Detection by monoclonal antibodies of an early membrane protein induced by Epstein-Barr virus

Interferon-γ-inducible protein 16 (IFI16) is required for the maintenance of Epstein-Barr virus latency

BACKGROUND

Epstein-Barr virus (EBV) exhibits both lytic and latent (Lat. I, II, and III) phases in an infected individual. It's during the latent phase of EBV that all EBV-associated cancers, including Burkitt's lymphoma, nasopharyngeal carcinoma and lymphoproliferative disease arise. Interferon-γ-inducible protein 16 (IFI16) is a well-established innate immune sensor and viral transcriptional regulator involved in response to invading DNA viruses. During latency, IFI16 remains in the nucleus, in part bound to the EBV genome; however, neither its role in EBV lytic cycle or latency has been established.

METHODS

Short interfering RNA against IFI16 and IFI16 overexpression were used to identify the role of IFI16 in the maintenance of EBV latency I. We also studied how induction of the lytic cycle affected IFI16 using the EBV positive, latently infected Akata or MUTU-1 cell lines. Akata cells were induced with TPA and MUTU-1 cells with TGF-β up to 96 h and changes in IFI16 protein were analyzed by Western blotting and immunofluorescence microscopy. To assess the mechanism of IFI16 decrease, EBV DNA replication and late lytic transcripts were blocked using the viral DNA polymerase inhibitor phosphonoacetic acid.

RESULTS

Knockdown of IFI16 mRNA by siRNA resulted in enhanced levels of EBV lytic gene expression from all temporal gene classes, as well as an increase in the total EBV genome abundance, whereas overexpression of exogenous IFI16 reversed these effects. Furthermore, 96 h after induction of the lytic cycle with either TPA (Akata) or TGF-β (MUTU-1), IFI16 protein levels decreased up to 80% as compared to the EBV-negative cell line BJAB. Reduction in IFI16 was observed in cells expressing EBV lytic envelope glycoprotein. The decreased levels of IFI16 protein do not appear to be dependent on late lytic transcripts of EBV but suggest involvement of the immediate early, early, or a combination of both gene classes.

CONCLUSIONS

Reduction of IFI16 protein levels following lytic cycle induction, as well as reactivation from latency after IFI16 mRNA knockdown suggests that IFI16 is crucial for the maintenance of EBV latency. More importantly, these results identify IFI16 as a unique host factor protein involved in the EBV lifecycle, making it a potential therapeutic target to combat EBV-related malignancies.

A Temporal Proteomic Map of Epstein-Barr Virus Lytic Replication in B Cells

Epstein-Barr virus (EBV) replication contributes to multiple human diseases, including infectious mononucleosis, nasopharyngeal carcinoma, B cell lymphomas, and oral hairy leukoplakia. We performed systematic quantitative analyses of temporal changes in host and EBV proteins during lytic replication to gain insights into virus-host interactions, using conditional Burkitt lymphoma models of type I and II EBV infection. We quantified profiles of >8,000 cellular and 69 EBV proteins, including >500 plasma membrane proteins, providing temporal views of the lytic B cell proteome and EBV virome. Our approach revealed EBV-induced remodeling of cell cycle, innate and adaptive immune pathways, including upregulation of the complement cascade and proteasomal degradation of the B cell receptor complex, conserved between EBV types I and II. Cross-comparison with proteomic analyses of human cytomegalovirus infection and of a Kaposi-sarcoma-associated herpesvirus immunoevasin identified host factors targeted by multiple herpesviruses. Our results provide an important resource for studies of EBV replication.

Cell-surface expression of a mutated Epstein-Barr virus glycoprotein B allows fusion independent of other viral proteins

Epstein-Barr virus (EBV) infects human B lymphocytes and epithelial cells. We have compared the requirements for EBV glycoprotein-induced cell fusion between Chinese hamster ovary effecter cells and human B lymphoblasts or epithelial cells by using a virus-free cell fusion assay. EBV-encoded gB, gH, gL, and gp42 glycoproteins were required for efficient B cell fusion, whereas EBV gB, gH, and gL glycoproteins were required for Chinese hamster ovary effecter cell fusion with epithelial cell lines (AGS and SCC68) or the human embryonic kidney cell line 293-P. Fusion with human embryonic kidney 293-P cells was greater than fusion observed with B cells, indicative of an important role for cell contact. An antibody directed against the gH and gL complex inhibited epithelial cell fusion. Increased surface expression of gB alone as a result of truncations or point mutants in the carboxyl-terminal tail allowed gB-mediated fusion with epithelial cells, albeit at a lower level than with coexpression of gB, gH, and gL. Overall, gB appears to be the critical component for EBV glycoprotein-mediated cell fusion.

Identification of a novel EBV-induced membrane glycoprotein of 43 kDa with H667 MAb

Using purified B95-8 Epstein-Barr virus (EBV), a MAb designated H667 was produced. We demonstrated by indirect membrane immunofluorescence (IF) on six EBV producer cell lines and by immunoelectron microscopy that H667 reacted with a membrane antigen. H667 recognized a 43-kDa EBV protein (p43) as determined by immunoblotting using purified EBV from the six producer cell lines. Phosphonoacetic acid treatment of B95-8 cells was associated with the disappearance of p43, indicating that it was a late antigen. This antigen was shown to be a glycoprotein by incorporation of [14C]glucosamine and was shown to contain an N-asparagine-linked glycosyl group by its sensitivity to tunicamycin. It was named gp43. The H667 MAb inhibited B95-8 EBV cord blood lymphocyte transformation only when a low inoculum was used but failed to inhibit EA induction in Raji cells by P3HR1 EBV. Human sera reactivity against the gp43 antigen was studied. By the immunoblotting method, using H667 immunoaffinity chromatography-purified gp43, we showed that 70.9% of the human sera tested had antibodies directed against gp43. By IF blocking tests, we found that only 12.5% of the sera tested were reactive, indicating that the epitope corresponding to the H667 MAb was not the most immunogenic gp43 epitope.

Characterization of envelope proteins of alcelaphine herpesvirus 1

Alcelaphine herpesvirus 1 is a gammaherpesvirus which causes malignant catarrhal fever, an acute lymphoproliferative disorder of cattle and other susceptible Bovidae, which is almost invariably fatal. A preliminary analysis of proteins induced by the virus indicated that as many as six glycoproteins and one nonglycosylated molecule might be present in the virus envelope. Monoclonal antibodies selected for recognition of virion envelope proteins included two that recognized a complex of infected cell proteins, designated the gp115 complex, and neutralized virus infectivity in the absence of complement. The gp115 complex consisted of five glycoproteins of 115, 110, 105, 78, and 48 kilodaltons (kDa), and all except the 48-kDa species reacted with antibody in Western blots (immunoblots). Pulse-chase experiments analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing and nonreducing conditions suggested that the 110-kDa protein was the precursor molecule which was processed by addition of sugars to 115 kDa. The 115-kDa protein was cleaved to form a disulfide-linked heterodimer of 78 and 48 kDa, which was the mature form of the molecule incorporated into the virion envelope. The glycoprotein contained N-linked sugars, but little or no O-linked sugar was present. The relative abundance of the mature protein and its ability to induce neutralizing antibodies suggest that it will prove useful to studies aimed at elucidating the biology and pathogenesis of alcelaphine herpesvirus 1.

Characterization and expression of a glycoprotein encoded by the Epstein-Barr virus BamHI I fragment

Computer-assisted analysis of the Epstein-Barr virus (EBV) open reading frame BILF2 (B95-8 nucleotides 150,525 to 149,782) predicts that it codes for a membrane-bound glycoprotein. [3H]glucosamine labeling of cells infected with vaccinia virus recombinants that expressed the BILF2 open reading frame revealed several diffuse species of glycoproteins of around 80,000 and 55,000 daltons. A monoclonal antibody derived from spleens of mice immunized with EBV immunoprecipitated the EBV-derived protein made by the vaccinia virus recombinants and also precipitated a late envelope glycoprotein with a mobility of 78,000 to 55,000 from EBV-producing cells. N-Glycanase treatment of the immunoprecipitated BILF2 product from EBV-producing cells resulted in a polypeptide of 28 kilodaltons, closely agreeing with the predicted molecular mass for the unmodified BILF2 gene product. Western (immuno-) blots using recombinant infected cells as a source of antigen showed that the majority of EBV-seropositive individuals have a serum antibody response to the BILF2-encoded gp78/55.

Intracellular trafficking of two major Epstein-Barr virus glycoproteins, gp350/220 and gp110

The processing and intracellular localization of the two predominant Epstein-Barr virus glycoproteins expressed in late lytic infection were investigated. Immune light or electron microscopy of frozen fixed sections revealed that gp110 colocalized to the endoplasmic reticulum and to the nuclear membrane with the endoplasmic reticulum-resident protein, heavy-chain-binding protein (BiP), while gp350/220 accumulated in low abundance in the endoplasmic reticulum and was present in higher abundance in cytoplasmic structures presumed to be Golgi and in plasma membranes. Consistent with endoplasmic reticulum and nuclear membrane localization, the bulk of gp110 was sensitive to endoglycosidase H, indicating high-mannose, pre-Golgi, N-linked glycosylation; while consistent with Golgi and plasma membrane localization, gp350/220 was mostly resistant to endoglycosidase H because of complex N- and O-linked glycosylation. gp350/220 was as abundant in extracellular enveloped virus as in the plasma membrane but was much less abundant or undetected in internal cytoplasmic or nuclear membranes. In contrast, gp110-specific antibodies did not label extracellular or intracellular virus. These data indicate that the major antigenic components of gp110 are not incorporated into or are occluded in virions and that gp350/220 is added to virus in cytoplasmic transit through a process of de-envelopment and re-envelopment at the plasma membrane or at post-Golgi vesicles. Consistent with cytoplasmic de-envelopment and re-envelopment at the plasma membrane was the finding of some free nucleocapsids in the cytoplasm of cells with intact nuclear membranes and nucleocapsids which appeared to bud through the plasma membrane.

Depletion of glycoprotein gp85 from virosomes made with Epstein-Barr virus proteins abolishes their ability to fuse with virus receptor-bearing cells

Entry of an enveloped virus such as Epstein-Barr virus (EBV) into host cells involves fusion of the virion envelope with host cell membranes either at the surface of the cell or within endocytic vesicles. Previous work has indirectly implicated the EBV glycoprotein gp85 in this fusion process. A neutralizing monoclonal antibody to gp85, F-2-1, failed to inhibit binding of EBV to its receptor but interfered with virus fusion as measured with the self-quenching fluorophore octadecyl rhodamine B chloride (R18) (N. Miller and L. M. Hutt-Fletcher, J. Virol. 62:2366-2372, 1988). To test further the hypothesis that gp85 functions as a fusion protein, EBV virion proteins including or depleted of gp85 were incorporated into lipid vesicles to form virosomes. Virosomes were labeled with R18, and those that were made with undepleted protein were shown to behave in a manner similar to that of R18-labeled virus. They bound to receptor-positive but not to receptor-negative cells and fused with Raji cells but not with receptor-positive, fusion-incompetent Molt 4 cells; monoclonal antibodies that inhibited binding or fusion of virus inhibited binding and fusion of virosomes, and virus competed with virosomes for attachment to cells. In contrast, virosomes made from virus proteins depleted of gp85 by immunoaffinity chromatography remained capable of binding to receptor-positive cells but failed to fuse. These results are compatible with the hypothesis that gp85 is actively involved in the fusion of EBV with lymphoblatoid cell lines and suggest that the ability of antibody F-2-1 to neutralize infectivity of EBV represents a direct effect on the function of gp85 as a fusion protein.

Characterization of a 75-kDa Epstein-Barr virus capsid protein using a new monoclonal antibody H250

A monoclonal antibody (mAb) designated H250, directed against an Epstein-Barr virus (EBV) capsid antigen, was obtained following immunization of BALB/c mice with naked particles from the producer cell line B95.8. This antigen was present in the producer lines B95.8, P3HR1, M81, RI and CA, and absent from the non-producer lines BJAB, Raji and 1022. H250 did not inhibit the transformation of cord blood lymphocytes by the B95.8 virus, nor did it inhibit EA induction on Raji cells by the P3HR1 virus. In addition, H250 showed no fluorescence on living B95.8 cells. This indicates that H250 does not recognize a membrane antigen. By indirect immunofluorescence, no fluorescence was observed on induced Raji cells or on PAA-treated B95.8 cells. Thus, H250 recognized a late antigen of the EBV virus replication cycle. Agglutination of naked virus by H250 showed it was directed against a capsid antigen. Positive fluorescence was observed on cells treated with tunicamycin, indicating that H250 recognized a protein. The molecular weight of this protein was obtained by Western blot and was approximately 75 kDa. The blocking tests carried out with H250 seemed to indicate that this Ab appeared late in patient sera during primary infection.

Identification of proteins specific for human herpesvirus 6-infected human T cells

Proteins specific for human herpesvirus 6 (HHV-6)-infected human T cells (HSB-2) were examined by using polyclonal rabbit antibodies and monoclonal antibodies against HHV-6-infected cells and human sera. More than 20 proteins and six glycoproteins specific for HHV-6-infected cells were identified from [35S]methionine- and [3H]glucosamine-labeled total-cell extracts. Polyclonal rabbit antibodies immunoprecipitated 33 [35S]methionine-labeled HHV-6-specific polypeptides with approximate molecular weights ranging from 180,000 to 31,000. In immunoprecipitation and Western immunoblot reactions, a patient's serum also recognized more than 30 HHV-6-specific proteins and seven glycoproteins. In contrast, sera from individuals with high-titered antibodies against other human herpesviruses reacted with fewer HHV-6-infected cell proteins, and only a 135,000-Mr polypeptide was prominent. Monoclonal antibodies to HHV-6-infected cells reacted with single and multiple polypeptides specific for virus-infected cells and immunoprecipitated three distinct sets of glycoproteins, which were designated gp105k and gp82k, gp116k, gp64k, and gp54k, and gp102k.

A monoclonal antibody to glycoprotein gp85 inhibits fusion but not attachment of Epstein-Barr virus

Epstein-Barr virus (EBV) codes for at least three glycoproteins, gp350, gp220, and gp85. The two largest glycoproteins are thought to be involved in the attachment of the virus to its receptor on B cells, but despite the fact that gp85 induces neutralizing antibody, no function has been attributed to it. As an indirect approach to understanding the role of gp85 in the initiation of infection, we determined the point at which a neutralizing, monoclonal antibody that reacted with the glycoprotein interfered with virus replication. The antibody had no effect on virus binding. To examine the effect of the antibody on later stages of infection, the fusion assay of Hoekstra and colleagues (D. Hoekstra, T. de Boer, K. Klappe, and J. Wilshaut, Biochemistry 23:5675-5681, 1984) was adapted for use with EBV. The virus was labeled with a fluorescent amphiphile that was self-quenched at the high concentration obtained in the virus membrane. When the virus and cell membrane fused, there was a measurable relief of self-quenching that could be monitored kinetically. Labeling had no effect on virus binding or infectivity. The assay could be used to monitor virus fusion with lymphoblastoid lines or normal B cells, and its validity was confirmed by the use of fixed cells and the Molt 4 cell line, which binds but does not internalize the virus. The monoclonal antibody to gp85 that neutralized virus infectivity, but not a second nonneutralizing antibody to the same molecule, inhibited the relief of self-quenching in a dose-dependent manner. This finding suggests that gp85 may play an active role in the fusion of EBV with B-cell membranes.

Induction of antibodies to the Epstein-Barr virus glycoprotein gp85 with a synthetic peptide corresponding to a sequence in the BXLF2 open reading frame

Epstein-Barr virus codes for at least three envelope glycoproteins, one of which, gp85, has not yet been mapped to the viral genome. The publication and analysis of the entire Epstein-Barr virus DNA sequence has allowed identification of open reading frames with potential for encoding membrane glycoproteins. To determine whether one of these candidate open reading frames, BXLF2, codes for gp85, an antibody was made to a 17-residue peptide derived from positions 518 to 533 of the predicted BXLF2 protein. The reactivity of the antipeptide antibody was then compared with that of the monoclonal antibody F-2-1, which was originally used to define and characterize gp85. Antipeptide antibody and F-2-1 immunoprecipitated glycosylated molecules with identical electrophoretic mobilities; digestion of the two immunoprecipitated proteins with V8 protease generated comparable peptides; and the antipeptide antibody reacted in Western immunoblots with the gp85 glycoprotein that had been immunoprecipitated by F-2-1 before transfer to nitrocellulose. In addition, a monospecific rabbit antibody, made against native gp85, reacted with the peptide used for immunization. These results are compatible with the hypothesis that the BXLF2 open reading frame codes for gp85.

Antigenic cross-reactions among herpes simplex virus types 1 and 2, Epstein-Barr virus, and cytomegalovirus

Polyvalent rabbit antisera against herpes simplex virus type 1 and 2 (HSV-1 and HSV-2), cytomegalovirus (CMV), and Epstein-Barr virus (EBV), monospecific antisera against affinity-purified HSV-2 glycoproteins gB and gG, and a panel of monoclonal antibodies against HSV and EBV proteins were used to analyze cross-reactive molecules in cells infected with the four herpesviruses. A combination of immunoprecipitation and Western blotting with these reagents was used to determine that all four viruses coded for a glycoprotein that cross-reacted with HSV-1 gB. CMV coded for proteins that cross-reacted with HSV-2 gC, gD, and gE. Both CMV and EBV coded for proteins that cross-reacted with HSV-2 gG. Antigenic counterparts to the p45 nucleocapsid protein of HSV-2 were present in HSV-1 and CMV, and counterparts of the major DNA-binding protein and the ribonucleotide reductase of HSV-1 were present in all the viruses. The EBV virion glycoprotein gp85 was immunoprecipitated by antisera to HSV-1, HSV-2, and CMV. Antisera to CMV and EBV neutralized the infectivity of both HSV-1 and HSV-2 at high concentrations. This suggests that cross-reactivity between these four human herpesviruses may have pathogenic as well as evolutionary significance.