Murine monoclonal antibody to a single protein neutralizes the infectivity of human cytomegalovirus

New therapies for human cytomegalovirus infections

The recent approval of letermovir marks a new era of therapy for human cytomegalovirus (HCMV) infections, particularly for the prevention of HCMV disease in hematopoietic stem cell transplant recipients. For almost 30 years ganciclovir has been the therapy of choice for these infections and by today's standards this drug exhibits only modest antiviral activity that is often insufficient to completely suppress viral replication, and drives the selection of drug-resistant variants that continue to replicate and contribute to disease. While ganciclovir remains the therapy of choice, additional drugs that inhibit novel molecular targets, such as letermovir, will be required as highly effective combination therapies are developed not only for the treatment of immunocompromised hosts, but also for congenitally infected infants. Sustained efforts, largely in the biotech industry and academia, have identified additional highly active lead compounds that have progressed into clinical studies with varying levels of success and at least two have the potential to be approved in the near future. Some of the new drugs in the pipeline inhibit new molecular targets, remain effective against isolates that have developed resistance to existing therapies, and promise to augment existing therapeutic regimens. Here, we will describe some of the unique features of HCMV biology and discuss their effect on therapeutic needs. Existing drugs will also be discussed and some of the more promising candidates will be reviewed with an emphasis on those progressing through clinical studies. The in vitro and in vivo antiviral activity, spectrum of antiviral activity, and mechanism of action of new compounds will be reviewed to provide an update on potential new therapies for HCMV infections that have progressed significantly in recent years.

A derivative of platelet-derived growth factor receptor alpha binds to the trimer of human cytomegalovirus and inhibits entry into fibroblasts and endothelial cells

Human cytomegalovirus (HCMV) is a widely distributed herpesvirus that causes significant morbidity in immunocompromised hosts. Inhibitors of viral DNA replication are available, but adverse effects limit their use. Alternative antiviral strategies may include inhibition of entry. We show that soluble derivatives of the platelet-derived growth factor receptor alpha (PDGFR-alpha), a putative receptor of HCMV, can inhibit HCMV infection of various cell types. A PDGFR-alpha-Fc fusion protein binds to and neutralizes cell-free virus particles at an EC50 of 10–30 ng/ml. Treatment of particles reduced both attachment to and fusion with cells. In line with the latter, PDGFR-alpha-Fc was also effective when applied postattachment. A peptide scan of the extracellular domain of PDGFR-alpha identified a 40mer peptide that inhibits infection at an EC50 of 1–2 nmol/ml. Both, peptide and fusion protein, were effective against various HCMV strains and are hence promising candidates for the development of novel anti-HCMV therapies.

Human cytomegalovirus (HCMV) depends on expression of platelet-derived growth factor receptor alpha (PDGFR-alpha) for infection of fibroblasts whereas this cell surface protein is not required for infection of endothelial cells. Surprisingly, pretreatment of HCMV with a soluble derivative of PDGFR-alpha prevents infection of both cell types, most probably via specific binding to the trimeric gH/gL/pUL74 complex. While adsorption is inhibited in both cell types, an additional penetration block occurs only in fibroblasts. The finding that an essential molecular interaction of HCMV with fibroblasts can be subverted for inhibition of the virus provides an antiviral strategy that may be hard to circumvent by the virus.

Vectored co-delivery of human cytomegalovirus gH and gL proteins elicits potent complement-independent neutralizing antibodies

Human cytomegalovirus (hCMV) is prevalent worldwide with infection generally being asymptomatic. Nevertheless, hCMV infection can lead to significant morbidity and mortality. Primary infection of seronegative women or reactivation/re-infection of seropositive women during pregnancy can result in transmission to the fetus, leading to severe neurological defects. In addition, hCMV is the most common viral infection in immunosuppressed organ transplant recipients and can produce serious complications. Hence, a safe and effective vaccine to prevent hCMV infection is an unmet medical need. Neutralizing antibodies to several hCMV glycoproteins, and complexes thereof, have been identified in individuals following hCMV infection. Interestingly, a portion of the CMV-specific neutralizing antibody responses are directed to epitopes found on glycoprotein complexes but not the individual proteins. Using an alphavirus replicon particle (VRP) vaccine platform, we showed that bicistronic VRPs encoding hCMV gH and gL glycoproteins produce gH/gL complexes in vitro. Furthermore, mice vaccinated with these gH/gL-expressing VRPs produced broadly cross-reactive complement-independent neutralizing antibodies to hCMV. These neutralizing antibody responses were of higher titer than those elicited in mice vaccinated with monocistronic VRPs encoding gH or gL antigens, and they were substantially more potent than those raised by VRPs encoding gB. These findings underscore the utility of co-delivery of glycoprotein components such as gH and gL for eliciting potent, broadly neutralizing immune responses against hCMV, and indicate that the gH/gL complex represents a potential target for future hCMV vaccine development.

Recombinant human IgG antibodies against human cytomegalovirus

OBJECTIVE

To study the passive immunization with human monoclonal antibodies as for prophylaxis of human cytomegalovirus (HCMV) infection.

METHODS

Fab monoclonal antibodies to HCMV were recovered by repertoire cloning of mRNA from a HCMV infected individual. Antigen binding specificity, CDR sequence of V(H) and V(L) and neutralizing activity on HCMV AD169 stain were analyzed in vitro. The light and heavy chain Fd fragment genes of Fab antibodies were further cloned into a recombinant baculovirus expression vector pAC-kappa-Fc to express intact IgG. Secreted products were purified with affinity chromatography using protein G.

RESULTS

SDS-PAGE and Western blot confirmed the expression of the intact IgG. Immuno-blotting and -precipitation were used to identify HCMV proteins. One Fab monoclonal antibody recognized a conformational HCMV protein.

CONCLUSION

IgG antibodies can neutralize the HCMV AD169 strain efficiently at a titer of 2.5 microg/mL and may prove valuable for passive immunoprophylaxis against HCMV infection in humans.

Developmental regulation of human cytomegalovirus receptors in cytotrophoblasts correlates with distinct replication sites in the placenta

Cytomegalovirus (CMV), the major viral cause of congenital disease, infects the uterus and developing placenta and spreads to the fetus throughout gestation. Virus replicates in invasive cytotrophoblasts in the decidua, and maternal immunoglobulin G (IgG)-CMV virion complexes, which are transcytosed by the neonatal Fc receptor across syncytiotrophoblasts, infect underlying cytotrophoblasts in chorionic villi. Immunity is central to protection of the placenta-fetal unit: infection can occur when IgG has a low neutralizing titer. Here we used immunohistochemical and function-blocking methods to correlate infection in the placenta with expression of potential CMV receptors in situ and in vitro. In placental villi, syncytiotrophoblasts express the virion receptor epidermal growth factor receptor (EGFR) but lack integrin coreceptors, and virion uptake occurs without replication. Focal infection can occur when transcytosed virions reach EGFR-expressing cytotrophoblasts that selectively initiate expression of alphaV integrin. In cell columns, proximal cytotrophoblasts lack receptors and distal cells express integrins alpha1beta1 and alphaVbeta3, enabling virion attachment. In the decidua, invasive cytotrophoblasts expressing coreceptors upregulate EGFR, thereby dramatically increasing susceptibility to infection. Our findings indicate that virion interactions with cytotrophoblasts expressing receptors in the placenta (i) change as the cells differentiate and (ii) correlate with spatially distinct sites of CMV replication in maternal and fetal compartments.

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.

Human cytomegalovirus virion protein complex required for epithelial and endothelial cell tropism

Human cytomegalovirus replicates in many different cell types, including epithelial cells, endothelial cells, and fibroblasts. However, laboratory strains of the virus, many of which were developed as attenuated vaccine candidates by serial passage in fibroblasts, have lost the ability to infect epithelial and endothelial cells. Their growth is restricted primarily to fibroblasts, due to mutations in the UL131-UL128 locus. We now demonstrate that two products of this locus, pUL130 and pUL128, form a complex with gH and gL, but not gO. The AD169 laboratory strain, which lacks a functional UL131 protein, produces virions containing only the gH-gL-gO complex. An epithelial and endothelial cell tropic AD169 variant in which the UL131 ORF has been repaired, termed BADrUL131, produces virions that carry both gH-gL-gO and gH-gL-pUL128-pUL130 complexes. Antibodies against pUL130 and pUL128 block infection of epithelial and endothelial cells by BADrUL131 and the fusion-inducing factor X clinical human cytomegalovirus isolate but do not affect the efficiency with which fibroblasts are infected.

Characterization of human cytomegalovirus glycoprotein-induced cell-cell fusion

Human cytomegalovirus (CMV) infection is dependent on the functions of structural glycoproteins at multiple stages of the viral life cycle. These proteins mediate the initial attachment and fusion events that occur between the viral envelope and a host cell membrane, as well as virion-independent cell-cell spread of the infection. Here we have utilized a cell-based fusion assay to identify the fusogenic glycoproteins of CMV. To deliver the glycoprotein genes to various cell lines, we constructed recombinant retroviruses encoding gB, gH, gL, and gO. Cells expressing individual CMV glycoproteins did not form multinucleated syncytia. Conversely, cells expressing gH/gL showed pronounced syncytium formation, although expression of gH or gL alone had no effect. Anti-gH neutralizing antibodies prevented syncytium formation. Coexpression of gB and/or gO with gH/gL did not yield detectably increased numbers of syncytia. For verification, these results were recapitulated in several cell lines. Additionally, we found that fusion was cell line dependent, as nonimmortalized fibroblast strains did not fuse under any conditions. Thus, the CMV gH/gL complex has inherent fusogenic activity that can be measured in certain cell lines; however, fusion in fibroblast strains may involve a more complex mechanism involving additional viral and/or cellular factors.

Genetic polymorphisms among human cytomegalovirus (HCMV) wild-type strains

Human cytomegalovirus (HCMV) clinical isolates display genetic polymorphisms in multiple genes. Some authors have suggested that those polymorphisms may be implicated in HCMV-induced immunopathogenesis, as well as in strain-specific behaviours, such as tissue-tropism and ability to establish persistent or latent infections. This review summarises the features of the main clustered HCMV polymorphic open reading frames and also briefly cites other variable loci within the viral genome. The implications of gene polymorphisms are discussed in terms of potentially advantageous higher fitness obtained by the strain, but also taking into account that the published data are often speculative. The last section of this review summarises and critically analyses the main literature reports about the linkage of strain specific genotypes with clinical manifestations of HCMV disease in different patient populations affected by severe cytomegalovirus infections, namely immunocompromised subjects and congenitally infected newborns.

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.

Neutralising human recombinant antibodies to human cytomegalovirus glycoproteins gB and gH

A phage antibody display library of single chain fragment variable (scFv) was applied to develop anti-HCMV glycoprotein B (gB) and glycoprotein H (gH) neutralising libraries. To enrich for specific scFvs, the phage antibody was panned against cytomegalovirus epitopes derived from the N-terminal part of gB, the C-terminal part of gB and the N-terminal part of gH (NETIYNTTLKYGDV, VTSGSTKD and AASEALDPHAFHLLLNTYGR). A number of clones were differentiated by Bst N1 fingerprinting. After isolation of specific clones against each peptide, the neutralising effect of each clone was assessed by plaque reduction assay. This resulted in the isolation of eight neutralising scFv antibodies with 51-63% neutralising effects. Sequence analysis of three neutralising clones revealed the amino acids specificity changes in heavy and light chains of antibody molecules.

The genes encoding the gCIII complex of human cytomegalovirus exist in highly diverse combinations in clinical isolates

The UL74 (glycoprotein O [gO])-UL75 (gH)-UL115 (gL) complex of human cytomegalovirus (CMV), known as the gCIII complex, is likely to play an important role in the life cycle of the virus. The gH and gL proteins have been associated with biological activities, such as the induction of virus-neutralizing antibody, cell-virus fusion, and cell-to-cell spread of the virus. The sequences of the two gH gene variants, readily recognizable by restriction endonuclease polymorphism, are well conserved among clinical isolates, but nothing is known about the sequence variability of the gL and gO genes. Sequencing of the full-length gL and gO genes was performed with 22 to 39 clinical isolates, as well as with laboratory strains AD169, Towne, and Toledo, to determine phylogenetically based variants of the genes. The sequence information provided the basis for identifying gL and gO variants by restriction endonuclease polymorphism. The predicted gL amino acid sequences varied less than 2% among the isolates, but the variability of gO among the isolates approached 45%. The variants of the genes coding for gCIII in laboratory strains Towne, AD169, and Toledo were different from those in most clinical isolates. When clinical isolates from different patient populations with various degrees of symptomatic CMV disease were surveyed, the gO1 variant occurred almost exclusively with the gH1 variant. The gL2 variant occurred with a significantly lower frequency in the gH1 variant group. There were no configurations of the gCIII complex that were specifically associated with symptomatic CMV disease or human immunodeficiency virus serologic status. The potential for the gCIII complex to exist in diverse genetic combinations in clinical isolates points to a new aspect that must be considered in studies of the significance of CMV strain variability.

Distinct glycoprotein O complexes arise in a post-Golgi compartment of cytomegalovirus-infected cells

Human cytomegalovirus (CMV) glycoproteins H, L, and O (gH, gL, and gO, respectively) form a heterotrimeric disulfide-bonded complex that participates in the fusion of the viral envelope with the host cell membrane. During virus maturation, this complex undergoes a series of intracellular assembly and processing events which are not entirely defined (M. T. Huber and T. Compton, J. Virol. 73:3886-3892, 1999). Here, we demonstrate that gO does not undergo the same posttranslational processing in transfected cells as it does in infected cells. We further determined that gO is modified by O-linked glycosylation and that this terminally processed form is highly enriched in virions. However, during studies of gO processing, novel gO complexes were discovered in CMV virions. The newly identified gO complexes, including gO-gL heterodimers, were not readily detected in CMV-infected cells. Further characterization of the trafficking of gO through the secretory pathway of infected cells localized gH, gL, and gO primarily to the Golgi apparatus and trans-Golgi network, supporting the conclusion that the novel virion-associated gO complexes arise in a post-Golgi compartment of infected cells.

Strong CD8 T-cell responses following coimmunization with plasmids expressing the dominant pp89 and subdominant M84 antigens of murine cytomegalovirus correlate with long-term protection against subsequent viral challenge

We previously showed that intradermal immunization with plasmids expressing the murine cytomegalovirus (MCMV) protein IE1-pp89 or M84 protects against viral challenge and that coimmunization has a synergistic protective effect (C. S. Morello, L. D. Cranmer, and D. H. Spector, J. Virol. 74:3696-3708, 2000). Using an intracellular gamma interferon cytokine staining assay, we have now characterized the CD8+ T-cell response after DNA immunization with pp89, M84, or pp89 plus M84. The pp89- and M84-specific CD8+ T-cell responses peaked rapidly after three immunizations. DNA immunization and MCMV infection generated similar levels of pp89-specific CD8+ T cells. In contrast, a significantly higher level of M84-specific CD8+ T cells was elicited by DNA immunization than by MCMV infection. Fusion of ubiquitin to pp89 enhanced the CD8+ T-cell response only under conditions where vaccination was suboptimal. Three immunizations with either pp89, M84, or pp89 plus M84 DNA also provided significant protection against MCMV infection for at least 6 months, with the best protection produced by coimmunization. A substantial percentage of antigen-specific CD8+ T cells remained detectable, and they responded rapidly to the MCMV challenge. These results underscore the importance of considering antigens that do not appear to be highly immunogenic during infection as DNA vaccine candidates.

A role for human cytomegalovirus glycoprotein O (gO) in cell fusion and a new hypervariable locus

A cell fusion assay using fusion-from-without (FFWO) recombinant adenoviruses (RAds) and specific antibody showed a role in fusion modulation for glycoprotein gO, the recently identified third component of the gH/gL gCIII complex of human cytomegalovirus (HCMV). As in HCMV, RAd gO expressed multiple glycosylated species with a mature product of 125 kDa. Coexpression with gH/gL RAds showed gCIII reconstitution in the absence of other HCMV products and stabilisation by intermolecular disulfide bonds. Properties of HCMV clinical isolate, Pt, also implicated gO in cell spread. Compared to laboratory strain AD169, Pt was resistant to gH antibody plaque inhibition, but mature gH was identical. However, the gO sequences were highly divergent (20%), with further variation in laboratory strain Towne gO (34%). Thus, gO forms gCIII with gH/gL, performs in cell fusion, and is a newly identified HCMV hypervariable locus which may influence gCIII's function in mediating infection.

In vitro selection of novel RNA ligands that bind human cytomegalovirus and block viral infection

Ribonuclease-resistant RNA molecules that bind to infectious human cytomegalovirus (HCMV) were isolated in vitro from a pool of randomized sequences after 16 cycles of selection and amplification. The two ligands (L13 and L19) characterized exhibited high HCMV-binding affinity in vitro and effectively inhibited viral infection in tissue culture. Their antiviral activity was also specific as they only reacted with two different strains of HCMV but not with the related herpes simplex virus 1 and human cells. These two ligands appeared to function as antivirals by blocking viral entry. Ultraviolet (UV) crosslinking studies suggested that L13 and L19 bind to HCMV essential glycoproteins B and H, respectively. Thus, RNA ligands that bind to different surface antigens of HCMV can be simultaneously isolated by the selection procedure. Our study demonstrates the feasibility of using these RNA ligands as a research tool to identify viral proteins required for infectivity and as an antiviral agent to block viral infection.

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.

Intracellular formation and processing of the heterotrimeric gH-gL-gO (gCIII) glycoprotein envelope complex of human cytomegalovirus

The human cytomegalovirus (HCMV) gCIII complex contains glycoprotein H (gH; gpUL75), glycoprotein L (gL; gpUL115), and glycoprotein O (gO; gpUL74). To examine how gH, gL, and gO interact within HCMV-infected cells to assemble the tripartite complex, pulse-chase experiments were performed. These analyses demonstrated that gH and gL associate by the end of the pulse period to form a disulfide dependent gH-gL complex. Subsequently, the gH-gL complex interacts with a 100-kDa precursor form of gO to form a 220-kDa precursor of the mature gH-gL-gO complex that contains a 125-kDa form of gO. The 220-kDa precursor complex (pgCIII) was sensitive to treatment with endoglycosidase H (endo H), while the mature gCIII complex was essentially resistant to digestion with this enzyme, suggesting that formation of pgCIII complex occurs in the endoplasmic reticulum (ER) and is processed to mature gH-gL-gO (gCIII) in a post-ER compartment. While the N-linked glycans on the 100-kDa form of gO were modified to endo H-resistant states as the 125-kDa gO formed, additional posttranslational modifications were detected on gO. These processing alterations were non-N-linked oligosaccharide modifications that could not be accounted for by phosphorylation or by O-glycosylation of the type sensitive to O-glycanase. Of gH, gL, gO, and the various complexes that they form, only the mature form of the complex was detectable at the infected cell membrane, as judged by surface biotinylation studies.

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.

Strain-specific neutralization of human cytomegalovirus isolates by human sera

Induction of an effective antibody response against human cytomegalovirus (HCMV) is an important defense mechanism since it is potentially capable of neutralizing infectious viruses. We have analyzed the extent of HCMV strain-specific neutralization capacity in human sera. Nine recent HCMV isolates and their corresponding sera were investigated in cross-neutralization assays. We observed differences, independent of the overall neutralization capacity, in the 50% neutralization titers of the sera against individual strains, differences that ranged from 8-fold to more than 60-fold. For one isolate, complete resistance to neutralization by two human sera was observed. The neutralization capacity of human sera was not influenced by the presence of various concentrations (up to 100-fold excess) of noninfectious envelope glycoproteins, an inherent contamination of virus preparations from recent HCMV isolates. This indicated that the decisive parameter for neutralization is the titer of the neutralizing antibodies and that neutralization is largely independent of the concentration of virus. Analysis with transplant patients revealed that during primary infection strain-specific and strain-common antibodies are produced asynchronously. Thus, our data demonstrate that the induction of strain-specific neutralizing antibodies is a common event during infection with HCMV and that it might have important implications for the course of the infection and the development of anti-HCMV vaccines.

The human cytomegalovirus UL74 gene encodes the third component of the glycoprotein H-glycoprotein L-containing envelope complex

The human cytomegalovirus (HCMV) gCIII envelope complex is composed of glycoprotein H (gH; gpUL75), glycoprotein L (gL; gpUL115), and a third, 125-kDa protein not related to gH or gL (M. T. Huber and T. Compton, J. Virol. 71:5391-5398, 1997; L. Li, J. A. Nelson, and W. J. Britt, J. Virol. 71:3090-3097, 1997). Glycosidase digestion analysis demonstrated that the 125-kDa protein was a glycoprotein containing ca. 60 kDa of N-linked oligosaccharides on a peptide backbone of 65 kDa or less. Based on these biochemical characteristics, two HCMV open reading frames, UL74 and TRL/IRL12, were identified as candidate genes for the 125-kDa glycoprotein. To identify the gene encoding the 125-kDa glycoprotein, we purified the gCIII complex, separated the components by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and subjected gH and the 125-kDa glycoprotein to amino acid microsequence analysis. Microsequencing of an internal peptide derived from purified 125-kDa glycoprotein yielded the amino acid sequence LYVGPTK. A FASTA search revealed an exact match of this sequence to amino acids 188 to 195 of the predicted product of the candidate gene UL74, which we have designated glycoprotein O (gO). Anti-gO antibodies reacted in immunoblots with a protein species migrating at ca. 100 to 125 kDa in lysates of HCMV-infected cells and with 100- and 125-kDa protein species in purified virions. Anti-gO antibodies also immunoprecipitated the gCIII complex and recognized the 125-kDa glycoprotein component of the gCIII complex. Positional homologs of the UL74 gene were found in other betaherpesviruses, and comparisons of the predicted products of the UL74 homolog genes demonstrated a number of conserved biochemical features.

Identification and characterization of the guinea-pig cytomegalovirus glycoprotein H gene

Subunit vaccines which target viral envelope glycoproteins offer promise for the prevention of congenital cytomegalovirus (CMV) infection. The guinea pig model of CMV infection is uniquely well suited to testing vaccines for prevention of congenital infection, since, in contrast to other animal cytomegaloviruses, the guinea pig CMV (GPCMV) crosses the placenta, producing intrauterine infection. Antibody to the CMV glycoproteins B (gB) and H (gH) appears to be important in conferring protective immunity. Unfortunately, little is known about specific GPCMV envelope glycoproteins. Sequencing of GPCMV genome fragments was therefore undertaken to test whether GPCMV encodes a gH homologue. Partial sequencing of the Hind III A fragment of the GPCMV genome revealed an open reading frame of 2,169 nucleotides capable of encoding a protein of 723 amino acids. Computer matrix analyses demonstrated identity between this ORF and the gH coding sequences of other herpesviruses. The GPCMV gH ORF encodes 12 highly conserved cysteine residues, contains 9 potential N-linked glycosylation sites, and has a predicted M(r) of 81.6 kDa. Northern blot hybridizations with gH-specific probes identified an abundant 5.1 kb mRNA with expression kinetics of an "early" gene. A polyclonal antiserum raised against a synthetic peptide derived from the deduced amino acid sequence of the gH ORF identified a virion-associated protein with an approximate M(r) of 85-kDa, the putative GPCMV gH, in immunoblot assays.

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).

Characterization of a novel third member of the human cytomegalovirus glycoprotein H-glycoprotein L complex

A prerequisite for understanding the molecular function of the human cytomegalovirus (HCMV) gH (UL75)-gL (UL115) complex is a detailed knowledge of the structure of this complex in its functional form, as it is present in mature virions. The gH protein is known to be a component of a 240-kDa envelope complex designated as gCIII (D. R. Gretch, B. Kari, L. Rasmussen, R. C. Gehrz, and M. F. Stinski, J. Virol. 62:875-881, 1988). However, the exact composition of the gCIII complex remains unknown. In this report, we attempted reconstitution of the gCIII complex by coexpression of gH and gL in the baculovirus expression system. Formation of recombinant gH-gL complexes of approximately 115 kDa was demonstrated; however, no higher-molecular-mass (approximately 240-kDa) recombinant gH-gL complexes were detected, suggesting that the presence of gH and gL alone is not sufficient for reconstitution of the gCIII complex. To identify other mammalian and/or HCMV factors which may be necessary for gCIII formation, immunoprecipitates of gH and gL from HCMV-infected fibroblasts and purified HCMV virions were examined. This analysis did reveal a number of coprecipitating proteins which associate either transiently or integrally with gH and gL. One coprecipitating protein of 145 kDa was shown to be an integral component of gCIII, along with gH and gL. Characterization of the 145-kDa protein demonstrates that it is structurally and antigenically unrelated to gH and gL and that it appears to be virally encoded. Together, these data indicate that the 145-kDa protein is a third novel component of the mature HCMV gH-gL complex.

Glycoprotein H-related complexes of human cytomegalovirus: identification of a third protein in the gCIII complex

Previous studies have described three disulfide-bonded glycoprotein complexes within the envelope of human cytomegalovirus (HCMV). These have been designated gCI, gCII, and gCIII. Although gCI has been identified as homodimeric glycoprotein B (gB, gpUL55), the compositions of gCII and gCIII remain incompletely defined. Earlier studies suggested that gCIII was composed of glycoprotein H (gH, gpUL75) complexed with a second glycoprotein, the gL homolog of HCMV. We characterized the gCIII complex of HCMV using recombinant vaccinia virus-expressed gH and gL. Our results indicated that authentic gCIII was not reconstituted by coexpression of gH and gL. The presence of a third, structurally and antigenically unique glycoprotein with an estimated molecular mass of 125,000 Da in virion-derived gCIII complexes suggested that at least three proteins were necessary for formation of this envelope glycoprotein complex. This third glycoprotein, gp125, contained both simple and complex N-linked carbohydrates and had an estimated deglycosylated mass of 64,000 Da. Furthermore, we demonstrated that mature gH existed as both a covalently complexed and noncovalently associated component of the gCIII complex within the envelope of infectious extracellular virions. These findings provide further evidence for the structural complexity of the envelope of HCMV and emphasize the uncertainties associated with the previous assignment of specific functions to envelope proteins of HCMV.

Transplacentally acquired antiviral antibodies and outcome in congenital human cytomegalovirus infection

The association between transplacentally acquired maternal antibodies and outcome in congenital human cytomegalovirus (HCMV) infection was investigated by analyzing antiviral antibodies in the cord blood from infants with permanent neurologic sequelae and from those without sequelae. Higher levels of antiglycoprotein B and neutralizing antibodies were observed in infants with sequelae. Infants with symptomatic infection and those with > or = 2 sequelae had higher levels of virus binding antibodies. No association between neutralizing titers and progressive hearing loss was noted. These results suggested that the development of sequelae following congenital HCMV infection was not associated with measurable deficits in the maternal antiviral antibody response. Higher levels of anti-gB and neutralizing antibodies in infants with sequelae also suggested that the natural history of this congenital infection is unlikely to be modified by the passive administration of antiviral antibodies in the postnatal period.

Postoligomerization folding of human cytomegalovirus glycoprotein B: identification of folding intermediates and importance of disulfide bonding

Human cytomegalovirus glycoprotein B (gB or UL55) has been demonstrated to be a disulfide-linked homodimer within the envelope of mature virions. Previously, it has been shown that gB undergoes a rapid dimerization nearly coincident with its synthesis. Following dimerization, the molecule slowly folds into a form which can be transported from the endoplasmic reticulum. In this study we have examined the prolonged folding of gB by using a set of defined gB-reactive murine monoclonal antibodies and gB expressed as a recombinant protein in the absence of other human cytomegalovirus proteins. Our results have documented a folding pathway consistent with the relatively rapid dimerization of the translation product followed by delayed conversion into a fully folded molecule. Assembly of the dominant antigenic domain of gB, AD-1, preceded dimerization and folding of the molecule. The fully folded dimer was heat stable, but its conformation was altered by treatment with 2% sodium dodecyl sulfate (SDS), whereas an oligomeric folding intermediate was both heat and SDS stable. Postoligomerization disulfide bond formation could be demonstrated during folding of gB, suggesting that the formation of these covalent bonds could contribute to the prolonged folding of this glycoprotein.

Transfer of humoral immunity against cytomegalovirus proteins following transplantation of T-cell-depleted allogeneic bone marrow from seropositive donors

Previous work by Grob et al. [Lancet i: 774, 1987] has demonstrated that allogeneic, T-cell-depleted bone marrow transplant recipients have a better prognosis for reactivated cytomegalovirus (CMV) infection if their donor is also immune. It was proposed that adoptively transferred humoral immunity was responsible for the protective effect of active infection. Immunoblot analysis using purified virions was used here to examine pre- and posttransplant antibody responses of seropositive recipients who had undergone active viral infection after transplantation. Immunoblots were assessed for the numbers of polypeptides recognised and reactivity against individual polypeptides. Immunoblots were also scanned by quantitative densitometry, and the intensity of antibody responses against total viral protein and individual polypeptides was determined. Sera from recipients with immune donors exhibited a secondary-type immune response in terms of both intensity and polypeptide specific pattern of antibody reactivity, compared with those recipients with nonimmune donors. In particular, recipients with immune donors appeared to show a greater reactivity against a protein of M(r) 55,000; this may represent the envelope glycoprotein gB, which is a major target for neutralising antibodies, and might also be utilised for preparing an effective vaccine for CMV.

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.

Fine specificity of the human immune response to the major neutralization epitopes expressed on cytomegalovirus gp58/116 (gB), as determined with human monoclonal antibodies

The humoral immune response to human cytomegalovirus (CMV) membrane glycoprotein gp58/116 (gB) has been studied by establishing cell lines producing specific human monoclonal antibodies. These cell lines were generated from peripheral blood lymphocytes obtained from a healthy carrier. Hybridomas producing gp58/116-specific antibodies were detected by reactivity to procaryotically expressed proteins containing the major neutralizing epitopes of this glycoprotein complex. One antibody, ITC88, which recognized an epitope located between amino acid residues 67 and 86 of gp116, potently neutralized the virus at 1 to 2 micrograms of immunoglobulin G per ml. Only four of the six human antibodies detecting the major neutralizing domain of gp58 neutralized the virus, and none of them required complement for activity. All antibodies that bound mature, processed gp58 recognized a conformational epitope involving sequences between residues 549 and 635. However, small differences existed between the antibodies in the actual minimal requirement for C- and N-terminal parts of this epitope. By peptide mapping with several of the antibodies, the epitope was shown to consist mainly of residues between amino acids 570 to 579 and 606 to 619. Despite the conformational nature of the epitope, the antibodies recognized both reduced and denatured native antigen. Presence of carbohydrates was not required for antigen binding of these gp58-specific human antibodies, but in at least one case, it greatly enhanced antigen recognition, indicating an importance of carbohydrate structures in some epitopes within the major neutralizing specificity of gp58.

Neutralizing monoclonal antibodies that distinguish three antigenic sites on human cytomegalovirus glycoprotein H have conformationally distinct binding sites

Seven neutralizing murine monoclonal antibodies specific for the glycoprotein H of human cytomegalovirus were produced and used to construct a topological map of two nonoverlapping antigenic sites that are bridged by a third antigenic site. Neutralization assays with 15 laboratory or clinical human cytomegalovirus strains indicated that the monoclonal antibodies recognize three antigenically variable and three conserved epitopes within the three antigenic sites. The variable-domain genes encoding monoclonal antibodies representing each of the three antigenic sites were cloned and sequenced, and molecular models of their binding sites were generated. Conformational differences in the antibody-binding sites suggested a structural basis for experimentally observed differences in gH epitope recognition.

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.

Development of human monoclonal antibodies against human cytomegalovirus

Human monoclonal antibodies (HMAbs) against human cytomegalovirus (HCMV) have been developed by fusion of human spleen cells and human lymphoblastoid cell lines (NP101 and NP197). The cell line NP101 had great advantages in its high fusion frequency and the stability of the resultant hybridomas. The specificity of HMAbs was confirmed by enzyme-linked immunosorbent assay (ELISA) and immunofluorescence staining. Two of the six HMAbs obtained, which were IgG3 subclass, neutralized viral infectivity in the absence of complement. The neutralizing activity of one of these two HMAbs was enhanced in the presence of human complement, whereas the other was not. Another IgG1 subclass HMAb neutralized viral infection only in the presence of complement. The remaining three HMAbs showed no neutralizing activity. Those HMAbs may provide an important approach to studying human immune responses to HCMV. HMAbs having neutralizing activity may prove to be useful for passive immunotherapy of HCMV diseases.

The dominant linear neutralizing antibody-binding site of glycoprotein gp86 of human cytomegalovirus is strain specific

Bacterial fusion proteins, constructed from overlapping fragments of the open reading frame coding for gp86 of human cytomegalovirus (HCMV) strain AD169, were used to localize antigenic regions recognized by antibodies from human convalescent sera. A major domain for binding of conformation-independent antibodies was localized on fusion protein AP86, containing amino acids 15 to 142 of gp86. Human antibodies, affinity purified on AP86, neutralized infectious virus in tissue culture. In addition, a mouse monoclonal antibody (AP86-SA4), raised against AP86, also neutralized HCMV. AP86-SA4 was reactive with viral gp86 in immunoblot assays and showed a plasma membrane staining on intact HCMV-infected fibroblasts late in infection. After exonuclease III deletions of the viral gene, the binding site of neutralizing human as well as mouse antibodies was localized between amino acid residues 34 and 43. The domain has sequence variation between laboratory strains AD169 and Towne, and binding of the antibodies was strain specific. To our knowledge, this is the first characterization of a strain-specific neutralizing epitope on HCMV.

Construction and properties of a mutant of herpes simplex virus type 1 with glycoprotein H coding sequences deleted

A mutant of herpes simplex virus type 1 (HSV-1) in which glycoprotein H (gH) coding sequences were deleted and replaced by the Escherichia coli lacZ gene under the control of the human cytomegalovirus IE-1 gene promoter was constructed. The mutant was propagated in Vero cells which contained multiple copies of the HSV-1 gH gene under the control of the HSV-1 gD promoter and which therefore provide gH in trans following HSV-1 infection. Phenotypically gH-negative virions were obtained by a single growth cycle in Vero cells. These virions were noninfectious, as judged by plaque assay and by expression of beta-galactosidase following high-multiplicity infection, but partial recovery of infectivity was achieved by using the fusogenic agent polyethylene glycol. Adsorption of gH-negative virions to cells blocked the adsorption of superinfecting wild-type virus, a result in contrast to that obtained with gD-negative virions (D. C. Johnson and M. W. Ligas, J. Virol. 62:4605-4612, 1988). The simplest conclusion is that gH is required for membrane fusion but not for receptor binding, a conclusion consistent with the conservation of gH in all herpesviruses.

Anti-idiotype antibodies that mimic gp86 of human cytomegalovirus inhibit viral fusion but not attachment

Human cytomegalovirus (CMV) infects cells by sequential processes involving attachment, fusion with the cell membrane, and penetration of the capsid. We used two monoclonal anti-idiotype that mimic one of the CMV envelope glycoproteins, gp86, to study its role in the early phases of CMV infection. Neither of two such antibodies inhibited virus binding to human embryonic lung (HEL) fibroblasts; however, both antibodies inhibited the fusion of CMV with HEL cells, as measured by an assay in which viral envelope is labeled with a fluorescent amphiphile (octadecyl rhodamine B chloride, or R18), resulting in increased fluorescence during fusion of virus with the cell membrane. Because these anti-idiotype antibodies were shown previously to bind to specific receptors on HEL cell membranes, these findings suggest that both gp86 and its cell membrane receptor may function in the fusion of human CMV with HEL cells.

Characterization of human monoclonal antibodies directed against the pp65-kD matrix antigen of human cytomegalovirus

Human monoclonal antibodies specific for human cytomegalovirus (CMV) antigens have been established using peripheral blood lymphocytes from a seropositive donor. Immortalization of antigen-specific B cells was achieved by Epstein-Barr virus transformation followed by somatic cell fusion of antigen-specific lymphoblastoid cells. Four clones producing high-affinity antibodies (0.2-7 x 10(9) M-1) specific for the viral matrix protein pp65 have been further characterized with respect to epitope specificity of secreted antibodies. The studied antigen represents a major protein produced by in vitro-cultivated virus, and is important in the serodiagnosis of CMV infection. The human monoclonal antibodies recognized different epitopes, some of which proved to be overlapping. The fine specificity of these antibodies was evaluated using synthetic peptides covering the sequence of pp65. The antibody MO58 recognized a linear epitope (residues 283-288) whereas antibody MO53 recognized a discontinuous epitope involving residues 208-216 and 280-285. Despite the close proximity of these epitopes, the antibodies did not compete with each other for the same binding site on intact antigen.

Sequence characteristics of a gene in equine herpesvirus 1 homologous to glycoprotein H of herpes simplex virus

A gene in equine herpesvirus 1 (EHV-1, equine abortion virus) homologous to the glycoprotein H gene of herpes simplex virus (HSV) was identified and characterised by its nucleotide and derived amino acid sequence. The EHV-1 gH gene is located at 0.47-0.49 map units and contains an open reading frame capable of specifying a polypeptide of 848 amino acids, including N- and C-terminal hydrophobic domains consistent with signal and membrane anchor regions respectively, and 11 potential sites for N-glycosylation. Alignment of the amino acid sequence with those published for HSV gH, varicella zoster virus gpIII, Epstein Barr virus gp85 and human cytomegalovirus p86 shows similarity of the EHV gene with the 2 other alpha-herpesviruses over most of the polypeptide, but only the C-terminal half could be aligned for all 5 viruses. The identical positioning of 6 cysteine residues and a number of highly conserved amino acid motifs supports a common evolutionary origin of this gene and is consistent with its role as an essential glycoprotein of the herpesvirus family. An origin of replication is predicted to occur at approximately 300 nucleotides downstream of the EHV-1 gH coding region, on the basis of similarity to other herpesvirus origins.

Cell surface expression of human cytomegalovirus (HCMV) gp55-116 (gB): use of HCMV-recombinant vaccinia virus-infected cells in analysis of the human neutralizing antibody response

Cell surface expression of the human cytomegalovirus (HCMV) major envelope glycoprotein complex, gp55-116 (gB), was studied by using monoclonal antibodies and an HCMV gp55-116 (gB) recombinant vaccinia virus. HCMV-infected human fibroblasts and recombinant vaccinia virus-infected HeLa cells expresses three electrophoretically distinct proteins of Mr 170,000, 116,000, and 55,000 on their surface. These species have been previously identified within infected cells and purified virions. Two unique neutralizing epitopes were shown to be present on the cell surface gp55-116 (gB). Utilizing HeLa cells infected with the gp55-116 recombinant vaccinia virus as a specific immunosorbent, we have shown that approximately 40 to 70% of the total serum virus-neutralizing activity of a group of individuals with past HCMV infections was directed against this single envelope glycoprotein. The implications of this finding for vaccine development are discussed.

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.

Antigenic properties and cellular localization of herpes simplex virus glycoprotein H synthesized in a mammalian cell expression system

Herpes simplex virus type 1 glycoprotein H (HSV-1 gH) was synthesized in an inducible mammalian cell expression system, and its properties were examined. The gH coding sequence, together with the stable 5' untranslated leader sequence from xenopus beta-globin, was placed under control of the strong promoter from the human cytomegalovirus major immediate-early gene in an amplifiable plasmid which contains the simian virus 40 (SV40) virus origin for replication (ori). This expression vector was transfected into ts COS cells constitutively expressing a temperature-sensitive SV40 T antigen which allows utilization of the SV40 ori at permissive temperatures. The results of transient expression assays at the permissive temperature showed that HSV-1 gH could be synthesized in greater amounts than those produced by a high-multiplicity virus infection. The proteins produced were detected in Western blots (immunoblots) with a HSV-1 gH-specific polyclonal serum raised against a TrpE-gH fusion protein. The transfected gH had an apparent molecular weight of approximately 105,000, intermediate in size to those of the precursor (100,000) and fully processed forms (110,000) of HSV-1 gH from infections. Antigenicity was investigated by reactions with three virus-neutralizing monoclonal antibodies specific for conformational epitopes on gH. Only one of these monoclonal antibodies could immunoprecipitate the synthesized gH. However, equal recognition of the transfected gH was achieved by superinfection with virus. In addition, detectable amounts of gH were not expressed on the cell surface unless the cells were superinfected with virus. Studies with a temperature-sensitive mutant, ts1201, defective in encapsidation showed that the changes in antigenic structure and cell surface expression caused by superinfection with virus were not due simply to incorporation of gH into virions. These results suggest that gH requires additional virus gene products for cell surface localization and formation of an antigenic structure important for its function in mediating infectivity.