Animal models of congenital cytomegalovirus infection: an overview of progress in the characterization of guinea pig cytomegalovirus (GPCMV)

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.

Animal Models of Congenital Cytomegalovirus Transmission: Implications for Vaccine Development

Although cytomegaloviruses (CMVs) are species-specific, the study of nonhuman CMVs in animal models can help to inform and direct research aimed at developing a human CMV (HCMV) vaccine. Because the driving force behind the development of HCMV vaccines is to prevent congenital infection, the animal model in question must be one in which vertical transmission of virus occurs to the fetus. Fortunately, two such animal models-the rhesus macaque CMV and guinea pig CMV-are characterized by congenital infection. Hence, each model can be evaluated in "proof-of-concept" studies of preconception vaccination aimed at blocking transplacental transmission. This review focuses on similarities and differences in the respective model systems, and it discusses key insights from each model germane to the study of HCMV vaccines.

Cytomegalovirus-induced pathology in human temporal bones with congenital and acquired infection

OBJECTIVE

Publications on histopathology of human temporal bones with cytomegalovirus (CMV) infection are limited. We aim to determine histopathology of the inner ears and the middle ears in human temporal bones with congenital and acquired CMV infections.

METHODS

Temporal bones from 2 infants with congenital and 2 adults with acquired CMV infection were evaluated by light microscopy.

RESULTS

Two infants with congenital CMV infection showed striking pathological changes in the inner ear. There was a hypervascularization of the stria vascularis in the cochlea of the first infant, but no obvious loss of outer and inner hair cells was seen in the organ of Corti. However, cytomegalic cells and a loss of outer hair cells were found in the cochlea of the second infant. The vestibular organs of both infants showed cytomegalic cells, mostly located on dark cells. There was a loss of type I and type II hair cells in the macula of the saccule and utricle. Loss of hair cells and degeneration of nerve fibers was also seen in the semicircular canals. Both infants with congenital infection showed abundant inflammatory cells and fibrous structures in the middle ear cavity. No evidence of cytomegalic cells and hair cell loss was found in the cochlea or vestibular labyrinth in acquired CMV infection.

CONCLUSIONS

In two infants with congenital CMV infection, the cochlea, vestibule, and middle ear were highly affected. Temporal bones of adult donors with acquired viral infection showed histological findings similar to donors of the same age without ear disease.

Cytomegalovirus host entry and spread

Cytomegaloviruses (CMVs) are large, complex pathogens that persistently and systemically colonize most mammals. Human cytomegalovirus (HCMV) causes congenital harm, and has proved hard to control. One problem is that key vaccine targets - virus entry and spread in naive hosts - remain ill-defined. As CMVs predate human speciation, those of other mammals can provide new insight. Murine CMV (MCMV) enters new hosts via olfactory neurons. Like HCMV it binds to heparan, which is lacking from most differentiated apical epithelia but is displayed on olfactory neuronal cilia. It then spreads via infected dendritic cells (DCs), which migrate to draining lymph nodes (LNs), rejoin the circulation by entering high endothelial venules (HEVs), and extravasate into other tissues. This migration depends quantitatively on M33, a constitutively active viral G protein-coupled receptor (GPCR). The homologous US28 GPCR of HCMV can substitute for M33 in allowing MCMV-infected DCs to leave LNs via HEVs, so HCMV could potentially use the same route. The capacity of DCs to seed MCMV to tissues, and for other DCs to collect it for redistribution, suggest that DC recirculation chronically maintains and links diverse CMV reservoirs through lytic exchange.

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.

A Guinea pig cytomegalovirus resistant to the DNA maturation inhibitor BDCRB

Herpesvirus DNA packaging is an essential step in virion morphogenesis and an important target for antiviral development. The halogenated benzimidazole 2-bromo-5,6-dichloro-1-β-d-ribofuranosyl-1H-benzimidazole (BDCRB) was the first compound found to selectively disrupt DNA packaging. It has activity against human cytomegalovirus as well as guinea pig cytomegalovirus. The latter provides a useful small animal model for congenital cytomegalovirus infection. To better understand the mechanism by which BDCRB acts, a guinea pig cytomegalovirus resistant to BDCRB was derived and characterized. An L406P substitution occurred within GP89, a subunit of the complex that cleaves and packages DNA, but transfer of this mutation to an otherwise wild type genetic background did not confer significant BDCRB resistance. The resistant virus also had a 13.4-kb deletion that also appeared to be unrelated to BDCRB-resistance as a virus with a similar spontaneous deletion was sensitive to BDCRB. Lastly, the BDCRB-resistant virus exhibited a dramatic increase in the number of reiterated terminal repeats at both genomic termini. The mechanism that underlies this change in genome structure is not known but may relate to the duplication of terminal repeats that is associated with DNA cleavage and packaging. A model is presented in which BDCRB impairs the ability of terminase to recognize cleavage site sequences, but repeat arrays overcome this impairment by presenting terminase with multiple opportunities to recognize the correct cleavage site sequences that lie within the repeats. Further elucidation of this phenomenon should prove valuable for understanding the molecular basis of herpesvirus DNA maturation and the mechanism of action of this class of drugs.

Chemokines encoded by herpesviruses

Viruses use diverse strategies to elude the immune system, including copying and repurposing host cytokine and cytokine receptor genes. For herpesviruses, the chemokine system of chemotactic cytokines and receptors is a common source of copied genes. Here, we review the current state of knowledge about herpesvirus-encoded chemokines and discuss their possible roles in viral pathogenesis, as well as their clinical potential as novel anti-inflammatory agents or targets for new antiviral strategies.

Detection of herpes simplex virus type 2 (HSV-2) -specific cell-mediated immune responses in guinea pigs during latent HSV-2 genital infection

Genital infections with herpes simplex virus type 2 (HSV-2) are a source of considerable morbidity and are a health concern for newborns exposed to virus during vaginal delivery. Additionally, HSV-2 infection diminishes the integrity of the vaginal epithelium resulting in increased susceptibility of individuals to infection with other sexually transmitted pathogens. Understanding immune protection against HSV-2 primary infection and immune modulation of virus shedding events following reactivation of the virus from latency is important for the development of effective prophylactic and therapeutic vaccines. Although the murine model of HSV-2 infection is useful for understanding immunity following immunization, it is limited by the lack of spontaneous reactivation of HSV-2 from latency. Genital infection of guinea pigs with HSV-2 accurately models the disease of humans including the spontaneous reactivation of HSV-2 from latency and provides a unique opportunity to examine virus-host interactions during latency. Although the guinea pig represents an accurate model of many human infections, relatively few reagents are available to study the immunological response to infection. To analyze the cell-mediated immune response of guinea pigs at extended periods of time after establishment of HSV-2 latency, we have modified flow-cytometry based proliferation assays and IFN-γ ELISPOT assays to detect and quantify HSV-specific cell-mediated responses during latent infection of guinea pigs. Here we demonstrate that a combination of proliferation and ELISPOT assays can be used to quantify and characterize effecter function of virus-specific immune memory responses during HSV-latency.

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.

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.

Models of vertical cytomegalovirus (CMV) transmission and pathogenesis

Despite the considerable clinical impact of congenital human cytomegalovirus (HCMV) infection, the mechanisms of maternal-fetal transmission and the resultant placental and fetal damage are largely unknown. Here, we discuss animal models for the evaluation of CMV vaccines and virus-induced pathology and particularly explore surrogate human models for HCMV transmission and pathogenesis in the maternal-fetal interface. Studies in floating and anchoring placental villi and more recently, ex vivo modeling of HCMV infection in integral human decidual tissues, provide unique insights into patterns of viral tropism, spread, and injury, defining the outcome of congenital infection, and the effect of potential antiviral interventions.

Glycoprotein B (gB) vaccines adjuvanted with AS01 or AS02 protect female guinea pigs against cytomegalovirus (CMV) viremia and offspring mortality in a CMV-challenge model

The transmission of cytomegalovirus (CMV) from mother to fetus can give rise to severe neurodevelopment defects in newborns. One strategy to prevent these congenital defects is prophylactic vaccination in young women. A candidate vaccine antigen is glycoprotein B (gB). This antigen is abundant on the virion surface and is a major target of neutralization responses in human infections. Here, we have evaluated in a challenge model of congenital guinea pig CMV (GPCMV) infection, GPCMV-gB vaccines formulated with the clinically relevant Adjuvant Systems AS01B and AS02V, or with Freund's adjuvant (FA). Fifty-two GPCMV-seronegative female guinea pigs were administered three vaccine doses before being mated. GPCMV-challenge was performed at Day 45 of pregnancy (of an estimated 65 day gestation). Pup mortality rates in the gB/AS01B, gB/AS02V, and gB/FA groups were 24% (8/34), 10% (4/39) and 36% (12/33), respectively, and in the unvaccinated control group was 65% (37/57). Hence, efficacies against pup mortality were estimated at 64%, 84% and 44% for gB/AS01B (p<0.001), gB/AS02V (p<0.001) and gB/FA (p=0.014), respectively. Efficacies against GPCMV viremia (i.e. DNAemia, detected by PCR) were estimated at 88%, 68% and 25% for the same vaccines, respectively, but were only significant for gB/AS01B (p<0.001), and gB/AS02V (p=0.002). In dams with viremia, viral load was approximately 6-fold lower with vaccination than without. All vaccines were highly immunogenic after two and three doses. In light of these results and of other results of AS01-adjuvanted vaccines in clinical development, vaccine immunogenicity was further explored using human CMV-derived gB antigen adjuvanted with either AS01B or the related formulation AS01E. Both adjuvanted vaccines were highly immunogenic after two doses, in contrast to the lower immunogenicity of the unadjuvanted vaccine. In conclusion, the protective efficacy and immunogenicity of adjuvanted vaccines in this guinea pig model are supportive of investigating gB/AS01 and gB/AS02 in the clinic.

A neutralizing anti-gH/gL monoclonal antibody is protective in the guinea pig model of congenital CMV infection

Human cytomegalovirus (HCMV) is the most common cause of congenital virus infection. Congenital HCMV infection occurs in 0.2–1% of all births, and causes birth defects and developmental abnormalities, including sensorineural hearing loss and developmental delay. Several key studies have established the guinea pig as a tractable model for the study of congenital HCMV infection and have shown that polyclonal antibodies can be protective [1]–[3]. In this study, we demonstrate that an anti-guinea pig CMV (GPCMV) glycoprotein H/glycoprotein L neutralizing monoclonal antibody protects against fetal infection and loss in the guinea pig. Furthermore, we have delineated the kinetics of GPCMV congenital infection, from maternal infection (salivary glands, seroconversion, placenta) to fetal infection (fetus and amniotic fluid). Our studies support the hypothesis that a neutralizing monoclonal antibody targeting an envelope GPCMV glycoprotein can protect the fetus from infection and may shed light on the therapeutic intervention of HCMV congenital infection in humans.

Human cytomegalovirus (HCMV) is the most common cause of congenital virus infection and causes developmental abnormalities, including hearing loss and developmental delay. Although there is no therapy for congenital HCMV disease, there is evidence from both human and animal studies that antibodies can have efficacy in this setting. Such studies have focused exclusively on polyclonal antibodies, in which the targets of protective antibodies are unknown. Guinea pigs have been used as a model of human maternal fetal transmission of infection because of similarities in placental anatomy between human and guinea pig. Furthermore, guinea pig CMV (GPCMV) has been demonstrated to cross the placenta and cause fetal infection and loss, similar to the effects of infection with HCMV. However, the kinetics of maternal and fetal infection in this model has not been carefully investigated. In this work, we have delineated the kinetics of maternal to fetal infection and found that congenital infection is rapid following maternal infection. Importantly, we demonstrate that a monoclonal antibody against a protein critical for viral entry protects pregnant guinea pigs against fetal infection. Thus, our studies may be informative for development of a therapeutic intervention to treat congenital HCMV infection in humans.

Cytomegalovirus pp65 limits dissemination but is dispensable for persistence

The most abundantly produced virion protein in human cytomegalovirus (HCMV) is the immunodominant phosphoprotein 65 (pp65), which is frequently included in CMV vaccines. Although it is nonessential for in vitro CMV growth, pp65 displays immunomodulatory functions that support a potential role in primary and/or persistent infection. To determine the contribution of pp65 to CMV infection and immunity, we generated a rhesus CMV lacking both pp65 orthologs (RhCMVΔpp65ab). While deletion of pp65ab slightly reduced growth in vitro and increased defective particle formation, the protein composition of secreted virions was largely unchanged. Interestingly, pp65 was not required for primary and persistent infection in animals. Immune responses induced by RhCMVΔpp65ab did not prevent reinfection with rhesus CMV; however, reinfection with RhCMVΔUS2-11, which lacks viral-encoded MHC-I antigen presentation inhibitors, was prevented. Unexpectedly, induction of pp65b-specific T cells alone did not protect against RhCMVΔUS2-11 challenge, suggesting that T cells targeting multiple CMV antigens are required for protection. However, pp65-specific immunity was crucial for controlling viral dissemination during primary infection, as indicated by the marked increase of RhCMVΔpp65ab genome copies in CMV-naive, but not CMV-immune, animals. Our data provide rationale for inclusion of pp65 into CMV vaccines but also demonstrate that pp65-induced T cell responses alone do not recapitulate the protective effect of natural infection.

What we have learned from animal models of HCMV

Although human cytomegalovirus (HCMV) primary infection is generally asymptomatic, in immune-compromised patients HCMV increases morbidity and mortality. As a member of the betaherpesvirus family, in vivo studies of HCMV are limited due to its species specificity. CMVs from other species are often used as surrogates to express HCMV genes/proteins or used as models for inferring HCMV protein function in humans. Using innovative experiments, these animal models have answered important questions about CMV's life cycle, dissemination, pathogenesis, immune evasion, and host immune response. This chapter provides CMV biologists with an overview of the insights gained using these animal models. Subsequent chapters will provide details of the specifics of the experimental methods developed for each of the animal models discussed here.

Characterization of the guinea pig CMV gH/gL/GP129/GP131/GP133 complex in infection and spread

In human cytomegalovirus (HCMV), the UL128-131A locus plays an essential role in cellular tropism and spread. Here, we report the complete annotation of the GP129-133 locus from guinea pig cytomegalovirus (GPCMV) and the discovery of the UL131A homolog, named GP133. We have found that similar to HCMV the GP129-133 proteins form a pentamer complex with the GPCMV glycoproteins gH and gL. In addition, we find that the GP129-133 proteins play a critical role in entry as the GP129-133 deletion mutant shows a defect in both endothelial and fibroblast cell entry. Although the GP129-133 deletion strain can propagate in vitro, we find that the deletion fails to spread in vivo. Interestingly, the wildtype strain can spontaneously give rise to the GP129-133 deletion strain during in vivo spread, suggesting genetic instability at this locus.

Human cytomegalovirus is protected from inactivation by reversible binding to villous trophoblasts

Human cytomegalovirus (HCMV) is the leading cause of congenital disease in the developed world. Transmission of HCMV to the fetus can occur through the villous placenta. Previously, we have shown that although syncytiotrophoblast (ST) can be productively infected, it is more likely that HCMV reaches the fetus through breaks in the ST than through basal release of progeny virus from infected ST. Progeny virus released on the maternal side could interact back with the ST and accumulate. In pregnancy, the organ distribution of disease burden is dramatically shifted, with the placenta reported as a reservoir for some pathogens. Thus, we propose that the ST layer functions as a viral reservoir, where HCMV is harbored and ultimately protected from degradation. Using primary cytotrophoblasts differentiated into an ST culture in vitro and challenged with HCMV, we have defined reversible binding between the virus and trophoblasts that protects the virus from degradation. This is blocked by treatment with low pH and neutralizing intravenous immunoglobulin. This reversible binding likely is to heparan sulfate proteoglycans, because heparin treatment blocks it. Importantly, we show that bound and released virus maintained in culture for at least 48 h results from inoculum and not progeny virus. Thus, the placenta has the potential to accumulate a relatively high steady-state level of virus within the intervillous space resulting from localized binding and release at the ST. A better understanding of the molecular interactions between HCMV and ST will provide insights regarding interventions to prevent or minimize congenital transmission.

Mouse embryonic stem cells inhibit murine cytomegalovirus infection through a multi-step process

In humans, cytomegalovirus (CMV) is the most significant infectious cause of intrauterine infections that cause congenital anomalies of the central nervous system. Currently, it is not known how this process is affected by the timing of infection and the susceptibility of early-gestational-period cells. Embryonic stem (ES) cells are more resistant to CMV than most other cell types, although the mechanism responsible for this resistance is not well understood. Using a plaque assay and evaluation of immediate-early 1 mRNA and protein expression, we found that mouse ES cells were resistant to murine CMV (MCMV) at the point of transcription. In ES cells infected with MCMV, treatment with forskolin and trichostatin A did not confer full permissiveness to MCMV. In ES cultures infected with elongation factor-1α (EF-1α) promoter-green fluorescent protein (GFP) recombinant MCMV at a multiplicity of infection of 10, less than 5% of cells were GFP-positive, despite the fact that ES cells have relatively high EF-1α promoter activity. Quantitative PCR analysis of the MCMV genome showed that ES cells allow approximately 20-fold less MCMV DNA to enter the nucleus than mouse embryonic fibroblasts (MEFs) do, and that this inhibition occurs in a multi-step manner. In situ hybridization revealed that ES cell nuclei have significantly less MCMV DNA than MEF nuclei. This appears to be facilitated by the fact that ES cells express less heparan sulfate, β1 integrin, and vimentin, and have fewer nuclear pores, than MEF. This may reduce the ability of MCMV to attach to and enter through the cellular membrane, translocate to the nucleus, and cross the nuclear membrane in pluripotent stem cells (ES/induced pluripotent stem cells). The results presented here provide perspective on the relationship between CMV susceptibility and cell differentiation.

Development of a guinea pig model of perinatal cytomegalovirus-induced hepatobiliary injury

To explore the possible link between perinatal cytomegalovirus infection and onset of biliary atresia, we build up a perinatal cytomeglovirus induced hepatobiliary system injury model in guinea pigs, verified the biliary tropism of gpCMV and the related inflammatory injury. It suggested host immune status, local response to inflammation, and activation of Th1-cell-mediated antiviral immune responses contributed to newborn geinea pigs' hepatobiliary damage. The model was helpful in study the CMV infection and biliary injury.

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.

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.

A live guinea pig cytomegalovirus vaccine deleted of three putative immune evasion genes is highly attenuated but remains immunogenic in a vaccine/challenge model of congenital cytomegalovirus infection

Live attenuated vaccines for prevention of congenital cytomegalovirus infections encode numerous immune evasion genes. Their removal could potentially improve vaccine safety and efficacy. To test this hypothesis, three genes encoding MHC class I homologs (presumed NK evasins) were deleted from the guinea pig cytomegalovirus genome and the resulting virus, 3DX, was evaluated as a live attenuated vaccine in the guinea pig congenital infection model. 3DX was attenuated in vivo but not in vitro. Vaccination with 3DX produced elevated cytokine levels and higher antibody titers than wild type (WT) virus while avidity and neutralizing titers were similar. Protection, assessed by maternal viral loads and pup mortality following pathogenic viral challenge during pregnancy, was comparable between 3DX and WT and significant compared to naïve animals. These results suggest that the safety and perhaps efficacy of live attenuated human cytomegalovirus vaccines could be enhanced by deletion of viral immunomodulatory genes.

Neuropathogenesis of congenital cytomegalovirus infection: disease mechanisms and prospects for intervention

Congenital cytomegalovirus (CMV) infection is the leading infectious cause of mental retardation and hearing loss in the developed world. In recent years, there has been an improved understanding of the epidemiology, pathogenesis, and long-term disabilities associated with CMV infection. In this review, current concepts regarding the pathogenesis of neurological injury caused by CMV infections acquired by the developing fetus are summarized. The pathogenesis of CMV-induced disabilities is considered in the context of the epidemiology of CMV infection in pregnant women and newborn infants, and the clinical manifestations of brain injury are reviewed. The prospects for intervention, including antiviral therapies and vaccines, are summarized. Priorities for future research are suggested to improve the understanding of this common and disabling illness of infancy.

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.

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.

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.

Engineering of cytomegalovirus genomes for recombinant live herpesvirus vaccines

The advances of sequence knowledge and genetic engineering hold a great promise for a rational approach to vaccine development. Herpesviruses are important pathogens of all vertebrates. They cause acute and chronic infections and persist in their hosts for life. In man there are eight herpesviruses known and most of them can be linked to diseases. To date only one licensed vaccine against a human herpesvirus exists and there is no proven successful concept on rational design for herpesvirus vaccines available. Here, we use new reverse genetic systems, based on the 230-kb mouse cytomegalovirus genome to explore new methods of vaccine delivery and of virus attenuation. With regard to virus delivery, we show that the bacterial transfer of the infectious DNA in vivo is theoretically possible but not yet a practical option. With regard to a rational approach of virus attenuation, we consider a selective deletion of viral genes that modulate the immune response of the host.

Human CMV vaccine trials: what if CMV caused a rash?

There are no apparent biologic obstacles to immunization against fetal/placental infection with cytomegalovirus (CMV), and vaccine candidates have been developed. The major obstacles for a human CMV vaccine are difficulties associated with the design and execution of efficacy trials. These trials will be prolonged with difficulties in recruiting subjects, and hampered by several factors: First is a lack of public awareness of CMV, making recruitment into vaccine trials difficult. Second is that trials using fetal infection as an endpoint will be prolonged since vaccine administration must occur preconception. Third, behavioral changes by subjects will affect infection rates, and controls for behavioral changes will have to be in place in any clinical trial. Fourth, not all women are at equal risk for CMV infections. High risk women may become infected by contact with young children or via sexual activity. Thus, the mode of acquisition may be a confounding variable in vaccine efficacy trials. In spite of these potential obstacles, successful evaluation of CMV vaccines is possible and likely when considered against similar obstacles encountered by several other recently licensed vaccines.

Human cytomegalovirus immediate-early-gene expression disrupts embryogenesis in transgenic Drosophila

Intrauterine infection with human cytomegalovirus (HCMV) is the leading viral cause of birth defects involving the central nervous system. Due to the highly species specific nature of the virus, its course of natural infection cannot be studied in animal models. Here we introduce a novel transgenic Drosophila model system for studying the effects of the major viral regulatory genes, the immediate-early genes, on normal embryonic development. We show that ectopic expression of the immediate-early genes in Drosophila led to increased embryonic lethality manifested in disintegration of the embryos. Further analysis suggested that immediate-early gene expression interfered with adherens junction maintenance, leading to the disruption of embryonic epithelial integrity. Owing to the evolutionary conservation of developmental mechanisms from invertebrates to mammals, we anticipate that the studies in Drosophila will be relevant also to humans and will ultimately provide a versatile system for studying different aspects of viral-host interactions.

Cyclosporine inhibits mouse cytomegalovirus infection via a cyclophilin-dependent pathway specifically in neural stem/progenitor cells

The potential of neural stem and progenitor cell (NSPC) transplantation in neurodegenerative disease raises a concern about immunosuppressive agents and opportunistic neurotropic pathogens that may interfere with engraftment. Cytomegalovirus (CMV) is an important opportunistic pathogen infecting the central nervous system, where it may remain latent for life, following transplacental transmission. Cyclosporine (Cs), an immunosuppressive drug used in organ transplantation, where its use is associated with CMV reactivation, suppressed murine CMV (MCMV) infection in cultured NSPCs but not in fibroblasts. This activity of Cs appears to be mediated via cyclophilin (CyP) rather than via calcineurin. First, the calcineurin-specific inhibitor FK506 failed to suppress replication. Second, the CyP-specific inhibitor NIM811 strongly suppressed replication in NSPC. NSPCs maintained in the presence of NIM811 retained viral genomes for several weeks without detectable viral gene expression or obvious deleterious effects. The withdrawal of NIM811 reactivated viral replication, suggesting that the inhibitory mechanism was reversible. Finally, inhibition of endogenous CyP A (CyPA) by small interfering RNA also inhibited replication in NSPCs. These results show that MCMV replication depends upon cellular CyPA pathways in NSPCs (in a specific cell type-dependent fashion), that CyPA plays an important role in viral infection in this cell type, and that inhibition of viral replication via CyP leads to persistence of the viral genome without cell damage. Further, the calcineurin-signaling pathway conferring immunosuppression in T cells does not influence viral replication in a detectable fashion.

Pathogenesis of cytomegalovirus-associated labyrinthitis in a guinea pig model

Cytomegalovirus infects fetuses through the placenta, resulting in various congenital disorders in newborns, including hearing loss. We developed a monoclonal antibody to guinea pig cytomegalovirus (GPCMV) that was available for immunohistochemistry, and investigated the expression of the GPCMV antigen in animal models of direct and congenital infections. Injection of GPCMV, directly to the inner ear, increased the sound pressure level and resulted in labyrinthitis with severe inflammation. Immunohistochemistry detected GPCMV-infected cells mainly in the scala tympani, scala vestibule and spinal ganglion, but rarely in the cochlear duct. Injection of GPCMV to 5-week pregnant guinea pigs resulted in severe labyrinthitis in fetuses. Immunohistochemistry detected GPCMV-infected cells in the perilymph area and spinal ganglion, but not in the endolymph area, including hair cells. These data suggest that the virus spreads via the perilymph and neural routes in the inner ear of both models of direct and congenital infections.

Predicting coding potential from genome sequence: application to betaherpesviruses infecting rats and mice

Prediction of protein-coding regions and other features of primary DNA sequence have greatly contributed to experimental biology. Significant challenges remain in genome annotation methods, including the identification of small or overlapping genes and the assessment of mRNA splicing or unconventional translation signals in expression. We have employed a combined analysis of compositional biases and conservation together with frame-specific G+C representation to reevaluate and annotate the genome sequences of mouse and rat cytomegaloviruses. Our analysis predicts that there are at least 34 protein-coding regions in these genomes that were not apparent in earlier annotation efforts. These include 17 single-exon genes, three new exons of previously identified genes, a newly identified four-exon gene for a lectin-like protein (in rat cytomegalovirus), and 10 probable frameshift extensions of previously annotated genes. This expanded set of candidate genes provides an additional basis for investigation in cytomegalovirus biology and pathogenesis.

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.

Vaccine development to prevent cytomegalovirus disease: report from the National Vaccine Advisory Committee

Cytomegalovirus (CMV) infection is the most common intrauterine infection in the United States, and it exacts a heavy toll when it infects children and immunocompromised individuals. A CMV vaccine was assigned the highest priority by the Institute of Medicine in its 1999 assessment of targets for vaccine development. The priority was based on the cost and human suffering that would be alleviated by reducing the disease burden of congenital CMV infection. The National Vaccine Advisory Committee and invited experts examined the prospects for a CMV vaccine and the actions needed to bring about successful vaccine development at a National Vaccine Program Office workshop in October 2000. This article summarizes information about the changing epidemiology of CMV and immune responses to infection and immunity, and it reviews the current status of several vaccine candidates. Support of government agencies for CMV vaccine research and development is critical to address this need.

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.

Listeriosis in the pregnant guinea pig: a model of vertical transmission

Feto-placental infections represent a major cause of pregnancy complications, and yet the underlying molecular and cellular mechanisms of vertical transmission are poorly understood. Listeria monocytogenes, a facultative intracellular pathogen, is one of a group of pathogens that are known to cause feto-placental infections in humans and other mammals. The purpose of this study was to evaluate possible mechanisms of vertical transmission of L. monocytogenes. Humans and guinea pigs have a hemochorial placenta, where a single layer of fetally derived trophoblasts separates maternal from fetal circulation. We characterized L. monocytogenes infection of the feto-placental unit in a pregnant guinea pig model and in primary human trophoblasts and trophoblast-derived cell lines. The clinical manifestations of listeriosis in the pregnant guinea pigs and the tropism of L. monocytogenes to the guinea pig placenta resembled those in humans. Trophoblast cell culture systems were permissive for listerial growth and cell-to-cell spread and revealed that L. monocytogenes deficient in internalin A, a virulence factor that mediates invasion of nonphagocytic cells, was 100-fold defective in invasion. However, crossing of the feto-placental barrier in the guinea pig model was independent of internalin A, suggesting a negligible role for internalin-mediated direct invasion of trophoblasts in vivo. Further understanding of vertical transmission of L. monocytogenes will help in designing more effective means of treatment and disease prevention.

Murine cytomegalovirus with a transposon insertional mutation at open reading frame M35 is defective in growth in vivo

We had previously constructed a pool of murine cytomegalovirus (MCMV) mutants that contained a Tn3-based transposon sequence randomly inserted in the viral genome. In the study reported here, one of the mutants, RvM35, which contains the transposon insertion at open reading frame M35, was characterized both in vitro in tissue cultures and in immunocompetent Balb/c and immunodeficient SCID mice. Our results provide the first direct evidence to suggest that M35 is not essential for viral replication in vitro in NIH 3T3 cells. Compared to the wild-type strain and a rescued virus that restored the M35 region, the viral mutant was attenuated in growth in both the intraperitoneally infected Balb/c and SCID mice. At 21 days postinfection, the titers of the mutant in the salivary glands, lungs, spleens, livers, and kidneys of the SCID mice were lower than the titers of the wild-type Smith strain and the rescued virus by 50,000-, 100-, 10-, 100-, and 50-fold, respectively. Moreover, the growth of RvM35 is severely attenuated in the salivary glands. The virulence of the mutant virus also appears to be attenuated, because no death was observed in SCID mice infected with RvM35 until 35 days postinfection, while all the animals infected with the wild-type and rescued viruses died 27 days postinfection. Our results suggest that M35 is important for MCMV virulence in killing SCID mice and is required for optimal viral growth in vivo, including in the salivary glands.

Cloning and expression of the guinea pig cytomegalovirus glycoprotein B (gB) in a recombinant baculovirus: utility for vaccine studies for the prevention of experimental infection

The guinea pig cytomegalovirus (GPCMV) is unique among the cytomegaloviruses of small mammals, insofar as during pregnancy it crosses the placenta, causing infection of the fetus. Although the guinea pig model is well suited to vaccine studies, the lack of cloned, recombinant forms of immunogenic GPCMV proteins, such as envelope glycoproteins, has hindered experimental evaluations of subunit immunization for prevention of fetal disease. Since the glycoprotein B (gB) is a major target of neutralizing antibody responses, the GPCMV gB was cloned and expressed in a recombinant baculovirus. A recombinant was generated which expressed gB, truncated at codon 692, upstream of the putative transmembrane domain. Processing and expression of the recombinant protein, designated Bac-gB, was assessed, and the protein was characterized immunologically. Anti-gB antibodies were immunoreactive with Bac-gB by enzyme linked immunosorbent assay (ELISA) and immunoblot assay. Immunoprecipitation with polyclonal anti-GPCMV antisera identified protein species of 120, 80 and 30 kDa by reducing SDS-PAGE, suggesting that authentic cleavage and processing of Bac-gB occurred in insect cells. Sera from guinea pigs immunized with lectin-column purified native glycoproteins had high ELISA titers to Bac-gB. Recombinant GPCMV gB expressed in insect cells should prove useful in defining correlates of protective immunity in the GPCMV congenital infection model.

Quantitative-competitive PCR monitoring of viral load following experimental guinea pig cytomegalovirus infection

Human cytomegalovirus (HCMV) is the most common cause of congenital viral infection in the developed world, and can lead to significant morbidity. Animal models of HCMV infection are required for study of pathogenesis, because of the strict species-specificity of cytomegalovirus (CMV). Among the small animal CMV models, the guinea pig CMV (GPCMV) has unique advantages, in particular its propensity to cross the placenta, causing disease in utero. In order to develop quantitative endpoints for vaccine and antiviral therapeutic studies in the GPCMV model, a quantitative-competitive PCR (qcPCR) assay was developed, based on the GPCMV homolog of the HCMV UL83 gene, GP83. Optimal amplification of GPCMV DNA was observed using primers spanning a 248 base pair (bp) region of this gene. A 91 bp deletion of this cloned fragment was generated for use as an internal standard (IS) for PCR amplification. Standard curves based upon the fluorescent intensity of full-length external target to IS were compared with signal intensity of DNA extracted from blood and organs of experimentally infected guinea pigs in order to quantify viral load. Viral load in newborn guinea pigs infected transplacentally was determined and compared with that of pups infected with GPCMV as neonates. Viral loads were highest in pups infected as neonates. The most consistent isolation and highest quantities of viral DNA were observed in liver and spleen, although viral genome could be readily identified in brain, lung, and salivary gland. Viral load determination should be useful for monitoring outcomes following vaccine studies, as well as responses to experimental antiviral agents.

A novel CC-chemokine homolog encoded by guinea pig cytomegalovirus

Infection with cytomegalovirus (CMV) is persistent, even in the normal host. Periodic viral reactivation may have serious consequences, particularly if the infected individual is immunosuppressed, or pregnant. A number of CMV genes appear to contribute to the phenomena of evasion of host immune clearance, including homologs of cellular immune effector proteins, such as chemokines (CKs), chemokine receptor-like G protein-coupled receptors (GPCRs), and MHC class I molecules. To examine whether the guinea pig cytomegalovirus (GPCMV) encodes homologs of these cellular immunoregulatory genes, regions of the viral genome were sequenced and analyzed for the presence of conserved and novel open reading frames (ORFs) with potential homology to GPCR and CK proteins. A region in the Hind III 'D' region of the genome was identified which had strong identity to multiple beta (CC) chemokines, particularly members of the macrophage inflammatory protein 1 (MIP-1) family. Northern blot analysis indicated that this region of the genome was transcriptionally active, encoding a transcript of 1.7 kbp, which was synthesized with 'late' gene kinetics. This is the first identification of a CK gene encoded by GPCMV, and adds to the growing list of putative CMV immunomodulatory genes which appear to have been transduced from the host genome during the co-evolution of host and pathogen.