Molecular cloning of the guinea pig cytomegalovirus (GPCMV) genome as an infectious bacterial artificial chromosome (BAC) in Escherichia coli

Combined Treatment with Host-Directed and Anticytomegaloviral Kinase Inhibitors: Mechanisms, Synergisms and Drug Resistance Barriers

Despite the availability of currently approved antiviral drugs, infections with human cytomegalovirus (HCMV) still cause clinically challenging, sometimes life-threatening situations. There is an urgent need for enhanced anti-HCMV drugs that offer improved efficacy, reduced dosages and options for long-term treatment without risk of the development of viral drug resistance. Recently, we reported the pronounced anti-HCMV efficacy of pharmacological inhibitors of cyclin-dependent kinases (CDKs), in particular, the potential of utilizing drug synergies upon combination treatment with inhibitors of host CDKs and the viral CDK-like kinase pUL97 (vCDK/pUL97). Here, we expand this finding by further assessing the in vitro synergistic antiviral interaction between vCDK and CDK inhibitors towards HCMV as well as non-human cytomegaloviruses. An extension of this synergy approach was achieved in vivo by using the recombinant MCMV-UL97/mouse model, confirming the high potential of combination treatment with the clinically approved vCDK inhibitor maribavir (MBV) and the developmental CDK7 inhibitor LDC4297. Moreover, mechanistic aspects of this synergistic drug combination were illustrated on the levels of intracellular viral protein transport and viral genome replication. The analysis of viral drug resistance did not reveal resistance formation in the case of MBV + LDC4297 combination treatment. Spanning various investigational levels, these new results strongly support our concept, employing the great potential of anti-HCMV synergistic drug treatment.

Endothelial Cell Infection by Guinea Pig Cytomegalovirus Is a Lytic or Persistent Infection Depending on Tissue Origin but Requires Viral Pentamer Complex and pp65 Tegument Protein

The guinea pig is the only small animal model for congenital cytomegalovirus (CMV) but requires species-specific guinea pig cytomegalovirus (GPCMV). Infection of epithelial cells and trophoblasts by GPCMV requires the viral glycoprotein pentamer complex (PC) and endocytic entry because of the absence of platelet-derived growth factor receptor alpha (PDGFRA). Endothelial cells represent an important cell type for infection, dissemination in the host, and disease but have been poorly evaluated for GPCMV. Novel endothelial cell lines were established from animal vascular systems, including aorta (EndoC) and placental umbilical cord vein (GPUVEC). Cell lines were characterized for endothelial cell protein markers (PECAM1, vWF, and FLI1) and evaluated for GPCMV infection. Only PC-positive virus was capable of infecting endothelial cells. Individual knockout mutants for unique PC components (GP129, GP131, and GP133) were unable to infect endothelial cells without impacting fibroblast infection. Ectopic expression of PDGFRA in EndoC cells enabled GPCMV(PC^-) infection via direct cell entry independent of the PC. Neutralizing antibodies to the essential viral gB glycoprotein were insufficient to prevent endothelial cell infection, which also required antibodies to gH/gL and the PC. Endothelial cell infection was also dependent upon viral tegument pp65 protein (GP83) to counteract the IFI16/cGAS-STING innate immune pathway, similar to epithelial cell infection. GPCMV endothelial cells were lytically (EndoC) or persistently (GPUVEC) infected dependent on tissue origin. The ability to establish a persistent infection in the umbilical cord could potentially enable sustained and more significant infection of the fetus in utero. Overall, results demonstrate the importance of this translationally relevant model for CMV research. IMPORTANCE Congenital CMV is a leading cause of cognitive impairment and deafness in newborns, and a vaccine is a high priority. The only small animal model for congenital CMV is the guinea pig and guinea pig cytomegalovirus (GPCMV) encoding functional HCMV homolog viral glycoprotein complexes necessary for cell entry that are neutralizing-antibody vaccine targets. Endothelial cells are important in HCMV for human disease and viral dissemination. GPCMV endothelial cell infection requires the viral pentamer complex (PC), which further increases the importance of this complex as a vaccine target, as antibodies to the immunodominant and essential viral glycoprotein gB fail to prevent endothelial cell infection. GPCMV endothelial cell infection established either a fully lytic or a persistent infection, depending on tissue origin. The potential for persistent infection in the umbilical cord potentially enables sustained infection of the fetus in utero, likely increasing the severity of congenital disease.

A Fully Protective Congenital CMV Vaccine Requires Neutralizing Antibodies to Viral Pentamer and gB Glycoprotein Complexes but a pp65 T-Cell Response Is Not Necessary

A vaccine against congenital cytomegalovirus infection is a high priority. Guinea pig cytomegalovirus (GPCMV) is the only congenital CMV small animal model. GPCMV encodes essential glycoprotein complexes for virus entry (gB, gH/gL/gO, gM/gN) including a pentamer complex (gH/gL/GP129/GP131/GP133 or PC) for endocytic cell entry. The cohorts for protection against congenital CMV are poorly defined. Neutralizing antibodies to the viral glycoprotein complexes are potentially more important than an immunodominant T-cell response to the pp65 protein. In GPCMV, GP83 (pp65 homolog) is an evasion factor, and the GP83 mutant GPCMV has increased sensitivity to type I interferon. Although GP83 induces a cell-mediated response, a GP83-only-based vaccine strategy has limited efficacy. GPCMV attenuation via GP83 null deletion mutant in glycoprotein PC positive or negative virus was evaluated as live-attenuated vaccine strains (GP83dPC+/PC-). Vaccinated animals induced antibodies to viral glycoprotein complexes, and PC+ vaccinated animals had sterilizing immunity against wtGPCMV challenge. In a pre-conception vaccine (GP83dPC+) study, dams challenged mid-2nd trimester with wtGPCMV had complete protection against congenital CMV infection without detectable virus in pups. An unvaccinated control group had 80% pup transmission rate. Overall, gB and PC antibodies are key for protection against congenital CMV infection, but a response to pp65 is not strictly necessary.

Guinea pig cytomegalovirus protective T cell antigen GP83 is a functional pp65 homolog for innate immune evasion and pentamer dependent virus tropism

The guinea pig is the only small animal model for congenital CMV but requires species-specific guinea pig cytomegalovirus (GPCMV). Tegument protein GP83 is the presumed homolog of HCMV pp65 but gene duplication in the UL82-UL84 homolog locus in various animal CMV made it unclear if GP83 was a functional homolog. A GP83 null deletion mutant GPCMV (GP83dPC+) generated in the backdrop of glycoprotein pentamer complex (PC) positive virus, required for non-fibroblast infection, had normal growth kinetics on fibroblasts but was highly impaired on epithelial and trophoblast cells. GP83dPC+ virus was highly sensitive to IFN-I suggesting GP83 had an innate immune evasion function. GP83 interacted with cellular DNA sensors guinea pig IFI16 and cGAS indicating a role in the cGAS/STING pathway. Ectopically expressed GP83 in trophoblast cells restored GP83dPC+ virus growth. Additionally, mutant virus growth was restored in epithelial cells by expression of bovine viral diarrhea virus (BVDV) N^PRO protein targeting IRF3 as part of the cGAS/STING pathway or alternatively by expression of fibroblast cell receptor PDGFRA. HCMV pp65 is a T cell target antigen and a recombinant adenovirus encoding GP83 was evaluated as a vaccine. In GPCMV challenge studies, vaccinated animals had varying levels of protection against wild type virus with a protective response against 22122 prototype strain but little protection against a novel clinical strain of GPCMV (TAMYC), despite 100% identity in GP83 protein sequences. Overall, GP83 is a functional pp65 homolog with novel importance for epithelial cell infection but a GP83 T cell response provides limited vaccine efficacy.ImportanceCongenital CMV (cCMV) is a leading cause of cognitive impairment and deafness in newborns and a vaccine is a high priority. The guinea pig is the only small animal model for cCMV but requires guinea pig cytomegalovirus (GPCMV). The translational impact of GPCMV research is potentially reduced if the virus does not encode functional HCMV homolog proteins. This study demonstrates that tegument protein GP83 (pp65 homolog) is involved in innate immune evasion and highly important for infection of non-fibroblast cells via the viral glycoprotein pentamer complex (PC)-dependent endocytic entry pathway. The PC pathway is highly significant for virus dissemination and disease in the host, including cCMV. A GP83 candidate Ad-vaccine strategy in animals induced a cell-mediated response but failed to provide cross strain protection against a novel clinical strain of GPCMV. Results suggest that the pp65 antigen provides very limited efficacy as a stand-alone vaccine, especially in cross strain protection.

Repair of an Attenuated Low-Passage Murine Cytomegalovirus Bacterial Artificial Chromosome Identifies a Novel Spliced Gene Essential for Salivary Gland Tropism

The cloning of herpesviruses as bacterial artificial chromosomes (BACs) has revolutionized the study of herpesvirus biology, allowing rapid and precise manipulation of viral genomes. Several clinical strains of human cytomegalovirus (HCMV) have been cloned as BACs; however, no low-passage strains of murine CMV (MCMV), which provide a model mimicking these isolates, have been cloned. Here, the low-passage G4 strain of was BAC cloned. G4 carries an m157 gene that does not ligate the natural killer (NK) cell-activating receptor, Ly49H, meaning that unlike laboratory strains of MCMV, this virus replicates well in C57BL/6 mice. This BAC clone exhibited normal replication during acute infection in the spleen and liver but was attenuated for salivary gland tropism. Next-generation sequencing revealed a C-to-A mutation at nucleotide position 188422, located in the 3' untranslated region of sgg1, a spliced gene critical for salivary gland tropism. Repair of this mutation restored tropism for the salivary glands. Transcriptional analysis revealed a novel spliced gene within the sgg1 locus. This small open reading frame (ORF), sgg1.1, starts at the 3' end of the first exon of sgg1 and extends exon 2 of sgg1. This shorter spliced gene is prematurely terminated by the nonsense mutation at nt 188422. Sequence analysis of tissue culture-passaged virus demonstrated that sgg1.1 was stable, although other mutational hot spots were identified. The G4 BAC will allow in vivo studies in a broader range of mice, avoiding the strong NK cell responses seen in B6 mice with other MCMV BAC-derived MCMVs.IMPORTANCE Murine cytomegalovirus (MCMV) is widely used as a model of human CMV (HCMV) infection. However, this model relies on strains of MCMV that have been serially passaged in the laboratory for over four decades. These laboratory strains have been cloned as bacterial artificial chromosomes (BACs), which permits rapid and precise manipulation. Low-passage strains of MCMV add to the utility of the mouse model of HCMV infection but do not exist as cloned BACs. This study describes the first such low-passage MCMV BAC. This BAC-derived G4 was initially attenuated in vivo, with subsequent full genomic sequencing revealing a novel spliced transcript required for salivary gland tropism. These data suggest that MCMV, like HCMV, undergoes tissue culture adaptation that can limit in vivo growth and supports the use of BAC clones as a way of standardizing viral strains and minimizing interlaboratory strain variation.

Guinea pig cytomegalovirus trimer complex gH/gL/gO uses PDGFRA as universal receptor for cell fusion and entry

Species-specific guinea pig cytomegalovirus (GPCMV) causes congenital CMV and the virus encodes homolog glycoprotein complexes to human CMV, including gH-based trimer (gH/gL/gO) and pentamer-complex (PC). Platelet-derived growth factor receptor alpha (gpPDGFRA), only present on fibroblast cells, was identified via CRISPR as the putative receptor for PC-independent GPCMV infection. Immunoprecipitation assays demonstrated direct interaction of gH/gL/gO with gpPDGFRA but not in absence of gO. Expression of viral gB also resulted in precipitation of gB/gH/gL/gO/gpPDGFRA complex. Cell-cell fusion assays determined that expression of gpPDGFRA and gH/gL/gO in adjacent cells enabled cell fusion, which was not enhanced by gB. N-linked gpPDGFRA glycosylation inhibition had limited effect and blocking tyrosine kinase (TK) transduction had no impact on infection. Ectopically expressed gpPDGFRA or TK-domain mutant in trophoblast or epithelial cells previously non-susceptible to GPCMV(PC-) enabled viral infection. In contrast, transient human PDGFRA expression did not complement GPCMV(PC-) infection, a potential basis for viral species specificity.

Requirements for guinea pig cytomegalovirus tropism and antibody neutralization on placental amniotic sac cells

Congenital cytomegalovirus (cCMV) is a leading cause of birth defects. The guinea pig is the only small cCMV animal model. Guinea pig cytomegalovirus (GPCMV) encodes similar glycoprotein complexes to human CMV (HCMV) including gB and the gH-based pentamer complex (PC). In HCMV, both gB and PC are neutralizing antibody antigens. The relevance of GPCMV PC for virus tropism and vaccine target remains controversial. A novel guinea pig placental amniotic sac epithelial (GPASE) cell-line did not express viral cell receptor platelet derived growth factor receptor alpha (PDGFRA) and resulted in requirement for the PC for GPCMV infection unless PDGFRA was ectopically expressed. High titer anti-gB sera from a GPCMV gB vaccine study was evaluated for GPCMV neutralizing capability on GPASE cells in comparison to convalescent sera from GPCMV(PC+) or GPCMV(PC-) infected animals. Anti-gB sera neutralized fibroblast infection but was less effective compared to anti-GPCMV(PC-), which had antibodies to gH/gL. However, both anti-GPCMV(PC-) and anti-gB sera similarly had reduced neutralizing capability on GPASE and renal epithelial cells in comparison to anti-GPCMV(PC+) sera, which had additional antibodies to PC. Overall, results demonstrate the importance of the PC for GPCMV tropism to various cell types that lack PDGFRA expression and the limited ability of anti-gB sera to neutralize GPCMV on non-fibroblast cells despite the essential nature of gB glycoprotein.

Inclusion of the Viral Pentamer Complex in a Vaccine Design Greatly Improves Protection against Congenital Cytomegalovirus in the Guinea Pig Model

A vaccine against congenital cytomegalovirus (cCMV) is a high priority. The guinea pig is a small-animal model for cCMV. A disabled infectious single-cycle (DISC) viral vaccine strain based on a guinea pig cytomegalovirus (GPCMV) capsid mutant was evaluated. A previous version of this vaccine did not express the gH/gL-based pentamer complex (PC) and failed to fully protect against cCMV. The PC is necessary for GPCMV epithelial cell/trophoblast tropism and congenital infection and is a potentially important neutralizing antigen. Here, we show that a second-generation PC-positive (PC⁺) DISC (DISCII) vaccine induces neutralizing antibodies to the PC and other glycoproteins and a cell-mediated response to pp65 (GP83). Additionally, a CRISPR/Cas9 strategy identified guinea pig platelet-derived growth factor receptor alpha (PDGFRA) to be the receptor for PC-independent infection of fibroblast cells. Importantly, PDGFRA was absent in epithelial and trophoblast cells, which were dependent upon the viral PC for infection. Virus neutralization by DISCII antibodies on epithelial and trophoblast cells was similar to that in sera from wild-type virus-infected animals and dependent in part on PC-specific antibodies. In contrast, sera from PC-negative virus-infected animals poorly neutralized virus on non-fibroblast cells. DISCII-vaccinated animals were protected against congenital infection, in contrast to a nonvaccinated group. The target organs of pups in the vaccine group were negative for wild-type virus, unlike those of pups in the control group, with GPCMV transmission being approximately 80%. Overall, the DISCII vaccine had 97% efficacy against cCMV. The complete protection provided by this PC⁺ DISC vaccine makes the possibility of the use of this approach against human cCMV attractive.IMPORTANCE Cytomegalovirus (CMV) is a leading cause of congenital disease in newborns, and an effective vaccine remains an elusive goal. The guinea pig is the only small-animal model for cCMV. Guinea pig cytomegalovirus (GPCMV) encodes a glycoprotein pentamer complex (PC) for entry into non-fibroblast cells, including placental trophoblasts, to enable cCMV. As with human cytomegalovirus (HCMV), GPCMV uses a specific cell receptor (PDGFRA) for fibroblast entry, but other receptors are required for non-fibroblast cells. A disabled infectious single-cycle (DISC) GPCMV vaccine strain induced an antibody immune response to the viral pentamer to enhance virus neutralization on non-fibroblast cells, and vaccinated animals were fully protected against cCMV. Inclusion of the PC as part of a vaccine design dramatically improved vaccine efficacy, and this finding underlines the importance of the immune response to the PC in contributing toward protection against cCMV. This vaccine represents an important milestone in the development of a vaccine against cCMV.

A fifty-year odyssey: prospects for a cytomegalovirus vaccine in transplant and congenital infection

INTRODUCTION

It has been almost fifty years since the Towne strain was used by Plotkin and collaborators as the first vaccine candidate for cytomegalovirus (CMV). While that approach showed partial efficacy, there have been a multitude of challenges to improve on the promise of a CMV vaccine. Efforts have been dichotomized into a therapeutic vaccine for patients with CMV-infected allografts, either stem cells or solid organ, and a prophylactic vaccine for congenital infection.

AREAS COVERED

This review will evaluate research prospects for a therapeutic vaccine for transplant recipients that recognizes CMV utilizing primarily T cell responses. Similarly, we will provide an extensive discussion on attempts to develop a vaccine to prevent the manifestations of congenital infection, based on eliciting a humoral anti-CMV protective response. The review will also describe newer developments that have upended the efforts toward such a vaccine through the discovery of a second pathway of CMV infection that utilizes an alternative receptor for entry using a series of antigens that have been determined to be important for prevention of infection.

EXPERT COMMENTARY

There is a concerted effort to unify separate therapeutic and prophylactic vaccine strategies into a single delivery agent that would be effective for both transplant-related and congenital infection.

Cytomegalovirus UL128 homolog mutants that form a pentameric complex produce virus with impaired epithelial and trophoblast cell tropism and altered pathogenicity in the guinea pig

Guinea pig cytomegalovirus (GPCMV) encodes a homolog pentameric complex (PC) for specific cell tropism and congenital infection. In human cytomegalovirus, the PC is an important antibody neutralizing target and GPCMV studies will aid in the development of intervention strategies. Deletion mutants of the C-terminal domains of unique PC proteins (UL128, UL130 and UL131 homologs) were unable to form a PC in separate transient expression assays. Minor modifications to the UL128 homolog (GP129) C-terminal domain enabled PC formation but viruses encoding these mutants had altered tropism to renal and placental trophoblast cells. Mutation of the presumptive CC chemokine motif encoded by GP129 was investigated by alanine substitution of the CC motif (codons 26-27) and cysteines (codons 47 and 62). GP129 chemokine mutants formed PC but GP129 chemokine mutant viruses had reduced epitropism. A GP129 chemokine mutant virus pathogenicity study demonstrated reduced viral load to target organs but highly extended viremia.

The essential role of guinea pig cytomegalovirus (GPCMV) IE1 and IE2 homologs in viral replication and IE1-mediated ND10 targeting

Guinea pig cytomegalovirus (GPCMV) immediate early proteins, IE1 and IE2, demonstrated structural and functional homologies with human cytomegalovirus (HCMV). GPCMV IE1 and IE2 co-localized in the nucleus with each other, the viral polymerase and guinea pig ND10 components (gpPML, gpDaxx, gpSp100, gpATRX). IE1 showed direct interaction with ND10 components by immunoprecipitation unlike IE2. Additionally, IE1 protein disrupted ND10 bodies. IE1 mutagenesis mapped the nuclear localization signal to the C-terminus and identified the core domain for gpPML interaction. Individual knockout of GPCMV GP122 or GP123 (IE2 and IE1 unique exons respectively) was lethal to the virus. However, an IE1 mutant (codons 234-474 deleted), was viable with attenuated viral growth kinetics and increased susceptibility to type I interferon (IFN-I). In HCMV, the IE proteins are important T cell target antigens. Consequently, characterization of the homologs in GPCMV provides a basis for their evaluation in candidate vaccines against congenital infection.

A Novel Non-Replication-Competent Cytomegalovirus Capsid Mutant Vaccine Strategy Is Effective in Reducing Congenital Infection

Congenital cytomegalovirus (CMV) infection is a leading cause of mental retardation and deafness in newborns. The guinea pig is the only small animal model for congenital CMV infection. A novel CMV vaccine was investigated as an intervention strategy against congenital guinea pig cytomegalovirus (GPCMV) infection. In this disabled infectious single-cycle (DISC) vaccine strategy, a GPCMV mutant virus was used that lacked the ability to express an essential capsid gene (the UL85 homolog GP85) except when grown on a complementing cell line. In vaccinated animals, the GP85 mutant virus (GP85 DISC) induced an antibody response to important glycoprotein complexes considered neutralizing target antigens (gB, gH/gL/gO, and gM/gN). The vaccine also generated a T cell response to the pp65 homolog (GP83), determined via a newly established guinea pig gamma interferon enzyme-linked immunosorbent spot assay. In a congenital infection protection study, GP85 DISC-vaccinated animals and a nonvaccinated control group were challenged during pregnancy with wild-type GPCMV (10(5) PFU). The pregnant animals carried the pups to term, and viral loads in target organs of pups were analyzed. Based on live pup births in the vaccinated and control groups (94.1% versus 63.6%), the vaccine was successful in reducing mortality (P = 0.0002). Additionally, pups from the vaccinated group had reduced CMV transmission, with 23.5% infected target organs versus 75.9% in the control group. Overall, these preliminary studies indicate that a DISC CMV vaccine strategy has the ability to induce an immune response similar to that of natural virus infection but has the increased safety of a non-replication-competent virus, which makes this approach attractive as a CMV vaccine strategy.

IMPORTANCE

Congenital CMV infection is a leading cause of mental retardation and deafness in newborns. An effective vaccine against CMV remains an elusive goal despite over 50 years of CMV research. The guinea pig, with a placenta structure similar to that in humans, is the only small animal model for congenital CMV infection and recapitulates disease symptoms (e.g., deafness) in newborn pups. In this report, a novel vaccine strategy against congenital guinea pig cytomegalovirus (GPCMV) infection was developed, characterized, and tested for efficacy. This disabled infectious single-cycle (DISC) vaccine strategy induced a neutralizing antibody or a T cell response to important target antigens. In a congenital infection protection study, animals were protected against CMV in comparison to the nonvaccinated group (52% reduction of transmission). This novel vaccine was more effective than previously tested gB-based vaccines and most other strategies involving live virus vaccines. Overall, the DISC vaccine is a safe and promising approach against congenital CMV infection.

Targeted Mutagenesis of Guinea Pig Cytomegalovirus Using CRISPR/Cas9-Mediated Gene Editing

The cytomegaloviruses (CMVs) are among the most genetically complex mammalian viruses, with viral genomes that often exceed 230 kbp. Manipulation of cytomegalovirus genomes is largely performed using infectious bacterial artificial chromosomes (BACs), which necessitates the maintenance of the viral genome in Escherichia coli and successful reconstitution of virus from permissive cells after transfection of the BAC. Here we describe an alternative strategy for the mutagenesis of guinea pig cytomegalovirus that utilizes clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)-mediated genome editing to introduce targeted mutations to the viral genome. Transient transfection and drug selection were used to restrict lytic replication of guinea pig cytomegalovirus to cells that express Cas9 and virus-specific guide RNA. The result was highly efficient editing of the viral genome that introduced targeted insertion or deletion mutations to nonessential viral genes. Cotransfection of multiple virus-specific guide RNAs or a homology repair template was used for targeted, markerless deletions of viral sequence or to introduce exogenous sequence by homology-driven repair. As CRISPR/Cas9 mutagenesis occurs directly in infected cells, this methodology avoids selective pressures that may occur during propagation of the viral genome in bacteria and may facilitate genetic manipulation of low-passage or clinical CMV isolates.

IMPORTANCE

The cytomegalovirus genome is complex, and viral adaptations to cell culture have complicated the study of infection in vivo Recombineering of viral bacterial artificial chromosomes enabled the study of recombinant cytomegaloviruses. Here we report the development of an alternative approach using CRISPR/Cas9-based mutagenesis in guinea pig cytomegalovirus, a small-animal model of congenital cytomegalovirus disease. CRISPR/Cas9 mutagenesis can introduce the same types of mutations to the viral genome as bacterial artificial chromosome recombineering but does so directly in virus-infected cells. CRISPR/Cas9 mutagenesis is not dependent on a bacterial intermediate, and defined viral mutants can be recovered after a limited number of viral genome replications, minimizing the risk of spontaneous mutation.

A Homolog Pentameric Complex Dictates Viral Epithelial Tropism, Pathogenicity and Congenital Infection Rate in Guinea Pig Cytomegalovirus

In human cytomegalovirus (HCMV), tropism to epithelial and endothelial cells is dependent upon a pentameric complex (PC). Given the structure of the placenta, the PC is potentially an important neutralizing antibody target antigen against congenital infection. The guinea pig is the only small animal model for congenital CMV. Guinea pig cytomegalovirus (GPCMV) potentially encodes a UL128-131 HCMV PC homolog locus (GP128-GP133). In transient expression studies, GPCMV gH and gL glycoproteins interacted with UL128, UL130 and UL131 homolog proteins (designated GP129 and GP131 and GP133 respectively) to form PC or subcomplexes which were determined by immunoprecipitation reactions directed to gH or gL. A natural GP129 C-terminal deletion mutant (aa 107-179) and a chimeric HCMV UL128 C-terminal domain swap GP129 mutant failed to form PC with other components. GPCMV infection of a newly established guinea pig epithelial cell line required a complete PC and a GP129 mutant virus lacked epithelial tropism and was attenuated in the guinea pig for pathogenicity and had a low congenital transmission rate. Individual knockout of GP131 or 133 genes resulted in loss of viral epithelial tropism. A GP128 mutant virus retained epithelial tropism and GP128 was determined not to be a PC component. A series of GPCMV mutants demonstrated that gO was not strictly essential for epithelial infection whereas gB and the PC were essential. Ectopic expression of a GP129 cDNA in a GP129 mutant virus restored epithelial tropism, pathogenicity and congenital infection. Overall, GPCMV forms a PC similar to HCMV which enables evaluation of PC based vaccine strategies in the guinea pig model.

Viral Glycoprotein Complex Formation, Essential Function and Immunogenicity in the Guinea Pig Model for Cytomegalovirus

Development of a cytomegalovirus (CMV) vaccine is a major public health priority due to the risk of congenital infection. A key component of a vaccine is thought to be an effective neutralizing antibody response against the viral glycoproteins necessary for cell entry. Species specificity of human CMV (HCMV) precludes direct studies in an animal model. The guinea pig is the only small animal model for congenital cytomegalovirus infection. Analysis of the guinea pig CMV (GPCMV) genome indicates that it potentially encodes homologs to the HCMV glycoproteins (including gB, gH, gL, gM, gN and gO) that form various cell entry complexes on the outside of the virus: gCI (gB); gCII (gH/gL/gO); gCIII (gM/gN). The gB homolog (GP55) has been investigated as a candidate subunit vaccine but little is known about the other homolog proteins. GPCMV glycoproteins were investigated by transient expression studies which indicated that homolog glycoproteins to gN and gM, or gH, gL and gO were able to co-localize in cells and generate respective homolog complexes which could be verified by immunoprecipitation assays. ELISA studies demonstrated that the individual complexes were highly immunogenic in guinea pigs. The gO (GP74) homolog protein has 13 conserved N-glycosylation sites found in HCMV gO. In transient expression studies, only the glycosylated protein is detected but in virus infected cells both N-glycosylated and non-glycosylated gO protein were detected. In protein interaction studies, a mutant gO that lacked N-glycosylation sites had no impact on the ability of the protein to interact with gH/gL which indicated a potential alternative function associated with these sites. Knockout GPCMV BAC mutagenesis of the respective glycoprotein genes (GP55 for gB, GP75 for gH, GP115 for gL, GP100 for gM, GP73 for gN and GP74 for gO) in separate reactions was lethal for virus regeneration on fibroblast cells which demonstrated the essential nature of the GPCMV glycoproteins. The gene knockout results were similar to HCMV, except in the case of the gO homolog, which was non-essential in epithelial tropic virus but essential in lab adapted GPCMV. Overall, the findings demonstrate the similarity between HCMV and GPCMV glycoproteins and strengthen the relevance of this model for development of CMV intervention strategies.

Vaccination with a Live Attenuated Cytomegalovirus Devoid of a Protein Kinase R Inhibitory Gene Results in Reduced Maternal Viremia and Improved Pregnancy Outcome in a Guinea Pig Congenital Infection Model

Development of a vaccine to prevent congenital cytomegalovirus infection is a major public health priority. Live vaccines attenuated through mutations targeting viral mechanisms responsible for evasion of host defense may be both safe and efficacious. Safety and vaccine efficacy were evaluated using a guinea pig cytomegalovirus (GPCMV) model. Recombinant GPCMV with a targeted deletion of gp145 (designated Δ145), a viral protein kinase R (PKR) inhibitor, was generated. Attenuation was evaluated following inoculation of 10(7) PFU of Δ145 or parental virus into guinea pigs immunosuppressed with cyclophosphamide. Efficacy was evaluated by immunizing GPCMV-naive guinea pigs twice with either 10(5) or 10(6) PFU of Δ145, establishing pregnancy, and challenging the guinea pigs with salivary gland-adapted GPCMV. The immune response, maternal viral load, pup mortality, and congenital infection rates in the vaccine and control groups were compared. Δ145 was substantially attenuated for replication in immunocompromised guinea pigs. Vaccination with Δ145 induced enzyme-linked immunosorbent assay (ELISA) and neutralizing antibody levels comparable to those achieved in natural infection. In the higher- and lower-dose vaccine groups, pup mortality was reduced to 1/24 (4%) and 4/29 (14%) pups, respectively, whereas it was 26/31 (81%) in unvaccinated control pups (P < 0.0001 for both groups versus the control group). Congenital infection occurred in 20/31 (65%) control pups but only 8/24 (33%) pups in the group vaccinated with 10(6) PFU (P < 0.05). Significant reductions in the magnitude of maternal DNAemia and pup viral load were noted in the vaccine groups compared to those in the controls. Deletion of a GPCMV genome-encoded PKR inhibitor results in a highly attenuated virus that is immunogenic and protective as a vaccine against transplacental infection.

IMPORTANCE

Previous attempts to develop successful immunization against cytomegalovirus have largely centered on subunit vaccination against virion proteins but have yielded disappointing results. The advent of bacterial artificial chromosome technologies has enabled engineering of recombinant cytomegaloviruses (CMVs) from which virus genome-encoded immune modulation genes have been deleted, toward the goal of developing a safe and potentially more efficacious live attenuated vaccine. Here we report the findings of studies of such a vaccine against congenital CMV infection based on a virus with a targeted deletion in gp145, a virus genome-encoded inhibitor of protein kinase R, using the guinea pig model of vertical CMV transmission. The deletion virus was attenuated for dissemination in immunocompromised guinea pigs but elicited ELISA and neutralizing responses. The vaccine conferred protection against maternal DNAemia and congenital transmission and resulted in reduced viral loads in newborn guinea pigs. These results provide support for future studies of attenuated CMV vaccines.

A novel CDK7 inhibitor of the Pyrazolotriazine class exerts broad-spectrum antiviral activity at nanomolar concentrations

Protein kinases represent central and multifunctional regulators of a balanced virus-host interaction. Cyclin-dependent protein kinase 7 (CDK7) plays crucial regulatory roles in cell cycle and transcription, both connected with the replication of many viruses. Previously, we developed a CDK7 inhibitor, LDC4297, that inhibits CDK7 in vitro in the nano-picomolar range. Novel data from a kinome-wide evaluation (>330 kinases profiled in vitro) demonstrate a kinase selectivity. Importantly, we provide first evidence for the antiviral potential of the CDK7 inhibitor LDC4297, i.e., in exerting a block of the replication of human cytomegalovirus (HCMV) in primary human fibroblasts at nanomolar concentrations (50% effective concentration, 24.5 ± 1.3 nM). As a unique feature compared to approved antiherpesviral drugs, inhibition occurred already at the immediate-early level of HCMV gene expression. The mode of antiviral action was considered multifaceted since CDK7-regulated cellular factors that are supportive of HCMV replication were substantially affected by the inhibitors. An effect of LDC4297 was identified in the interference with HCMV-driven inactivation of retinoblastoma protein (Rb), a regulatory step generally considered a hallmark of herpesviral replication. In line with this finding, a broad inhibitory activity of the drug could be demonstrated against a selection of human and animal herpesviruses and adenoviruses, whereas other viruses only showed intermediate drug sensitivity. Summarized, the CDK7 inhibitor LDC4297 is a promising candidate for further antiviral drug development, possibly offering new options for a comprehensive approach to antiviral therapy.

Guinea pig cytomegalovirus GP129/131/133, homologues of human cytomegalovirus UL128/130/131A, are necessary for infection of monocytes and macrophages

The GP129, GP131 and GP133 genes of guinea pig cytomegalovirus (GPCMV) are homologues of human cytomegalovirus UL128, UL130 and UL131A, respectively, which are essential for infection of endothelial and epithelial cells, and for viral transmission to leukocytes. Our previous study demonstrated that a GPCMV strain lacking the 1.6 kb locus that contains the GP129, GP131 and GP133 genes had a growth defect in animals. Here, we demonstrated that the WT strain, but not the 1.6 kb-deleted strain, formed capsids in macrophages prepared from the peritoneal fluid. To understand the mechanism, we prepared GPCMV strains defective in each of GP129, GP131 and GP133, and found that they were all essential for the infection of peritoneal, splenic and PBMC-derived macrophages/monocytes, and for expression of immediate-early antigens in the macrophages/monocytes, although they were dispensable for infection of fibroblasts. Monocyte/macrophage tropism could be one of the important determinants for viral dissemination in vivo.

Identification by mass spectrometry and immune response analysis of guinea pig cytomegalovirus (GPCMV) pentameric complex proteins GP129, 131 and 133

Development of a vaccine against congenital infection with human cytomegalovirus (HCMV) is a major public health priority. A potential vaccine target receiving considerable recent attention is the pentameric complex (PC) of HCMV proteins consisting of gL, gH, UL128, UL130, and UL131, since some antibodies against these target proteins are capable of potently neutralizing virus at epithelial and endothelial cell surfaces. Recently, homologous proteins have been described for guinea pig cytomegalovirus (GPCMV), consisting of gH, gL, and the GPCMV proteins GP129, GP131, and GP133. To investigate these proteins as potential vaccine targets, expression of GP129-GP133 transcripts was confirmed by reverse-transcriptase PCR. Mass spectrometry combined with western blot assays demonstrated the presence of GP129, GP131, and GP133 proteins in virus particles. Recombinant proteins corresponding to these PC proteins were generated in baculovirus, and as GST fusion proteins. Recombinant proteins were noted to be immunoreactive with convalescent sera from infected animals, suggesting that these proteins are recognized in the humoral immune response to GPCMV infection. These analyses support the study of PC-based recombinant vaccines in the GPCMV congenital infection model.

An attenuated cytomegalovirus vaccine with a deletion of a viral chemokine gene is protective against congenital CMV transmission in a guinea pig model

Development of a vaccine against congenital cytomegalovirus (CMV) infection is a public health priority, but CMVs encode immune evasion genes that complicate live virus vaccine design. To resolve this problem, this study employed guanosyl phosphoribosyl transferase (gpt) mutagenesis to generate a recombinant guinea pig CMV (GPCMV) with a knockout of a viral chemokine gene, GPCMV MIP (gp1). MIP deletion virus replicated with wild-type kinetics in cell culture but was attenuated in nonpregnant guinea pigs, demonstrating reduced viremia and reduced inflammation and histopathology (compared to a control virus with an intact GPCMV MIP gene) following footpad inoculation. In spite of attenuation, the vaccine was immunogenic, eliciting antibody responses comparable to those observed in natural infection. To assess its protective potential as a vaccine, either recombinant virus or placebo was used to immunize seronegative female guinea pigs. Dams were challenged in the early 3rd trimester with salivary gland-adapted GPCMV. Immunization protected against DNAemia (1/15 in vaccine group versus 12/13 in the control group, P < 0.01). Mean birth weights were significantly higher in pups born to vaccinated dams compared to controls (98.7 g versus 71.2 g, P < 0.01). Vaccination reduced pup mortality, from 35/50 (70%) in controls to 8/52 (15%) in the immunization group. Congenital GPCMV infection was also reduced, from 35/50 (70%) in controls to 9/52 (17%) in the vaccine group (P < 0.0001). We conclude that deletion of an immune modulation gene can attenuate the pathogenicity of GPCMV while resulting in a viral vaccine that retains immunogenicity and demonstrates efficacy against congenital infection and disease.

Human cytomegalovirus: bacterial artificial chromosome (BAC) cloning and genetic manipulation

The understanding of human cytomegalovirus (HCMV) biology was long hindered by the inability to perform efficient viral genetic analysis. This hurdle was recently overcome when the genomes of multiple HCMV strains were cloned as infectious bacterial artificial chromosomes (BACs). The BAC system takes advantage of the single-copy F plasmid of E. coli that can stably carry large pieces of foreign DNA. In this system, a recombinant HCMV virus carrying a modified F plasmid is first generated in eukaryotic cells. Recombinant viral genomes are then isolated and recovered in E. coli as BAC clones. BAC-captured viral genomes can be manipulated using prokaryotic genetics, and recombinant virus can be reconstituted from BAC transfection in eukaryotic cells. The BAC reverse genetic system provides a reliable and efficient method to introduce genetic alterations into the viral genome in E.coli and subsequently analyze their effects on virus biology in eukaryotic cells.

Viral bacterial artificial chromosomes: generation, mutagenesis, and removal of mini-F sequences

Maintenance and manipulation of large DNA and RNA virus genomes had presented an obstacle for virological research. BAC vectors provided a solution to both problems as they can harbor large DNA sequences and can efficiently be modified using well-established mutagenesis techniques in Escherichia coli. Numerous DNA virus genomes of herpesvirus and pox virus were cloned into mini-F vectors. In addition, several reverse genetic systems for RNA viruses such as members of Coronaviridae and Flaviviridae could be established based on BAC constructs. Transfection into susceptible eukaryotic cells of virus DNA cloned as a BAC allows reconstitution of recombinant viruses. In this paper, we provide an overview on the strategies that can be used for the generation of virus BAC vectors and also on systems that are currently available for various virus species. Furthermore, we address common mutagenesis techniques that allow modification of BACs from single-nucleotide substitutions to deletion of viral genes or insertion of foreign sequences. Finally, we review the reconstitution of viruses from BAC vectors and the removal of the bacterial sequences from the virus genome during this process.

Back to BAC: the use of infectious clone technologies for viral mutagenesis

Bacterial artificial chromosome (BAC) vectors were first developed to facilitate the propagation and manipulation of large DNA fragments in molecular biology studies for uses such as genome sequencing projects and genetic disease models. To facilitate these studies, methodologies have been developed to introduce specific mutations that can be directly applied to the mutagenesis of infectious clones (icBAC) using BAC technologies. This has resulted in rapid identification of gene function and expression at unprecedented rates. Here we review the major developments in BAC mutagenesis in vitro. This review summarises the technologies used to construct and introduce mutations into herpesvirus icBAC. It also explores developing technologies likely to provide the next leap in understanding these important viruses.

Recent advances in cloning herpesviral genomes as infectious bacterial artificial chromosomes

Herpesviruses are common but important pathogens in humans and animals. These viruses have large complex genomes encoding genes with diverse functions in different phases of their life cycle and associated diseases. In the last decade, genomes of herpesviruses cloned as infectious bacterial artificial chromosomes (BACs) have become powerful tools for delineating the functions of viral genes and understanding the pathogenesis of their associated diseases. Here we review the history of herpesviral genetics and recent advances in methods for cloning herpesviral genomes as infectious BACs.

Guinea pig cytomegalovirus GP84 is a functional homolog of the human cytomegalovirus (HCMV) UL84 gene that can complement for the loss of UL84 in a chimeric HCMV

The guinea pig cytomegalovirus (GPCMV) co-linear gene and potential functional homolog of HCMV UL84 (GP84) was investigated. The GP84 gene had delayed early transcription kinetics and transient expression studies of GP84 protein (pGP84) demonstrated that it targeted the nucleus and co-localized with the viral DNA polymerase accessory protein as described for HCMV pUL84. Additionally, pGP84 exhibited a transdominant inhibitory effect on viral growth as described for HCMV. The inhibitory domain could be localized to a minimal peptide sequence of 99 aa. Knockout of GP84 generated virus with greatly impaired growth kinetics. Lastly, the GP84 ORF was capable of complementing for the loss of the UL84 coding sequence in a chimeric HCMV. Based on this research and previous studies we conclude that GPCMV is similar to HCMV by encoding single copy co-linear functional homologs of HCMV UL82 (pp71), UL83 (pp65) and UL84 genes.

Herpesvirus BACs: past, present, and future

The herpesviridae are a large family of DNA viruses with large and complicated genomes. Genetic manipulation and the generation of recombinant viruses have been extremely difficult. However, herpesvirus bacterial artificial chromosomes (BACs) that were developed approximately 10 years ago have become useful and powerful genetic tools for generating recombinant viruses to study the biology and pathogenesis of herpesviruses. For example, BAC-directed deletion mutants are commonly used to determine the function and essentiality of viral genes. In this paper, we discuss the creation of herpesvirus BACs, functional analyses of herpesvirus mutants, and future applications for studies of herpesviruses. We describe commonly used methods to create and mutate herpesvirus BACs (such as site-directed mutagenesis and transposon mutagenesis). We also evaluate the potential future uses of viral BACs, including vaccine development and gene therapy.

Guinea Pig Cytomegalovirus (GPCMV): A Model for the Study of the Prevention and Treatment of Maternal-Fetal Transmission

A major public health challenge today is the problem of congenital cytomegalovirus (CMV) transmission. Maternal-fetal CMV infections are common, occurring in 0.5-2% of pregnancies, and these infections often lead to long-term injury of the newborn infant. In spite of the well-recognized burden that these infections place on society, there are as yet no clearly established interventions available to prevent transmission of CMV. In order to study potential interventions, such as vaccines or antiviral therapies, an animal model of congenital CMV transmission is required. The best small animal model of CMV transmission is the guinea pig cytomegalovirus (GPCMV) model. This article summarizes the GPCMV model, putting it into the larger context of how studies in this system have relevance to human health. An emphasis is placed on how the vertical transmission of GPCMV recapitulates the pathogenesis of congenital CMV in infants, making this a uniquely well-suited model for the study of potential CMV vaccines.

Autoexcision of bacterial artificial chromosome facilitated by terminal repeat-mediated homologous recombination: a novel approach for generating traceless genetic mutants of herpesviruses

Infectious bacterial artificial chromosomes (BACs) of herpesviruses are powerful tools for genetic manipulation. However, the presence of BAC vector sequence in the viral genomes often causes genetic and phenotypic alterations. While the excision of the BAC vector cassette can be achieved by homologous recombination between extra duplicate viral sequences or loxP site-mediated recombination, these methods either are inefficient or leave a loxP site mark in the viral genome. Here we describe the use of viral intrinsic repeat sequences, which are commonly present in herpesviral genomes, to excise the BAC vector cassette. Using a newly developed in vitro transposon-based cloning approach, we obtained an infectious BAC of rhesus rhadinovirus (RRV) strain RRV26-95 with the BAC vector cassette inserted in the terminal repeat (TR) region. We showed that the BAC vector cassette was rapidly excised upon reconstitution in cells predominantly through TR-mediated homologous recombination. Genetic and phenotypic analysis showed that the BAC-excised virus was reversed to wild-type RRV. Using this autoexcisable BAC clone, we successfully generated an RRV mutant with a deletion of Orf50, which encodes a replication and transcription activator (RTA) protein. Together, these results illustrate the usefulness of TR for genetic manipulation of herpesviruses when combined with the novel transposon-based cloning approach.

Analysis of the nucleotide sequence of the guinea pig cytomegalovirus (GPCMV) genome

In this report we describe the genomic sequence of guinea pig cytomegalovirus (GPCMV) assembled from a tissue culture-derived bacterial artificial chromosome clone, plasmid clones of viral restriction fragments, and direct PCR sequencing of viral DNA. The GPCMV genome is 232,678 bp, excluding the terminal repeats, and has a GC content of 55%. A total of 105 open reading frames (ORFs) of > 100 amino acids with sequence and/or positional homology to other CMV ORFs were annotated. Positional and sequence homologs of human cytomegalovirus open reading frames UL23 through UL122 were identified. Homology with other cytomegaloviruses was most prominent in the central ~60% of the genome, with divergence of sequence and lack of conserved homologs at the respective genomic termini. Of interest, the GPCMV genome was found in many cases to bear stronger phylogenetic similarity to primate CMVs than to rodent CMVs. The sequence of GPCMV should facilitate vaccine and pathogenesis studies in this model of congenital CMV infection.

A sequence-independent in vitro transposon-based strategy for efficient cloning of genomes of large DNA viruses as bacterial artificial chromosomes

Bacterial artificial chromosomes (BACs) derived from genomes of large DNA viruses are powerful tools for functional delineation of viral genes. Current methods for cloning the genomes of large DNA viruses as BACs require prior knowledge of the viral sequences or the cloning of viral DNA fragments, and are tedious because of the laborious process of multiple plaque purifications, which is not feasible for some fastidious viruses. Here, we describe a novel method for cloning the genomes of large DNA viruses as BACs, which entails direct in vitro transposition of viral genomes with a BAC cassette, and subsequent recovery in Escherichia coli. Determination of insertion sites and adjacent viral sequences identify the BAC clones for genetic manipulation and functional characterization. Compared to existing methods, this new approach is highly efficient, and does not require any information on viral sequences or cloning of viral DNA fragments, and plaque purifications. This method could potentially be used for discovering previously unidentified viruses.

Identification of a 1.6 kb genome locus of guinea pig cytomegalovirus required for efficient viral growth in animals but not in cell culture

Guinea pig cytomegalovirus (GPCMV) provides a useful model for studies of congenital CMV infection. During characterization of the GPCMV genome sequence, we identified two types of strains in a virus stock purchased from ATCC. One of them, GPCMV/del, lacks a 1.6 kb locus that positionally corresponds to murine CMV (MCMV) M129-M133. Growth of GPCMV/del in cell culture was marginally better than that of the other strain, GPCMV/full, which harbors the 1.6 kb locus. However, in animals infected intraperitoneally with virus stocks containing both strains, GPCMV/full disseminated more efficiently than GPCMV/del, including 200-fold greater viral load in salivary glands. Viral DNA, transcripts of the immediate-early 2 gene homolog, and viral antigens were more abundant in animals infected with GPCMV/full than in those infected with GPCMV/del. Although the observed phenomena have some similarity with the growth properties of MCMV strains defective in mck-1/mck-2(M129/131) and those defective in sgg(M132), no M129-M132 homologs were found in the 1.6 kb locus. Since one of the ORFs in the locus has a weak sequence similarity with HCMV UL130, which relates to cell tropism, further studies will be required to learn the mechanism for efficient GPCMV growth in animal.

Cloning the complete guinea pig cytomegalovirus genome as an infectious bacterial artificial chromosome with excisable origin of replication

Congenital human cytomegalovirus infections are the major infectious cause of birth defects in the United States. How this virus crosses the placenta and causes fetal disease is poorly understood. Guinea pig cytomegalovirus (GPCMV) is a related virus that provides an important model for studying cytomegaloviral congenital transmission and pathogenesis. In order to facilitate genetic analysis of GPCMV, the 232kb GPCMV genome was cloned as an infectious bacterial artificial chromosome (BAC). The BAC vector sequences were flanked by LoxP sites to allow efficient excision using Cre recombinase. All initial clones contained spontaneous deletions of viral sequences and reconstituted mutant viruses with impaired growth kinetics in vitro. The deletions in one BAC were repaired using Escherichia coli genetics. The resulting repaired BAC reconstituted a virus with in vitro replication kinetics identical to the wild type parental virus; moreover, its genome was indistinguishable from that of the wild type parental virus by restriction pattern analysis using multiple restriction enzymes. These results suggest that the repaired BAC is an authentic representation of the complete GPCMV genome. It should provide a valuable tool for evaluating the impact of genetic modifications on the safety and efficacy of live attenuated vaccines and for identifying genes important for congenital transmission and fetal disease.

Protein kinase inhibitors of the quinazoline class exert anti-cytomegaloviral activity in vitro and in vivo

Cytomegalovirus infection is associated with severe disease in immunocompromised individuals. Current antiviral therapy faces several limitations. In a search of novel drug candidates, we describe here the anti-cytomegaloviral properties of two compounds of the chemical class of quinazolines, gefitinib (Iressa) and Ax7396 (RGB-315389). Both compounds showed strong inhibitory effects in vitro against human and animal cytomegaloviruses with IC(50)s in a low micromolar range. Cytotoxicity did not occur at these effective concentrations. The antiviral mode of action was based on the inhibition of protein kinase activity, mainly directed to a viral target kinase (UL97/M97) in addition to cellular target candidates. This was demonstrated by a high sensitivity of the respective protein kinases in vitro and by infection experiments with viral mutants carrying genomic alterations in the ORF UL97/M97 modulating viral drug sensitivity. In a guinea pig model, gefitinib showed inhibition of cytomegaloviral loads in blood and lung tissue. Importantly, the rate of mortality of infected animals was reduced by gefitinib treatment. In contrast to the in vitro data, Ax7396 showed no significant antiviral activity in a mouse model. Further in vivo analyses have to assess the potential use of gefitinib in the treatment of cytomegalovirus disease.

Expression of the human cytomegalovirus UL97 gene in a chimeric guinea pig cytomegalovirus (GPCMV) results in viable virus with increased susceptibility to ganciclovir and maribavir

In lieu of a licensed vaccine, antivirals are being considered as an intervention to prevent congenital human cytomegalovirus (HCMV) infection. Ideally, antiviral therapies should undergo pre-clinical evaluation in an animal model prior to human use. Guinea pig cytomegalovirus (GPCMV) is the only small animal model for congenital CMV. However, GPCMV is not susceptible to the most commonly used HCMV antiviral, ganciclovir (GCV), rendering in vivo study of this agent problematic in the guinea pig model. Human cytomegalovirus (HCMV) susceptibility to GCV is linked to the UL97 gene. We hypothesized that GPCMV susceptibility to GCV could be improved by inserting the HCMV (Towne) UL97 gene into the GPCMV genome in place of the homolog, GP97. A chimeric GPCMV (GPCMV::UL97) expressed UL97 protein, and replicated efficiently in cell culture, with kinetics similar to wild-type GPCMV. In contrast, deletion of GP97 resulted in a virus (GPCMVdGP97) that grew poorly in culture. GPCMV::UL97 had substantially improved susceptibility to the inhibitory effects of GCV in comparison to wild-type GPCMV. Additionally, GPCMV::UL97 exhibited improved susceptibility to another antiviral undergoing clinical trials, maribavir (MBV; benzimidazole riboside 1263W94), which also acts through UL97.

Novel bacterial artificial chromosome vector pUvBBAC for use in studies of the functional genomics of Listeria spp

Bacterial artificial chromosome (BAC) vectors are important tools for microbial genome research. We constructed a novel BAC vector, pUvBBAC, for replication in both gram-negative and gram-positive bacterial hosts. The pUvBBAC vector was used to generate a BAC library for the facultative intracellular pathogen Listeria monocytogenes EGD-e. The library had insert sizes ranging from 68 to 178 kb. We identified two recombinant BACs from the L. monocytogenes pUvBBAC library that each contained the entire virulence gene cluster (vgc) of L. monocytogenes and transferred them to a nonpathogenic Listeria innocua strain. Recombinant L. innocua strains harboring pUvBBAC+vgc1 and pUvBBAC+vgc2 produced the vgc-specific listeriolysin (LLO) and actin assembly protein ActA and represent the first reported cloning of the vgc locus in its entirety. The use of the novel broad-host-range BAC vector pUvBBAC extends the versatility of this technology and provides a powerful platform for detailed functional genomics of gram-positive bacteria as well as its use in explorative functional metagenomics.

Macrophage inflammatory proteins in cytomegalovirus-related inner ear injury

OBJECTIVE

Inner ear inflammation triggered by CMV infection may play a role in CMV-related auditory pathogenesis. The purpose of the study was to determine if a virally encoded macrophage inflammatory protein played a role in CMV-related hearing loss.

DESIGN

Mutagenesis was performed with deletion of a guinea pig CMV macrophage inflammatory protein. Intracochlear inoculations were performed on three groups of animals (n = 18). Group 1 received sterile viral media, Group 2 received wild-type CMV virus, and Group 3 received "knockout" (KO) virus with a deleted immunomodulation gene. Baseline and postinoculation ABRs were obtained. ELISA and PCR were performed and temporal bones examined.

SUBJECTS

Eighteen guinea pigs.

RESULTS

The KO group had significantly better hearing than the WT group. There were no significant differences between the KO and sham groups. The WT group had significant hearing loss at all frequencies. Inflammation and fibrosis were noted in the WT temporal bones only.

CONCLUSIONS

Virally encoded macrophage inflammatory proteins appear to play a significant role in CMV-related hearing loss.

A self-excisable infectious bacterial artificial chromosome clone of varicella-zoster virus allows analysis of the essential tegument protein encoded by ORF9

In order to facilitate the generation of mutant viruses of varicella-zoster virus (VZV), the agent causing varicella (chicken pox) and herpes zoster (shingles), we generated a full-length infectious bacterial artificial chromosome (BAC) clone of the P-Oka strain. First, mini-F sequences were inserted into a preexisting VZV cosmid, and the SuperCos replicon was removed. Subsequently, mini-F-containing recombinant virus was generated from overlapping cosmid clones, and full-length VZV DNA recovered from the recombinant virus was established in Escherichia coli as an infectious BAC. An inverted duplication of VZV genomic sequences within the mini-F replicon resulted in markerless excision of vector sequences upon virus reconstitution in eukaryotic cells. Using the novel tool, the role in VZV replication of the major tegument protein encoded by ORF9 was investigated. A markerless point mutation introduced in the start codon by two-step en passant Red mutagenesis abrogated ORF9 expression and resulted in a dramatic growth defect that was not observed in a revertant virus. The essential nature of ORF9 for VZV replication was ultimately confirmed by restoration of the growth of the ORF9-deficient mutant virus using trans-complementation via baculovirus-mediated gene transfer.

Protection against congenital cytomegalovirus (CMV) disease, conferred by a replication-disabled, bacterial artificial chromosome (BAC)-based DNA vaccine

It is unclear if protective immunity can be conferred by a cytomegalovirus (CMV) vaccine encoding a single protein subunit, or if multiple viral genes need to be targeted. Using the guinea pig model of congenital CMV infection, these studies examined the immunogenicity and efficacy of a DNA vaccine based on the guinea pig cytomegalovirus (GPCMV) genome cloned as a non-infectious BAC plasmid, modified by transposon insertion into the homolog of the HCMV tegument protein, UL48. Following vaccination of female Hartley guinea pigs with BAC DNA, adverse GPCMV-related pregnancy outcome were assessed after establishment of pregnancy, followed by GPCMV third-trimester challenge. Animals immunized with recombinant BACmid engendered anti-GPCMV antibodies by ELISA assay. Immunogenicity of BAC plasmid DNA was augmented by inclusion of the lipid adjuvant, DOTMA/DOPE, in the vaccine regimen. Among pups born to 12 control (sham-immunized) dams challenged with GPCMV in the third trimester, mortality was 23/35 (66%). In contrast, among evaluable pregnancy outcomes in pups born to 10 BAC-immunized pregnant dams, preconception immunization resulted in reduced pup mortality, to 10/34 pups (29%; p<0.005 versus control, Fisher's exact test). In addition, vaccinated dams had reduced viral load, compared to controls, as assessed by quantitative, real-time PCR.

Cyclic cidofovir (cHPMPC) prevents congenital cytomegalovirus infection in a guinea pig model

Background

Congenital cytomegalovirus (CMV) infection is a major public health problem. Antiviral therapies administered during pregnancy might prevent vertical CMV transmission and disease in newborns, but these agents have not been evaluated in clinical trials. The guinea pig model of congenital CMV infection was therefore used to test the hypothesis that antiviral therapy, using the agent agent cyclic cidofovir (cHPMPC), could prevent congenital CMV infection.

Results

Pregnant outbred Hartley guinea pigs were challenged in the early-third trimester with guinea pig CMV (GPCMV) and treated with placebo, or the antiviral agent, cyclic cidofovir. To optimize detection of vertical infection, an enhanced green fluorescent protein (eGFP)-tagged virus was employed. Compared to placebo, cyclic cidofovir-treated dams and pups had reduced mortality following GPCMV challenge. The magnitude of GPCMV-induced maternal and fetal mortality in this study was reduced from 5/25 animals in the placebo group to 0/21 animals in the treatment group (p = 0.05, Fisher's exact test). By viral culture assay, antiviral therapy was found to completely prevent GPCMV transmission to the fetus. In control pups, 5/19 (26%) were culture-positive for GPCMV, compared to 0/16 of pups in the cyclic cidofovir treatment group (p < 0.05, Fisher's exact test).

Conclusion

Antiviral therapy with cyclic cidofovir improves pregnancy outcomes in guinea pigs, and eliminates congenital CMV infection, following viral challenge in the third trimester. This study also demonstrated that an eGFP-tagged recombinant virus, with the reporter gene inserted into a dispensable region of the viral genome, retained virulence, including the potential for congenital transmission, facilitating tissue culture-based detection of congenital infection. These observations provide support for clinical trials of antivirals for reduction of congenital CMV infection.

Construction of an infectious clone of canine herpesvirus genome as a bacterial artificial chromosome

Canine herpesvirus (CHV) is an attractive candidate not only for use as a recombinant vaccine to protect dogs from a variety of canine pathogens but also as a viral vector for gene therapy in domestic animals. However, developments in this area have been impeded by the complicated techniques used for eukaryotic homologous recombination. To overcome these problems, we used bacterial artificial chromosomes (BACs) to generate infectious BACs. Our findings may be summarized as follows: (i) the CHV genome (pCHV/BAC), in which a BAC flanked by loxP sites was inserted into the thymidine kinase gene, was maintained in Escherichia coli; (ii) transfection of pCHV/BAC into A-72 cells resulted in the production of infectious virus; (iii) the BAC vector sequence was almost perfectly excisable from the genome of the reconstituted virus CHV/BAC by co-infection with CHV/BAC and a recombinant adenovirus that expressed the Cre recombinase; and (iv) a recombinant virus in which the glycoprotein C gene was deleted was generated by lambda recombination followed by Flp recombination, which resulted in a reduction in viral titer compared with that of the wild-type virus. The infectious clone pCHV/BAC is useful for the modification of the CHV genome using bacterial genetics, and CHV/BAC should have multiple applications in the rapid generation of genetically engineered CHV recombinants and the development of CHV vectors for vaccination and gene therapy in domestic animals.

[BAC system: A novel method for manipulation of herpesvirus genomes based on bacterial genetics]

Although methods for reverse genetics of herpesviruses have been established in early 1980s, the steps are laborious and time-consuming. In 1997, Dr. Koszinwski's group reported a novel approach for the construction of herpesvirus mutants, based on cloning the viral genome as a bacterial artificial chromosome (BAC) in E. coli. This technique allows the maintenance of viral genomes as plasmid in E. coli and the reconstitution of viral progeny by transfection of the BAC plasmid into eukaryotic cells. Any genetics modification of the viral genome in E. coli using bacterial genetics is possible, thereby facilitating the introduction of mutagenesis into herpesvirus genome. This 'BAC system' has opened new avenues for reverse and forward genetics of herpesviruses in basic research and in vector development for human therapy. Here we describe the principle of the 'BAC system' in herpesvirus researches.

The non-nucleoside antiviral, BAY 38-4766, protects against cytomegalovirus (CMV) disease and mortality in immunocompromised guinea pigs

New antiviral drugs are needed for the treatment of cytomegalovirus (CMV) infections, particularly in immunocompromised patients. These studies evaluated the in vitro and in vivo activity of the non-nucleosidic CMV inhibitor, BAY 38-4766, against guinea pig cytomegalovirus (GPCMV). Plaque reduction assays indicated that BAY 38-4766 was active against GPCMV, with an IC(50) of 0.5muM. Yield reduction assays demonstrated an ED(90) and ED(99) of 0.4 and 0.6muM, respectively, of BAY 38-4766 against GPCMV. Guinea pigs tolerated oral administration of 50mg/kg/day of BAY 38-4766 without evidence of biochemical or hematologic toxicity. Plasma concentrations of BAY 38-4766 were high following oral dosing, with a mean peak level at 1-h post-dose of 26.7mg/ml (n=6; range, 17.8-35.4). Treatment with BAY 38-4766 reduced both viremia and DNAemia, as determined by a real-time PCR assay, following GPCMV infection of cyclophosphamide-immunosuppressed strain 2 guinea pigs (p<0.05, Mann-Whitney test). BAY 38-4766 also reduced mortality following lethal GPCMV challenge in immunosuppressed Hartley guinea pigs, from 83% (20/24) in placebo-treated guinea pigs, to 17% (4/24) in BAY 38-4766-treated animals (p<0.0001, Fisher's exact test). Mortality differences were accompanied by reduction in DNAemia in Hartley guinea pigs. Based upon its favorable safety, pharmacokinetic, and therapeutic profiles, BAY 38-4766 warrants further investigation in the GPCMV model.

Cloning of the varicella-zoster virus genome as an infectious bacterial artificial chromosome in Escherichia coli

The complete genome of the varicella-zoster virus (VZV) Oka strain has been cloned as a bacterial artificial chromosome (BAC). Following electroporation into Escherichia coli (E. coli) strain DH10B, the VZV BAC was stably propagated over multiple generations of its host. Human embryonic lung (HEL) cells transfected with VZV BAC DNA recovered from DH10B showed cytopathic effect (CPE), and virus spread to neighbouring cells was observed. BAC vector sequences are flanked by loxP sites and, coinfection of the reconstituted virus, with a recombinant adenovirus expressing Cre recombinase removed the bacterial sequences. The resulting recombinant rV02 grew as well as the parental virus in HEL cells. The recombinant VZV will promote VZV research and increase use of the viral genome as an investigative tool.

Molecular, biological, and in vivo characterization of the guinea pig cytomegalovirus (CMV) homologs of the human CMV matrix proteins pp71 (UL82) and pp65 (UL83)

We recently identified the genes encoding the guinea pig cytomegalovirus (GPCMV) homologs of the upper and lower matrix proteins of human CMV, pp71 (UL82) and pp65 (UL83), which we designated GP82 and GP83, respectively. Transient-expression studies with a GP82 plasmid demonstrated that the encoded protein targets the nucleus and that the infectivity and plaquing efficiency of cotransfected GPCMV viral DNA was enhanced by GP82. The transactivation function of GP82 was not limited to GPCMV, but was also observed for a heterologous virus, herpes simplex virus type 1 (HSV-1). This was confirmed by its ability to complement the growth of an HSV-1 VP16 transactivation-defective mutant virus in an HSV viral DNA cotransfection assay. Study of a GP82 "knockout" virus (and its attendant rescuant), generated on a GPCMV bacterial artificial chromosome construct, confirmed the essential nature of the gene. Conventional homologous recombination was used to generate a GP83 mutant to examine the role of GP83 in the viral life cycle. Comparison of the one-step growth kinetics of the GP83 mutant (vAM409) and wild-type GPCMV indicated that GP83 protein is not required for viral replication in tissue culture. The role of GP83 in vivo was examined by comparing the pathogenesis of wild-type GPCMV, vAM409, and a control virus, vAM403, in guinea pigs. The vAM409 mutant was significantly attenuated for dissemination in immunocompromised strain 2 guinea pigs, suggesting that the GP83 protein is essential for full pathogenicity in vivo.

Identification of essential and non-essential genes of the guinea pig cytomegalovirus (GPCMV) genome via transposome mutagenesis of an infectious BAC clone

We report application of a transposition methodology that allows the easy characterization and mutation of genes encoded on an infectious bacterial artificial chromosome (BAC) clone. We characterized mutants generated by transposome (Tn) mutagenesis of a BAC clone of guinea pig cytomegalovirus (GPCMV). A pool of Tn mutant GPCMV BACs were screened initially by restriction profile analysis to verify they were full-length, and subsequently GPCMV BAC DNA from individual mutants was transfected onto guinea pig lung fibroblast cells in order to generate virus. Tn GPCMV BAC mutants were classed as either essential or non-essential gene insertions, depending upon their ability to regenerate viable, replication-competent virus. Representative mutants were more fully characterized. Analysis by sequencing the Tn insertion site on the mutated BACs, and by regeneration of virus using transfection of guinea pig fibroblasts (GPL), demonstrated that a recombinant with a Tn insertion in the UL35 homolog gene (GP35) was a non-essential gene for viral replication in tissue culture. A mutant with an insertion in the UL46 homolog (GP46) was nonviable, a phenotype which could be rescued by homologous recombination of BAC DNA with wild-type UL46 sequences, suggesting an essential role of this putative capsid gene in virus replication.

Dramatic effects of 2-bromo-5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole riboside on the genome structure, packaging, and egress of guinea pig cytomegalovirus

The halogenated benzimidazoles BDCRB (2-bromo-5,6-dichloro-1-beta-D-riborfuranosyl benzimidazole riboside) and TCRB (2,5,6-trichloro-1-beta-D-riborfuranosyl benzimidazole riboside) were the first compounds shown to inhibit cleavage and packaging of herpesvirus genomes. Both inhibit the formation of unit length human cytomegalovirus (HCMV) genomes by a poorly understood mechanism (M. R. Underwood et al., J. Virol. 72:717-715, 1998; P. M. Krosky et al., J. Virol. 72:4721-4728, 1998). Because the simple genome structure of guinea pig cytomegalovirus (GPCMV) provides a useful model for the study of herpesvirus DNA packaging, we investigated the effects of BDCRB on GPCMV. GPCMV proved to be sensitive to BDCRB (50% inhibitory concentration = 4.7 microM), although somewhat less so than HCMV. In striking contrast to HCMV, however, a dose of BDCRB sufficient to reduce GPCMV titers by 3 logs (50 microM) had no effect on the quantity of GPCMV genomic DNA that was formed in infected cells. Electron microscopy revealed that this DNA was in fact packaged within intranuclear capsids, but these capsids failed to egress from the nucleus and failed to protect the DNA from nuclease digestion. The terminal structure of genomes formed in the presence of BDCRB was also altered. Genomes with ends lacking a terminal repeat at the right end, which normally exist in an equimolar ratio with those having one copy of the repeat at the right end, were selectively eliminated by BDCRB treatment. At the left end, BDCRB treatment appeared to induce heterogeneous truncations such that 2.7 to 4.9 kb of left-end-terminal sequences were missing. These findings suggest that BDCRB induces premature cleavage events that result in truncated genomes packaged within capsids that are permeable to nuclease. Based on these and other observations, we propose a model for duplication of herpesvirus terminal repeats during the cleavage and packaging process that is similar to one proposed for bacteriophage T7 (Y. B. Chung, C. Nardone, and D. C. Hinkle, J. Mol. Biol. 216:939-948, 1990).

A macrophage inflammatory protein homolog encoded by guinea pig cytomegalovirus signals via CC chemokine receptor 1

Cytomegaloviruses encode homologs of cellular immune effector proteins, including chemokines (CKs) and CK receptor-like G protein-coupled receptors (GPCRs). Sequence of the guinea pig cytomegalovirus (GPCMV) genome identified an open reading frame (ORF) which predicted a 101 amino acid (aa) protein with homology to the macrophage inflammatory protein (MIP) subfamily of CC (beta) CKs, designated GPCMV-MIP. To assess functionality of this CK, recombinant GPCMV-MIP was expressed in HEK293 cells and assayed for its ability to bind to and functionally interact with a variety of GPCRs. Specific signaling was observed with the hCCR1 receptor, which could be blocked with hMIP -1alpha in competition experiments. Migration assays revealed that GPCMV-MIP was able to induce chemotaxis in hCCR1-L1.2 cells. Antisera raised against a GST-MIP fusion protein immunoprecipitated species of approximately 12 and 10 kDa from GPCMV-inoculated tissue culture lysates, and convalescent antiserum from GPCMV-infected animals was immunoreactive with GST-MIP by ELISA assay. These results represent the first substantive in vitro characterization of a functional CC CK encoded by a cytomegalovirus.

Cloning of herpesviral genomes as bacterial artificial chromosomes

Herpesviruses, which are important pathogens for both animals and humans, have large and complex genomes with a coding capacity for up to 225 open reading frames (ORFs). Due to the large genome size and the slow replication kinetics in vitro of some herpesviruses, mutagenesis of viral genes in the context of the viral genome by conventional recombination methods in cell culture has been difficult. Given that mutagenesis of viral genes is the basic strategy to investigate function, many of the herpesvirus ORFs could not be defined functionally. Recently, a completely new approach for the construction of herpesvirus mutants has been developed, based on cloning of the virus genome as a bacterial artificial chromosome (BAC) in E. coli. This technique allows the maintenance of viral genomes as a plasmid in E. coli and the reconstitution of viral progeny by transfection of the BAC plasmid into eukaryotic cells. Any genetic modification of the viral genome in E. coli using prokaryotic recombination proteins is possible, thereby allowing the generation of mutant viruses and facilitating the analysis of herpesvirus genomes cloned as infectious BACs. In this review, we describe the principle of cloning a viral genome as a BAC using murine gammaherpesvirus 68 (MHV-68), a mouse model for gammaherpesvirus infections, as an example.

Cloning of the genomes of human cytomegalovirus strains Toledo, TownevarRIT3, and Towne long as BACs and site-directed mutagenesis using a PCR-based technique

The 230-kb human cytomegalovirus genome is among the largest of the known viruses. Experiments to determine the genetic determinants of attenuation, pathogenesis, and tissue tropism are underway; however, a lack of complete sequence data for multiple strains and substantial problems with genetic instability during in vitro propagation create serious complications for such studies. For example, recent findings suggest that common laboratory strains Towne and AD169 passaged in cultured human fibroblasts are missing up to 15 kb of genetic information relative to clinical isolates. To establish standard, genetically stable genomes that can be sequenced, disseminated, and repeatedly reconstituted to produce virus stocks, we have undertaken to clone two variants of Towne, designated Towne(long) and Towne(short) (referred to as TownevarRIT3) (A., Proc. Natl. Acad. Sci. USA 98, 7829-7834), and the pathogenic strain Toledo into bacterial artificial chromosomes (BACs). We further demonstrate the ease with which mutagenesis can be achieved by deleting 13.5 kb from the Toledo genome using a PCR-based technique.