Detection of a high-affinity binding site for the globular head regions of the C1q complement protein on a human diploid fibroblast subtype

Differential chemokine response of fibroblast subtypes to complement C1q

BACKGROUND AND OBJECTIVE

The pathogenesis of periodontitis includes an inappropriate activation of the classical complement cascade (C') with accumulation of inflammatory C' products in fluids and tissues. Our hypothesis is that in vivo the C' product, C1q, may act as a regulatory component of the innate immune response of distinct matrix fibroblasts to the inflammatory environment. This study analyzed the C1q induction of pro-inflammatory cytokine secretion in fibroblast subtypes derived from distinct periodontal tissues, and identified a mechanism of the cell response.

MATERIAL AND METHODS

Primary human gingival fibroblast, periodontal ligament fibroblast, and granulation tissue fibroblast cultures were treated for 24 h with C1q. Protein arrays assessed the secretory profile of constitutive and C1q-inducible pro-inflammatory cytokines, and enzyme-linked immunosorbent assays were used to quantify the kinetics of each inducible cytokine.

RESULTS

Granulation tissue fibroblast cultures were unresponsive to C1q challenge. In contrast, periodontal ligament fibroblasts responded with a release of monocyte chemoattractant protein (MCP)-1, interleukin-6, interleukin-8, and macrophage inflammatory protein (MIP)-1beta higher than the basal level by 8.2-, 7.0-, 3.8-, and 7.2-fold, respectively. Human gingival fibroblast cultures increased secretion of these chemokines by 5.2-, 4.5-, 3.0-, and 9.8-fold, respectively. Inhibitor studies revealed that C1q-inducible release of chemokines by the human gingival fibroblast and periodontal ligament cultures was contingent upon p38 mitogen-activated protein kinase activity.

CONCLUSION

The ability of C1q to stimulate secretion of pro-inflammatory chemokines depends upon which specific fibroblast subtype is involved. Targeting C1q-activated intracellular signaling pathways may be an effective means to inhibit the production of chemokines that promote inflammatory cell infiltration into gingival and periodontal ligament tissues.

Complement C1q inhibits cellular spreading and stimulates adenylyl cyclase activity of fibroblasts

C1q selectively localizes at injured tissues, where it may function as a regulator of cell-matrix interactions. We show here that purified C1q, added to the culture medium of human gingival fibroblasts (HF) spread onto fibronectin substrates, elicited a round morphology that was accompanied by altered F-actin and correlated with inhibition of cellular spreading. Shape modification required integrity of the molecule and was specific, dose dependent, nontoxic, and reversible. Antispreading activity was mediated, at least in part, by specific cell-surface C1q receptors. We hypothesized that ligand occupancy of C1q receptors could influence shape by affecting intracellular levels of cyclic AMP (cAMP). Within 20 min of exposure of adhering HF to C1q, we detected an increase in adenylyl cyclase activity (six- to ninefold) in cAMP accumulation (by 20%) and in cAMP-dependent protein kinase activity (by 20%). These changes suggested that the rounding effect of C1q may be associated with activation of the adenylyl cyclase pathway.

The Multiligand-Binding Protein gC1qR, Putative C1q Receptor, Is a Mitochondrial Protein

A protein of 33 kDa (p33) that tightly binds to the globular domains of the first complement component, C1q, is thought to serve as the major C1q receptor (gC1qR) on B cells, neutrophils, and mast cells. However, the cellular routing and the subcellular localization of p33/gC1qR are unknown. We have performed confocal laser-scanning microscopy and found that p33/gC1qR is present in intracellular compartments, where it colocalizes with the mitochondrial marker protein, pyruvate dehydrogenase. No surface staining for p33/gC1qR on endothelial EA.hy926 cells was observed. A fusion protein of the p33/gC1qR presequence with green fluorescent protein translocated to the mitochondria of transfected COS-7 cells. Concomitantly, a 6-kDa portion of the fusion protein was proteolytically removed. The 33 amino-terminal residues of the presequence proved sufficient to direct reporter constructs to mitochondria. Association of p33/gC1qR with mitoplasts indicated that the mature protein of 209 residues resides in the matrix and/or the inner membrane of mitochondria. Immunocytochemistry of fetal mice tissues revealed a ubiquitous expression of p33/gC1qR, most prominently in tissues that are rich in mitochondria. Thus, the candidate complement receptor p33/gC1qR of intact cells cannot interact with plasma C1q due to mutually exclusive localizations of the components. The functional role of p33/gC1qR needs to be reconsidered.

Characterisation of the rat and mouse homologues of gC1qBP, a 33 kDa glycoprotein that binds to the globular 'heads' of C1q

gC1qBP is a 33 kDa glycoprotein that binds to the globular 'heads' of C1q. We have cloned cDNAs encoding the rat and mouse homologues of gC1qBP. Comparison of the cDNA-derived amino acid sequences of gC1qBP reveals that either of the rodent sequences is 89.9% identical to the reported human sequence. Recombinant rat gC1qBP binds avidly to human C1q. gC1qBP mRNA is abundantly expressed in every rat and mouse tissue analysed. Rat mesangial cells synthesise gC1qBP, but do not express gC1qBP on the cell surface. In rat serum, gC1qBP is present at low levels.

Complement receptors in HIV infection

The complement system plays an important role in the antimicrobial defense of the organism. Its components recognize a large variety of pathogens and target them for destruction, either directly by formation of a membrane attack complex or indirectly by recruiting phagocytic cells. In addition, it has several functions in cell activation, clearance of immune complexes, control of inflammatory reactions, chemotaxis and autoimmunity. For mediation of all these tasks of the complement system, complement receptor molecules on the cell surface play a key role. Current knowledge on structure, function, signal transduction and associated molecules is briefly summarized here. The role of complement receptors for human immunodeficiency virus (HIV)-associated pathogenesis is ambiguous and varies depending on cell type. On the one hand, complement receptors support the infected host to manage HIV infection and to defend itself, at least partially, against viral spreading throughout the organism. Such complement receptor-mediated supporting mechanisms are activation of immune cells and lysis of viral particles and infected host cells. On the other hand, HIV employs complement receptors to intrude more easily into various cell types, to become localized into lymph follicles and to activate viral replication in latently infected cells. This review summarizes the complex interaction of virus and complement receptors in HIV infection for different cell types.

Distribution of receptors of collagen and globular domains of C1q in human lung fibroblasts

Fibroblasts are the predominant cell type responsible for the synthesis of collagen and other matrix elements in normal and fibrotic lungs. We have previously reported that human lung fibroblasts are heterogeneous in C1q binding and that subpopulations differing in C1q binding can be isolated and subcultured. We have investigated the distribution of receptors for C1q-collagen domain (cC1q-R) and globular domain (gC1q-R) in adult human lung fibroblasts. Fibroblasts were isolated from cultures of adult human lung explants in medium containing fresh- or heated plasma-derived human sera and separated by FACS-cell sorting into populations binding to C1q with high- (HF) and low- (LF) fluorescence. The cC1q-R was obtained from fibroblast membrane preparations by affinity chromatography through an anti-cC1q-R antibody column and its distribution was determined by Western analysis. The presence of gC1q-R was determined by immunoblots using an anti-gC1q-R antibody raised against a synthetic peptide. The results showed that a 54 kD protein crossreacting with anti-cC1q-R antibody was produced by LF cells, but it was barely detectable in HF cultures. Immunostaining with anti-cC1q-R antibody revealed that most of the cells in LF cultures were positive while the HF cells were negative. A 38 kD protein recognized by anti-gC1q-R antibody was produced by lung fibroblasts; however, no differences were detected in its distribution between LF and HF cultures. SDS-polyacrylamide gel electrophoresis of membrane proteins binding to an affinity column of C1q-globular fragment showed that the HF cultures contain a approximately 51 kD protein, which was a minor component in LF membranes. These data show that cC1q-R is expressed predominantly by a population of human lung fibroblasts, while the 38 kD gC1q-R is produced by all cells. Another 51 kD protein appears to be produced by a separate population of fibroblasts which does not express cC1q-R. Our results indicate that two lung fibroblast subtypes may be distinguished based on production of the 54 kD putative cC1q-R and another 51 kD protein which binds to C1q-globular domain.

The C1q-binding cell membrane proteins cC1q-R and gC1q-R are released from activated cells: subcellular distribution and immunochemical characterization

Two types of widely coexpressed cell surface C1q-binding proteins (C1q-R): a 60-kDa calreticulin-homolog which binds to the collagen-like "stalk" of C1q and a 33-kDa protein with affinity for the globular "heads" of the molecule, have been described. In this report, we show that the two molecules are also secreted by Raji cells and peripheral blood lymphocytes and can be isolated in soluble form from serum-free culture supernatant by HPLC purification using a Mono-Q column. The two purified soluble proteins had immunochemical and physical characteristics similar to their membrane counterparts in that both bound to intact C1q and to their respective C1q ligands, cC1q and gC1q. In addition, N-terminal amino acid sequence analyses of the soluble cC1q-R and gC1q-R were found to be identical to the reported sequences of the respective membrane-isolated proteins. Ligand blot analyses using biotinylated membrane or soluble cC1q-R and gC1q-R showed that both bind to the denatured and nondenatured A-chain and moderately to the C-chain of C1q. Moreover, like their membrane counterparts, the soluble proteins were found to inhibit serum C1q hemolytic activity. Although cC1q-R was released when both peripheral blood lymphocytes and Raji cells were incubated in phosphate-buffered saline for 1 hr under tissue culture conditions, gC1q-R was releasable only from Raji cells, suggesting that perhaps activation or transformation leading to immortalization is required for gC1q-R release. Subcellular fractionation of Raji cells and analyses by enzyme-linked immunosorbent assay and Western blotting showed that the two molecules are present in the cytosolic fractions as well as on the membrane. The data suggest that soluble forms of both C1q-binding molecules are released from cells and that these molecules may play important roles in vivo as regulators of complement activation.

Proteolytic cleavage of MHC class I by complement C1-esterases--an overlooked mechanism?

The complement C1-esterases have been shown to cleave the MHC class I molecules, which are important participants in the activation of T lymphocytes, between the alpha 2- and the alpha 3-domain of the heavy chain. The possible involvement of the C1-esterases in the regulation of peripheral self-tolerance is discussed. It is hypothesized that the C1-esterase-mediated cleavage of the MHC class I molecules either induces: a soluble fragment of the outer two domains of the MHC class I molecule, in association with beta 2-microglobulin, to bind to the T cell receptors and prevent the cells from being activated, or produces a change in the exposure of the alpha 3-domain that remain on the cell surface, acting as mediator of a 'veto signal' that prevents these cells from being activated.

Identification of a gC1q-binding protein (gC1q-R) on the surface of human neutrophils. Subcellular localization and binding properties in comparison with the cC1q-R

Human neutrophils have multiple C1q-binding proteins. Direct ligand-binding studies with the globular domain of C1q and two-dimensional Western blot analysis revealed two gC1q-binding proteins (gC1q-R): a 33,000 M(r) protein (pI 4.5) mainly in the neutrophil plasma membrane and an 80,000-90,000 M(r) protein (pI 4.1-4.2) located mainly in the granules. Direct binding studies showed that C1q bound to this higher molecular weight protein under physiological conditions. In contrast, anti-cC1q-R antibody, which recognizes a protein binding to collagenous tails of C1q, detected only a 68,000 M(r) protein in the plasma membrane. Both the 33,000 and 68,000 M(r) receptors appear early on the surface of differentiating HL-60 cells. On mature neutrophils, surface expression of both C1q receptors was evident, but no upregulation was observed upon stimulation. Phorbol myristate acetate treatment of neutrophils downregulated both the receptors from cell surface, and significant amounts of soluble gC1q-R were in cell media supernatants, suggesting receptor shedding or secretion. gC1q-R, unlike cC1q-R, did not bind to other C1q-like ligands, namely mannose binding protein, surfactant protein-A, surfactant protein-D, or conglutinin under normal ionic conditions, suggesting a greater specificity for C1q than the "collectin" type receptor (cC1q-R). Rather, gC1q-R only bound purified C1q, and the binding was enhanced under low ionic conditions and in the absence of calcium. The role of C1q receptor shedding and its biologic consequence remain to be defined, but may contribute to the diversity of C1q-mediated responses observed in many cell types.

Beta-sheet secondary structure of the trimeric globular domain of C1q of complement and collagen types VIII and X by Fourier-transform infrared spectroscopy and averaged structure predictions

C1q plays a key role in the recognition of immune complexes, thereby initiating the classical pathway of complement activation. Although the triple-helix conformation of its N-terminal segment is well established, the secondary structure of the trimeric globular C-terminal domain is as yet unknown. The secondary structures of human C1q and C1q stalks and pepsin-extracted human collagen types I, III and IV (with no significant non-collagen-like structure) were studied by Fourier-transform i.r. spectroscopy in 2H2O buffers. After second-derivative calculation to resolve the fine structure of the broad amide I band, the Fourier-transform i.r. spectrum of C1q showed two major bands, one at 1637 cm-1, which is a characteristic frequency for beta-sheets, and one at 1661 cm-1. Both major bands were also detected for Clq in H2O buffers. Only the second major band was observed at 1655 cm-1 in pepsin-digested C1q which contains primarily the N-terminal triple-helix region. The Fourier-transform i.r. spectra of collagen in 2H2O also showed a major band at 1659 cm-1 (and minor bands at 1632 cm-1 and 1682 cm-1). It is concluded that the C1q globular heads contain primarily beta-sheet structure. The C-terminal domains of C1q show approximately 25% sequence identity with the non-collagen-like C-terminal regions of the short-chain collagen types VIII and X. To complement the Fourier-transform-i.r. spectroscopic data, averaged Robson and Chou-Fasman structure predictions on 15 similar sequences for the globular domains of C1q and collagen types VIII and X were performed. These showed a clear pattern of ten beta-strands interspersed by beta-turns and /or loops. Residues thought to be important for C1q-immune complex interactions with IgG and IgM were predicted to be at a surface-exposed loop. Sequence insertions and deletions, glycosylation sites, the free cysteine residue and RGD recognition sequences were also predicted to be at surface-exposed positions.

Isolation, cDNA cloning, and overexpression of a 33-kD cell surface glycoprotein that binds to the globular "heads" of C1q

This work describes the functional characterization, cDNA cloning, and expression of a novel cell surface protein. This protein designated gC1q-R, was first isolated from Raji cells and was found to bind to the globular "heads" of C1q molecules, at physiological ionic strength, and also to inhibit complement-mediated lysis of sheep erythrocytes by human serum. The NH2-terminal amino acid sequence of the first 24 residues of the C1q-binding protein was determined and this information allowed the synthesis of two degenerate polymerase chain reaction primers for use in the preparation of a probe in the screening of a B cell cDNA library. The cDNA isolated, using this probe, was found to encode a pre-pro protein of 282 residues. The NH2 terminus of the protein isolated from Raji cells started at residue 74 of the predicted pre-pro sequence. The cDNA sequence shows that the purified protein has three potential N-glycosylation residues and is a highly charged, acidic molecule. Hence, its binding to C1q may be primarily but not exclusively due to ionic interactions. The "mature" protein, corresponding to amino acid residues 74-282 of the predicted pre-pro sequence, was overexpressed in Escherichia coli and was purified to homogeneity. This recombinant protein was also able to inhibit the complement-mediated lysis of sheep erythrocytes by human serum and was shown to be a tetramer by gel filtration in nondissociating conditions. Northern blot and RT-PCR studies showed that the C1q-binding protein is expressed at high levels in Raji and Daudi cell lines, at moderate levels in U937, Molt-4, and HepG2 cell lines, and at a very low level in the HL60 cell line. However, it is not expressed in the K562 cell line. Comparison of gC1q-R NH2-terminal sequence with that of the receptor for the collagen-like domain of C1q (cC1q-R) showed no similarity. Furthermore, antibodies to gC1q-R or an 18-amino acid residue-long NH2-terminal synthetic gC1q-R peptide did not cross-react with antibodies to cC1q-R. Anti-gC1q-R immunoblotted a 33-kD Raji cell membrane protein, whereas anti cC1q-R recognized a molecule of approximately 60 kD. The NH2-terminal sequence of gC1g-R appears to be displayed extracellularly since anti-gC1g-R peptide reacted with surface molecules on lymphocytes, polymorphonuclear leukocytes, and platelets, as assessed by flow cytometric and confocal laser scanning microscopic analyses.(ABSTRACT TRUNCATED AT 400 WORDS)

Human lung fibroblast subpopulations with different C1q binding and functional properties

Human lung fibroblasts differing in C1q binding, steady-state levels of collagen synthesis, and other functional properties were isolated. Explants of normal human lung specimens were cultured in medium containing complement-inactivated plasma-derived human serum or complete human serum. Cells obtained were treated with C1q and fluorescein isothiocyanate-anti-C1q antibody and separated based on fluorescence intensity in a fluorescence-activated cell sorter (FACS). FACS profiles showed that fibroblasts obtained in the presence of plasma-derived serum (HF cells) displayed higher fluorescence intensity than those obtained in complete serum (LF cells). The unsorted and sorted HF and LF fibroblasts retained their respective fluorescence phenotypes after subculture. The LF fibroblasts proliferated faster than HF cells and contained more cycling cells. However, whereas the sorted HF cells grew normally, sorted LF cells grew poorly. Collagen production and pro alpha l[I] mRNA levels in HF cells were 2.6 +/- 0.7 and 2.1 +/- 0.6 times as high as LF cells (n = 4). Collagen synthesis in both HF and LF cells was stimulated by transforming growth factor-beta and inhibited by interferon-gamma, but the stimulation was greater and inhibition less in LF cells. Our results indicate that C1q binding and the type of C1q receptors can serve as markers for fibroblast subpopulations differing in collagen synthesis, and that selection of subpopulations and their differential sensitivity to regulatory molecules can contribute to collagen alterations associated with inflammation, fibrosis, and other acquired diseases.

Smooth muscle and epithelial cells express specific binding sites for the C1q component of complement

In injury and inflammation, interactions of complement C1q with C1q receptors may provide attachment sites for cell localization and tissue regeneration. Cultured smooth muscle cells (58%), epithelial cells (26%), and endothelial cells (25%) attach to C1q-coated surfaces, while only 6% of cultured B cells (Raji) attach. Endothelial and Raji cells express C1q receptors, but C1q receptors (C1qR) on smooth muscle cells and epithelial cells have not previously been demonstrated. Evidence is provided that smooth muscle cells express an average of 1.5 x 10(6) C1qR/cell (K alpha = 10(8) M-1) and that epithelial cells express an average of 0.7 x 10(6) C1qR/cell (K alpha = 1.4 x 10(8) M-1). Binding properties of C1qR, and immunoreactivity to anti-C1qR antibodies, are characterized. The antibodies specifically recognize a 67-kDa component of smooth muscle cell lysates and inhibit cell attachment to C1q substrates. We conclude that distribution of C1qR may be ubiquitous; binding properties, size, and antigenicity of various C1qR may be related, but adhesive function may be tissue specific.

Role of fibroblast subpopulations in periodontal physiology and pathology

Fibroblasts are the principal cell type in the soft connective tissues of the periodontium; they perform important functions in development, physiology, and disease. A growing number of reports have indicated site-specific phenotypic variation of fibroblasts. Heterogeneity of metabolic traits has been demonstrated in cells from healthy and diseased tissues. The tissue distribution and relative proportions of fibroblast subpopulations have a significant impact on the regulation of connective tissue function in health and disease.