The identification of N-linked oligosaccharides on the human CR2/Epstein-Barr virus receptor and their function in receptor metabolism, plasma membrane expression, and ligand binding

New aspects in the regulation of human B cell functions by complement receptors CR1, CR2, CR3 and CR4

The involvement of complement in the regulation of antibody responses has been known for long. By now several additional B cell functions - including cytokine production and antigen presentation - have also been shown to be regulated by complement proteins. Most of these important activities are mediated by receptors interacting with activation fragments of the central component of the complement system C3, such as C3b, iC3b and C3d, which are covalently attached to antigens and immune complexes. This review summarizes the role of complement receptors interacting with these ligands, namely CR1 (CD35), CR2 (CD21), CR3 (CD11b/CD18) and CR4 (CD11c/CD18) expressed by B cells in health and disease. Although we focus on human B lymphocytes, we also aim to call the attention to important differences between human and mouse systems.

The 15 SCR Flexible Extracellular Domains of Human Complement Receptor Type 2 can Mediate Multiple Ligand and Antigen Interactions

Complement receptor type 2 (CR2, CD21) is a cell surface protein that links the innate and adaptive immune response during the activation of B cells. The extracellular portion of CR2 comprises 15 or 16 short complement regulator (SCR) domains, for which the overall arrangement in solution is unknown. This was determined by constrained scattering and ultracentrifugation modelling. The radius of gyration of CR2 SCR 1-15 was determined to be 11.5 nm by both X-ray and neutron scattering, and that of its cross-section was 1.8 nm. The distance distribution function P(r) showed that the overall length of CR2 SCR 1-15 was 38 nm. Sedimentation equilibrium curve fits gave a mean molecular weight of 135,000 (+/- 13,000) Da, in agreement with a fully glycosylated structure. Velocity experiments using the g*(s) derivative method gave a sedimentation coefficient of 4.2 (+/- 0.1) S. In order to construct a model of CR2 SCR 1-15 for constrained fitting, homology models for the 15 SCR domains were combined with randomised linker peptides generated by molecular dynamics simulations. Using an automated procedure, the analysis of 15,000 possible CR2 SCR 1-15 models showed that only those models in which the 15 SCR domains were flexible but partially folded back accounted for the scattering and sedimentation data. The best-fit CR2 models provided a visual explanation for the versatile interaction of CR2 with four ligands C3d, CD23, gp350 and IFN-alpha. The flexible location of CR2 SCR 1-2 is likely to facilitate interactions of C3d-antigen complexes with the B cell receptor.

Solution structure of the complex between CR2 SCR 1-2 and C3d of human complement: an X-ray scattering and sedimentation modelling study

Complement receptor type 2 (CR2, CD21) forms a tight complex with C3d, a fragment of C3, the major complement component. Previous crystal structures of the C3d-CR2 SCR 1-2 complex and free CR2 SCR 1-2 showed that the two SCR domains of CR2 form contact with each other in a closed V-shaped structure. SCR 1 and SCR 2 are connected by an unusually long eight-residue linker peptide. Medium-resolution solution structures for CR2 SCR 1-2, C3d, and their complex were determined by X-ray scattering and analytical ultracentrifugation. CR2 SCR 1-2 is monomeric. For CR2 SCR 1-2, its radius of gyration R(G) of 2.12(+/-0.05) nm, its maximum length of 10nm and its sedimentation coefficient s20,w(o) of 1.40(+/-0.03) S do not agree with those calculated from the crystal structures, and instead suggest an open structure. Computer modelling of the CR2 SCR1-2 solution structure was based on the structural randomisation of the eight-residue linker peptide joining SCR 1 and SCR 2 to give 9950 trial models. Comparisons with the X-ray scattering curve indicated that the most favoured arrangements for the two SCR domains corresponded to an open V-shaped structure with no contacts between the SCR domains. For C3d, X-ray scattering and sedimentation velocity experiments showed that it exists as a monomer-dimer equilibrium with a dissociation constant of 40 microM. The X-ray scattering curve for monomeric C3d gave an R(G) value of 1.95 nm, and this together with its s20,w(o) value of 3.17 S gave good agreement with the monomeric C3d crystal structure. Modelling of the C3d dimer gave good agreements with its scattering and ultracentrifugation parameters. For the complex, scattering and ultracentrifugation experiments showed that there was no dimerisation, indicating that the C3d dimerisation site was located close to the CR2 SCR 1-2 binding site. The R(G) value of 2.44(+/-0.1) nm, its length of 9 nm and its s20,w(o) value of 3.45(+/-0.01) S showed that its structure was not much more elongated than that of C3d. Calculations with 9950 models of CR2 SCR 1-2 bound to C3d through SCR 2 showed that SCR 1 formed an open V-shaped structure with SCR 2 and was capable of interacting with the surface of C3d. We conclude that the open V-shaped structures formed by CR2 SCR 1-2, both when free and when bound to C3d, are optimal for the formation of a tight two-domain interaction with its ligand C3d.

Complement receptor 2 in the regulation of the immune response

Antigens coated with split products of C3, the result of complement activation, are capable of crosslinking the complement receptor 2 (CR2, CD21) and the antigen receptor on the surface of B cells simultaneously. This dual recognition leads to increased cell proliferation and differentiation and enhanced antibody production. CR2 is also considered to be a regulator of the B cell response to antigen. In this review we summarize the biology of the CR2 and focus on its essential role in generating an effective B cell response to antigenic stimuli. The involvement of CR2 in the pathophysiology of infectious and autoimmune diseases is also discussed.

Epitope tagging permits cell surface detection of functional CFTR

The cystic fibrosis transmembrane conductance regulator (CFTR) is a phosphorylation-activated Cl channel responsible for adenosine 3',5'-cyclic monophosphate (cAMP)-induced Cl secretion across the apical membranes of epithelial cells. To optimize its detection for membrane localization studies, we tagged CFTR with epitope sequences at the carboxy terminus or in the fourth external loop. When epitopes were added to the fourth external loop, the N-linked glycosylation sites in that loop were either preserved or they were mutated to produce a deglycosylated CFTR (dgCFTR). Tagged CFTRs were expressed in HeLa cells, and their cAMP-sensitive Cl permeability was assayed using the halide-sensitive fluorophore SPQ. CFTRs containing the M2 epitope showed halide permeability responses to cAMP, whereas cells expressing CFTR with the hemagglutinin (HA) tag showed little or no cAMP response. Xenopus oocytes expressing dgCFTR, with or without the M2 epitope, showed Cl conductance responses that were 20% of the wild-type response, whereas M2-tagged constructs retaining the glycosylation sites responded like wild-type CFTR. External M2-tagged CFTR was detected in the surface membrane of nonpermeabilized cells. The surface expression of the mutant M2-tagged CFTRs correlated with processing of these mutants (Gregory et al. Mol. Cell. Biol. 11:3886-3893, 1991). M2-901/CFTR is a useful reporter for the trafficking of wild-type and mutant CFTRs to the cell surface.

The CD19-CR2-TAPA-1 complex, CD45 and signaling by the antigen receptor of B lymphocytes

A paradigm describing the response of T lymphocytes to antigen holds that signals from antigen receptors must be modulated by non-antigen-specific, accessory membrane proteins for an appropriate cellular response to occur, such as differentiation, activation and tolerance. Recent studies suggest that this paradigm applies also to B lymphocytes. Signaling through membrane IgM in these cells requires CD45, a phosphotyrosine phosphatase, and is amplified by a complex containing CD19, complement receptor 2 (CD21), and TAPA-1, which recruits the intracellular enzyme, phosphatidylinositol 3-kinase.

Characterization of a cDNA clone coding for human testis membrane cofactor protein (MCP, CD46)

Membrane cofactor protein (MCP) is a complement regulatory protein that acts as a cofactor for the cleavage of C3b and C4b by the serine protease factor I. We have previously reported the characterization of a functional MCP molecule on the acrosomal membrane. This protein migrated as a single band with a molecular weight of 40,000 Da, which is 10,000-20,000 Da smaller than the known MCP molecules, and is devoid of N- and O-linked sugars. We have proposed that the difference in molecular weight resulted from the lack of sugars. To investigate if this is due to the absence of glycosylation sites, we have characterized a cDNA clone from a human testis cDNA library. This cDNA corresponds to a peculiar MCP form previously described, which is characterized by the presence of the serine/threonine/proline-rich exon C (STPC) and the cytoplasmic tail known as CYT2, and we conclude that the absence of mature oligosaccharide of the sperm MCP cannot be totally attributed to a defect of N- and O-glycosylation sequences but rather reflects an alteration of the mechanisms of glycosylation in spermatozoa. The presence of functional MCP on the acrosomal membrane, as well as the other complement regulatory protein, decay-accelerating factor, strongly suggests that these proteins may act concomitantly to protect the acrosome-reacted spermatozoa from the attack of the complement present in the female genital tract.

Complement

The complement system mediates a wide range of important biological functions. The use of modern techniques in protein chemistry and molecular biology has greatly facilitated our understanding of the interactions between the fluid phase and cell-bound components of the system. Structural and genetic analysis has shown that while many of these components are polymorphic, there are major similarities between many of the proteins serving enzymatic and regulatory roles in both the alternative and classical pathways. The regulation of complement activation and Class III genes, on chromosomes 1 and 6 respectively, encode nine of the major proteins in the system. The genetic basis of C4 and C3 polymorphisms is now well established, and further study may reveal functional differences between polymorphic variants of other components. The study of individuals with either genetic or acquired deficiencies of complement proteins and receptors has provided insight into the function of these components, leukocyte adherence deficiency (LAD) providing the best example. An appreciation of the genetics, structure and functions of the regulatory proteins decay-accelerating factor (DAF) and homologous restriction factor has enhanced our understanding of the pathogenesis of paroxysmal nocturnal haemoglobinuria. The full importance of CD59 glycoprotein, the newest member of the complement family, remains to be determined.

Adherence and receptor relationships of Candida albicans

The cell surface of Candida albicans is composed of a variety of polysaccharides such as glucan, chitin, and mannan. The first two components primarily provide structure, while the mannan, often covalently linked to protein, constitutes the major antigen of the organism. Mannoproteins also have enzymatic activity (acid protease) and ligand-receptor functions. The complement receptors of C. albicans appear to be mannoproteins that are required for the adherence of the organism to endothelial cells. This is certainly true of the CR3-like protein of C. albicans. Proof that the CR3 is the Candida receptor for endothelial cells is derived from two observations. First, mutants lacking CR3 activity are less adherent in vitro and, in fact, less virulent. Second, the ligand recognized by the CR3 receptor (C3bi) as well as anti-CR3 antibodies blocks adherence of the organism to endothelial cells. The CR2 of C. albicans appears to promote the adherence of the organism to plastic substrates. Unlike the CR2 of mammalian cells, the Candida CR2 recognizes ligands containing the RGD sequence of amino acids in addition to the C3d ligand, which does not contain the RGD sequence. There is uncertainty as to whether the Candida CR2 and CR3 are, in fact, different proteins. A mannoprotein has also been described as the adhesin for epithelial cells. In this case, the receptor has a lectinlike activity and recognizes fucose- or glucosamine-containing glycoproteins of epithelial cells, depending on the strain of C. albicans. The oligosaccharide component of the receptor is probably not involved in ligand recognition and may serve to stabilize the receptor. However, the oligosaccharide factor 6 epitope of mannan may also provide adhesin activity in the recognition of epithelial cells. Mannoproteins can be extracted from cells by a number of reagents. Zymolyase, for instance, tends to remove structural mannoproteins, which contain relatively little protein and are linked to glucan. Reagents such as dithiothreitol, on the other hand, tend to extract mannoproteins containing higher amounts of protein that appear to have receptor function. The mannoproteins of C. albicans are dynamically expressed and may be growth phase and growth form specific.

Purification and characterization of the extracellular C3d-binding protein of Candida albicans

A C3d-binding glycoprotein was purified from the culture filtrate of Candida albicans by preparative isoelectric focusing. The protein possessed a pI of 3.9 to 4.1 and could inhibit rosetting of EAC3d (sheep erythrocytes conjugated to C3d) by pseudohyphae of C. albicans. When analyzed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and mercaptoethanol, the protein migrated as a doublet with apparent molecular masses of 55 and 60 kilodaltons (kDa) and as a 50-kDa band in nonreducing gels. These results were observed with Aurodye stain for proteins. Western immunoblot, and concanavalin A stain, which indicates that both bands contain carbohydrate as well as antigenic determinants. The treatment of purified glycoprotein with endoglycosidase F but not endoglycosidases H, N, and O resulted in a complete conversion of the doublet into a faster-migrating broad band with an apparent molecular mass of 45 kDa. When the amino acid analysis of the C3d-binding protein was compared with that of the CR2 from B lymphocytes, significant differences were observed. These data indicate that C. albicans secretes a C3d-binding protein during growth in vitro which appears to be different from the mammalian C3d receptor.

Proliferation of resting B cells is modulated by CR2 and CR1

Absence of the third component of complement, C3, is associated with impaired ability to synthesize antibody, particularly in the presence of limiting antigen [1-9]. The mature B lymphocyte bearing the surface immunoglobulin receptor transduces signals for proliferation and differentiation upon binding of specific antigen. This mature B cell also bears two related membrane proteins, CR2 (the C3d/Epstein-Barr virus receptor) (CD35) [15], which can mediate the binding of ligands to which appropriate cleavage fragments of C3 have become attached [16]. It has been suggested that these receptors play a direct role(s) in B cell activation [17-25]. In light of previous in vivo observations we decided to assess the function of CR2 and CR1 in relation to B cell activation through the membrane IgM receptor. Highly purified splenic B cells were prepared. No contaminating T cells or macrophages were detected by flow cytometric analysis and no proliferative activity was present upon PHA or ConA stimulation of the purified cells. The B cells were separated into low (activated), medium (preactivated) and high density (resting) fractions by Percoll gradient density centrifugation [26]. The responses of the B cell subpopulations to various concentrations of anti mu (DA4.4 monoclonal antibody) [27] were examined for proliferation at 72 h and for IgM/IgG production at 7 days. Low density B cells were maximally stimulated and no concentration of anti-mu was effective in enhancing their responses. High density B cells proliferated to anti-mu in a concentration dependent manner. When substimulatory concentrations of anti-mu were employed, concomitant crosslinking of CR2 (with either of 2 distinct monoclonal antibodies HB-5 [28] or OKB7 [17]) resulted in a 45% enhancement of B cell proliferation above that observed by crosslinking of SIgM alone. In these studies, total IgM and IgG did not increase in the absence of T cells or T cell factors, indicating that terminal differentiation did not occur. In contrast, when a monoclonal antibody to CR1(44D) [29] was employed in an identical experiment, B cell proliferation was completely inhibited. Antibodies to CR2 or CR1 either alone or in crosslinked form did not enhance B cell proliferation. Immune complexes may crosslink the B cell surface in a manner analogous to our model when the immunoglobulin receptor and CR2 are simultaneously engaged. This activation signal may be particularly important in eliciting antibody responses when the quantity of specific antigen or the affinity for antigen is low. The marked inhibition of proliferation induced by CR1 suggests an alternate role for this receptor in modulation of B cell responses.

Candida albicans C3d receptor, isolated by using a monoclonal antibody

Pseudohyphae of Candida albicans possess a receptor for C3d, a fragment of the complement component C3. This receptor was partially purified by using a monoclonal antibody (CA-A) that previously had been shown to inhibit the binding of C3d to C. albicans pseudohyphae. Purified immunoglobulin G from ascites fluid (CA-A) was coupled to a cyanogen bromide-activated Sepharose column, and an affinity-purified fraction (A2) from C. albicans pseudohyphae was obtained. This fraction inhibited rosetting of the EAC3d receptor by pseudohyphae and appeared to contain glycoprotein, since receptor activity could be removed when A2 was incubated with lectins specific for mannose and glucose. A2 was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and two polypeptides of approximately 60 and 70 kilodaltons (kDa) were consistently identified in reducing gels. The 60-kDa protein was identified as a glycoprotein by concanavalin A binding. A2 was further analyzed by high-pressure liquid chromatography (HPLC). Of three fractions obtained by HPLC, one containing the 60-kDa protein was found to have receptor activity. When analyzed by HPLC, this protein was found to contain mannose and glucose in approximately equal amounts. Both immunofluorescence and electron microscopy of pseudohyphae treated with CA-A identified A2 as a surface moiety. Thus, the C3d receptor of C. albicans, isolated with CA-A, is a glycoprotein of approximately 60 kDa.

Identification of C3d receptors on Candida albicans

Pseudohyphal but not yeast forms of Candida albicans possess both iC3b and C3d receptors, as determined by rosetting with erythrocytes carrying iC3b (EAC3bi) or C3d (EAC3d). Rosetting with EAC3d was markedly reduced when pseudohyphae were heat killed or treated with trypsin or pronase but was not inhibited by several saccharides or aminosaccharides, including alpha-methyl-D-mannoside, or by pretreatment of pseudohyphae with concanavalin A. However, mannoproteins obtained by concanavalin A affinity chromatography of whole pseudohyphal extracts inhibited the attachment of EAC3d to C. albicans, whereas soluble (nonmannosylated) proteins were less active. Thus, although the C3d receptors appeared to be glycosylated, the oligosaccharide component of the receptor was apparently not involved in the recognition of C3d. To isolate these receptors, whole-cell extracts were separated by DEAE-Trisacryl chromatography. Fractions that inhibited rosetting were pooled and affinity purified by C3d-Thiol-Sepharose chromatography. The eluate from this affinity column inhibited attachment of C. albicans to EAC3d. Monoclonal antibodies to C. albicans were prepared, and three of these antibodies blocked rosetting. Western blotting (immunoblotting) with these antibodies indicated the presence of 62- and 70-kilodalton receptors for C3d in the extracts purified by C3d affinity chromatography and separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

gp140, the EBV/C3d receptor (CR2) of human B lymphocytes, is involved in cell-free phosphorylation of p120, a nuclear ribonucleoprotein

gp140, the EB/C3d receptor (EBV/C3dR; CR2), is a membrane site involved in human B cell regulation. Cross-linking of this receptor on the cell surface by its specific ligands led to the enhancement of B cell proliferation in synergy with T cell factors. In vitro activation of human peripheral B lymphocytes by cross-linking membrane immunoglobulins with anti-mu antibody induced EBV/C3dR phosphorylation. These studies were pursued by analyzing cell-free phosphorylation of EBV/C3dR isolated from Raji cell fractions, and immobilized on OKB7, a monoclonal anti-EBV/C3dR antibody. Three EBV/C3dR-related antigens which could be cell-free phosphorylated were detected: gp140, the EBV/C3dR, p130 and p120. gp140, the mature form of EBV/C3dR, was isolated from plasma membrane and from purified nuclei. p130 was identified as an intracellular intermediate of EBV/C3dR glycosylation, localized in low-density microsomes. Phosphoamino acid analysis of EBV/C3dR allowed the detection of phosphotyrosine and phosphoserine residues. These data suggest that EBV/C3dR could carry an autophosphorylation activity and could be associated to serine kinases. Using polyclonal anti-p120 antibody and anti-120 kDa nuclear ribonucleoprotein monoclonal antibody (mAb), p120 was identified as a nuclear ribonucleoprotein antigenically not related to EBV/C3dR. Detection of p120 on EBV/C3dR, immobilized on OKB7, was due to interactions between both antigens, instead of anti-EBV/C3dR mAb cross-reactivity with p120. Cell-free phosphorylation of p120 was under the control of EBV/C3dR. However, it is not yet established whether other nuclear or membrane components were involved in the control of p120 cell-free phosphorylation by EBV/C3dR. From the data presented herein, we propose that phosphorylation of a 120-kDa nuclear ribonucleoprotein by EBV/C3dR-associated kinases could represent a crucial step in in vivo regulation of human B cell activation.

Epstein-Barr virus gp350/220 binding to the B lymphocyte C3d receptor mediates adsorption, capping, and endocytosis

The type 2 complement receptor, CR2, a B lymphocyte surface glycoprotein, is known to be a component of the EBV receptor. We now demonstrate that the major EBV outer membrane glycoprotein, gp350/220, is a highly specific ligand for CR2. EBV or beads coated with purified recombinant gp350/220 adsorb to normal B lymphocytes, cap with CR2, become endocytosed into vesicles, and are released into the cytoplasm. This is the first demonstration of herpesvirus glycoprotein-cell glycoprotein receptor interaction in viral adsorption and penetration. The capping of CR2 in response to virus, gp350/220-coated beads, or anti-CR2 monoclonal antibodies is associated with cocapping of surface immunoglobulin. Interaction between CR2 and surface immunoglobulin may be important in modulating the B cell activation that normally follows EBV infection or exposure to antigen.

Current ideas on the significance of protein glycosylation

Carbohydrate has been removed from a number of glycoproteins without major effect on the structure or enzyme activity of the protein. Thus carbohydrate has been suggested to underly a non-primary function for proteins, such as in relatively non-specific interactions with other carbohydrates or macromolecules, stabilization of protein conformation, or protection from proteolysis. This non-specific concept is consistent with both the general similarity in carbohydrate structure on very diverse glycoproteins and the frequent structural microheterogeneity of carbohydrate chains at given sites. The concept is supported in a general sense by the viability of cells whose glycosylation processes have been globally disrupted by mutation or pharmacological inhibitors. In contrast to the above observations, other studies have revealed the existence of specific, selective receptors for discrete oligosaccharide structures on glycoproteins which seem to be important for compartmentalization of the glycoprotein, or the positioning of cells on which the glycoprotein is concentrated. Sometimes multivalency in the carbohydrate-receptor interaction is crucial. There are additional possible roles for carbohydrate in the transduction of information upon binding to a receptor. The possibility of specific roles for carbohydrate is supported by the existence of numerous unique carbohydrate structures, many of which have been detected as glycoantigens by monoclonal antibodies, with unique distributions in developing and differentiated cells. This article attempts to summarize and rationalize the contradictory results. It appears that in general carbohydrate does in fact underlie only roles secondary to a protein's primary function. These secondary roles are simple non-specific ones of protection and stabilization, but often also satisfy the more sophisticated needs of spatial position control and compartmentalization in multicellular eukaryotic organisms. It is suggested that there are advantages, evolutionarily speaking, for the shared use of carbohydrate for non-specific roles and for specific roles primarily as luxury functions to be executed during the processes of cell differentiation and morphogenesis.

Purification of the B lymphocyte receptor for the C3d fragment of complement and the Epstein-Barr virus by monoclonal antibody affinity chromatography, and assessment of its functional capacities

The human C3d receptor (complement receptor type 2, CR2), that also serves as the B lymphocyte receptor for the Epstein-Barr virus, was purified from detergent lysates from the B lymphoblastoid cell lines, SB and Raji, by monoclonal antibody affinity chromatography using the anti-CR2 monoclonal antibody, HB-5. Relative to the concentration of cellular protein and receptor that was initially solubilized by detergent, the procedure provided a 37,000-fold purification with a 40-50% recovery of CR2. The purified receptor presented a single Coomassie blue-stained band when analyzed by SDS-PAGE, and it retained its function of binding to C3-Sepharose. The N-terminus of CR2 was blocked. The amino acid composition was significantly similar to that of the C3b/C4b receptor, factor H and C4 binding protein, suggesting that CR2 may be a member of this newly defined protein family. However, CR2 did not exhibit the regulatory functions of these proteins, namely, the decay dissociation of the classical or alternative pathway C3 convertases and serving as a cofactor for the cleavage of C3b.