C1q is the first subcomponent of the classical pathway of the complement system and a major connecting link between innate and acquired immunity. As a versatile charge pattern recognition molecule, C1q is capable of engaging a broad range of ligands via its heterotrimeric globular domain (gC1q) which is composed of the C-terminal regions of its A (ghA), B (ghB) and C (ghC) chains. Recent studies using recombinant forms of ghA, ghB and ghC have suggested that the gC1q domain has a modular organization and each chain can have differential ligand specificity. The crystal structure of the gC1q, molecular modeling and protein engineering studies have combined to illustrate how modular organization, charge distribution and the spatial orientation of the heterotrimeric assembly offer versatility of ligand recognition to C1q. Although the biochemical and structural studies have provided novel insights into the structure-function relationships within the gC1q domain, they have also raised many unexpected issues for debate.

The host defense functions of human C-reactive protein (CRP) depend to a great extent on its ability to activate the classical complement pathway. The aim of this study was to define the topology and structure of the CRP site that binds C1q, the recognition protein of the classical pathway. We have previously reported that residue Asp112 of CRP plays a major role in the formation of the C1q-binding site, while the neighboring Lys114 hinders C1q binding. The three-dimensional structure of CRP shows the presence of a deep, extended cleft in each protomer on the face of the pentamer opposite that containing the phosphocholine-binding sites. Asp112 is part of this marked cleft that is deep at its origin but becomes wider and shallower close to the inner edge of the protomer and the central pore of the pentamer. The shallow end of the pocket is bounded by the 112–114 loop, residues 86–92 (the inner loop), the C terminus of the protomer, and the C terminus of the pentraxin α-helix 169–176, particularly Tyr175. Mutational analysis of residues participating in the formation of this pocket demonstrates that Asp112 and Tyr175 are important contact residues for C1q binding, that Glu88 influences the conformational change in C1q necessary for complement activation, and that Asn158 and His38 probably contribute to the correct geometry of the binding site. Thus, it appears that the pocket at the open end of the cleft is the C1q-binding site of CRP.

C1q is the first subcomponent of the classical complement pathway that can interact with a range of biochemically and structurally diverse self and nonself ligands. The globular domain of C1q (gC1q), which is the ligand-recognition domain, is a heterotrimeric structure composed of the Cterminal regions of A (ghA), B (ghB), and C (ghC) chains. The expression and functional characterization of ghA, ghB, and ghC modules have revealed that each chain has specific and differential binding properties toward C1q ligands. It is largely considered that C1q-ligand interactions are ionic in nature; however, the complementary ligand-binding sites on C1q and the mechanisms of interactions are still unclear. To identify the residues on the gC1q domain that are likely to be involved in ligand recognition, we have generated a number of substitution mutants of ghA, ghB, and ghC modules and examined their interactions with three selected ligands: IgG1, Creactive protein (CRP), and pentraxin 3 (PTX3). Our results suggest that charged residues belonging to the apex of the gC1q heterotrimer (with participation of all three chains) as well as the side of the ghB are crucial for C1q binding to these ligands, and their contribution to each interaction is different. It is likely that a set of charged residues from the gC1q surface participate † This study was supported by the National Science Foundation of Bulgaria, Grant MY-K-1303 to L.T.R. and L-1000 to M.S.K. R.G.is sponsored by the Deutsche Forschungsgemeinschaft through the Graduiertenkolleg GK370. U.K. is funded by the European Commission, University of Oxford, and the Alexander von Humboldt Foundation. A.M. and B.B. are funded by TELETHON, MIUR (FIRB Fund), the European Commission, and AIRC.© 2006 American Chemical Society * Corresponding author. Phone: +44-1865+44- -222325. Fax: +44-1865 +49-641-9941259. ukishore@hotmail.comu.kishore@rediffmail.com. NIH Public Access Author ManuscriptBiochemistry. Author manuscript; available in PMC 2013 December 29. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript via different ionic and hydrogen bonds with corresponding residues from the ligand, instead of forming separate binding sites. Thus, a recently proposed model suggesting the rotation of the gC1q domain upon ligand recognition may be extended to C1q interaction with CRP and PTX3 in addition to IgG1.C1q is the first subcomponent of the classical complement pathway and its binding to IgGor IgM-containing immune complexes leads to the autoactivation of C1r, which in turn, activates C1s. C1r and C1s, the two serine protease proenzymes, together with C1q constitute C1, the first component of the classical pathway. The activation of the C1 complex (C1q + C1r2 + C1s2) subsequently leads to the activation of the C2−C9 components of the classical pathway and the formation of the terminal-membrane-attack complex (MAC) (1, 2). C1q is a versatile innate immune molecule that can bind a diverse range of self and nonself ligands, ranging from proteins to lipids ...

C-reactive protein (CRP)2 is an acute phase reactant that is markedly increased during infection, inflammation, and tissue injury (1-5). It is synthesized and secreted mainly by the liver in response to circulating inflammatory mediators (6, 7). Elevated serum CRP levels serve as a risk marker for cardiovascular disease and predict future cardiovascular events and mortality (8, 9).Data obtained both in vivo and in vitro indicate that CRP plays a role in vascular inflammation (10 -12). CRP can be detected in human atherosclerotic plaques co-localized with modified low density lipoprotein (13,14). It can also associate with the terminal complex of complement in the arterial wall, inducing its activation in plaques. CRP promotes the uptake of low density lipoprotein by macrophages (15) and exerts a mitogenic effect on vascular smooth muscle cells (16). CRP stimulates chemokine and adhesion molecule expression in vascular endothelial cells and enhances platelet adhesion to endothelial cells (17). These data suggest that CRP is not just a marker of cardiovascular risk but is a risk factor in its own right, and CRP plays a causal role in atherosclerosis and thrombosis. In fact, transgenic overexpression of human CRP has been shown to promote atherosclerosis in apoE Ϫ/Ϫ mice (18), as does chronic administration (19). These data support an hypothesis that CRP is a proinflammatory and pro-atherogenic factor.Inflammation is an important component in all stages of atherosclerosis, with proinflammatory cytokines and chemokines playing critical roles. IL-17 is a member of a novel group of proinflammatory cytokines that is composed of six major isoforms, IL-17A, -B, -C, -D, -E (also known as IL-25), and -F (20). These isoforms are encoded by unique genes and share little homology with other interleukins. IL-17 signals via IL-17 receptors, products of unique genes, and includes IL-17RA, -B (also known as IL-25R), -C, -D, and -E (20).IL-17A is the most widely studied cytokine of the IL-17 family. It signals via IL-17RA and exerts proinflammatory, pro-apoptotic, and pro-mitogenic effects. Unlike IL-17, which is considered a T-cell-specific cytokine (21), many cell types in the body express the receptors and are therefore targets of . In this study we investigated whether IL-17 stimulates CRP expression in human hepatocytes and CASMC, and we determined the signal transduction pathways involved in * This work was supported in part by the Research Service of the Department of Veterans Affairs and NHLBI Grant HL68020 from the National Institutes of Health (to B. C.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 To whom correspondence should be addressed: Medicine/Cardiology,

C-reactive protein (CRP) is an evolutionarily conserved protein. From arthropods to humans, CRP has been found in every organism where the presence of CRP has been sought. Human CRP is a pentamer made up of five identical subunits which binds to phosphocholine (PCh) in a Ca 2+ -dependent manner. In various species, we define a protein as CRP if it has any two of the following three characteristics: First, it is a cyclic oligomer of almost identical subunits of molecular weight 20–30 kDa. Second, it binds to PCh in a Ca 2+ -dependent manner. Third, it exhibits immunological cross-reactivity with human CRP. In the arthropod horseshoe crab, CRP is a constitutively expressed protein, while in humans, CRP is an acute phase plasma protein and a component of the acute phase response. As the nature of CRP gene expression evolved from a constitutively expressed protein in arthropods to an acute phase protein in humans, the definition of CRP became distinctive. In humans, CRP can be distinguished from other homologous proteins such as serum amyloid P, but this is not the case for most other vertebrates and invertebrates. Literature indicates that the binding ability of CRP to PCh is less relevant than its binding to other ligands. Human CRP displays structure-based ligand-binding specificities, but it is not known if that is true for invertebrate CRP. During evolution, changes in the intrachain disulfide and interchain disulfide bonds and changes in the glycosylation status of CRP may be responsible for different structure-function relationships of CRP in various species. More studies of invertebrate CRP are needed to understand the reasons behind such evolution of CRP. Also, CRP evolved as a component of and along with the development of the immune system. It is important to understand the biology of ancient CRP molecules because the knowledge could be useful for immunodeficient individuals.

Human C-reactive protein is a Ca2+-binding acute phase-protein with binding specificity for phosphocholine. Recent crystallographic and mutagenesis studies have provided a solid understanding of the structural biology of the protein, while experiments using transgenic mice have confirmed its host-defense function. The protein consists of five identical protomers in cyclic symmetry. On one face of each protomer there is a binding site for phosphocholine consisting of two Ca2+ ions that ligate the phosphate group and a hydrophobic pocket that accommodates the methyl groups of phosphocholine. On the opposite face is a deep cleft formed by parts of the N and C termini and bordered by an alpha-helix. Mutational studies indicate that the C1q-binding site of the molecule is located at the open end of this cleft with Asp112 and Tyr175 representing contact residues. Using human C-reactive protein transgenic mice, we investigated the host defense functions of the protein. Transgenic mice infected with Streptococcus pneumoniae had increased lifespan and lowered mortality compared to wild-type mice. This was attributable to an up to 400-fold reduction in bacteremia mediated mainly by the interaction of C-reactive protein with complement. A complement-independent host protective effect was also demonstrated.

SUMMARYC-reactive protein (CRP), the prototypical human acute phase protein, is produced primarily by hepatocytes. Its expression is modestly induced by interleukin (IL)-6 in Hep3B cells while IL-1, which alone has no effect, synergistically enhances the effects of IL-6. In previous studies of the proximal CRP promoter, we found that signal transducer and activator of transcription-3 (STAT3) and C/EBPb -mediated IL-6-induced transcription and that Rel p50 acted synergistically with C/ EBPb, in the absence of p65, to enhance CRP transcription. Neither a requirement nor a binding site for the classic nuclear factor (NF)-kB heterodimer p50/p65 were found. The current studies were undertaken to determine whether similar novel transcription factor interactions might regulate the endogenous CRP gene. Transiently overexpressed p50 or p65 induced CRP mRNA accumulation in Hep3B cells. The heterodimer p50/p65 was markedly more effective than p50 or p65 homodimers. Co-overexpression of p50 or p65 with C/EBPb or STAT3 synergistically enhanced CRP expression. Maximal expression was observed with overexpression of all four transcription factors; comparable effects were observed with IL-1b treatment of cells overexpressing STAT3 C/EBPb. Data from the Human Genome Project revealed 13 potential kB sites in the ®rst 4000 bases of the CRP promoter, only one of which, centred at À2652, bound nuclear p50/p65 heterodimer activated by IL-1b. Our ®ndings indicate that classical NF-kB activation can participate in endogenous CRP induction, and that activated NF-kB may synergistically enhance the effects of C/EBPb and STAT3. They raise the possibility, not as yet established, that NF-kB activation may be responsible for the synergistic effect of IL-1b on IL-6-induced CRP expression.

The connection between C-reactive protein (CRP) and atherosclerosis lies on three grounds. First, the concentration of CRP in the serum, which is measured by using highly sensitive (a.k.a. 'hs') techniques, correlates with the occurrence of cardiovascular disease. Second, although CRP binds only to Fcgamma receptor-bearing cells and, in general, to apoptotic and damaged cells, almost every type of cultured mammalian cells has been shown to respond to CRP treatment. Many of these responses indicate proatherogenic functions of CRP but are being reinvestigated using CRP preparations that are free of endotoxins, sodium azide, and biologically active peptides derived from the protein itself. Third, CRP binds to modified forms of low-density lipoprotein (LDL), and, when aggregated, CRP can bind to native LDL as well. Accordingly, CRP is seen with LDL and damaged cells at the atherosclerotic lesions and myocardial infarcts. In experimental rats, human CRP was found to increase the infarct size, an effect that could be abrogated by blocking CRP-mediated complement activation. In the Apob (100/100) Ldlr (-/-) murine model of atherosclerosis, human CRP was shown to be atheroprotective, and the importance of CRP-LDL interactions in this protection was noted. Despite all this, at the end, the question whether CRP can protect humans from developing atherosclerosis remains unanswered.