Soluble recombinant CR2 (CD21) inhibits Epstein-Barr virus infection

Structural basis of Epstein-Barr virus gp350 receptor recognition and neutralization

Epstein-Barr virus (EBV) is an oncogenic virus associated with multiple lymphoid malignancies and autoimmune diseases. During infection in B cells, EBV uses its major glycoprotein gp350 to recognize the host receptor CR2, initiating viral attachment, a process that has lacked direct structural evidence for decades. In this study, we resolved the structure of the gp350-CR2 complex, elucidated their key interactions, and determined the site-specific N-glycosylation map of gp350. Our findings reveal that CR2 primarily binds to gp350 through an electrostatically complementary and glycan-free interface and that the diversity of key residues in CR2 across different species influences EBV host selectivity mediated by gp350. With the confirmed binding, we constructed a CR2-Fc antibody analog that targets the vulnerable site of gp350, demonstrating a potent neutralization effect against EBV infection in B cells. Our work provides essential structural insights into the mechanism of EBV infection and host tropism, suggesting a potential antiviral agent.

How EBV Infects: The Tropism and Underlying Molecular Mechanism for Viral Infection

The Epstein-Barr virus (EBV) is associated with a variety of human malignancies, including Burkitt's lymphoma, Hodgkin's disease, nasopharyngeal carcinoma and gastric cancers. EBV infection is crucial for the oncogenesis of its host cells. The prerequisite for the establishment of infection is the virus entry. Interactions of viral membrane glycoproteins and host membrane receptors play important roles in the process of virus entry into host cells. Current studies have shown that the main tropism for EBV are B cells and epithelial cells and that EBV is also found in the tumor cells derived from NK/T cells and leiomyosarcoma. However, the process of EBV infecting B cells and epithelial cells significantly differs, relying on heterogenous glycoprotein-receptor interactions. This review focuses on the tropism and molecular mechanism of EBV infection. We systematically summarize the key molecular events that mediate EBV cell tropism and its entry into target cells and provide a comprehensive overview.

Antiviral strategies targeting herpesviruses

Herpesviruses, known as large DNA viruses, have a wide host range. In addition to human beings, cattle, and horses, even carp can be hosts for herpesvirus infection. Herpesviruses are pathogens possessing elaborate mechanisms that regulate host cell components for its replication, assembly and generating mature virus particles that can infect humans and most animals, usually causing multiple and lifelong infections. In addition, several human diseases, such as genital or mouth herpes, mononucleosis, and Burkitt lymphoma, are usually associated with herpesvirus infection. Blocking the steps of viral infection, such as entry, replication and assembly, may be an effective way for many different herpes viruses and their related diseases. Therefore, we aim to describe antiviral agents that are able to prevent herpesvirus entry, replication and assembly in host cells. We summarize antiviral strategies, including certain small molecular drugs, RNA interference and CRISPR/Cas9 system-based antiviral approaches, which represent promising approaches.

Immunization with a Self-Assembled Nanoparticle Vaccine Elicits Potent Neutralizing Antibody Responses against EBV Infection

Epstein-Barr virus (EBV) infection is a global health concern infecting over 90% of the population. However, there is no currently available vaccine. EBV primarily infects B cells, where the major glycoprotein 350 (gp350) is the main target of neutralizing antibodies. Given the advancement of nanoparticle vaccines, we describe rationally designed vaccine modalities presenting 60 copies of gp350 on self-assembled nanoparticles in a repetitive array. In a mouse model, gp350s on lumazine synthase (LS) and I3-01 adjuvanted with MF59 or aluminum hydroxide (Alhydrogel) elicited over 65- to 133-fold higher neutralizing antibody titers than the corresponding gp350 monomer to EBV. Furthermore, immunization with gp350D₁₂₃-LS and gp350D₁₂₃-I3-01 vaccine induced a Th2-biased response. For the nonhuman primate model, gp350D₁₂₃-LS in MF59 elicited higher titers of total IgG and neutralizing antibodies than the monomeric gp350D₁₂₃. Overall, these results support gp350D₁₂₃-based nanoparticle vaccine design as a promising vaccine candidate for potent protection against EBV infection.

Illuminating the in vitro effects of Epstein-Barr virus and vitamin D on immune response in multiple sclerosis patients

Given the complexity of immune complex diseases including multiple sclerosis (MS) and the plausible interactions between different risk factors, delineating the interplay between them would be imperative. The current study aimed to evaluate the in vitro effects of Epstein-Barr virus (EBV) and vitamin D on immune response in MS patients and healthy controls. The status of vitamin D and EBV load was evaluated using multiple techniques. In vitro EBV-infected peripheral blood mononuclear cells (PBMCs), in the presence or absence of vitamin D, were checked for IL-10, IFN-γ, and vitamin D receptor. MS patients showed significantly higher plasma levels of 1,25-(OH)2D but not 25-OHD, increased EBV load, and lower levels of vitamin D receptor (VDR) expression compared with healthy controls. Interestingly, an inverse correlation was observed between VDR expression and EBV load in PBMCs. Indeed, the levels of IFN-γ and IL-10 production were significantly higher in supernatant collected from in vitro EBV-infected PBMCs in MS patients compared with controls. While all vitamin D-treated PBMCs showed reduced levels of IFN-γ production, in vitro treatment of vitamin D showed no influence in IL-10 production. EBV and vitamin D were found to exert opposite in vitro effects on immune dysregulation in these patients. Our results highlight the complex interactions of different risk factors with immune system.

From Classical to Unconventional: The Immune Receptors Facilitating Platelet Responses to Infection and Inflammation

Platelets have long been recognized for their role in maintaining the balance between hemostasis and thrombosis. While their contributions to blood clotting have been well established, it has been increasingly evident that their roles extend to both innate and adaptive immune functions during infection and inflammation. In this comprehensive review, we describe the various ways in which platelets interact with different microbes and elicit immune responses either directly, or through modulation of leukocyte behaviors.

PD-1 immunobiology in systemic lupus erythematosus

Programmed death (PD)-1 receptors and their ligands have been identified in the pathogenesis and development of systemic lupus erythematosus (SLE). Two key pathways, toll-like receptor and type I interferon, are significant to SLE pathogenesis and modulate the expression of PD-1 and the ligands (PD-L1, PD-L2) through activation of NF-κB and/or STAT1. These cell signals are regulated by tyrosine kinase (Tyro, Axl, Mer) receptors (TAMs) that are aberrantly activated in SLE. STAT1 and NF-κB also exhibit crosstalk with the aryl hydrocarbon receptor (AHR). Ligands to AHR are identified in SLE etiology and pathogenesis. These ligands also regulate the activity of the Epstein-Barr virus (EBV), which is an identified factor in SLE and PD-1 immunobiology. AHR is important in the maintenance of immune tolerance and the development of distinct immune subsets, highlighting a potential role of AHR in PD-1 immunobiology. Understanding the functions of AHR ligands as well as AHR crosstalk with STAT1, NF-κB, and EBV may provide insight into disease development, the PD-1 axis and immunotherapies that target PD-1 and its ligand, PD-L1.

Immunization With Fc-Based Recombinant Epstein-Barr Virus gp350 Elicits Potent Neutralizing Humoral Immune Response in a BALB/c Mice Model

Epstein–Barr virus (EBV) was the first human virus proved to be closely associated with tumor development, such as lymphoma, nasopharyngeal carcinoma, and EBV-associated gastric carcinoma. Despite many efforts to develop prophylactic vaccines against EBV infection and diseases, no candidates have succeeded in effectively blocking EBV infection in clinical trials. Previous investigations showed that EBV gp350 plays a pivotal role in the infection of B-lymphocytes. Nevertheless, using monomeric gp350 proteins as antigens has not been effective in preventing infection. Multimeric forms of the antigen are more potently immunogenic than monomers; however, the multimerization elements used in previous constructs are not approved for human clinical trials. To prepare a much-needed EBV prophylactic vaccine that is potent, safe, and applicable, we constructed an Fc-based form of gp350 to serve as a dimeric antigen. Here, we show that the Fc-based gp350 antigen exhibits dramatically enhanced immunogenicity compared with wild-type gp350 protein. The complete or partial gp350 ectodomain was fused with the mouse IgG2a Fc domain. Fusion with the Fc domain did not impair gp350 folding, binding to a conformation-dependent neutralizing antibody (nAb) and binding to its receptor by enzyme-linked immunosorbent assay and surface plasmon resonance. Specific antibody titers against gp350 were notably enhanced by immunization with gp350-Fc dimers compared with gp350 monomers. Furthermore, immunization with gp350-Fc fusion proteins elicited potent nAbs against EBV. Our data strongly suggest that an EBV gp350 vaccine based on Fc fusion proteins may be an efficient candidate to prevent EBV infection in clinical applications.

Expression and immunogenic characterization of recombinant gp350 for developing a subunit vaccine against Epstein-Barr virus

Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus that is linked to the development of various malignancies. There is an urgent need for effective vaccines against EBV. EBV envelope glycoprotein gp350 is an attractive candidate for a prophylactic vaccine. This study was undertaken to produce the truncated (codons 1-443) gp350 protein (gp350(1-443)) in Pichia pastoris and evaluate its immunogenicity. The gp350(1-443) protein was expressed as a secretory protein with an N-terminal His-tag in P. pastoris and purified through Ni-NTA chromatography. Immunization with the recombinant gp350(1-443) could elicit high levels of gp350(1-443)-specific antibodies in mice. Moreover, gp350(1-443)-immunized mice developed strong lymphoproliferative and Th1/Th2 cytokine responses. Furthermore, the recombinant gp350(1-443) could stimulate CD4(+) and CD8(+) T cell responses in vaccinated mice. Collectively, these findings demonstrated that the yeast-expressed gp350(1-443) retained strong immunogenicity. This study will provide a useful source for developing EBV subunit vaccine candidates.

In-cell infection: a novel pathway for Epstein-Barr virus infection mediated by cell-in-cell structures

Epstein-Barr virus (EBV) can infect both susceptible B lymphocytes and non-susceptible epithelial cells (ECs). Viral tropism analyses have revealed two intriguing means of EBV infection, either by a receptor-mediated infection of B cells or by a cell-to-cell contact-mediated infection of non-susceptible ECs. Herein, we report a novel "in-cell infection" mechanism for EBV infection of non-susceptible ECs through the formation of cell-in-cell structures. Epithelial CNE-2 cells were invaded by EBV-infected Akata B cells to form cell-in-cell structures in vitro. Such unique cellular structures could be readily observed in the specimens of nasopharyngeal carcinoma. Importantly, the formation of cell-in-cell structures led to the autonomous activation of EBV within Akata cells and subsequent viral transmission to CNE-2 cells, as evidenced by the expression of viral genes and the presence of virion particles in CNE-2 cells. Significantly, EBV generated from in-cell infected ECs displayed altered tropism with higher infection efficacy to both B cells and ECs. In addition to CNE-2 tumor cells, cell-in-cell structure formation could also mediate EBV infection of NPEC1-Bmi1 cells, an immortalized nasopharyngeal epithelial cell line. Furthermore, efficient infection by this mechanism involved the activation of the PI3K/AKT signaling pathway. Thus, our study identified "in-cell infection" as a novel mechanism for EBV infection. Given the diversity of virus-infected cells and the prevalence of cell-in-cell structures during chronic infection, we speculate that "in-cell infection" is likely a general mechanism for EBV and other viruses to infect non-susceptible ECs.

The soluble form of the EIAV receptor encoded by an alternative splicing variant inhibits EIAV infection of target cells

Equine lentivirus receptor 1 (ELR1) has been identified as the sole receptor for equine infectious anemia virus (EIAV) and is a member of the tumor necrosis factor receptor (TNFR) superfamily. In addition to the previously described membrane-associated form of ELR1, two other major alternative splicing variant mRNAs were identified in equine monocyte-derived macrophages (eMDMs). One major spliced species (ELR1-IN) contained an insertion of 153 nt, which resulted in a premature stop codon situated 561 nt upstream of the predicted membrane spanning domain. The other major species (ELR1-DE) has a deletion of 109 nt that causes a shift of the open reading frame and generates a stop codon 312 nt downstream. Because ELR1-DE presumably encodes a peptide of a mere 23 residues, only ELR1-IN was further analyzed. The expression of a soluble form of ELR1 (sELR1) by ELR1-IN was confirmed by Western blot and immunofluorescence analyses. Similar to ELR1, the transcription level of ELR1-IN varied among individual horses and at different time points in the same individuals. The ratio of ELR1-IN mRNA species to ELR1 mRNA was approximately 1∶2.5. Pre-incubation of the recombinant sELR1 with EIAV significantly inhibited EIAV infection in equine macrophages, the primary in vivo target cell of the virus. Fetal equine dermal (FED) cells are susceptible to EIAV in vitro, and the replication of EIAV in FED cells transiently transfected with ELR1-IN was markedly reduced when compared with replication in cells transfected with the empty vector. Finally, the expression levels of both forms of the EIAV receptor were significantly regulated by infection with this virus. Taken together, our data indicate that sELR1 acts as a secreted cellular factor that inhibits EIAV infection in host cells.

Epstein-Barr viruses that express a CD21 antibody provide evidence that gp350's functions extend beyond B-cell surface binding

The gp350 glycoprotein encoded by BLLF1 is crucial for efficient Epstein-Barr virus (EBV) infection of resting B cells. Gp350 binds to CD21, but whether this interaction sums up its functions remains unknown. We generated gp350-null EBVs that display CD19-, CD21-, or CD22-specific antibodies at their surface (designated as DeltaBLLF1-Ab). Gp350-complemented (DeltaBLLF1-C) and DeltaBLLF1-Ab were found to bind equally well to B cells. Surprisingly, DeltaBLLF1 binding was reduced only 1.7-fold relative to its complemented counterparts. Furthermore, B cells exposed to DeltaBLLF1-Ab or DeltaBLLF1 viruses presented structural antigens with comparable efficiency and achieved 25 to 80% of the T-cell activation elicited by DeltaBLLF1-C. These findings show that the gp350-CD21 interaction pair plays only a modest role during virus transfer to the endosomal compartment. However, primary B cells or Raji B cells infected with DeltaBLLF1-C viruses displayed a 35- to 70-fold higher infection rates than those exposed to DeltaBLLF1, DeltaBLLF1-CD22Ab, or DeltaBLLF1-CD19Ab viruses. Complementation of the gp350 knockout phenotype with CD21Ab substantially enhanced infection rates relative to DeltaBLLF1 but remained sevenfold (Raji B-cell line) to sixfold (primary B cells) less efficient than with gp350. We therefore infer that gp350 mainly exerts its functions after the internalization step, presumably during release of the viral capsid from the endosomal compartment, and that CD21-dependent but also CD21-independent molecular mechanisms are involved in this process. The latter appear to be characteristic of B-cell infection since transfection of CD21 in 293 cells improved the infection rates with both DeltaBLLF1-CD21Ab and DeltaBLLF1-C to a similar extent.

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.

Inhibitory effects of some derivatives of glycyrrhizic acid against Epstein-Barr virus infection: structure-activity relationships

Glycyrrhizic acid (18β-GL or GL) is a herbal drug with a broad spectrum of antiviral activities and pharmacological effects and multiple sites of action. Previously we showed that GL inhibits Epstein-Barr virus (EBV) infection in vitro by interfering with an early step of the EBV replication cycle (possibly attachment/penetration). Here we tested the effects of 15 GL derivatives against EBV infection by scoring the numbers of cell expressing viral antigens and quantifying EBV DNA copy numbers in superinfected Raji cells. The derivatives were made either by transformation of GL on carboxyl and hydroxyl groups or by conjugation of amino acid residues into the carbohydrate part. We identified seven compounds active against EBV and all showed dose-dependent inhibition as determined by both assays. Among these active compounds, the introduction of amino acid residues into the GL carbohydrate part enhanced the antiviral activity in three of the seven active compounds. However, when Glu(OH)-OMe was substituted by Glu(OMe)-OMe, its antiviral activity was completely abolished. Introduction of potassium or ammonium salt to GL reduced the antiviral activity with no significant effect on cytotoxicity. The α-isomer (18α-GL) of 18β-GL was as potent as the β-form, but its sodium salt lost antiviral activity. The metabolic product of GL, 18β-glycyrrhetinic acid (18β-GA or GA), was 7.5-fold more active against EBV than its parental compound GL but, concomitantly, exhibited increased cytotoxicity resulting in a decreased therapeutic index.

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.

Purification and characterization of soluble CD21 from human plasma by affinity chromatography and density gradient centrifugation

Complement receptor II (CD21) is the receptor for C3d fragments on immune complexes. It also serves as a receptor for Epstein-Barr virus (EBV) and on B-lymphocytes CD21 amplifies signalling through the B cell receptor. CD21 is shed from the surface of the cell and is found circulating in plasma. There, soluble CD21 (sCD21) binds to CD23 and complement fragments, thereby modulating the immune response. sCD21 activates monocytes through binding to membrane CD23. The clinical significance of sCD21 is shown by the increased levels found in the sera of patients with B lymphomas, EBV infections and other lymphoblastoid tumors. In this paper, we report the isolation of soluble CD21 from human plasma using affinity chromatography and density gradient centrifugation. sCD21 was found to be a single 126 kDa molecular species. By determining the sedimentation coefficient, we have calculated the partial specific volume, diffusion coefficient and frictional coefficient of the protein. These values show that the sCD21 isolated from human plasma is an elongated rod-shaped molecule.

EGF/ErbB receptor family in ovarian cancer

In summary, the EGF/ErbB family of receptor tyrosine kinases has been shown to play a key role in normal ovarian follicle development, and cell growth regulation of the ovarian surface epithelium. Disregulation of these normal growth regulatory pathways, including overexpression and/or mutation of EGFR/ErbB receptor family members, as well as elements of their downstream signalling pathways, have been shown to contribute to the etiology and progression of epithelial ovarian cancer. It is, therefore, not surprising that these gene products, and their related soluble receptor isoforms may have clinical utility as tumor and/or serum biomarkers of disease activity. Moreover, since several of these soluble receptor isoforms have potent growth inhibitory activity, and are naturally occurring in the circulation, they are ideal candidates for the development of novel therapeutics for the treatment of ovarian cancer patients.

Identification of key residues in subgroup A avian leukosis virus envelope determining receptor binding affinity and infectivity of cells expressing chicken or quail Tva receptor

To better understand retroviral entry, we have characterized the interactions between subgroup A avian leukosis virus [ALV(A)] envelope glycoproteins and Tva, the receptor for ALV(A), that result in receptor interference. We have recently shown that soluble forms of the chicken and quail Tva receptor (sTva), expressed from genes delivered by retroviral vectors, block ALV(A) infection of cultured chicken cells ( approximately 200-fold antiviral effect) and chickens (>98% of the birds were not infected). We hypothesized that inhibition of viral replication by sTva would select virus variants with mutations in the surface glycoprotein (SU) that altered the binding affinity of the subgroup A SU for the sTva protein and/or altered the normal receptor usage of the virus. Virus propagation in the presence of quail sTva-mIgG, the quail Tva extracellular region fused to the constant region of the mouse immunoglobulin G (IgG) protein, identified viruses with three mutations in the subgroup A hr1 region of SU, E149K, Y142N, and Y142N/E149K. These mutations reduced the binding affinity of the subgroup A envelope glycoproteins for quail sTva-mIgG (32-, 324-, and 4,739-fold, respectively) but did not alter their binding affinity for chicken sTva-mIgG. The ALV(A) mutants efficiently infected cells expressing the chicken Tva receptor but were 2-fold (E149K), 10-fold (Y142N), and 600-fold (Y142N/E149K) less efficient at infecting cells expressing the quail Tva receptor. These mutations identify key determinants of the interaction between the ALV(A) glycoproteins and the Tva receptor. We also conclude from these results that, at least for the wild-type and variant ALV(A)s tested, the receptor binding affinity was directly related to infection efficiency.

Natural history of primary Epstein-Barr virus infection in children of mothers infected with human immunodeficiency virus type 1

The natural history of Epstein-Barr virus (EBV) infection in 556 infants born to 517 human immunodeficiency virus (HIV) type 1-infected mothers was studied in a prospective, multicenter, cohort study. HIV-1-infected children had a cumulative EBV infection rate similar to HIV-1-uninfected children at age 3 years (77.8% vs. 84. 9%) but had more frequent oropharyngeal EBV shedding (50.4% vs. 28. 2%; P<.001). The probability of shedding decreased with longer time from EBV seroconversion and was similar to that of HIV-1-uninfected children 3 years after seroconversion. HIV-1-infected children identified as rapid progressors shed EBV more frequently than nonrapid progressors (69.4% vs.41.0%; P=.01). HIV-1-infected children with EBV infection had higher mean CD8 cell counts. EBV infection did not have an independent effect on mean CD4 cell counts, percent CD4, IgG levels, HIV-1 RNA levels, lymphadenopathy, hepatomegaly, or splenomegaly. Early EBV infection is common in children born to HIV-1-infected mothers. Children with rapidly progressive HIV-1 disease have more frequent EBV shedding.

Production and characterization of a soluble, active form of Tva, the subgroup A avian sarcoma and leukosis virus receptor

The receptor for the subgroup A avian sarcoma and leukosis viruses [ASLV(A)] is the cellular glycoprotein Tva. A soluble form of Tva, sTva, was produced and purified with a baculovirus expression system. Using this system, 7 to 10 mg of purified sTva per liter of cultured Sf9 cells was obtained. Characterization of the carbohydrate modification of sTva revealed that the three N glycosylation sites in sTva were differentially utilized; however, the O glycosylation common to Tva produced in mammalian and avian cells was not observed. Purified sTva demonstrates significant biological activity, specifically blocking infection of avian cells by ASLV(A) with a 90% inhibitory concentration of approximately 25 pM. A quantitative enzyme-linked immunosorbent assay, developed to assess the binding of sTva to ASLV envelope glycoprotein, demonstrates that sTva has a high affinity for EnvA, with an apparent dissociation constant of approximately 0.3 nM. Once they are bound, a very stable complex is formed between EnvA and sTva, with an estimated complex half-life of 6 h. The soluble receptor protein described here represents a valuable tool for analysis of the receptor-envelope glycoprotein interaction and for structural analysis of Tva.

Origin and properties of soluble CD21 (CR2) in human blood

By analysis with a panel of CD21 MoAbs it is shown that a large part of the soluble CD21 in human blood plasma is of the long isoform (CD21L), as judged by comparison with antigen produced by mouse L cells transfected with CD21L-cDNA and reactivity with the restricted CD21 MoAb R4/23. This is compatible with the hypothesis that soluble CD21 in the blood is mainly derived from follicular dendritic cells (FDC). Cells from a human keratinocyte cell line transfected with cDNA from the Burkitt lymphoma cell line Raji also produced soluble CD21L (sCD21L), whereas the short form of sCD21 (sCD21S) was the major component of sCD21 produced by the B lymphoblastoid cell line LICR-LON-HMy and the T cell line Jurkat. Confocal studies of FDC isolated from human tonsil revealed that CD21 was present in the cytoplasm. On gel filtration sCD21 from untreated serum has an apparent size considerably greater than the 130kD found by SDS-PAGE analysis. This may be partly accounted for by the non-globular shape of the molecule, but may also indicate, as reported by others, that in its native state sCD21 is complexed with other proteins. However, no evidence of complexing with sCD23 or C3d could be found.

Epstein-Barr viral gene expression in B-lymphocytes

The strategy of the Epstein-Barr virus to persist lifelong in the host depends on establishing a reservoir, which cannot be detected by the immune system but allows reactivation of the virus for shedding and transmission to a new host. Epithelial cells and B-cells play a major role in this viral strategy of EBV, since differentiating epithelial tissues were shown to be permissive for lytic replication in vivo, whereas the B-lymphocytes become predominantly latently infected. However, which cells are the reservoir and which the sites of lytic replication are not quite clear. With the technique of reverse transcription, PCR and immunohistochemistry, we demonstrated that the B-cells of the peripheral blood are a major site of virus production during the primary infection during infectious mononucleosis. These permissive B-cells were also detected after convalescence, however, the absence of any lytic transcripts suggested an efficient immunological control very early in the viral lytic cycle. Serological data on reactivation of EBV correlated with the detection of lytic cycle transcripts in the blood and thus demonstrated that the site of virus production during infectious mononucleosis must be different from that of the persistent state. In those cases, where the infection takes a chronic active course, control of lytic replication is insufficient, either on the level of immune surveillance or of viral gene regulation. We have demonstrated a virus strain with a lytic phenotype in an individual suffering chronic active infection. The impaired capability of this virus to immortalise B-cells correlated with an enhanced expression of the lytic switch gene BZLF-1 and down-regulation of latent regulatory genes in the early phase of infection.

Expression in insect cells of the functional domain of CD21 (complement receptor type two) as a truncated soluble molecule using a baculovirus vector

We have made use of the plasmid vector pBSV-8His recently established for baculovirus-mediated expression of His-tagged proteins for the production of a truncated soluble complement regulator protein. The protein comprised the N-terminal part, i.e. short consensus repeats (SCRs) 1-4, of the B-cell membrane protein complement receptor type two (CR2; CD21) and contained the functional epitopes which mediate the binding of the complement component C3 fragments C3dg and iC3b. This recombinant protein, termed rsCR2.1-4, was furnished with a C-terminal histidine-tag for easy purification from insect cell supernatant. The yield of > 90% pure rsCR2.1-4 was 3 micrograms/ml supernatant at day eight p.i. RsCR2.1-4 was expressed as two proteins with a M(r) of 29 and 31 representing differentially glycosylated forms. Both reacted specifically with anti-CR2 mAb HB5 directed against SCRs 3-4, but not with anti-CR2 mAbs recognizing SCRs beyond SCR 4. RsCR2.1-4 was able to bind C3dg and to block binding of C3dg-coated beads to Raji cells. Used as antigen for immunization, it allowed the efficient and well-aimed generation of antisera which specifically blocked attachment of C3dg-coated beads to Raji B cells. Thus, insect cell derived rsCR2.1-4 has proved a valuable tool to study the functional domain of CR2 and its immunoregulatory capacity in B-lymphocytes.

Function of CD23 in the response of human B cells to antigen

Co-ligation of antigen receptor and complement receptor 2 (CD21) in the B cell membrane is important in the immune response to T-dependent antigens. Four CD21 ligands have so far been identified, but only the activated products of the third component of complement (C3) are known to augment the immune response to specific antigens. The most recently discovered ligand for CD21 is CD23. We have generated a CD32+ CD23+ fibroblast cell line which presents a surrogate antigen (anti-IgM) to human tonsil B cells in vitro. Incubation with these cells causes a 10- to 100-fold reduction in the threshold concentration of anti-IgM required for B cell proliferation. Anti-CD19 further enhances the response to antigen and induces proliferation in the absence of anti-IgM. Addition of soluble CD21 totally inhibits the effect of CD23, suggesting that CD21 mediates synergistic signaling by CD23.

Sequence, expression and function of an mRNA encoding a soluble form of the human interleukin-6 receptor (sIL-6R)

Soluble receptors have been shown to be potent immunomodulators of their respective ligands. Since IL-6 is a central growth factor for myeloma cells, an sIL-6R may modulate IL-6 activity. We have previously reported a novel IL-6R mRNA from myeloma cells that exhibits a 94-nt deletion of the entire transmembrane domain from codons 356 (G-TG) to 387 (AG-G). The transmembrane domain deletion results in a shift in the translational reading frame with the insertion of 10 new amino acids followed by a stop codon. Sequence analysis shows the ligand-binding domain of the sIL-6R to be identical to that of the membrane-bound IL-6R up to the transmembrane domain deletion. The sIL-6R cDNA was expressed in QT-6 fibroblasts and PA-1 ovarian cells using the expression vector pCDM8. Supernates were immunoprecipitated with anti-IL-6R antibody and cells transfected with the sIL-6R cDNA produced a single band with a molecular weight of 50-55 kDa. This molecular weight corresponds to the size of the sIL-6R protein observed in normal human urine. Supernates were collected from mock or sIL-6R transfected PA-1 cells after 48 hours and assayed for their ability to stimulate or suppress the growth of an IL-6 dependent cell line, ANBL-6. Soluble IL-6R alone had no effect on the growth of the ANBL-6 cells. However, the growth of ANBL-6 cells by sIL-6R was potentiated in the presence of IL-6 and could be blocked by anti-IL-6 antibody. The above results suggest that, in the presence of IL-6, sIL-6R associates with gp130 leading to signal transduction and cell growth.

Epstein-Barr virus, infectious mononucleosis, and posttransplant lymphoproliferative disorders

PTLD may be considered as an "opportunistic cancer" in which the immunodeficiency state of the host plays a key role in fostering the environment necessary for abnormal lymphoproliferation. The following discussion reflects our own current thoughts regarding events which may result in PTLD and its sequelae. Many of the individual steps have not been rigorously proved or disproved at this point in time. Following transplantation and iatrogenic immunosuppression, the host:EBV equilibrium is shifted in favor of the virus. Most seronegative patients will become infected either via the graft or through natural means; seropositive patients will begin to shed higher levels of virus and may become secondarily superinfected via the graft. There is a "grace" period of approximately one month posttransplant before increased viral shedding begins. PTLD is almost never seen during this interval. In many cases infection continues to be silent whereas in rare individuals there is an overwhelming polyclonal proliferation of infected B lymphocytes. This is the parallel of infectious mononucleosis occurring in patients with a congenital defect in virus handling (X-linked lymphoproliferative disorder). It is possible that transplant patients with this presentation also suffer a defect in virus handling. In other cases excessive iatrogenic immunosuppression may paralyze their ability to respond to the infection. With CsA and FK506 regimens, individual tumors may occur within a matter of months following transplant. The short time of incubation suggests that these are less than fully developed malignancies. It may be that local events conspire to allow outgrowth of limited numbers of B-lymphocyte clones. A cytokine environment favoring B-lymphocyte growth may be one factor and differential inhibition by the immuno-suppressive drugs of calcium-dependent and -independent B-cell stimulation may be another. In addition, there is some evidence that CsA itself may inhibit apoptosis within B cells. Since most patients do not develop PTLDs, an additional signal(s) for B-cell stimulation may be required. Indeed, it is possible that the virus may simply serve to lower the threshold for B-cell activation and/or provide a survival advantage to these cells. The ability of individual cell clones to evade a weakened immune system may set into play a Darwinian type of competition in which the most rapidly proliferating cells with the least number of antigenic targets predominate. In this regard, differences in host HLA types may determine the repertoire of viral antigens which are subject to attack.(ABSTRACT TRUNCATED AT 400 WORDS)

Epstein-Barr virus infection and associated diseases in children. II. Diagnostic and therapeutic strategies

Epstein-Barr virus (EBV), an ubiquitous human B lymphotropic virus, is the cause of infectious mononucleosis. Moreover, EBV infection can be followed by lymphoproliferative diseases in patients with inherited and acquired immunodeficiencies. Primary EBV infection may be a threat to all children after marrow or organ transplantation or those receiving chronic immunosuppressive treatment for various other reasons. The virus has been also implicated in the pathogenesis of different malignant tumours such as Burkitt lymphoma, nasopharyngeal carcinoma, Hodgkin disease and also some T-cell lymphomas. This review focuses on various aspects of virus-host interactions, immune mechanisms of the host, and the still experimental therapeutic approaches in EBV-associated diseases.

Properties of soluble CR2 in human serum

A soluble form of complement receptor number 2 (sCR2) found in human serum closely resembles that produced in culture by B lymphoblastoid cells. Epitope analysis with a panel of CD21 monoclonal antibodies revealed only minor differences between antigen from the two sources. Purified sCR2 from both sources bound to C3dg prepared from human or mouse serum and to u.v.-inactivated Epstein-Barr virus. SDS-PAGE analysis of culture supernates of B-lymphoid cells labelled by growth in medium containing 35S-methionine revealed a major component of molecular weight approximately 130 kDa and another band at 30 kDa. Incubation with endoglycosidase F reduced the size of the high molecular weight component. Gel filtration of untreated serum or culture supernate revealed that, in its native state, sCR2 behaved as a molecule or complex of apparent molecular weight 320 kDa. Possible explanations are discussed.

The fibroblast growth factor receptor is not required for herpes simplex virus type 1 infection

The early events mediating herpes simplex virus type 1 (HSV-1) infection include virion attachment to cell surface heparan sulfates and subsequent penetration. Recent evidence has suggested that the high-affinity fibroblast growth factor (FGF) receptor mediates HSV-1 entry. This report presents three lines of experimental evidence showing that the high-affinity FGF receptor is not required for HSV-1 infection. First, rat L6 myoblasts lacking FGF receptors were as susceptible to HSV-1 infection as L6 cells genetically engineered to express the FGF receptor. Second, a soluble FGF receptor fragment that inhibited FGF binding and receptor activation did not inhibit HSV-1 infection. Finally, basic FGF (but not acidic FGF) inhibited HSV-1 infection in L6 cells lacking FGF receptors, presumably by blocking cell surface heparan sulfates also required for HSV-1 infection. These results show that the high-affinity FGF receptor is not required for HSV-1 infection but instead that specific low-affinity basic FGF binding sites are used for HSV-1 infection.

Inhibition of Epstein-Barr virus infection in vitro and in vivo by soluble CR2 (CD21) containing two short consensus repeats

The extracellular domain of CR2, the Epstein-Barr virus (EBV)/C3d receptor of B lymphocytes, contains 15 or 16 tandemly arranged short consensus repeat elements (SCR). Recombinant CR2 proteins containing SCR 1 and 2 fused to Staphylococcus aureus protein A (PA-CR2) and to murine complement factor H SCR 20 (CR2FH) were expressed in Escherichia coli and in insect cells, respectively. These recombinant CR2 molecules retained functional activity as indicated by their ability to bind to C3dg in an enzyme-linked immunosorbent assay and to inhibit EBV gp350/220 binding to B cells. PA-CR2 and CR2FH were as efficient in blocking EBV gp350/220 binding as the full-length CR2 extracellular domain, indicating that the first two SCR of CR2 contain the majority of the ligand binding activity of the receptor. PA-CR2 and CR2FH inhibited EBV-induced B-cell proliferation in vitro and blocked the development of EBV-induced lymphoproliferative disease in severe combined immunodeficient mice reconstituted with human lymphocytes. These studies indicate that soluble forms of truncated CR2 proteins may have potential therapeutic value in the treatment of EBV-induced lymphoproliferative disorders in humans that involve viral replication.

Expression of a human cytomegalovirus receptor correlates with infectibility of cells

Previous studies have demonstrated that human cytomegalovirus (HCMV) specifically binds to a fibroblast membrane glycoprotein(s) with a molecular mass from 30 to 34 kDa. In this study, the distribution of the putative receptor proteins was analyzed in a variety of cell types, including cell types representative of those that are infected in vivo. Using a sensitive microbinding assay (to score virus attachment) and an indirect detection method (to score HCMV-binding proteins), we found that the 34- and 32-kDa HCMV binding proteins are ubiquitous molecules, broadly distributed among diverse cell types. In addition, the level of virus attachment was found to correlate with the abundance of the 34- and 32-kDa cellular proteins, while the ability of the virus to penetrate cells and initiate infection did not. The results support the hypothesis that the 34- and 32-kDa cellular proteins represent the HCMV (attachment) receptor. The data also support the notion that additional cellular components are required for virus entry and fusion.

Poliovirus mutants resistant to neutralization with soluble cell receptors

Poliovirus mutants resistant to neutralization with soluble cellular receptor were isolated. Replication of soluble receptor-resistant (srr) mutants was blocked by a monoclonal antibody directed against the HeLa cell receptor for poliovirus, indicating that the mutants use this receptor to enter cells. The srr mutants showed reduced binding to HeLa cells and cell membranes. However, the reduced binding phenotype did not have a major impact on viral replication, as judged by plaque size and one-step growth curves. These results suggest that the use of soluble receptors as antiviral agents could lead to the selection of neutralization-resistant mutants that are able to bind cell surface receptors, replicate, and cause disease.

The human cytomegalovirus receptor on fibroblasts is a 30-kilodalton membrane protein

Previous studies have demonstrated that human cytomegalovirus (HCMV) binding to human foreskin fibroblasts (HFF) is mediated by a single type of molecule, likely a glycoprotein, which serves as a specific receptor for the virus. In the present experiments, HCMV was found to bind to an HFF membrane protein with an approximate molecular mass of 30 kilodaltons (kDa); weak binding to 28- and 92-kDa membrane components was also observed. Binding was specific, as it was inhibited by excess unlabeled HCMV. Radiolabeled HCMV also bound selectively to Raji and Daudi lymphoblastoid cell membrane proteins of the same molecular masses. The 30-kDa radiolabeled HFF membrane protein bound to HCMV in solution; this binding was also specific, as it was blocked by an excess of HCMV. These data suggest that a membrane protein with a molecular mass of approximately 30 kDa mediates HCMV binding to several cell types.