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Clin Exp Immunol. Jun 2000; 120(3): 406–412.
PMCID: PMC1905565

C1q receptors

Introduction

The complement system consists of over 20 plasma proteins, which are activated and regulated by enzymes of the serine protease type. There are three recognized routes leading to activation of the complement system, the classical, and the alternative and mannan-binding lectin (MBL) pathways. Upon activation, some of the complement proteins are cleaved into small fragments that can be recognized and bound by complement receptors on the surfaces of various cell types. A primary function of the complement cell surface receptors is to enhance the responsiveness of the innate immune system in destroying and removing proteins, microorganisms and other foreign material as well as cellular debris from the circulation. A number of complement receptors have been found on specialized cell types and characterized at the molecular and biochemical level, including complement receptor 1 (CR1, CD35), complement receptor 2 (CR2, CD21), complement receptor 3 (CR3, CD11b/CD18) complement receptor 4 (CR4, CD11c/CD18) and the C5a receptor (CD88). In 1977, Sobel and co-workers observed saturable and concentration-dependent binding of C1q to B lymphocytes and other cells of a lymphoblastoid lineage [1]. Since then, a number of researchers have observed that C1q, a subcomponent of the first component of complement, is also capable of modulating diverse cellular responses, and this has resulted in an intense search for C1q binding proteins which may be C1q receptors. A number of intracellular and cell surface proteins which interact with C1q and may or may not be receptors for C1q have been found (Table 1).

Table 1
Proteins that have been proposed as receptors for the first component of complement, C1q

The most characterized immunological function of C1q is as the recognition component for the activation of the classical complement pathway. However, it is known that C1q is synthesized and secreted in the absence of C1r and C1s, the other components of C1, the classical complement pathway initiation complex. In addition, C1q is found in vivo in the absence of C1r and C1s, and thus additional roles for C1q are plausible. The proteins discussed in this review that have been proposed to be C1q receptors involved in these alternative roles are diverse, consisting of novel cell surface proteins present on specific cells, ubiquitous intracellular proteins, or known receptors for complement components other than C1q. There is still much work required before immunologists can be confident that specific C1q receptors are responsible for triggering or modulating host responses or for initiating specific signal transduction mechanisms within the cell. Nevertheless, this review describes the evidence so far that implicates a role for each of these C1q binding proteins either as genuine receptors or as modulators of pathological and physiological processes in which C1q is involved.

MULTIFUNCTIONAL ROLE OF C1q DURING ACTIVATION OF THE CLASSICAL PATHWAY

The C1 complex of the classical pathway comprises three glycoproteins, C1q, C1r and C1s. The trigger for complement activation occurs in vivo by the interaction of the C1q portion of the C1 complex with immune complexes (IC) or other known non-immunoglobulin C1 activators [2,3] The C1q molecule is a 460 000-D glycoprotein composed of six globular ‘heads’, each of which are 5 nm across and 7 nm in depth and held together by six collagen-like ‘stalks’. Each head is composed of the C-terminal halves of three distinct polypeptide A, B and C chains, and these heads are connected to collagen-like strands which extend 11 nm and link to a central elongated triple helical fibril structure of 11·2 nm in length also composed of a collagen-like repeating sequence of Gly-X-Y triplets. Dimers of the C1r and C1s subcomponents contain the enzymatic activities associated with the C1 complex and form a tetrameric proenzyme structure around the collagen-like stands of C1q. The heads of C1q function in molecular recognition of defined regions within the CH2 domain of antigen-complexed IgG molecules. Recent studies employing portions of recombinant C1q heads have indicated that the C1qB chain is important in IgG binding [4], while chemical modification studies imply all three chains may be involved for optimum binding. The recent crystal structure of the globular head of adipocyte complement-related protein of 30 kD (ACRP30), a C1q-like protein, reveals that it is analogous to regions of the tumour necrosis factor family (TNF) that are directly involved in interaction with their receptors [5]. The head region of C1q is clearly involved in specific protein–protein interactions, but less definitive information has been reported concerning possible functions for the collagen-like tail region of C1q. After activation of the C1 complex by immune aggregates the activated C1r2-C1s2 enzymes are rapidly removed from the collagen-like region of C1q by covalent interaction with the serum C1-inhibitor. This, as well as preventing excessive activation of the complement system, exposes the collagen-like regions of C1q and makes them available for interaction with cell surfaces. Recent evidence from a number of laboratories indicates that C1q binds to a number of different cell types and triggers a variety of cellular responses, including phagocytosis, enhanced microbial killing by phagocytes, induction of chemotaxis and stimulation of increased oxidative metabolism. This has focused attention on both the head and tail regions of C1q as possible ligands for cell surface C1q binding proteins and receptors [6]. The collagen-like tail regions of C1q have been proposed to bind to a number of proteins including C1qRp, C1qRO2−, calreticulin (also termed cC1qR and collectin receptor) and CR1. The globular head region of C1q has been observed to bind to the globular head C1q binding protein (gC1qbp) and calreticulin.

C1qRp

C1q affinity chromatography in which C1q collagen tails were coupled to Sepharose was initially used to identify successfully a novel C1q binding protein with a mol. wt of 126 000 which binds specifically to the collagen-like region of C1q. Recent characterization studies have led to the cloning and sequencing of the cDNA encoding the C1qRp[7]. The precursor protein is 652 amino acids in length and consists of a signal peptide of 21 amino acids followed by a mature protein of 631 residues. While the predicted molecular mass of the protein is only 66·5 kD, extensive O-linked glycosylation has been shown to account for a substantial proportion of the aberrant migration behaviour in SDS–PAGE (relative molecular migration of 126 kD) [8]. The primary structure, predicted from the cDNA sequence (Fig. 1), consists of a C-terminal 47 residue cytoplasmic tail that contains the tyrosine kinase recognition motif, RAMENQY. This is followed by a 25 amino acid transmembrane domain, with the majority of the N-terminal region of the protein (559 amino acids) positioned extracellularly. The portion of the protein juxtaposed to the membrane is a serine/threonine-rich mucin-like domain that serves to extend the N-terminal region of the molecule away from the surface of the cell, thus being more accessible for ligand interaction. This region is followed by five epidermal growth factor (EGF)-like modules, ranging between 40 and 43 amino acids in length. The two C1q-associated serine proteases C1r and C1s, as well as the proteases MASP 1 and 2 which bind to the structurally related MBL, also contain EGF-like domains [9,10]. Such motifs are capable of binding calcium and are found in a number of proteins of diverse function, where they may serve to stabilize protein–protein interactions [11]. Finally, the most N-terminal domain shows sequence identity to the carbohydrate recognition domain (CRD) found in the C-type lectins. These CRDs are found in other membrane receptors thought to regulate endocytosis, including the human mannose macrophage receptor [12].

Fig. 1
Amino acid sequence and domain structure of C1qRp. This cell surface glycoprotein contains a number of domains: a C-type lectin carbohydrate recognition domain (CRD), five epidermal growth factor (EGF)-like domains, three of which bind calcium (cbEGF), ...

There are a number of cell surface molecules present on various phagocytes that enhance the uptake of microorganisms, IC and antigens, including CR1 and Fc receptors. Early studies employing collagen-like fragments of C1q demonstrated that this region of C1q interacting with the cell surface functioned as a ligand to enhance FcR-mediated phagocytosis in human monocytes and macrophages [13]. Unfortunately, a large number of cell surface-associated proteins bind to C1q, which has made it difficult to identify the specific protein responsible for enhanced phagocytosis. However, a number of MoAbs raised against C1qRp have been shown to inhibit C1q-mediated phagocytosis, providing strong evidence that regions of this receptor are likely to be involved in the C1q-mediated enhancement of phagocytosis [14]. The presence of this receptor on endothelial cells is particularly interesting in light of the recent report of Hess et al. suggesting that at least some subtypes of these cells are capable of phagocytosing apoptotic lymphocytes [15]. In addition, the development of autoimmunity in humans and mice lacking C1q [16,17] is consistent with a role for this receptor system in the clearance of cellular debris. Finally, a recent report in which a mouse C1QRp homologue has been described as a fetal stem cell surface marker raises the possibility of the functional importance of this receptor through several stages of development and/or differentiation [18].

C1qR02–

For many years the importance of the complement cascade cleavage fragments of C3 and C5 to function as intermediates for initiating cellular activation has been known. Independent investigators have noted the generation of superoxide (O2) by human leucocytes when exposed to multivalent C1q. The production of superoxide and other oxygen intermediates, such as hydrogen peroxide and singlet oxygen, is a protective mechanism elicited by these immune cells to aid in the killing of pathogens. The activation of this ‘respiratory burst’ by neutrophils and macrophages can lead to local tissue destruction, is normally regulated by a complex series of signalling pathways, and is often accompanied by receptor up-regulation, increased adherence and motility through the tissues by the cells. Many of these processes are linked to G-protein coupled signal transduction pathways. What is unusual about C1q activation of superoxide production is that it does not lead to a general degranulation of the leucocyte, which normally results in the active secretion of proteolytic enzymes and other anti-microbial components from the primary and secondary granules [19]. Moreover, the C1q-mediated O2 production is not inhibited by the G-protein-sensitive inhibitor, pertussis toxin, nor has it a requirement for stable cell–cell adherence for activation of the oxidase [20]. This latter point is of concern, since C1q-mediated reactivity of the respiratory burst may cause significant non-specific host tissue damage such as occurs in sufferers from systemic lupus erythematosus [21,22].

Given the functional differences in events such as phagocytosis and superoxide production, one might expect different C1q receptors to be implicated in their activation. Tenner and colleagues have investigated this possibility, and several lines of evidence suggest distinct pathways of activation are induced. First, MoAbs that specifically recognize C1qRp on neutrophils failed to inhibit C1q-induced production of O2− by neutrophils [14]. Second, MBL and SP-A which have been shown to interact with C1qRp do not induce a superoxide response by neutrophils [19], suggesting the ‘receptor-interaction’ site for C1qR02− is specific for C1q. In a series of studies employing C1q fragments as agonists, the region of C1q responsible for triggering superoxide production has been identified as a motif flanked by amino acids 42–61 of the C chain of the collagen-like region of C1q [23,24]. This region of C1q differs from the section of the collagen-like region of the molecule that shares most similarity with sections of MBL and SPA which both trigger C1qRp, possibly through a common interaction site.

gC1qbp

The cDNA for gC1qbp was initially sequenced by Krainer et al. [25], who speculated that the protein was an intracellular pre-mRNA splicing factor, which they designated as p32 or splicing factor 2 (SF2) in HeLa cells. After additional work, the same authors withdrew the claims that the protein was a mRNA splicing factor and the function of the protein remained undetermined for several years [26]. The 33-kD protein was later isolated from Raji cells, shown to exist as a tetramer in the native form [27] and was found to bind to the globular ‘heads’ of C1q. The possibility that gC1qbp might be a receptor for C1q was highlighted when confocal microscopy, flow cytometry and immunoblotting revealed that the protein appeared to be associated with the cell surface of fibroblasts [28], neutrophils [29], endothelial cells [30] and platelets [31] under certain conditions. More recent studies [32] have suggested that only low levels of gC1qbp are actually on the cell surface, whilst the majority of gC1qbp is predominantly cytoplasmic and is detectable after permeabilization of the cell membrane. The same authors [32] also showed by reflection contrast microscopy and electron microscopy on ultra-thin sections of permeabilized Raji cells, that gC1qbp is present in double membranous cytoplasmic vesicles located in the proximity of the plasma membrane. This in itself is not unusual, since most complement receptors are contained within intracellular stores, and can be up-regulated upon stimulation. However, gC1qbp is not easily up-regulated, nor does it contain a transmembrane domain. These features, together with the fact that it has been recently reported to be a mitochondrial protein [33] which is found in all the mammalian cells examined, except erythrocytes, is suggestive of a lack of expression on the surface of specific cell types in a manner normally required of a classical membrane receptor. In this report, the subcellular distribution of gC1qbp in transfected COS-7 cells was followed and it was demonstrated that the amino-terminal portion of gC1qbp represents a target sequence that directs the protein to mitochondria. However, a recent study has shown that a soluble form of gC1qbp is released from Raji cells in vitro [34]. Thus, gC1qbp may be actively secreted into the extracellular environment in vivo, although the mechanism of secretion remains unknown. The release of gC1qbp may also arise when cells are stressed or undergoing defective apoptosis, or during necrosis. Whatever the means of release, gC1qbp is highly acidic in nature, which may make it a target for protein–protein charge interactions. In this respect, several extracellular roles for gC1qbp as a ‘response modifier’ of a number of C1q and other plasma protein-mediated functions have been recently reported [27,3539, ]. So although gC1qbp cannot be expected to serve as a cell surface receptor, especially in view of its mitochondrial location in normal cells [33], its release from cells can result in the modification of a number of cellular and vascular protein responses.

CALRETICULIN, cC1qR and COLLECTIN RECEPTOR

The term ‘C1q receptor’ has been loosely applied to many proteins that have been observed to bind to C1q. However, this area of research has been made more confusing due to the multiple naming of a single C1q binding protein, calreticulin (CRT), and requires some explanation. Prior to the cDNA sequencing of CRT, a highly charged 46-kD protein which ran aberrantly on SDS–PAGE gels as a 55–60-kD band was isolated from a number of cell lysate preparations as a C1q binding protein or receptor by several research groups by C1q-affinity chromatography [4042]. This protein was initially believed to bind to the collagen-like region of C1q and was termed cC1qR (collagen C1q receptor) [31]. C1q bears structural similarity to a family of carbohydrate binding protein termed the ‘collectins’, which includes MBL, pulmonary surfactant protein A (SP-A) and conglutinin. These proteins also possess amino terminal collagen-like tail regions and carboxyl-terminal globular head domains. There are however distinct functional differences between the globular heads of collectins compared with C1q. The collectin head regions contain C-type lectin carbohydrate recognition domains (CRD) that target and bind to terminal sugars present on microorganisms, while the head region of C1q targets protein recognition sites on ICs. As C1q and certain collectins (SP-A and MBL) enhance leucocyte FcR- and CR1-mediated phagocytosis [4345], the structural similarity between the collagenous-like region of C1q and the collectins makes the possibility that they share a common receptor an attractive one. Examination of the binding specificity of this particular putative ‘C1q receptor’ revealed that it could also bind to solid-phase SP-A and MBL in vitro. Since all three of these molecules possess a collagen-like region, a more general term for this ‘C1q-binding protein’ was adopted and it was termed the ‘collectin receptor’[46].

However, it is now know that CRT is a 46-kD, highly abundant and soluble calcium-binding protein located primarily in the endoplasmic reticulum of most nucleated animal and plant cells, where it participates as a quality controller of glycoprotein formation together with calnexin [47]. Its location within the cell makes CRT a doubtful candidate as a C1q receptor. However, given CRT’s ability to associate with many other proteins, in hindsight it is not surprising that it was reported to bind to C1q and many other cellular components. Two independent groups identified a ‘putative’ C1q or collectin receptor protein that was shown to be highly acidic. Both groups of workers performed amino acid composition analysis on the protein and it was shown to be almost identical [40,42]. During this period, the successful molecular cloning of the calcium binding protein, calreticulin, was completed [48] and it was initially believed that CRT was the 46-kD protein component of the autoantigen complex, Ro/SSA, which is comprised of a mixture of 46-, 52- and 60-kD proteins that associate with cytoplasmic forms of RNA. It was however later revealed that CRT is not one of the three major protein components of Ro/SSA [49], but may be instrumental in the formation of the Ro/SSA complex [50]. The determination of the N-terminal protein sequence of the ‘putative’ cC1q receptor at this time [51] suggested the ‘cC1q receptor’ and calreticulin were identical. Further examination of the internal sequences of the collectin receptor employing proteolytic cleavage by Staphylococci V8 protease revealed at least two potentially unique short sequences (six to eight residues) which suggested the collectin’s receptor may be similar but not entirely identical to CRT [51]. However, the same authors subsequently reported that one of these potential unique sequences was likely to be a contaminant [52], while the remaining sequence remained a potential different sequence from that found in CRT. On closer examination, this latter sequence bears remarkable homology to an internal amino acid sequence present in the Staphylococcal V8 protease used initially to digest the collectin receptor, and is therefore probably also derived from a minor contaminant. Therefore, to date there is no biochemical or molecular evidence to suggest CRT and the collectin receptor/cC1qR are different proteins. In addition, recombinant CRT of known molecular structure binds to C1q and the collectins [53,54]. How and why a predominantly intracellular protein such as CRT binds to the extracellular complement components is uncertain. However, evidence suggests CRT can translocate to the cell surface during cell stress, such as inflammation, heat shock or viral infection [55]. Once released from cells, it may have a role to play in autoimmunity, such as participating in the pathology of systemic lupus erythematosus (SLE) [56,57]. Recently, using recombinant CRT domains, several C1q binding sites were revealed to be present in the N-terminal half of CRT [58]. Some of these sites contained the amino acid sequence ExKxK, an IgG-like motif present in the CH2 domain of IgG which is known to be one of the C1q binding sites on IgG [59]. When expressed as synthetic peptides, some of these CRT IgG-like motifs are able to inhibit C1q binding to IgG and also prevent C1q-mediated complement activation [58], which may be important in the formation and clearance of IC in autoimmune disease. Therefore CRT, although not a classical receptor for C1q, may play an important role in C1q-mediated function during complement activation in certain inflammatory conditions.

CR1 (CD35) AS A C1q RECEPTOR

The interaction between complement activation products and cells of innate and acquired immunity appears to mediate specialized responses by T and B lymphocytes and professional phagocytes. One of the major ‘house keeping’ functions of classical complement activation is to target ICs for clearance. The human CR1 receptor (CD35) present on leucocytes and erythrocytes recognizes ICs opsonized with the activated complement components C3b and C4b, leading to their clearance in the liver and spleen [60,61]. In this regard, CR1 can be regarded as a hybrid receptor, since the most common allotypes (A and F) each have one site for C4b and two sites for C3b binding. ICs are also opsonized with C1q, and recent evidence suggests the CR1 receptor is a trihybrid receptor capable of recognizing all these complement opsonins, C1q, C3b and C4b [62]. The collagen tail portion of C1q appears to be the target for CR1 and may be a common binding site for SCR-containing proteins (short consensus repeats), although this remains to be proven. The function of CR1 as a proposed C1q receptor adds further to the complexity of the many biological effects of C1q mediated through different cell surface receptors. If CR1 acts as a receptor for uptake of C1q–immunoglobulin complexes, one might expect such a receptor to function in this manner in patients with deficiencies in complement components downstream of C1q, such as patients with C4 or C2 deficiencies who are incapable of generating C3b- or C4b-associated ICs for clearance. However, C1q is apparently unable to provide sufficient IC removal in these patients, suggesting that the interaction of CR1 with C1q does not play a major role in complement-mediated enhancement of the clearance of ICs.

In conclusion, free C1q is one of several activation products of complement that interact with proteins on the cell surface of neutrophils, macrophages and related cells to bring about an array of cellular responses. Over the past 15 years or so, a number of C1q binding proteins have been identified and proposed as putative C1q receptors. One of these proteins, C1qRp, has recently been characterized at the molecular level and is composed of a 556-amino acid extracellular domain connected to a hydrophobic transmembrane segment and a cytoplasmic domain which contains a tryrosine kinase recognition motif (Fig. 1). This receptor is considered to mediate the cellular signalling of not only C1q, but also a number of structurally similar molecules, namely SP-A and MBL, possibly via the same site, as all three molecules share similar collagen-like structures, which may act as ligands for the receptor. Other molecules, CRT (cC1qR/collectin receptor) and gC1qbp have also been proposed as C1q ‘receptors’. However, neither protein has a transmembrane domain and both are predominantly intracellular in nature. Nevertheless, they have been reported to appear on the cell surface during cell activation and stress and therefore may have a role to play in modulating C1q-mediated cell function. CR1 is known to bind to C3b and C4b, employing different regions of the receptor to bind to each ligand. Recent evidence suggests C1q complexed with ICs may also bind to CR1, which implies that CR1 is a multifunctional receptor capable of binding to multiple ligands unrelated with respect to structure. Finally, other C1q binding proteins remain to be identified and characterized which may direct other cellular responses such as superoxide production and chemotaxis, which also appear to be triggered by C1q.

Acknowledgments

This work was supported by a grant (E0521) from the Arthritis Research Campaign of Great Britain (P.E. and K.B.M.R.) and grants from the Arthritis Foundation, American Heart Association and NIH AI41090 (A.J.T.). We also thank Miss A. J. Marsland for secretarial assistance. P.E. is an ARC Research Fellow.

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