Viral polypeptides detected by a complement-dependent neutralizing murine monoclonal antibody to human cytomegalovirus

Direct enhancement of viral neutralising antibody potency by the complement system: a largely forgotten phenomenon

Neutralisation assays are commonly used to assess vaccine-induced and naturally acquired immune responses; identify correlates of protection; and inform important decisions on the screening, development, and use of therapeutic antibodies. Neutralisation assays are useful tools that provide the gold standard for measuring the potency of neutralising antibodies, but they are not without limitations. Common methods such as the heat-inactivation of plasma samples prior to neutralisation assays, or the use of anticoagulants such as EDTA for blood collection, can inactivate the complement system. Even in non-heat-inactivated samples, the levels of complement activity can vary between samples. This can significantly impact the conclusions regarding neutralising antibody potency. Restoration of the complement system in these samples can be achieved using an exogenous source of plasma with preserved complement activity or with purified complement proteins. This can significantly enhance the neutralisation titres for some antibodies depending on characteristics such as antibody isotype and the epitope they bind, enable neutralisation with otherwise non-neutralising antibodies, and demonstrate a better relationship between in vitro and in vivo findings. In this review, we discuss the evidence for complement-mediated enhancement of antibody neutralisation against a range of viruses, explore the potential mechanisms which underpin this enhancement, highlight current gaps in the literature, and provide a brief summary of considerations for adopting this approach in future research applications.

Complement in malaria immunity and vaccines

Developing efficacious vaccines for human malaria caused by Plasmodium falciparum is a major global health priority, although this has proven to be immensely challenging over the decades. One major hindrance is the incomplete understanding of specific immune responses that confer protection against disease and/or infection. While antibodies to play a crucial role in malaria immunity, the functional mechanisms of these antibodies remain unclear as most research has primarily focused on the direct inhibitory or neutralizing activity of antibodies. Recently, there is a growing body of evidence that antibodies can also mediate effector functions through activating the complement system against multiple developmental stages of the parasite life cycle. These antibody‐complement interactions can have detrimental consequences to parasite function and viability, and have been significantly associated with protection against clinical malaria in naturally acquired immunity, and emerging findings suggest these mechanisms could contribute to vaccine‐induced immunity. In order to develop highly efficacious vaccines, strategies are needed that prioritize the induction of antibodies with enhanced functional activity, including the ability to activate complement. Here we review the role of complement in acquired immunity to malaria, and provide insights into how this knowledge could be used to harness complement in malaria vaccine development.

Complement enhances in vitro neutralizing potency of antibodies to human cytomegalovirus glycoprotein B (gB) and immune sera induced by gB/MF59 vaccination

Human cytomegalovirus (HCMV) is the leading cause of in utero viral infection in the United States. Since congenital HCMV infection can lead to birth defects in newborns, developing a prophylactic vaccine is a high priority. One of the early experimental vaccines, composed of a recombinant glycoprotein B (gB) formulated with MF59 adjuvant, has demonstrated approximately 50% efficacy against HCMV infection in seronegative women. Using immune sera from two gB/MF59 Phase 1 studies in humans we showed that complement can enhance the in vitro HCMV neutralizing potency of antibodies induced by the gB/MF59 vaccination. To characterize this complement-dependent antiviral activity, we analyzed three rabbit non-neutralizing gB monoclonal antibodies (mAbs) with different biochemical profiles including epitope specificity. Two of the three mAbs, r272.7 and r210.4, exhibited neutralizing activity when complement was added to the assays, and this complement-dependent antiviral activity was not related to the antibody's affinity to gB but appeared to be associated with their epitope specificities. Moreover, neutralization could only be demonstrated when complement was present at or before viral entry, suggesting that IgG Fc-mediated function was not the basis for this antiviral activity. Lastly, we demonstrated that gB/MF59 immune sera contained antibodies that can cross-compete with r272.7 for gB binding and that the titers of these antibodies correlated with complement-dependent neutralization titers. These results suggested that gB antibodies with certain biochemical properties have neutralizing potency when complement is present and that this complement-dependent antiviral activity may be a part of immune components which conferred protection against HCMV infection by gB/MF59 vaccination.

Human Immunodeficiency Virus Proteins Mimic Human T Cell Receptors Inducing Cross-Reactive Antibodies

Human immunodeficiency virus (HIV) hides from the immune system in part by mimicking host antigens, including human leukocyte antigens. It is demonstrated here that HIV also mimics the V-β-D-J-β of approximately seventy percent of about 600 randomly selected human T cell receptors (TCR). This degree of mimicry is greater than any other human pathogen, commensal or symbiotic organism studied. These data suggest that HIV may be evolving into a commensal organism just as simian immunodeficiency virus has done in some types of monkeys. The gp120 envelope protein, Nef protein and Pol protein are particularly similar to host TCR, camouflaging HIV from the immune system and creating serious barriers to the development of safe HIV vaccines. One consequence of HIV mimicry of host TCR is that antibodies against HIV proteins have a significant probability of recognizing the corresponding TCR as antigenic targets, explaining the widespread observation of lymphocytotoxic autoantibodies in acquired immunodeficiency syndrome (AIDS). Quantitative enzyme-linked immunoadsorption assays (ELISA) demonstrated that every HIV antibody tested recognized at least one of twelve TCR, and as many as seven, with a binding constant in the 10-8 to 10-9 m range. HIV immunity also affects microbiome tolerance in ways that correlate with susceptibility to specific opportunistic infections.

Conservation of the glycoprotein B homologs of the Kaposi׳s sarcoma-associated herpesvirus (KSHV/HHV8) and old world primate rhadinoviruses of chimpanzees and macaques

The envelope-associated glycoprotein B (gB) is highly conserved within the Herpesviridae and plays a critical role in viral entry. We analyzed the evolutionary conservation of sequence and structural motifs within the Kaposi׳s sarcoma-associated herpesvirus (KSHV) gB and homologs of Old World primate rhadinoviruses belonging to the distinct RV1 and RV2 rhadinovirus lineages. In addition to gB homologs of rhadinoviruses infecting the pig-tailed and rhesus macaques, we cloned and sequenced gB homologs of RV1 and RV2 rhadinoviruses infecting chimpanzees. A structural model of the KSHV gB was determined, and functional motifs and sequence variants were mapped to the model structure. Conserved domains and motifs were identified, including an "RGD" motif that plays a critical role in KSHV binding and entry through the cellular integrin αVβ3. The RGD motif was only detected in RV1 rhadinoviruses suggesting an important difference in cell tropism between the two rhadinovirus lineages.

Cytomegalovirus vaccine strain towne-derived dense bodies induce broad cellular immune responses and neutralizing antibodies that prevent infection of fibroblasts and epithelial cells

Human cytomegalovirus (HCMV), a betaherpesvirus, can cause severe disease in immunosuppressed patients and following congenital infection. A vaccine that induces both humoral and cellular immunity may be required to prevent congenital infection. Dense bodies (DBs) are complex, noninfectious particles produced by HCMV-infected cells and may represent a vaccine option. As knowledge of the antigenicity and immunogenicity of DB is incomplete, we explored characterization methods and defined DB production methods, followed by systematic evaluation of neutralization and cell-mediated immune responses to the DB material in BALB/c mice. DBs purified from Towne-infected cultures treated with the viral terminase inhibitor 2-bromo-5,6-dichloro-1-beta-d-ribofuranosyl benzimidazole riboside (BDCRB) were characterized by nanoparticle tracking analysis (NTA), two-dimensional fluorescence difference gel electrophoresis (2D-DIGE), immunoblotting, quantitative enzyme-linked immunosorbent assay, and other methods. The humoral and cellular immune responses to DBs were compared to the immunogenicity of glycoprotein B (gB) administered with the adjuvant AddaVax (gB/AddaVax). DBs induced neutralizing antibodies that prevented viral infection of cultured fibroblasts and epithelial cells and robust cell-mediated immune responses to multiple viral proteins, including pp65, gB, and UL48. In contrast, gB/AddaVax failed to induce neutralizing antibodies that prevented infection of epithelial cells, highlighting a critical difference in the humoral responses induced by these vaccine candidates. Our data advance the potential for the DB vaccine approach, demonstrate important immunogenicity properties, and strongly support the further evaluation of DBs as a CMV vaccine candidate.

Cytomegalovirus (CMV) genotype in allogeneic hematopoietic stem cell transplantation

Background

Based on sequence variation in the UL55 gene that encodes glycoprotein B (gB), human cytomegalovirus (CMV) can be classified into four gB genotypes. Previous studies have suggested an association between CMV gB genotype and clinical outcome in patients who underwent an allogeneic hematopoietic stem cell transplant (HSCT). The goals of this study were identify patients with active infection caused by CMV in recipients of HSCT; determine the prevalence of CMV genotypes in the study group; correlate genotype with CMV disease, acute GVHD and overall survival.

Methods

The diagnosis of active CMV infection after allogeneic HSCT was detected by antigenemia (AGM) and/or nested-PCR (N-PCR). Positive samples from patients with active CMV infection were submitted to genotyping using N-PCR to amplify a region of UL55, followed by restriction analysis based on HinfI and RsaI digestion. Real-time PCR (qPCR) was used to determine the viral load during active CMV infection and antiviral treatment.

Results

Sixty-three allogeneic HSCT recipients were prospectively evaluated; 49/63 (78%) patients were infected with CMV genotypes – gB1 19/49 (39%), gB2 17/49 (35%), gB3 3/49 (6%), gB4 7/49 (14%) – and 3 (6%) had mixed CMV genotypes (gB1 + gB3, gB1 + gB4 and gB2 + gB4). Characterized by gastrointestinal disease, CMV disease occurred in 3/49 (6.1%) patients, who had CMV gB3 genotype. These gB3 genotype patients presented an increasing AGM number, mean 125 (± 250) (P = 0.70), and qPCR copies/ml, mean 37938 (SD ± 50542) (P = 0.03), during antiviral treatment, when compared with other CMV genotypes. According to CMV genotypes, stratified overall survival was 55% for gB1, 43% for gB2; 0% for gB3 and 57% for gB4 (P = 0.03).

Conclusions

One of the restrictions of the presented study was the low number of CMV gB sub-cohorts). However, we demonstrated that the frequency of active CMV infection in this HSCT population was high, and the most prevalent genotype in these patients with active CMV infection was gB1 and gB2 genotype (74%). In Brazil, HSCT recipients seem to carry mainly gB1 and gB2 CMV genotype.

DNA vaccines expressing glycoprotein complex II antigens gM and gN elicited neutralizing antibodies against multiple human cytomegalovirus (HCMV) isolates

Human cytomegalovirus (HCMV) glycoprotein complex II (gcII) consists of two glycoproteins, gM and gN. Although gcII specific IgG purified from HCMV positive patient sera can neutralize HCMV, there has been no report describing the generation of virus-neutralizing antibodies by immunization with individual recombinant gM or gN antigens. In the current study, gM and gN antigens were expressed by the mammalian expression vector pJW4303 and used as DNA vaccines to determine the immunogenicity of these proteins. Sera from mice or rabbits immunized with individual or combinations of gM and gN DNA vaccines contained gM and gN specific antibodies as confirmed by ELISA and Western blot analyses. The combined gM and gN antigens induced the strongest antibody responses that recognized both gM and gcII complex while gM DNA vaccine alone could only elicit antibody specific for gM antigen. When given alone, the gN DNA vaccine did not induce detectable gcII specific antibody even though in vitro gN expression was confirmed by the formation of gM/gN complex in FSK cells using a gN-specific monoclonal antibody 14-16A. The neutralizing antibody titer of anti-gM/gN sera (1:128) was higher than that of anti-gM sera (1:32) against the autologous virus, HCMV AD169. Heterologous HCMV strains including Towne and Davis could also be neutralized by the anti-gM/gN antisera. Our data supported the rationale for the use of the HCMV gM/gN protein complex as protective antigens for subunit based HCMV vaccine development. DNA vaccination is an effective approach to express the gM/gN antigen complex in vivo without the need to express and purify these highly insoluble and structurally complicated antigens.

Neutralizing antibody to gB2 human cytomegalovirus does not prevent reactivation in patients with human immunodeficiency virus infection

The incidence of human cytomegalovirus (CMV) genotype gB2 (UL55) is high in patients with human immunodeficiency virus (HIV) infection in the San Francisco Bay area of California. Virus neutralizing antibody (NAb) to human CMV strain Ad169, a gB2 laboratory strain, was measured prospectively in HIV-infected patients, with CD4 T-lymphocyte counts <200, who were at risk for CMV-associated disease. Patients were grouped according to CMV DNA copy number, as quantified by PCR, and presence or absence of CMV-induced retinitis. Mean NAb titres were similar in all patient groups and unrelated to either virus load or outcome of CMV infection. Both gB2 and mixtures of gB2 with other gB genotypes were represented in isolates from blood and/or urine, even in the presence of high titres of antibody to the gB2 genotype challenge virus.

Replication of rhesus cytomegalovirus in life-expanded rhesus fibroblasts expressing human telomerase

The kinetics of rhesus cytomegalovirus (RhCMV) infection were compared in primary and telomerase-immortalized (Telo-) rhesus fibroblasts (RF). Equivalent viral titers were achieved with both cell types. However, the production of infectious virions was slightly faster and plaque size was larger in Telo-RF, compared with primary cells. Comparable RhCMV growth curves and viral susceptibility were observed using Telo-RF passaged for different time periods in culture, whereas the ability of primary cells to support robust RhCMV replication declined as the cells approached senescence. Analysis of cell growth kinetics suggested that the rate of RhCMV replication was directly related to the rate of cell proliferation. RT-PCR analysis of representative RhCMV genes demonstrated that the presence of telomerase did not alter the temporal profile of RhCMV gene expression. In addition, Telo-RF cells were observed to have a significantly increased efficiency of transfection with cationic lipids, compared with primary RF. These results demonstrated that Telo-RF represents a stable, permissive cell line for RhCMV infection, facilitating standardization of in vitro assays for this important non-human primate CMV. The ease of transfection will enable molecular analyses and the generation of complementing cell lines for the propagation of defective RhCMV variants.

Cloning and epitope mapping of a functional partial fusion receptor for human cytomegalovirus gH

A cDNA clone encoding a partial putative human cytomegalovirus (HCMV) gH fusion receptor (CMVFR) was previously identified. In this report, the cDNA sequence of CMVFR was determined and the role of this CMVFR in HCMV/cell fusion was confirmed by rendering fusion-incompetent MOLT-4 cells susceptible to fusion following transfection with receptor cDNA. Blocking experiments using recombinant gH or either of two MAbs (against recombinant gH or purified viral gH:gL) provided additional evidence for the role of gH binding to this protein in virus fusion. An HCMV-binding domain of 12 aa in the middle hydrophilic region of CMVFR was identified by fusion blocking studies using synthetic receptor peptides. The 1368 bp cDNA of CMVFR contained a predicted ORF of 345 aa with two potential membrane-spanning domains and several possible nuclear localization signals. A search of sequence databases indicated that CMVFR is a novel protein. Further characterization of this cell membrane protein that confers susceptibility to fusion with the viral envelope should provide important information about the mechanism by which HCMV infects cells.

Human Cytomegalovirus Binding to Human Monocytes Induces Immunoregulatory Gene Expression

To continue our investigation of the cellular events that occur following human CMV (HCMV) infection, we focused on the regulation of cellular activation following viral binding to human monocytes. First, we showed that viral binding induced a number of immunoregulatory genes (IL-1β, A20, NF-κB-p105/p50, and IκBα) in unactivated monocytes and that neutralizing Abs to the major HCMV glycoproteins, gB (UL55) and gH (UL75), inhibited the induction of these genes. Next, we demonstrated that these viral ligands directly up-regulated monocyte gene expression upon their binding to their appropriate cellular receptors. We then investigated if HCMV binding also resulted in the translation and secretion of cytokines. Our results showed that HCMV binding to monocytes resulted in the production and release of IL-1β protein. Because these induced gene products have NF-κB sites in their promoter regions, we next examined whether there was an up-regulation of nuclear NF-κB levels. These experiments showed that, in fact, NF-κB was translocated to the nucleus following viral binding or purified viral ligand binding. Changes in IκBα levels correlated with the changes in NF-κB translocation. Lastly, we demonstrated that p38 kinase activity played a central role in IL-1β production and that it was rapidly up-regulated following infection. These results support our hypothesis that HCMV initiates a signal transduction pathway that leads to monocyte activation and pinpoints a potential mechanism whereby HCMV infection of monocytes can result in profound pathogenesis, especially in chronic inflammatory-type conditions.

Induction of human cytomegalovirus (HCMV)-glycoprotein B (gB)-specific neutralizing antibody and phosphoprotein 65 (pp65)-specific cytotoxic T lymphocyte responses by naked DNA immunization

Plasmids expressing the human cytomegalovirus (HCMV) glycoprotein B (gB) (UL55) or phosphoprotein 65 (pp65) (UL83) were constructed and evaluated for their ability to induce immune responses in mice. The full-length gB as well as a truncated form expressing amino acids 1-680 of gB, and lacking the fragment encoding amino acids 681 907 including the transmembrane domain of gB (gB680) were evaluated. Immunization of mice with plasmids coding for gB or gB680 induced ELISA and neutralizing antibodies, with the highest titres in mice immunized with the gB680 plasmid. Mice immunized with the gB plasmid predominantly produced IgG2a gB-specific antibody, while the gB680 plasmid raised mostly IgG1 anti-gB antibody. Mice immunized with the pp65 plasmid developed pp65-specific cytotoxic T lymphocytes (CTL) and ELISA antibodies. Immunization with a mixture of both gB and pp65 plasmids raised antibodies to both proteins and pp65-specific CTL, indicating a lack of interference between these two plasmids. These results suggest that DNA immunization is a useful approach for vaccination against HCMV disease.

Expression, purification, and characterization of a soluble form of human cytomegalovirus glycoprotein B

The human cytomegalovirus glycoprotein B gene (gB; gpUL55) was truncated at amino acid 692 and recombined into Autographa californica nuclear polyhedrosis virus (baculovirus). Infection of insect cells with the recombinant baculovirus resulted in high-level expression and secretion of the truncated gB protein (gB-S) into the culture medium. Purification of gB-S by monoclonal antibody affinity chromatography yielded a protein of ca. 200 kDa. Characterization of the 200-kDa purification product indicated that the recombinant gB protein retained many structural and functional features of the viral gB. Comparison of electrophoretic migration patterns in reduced versus nonreduced protein samples and immune blotting analysis with antibodies specific for the amino or carboxy-terminus of gB demonstrated that the recombinant protein was composed of disulfide linked 69 kDa amino terminal and 35-kDa carboxy-terminal fragments. In addition, recognition of the 200-kDa gB-S by a conformational-dependent, oligomer-specific monoclonal antibody suggested that gB-S was properly folded and dimeric. Like the viral gB, gB-S had heparin binding ability. One heparin binding site was found to reside within the 35-kDa carboxy-terminal fragment (aa 492-692). Heparin binding was abolished when gB-S was denatured. These data suggest that gB contains a novel heparin binding motif that is at least partially conformational dependent.

Lobucavir is phosphorylated in human cytomegalovirus-infected and -uninfected cells and inhibits the viral DNA polymerase

Lobucavir (LBV) is a deoxyguanine nucleoside analog with broad-spectrum antiviral activity. LBV was previously shown to inhibit herpes simplex virus (HSV) DNA polymerase after phosphorylation by the HSV thymidine kinase. Here we determined the mechanism of action of LBV against human cytomegalovirus (HCMV). LBV inhibited HCMV DNA synthesis to a degree comparable to that of ganciclovir (GCV), a drug known to target the viral DNA polymerase. The expression of late proteins and RNA, dependent on viral DNA synthesis, was also inhibited by LBV. Immediate-early and early HCMV gene expression was unaffected, suggesting that LBV acts temporally coincident with HCMV DNA synthesis and not through cytotoxicity. In vitro, the triphosphate of LBV was a potent inhibitor of HCMV DNA polymerase with a Ki of 5 nM. LBV was phosphorylated to its triphosphate form intracellularly in both infected and uninfected cells, with phosphorylated metabolite levels two- to threefold higher in infected cells. GCV-resistant HCMV isolates, with deficient GCV phosphorylation due to mutations in the UL97 protein kinase, remained sensitive to LBV. Overall, these results suggest that LBV-triphosphate halts HCMV DNA replication by inhibiting the viral DNA polymerase and that LBV phosphorylation can occur in the absence of viral factors including the UL97 protein kinase. Furthermore, LBV may be effective in the treatment of GCV-resistant HCMV.

The human cytomegalovirus UL55 (gB) and UL75 (gH) glycoprotein ligands initiate the rapid activation of Sp1 and NF-kappaB during infection

The cellular transcription factors Sp1 and NF-kappaB were upregulated shortly after the binding of purified live or UV-inactivated human cytomegalovirus (HCMV) to the cell surface. The rapid time frame of transcription factor induction is similar to that seen in other systems in which cellular factors are induced following receptor-ligand engagement. This similarity suggested that a cellular receptor-viral ligand interaction might be involved in Sp1 and NF-kappaB activation during the earliest stages of HCMV infection. To focus on the possible role viral ligands play in initiating cellular events following infection, we first used purified viral membrane extracts to demonstrate that constituents on the membrane are responsible for cellular activation. Additionally, these studies showed, through the use of neutralizing antibodies, that the viral membrane mediators of this activation are the major envelope glycoproteins gB (UL55) and gH (UL75). To confirm these results, neutralizing anti-gB and -gH antibodies were used to block the interactions of these glycoproteins on whole purified virus with their cell surface receptors. In so doing, we found that Sp1 and NF-kappaB induction was inhibited. Lastly, through the use of purified viral gB protein and an anti-idiotypic antibody that mimics the image of the viral gH protein, it was found that the engagement of individual viral ligands with their appropriate cell surface receptors was sufficient to activate cellular Sp1 and NF-kappaB. These results support our hypothesis that HCMV glycoproteins mediate an initial signal transduction pathway which leads to the upregulation of host cell transcription factors and suggests a model wherein the orderly sequence of virus-mediated changes in cellular activation initiates with viral binding via envelope glycoproteins to the cognate cellular receptor(s).

The human cytomegalovirus UL37 immediate-early regulatory protein is an integral membrane N-glycoprotein which traffics through the endoplasmic reticulum and Golgi apparatus

The human cytomegalovirus (HCMV) UL37 immediate-early gene is predicted to encode a type I membrane-bound glycoprotein, gpUL37. Following expression of the UL37 open reading frame in vitro, its signals for translocation and N-glycosylation were recognized by microsomal enzymes. Its orientation in the microsomes is that of a type I protein. gpUL37 produced in HCMV-infected human cells was selectively immunoprecipitated by rabbit polyvalent antiserum generated against the predicted unique domains of the UL37 open reading frame and migrated as an 83- to 85-kDa protein. Tunicamycin treatment, which inhibits N-glycosylation, increased the rate of migration of the UL37 protein to 68 kDa, verifying its modification by N-glycosylation in HCMV-infected cells. Consistent with this observation, gpUL37 was found to be resistant to digestion with either endoglycosidase F or H but sensitive to peptide N-glycosidase F digestion. These results suggested that gpUL37 is N-glycosylated and processed in both the endoplasmic reticulum (ER) and the Golgi apparatus. Direct demonstration of passage of gpUL37 through the ER and the Golgi was obtained by confocal microscopy. gpUL37 colocalized with protein disulfide isomerase, a protein resident in the ER, and with a Golgi protein. Subcellular fractionation of HCMV-infected cells demonstrated that gpUL37 is an integral membrane protein. Taken together, our results demonstrate that the HCMV gpUL37 immediate-early regulatory protein is a type I integral membrane N-glycoprotein which traffics through the ER and the Golgi network.

Increases in [Ca2+]i mediated by the 92.5-kDa putative cell membrane receptor for HCMV gp86

We determined that changes in intracellular Ca2+ concentration ([Ca2+]i) occur in human fibroblasts within the first hour of human cytomegalovirus (HCMV) infection when viral adsorption and fusion take place, and we investigated whether such changes also occur in response to monoclonal anti-idiotype antibodies (MAb2) that mimic HCMV gp86 and bind to a 92.5-kDa putative cell membrane receptor for gp86. Digitized image analysis of fura 2-loaded human embryonic lung fibroblasts indicated specific transient increases in [Ca2+]i beginning in some cells within the first 5 min of incubation with cross-linked MAb2 (70-750 nM, P < 0.01), which were similar in timing and intracellular distribution to those induced by HCMV. A primary source of Ca2+ appeared to be intracellular Ca2+ stores, since prior depletion of these stores with 30 nM thapsigargin inhibited the response (91.7 +/- 8.6%, P < 0.01); influx of Ca2+ from the extracellular medium was apparently necessary to maintain the intracellular Ca2+ stores. Transient increases in inositol trisphosphate (IP3) occurred in response to MAb2 (up to 3,329 +/- 84%, P < 0.001) or HCMV (92.8 +/- 19%, P < 0.01) during this same time period. These data suggest that the 92.5-kDa receptor for HCMV gp86 mediates an increase in IP3 and subsequent release of Ca2+ from intracellular stores.

Identification of framework residues required to restore antigen binding during reshaping of a monoclonal antibody against the glycoprotein gB of human cytomegalovirus

Introduction of the complementary determining regions (CDRs) from a murine antibody to a human monoclonal antibody is an important technique (humanization) in the development of human immunotherapeutics. We have humanized murine monoclonal antibody HCMV37 which binds to the gB envelope glycoprotein of human cytomegalovirus. Simple transfer of the murine HCMV37 CDRs into heavy- and light-chain framework regions (FRs) based on human NEW and REI, respectively, together with human IgGI and K constant regions, abolished antigen binding because of a suboptimal heavy chain. Replacement of human VH amino acids Leu70, Val71 and Arg94 with murine residues Thr70, Arg71 and Asn94 was insufficient to improve affinity. However, significant restoration of binding was obtained by substitution of human VH amino acids Thr28, Phe29, Ser30 with murine residues Ser28, Ile29, Thr30, in conjunction with the position 94 change. Residue 71, often regarded as critical for antigen binding, was not a major factor.

Characterization of immunoreactive octapeptides of human-cytomegalovirus gp58

We have mapped continuous epitopes, for positions 591-673 of the human cytomegalovirus 58-kDa glycoprotein using overlapping synthetic peptides and human sera. This region contains a fragment previously described as including the dominant site for induction of human-cytomegalovirus antibodies. Since the selected sequence is highly conserved among herpes viruses, we have considered the possible presence of antigenic cross-reactivity, particularly with the Epstein-Barr virus. Several peptides in the studied region were antigenic and two main continuous epitopes have been identified. Serological cross-reactions observed with Epstein-Barr virus are discussed, focusing on the possible implications of structural features and sequence similarity between human-cytomegalovirus and Epstein-Barr-virus glycoproteins.

An ELISA using recombinant proteins for the detection of neutralizing antibodies against human cytomegalovirus

Two prokaryotically expressed fusion proteins encompassing amino acids 484-650 (AD-1) and 27-100 (AD-2) of glycoprotein gp58/116 of human cytomegalovirus (HCMV) were purified from E. coli lysates and used in ELISA to determine antibody levels in human sera. The specificity of the test was established by comparison of 116 randomly selected sera with commercially available HCMV-ELISA tests. The recombinant polypeptides were then used for the analysis of antibody titers in 112 human sera and were compared to the capacity to neutralize HCMV. A strong correlation between the neutralization titer and antibody levels against AD-1 and a weaker correlation for AD-2 was observed. Of 29 sera with a high neutralization titer (> 1:128), 96% and 62% were positive for AD-1 and AD-2, respectively, while 44% and 19% were positive in sera with low neutralization titer (< 1:8). Serum pools prepared from human sera selected on the basis of recognition of the recombinant antigens had a 10-fold higher neutralization capacity than pools prepared from sera with a high titer in commercially available HCMV tests. A synchronous increase in neutralization capacity and titer against recombinant antigens was observed in transplant patients.

Antibody responses to human cytomegalovirus-specific polypeptides studied by immunoblotting in relation to viral load during cytomegalovirus infection

The humoral immune response to individual proteins of human cytomegalovirus (CMV) was studied by immunoblotting. CMV polypeptides present in an extract of CMV-infected fibroblasts in a late stage of infection were recognized by sera of healthy seropositive individuals and transplant recipients who suffered from a primary or secondary CMV infection. The results showed that the sera reacted with a maximum number of 18 polypeptides ranging in molecular weight from 28-235 kDa. In 60% or more of the healthy seropositives, polypeptides were recognized with an apparent molecular weight of 150, 98, 94, 58, 50, 44, 38, and 32 kDa. In the patient group, the most immunogenic polypeptides were those with an apparent molecular weight of 150, 104, 94, 66, 50, 38, and 32 kDa. A correlation was found between the antibody levels in sera from the healthy seropositives and the number of recognized polypeptides but no such relationship was seen in the transplant recipients. However, sera of patients with a high virus load during their secondary CMV infection, as detected by the CMV-antigenemia test, appeared to react less frequently and less intensely with the polypeptides than those with low viremia. A high number of antigen-positive leukocytes in the CMV-antigenemia test was related to a low frequency of polypeptides with molecular weights of 85, 76, 66, 44, 38, and 32 kDa recognized before transplantation.

The UL16 gene of human cytomegalovirus encodes a glycoprotein that is dispensable for growth in vitro

The UL16 gene of human cytomegalovirus (HCMV) encodes a predicted translation product with features characteristic of glycoproteins (signal and anchor sequences and eight potential N-linked glycosylation sites). Antisera were raised against the UL16 gene product expressed in Escherichia coli as a beta-galactosidase fusion protein. The antisera detected a 50-kDa glycoprotein in HCMV-infected cells that was absent from purified virions. The UL16 glycoprotein was synthesized at early times after infection and accumulated to the highest levels at late times after infection. A recombinant HCMV in which UL16 coding sequences were interrupted by a lacZ expression cassette was constructed by insertional mutagenesis. Analysis of the phenotype of the recombinant virus indicated that the UL16 gene product is nonessential for virus infectivity and growth in tissue culture.

Comparative sequence analysis of human cytomegalovirus strains

Three regions of DNA from five low-passage clinical isolates of human cytomegalovirus were amplified by polymerase chain reaction. The DNA sequences as well as the predicted amino acid sequences were compared with those of the laboratory strains AD169 and Towne. The genomic regions consisted of (i) three regions from the major glycoprotein (gp58/116, unique long [UL]55), (ii) three regions from the integral membrane protein (IMP, UL100), and (iii) a region from the major immediate-early 1 and 2 (IE-1/2) enhancer/promoter. Homologies ranged from 75.8 to 100.0% on the nucleotide level and from 47 to 100% on the amino acid level. The following two patterns were observed. (i) There are regions with a high degree of conservation with few scattered point mutations (mainly in the IE-1/2 enhancer/promoter and in the IMP gene). (ii) There are clusters of highly variable regions (parts of the gp58/116 gene and of the IMP gene). Within the areas of high variability, the strains could be classified into a limited number of subtypes.

High expression of human cytomegalovirus (HCMV)-gB protein in cells infected with a vaccinia-gB recombinant: the importance of the gB protein in HCMV immunity

The human cytomegalovirus (HCMV), Towne strain, glycoprotein B (gB) gene was cloned into a vaccinia vector (Copenhagen strain) under the control of the H6 early and late vaccinia promoters (Vac-gB recombinant). The gB protein was expressed in a high percentage of the Vac-gB-infected cells throughout the virus replication cycle. Cytosine-arabinoside (ara-C) did not influence the expression of the gB protein early after infection (5 h), but did inhibit it later in viral replication (7-29 h). The Vac-gB recombinant induced HCMV neutralizing antibodies in guinea-pigs. Cells infected with the Vac-gB recombinant absorbed 50-88% of neutralizing activity of human sera obtained from volunteers previously inoculated with the Towne or Toledo strains and from naturally seropositive individuals.

Analysis of human antibody responses to human cytomegalovirus envelope glycoproteins found in two families of disulfide linked glycoprotein complexes designated gC-I and gC-II

Human antibody responses to human cytomegalovirus (HCMV) envelope glycoproteins were analyzed using immunoaffinity purified glycoproteins and Western blotting. Two families of disulfide linked glycoprotein complexes, designated gC-I and gC-II, were isolated. These complexes were reduced and their individual glycoproteins separated by polyacrylamide gel electrophoresis prior to electroblotting. The reactivity of adult convalescent sera with individual glycoproteins was compared to that of sera from congenitally infected infants. All sera tested reacted with a 52,000 molecular weight glycoprotein from these complexes, but only 75% reacted with a 93,000 to 130,000 molecular weight glycoprotein from gC-I complexes. Most adult convalescent sera reacted with glycoproteins from gC-II complexes. However, 14 of 16 infant sera did not have high enough levels of gC-II antibodies to give a positive reaction with Western blotting. A longitudinal study was done with several infants and their mothers. These studies indicated a failure by the infants and their mothers to develop detectable levels of gC-II antibodies months to years after the initial infection or after repeated stimulation with HCMV due to persistent infection. The inability of these infants to develop significant levels of gC-II antibodies was not due to an inability to respond to viral glycoproteins since they had antibodies to gC-I glycoproteins. We also determined that the strains of HCMV infecting some of these infants expressed gC-II glycoproteins. Thus, the lack of response by these infants was not due to lack of expression of gC-II glycoproteins by their infecting strain.

Preparation of human monoclonal antibodies against a cytomegalovirus glycoprotein complex of 130 and 55 kDa

Nine human monoclonal antibodies (MAbs) with neutralization activity against cytomegalovirus (CMV) were obtained by screening human MAbs using a CMV glycoprotein complex of 130 and 55 kDa (gp130/55). The gp130/55 antigen was purified by immunoaffinity chromatography and the purified antigen used to detect anti-gp130/55 MAbs in an enzyme-linked immunosorbent assay. Relatively few of the human anti-CMV MAbs were directed against gp130/55 but all showed high neutralization activities to a variety of clinical isolates with titers (ED50 values) ranging from 0.15 to 7.9 micrograms/ml. Six of the nine anti-gp130/55 MAbs required complement for virus neutralization. Such human MAbs may prove to be useful for passive immunotherapy against CMV infection.

Sequence requirements for proteolytic processing of glycoprotein B of human cytomegalovirus strain Towne

Truncated versions of the human cytomegalovirus (CMV) strain Towne glycoprotein B (gB) gene were stably expressed in CHO cell lines. The calcium-specific ionophore A23187 inhibited proteolytic cleavage of C-terminal-truncated gB expressed by cell line 67.77. These inhibition studies also showed that the 93-kilodalton cleavage product most likely represents the N-terminal cleavage fragment of gB. The ionophore carboxyl cyanide m-chlorophenyl-hydrazone was used to show that proteolytic cleavage of gB did not occur in the endoplasmic reticulum. Two-dimensional polyacrylamide gel electrophoresis demonstrated that the N- and C-terminal cleavage products of gB remained associated by disulfide linkages after cleavage. Expression studies using constructs in which 80% or all of the N terminus was deleted demonstrated that the N terminus was required for secretion of the gB molecule. The amino acid sequence at the site of cleavage was shown to be critical for cleavage by a cellular protease. Our results indicate that an arginine-to-threonine change at either amino acid 457 or 460, a lysine-to-glutamine change at amino acid 459, or all three substitutions together block gB cleavage. The effect on proteolysis of the arginine-to-threonine amino acid change at residue 457 (position -4 relative to the cleavage site) demonstrated that a basic pair of amino acids at the endoproteolytic processing site is not the only requirement in cis for gB cleavage.

Isolated gA/gB glycoprotein complex of human cytomegalovirus envelope induces humoral and cellular immune-responses in human volunteers

Three human cytomegalovirus (HCMV) seronegative individuals were immunized with a single dose of HCMV envelope; two individuals developed neutralizing antibodies. Two naturally HCMV seropositive and three HCMV seronegative human volunteers were immunized with a major glycoprotein complex, gA/gB, of HCMV that had been purified by immunoadsorbent column chromatography. After a single injection of the gA/gB preparation, the naturally seropositive individuals developed higher titres of neutralizing antibodies and temporarily higher HCMV-specific lymphocyte proliferation (HCMV-LP) responses in vitro. The seronegative individuals developed neutralizing antibodies after the third injection of gA/gB, which were present only transiently, but showed a rapid reappearance and increase in titre after the fourth injection. At 1 year after the first injection, the neutralizing antibody titres were still comparable with those of the naturally seropositive individuals. HCMV-LP responses to HCMV in the initially seronegative individuals developed after the second or third injection with the gA/gB preparation and remained positive during the 1-year observation period. These results show that the gA/gB protein induces both humoral and cellular immune responses in humans, and might serve as the basis of a subunit vaccine.

Identification of cell surface receptors for the 86-kilodalton glycoprotein of human cytomegalovirus

Cell surface receptors for the 86-kDa glycoprotein (gp86) of human cytomegalovirus (HCMV) were identified by using two monoclonal anti-idiotype antibodies that bear the internal image of gp86. These antibodies bound to cells permissive for HCMV infection by both ELISA and immunofluorescence assay and inhibited HCMV plaque formation in human embryonic lung (HEL) cells. Immunoblot analysis showed specific binding of both internal image anti-idiotype antibodies as well as gp86 to an HEL cell membrane protein with an approximate molecular mass of 92.5 kDa. In addition, immunoprecipitation of radiolabeled membrane and cell surface proteins from human foreskin tissue, human foreskin fibroblasts, or HEL cells showed specific binding of anti-idiotype antibody predominantly to the 92.5-kDa protein.

Complement-independent neutralising monoclonal antibody with differential reactivity for strains of human cytomegalovirus

A mouse monoclonal antibody with complement-independent neutralising activity against cytomegalovirus (CMV) and reactive with the 86 kilodalton (kDa) viral glycoprotein H is described. Neutralisation tests against a range of different strains of CMV showed significant crossreactivity, but clear differences were evident between the two prototype viruses AD169 and Davis, and particularly between AD169 and several low-passage recent clinical isolates; CMV present in urine was neutralised weakly if at all.

Identification and expression of a human cytomegalovirus early glycoprotein

A human cytomegalovirus early gene which possesses three temporally regulated promoters is located in the large unique component of the viral genome between 0.054 and 0.064 map units (C.-P. Chang, C.L. Malone, and M.F. Stinski, J. Virol. 63:281-290, 1989). This gene contains a major open reading frame (ORF) located 233 bases downstream of the cap site of an early unspliced RNA. The major ORF predicts a polypeptide of 17 kilodaltons (kDa) which contains a glycoproteinlike signal and anchor domains as well as potential N-glycosylation sites. Antisera were prepared against synthetic peptides derived from amino acid sequences within the major ORF. The antisera detected a viral glycoprotein of 48 kDa in infected cells and recognized the in vitro-translated 17-kDa protein early-gene product. The viral glycoprotein, designated gp48, was modified by N-linked glycans and possibly O-linked glycans. The synthesis of gp48 occurred in the absence of viral DNA replication but accumulated to the highest levels at late times after infection. Since gp48 was found in the virion, it is considered an early structural glycoprotein.

Identification of a neutralizing epitope on glycoprotein gp58 of human cytomegalovirus

Human cytomegalovirus contains an envelope glycoprotein of 58 kilodaltons (gp58). The protein, which is derived from a glycosylated precursor molecule of 160 kilodaltons via proteolytic cleavage, is capable of inducing neutralizing antibodies. We have mapped the epitopes recognized by the neutralizing monoclonal antibody 7-17 and a second antibody (27-287) which is not neutralizing. Overlapping fragments of the carboxy-terminal part of the open reading frame coding for gp58 were expressed in Escherichia coli as beta-galactosidase fusion proteins. The reactivities of antibodies 7-17 and 27-287 were determined by Western blot (immunoblot) analysis. Both antibodies recognized sequences between amino acids 608 and 625 of the primary gp58 translation product. The antibodies almost completely inhibited one another in a competitive binding assay with intact virus as antigen. Moreover, antibody 27-287 was able to inhibit the complement-independent neutralizing activity of antibody 7-17.

The human cytomegalovirus strain Towne glycoprotein H gene encodes glycoprotein p86

The gene encoding the glycoprotein H (gH) homologue of CMV strain Towne was cloned, sequenced, and expressed. The predicted 742 amino acid gH protein had characteristics typical of a membrane glycoprotein including hydrophobic signal and transmembrane domains and six possible N-linked glycosylation sites. The CMV (Towne) gH gene had a 95% nucleotide identity and a 96.6% amino acid identity with the CMV (AD169) gH gene, as described by M. P. Cranage, G. L. Smith, S. E. Bell, H. Hart, C. Brown, A. T. Bankier, P. Tomlinson, B. G. Barrell, and T. C. Minson (1988, J. Virol. 62, 1416-1422). Transcriptional analysis of the gH gene revealed that the 2.9-kilobase (kb) gH transcript was not detected until late after CMV infection, indicating that the kinetics of gH expression were typical of the late class of CMV genes. The gH gene was expressed in COS cells using a vector in which transcription was driven by the SV40 early promoter. The expression of gH was detected by immunofluorescence using the virus neutralizing murine monoclonal antibody 1G6, which is specific for an 86-kilodalton (kDa) CMV virion membrane protein (p86). Amino acid sequence analysis of p86 tryptic peptides revealed sequence identity with peptides from the deduced gH amino acid sequence, confirming that the gH gene encodes p86. These results indicate that CMV gH can induce virus neutralizing antibodies and establishes gH as a candidate antigen for a subunit vaccine against CMV.

Characterization of membrane antigens on human cytomegalovirus-infected fibroblasts recognized by human antibodies

The antigens on the surface of human cytomegalovirus (HCMV)-infected fibroblasts which are recognized by human HCMV antibody-positive sera were characterized. Three HCMV-induced polypeptides, with apparent molecular masses of 53 to 63, 94, and 94 to 120 kilodaltons, were precipitated from 125I-surface-labeled cell extracts with different sera obtained from healthy individuals. Renal transplant recipients who were suffering from active HCMV infections recognized the same set of antigens. By the use of monoclonal antibodies, these antigens were identified as polypeptides belonging to the gcI and gcIII families of HCMV glycoproteins.

Processing of the gp55-116 envelope glycoprotein complex (gB) of human cytomegalovirus

The processing pathway of the major envelope glycoprotein complex, gp55-116 (gB), of human cytomegalovirus was studied using inhibitors of glycosylation and endoglycosidases. The results of these studies indicated that the mature gp55-116 is synthesized by the addition of both simple and complex N-linked sugars to a nonglycosylated precursor of estimated Mr 105,000. In a rapid processing step, the Mr 105,000 precursor is glycosylated to a protein of Mr 150,000 (gp150) which contains only endoglycosidase H-sensitive sugar linkages. The gp150 is then processed relatively slowly to a Mr 165,000 to 170,000 species (gp165-170), which is then cleaved to yield the mature gp55-116. Monensin prevented the final processing steps of the gp150, including cleavage, suggesting that transport through the Golgi apparatus is required for complete processing. Digestion of the intracellular forms of this complex as well as the virion forms confirmed the above findings and indicated that the mature virion form of gp55 contains 8,000 daltons of N-linked sugars. The virion gp116 contains some 52,000 to 57,000 daltons of N-linked carbohydrates and approximately 5,000 daltons of O-linked sugars.

Human cytomegalovirus strain Towne glycoprotein B is processed by proteolytic cleavage

The gene encoding glycoprotein B of human cytomegalovirus (CMV) strain Towne was cloned, sequenced, and expressed in order to study potential targets for viral neutralization. Secondary structure analysis of the 907 amino acid protein predicted a 24 amino acid N-terminal signal sequence and a potential transmembrane region composed of two domains, 34 and 21 amino acids. The CMV (Towne) gB gene had a 94% nucleotide similarity and a 95% amino acid similarity to the CMV (AD169) gB gene [as described by M.P. Cranage et al. (1986, EMBO J. 5, 3057-3063)]. Transcriptional analysis of the CMV (Towne) gB coding strand revealed that the gB message (3.9 kb), was transcribed from this region as early as 4 hr postinfection, and well in advance of gB protein synthesis. Full-length and truncated versions of the gB gene were expressed in COS cells using expression vectors where transcription was driven by the SV40 early promoter or the CMV major immediate early promoter. Expression was detected by immunofluorescence and ELISA using the virus neutralizing murine monoclonal antibody 15D8 (L. Rasmussen, J. Mullenax, R. Nelson, and T.C. Merigan, 1985, J. Virol. 55, 274-280). This antibody had been shown previously to recognize a 55-kDa CMV virion protein and a related 130-kDa intracellular precursor. Amino acid sequence analysis of the N-terminus of the 55-kDa viral glycoprotein (gp55) showed that gp55 is derived from gB (gp130) by proteolytic cleavage and represents the C-terminal region of gp130. The truncated version of gB expressed in COS and CHO cells was also processed by proteolytic cleavage as demonstrated by Western blotting. Our study localizes the epitope recognized by 15D8 to within a 186 amino acid fragment of the gp55 protein. These results indicate that CMV gB is a target for neutralization and establishes gp55 as a candidate component for use in a subunit vaccine.

Loss of cytomegalovirus infectivity after treatment with castanospermine or related plant alkaloids correlates with aberrant glycoprotein synthesis

Many plants contain polyhydroxyalkaloids which are potent inhibitors of glucosidases, enzymes involved in oligosaccharide trimming. These are important in determining the final configuration of specific glycoproteins. Human cytomegalovirus (CMV) encodes a number of glycoproteins, some of which ultimately reside in the outer envelope of the mature virion and are important for virus infectivity. Treatment with three polyhydroxyalkaloids, castanospermine (CAST), deoxynojirimycin (DNJ) and 2R,5R-dihydroxymethyl-3R,4R-dihydroxypyrrolidine (DMDP) blocked the growth of infectious virus, as determined by yield reduction and plaque reduction assays. However, in the presence of CAST, CMV infected cells continued to shed virions into the extracellular medium, as determined by electron microscopy. Envelope glycoproteins of virions produced after treatment with CAST (2.5 mM) were immunoprecipitated with a monoclonal antibody (F5) specific for the gcI family of glycoproteins. Analysis by PAGE-SDS showed an absence of gcI complex 2 (gp52 disulphide-linked to gp130) with a proportional increase in gcI complex 1 (gp52 disulphide-linked to gp95). The results indicated that gp130 alone, or linked to gp52, was important for CMV infectivity. As well as being potential targets for antiviral agents against CMV, inhibitors of glycoprotein trimming reactions may define components of the virion surface important for infectivity.

Induction of complement-dependent and -independent neutralizing antibodies by recombinant-derived human cytomegalovirus gp55-116 (gB)

The human cytomegalovirus (HCMV) envelope glycoprotein complex gp55-116 was expressed in both Escherichia coli and cells infected with a recombinant vaccinia virus. E. coli produced a single protein of Mr 100,000 which approximated the size of the nonglycosylated gp55-116 precursor found in HCMV-infected cells. Cells infected with the recombinant vaccinia virus contained three intracellular forms of Mr 160,000, 150,000, and 55,000 which were detected by a monoclonal antibody reactive with gp55. Comparison of the immunological properties of these recombinant proteins indicated that several of the HCMV gp55-116 monoclonal antibodies and sera from patients infected with HCMV reacted with the vaccinia virus-derived proteins whereas a more restricted group of monoclonal antibodies recognized the E. coli-produced protein. Immunization of mice with either E. coli or vaccinia virus recombinant HCMV gp55-116 resulted in production of virus-neutralizing antibodies. In contrast to the almost exclusive production of complement-dependent neutralizing antibodies following immunization with recombinant vaccinia virus, the E. coli-derived protein induced complement-independent neutralizing antibodies.

Isolation and characterization of a human cytomegalovirus glycoprotein containing a high content of O-linked oligosaccharides

Several disulfide linked glycoprotein complexes were extracted from human cytomegalovirus with a non-ionic detergent and separated by anion exchange high performance liquid chromatography (HPLC). One complex had a molecular weight of 93,000 and was classified as gCII-93. Another complex had a molecular weight greater than 200,000 and was classified as gCII-200. Both complexes immunoprecipitated with a monoclonal antibody (9E10). A third set of complexes (classified as gC-I) immunoprecipitated with another monoclonal antibody (41C2). Isolated complexes were reduced, alkylated, and individual glycoproteins separated by gel-filtration HPLC. Glycoproteins with molecular weights of 50-52,000 from gCII-93 and gCII-200 appeared to be the same glycoprotein since they could be immunoprecipitated by 9 E 10 and had identical peptide maps. The amino sugar content of these glycoproteins was compared to that of higher molecular weight glycoproteins obtained from gCII-200 and to a glycoprotein with a molecular weight of 93,000 (gp93 (I] from gCI. Glycoproteins with molecular weights of 50-52,000 from gCII-93 and gCII-200 contained similar amounts of galactosamine (GalN), glucosamine (GlcN) and sialic acid. However, they contained 2-3 times more GalN than any other glycoprotein from gCII-200 and 10 times more GalN than was detected in gp93 (I). All glycoproteins from gCII-93 or gCII-200 also contained more sialic acid when compared to gp93 (I). GalN in these glycoproteins was present in O-linked oligosaccharides. This was demonstrated by release of low molecular weight oligosaccharides from high molecular weight glycopeptides by mild base hydrolysis and the conversion of GalN to galactosaminitol. Thus, gp52(II) appears to have a unique phenotype marked by a high amount of O-linked oligosaccharides.