogenic properties in addition to its acknowledged roles in insulin sensitivity and energy homeostasis. These properties include the suppression of lipopolysaccharide [LPS]-mediated inflammatory events. We demonstrated that both recombinant and native adiponectin directly bind LPS derived from three different bacteria. The interaction occurred at pH 5.0-6.0 and was inhibited by the presence of Ca(2+) and Mg(2+), but enhanced by the sequestration of these cations. Maximal binding occurred at pH 6.0 in the presence of ethylenediaminetetraacetic acid. Lipid A and C1q were not inhibitory, although LPS, heparin, zymosan, and individual sugars all inhibited the reaction. Periodate-mediated deglycosylation of adiponectin, and reduction and alkylation also inhibited binding. Since adiponectin infiltrates into [relatively] acidic sites of inflammation, it may act as a scavenging anti-inflammatory agent in atherosclerosis and vascular damage where LPS [and other pro-inflammatory molecules] are present.

Unusual electrostatic effects on binding of C1q to anionic liposomes: role of anionic phospholipid domains and their line tension

The binding of 125I-C1q to anionic liposomes was studied as a function of protein concentration, pH, ionic strength, and anionic lipid composition. The maximum amount of protein bound per micromole of lipid was very sensitive to electrostatic factors, increasing strongly with decreased pH and ionic strength or increased anionic lipid content. The apparent association constant was independent of these electrostatic factors, however, in marked contrast to studies on basic peptide binding to anionic lipid vesicles. Microscopic observations of large unilamellar liposomes containing fluorescently labeled C1q or phosphatidylglycerol demonstrated, under conditions causing strong electrostatic interactions, that C1q and anionic lipids colocalized into domains whose radii of curvature were higher than that of the surrounding lipid. These domains were observed to bud and pinch off into brightly fluorescent vesicles. We propose a model for all of these observations in which the line tension or edge energy at the boundary of the domain resists its increase in circumference as the domain grows by electrostatic effects on binding, eventually resulting in vesiculation. We propose that under favorable electrostatic conditions, as larger domains form the edge energy balances the increases in the electrostatic contribution to binding, resulting in a net binding energy independent of electrostatic factors.

C1q binding to liposomes is surface charge dependent and is inhibited by peptides consisting of residues 14-26 of the human C1qA chain in a sequence independent manner

Complement activation by anionic liposomes proceeds by antibody-independent, C1q-initiated activation of the classical pathway. Purified C1q bound to anionic liposomes in an acidic lipid concentration-dependent manner. Saturation binding, but not the apparent association constant, was enhanced by increasing the cardiolipin content of the liposomes or decreasing either the pH or ionic strength of the reaction mixture. These observations indicate the involvement of electrostatic factors in the binding. A highly cationic region in the collagen-like domain of C1q comprised of residues 14-26 of the C1qA polypeptide chain was assessed for involvement in liposome binding. This region has previously been shown to mediate C1q binding to other immunoglobulin-independent activators of the classical pathway of complement. Peptides containing residues 14-26 of C1qA, denoted C1qA14-26, inhibited C1q binding to and complement activation by anionic liposomes. The inhibitory capacity of these cationic peptides had no sequence or conformation specificity. Rather, the amount of positive charge on the peptides was the determining factor. When present in excess, peptides with five cationic residues inhibited C1q binding and complement activation; however, C1q peptides with only two cationic residues did not. In addition to the C1qA14-26 region, other parts of C1q that contain cationic residues may also be involved in C1q binding to anionic liposomes.

Interaction between complement subcomponent C1q and the Klebsiella pneumoniae porin OmpK36

The interaction between C1q, a subcomponent of the complement classical pathway component C1, and OmpK36, a porin protein from Klebsiella pneumoniae, was studied in a solid-phase direct-binding assay, inhibition assays with the purified globular and collagen-like regions of C1q, and cross-linking experiments. We have shown that the binding of C1q to the OmpK36 porin of the serum-sensitive strain K. pneumoniae KT707 occurs in an in vivo situation and that this binding leads to activation of the complement classical pathway and the subsequent deposition of complement components C3b and C5b-9 on the OmpK36 porin. Scatchard analysis of the binding of [125I]C1q to the OmpK36 porin showed two binding sites with dissociation constants of 1.5 and 75 nM. The decrease of [125I]C1q binding to the OmpK36 porin in buffer with increasing salt concentrations and the pIs of the C1q subcomponent (10.3) and OmpK36 porin (4.5) suggest that charged amino acids are involved in the binding phenomenon. In inhibition assays, only the globular regions of C1q inhibited the interaction between C1q and OmpK36 porin, demonstrating that C1q binds to porin through its globular region and not through the collagen-like stalks.

Chemical characterization and location of ionic interactions involved in the assembly of the C1 complex of human complement

The C1 complex of human complement comprises two loosely interacting subunits, C1q and the Ca(2+)-dependent C1s-C1r-C1r-C1s tetramer. With a view to gain information on the nature of the ionic interactions involved in C1 assembly, we have studied the effects of the chemical modifications of charged residues of C1q or the tetramer on their ability to reconstitute the C1 complex. Treatment of C1q with pyridoxal-5'-phosphate, acetic anhydride, and citraconic anhydride, as well as with cyclohexanedione and diethylpyrocarbonate, inhibited its ability to associate with C1s-C1r-C1r-C1s. Treatment of the collagen-like fragments of C1q with the same reagents yielded the same effects. Treatment of C1s-C1r-C1r-C1s with 1-ethyl-3-[-3-(dimethylamino) propyl] carbodiimide also prevented C1 assembly, through modification of acidic amino acids which were shown to be located in C1r. Further studies on the location of the interaction sites within C1q, using ligand-blotting and competition experiments with synthetic peptides, were unsuccessful, suggesting that these sites are contributed to by two or three of the C1q chains. It is concluded that C1 assembly involves interactions between acidic amino acids of C1r and lysine (hydroxylysine) and arginine residues located within the collagen-like region of C1q. Sequence comparison with mannan binding protein, another collagen-like molecule which binds the C1s-C1r-C1r-C1s tetramer, suggests Arg A38, and HyL B32, B65, and C29 of C1q as possible interaction sites.

Localization of sites through which C-reactive protein binds and activates complement to residues 14-26 and 76-92 of the human C1q A chain

Studies were initiated to localize the C-reactive protein (CRP) binding site on the collagen-like region (CLR) of C1q. CRP bound preferentially to the A chain of reduced C1q, in contrast to aggregated immunoglobulin G (Agg-IgG), which reacted preferentially with the C chain. A group of C1q A chain peptides, including peptides identical to residues 81-97, 76-92, and 14-26, respectively, were synthesized from predicted binding regions. Peptide 76-92 contained two proximal lysine groups, and peptide 14-26 contained four proximal arginine groups. CRP-trimers and CRP-ligand complexes did not bind to immobilized peptide 81-97, but bound avidly to immobilized peptides 76-92 and 14-26. Agg-IgG did not bind to any of the peptides. Peptide 76-92 partially, and peptide 14-26 completely, inhibited binding of CRP to intact C1q. Peptide 14-26 also blocked C consumption initiated by CRP, but not by IgG. Replacement of the two prolines with alanines, or scrambling the order of the amino acids, resulted in loss of ability of peptide 14-26 to inhibit C1q binding and C activation by CRP, indicating a sequence specificity, and not a charge specificity alone, as the basis for the inhibitory activity of the peptide. Similar investigations with scrambled peptides showed a sequence specificity for the effects of peptide 76-92 as well. DNA and heparin inhibited binding of CRP trimers to intact C1q, as well as to each peptide 14-26 and 76-92, suggesting involvement of these regions in C1q-CLR binding reactions generally. Collectively, these data identify two cationic regions within residues 14-26 and 76-92 of the C1q A chain CLR as sites through which CRP binds and activates the classical C pathway, and suggest that these residues represent significant regions for C1q CLR binding reactions generally. To our knowledge, this represents the first delineation of sites on C1q through which binding and activation of the classical C pathway can occur.

C1: molecular interactions with activating systems

The molecular events controlling complement activation have been gradually unravelled over the past three decades, stimulated by improved isolation procedures and a better understanding of the roles of individual proteins. In this review, Bob Sim and Ken Reid examine the interactions between C1q and its numerous ligands in the initiation of the classical pathway cascade.