Determination of the structural basis for selective binding of Epstein-Barr virus to human complement receptor type 2

A novel C3d-containing oligomeric vaccine provides insight into the viability of testing human C3d-based vaccines in mice

The use of C3d, the final degradation product of complement protein C3, as a "natural" adjuvant has been widely examined since the initial documentation of its immunogenicity-enhancing properties as a consequence of binding to complement receptor 2. Subsequently it was demonstrated that these effects are most evident when oligomeric, rather than when monomeric forms of C3d, are linked to various test protein antigens. In this study, we examined the feasibility of enhancing the adjuvant properties of human C3d further by utilizing C4b-binding protein (C4BP) to provide an oligomeric arrayed scaffold fused to the model antigen, tetanus toxin C fragment (TTCF). High molecular weight, C3d-containing oligomeric vaccines were successfully expressed, purified from mammalian cells and used to immunize groups of mice. Surprisingly, anti-TTCF antibody responses measured in these mice were poor. Subsequently we established by in vitro and in vivo analysis that, in the presence of mouse C3, human C3d does not interact with either mouse or even human complement receptor 2. These data confirm the requirement to develop murine versions of C3d based adjuvant compounds to test in mice or that mice would need to be developed that express both human C3 and human CR2 to allow the testing of human C3d based adjuvants in mouse in any capacity.

The BDLF3 gene product of Epstein-Barr virus, gp150, mediates non-productive binding to heparan sulfate on epithelial cells and only the binding domain of CD21 is required for infection

The cell surface molecules used by Epstein-Barr virus (EBV) to attach to epithelial cells are not well-defined, although when CD21, the B cell receptor for EBV is expressed epithelial cell infection increases disproportionately to the increase in virus bound. Many herpesviruses use low affinity charge interactions with molecules such as heparan sulfate to attach to cells. We report here that the EBV glycoprotein gp150 binds to heparan sulfate proteoglycans, but that attachment via this glycoprotein is not productive of infection. We also report that only the aminoterminal two short consensus repeats of CD21 are required for efficient infection, This supports the hypothesis that, when expressed on an epithelial cell CD21 serves primarily to cluster the major attachment protein gp350 in the virus membrane and enhance access of other important glycoproteins to the epithelial cell surface.

Viral Entry

Epstein-Barr virus primarily, though not exclusively, infects B cells and epithelial cells. Many of the virus and cell proteins that are involved in entry into these two cell types in vitro have been identified, and their roles in attachment and fusion are being explored. This chapter discusses what is known about entry at the cellular level in vitro and describes what little is known about the process in vivo. It highlights some of the questions that still need to be addressed and considers some models that need further testing.

A novel tetrameric gp350 1-470 as a potential Epstein-Barr virus vaccine

Infectious mononucleosis and B-cell transformation in response to infection with Epstein-Barr virus (EBV) is dependent upon binding of the EBV envelope glycoprotein gp350 to CD21 on B-cells. Gp350-specific antibody comprises most of the EBV neutralizing activity in the serum of infected patients, making this protein a promising target antigen for a prophylactic EBV vaccine. We describe a novel, tetrameric gp350-based vaccine that exhibits markedly enhanced immunogenicity relative to its monomeric counterpart. Plasmid DNA was constructed for synthesis, within transfected CHO cells, of a tetrameric, truncated (a.a. 1-470) gp350 protein (gp350(1-470)). Tetrameric gp350(1-470) induced ≈ 20-fold higher serum titers of gp350(1-470)-specific IgG and >19-fold enhancements in neutralizing titers at the highest dose, and was >25-fold more immunogenic on a per-weight basis than monomeric gp350(1-470). Further, epidermal immunization with plasmid DNA encoding gp350(1-470) tetramer induced 8-fold higher serum titers of gp350(1-470)-specific IgG relative to monomer. Tetrameric gp350(1-470) binding to human CD21 was >24-fold more efficient on a per-weight basis than monomer, but neither tetramer nor monomer mediated polyclonal human B-cell activation. Finally, the introduction of strong, universal tetanus toxoid (TT)-specific CD4+ T-cell epitopes into the tetrameric gp350(1-470) had no effect on the gp350(1-470)-specific IgG response in naïve mice, and resulted in suppressed gp350(1-470)-specific IgG responses in TT-primed mice. Collectively, these data suggest that tetrameric gp350(1-470) is a potentially promising candidate for testing as a prophylactic EBV vaccine, and that protein multimerization, using the approach described herein, is likely to be clinically relevant for enhancing the immunogenicity of other proteins of vaccine interest.

Human complement receptor 2 (CR2/CD21) as a receptor for DNA: implications for its roles in the immune response and the pathogenesis of systemic lupus erythematosus (SLE)

Human CR2 is a B cell membrane glycoprotein that plays a central role in autoimmunity. Systemic lupus erythematosus (SLE) patients show reduced CR2 levels, and complete deficiency of CR2 and CR1 promotes the development of anti-DNA antibodies in mouse models of SLE. Here we show that multiple forms of DNA, including bacterial, viral and mammalian DNA, bind to human CR2 with moderately high affinity. Surface plasmon resonance studies showed that methylated DNA bound with high affinity with CR2 at a maximal K(D) of 6nM. DNA was bound to the first two domains of CR2 and this binding was blocked by using a specific inhibitory anti-CR2 mAb. DNA immunization in Cr2(-/-) mice revealed a specific defect in immune responses to bacterial DNA. CR2 can act as a receptor for DNA in the absence of complement C3 fixation to this ligand. These results suggest that CR2 plays a role in the recognition of foreign DNA during host-immune responses. This recognition function of CR2 may be a mechanism that influences the development of autoimmunity to DNA in SLE.

Complement receptor 2 is expressed in neural progenitor cells and regulates adult hippocampal neurogenesis

Injury and inflammation are potent regulators of adult neurogenesis. As the complement system forms a key immune pathway that may also exert critical functions in neural development and neurodegeneration, we asked whether complement receptors regulate neurogenesis. We discovered that complement receptor 2 (CR2), classically known as a coreceptor of the B-lymphocyte antigen receptor, is expressed in adult neural progenitor cells (NPCs) of the dentate gyrus. Two of its ligands, C3d and interferon-α (IFN-α), inhibited proliferation of wild-type NPCs but not NPCs derived from mice lacking Cr2 (Cr2(-/-)), indicating functional Cr2 expression. Young and old Cr2(-/-) mice exhibited prominent increases in basal neurogenesis compared with wild-type littermates, whereas intracerebral injection of C3d resulted in fewer proliferating neuroblasts in wild-type than in Cr2(-/-) mice. We conclude that Cr2 regulates hippocampal neurogenesis and propose that increased C3d and IFN-α production associated with brain injury or viral infections may inhibit neurogenesis.

Delineation of the complement receptor type 2-C3d complex by site-directed mutagenesis and molecular docking

The interactions between the complement receptor type 2 (CR2) and the C3 complement fragments C3d, C3dg, and iC3b are essential for the initiation of a normal immune response. A crystal-derived structure of the two N-terminal short consensus repeat (SCR1-2) domains of CR2 in complex with C3d has previously been elucidated. However, a number of biochemical and biophysical studies targeting both CR2 and C3d appear to be in conflict with these structural data. Previous mutagenesis and heteronuclear NMR spectroscopy studies directed toward the C3d-binding site on CR2 have indicated that the CR2-C3d cocrystal structure may represent an encounter/intermediate or nonphysiological complex. With regard to the CR2-binding site on C3d, mutagenesis studies by Isenman and coworkers [Isenman, D. E., Leung, E., Mackay, J. D., Bagby, S. & van den Elsen, J. M. H. (2010). Mutational analyses reveal that the staphylococcal immune evasion molecule Sbi and complement receptor 2 (CR2) share overlapping contact residues on C3d: Implications for the controversy regarding the CR2/C3d cocrystal structure. J. Immunol. 184, 1946-1955] have implicated an electronegative "concave" surface on C3d in the binding process. This surface is discrete from the CR2-C3d interface identified in the crystal structure. We generated a total of 18 mutations targeting the two (X-ray crystallographic- and mutagenesis-based) proposed CR2 SCR1-2 binding sites on C3d. Using ELISA analyses, we were able to assess binding of mutant forms of C3d to CR2. Mutations directed toward the concave surface of C3d result in substantially compromised CR2 binding. By contrast, targeting the CR2-C3d interface identified in the cocrystal structure and the surrounding area results in significantly lower levels of disruption in binding. Molecular modeling approaches used to investigate disparities between the biochemical data and the X-ray structure of the CR2-C3d cocrystal result in highest-scoring solutions in which CR2 SCR1-2 is docked within the concave surface of C3d.

Biophysical investigations of complement receptor 2 (CD21 and CR2)-ligand interactions reveal amino acid contacts unique to each receptor-ligand pair

Human complement receptor type 2 (CR2 and CD21) is a cell membrane receptor, with 15 or 16 extracellular short consensus repeats (SCRs), that promotes B lymphocyte responses and bridges innate and acquired immunity. The most distally located SCRs, SCR1-2, mediate the interaction of CR2 with its four known ligands (C3d, EBV gp350, IFNalpha, and CD23). To ascertain specific interacting residues on CR2, we utilized NMR studies wherein gp350 and IFNalpha were titrated into (15)N-labeled SCR1-2, and chemical shift changes indicative of specific inter-molecular interactions were identified. With backbone assignments made, the chemical shift changes were mapped onto the crystal structure of SCR1-2. With regard to gp350, the binding region of CR2 is primarily focused on SCR1 and the inter-SCR linker, specifically residues Asn(11), Arg(13), Ala(22), Arg(28), Ser(32), Arg(36), Lys(41), Lys(57), Tyr(64), Lys(67), Tyr(68), Arg(83), Gly(84), and Arg(89). With regard to IFNalpha, the binding is similar to the CR2-C3d interaction with specific residues being Arg(13), Tyr(16), Arg(28), Ser(42), Lys(48), Lys(50), Tyr(68), Arg(83), Gly(84), and Arg(89). We also report thermodynamic properties of each ligand-receptor pair determined using isothermal titration calorimetry. The CR2-C3d interaction was characterized as a two-mode binding interaction with K(d) values of 0.13 and 160 microm, whereas the CR2-gp350 and CR2-IFNalpha interactions were characterized as single site binding events with affinities of 0.014 and 0.035 microm, respectively. The compilation of chemical binding maps suggests specific residues on CR2 that are uniquely important in each of these three binding interactions.

Mapping of the C3d ligand binding site on complement receptor 2 (CR2/CD21) using nuclear magnetic resonance and chemical shift analysis

Complement receptor 2 (CR2, CD21) is a cell membrane protein, with 15 or 16 extracellular short consensus repeats (SCRs), that promotes B lymphocyte responses and bridges innate and acquired immunity. The most distally located SCRs (SCR1-2) mediate the interaction of CR2 with its four known ligands (C3d, Epstein-Barr virus gp350, interferon-alpha, and CD23). Inhibitory monoclonal antibodies against SCR1-2 block binding of all ligands. To develop ligand-specific inhibitors that would also assist in identifying residues unique to each receptor-ligand interaction, phage were selected from randomly generated libraries by panning with recombinant SCR1-2, followed by specific ligand-driven elution. Derived peptides were tested by competition ELISA. One peptide, C3dp1 (APQHLSSQYSRT) exhibited ligand-specific inhibition at midmicromolar IC(50). C3d was titrated into (15)N-labeled SCR1-2, which revealed chemical shift changes indicative of specific intermolecular interactions. With backbone assignments made, the chemical shift changes were mapped onto the crystal structure of SCR1-2. With regard to C3d, the binding surface includes regions of SCR1, SCR2, and the inter-SCR linker, specifically residues Arg(13), Tyr(16), Arg(28), Tyr(29), Ser(32), Thr(34), Lys(48), Asp(56), Lys(57), Tyr(68), Arg(83), Gly(84), Asn(101), Asn(105), and Ser(109). SCR1 and SCR2 demonstrated distinct binding modes. The CR2 binding surface incorporating SCR1 is inconsistent with a previous x-ray CR2-C3d co-crystal analysis but consistent with mutagenesis, x-ray neutron scattering, and inhibitory monoclonal antibody epitope mapping. Titration with C3dp1 yielded chemical shift changes (Arg(13), Tyr(16), Thr(34), Lys(48), Asp(56), Lys(57), Tyr(68), Arg(83), Gly(84), Asn(105), and Ser(109)) overlapping with C3d, indicating that C3dp1 interacts at the same CR2 site as C3d.

Molecular basis of the interaction between complement receptor type 2 (CR2/CD21) and Epstein-Barr virus glycoprotein gp350

The binding of the Epstein-Barr virus glycoprotein gp350 by complement receptor type 2 (CR2) is critical for viral attachment to B lymphocytes. We set out to test hypotheses regarding the molecular nature of this interaction by developing an enzyme-linked immunosorbent assay (ELISA) for the efficient analysis of the gp350-CR2 interaction by utilizing wild-type and mutant forms of recombinant gp350 and also of the CR2 N-terminal domains SCR1 and SCR2 (designated CR2 SCR1-2). To delineate the CR2-binding site on gp350, we generated 17 gp350 single-site substitutions targeting an area of gp350 that has been broadly implicated in the binding of both CR2 and the major inhibitory anti-gp350 monoclonal antibody (MAb) 72A1. These site-directed mutations identified a novel negatively charged CR2-binding surface described by residues Glu-21, Asp-22, Glu-155, Asp-208, Glu-210, and Asp-296. We also identified gp350 amino acid residues involved in non-charge-dependent interactions with CR2, including Tyr-151, Ile-160, and Trp-162. These data were supported by experiments in which phycoerythrin-conjugated wild-type and mutant forms of gp350 were incubated with CR2-expressing K562 cells and binding was assessed by flow cytometry. The ELISA was further utilized to identify several positively charged residues (Arg-13, Arg-28, Arg-36, Lys-41, Lys-57, Lys-67, Arg-83, and Arg-89) within SCR1-2 of CR2 that are involved in the binding interaction with gp350. These experiments allowed a comparison of those CR2 residues that are important for binding gp350 to those that define the epitope for an effective inhibitory anti-CR2 MAb, 171 (Asn-11, Arg-13, Ser-32, Thr-34, Arg-36, and Tyr-64). The mutagenesis data were used to calculate a model of the CR2-gp350 complex using the soft-docking program HADDOCK.

Solution structure of the complex formed between human complement C3d and full-length complement receptor type 2

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 through its binding to C3d, a cleavage fragment of the major complement component C3. The extracellular portion of CR2 comprises 15 or 16 short complement regulator (SCR) domains in a partially folded-back but flexible structure. Here, the effect of C3d binding to CR2 was determined by analytical ultracentrifugation and X-ray scattering. The sedimentation coefficient of unbound CR2 is 4.03 S in 50 mM NaCl. Because this agrees well with a value of 3.93 S in 137 mM NaCl, the overall CR2 structure is unaffected by change in ionic strength. Unbound C3d exists in monomer-dimer and monomer-trimer equilibria in 50 mM NaCl, but as a monomer only in 137 mM NaCl. In c(s) size-distribution analyses, an equimolar mixture of the CR2-C3d complex in 50 mM NaCl revealed a single peak shifted to 4.52 S when compared to unbound CR2 at 4.03 S to show that the complex had formed. The CR2-C3d complex in 137 mM NaCl showed two peaks at 2.52 S and 4.07 S to show that this had dissociated. Solution structural models for the CR2 SCR-1/2 complex with C3d and CR2 SCR-1/15 were superimposed. These gave an average sedimentation coefficient of 4.57 S for the complex, in good agreement with the observed value of 4.52 S. It is concluded that CR2 does not detectably change conformation when C3d is bound to it. Consistent with previous analyses, its C3d complex is not formed in physiological salt conditions. The implications of these solution results for its immune role are discussed. To our knowledge, this is the first solution structural study of a large multidomain SCR protein CR2 bound to its physiological ligand C3d.

Laser-capture microdissection of oropharyngeal epithelium indicates restriction of Epstein-Barr virus receptor/CD21 mRNA to tonsil epithelial cells

BACKGROUND

Epstein-Barr virus colonizes the oropharynx of a majority of individuals. It infects B lymphocytes and epithelial cells and can contribute to the development of both lymphoid and epithelial tumors. The virus uses CD21 for attachment to B cells which constitutively express the protein. Infection of epithelial cells in vitro is also more efficient if CD21 is available. However, its potential contribution to infection in vivo has been difficult to evaluate as discrepant results with antibodies have made it difficult to determine which, if any, epithelial cells in the oropharynx express CD21.

METHODS

To reevaluate CD21 expression by an alternative method, epithelial cells were isolated by laser-capture microdissection from formalin-fixed sections of tissues from various parts of the oropharynx and mRNA was amplified with primers specific for the exons of CD21 which code for the Epstein-Barr virus binding site.

RESULTS

CD21 mRNA was expressed in tonsil epithelium, but not in epithelium from buccal mucosa, uvula, soft palate or tongue.

CONCLUSIONS

CD21 does not contribute to infection of most normal epithelial tissues in the oropharynx, but may contribute to infection of epithelial cells in the tonsil, where virus has been demonstrated in healthy carriers.

Isolating the Epstein-Barr virus gp350/220 binding site on complement receptor type 2 (CR2/CD21)

Complement receptor type 2 (CR2/CD21) is essential for the attachment of Epstein-Barr virus (EBV) to the surface of B-lymphocytes in an interaction mediated by the viral envelope glycoprotein gp350. The heavily glycosylated structure of EBV gp350 has recently been elucidated by x-ray crystallography, and the CR2 binding site on this protein has been characterized. To identify the corresponding gp350 binding site on CR2, we have undertaken a site-directed mutagenesis study targeting regions of CR2 that have previously been implicated in the binding of CR2 to the C3d/C3dg fragments of complement component C3. Wild-type or mutant forms of CR2 were expressed on K562 cells, and the ability of these CR2-expressing cells to bind gp350 was measured using flow cytometry. Mutations directed toward the two N-terminal extracellular domains of CR2 (SCR1-2) reveal that a large contiguous surface of CR2 SCR1-2 is involved in gp350 binding, including a number of positively charged residues (Arg-13, (Arg-28, (Arg-36, Lys-41, Lys-57, Lys-67, and Arg-83). These data appear to complement the CR2 binding site on gp350, which is characterized by a preponderance of negative charge. In addition to identifying the importance of charge in the formation of a CR2-gp350 complex, we also provide evidence that both SCR1 and SCR2 make contact with gp350. Specifically, two anti-CR2 monoclonal antibodies, designated as monoclonal antibodies 171 and 1048 whose primary epitopes are located within SCR2, inhibit binding of wild-type CR2 to EBV gp350; with regard to SCR1, both K562 cells expressing an S15P mutation and recombinant S15P CR2 proteins exhibit diminished gp350 binding.

Structure of the Epstein-Barr virus major envelope glycoprotein

Epstein-Barr virus (EBV) infection of B cells is associated with lymphoma and other human cancers. EBV infection is initiated by the binding of the viral envelope glycoprotein (gp350) to the cell surface receptor CR2. We determined the X-ray structure of the highly glycosylated gp350 and defined the CR2 binding site on gp350. Polyglycans shield all but one surface of the gp350 polypeptide, and we demonstrate that this glycan-free surface is the receptor-binding site. Deglycosylated gp350 bound CR2 similarly to the glycosylated form, suggesting that glycosylation is not important for receptor binding. Structure-guided mutagenesis of the glycan-free surface disrupted receptor binding as well as binding by a gp350 monoclonal antibody, a known inhibitor of virus-receptor interactions. These results provide structural information for developing drugs and vaccines to prevent infection by EBV and related viruses.

Characterization of human complement receptor type 2 (CR2/CD21) as a receptor for IFN-alpha: a potential role in systemic lupus erythematosus

Human complement receptor type 2 (CR2/CD21) is a B lymphocyte membrane glycoprotein that plays a central role in the immune responses to foreign Ags as well as the development of autoimmunity to nuclear Ags in systemic lupus erythematosus. In addition to these three well-characterized ligands, C3d/iC3b, EBV-gp350, and CD23, a previous study has identified CR2 as a potential receptor for IFN-alpha. IFN-alpha, a multifunctional cytokine important in the innate immune system, has recently been proposed to play a major pathogenic role in the development of systemic lupus erythematosus in humans and mice. In this study, we have shown using surface plasmon resonance and ELISA approaches that CR2 will bind IFN-alpha in the same affinity range as the other three well-characterized ligands studied in parallel. In addition, we show that IFN-alpha interacts with short consensus repeat domains 1 and 2 in a region that serves as the ligand binding site for C3d/iC3b, EBV-gp350, and CD23. Finally, we show that treatment of purified human peripheral blood B cells with the inhibitory anti-CR2 mAb 171 diminishes the induction of IFN-alpha-responsive genes. Thus, IFN-alpha represents a fourth class of extracellular ligands for CR2 and interacts with the same domain as the other three ligands. Defining the role of CR2 as compared with the well-characterized type 1 IFN-alpha receptor 1 and 2 in mediating innate immune and autoimmune roles of this cytokine should provide additional insights into the biologic roles of this interaction.

Complement receptor 2, natural antibodies and innate immunity: Inter-relationships in B cell selection and activation

Complement receptor type 2 (CR2) is a receptor that serves as an important interface between the complement system and adaptive immunity. Recent studies have shown that CR2 is also centrally involved in innate immunity, and one key area is the development of potentially pathogenic natural antibodies that target neo-epitopes revealed in ischemic tissue undergoing reperfusion. Mice lacking either total immunoglobulins or CR2 alone are protected from the development of ischemia-reperfusion injury, and this effect can be reversed by introducing CR2-sufficient B-1 cells or by transferring polyclonal natural IgM antibody from wild type mice as well as monoclonal antibodies that recognize phospholipids, DNA or non-muscle myosin. We will report at the XXI ICW an additional membrane-associated protein to which pathogenic IgM antibodies are directed. Whether B cells producing these natural antibodies are differentially selected in CR2-deficient mice is as yet not well understood, and the complement-related mechanism(s) whereby this differential repertoire selection process could occur have yet to be explored in any detail. In addition to this important role in innate immunity, CR2 can also act as a receptor for other components or activators of innate immunity. One such component is interferon-alpha, an anti-viral cytokine that binds CR2 and induces a component of its mRNA signature in B cells through this receptor. Other potential CR2 ligands are DNA and DNA-containing complexes such as chromatin. The biologic role of these CR2 interactions with interferon-alpha and DNA-containing complexes is not well understood, but may be important in the development of the autoimmune disease systemic lupus erythematosus that is characterized by enhanced interferon-alpha levels and loss of self tolerance to DNA-containing self antigens.

A B-lymphocyte binding peptide from BNRF1 induced antibodies inhibiting EBV-invasion of B-lymphocytes

Epstein-Barr virus (EBV) infects human target cells mainly through gp350/220-CD21 and gp42-MHCII interactions; however, it has been shown that these interactions are dispensable for EBV-invasion of susceptible cells, suggesting that other viral proteins are involved in this process. It is probable that tegument BNRF1/p140 protein is involved in EBV-invasion of target cells, since anti-p140 antibodies inhibit EBV-infection of B-lymphocytes and there is evidence that part of the protein is located on virus surface. Sixty-six peptides, covering the entire BNRF1/p140 sequence, were synthesised and tested in lymphoblastoid cell line binding assays. Peptides 11465 and 11521 bound with high affinity to Raji, Ramos and P3HR-1 cells but not to erythrocytes, showing cell-binding behaviour similar to EBV. These two peptides induced antibodies recognising live EBV-infected cells. Interestingly, peptide-11521 (YVLQNAHQIACHFHSNGTDA) or antibodies induced by this peptide inhibited EBV-binding to B-lymphocytes, suggesting that this p140-region could be involved in EBV and B-lymphocyte interaction.

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.

Mutational analysis of the complement receptor type 2 (CR2/CD21)-C3d interaction reveals a putative charged SCR1 binding site for C3d

We have characterized the interaction between the first two short consensus repeats (SCR1-2) of complement receptor type 2 (CR2, CD21) and C3d in solution, by utilising the available crystal structures of free and C3d-bound forms of CR2 to create a series of informative mutations targeting specific areas of the CR2-C3d complex. Wild-type and mutant forms of CR2 were expressed on the surface of K562 erythroleukemia cells and their binding ability assessed using C3dg-biotin tetramers complexed to fluorochrome conjugated streptavidin and measured by flow cytometry. Mutations directed at the SCR2-C3d interface (R83A, R83E, G84Y) were found to strongly disrupt C3dg binding, supporting the conclusion that the SCR2 interface reflected in the crystal structure is correct. Previous epitope and peptide mapping studies have also indicated that the PILN11GR13IS sequence of the first inter-cysteine region of SCR1 is essential for the binding of iC3b. Mutations targeting residues within or in close spatial proximity to this area (N11A, N11E, R13A, R13E, Y16A, S32A, S32E), and a number of other positively charged residues located primarily on a contiguous face of SCR1 (R28A, R28E, R36A, R36E, K41A, K41E, K50A, K50E, K57A, K57E, K67A, K67E), have allowed us to reassess those regions on SCR1 that are essential for CR2-C3d binding. The nature of this interaction and the possibility of a direct SCR1-C3d association are discussed extensively. Finally, a D52N mutant was constructed introducing an N-glycosylation sequence at an area central to the CR2 dimer interface. This mutation was designed to disrupt the CR2-C3d interaction, either directly through steric inhibition, or indirectly through disruption of a physiological dimer. However, no difference in C3dg binding relative to wild-type CR2 could be observed for this mutant, suggesting that the dimer may only be found in the crystal form of CR2.

Expression of human complement receptor type 2 (CD21) in mice during early B cell development results in a reduction in mature B cells and hypogammaglobulinemia

Complement receptor (CR) type 2 (CR2/CD21) is normally expressed only during the immature and mature stages of B cell development. In association with CD19, CR2 plays an important role in enhancing mature B cell responses to foreign Ag. We used a murine Vlambda2 promoter/Vlambda2-4 enhancer minigene to develop transgenic mice that initiate expression of human CR2 (hCR2) during the CD43(+)CD25(-) late pro-B cell stage of development. We found peripheral blood B cell numbers reduced by 60% in mice expressing high levels of hCR2 and by 15% in mice with intermediate receptor expression. Splenic B cell populations were altered with an expansion of marginal zone cells, and basal serum IgG levels as well as T-dependent immune responses were also significantly decreased in transgenic mice. Mice expressing the highest levels of hCR2 demonstrated in the bone marrow a slight increase in B220(int)CD43(+)CD25(-) B cells in association with a substantial decrease in immature and mature B cells, indicative of a developmental block in the pro-B cell stage. These data demonstrate that stage-specific expression of CR2 is necessary for normal B cell development, as premature receptor expression substantially alters this process. Alterations in B cell development are most likely due to engagement of pre-B cell receptor-mediated or other regulatory pathways by hCR2 in a CD19- and possibly C3 ligand-dependent manner.

The crystal structure of human CD21: Implications for Epstein-Barr virus and C3d binding

Human complement receptor type 2 (CD21) is the cellular receptor for Epstein-Barr virus (EBV), a human tumor virus. The N-terminal two short consensus repeats (SCR1-SCR2) of the receptor interact with the EBV glycoprotein gp350/220 and also with the natural CD21 ligand C3d. Here we present the crystal structure of the CD21 SCR1-SCR2 fragment in the absence of ligand and demonstrate that it is able to bind EBV. Based on a functional analysis of wild-type and mutant CD21 and molecular modeling, we identify a likely region for EBV attachment and demonstrate that this region is not involved in the interaction with C3d. A comparison with the previously determined structure of CD21 SCR1-SCR2 in complex with C3d shows that, in both cases, CD21 assumes compact V-shaped conformations. However, our analysis reveals a surprising degree of flexibility at the SCR1-SCR2 interface, suggesting interactions between the two domains are not specific. We present evidence that the V-shaped conformation is induced by deglycosylation of the protein, and that physiologic glycosylation of CD21 would result in a more extended conformation, perhaps with additional epitopes for C3d binding.

Generation of recombinant human C3dg tetramers for the analysis of CD21 binding and function

CD21 (complement receptor 2, CR2) binds the terminal proteolytic fragments of the third component of complement (C3) that have been covalently attached to immune complexes or other targets during the activation of complement. We used the technique of in vivo biotinylation to create a recombinant multivalent ligand for CD21. A sequence coding for a biotinylation signal peptide was added to the 3' end of the human C3dg cDNA. The modified C3dg was expressed in Escherichia coli and biotinylated intracellularly by the bacterial biotin holoenzyme synthetase (BirA) enzyme. Monomeric C3dg was unable to bind to CD21 as determined by flow cytometry, while biotinylated recombinant C3dg (rC3dg) complexed with fluorochrome-conjugated streptavidin bound tightly. Binding was observed using CD21 positive B cells but not seen on pre-B cells that do not express this complement receptor. Two assays were used to assess the functional capacity of the recombinant C3dg. First, multimeric C3dg caused the phosphorylation of the mitogen-activated kinase, p38, in mature B lymphoma cells. Second, C3dg greatly enhanced the activation of primary B cells in combination with a sub-stimulatory concentration of anti-IgM monoclonal antibody. These results illustrate the utility of the technique of in vivo biotinylation to generate ligands for cell surface receptors that require multimerization for high avidity binding and function.

Epitope mapping using the X-ray crystallographic structure of complement receptor type 2 (CR2)/CD21: identification of a highly inhibitory monoclonal antibody that directly recognizes the CR2-C3d interface

Complement receptor type 2 (CR2)/CD21 is a B lymphocyte cell membrane C3d/iC3b receptor that plays a central role in the immune response. Human CR2 is also the receptor for the EBV viral membrane glycoprotein gp350/220. Both C3d and gp350/220 bind CR2 within the first two of 15-16 repetitive domains that have been designated short consensus/complement repeats. Many mAbs react with human CR2; however, only one currently available mAb is known to block both C3d/iC3b and gp350/220 binding. We have used a recombinant form of human CR2 containing the short consensus/complement repeat 1-2 ligand-binding fragment to immunize Cr2(-/-) mice. Following fusion, we identified and further characterized four new anti-CR2 mAbs that recognize this fragment. Three of these inhibited binding of CR2 to C3d and gp350/220 in different forms. We have determined the relative inhibitory ability of the four mAbs to block ligand binding, and we have used overlapping peptide-based approaches to identify linear epitopes recognized by the inhibitory mAbs. Placement of these epitopes on the recently solved crystal structure of the CR2-C3d complex reveals that each inhibitory mAb recognizes a site either within or adjacent to the CR2-C3d contact site. One new mAb, designated 171, blocks CR2 receptor-ligand interactions with the greatest efficiency and recognizes a portion of the C3d contact site on CR2. Thus, we have created an anti-human CR2 mAb that blocks the C3d ligand by direct contact with its interaction site, and we have provided confirmatory evidence that the C3d binding site seen in its crystal structure exists in solution.

The role of the CD19/CD21 complex in B cell processing and presentation of complement-tagged antigens

The CD19/CD21 complex is an essential B cell coreceptor that functions synergistically to enhance signaling through the B cell Ag receptor in response to T cell-dependent, complement-tagged Ags. In this study, we use a recombinant protein containing three tandemly arranged copies of C3d and the Ag hen egg lysozyme, shown to be a highly effective immunogen in vivo, to evaluate the role of the CD19/CD21 complex in Ag processing in B cells. Evidence is provided that coengagement of the CD19/CD21 complex results in more rapid and efficient production of antigenic peptide/class II complexes as compared with B cell Ag receptor-mediated processing alone. The CD19/CD21 complex does not itself target complement-tagged Ags for processing, but rather appears to influence B cell Ag processing through its signaling function. The ability of the CD19/CD21 complex to augment processing may be an important element of the mechanism by which the CD19/CD21 complex functions to promote B cell responses to T cell-dependent complement-tagged Ags in vivo.

Structural Studies in Solution of the Recombinant N-Terminal Pair of Short Consensus/Complement Repeat Domains of Complement Receptor Type 2 (CR2/CD21) and Interactions with Its Ligand C3dg

Human complement receptor type 2 (CR2, CD21) is a cell surface receptor that binds three distinct ligands (complement C3d, Epstein-Barr virus gp350/220, and the low-affinity IgE receptor CD23) via the N-terminal two of fifteen or sixteen short consensus/complement repeat (SCR) domains. Here, we report biophysical studies of the CR2 SCR 1-2 domain binding to its ligand C3dg. Two recombinant forms of CR2 containing the SCR 1-2 and SCR 1-15 domains were expressed in high yield in Pichia pastoris and baculovirus, respectively. Circular dichroism spectroscopy showed that CR2 SCR 1-2 receptor possessed a beta-sheet secondary structure with a melting temperature of 59 degrees C. Using surface plasmon resonance, kinetic parameters for the binding of either CR2 SCR 1-2 or the full-length SCR 1-15 form of CR2 showed that the affinity of binding to immobilized C3d is comparable for the SCR 1-15 compared to the SCR 1-2 form of CR2. Unexpectedly, both the association and dissociation rates for the SCR 1-15 form were slower than for the SCR 1-2 form. These data show that the SCR 1-2 domains account for the primary C3dg binding site of CR2 and that the additional SCR domains of full-length CR2 influence the ability of CR2 SCR 1-2 to interact with its ligand. Studies of the pH and ionic strength dependence of the interaction between SCR 1-2 and C3d by surface plasmon resonance showed that this is influenced by charged interactions, possibly involving the sole His residue in CR2 SCR 1-2. Sedimentation equilibrium studies of CR2 SCR 1-2 gave molecular weights of 17 000, in good agreement with its sequence-derived molecular weight to show that this was monomeric. Its sedimentation coefficient was determined to be 1.36 S. The complex with C3d gave molecular weights in 50 mM and 200 mM NaCl buffer that agreed closely with its sequence-derived molecular weight of 50 600 and showed that a 1:1 complex had been formed. Molecular graphics views of homology models for the separate CR2 SCR 1 and SCR 2 domains showed that both SCR domains exhibited a distribution of charged groups throughout its surface. The single His residue is located near a long eight-residue linker between the two SCR domains and may influence the linker conformation and the association of C3d and CR2 SCR 1-2 into their complex. Sedimentation modeling showed that the arrangement of the two SCR domains in CR2 SCR 1-2 is highly extended in solution.

Activation of the EBV/C3d receptor (CR2, CD21) on human B lymphocyte surface triggers tyrosine phosphorylation of the 95-kDa nucleolin and its interaction with phosphatidylinositol 3 kinase

We previously demonstrated that CR2 activation on human B lymphocyte surface triggered tyrosine phosphorylation of a p95 component and its interaction with p85 subunit of phosphatidylinositol 3' (PI 3) kinase. Despite identical molecular mass of 95 kDa, this tyrosine phosphorylated p95 molecule was not CD19, the proto-oncogene Vav, or the adaptator Gab1. To identify this tyrosine phosphorylated p95 component, we first purified it by affinity chromatography on anti-phosphotyrosine mAb covalently linked to Sepharose 4B, followed by polyacrylamide gel electrophoresis. Then, the isolated 95-kDa tyrosine phosphorylated band was submitted to amino acid analysis by mass spectrometry; the two different isolated peptides were characterized by amino acid sequences 100% identical with two different domains of nucleolin, localized between aa 411--420 and 611--624. Anti-nucleolin mAb was used to confirm the antigenic properties of this p95 component. Functional studies demonstrated that CR2 activation induced, within a brief span of 2 min, tyrosine phosphorylation of nucleolin and its interaction with Src homology 2 domains of the p85 subunit of PI 3 kinase and of 3BP2 and Grb2, but not with Src homology 2 domains of Fyn and Gap. These properties of nucleolin were identical with those of the p95 previously described and induced by CR2 activation. Furthermore, tyrosine phosphorylation of nucleolin was also induced in normal B lymphocytes by CR2 activation but neither by CD19 nor BCR activation. These data support that tyrosine phosphorylation of nucleolin and its interaction with PI 3 kinase p85 subunit constitute one of the earlier steps in the specific intracellular signaling pathway of CR2.

Functional characterization of soluble and membrane-bound forms of vaccinia virus complement control protein (VCP)

Vaccinia virus secretes a 35 kD protein, vaccinia virus complement control protein (VCP), that inhibits the classical and alternative pathways of complement at several points, indicating that it may be a viral analogue of human complement receptor type 1 (CR1; CD35). Structurally, however, CR1 is composed of 30 short consensus repeats (SCRs), whereas VCP consists entirely of four SCRs. We have begun a structure-function analysis of VCP to define the minimum number of SCRs necessary for function, the functional differences between VCP and CR1, and the potential therapeutic roles for VCP. We addressed these questions by creating and characterizing recombinant soluble and membrane-bound forms of VCP. We have determined that (1) VCP requires all four SCRs to bind C3b, (2) whereas CR1 binds C3b and iC3b, VCP binds C3b but not iC3b, and (3) although normally secreted, if expressed on the membrane of mammalian cells, VCP effectively protects the cells from complement-mediated lysis. Thus, VCP appears to be a compact and unique complement regulatory protein with the ability to inhibit both arms of the complement cascade, but lacking affinity for iC3b. By releasing rather than capturing iC3b-bearing complexes following inactivation of C3b, VCP may 'recycle' its active site locally among infected cells, and thereby enable the virus to evade more efficiently host immune and inflammatory responses. The unique function, compact structure, and capacity of VCP to protect mammalian cells from complement-mediated attack, suggests that it could be used both to better understand the structure-function relationship of complement regulatory proteins, in general, and also to rationally design and develop novel therapeutic agents.

Independently melting modules and highly structured intermodular junctions within complement receptor type 1

A segment of complement receptor type 1 (CR1) corresponding to modules 15-17 was overexpressed as a functionally active recombinant protein with N-glycosylation sites ablated by mutagenesis (referred to as CR1 approximately 15-17(-)). A protein consisting of modules 15 and 16 and another corresponding to module 16 were also overexpressed. Comparison of heteronuclear nuclear magnetic resonance (NMR) spectra for the single, double, and triple module fragments indicated that module 16 makes more extensive contacts with module 15 than with module 17. A combination of NMR, differential scanning calorimetry, circular dichroism, and tryptophan-derived fluorescence indicated a complex unfolding pathway for CR1 approximately 15-17(-). As temperature or denaturant concentration was increased, the 16-17 junction appeared to melt first, followed by the 15-16 junction, and module 17 itself; finally, modules 15 and 16 became denatured. Modules 15 and 16 adopted an intermediate state prior to total denaturation. These results are compared with a previously published study [Clark, N. S., Dodd, I, Mossakowska, D. E., Smith, R. A. G., and Gore, M. G. (1996) Protein Eng. 9, 877-884] on a fragment consisting of the N-terminal three CR1 modules which appeared to melt as a single unit.

Signaling Through the EBV/C3d Receptor (CR2, CD21) in Human B Lymphocytes: Activation of Phosphatidylinositol 3-Kinase via a CD19-Independent Pathway

We herein analyzed the regulation of phosphatidylinositol 3-kinase (PI 3-kinase) activity by CR2 activated on B lymphocyte cell surface. We demonstrated that CR2 activation triggered in vivo PI 3-kinase activity and interaction of PI 3-kinase p85 subunit with a tyrosine-phosphorylated p95 component. The specificity of PI 3-kinase activity was controlled using wortmannin and LY294002. CR2 activation did not trigger tyrosine phosphorylation of PI 3-kinase p85 subunit, but induced direct interaction of tyrosine phosphorylated p95 with the Src homology 2 domain of p85 subunit, as shown using glutathione-S-transferase fusion proteins. Despite identical molecular masses, immunoblotting analysis demonstrated that tyrosine-phosphorylated p95 that interacted in vivo and in vitro with p85 was neither CD19, the 95-kDa proto-oncogene vav, nor Gab1 (a 95-kDa adaptor molecule). Furthermore, p95 tyrosine phosphoprotein also expressed in K562A cells (CR2+ CD19− cells) interacted with Src homology 2 domain of PI 3-kinase p85 subunit after CR2 activation. Activated CR2 did not interact directly with p85 subunit or tyrosine-phosphorylated p95. This suggests the presence of an intermediate molecule between activated CR2 and tyrosine-phosphorylated p95, which may be 3BP2. In addition, in contrast to CD19 activation, CR2 activation did not trigger interaction of CD19 or Vav with PI 3-kinase p85 subunit or coprecipitation of PI 3-kinase activity with CD19. Together, these data clearly demonstrated that CR2 activation triggered in vivo PI 3-kinase activation through a pathway distinct from that triggered through CD19 activation.

Complement receptor type two (CR2,CR21): a target for influencing the humoral immune response and antigen-trapping

Cellular receptors for complement C3 fragments deposited on antigens are important bricks in the wall defending against microbial pathogens. The part of complement receptor type 2 (CR2; CD21) deals with enhancing humoral immune responses and with long-term trapping of C3d-coated antigen by follicular dendritic cells. CR2 is also pivotal for Epstein-Barr virus (EBV) infection. Here, the current understanding, how CR2 interacts with its ligands C3d, EBV, and CD23 is summarized. The potential to target CR2 for clinical therapy or immunization purposes are discussed.

Characterization of C3dg Binding to a Recess Formed Between Short Consensus Repeats 1 and 2 of Complement Receptor Type 2 (CR2; CD21)

To allow for a better characterization of the ligand binding structures of human complement receptor type 2 (CR2; CD21), we have established an IgG1 κ mouse mAb, FE8, that interferes efficiently with binding of C3dg and EBV to CR2. In contrast to mAb OKB7, the only well-characterized mAb with similar specificity, mAb FE8 blocked binding of soluble C3dg or particles carrying multiple copies of surface-bound C3dg to CR2 or induced complete removal of these ligands from the receptor. In vitro EBV infection of B lymphocytes, on the other hand, was abrogated by mAbs FE8 and OKB7 with similar dose-response characteristics. As FE8 was shown to recognize a discontinuous epitope, a series of overlapping peptides derived from SCR1 and -2 and immobilized on cellulose was screened with FE8. The results suggest that up to five discontinuous sequences contributed to the epitope. The sequence 63-EYFNKYS-69, located between the two SCR units, reacted most intensively. Two other sequences, 16-YYSTPI-21 and 105-NGNKSVWCQANN-116, are located between Cys1 and Cys2 of SCR1 and around Cys3 of SCR2, respectively. Based on the solution structure for two factor H SCRs, a three-dimensional model of SCR1 and -2 was generated. The FE8 binding peptide sequences were located in relative proximity to each other, bounding the recess formed between SCR1 and -2. This potential of mAb FE8 is currently unique and may be exploited for interfering with conditions of unwanted recognition of C3dg-coated structures by the immune system.

NMR studies of a viral protein that mimics the regulators of complement activation

Vaccinia virus complement control protein (VCP) is a 243-residue protein that is similar in sequence to the regulators of complement activation; its role is to defend the virus against attack by the host complement system. A fragment of this protein spanning the two complement protein (CP)-modules (residues 126 to 243) which make up the C-terminal half of VCP has been expressed in Pichia pastoris. A 15N-labelled sample was purified for the purposes of structure determination and measurements of dynamics in solution using NMR. Structures were calculated on the basis of 1767 NMR-derived distance and angle restraints, with a longer than normal high-temperature simulated annealing (SA) protocol which improved convergence. The viral CP-modules are structurally very similar to the 15th and 16th CP-modules of human factor H (fH; average r.m.s.d., for invariant Trp and Cys, four pair-wise comparisons,=1.2 A) but less similar to the fifth CP-module of fH (average r.m.s.d.=2.2 A). In the VCP fragment, the orientation of one module with respect to the other is clearly defined by the experimental data, and T1 measurements are consistent with only limited flexibility at the module-module interface. The r.m.s.d. over all of the 118 residues (backbone atoms) is 0.73 A. The intermodular orientation is better defined than, and significantly different from, that observed in a CP-module pair from fH (re-calculated using the extended SA protocol). In VCP the long axis of the second module is tilted by 59(+/-4) degrees with respect to the first module (50(+/-13) degrees in the fH pair), and twisted with respect to the first module by 22(+/-6) degrees (223(+/-17) degrees in fH). The differences between the human and viral proteins may be rationalised in terms of the lack of hydrogen-bond stabilised secondary structure in the N-terminal portion of fH module 16, and the number and type of amino acid side-chains which make up the interface. A similar intermodular interface may be predicted between the third and fourth module of human C4 binding protein and, probably, between the third and fourth modules of the guinea pig acrosomal matrix protein 67; but the formulation of general rules for predicting the structure of interfaces between CP-modules awaits further experimental data.

Mapping of the primary binding site of measles virus to its receptor CD46

The measles virus (MV) hemagglutinin binds to the complement control protein (CCP) CD46 primarily through the two external modules, CCP-I and -II. To define the residues involved in binding, 40 amino acids predicted to be solvent-exposed on the CCP-I-II module surface were changed to either alanine or serine. Altered proteins were expressed on the cell surface, and their abilities to bind purified MV particles, a soluble form of hemagglutinin (sH) and nine CD46-specific antibodies competing to different levels with sH attachment, were measured. All proteins retained, at least in part, MV and sH binding, but some completely lost binding to certain antibodies. Amino acids essential for binding of antibodies weakly or moderately competing with sH attachment are situated in the membrane-distal tip of CCP-I, whereas residues involved in binding of strongly sH competing antibodies cluster in the center of CCP-I (Arg-25, Asp-27) or in CCP-II (Arg-69, Asp-70). Both clusters face the same side of CCP-I-II and map close to amino acid exchanges impairing sH binding (E11A, R29A, P39A, and D70A) or MV binding (D70A and E84A) and to a six-amino acid loop, previously shown to be necessary for sH binding.

EBV binds to lymphocytes of transgenic mice that express the human CR2 gene

Epstein Barr virus (EBV) is unable to bind to or infect normal mouse lymphocytes. A construct containing the human complement receptor type 2 (CR2) gene, the receptor for EBV, was placed under the control of the IgH/c-fos enhancer/promoter and microinjected into single cell embryos. A total of five transgenic mouse lines were established and four expressed hCR2 mRNA. Flow cytometry and immunostaining revealed that approximately 15-30% of the lymphocytes from the thymus, spleen and lymph nodes expressed hCR2 protein on their surface and bound EBV. Despite this binding, less than 1% of the cells showed evidence that the virus was internalized or replicated. Transgenic mouse lymphocytes, expressing hCR2, could not be immortalized with EBV. It is concluded that the simple expression of hCR2 receptor on mouse lymphocytes is not sufficient for efficient infection.

A 3D model for the measles virus receptor CD46 based on homology modeling, Monte Carlo simulations, and hemagglutinin binding studies

The two terminal complement control protein (CCP) modules of the CD46 glycoprotein mediate measles virus binding. Three-dimensional models for these two domains were derived based on the NMR structures of two CCP modules of factor H. Both CD46 modules are about 35 A long, and form a five-stranded antiparallel beta-barrel structure. Monte Carlo simulations, sampling the backbone torsion angles of the linker peptide and selecting possible orientations on the basis of minimal solvent-exposed hydrophobic area, were used to predict the orientation of CCP-I relative to CCP-II. We tested this procedure successfully for factor H. For CD46, three clusters of structures differing in the tilt angle of the two domains were obtained. To test these models, we mutagenized the CCP modules. Four proteins, two without an oligosaccharide chain and two with mutated short amino acid segments, reached the cell surface efficiently. Only the protein without the CCP-I oligosaccharide chain maintained binding to the viral attachment protein hemagglutinin. These results are consistent with one of our models and suggest that the viral hemagglutinin does not bind at the membrane-distal tip of CD46, but near the concave CCP-II interface region.

Infection of primary CD4+ and CD8+ T lymphocytes by Epstein-Barr virus enhances human immunodeficiency virus expression

CD4+ and CD8+ T lymphocytes purified from normal adult donors by flow cytometry could be infected with Epstein-Barr virus (EBV) as measured by the accumulation of components of the EBV replicative cycle, viral DNA and viral transcripts encoding EBER1 and BRLF1. EBV infection resulted in enhanced replication of human immunodeficiency virus type 1 (HIV-1) IIIB in CD4+ lymphocytes as measured by accumulation of reverse transcriptase and formation of syncytia. Furthermore, a small percentage of CD8+ T cells became permissive after infection with EBV. Inactivation of transforming functions by irradiation with UV light greatly reduced the ability of EBV to enhance HIV-1 replication in T4+ T cell, suggesting that live virus is needed for enhancement. These results demonstrate a direct synergy between EBV and HIV-1 during coinfection of T cells in vitro and may explain the beneficial effect of acyclovir in combination with antiretroviral chemotherapy as well as the increased incidence of T-cell lymphomas associated with EBV in patients with AIDS.

Induction of interleukin-6 after stimulation of human B-cell CD21 by Epstein-Barr virus glycoproteins gp350 and gp220

The cellular receptor for Epstein-Barr virus (EBV) is the type 2 complement receptor, CD21. At initial infection, EBV virion glycoproteins gp350 and gp220 bind to CD21. We report here that the cross-linking of CD21 by gp350/220 results in increased amounts of interleukin 6 (IL-6) RNA and IL-6 protein. This effect could be blocked with anti-gp350/220 and anti-CD21 monoclonal antibodies. Induction of IL-6 in B cells by EBV could be mimicked by treatment with the protein kinase C (PKC) activator phorbol 12,13-dibutyrate but not with the calcium ionophore ionomycin. IL-6 induction by EBV was inhibited with the PKC-specific inhibitor bisindolylmaleimide or the protein tyrosine kinase inhibitors methyl 2,5-dihydroxycinnamate and herbimycin A, indicating that the induction of IL-6 following CD21 cross-linking is mediated through PKC- and protein tyrosine kinase-dependent pathways.

Epstein-Barr virus/C3d receptor (CR2, CD21) activated by its extracellular ligands regulates pp105 phosphorylation through two distinct pathways

We previously demonstrated that human C3d or pep16, a 16-amino acid synthetic peptide derived from human C3d, induced in vivo and in vitro tyrosine phosphorylation of pp105, an intracellular component found only in human cells that express CR2 at their surface. To determine the contribution of CR2 molecules to this enzymatic regulation, we first analyzed whether activation of CR2 by other extracellular CR2 ligands could trigger such regulation in cell extracts. Subsequently, we used cell extracts of either CR2-positive cells depleted in CR2 molecules by absorption with anti-CR2 antibodies or CR2-negative cells transfected with CR2 cDNA. We demonstrate here that pp105 phosphorylation was induced when CR2 was activated by C3d and pep16 as well as by gp350, the Epstein-Barr virus capsid protein or OKB7, an anti-CR2 monoclonal antibody (mAb). HB5, another anti-CR2 mAb, which did not activate B lymphocytes through CR2, did not induce pp105 phosphorylation. Thus, C3d, pep16, gp350, and OKB7 presented similar properties in activating CR2 to trigger pp105 phosphorylation and in regulating B lymphocyte proliferation, while HB-5 had no effect on either assays. Furthermore, our data demonstrate that the presence of CR2 activated by its extracellular ligands regulates pp105 phosphorylation through two distinct pathways: one which also requires the presence of non-activated CD19, and one which is independent of CD19. The involvement of CD19 in the first pathway was not due to the formation of putative CR2-CD19 complexes. Both pathways were TAPA-1 independent. This is the first demonstration that activated CR2 molecules can play a regulatory role in enzymatic function, such as phosphorylation, despite the absence of CD19 and TAPA-1.

Host cell requirements for efficient infection of quiescent primary B lymphocytes by Epstein-Barr virus

Quiescent primary B lymphocytes are efficiently immortalized by Epstein-Barr virus (EBV). This process requires both the delivery and expression of the viral genome and results in activation of the cell division cycle. Infection of B lymphocytes depends on a direct interaction between the viral glycoprotein gp340/220 and CD21, the C3dg complement receptor. This interaction is required for the adsorption of EBV. In addition, several lines of evidence suggest that the interaction of EBV with CD21 modulates the phenotype of cells. CD21 forms part of a multimeric signal transduction complex with CD19, TAPA-1, and Leu-13. In normal B lymphocytes, CD19 becomes tyrosine phosphorylated following stimulation of the antigen receptor and recruits the signal-transducing enzyme phosphatidylinositol 3-kinase kinase. Here, we investigated the involvement of signal transduction pathways in efficient infection. Protein synthesis is not required for events leading to the transcription of the viral genome, suggesting that the early stages of infection do not depend on the expression of novel cell genes and consistent with the Wp promoter being the first viral promoter used upon infection. Since the stimulation of cells with gp340/220 leads to an increase in the level of CD19 tyrosine phosphorylation, we investigated the potential contribution of both tyrosine and phosphatidylinositol 3-kinase kinases to efficient infection. Both kinases contribute to the posttranscriptional control of viral gene expression following infection, but neither is required for the entry or initial transcription of the virus. Thus, it appears that EBV exploits a host signal transduction pathway to efficiently infect primary cells.

Virus receptors: implications for pathogenesis and the design of antiviral agents

A virus initiates infection by attaching to its specific receptor on the surface of a susceptible host cell. This prepares the way for the virus to enter the cell. Consequently, the expression of the receptor on specific cells and tissues of the host is a major determinant of the route of entry of the virus into the host and of the patterns of virus spread and pathogenesis in the host. This review emphasizes the virus-receptor interactions of human immunodeficiency virus, the rhinoviruses, the herpesviruses, and the coronaviruses. These interactions are often found to be complex and dynamic, involving multiple sites or factors on both the virus and the host cell. Also, the receptor may play an important role in virus entry per se in addition to its role in virus binding. In the cases of human immunodeficiency virus and the rhinoviruses, ingenious approaches to therapeutic strategies based on inhibiting virus attachment and entry are under development and in clinical trials.

B-cell signalling via the C-type lectins CD23 and CD72

Although CD23 and CD72 are well-known B-cell signalling molecules, the intracellular signal transduction pathways through which they operate remain poorly elucidated. This may partly reflect their somewhat dubious histories, with claims and counterclaims being made for functions and ligands. Here, John Gordon discusses why such controversy should surround the two B-cell-associated C-type lectins and provides speculation as to their respective roles in regulating an immune response that may be different in mice and humans.

Determination of the role for CD21 during Epstein-Barr virus infection of B-lymphoblastoid cells

Epstein-Barr virus (EBV), a herpesvirus with oncogenic potential, is camouflaged with glycoprotein 350/220, which mimics the human ligand C3dg and thereby binds to and exploits complement receptor type 2 (CR2; CD21), the EBV receptor. It has not been possible to determine the role of CR2 during postbinding events of viral infection because all B lymphocytes express endogenous CR2, precluding an informative study of receptor mutants. We have overcome this obstacle through creation of a novel experimental system based on molecular dissection of the ligand-binding domains of human CR2 and murine CR2. Our results demonstrate first, that two discontinuous amino acid substitutions within the ligand-binding domain of murine CR2 render it capable of mediating EBV infection of human B-lymphoblastoid cells, and second, that the specific role of CR2 during EBV infection is to capture virions at the cell surface, after which cofactors not associated with CR2 mediate postbinding events. These are the first studies to be described in which a cell that is normally susceptible to viral infection can be manipulated so as to direct entry of virions via recombinant or endogenous receptors.

Building protein structure and function from modular units

Many proteins in multicellular organisms are made from combinations of several, clearly identifiable, autonomously folding domains or modules. The structures of many of the constituent modules and some module pairs are now known. This review briefly describes some of the recent X-ray crystallographic and nuclear magnetic resonance (NMR) structural work on modules 'dissected' from proteins that are often large, membrane-bound and glycosylated. These include important proteins involved in cell adhesion, clotting, fibrinolysis and signalling. The structure and function of the intact proteins is discussed in the light of the recent structural work.

Functional dissection of the CD21/CD19/TAPA-1/Leu-13 complex of B lymphocytes

The CD21/CD19/TAPA-1 complex of B lymphocytes amplifies signal transduction through membrane immunoglobulin (mIg), recruits phosphatidylinositol 3-kinase (PI3-kinase), and induces homotypic cellular aggregation. The complex is unique among known membrane protein complexes of the immune system because its components represent different protein families, and can be expressed individually. By constructing chimeric molecules replacing the extracellular, transmembrane, and cytoplasmic regions of CD19 and CD21 with those of HLA-A2 and CD4, we have determined that CD19 and TAPA-1 interact through their extracellular domains, CD19 and CD21 through their extracellular and transmembrane domains, and, in a separate complex, CD21 and CD35 through their extracellular domains. A chimeric form of CD19 that does not interact with CD21 or TAPA-1 was expressed in Daudi B lymphoblastoid cells and was shown to replicate two functions of wild-type CD19 contained within the complex: synergistic interaction with mIgM to increase intracellular free calcium and tyrosine phosphorylation and association with the p85 subunit of PI3-kinase after ligation of mIgM. The chimeric CD19 lacked the capacity of the wild-type CD19 to induce homotypic cellular aggregation, a function of the complex that can be ascribed to the TAPA-1 component. The CD21/CD19/TAPA-1 complex brings together independently functioning subunits to enable the B cell to respond to low concentrations of antigen.

Mutational analysis of the CD2/CD58 interaction: the binding site for CD58 lies on one face of the first domain of human CD2

The adhesion interaction between the immunoglobulin superfamily molecules CD2 and CD58 (lymphocyte function-associated antigen 3) plays an important role in T cell and natural killer cell interaction with various antigen-presenting and target cells. Determination of the solution structure of rat CD2 domain 1 has allowed a model of human CD2 domain 1 to be generated, and a series of mutants based on this model have been made. Residues of domain 1 of human CD2 predicted to be solvent exposed were substituted with the equivalent residues present in the rat CD2 molecule. The ability of these mutants to mediate rosetting with human and sheep erythrocytes was studied. Results show that the binding site of CD2 for both human and sheep CD58 maps to the beta sheet containing beta strands CC'C"F and G. Residues K34 and E36 in beta strand C, R48 and K49 in beta strand C', and K91 and N92 in the loop connecting beta strands F and G are shown to be critical in the interaction. The data support the proposition that the interaction between CD2 and CD58 involves the major beta sheet face of CD2.

Cutaneous anaplastic T-cell lymphoma in a patient with human immunodeficiency virus infection: detection of Epstein-Barr virus DNA

The Epstein-Barr virus (EBV) genome exists in tumour cells of T-cell lymphomas in non-immunosuppressed patients. We identified EBV-DNA by in situ hybridization in a case of anaplastic T-cell lymphoma associated with acquired immunodeficiency syndrome (AIDS). EBV-DNA has been reported in AIDS-related Hodgkin's disease or B-cell lymphoma, but never in T-cell lymphoma. Although our results suggest that EBV could play a role in the development of these anaplastic T-cell lymphomas, the mechanism of EBV penetration into T-cells remains uncertain.