Abortive infection of human diploid cells by murine cytomegalovirus

Tumor Control by Cytomegalovirus: A Door Open for Oncolytic Virotherapy?

Belonging to the herpesviridae family, human cytomegalovirus (HCMV) is a well-known ubiquitous pathogen that establishes a lifelong infection in humans. Recently, a beneficial tumor-cytoreductive role of CMV infection has been defined in human and animal models. Described as a potential anti-tumoral activity, HCMV modulates the tumor microenvironment mainly by inducing cell death through apoptosis and prompting a robust stimulatory effect on the immune cells infiltrating the tumor tissue. However, major current limitations embrace transient protective effect and a viral dissemination potential in immunosuppressed hosts. The latter could be counteracted through direct viral intratumoral delivery, use of non-human strains, or even defective CMV vectors to ascertain transformed cells-selective tropism. This potential oncolytic activity could be complemented by tackling further platforms, namely combination with immune checkpoint inhibitors or epigenetic therapy, as well as the use of second-generation chimeric oncovirus, for instance HCMV/HSV-1 oncolytic virus. Overall, preliminary data support the use of CMV in viral oncolytic therapy as a viable option, establishing thus a potential new modality, where further assessment through extensive basic research armed by molecular biotechnology is compulsory.

Uncovering the Anticancer Potential of Murine Cytomegalovirus against Human Colon Cancer Cells

Human cytomegalovirus (HCMV) components are often found in tumors, but the precise relationship between HCMV and cancer remains a matter of debate. Pro-tumor functions of HCMV were described in several studies, but an association between HCMV seropositivity and reduced cancer risk was also evidenced, presumably relying on recognition and killing of cancer cells by HCMV-induced lymphocytes. This study aimed at deciphering whether CMV influences cancer development in an immune-independent manner. Using immunodeficient mice, we showed that systemic infection with murine CMV (MCMV) inhibited the growth of murine carcinomas. Surprisingly, MCMV, but not HCMV, also reduced human colon carcinoma development in vivo. In vitro, both viruses infected human cancer cells. Expression of human interferon-β (IFN-β) and nuclear domain (ND10) were induced in MCMV-infected, but not in HCMV-infected human colon cancer cells. These results suggest a decreased capacity of MCMV to counteract intrinsic defenses in the human cellular host. Finally, immunodeficient mice receiving peri-tumoral MCMV therapy showed a reduction of human colon cancer cell growth, albeit no clinical sign of systemic virus dissemination was evidenced. Our study, which describes a selective advantage of MCMV over HCMV to control human colon cancer, could pave the way for the development of CMV-based therapies against cancer.

The Susceptibility of Primary Dermis Fibroblasts from the Chinese Tree Shrew to Human Cytomegalovirus Infection

As a universal pathogen leading to neonatal defects and transplant failure, human cytomegalovirus (HCMV) has strict species specificity and this has prevented the development of a suitable animal model for the pathogenesis study. The mechanism of cross-species barrier remains elusive and there are so far no non-human cell culture models that support HCMV replication. The Chinese tree shrew (Tupaia belangeri chinensis) is a small laboratory animal and evolutionary closely related with primates. We investigated the susceptibility of primary tree shrew dermis fibroblasts (TSDF) to HCMV infection. Infection with a GFP-expressing HCMV virus resulted in green fluorescence in infected cells with the expression of IE1, UL44 and pp28. The titers of cell-free viruses reached 10³ PFU/mL at 96 hpi, compared to titers of 10⁴ PFU/mL observed in primary human foreskin fibroblasts. Our results suggested that TSDF was semi-permissive for HCMV infection. The TSDF model could be further used to investigate key factors influencing cross-species multiplication of HCMV.

Murine cytomegalovirus IE3-dependent transcription is required for DAI/ZBP1-mediated necroptosis

DNA-dependent activator of interferon regulatory factors/Z-DNA binding protein 1 (DAI/ZBP1) is a crucial sensor of necroptotic cell death induced by murine cytomegalovirus (MCMV) in its natural host. Here, we show that viral capsid transport to the nucleus and subsequent viral IE3-dependent early transcription are required for necroptosis. Necroptosis induction does not depend on input virion DNA or newly synthesized viral DNA A putative RNA-binding domain of DAI/ZBP1, Zα2, is required to sense virus and trigger necroptosis. Thus, MCMV IE3-dependent transcription from the viral genome plays a crucial role in activating DAI/ZBP1-dependent necroptosis. This implicates RNA transcripts generated by a large double-stranded DNA virus as a biologically relevant ligand for DAI/ZBP1 during natural viral infection.

UL36 Rescues Apoptosis Inhibition and In vivo Replication of a Chimeric MCMV Lacking the M36 Gene

Apoptosis is an important defense mechanism mounted by the immune system to control virus replication. Hence, cytomegaloviruses (CMV) evolved and acquired numerous anti-apoptotic genes. The product of the human CMV (HCMV) UL36 gene, pUL36 (also known as vICA), binds to pro-caspase-8, thus inhibiting death-receptor apoptosis and enabling viral replication in differentiated THP-1 cells. In vivo studies of the function of HCMV genes are severely limited due to the strict host specificity of cytomegaloviruses, but CMV orthologues that co-evolved with other species allow the experimental study of CMV biology in vivo. The mouse CMV (MCMV) homolog of the UL36 gene is called M36, and its protein product (pM36) is a functional homolog of vICA that binds to murine caspase-8 and inhibits its activation. M36-deficient MCMV is severely growth impaired in macrophages and in vivo. Here we show that pUL36 binds to the murine pro-caspase-8, and that UL36 expression inhibits death-receptor apoptosis in murine cells and can replace M36 to allow MCMV growth in vitro and in vivo. We generated a chimeric MCMV expressing the UL36 ORF sequence instead of the M36 one. The newly generated MCMV^UL36 inhibited apoptosis in macrophage lines RAW 264.7, J774A.1, and IC-21 and its growth was rescued to wild type levels. Similarly, growth was rescued in vivo in the liver and spleen, but only partially in the salivary glands of BALB/c and C57BL/6 mice. In conclusion, we determined that an immune-evasive HCMV gene is conserved enough to functionally replace its MCMV counterpart and thus allow its study in an in vivo setting. As UL36 and M36 proteins engage the same molecular host target, our newly developed model can facilitate studies of anti-viral compounds targeting pUL36 in vivo.

Stepwise adaptation of murine cytomegalovirus to cells of a foreign host for identification of host range determinants

Ever since their first isolation 60 years ago, cytomegaloviruses have been recognized as being highly species specific. They replicate only in cells of their own or a closely related host species, while cells of phylogenetically more distant hosts are usually not permissive for viral replication. For instance, human cytomegalovirus replicates in human and chimpanzee fibroblasts but not in rodent cells, and murine cytomegalovirus (MCMV) replicates in cells of mice and rats but not in primate cells. However, the viral and cellular factors determining the narrow host range of cytomegaloviruses have remained largely unknown. We show that MCMV can be adapted stepwise to replicate in cultured human retinal pigment epithelial (RPE-1) cells and human fibroblasts. The human RPE-1 cells used for the initial adaptation step showed a pronounced contact inhibition and produced very low level of interferon-β transcripts upon cytomegalovirus infection, suggesting that these cells provide a particularly favorable environment for adaptation. By whole genome sequencing of the 230 kbp viral genomes of several adapted mutants, a limited number of mutations were detected. Comparison of several human cell-adapted MCMV clones and introduction of specific mutations into the wild-type MCMV genome by site-directed mutagenesis allows for the identification of viral host range determinants and provides the basis for elucidating the molecular basis of the cytomegalovirus host species specificity.

Developing a Vaccine against Congenital Cytomegalovirus (CMV) Infection: What Have We Learned from Animal Models? Where Should We Go Next?

Congenital human cytomegalovirus (HCMV) infection can lead to long-term neurodevelopmental sequelae, including mental retardation and sensorineural hearing loss. Unfortunately, CMVs are highly adapted to their specific species, precluding the evaluation of HCMV vaccines in animal models prior to clinical trials. Several species-specific CMVs have been characterized and developed in models of pathogenesis and vaccine-mediated protection against disease. These include the murine CMV (MCMV), the porcine CMV (PCMV), the rhesus macaque CMV (RhCMV), the rat CMV (RCMV), and the guinea pig CMV (GPCMV). Because of the propensity of the GPCMV to cross the placenta, infecting the fetus in utero, it has emerged as a model of particular interest in studying vaccine-mediated protection of the fetus. In this paper, a review of these various models, with particular emphasis on the value of the model in the testing and evaluation of vaccines against congenital CMV, is provided. Recent exciting developments and advances in these various models are summarized, and recommendations offered for high-priority areas for future study.

Efficiency of porcine endothelial cell infection with human cytomegalovirus depends on both virus tropism and endothelial cell vascular origin

BACKGROUND

Human cytomegalovirus (HCMV) infection or reactivation has been linked to allograft rejection resulting from endothelial injury and immune activation. In pig-to-human xenotransplantation, currently investigated to circumvent the shortage of human organs in transplantation medicine, the porcine endothelium will inevitably be exposed to human pathogens such as HCMV. We investigated the susceptibility of porcine endothelial cells (pEC) to HCMV infection.

METHODS

Immortalized porcine aortic (PEDSV15) and porcine microvascular bone-marrow derived EC (2A2) as well as a panel of primary pEC originated from different vascular beds were inoculated with the endotheliotropic (TB40/E) and the fibroblast propagated (TB40/F) HCMV strains at multiplicity of infection (MOI) ranging from 0.1 to 5. Viral replication kinetics, development of cytopathology and release of viral progeny were analyzed.

RESULTS

All viral strains infected pEC with differences in both infection efficiency and kinetics of cytopathology. Moreover, differences in susceptibility of pEC derived from distinct vascular beds were observed. HCMV underwent a complete replication cycle in about 5% of the infected pEC. Comparing the permissiveness of pEC to human aortic EC (HAEC) revealed differences in strain susceptibility and lower rates of late antigen expression in pEC. Finally, HCMV-infected pEC released viral particles but with a lower efficiency than infected HAEC.

CONCLUSIONS

Our data demonstrate that HCMV productively infects pEC, therefore finding strategies to render pEC resistant to HCMV infection will be of interest to reduce the potential risk carried by HCMV reactivation in xenotransplantation.

Mutations in the M112/M113-coding region facilitate murine cytomegalovirus replication in human cells

Cytomegaloviruses, representatives of the Betaherpesvirinae, cause opportunistic infections in immunocompromised hosts. They infect various cells and tissues in their natural host but are highly species specific. For instance, human cytomegalovirus (HCMV) does not replicate in mouse cells, and human cells are not permissive for murine cytomegalovirus (MCMV) infection. However, the underlying molecular mechanisms are so far poorly understood. In the present study we isolated and characterized a spontaneously occurring MCMV mutant that has gained the capacity to replicate rapidly and to high titers in human cells. Compared to the parental wild-type (wt) virus, this mutant formed larger nuclear replication compartments and replicated viral DNA more efficiently. It also disrupted promyelocytic leukemia (PML) protein nuclear domains with greater efficiency but caused less apoptosis than did wt MCMV. Sequence analysis of the mutant virus genome revealed mutations in the M112/M113-coding region. This region is homologous to the HCMV UL112-113 region and encodes the viral early 1 (E1) proteins, which are known to play an important role in viral DNA replication. By introducing the M112/M113 mutations into wt MCMV, we demonstrated that they are sufficient to facilitate MCMV replication in human cells and are, at least in part, responsible for the efficient replication capability of the spontaneously adapted virus. However, additional mutations probably contribute as well. These results reveal a previously unrecognized role of the viral E1 proteins in regulating viral replication in different cells and provide new insights into the mechanisms of the species specificity of cytomegaloviruses.

Cell cycle-independent expression of immediate-early gene 3 results in G1 and G2 arrest in murine cytomegalovirus-infected cells

The infectious cycle of human cytomegalovirus (HCMV) is intricately linked to the host's cell cycle. Viral gene expression can be initiated only in G(0)/G(1) phase. Once expressed, the immediate-early gene product IE2 prevents cellular DNA synthesis, arresting infected cells with a G(1) DNA content. This function is required for efficient viral replication in vitro. A prerequisite for addressing its in vivo relevance is the characterization of cell cycle-regulatory activities of CMV species for which animal models have been established. Here, we show that murine CMV (MCMV), like HCMV, has a strong antiproliferative capacity and arrests cells in G(1). Unexpectedly, and in contrast to HCMV, MCMV can also block cells that have passed through S phase by arresting them in G(2). Moreover, MCMV can also replicate in G(2) cells. This is made possible by the cell cycle-independent expression of MCMV immediate-early genes. Transfection experiments show that of several MCMV candidate genes, only immediate-early gene 3 (ie3), the homologue of HCMV IE2, exhibits cell cycle arrest activity. Accordingly, an MCMV ie3 deletion mutant has lost the ability to arrest cells in either G(1) or G(2). Thus, despite interspecies variations in the cell cycle dependence of viral gene expression, the central theme of HCMV IE2-induced cell cycle arrest is conserved in the murine counterpart, raising the possibility of studying its physiological relevance at the level of the whole organism.

Induction of apoptosis limits cytomegalovirus cross-species infection

Cross-species infections are responsible for the majority of emerging and re-emerging viral diseases. However, little is known about the mechanisms that restrict viruses to a certain host species, and the factors viruses need to cross the species barrier and replicate in a different host. Cytomegaloviruses (CMVs) are representatives of the beta-herpesviruses that are highly species specific. They replicate only in cells of their own or a closely related species. In this study, the molecular mechanism underlying the cytomegalovirus species specificity was investigated. We show that infection of human cells with the murine cytomegalovirus (MCMV) triggers the intrinsic apoptosis pathway involving caspase-9 activation. MCMV can break the species barrier and replicate in human cells if apoptosis is blocked by Bcl-2 or a functionally analogous protein. A single gene of the human cytomegalovirus encoding a mitochondrial inhibitor of apoptosis is sufficient to allow MCMV replication in human cells. Moreover, the same principle facilitates replication of the rat cytomegalovirus in human cells. Thus, induction of apoptosis serves as an innate immune defense to inhibit cross-species infections of rodent CMVs.

Propagation and titration of murine cytomegalovirus in a continuous bone marrow-derived stromal cell line (M2-10B4)

Murine cytomegalovirus (MCMV) can only be propagated effectively in mouse embryo fibroblast (MEF) cells. We demonstrate that MCMV replicates significantly better in M2-10B4 cells, a continuous line of murine bone marrow stromal cells. M2-10B4 cells were also comparable to MEF cells for detection of small amounts of MCMV reactivating from latently infected spleen explants. M2-10B4 cells will be very useful for studies of MCMV pathogenesis.

Analysis of immediate-early and early proteins of murine cytomegalovirus in permissive and nonpermissive cells

The immediate-early (IE) and early proteins induced by murine cytomegalovirus (Smith strain) in permissively infected 3T3-L1 murine fibroblasts were identified by SDS polyacrylamide gel electrophoresis. Ten proteins were classified as IE by their time of synthesis and by their synthesis in the presence of actinomycin D following reversal of a cycloheximide mediated protein synthesis block. By exclusion, seven proteins were classified as early class. Eleven of these proteins were precipitated by MCMV antiserum. The role of IE and early proteins in the replication of the virus was studied by infection of a murine macrophage cell line (J 774A.1) and human foreskin fibroblast (HFF) cells. The infections were characterized as nonpermissive by several criteria, including lack of production of infectious virus or viral DNA. However, the major IE and early MCMV proteins were detected in the nonpermissively infected cells. The block to virus replication in the macrophage and human fibroblast cells appeared to occur after the switch from IE to early protein synthesis, but before viral DNA replication.

Genetic analysis of the susceptibility of mouse cytomegalovirus to acyclovir

Eight independently derived mouse cytomegalovirus (MCMV) mutants resistant to acyclovir (ACV) were obtained by the sequential plating of wild-type virus in increasing concentrations of ACV. Results of complementation studies among these eight mutants suggest that all had mutations within the same or closely associated genes. A ninth MCMV mutant resistant to phosphonoacetate (PAA) derived by plating wild-type virus in the presence of 100 micrograms of PAA per ml displayed coresistance to ACV and was unable to complement any of the ACV-derived mutants. Recombination experiments among all combinations of the nine MCMV mutants were performed and supported the complementation data in that no recombination could be detected. Seven of the eight ACV-resistant mutants demonstrated cross-resistance to PAA and hypersensitivity to aphidicolin. The one mutant not coresistant to PAA was more susceptible to PAA than was the parent virus. Only a few mutants demonstrated coresistance when the mutants were tested against 9-beta-D-arabinofuranosyladenine (ara-A). The ACV mutant that demonstrated increased susceptibility to PAA was 30-fold more susceptible to ara-A but remained unchanged in susceptibility to aphidicolin. Two of the parent-mutant combinations were selected for DNA synthesis analysis in the presence of ACV (5 microM). A significant decrease in DNA synthesis was demonstrated for both parent viruses, and there was little effect on mutant virus DNA synthesis at the same drug concentration. These results suggest that susceptibility of MCMV to ACV is confined to a product of a single gene and that a mutation of this gene can lead to an altered phenotype when compared with parent virus in susceptibility of DNA synthesis to PAA, ara-A, and aphidicolin, drugs that are known to inhibit DNA polymerase activity.

Growth of murine cytomegalovirus in murine and heterologous brain cell cultures. Brief report

Murine cytomegalovirus (MCMV) produced a cytopathic effect in mouse brain, guinea pig embryonic brain, human brain and fibroblast cells. Virus-specific antigen was detected by immunofluorescence in these cells after primary infection with MCMV. MCMV also replicated in ouse embryo brain and guinea pig brain cells. Although definite evidence of MCMV replication could not be demonstrated in human cells, MCMV infectivity was maintained for 12 days in human cell cultures.

Murine cytomegalovirus infection in a non-permissive line of mouse fibroblasts

A line of murine cytomegalovirus (MCMV)-resistant cells was derived from mouse embryo cultures. The virus penetrated the nuclei of these resistant cells efficiently and a small amount of viral gene transcription occurred, although no viral DNA synthesis was detected. Synthesis of cellular DNA, RNA and protein continued unabated, and the infected cells grew at the same rate as uninfected cells. The block in virus replication in these cells appears to be between early transcription and viral DNA synthesis.

Analysis of structural proteins of purified murine cytomegalovirus

Murine cytomegalovirus propagated in mouse embryo fibroblasts was purified by the following procedures. (i) Extracellular virus was concentrated by centrifugation at 100,000 x g for 90 min. (ii) The concentrated virus was passed through a Bio-Rad Bio-Gel A-15m column to eliminate contaminating materials smaller than 15 x 10(6) daltons. Most of the virus was recovered in the void volume of the column. (iii) Two consecutive centrifugations through 20 to 50% potassium tartrate gradients were performed. After the second tartrate gradient centrifugation, symmetrical, coinciding peaks of plaque titer, protein, and radioactivity were found at a density between 1.20 g/cm3 and 1.21 g/cm3. To establish purification criteria, virus was purified from two different mixtures: [35S]methionine-labeled extracellular virus, mixed with an equal volume of unlabeled normal culture fluid, and unlabeled extracellular virus mixed with an equal volume of [35S]methionine-labeled normal culture fluid. At the end of the procedure, the extent of purification, as judged by the ratio of cellular to viral radioactivity was at least 70-fold. Virus proteins were analyzed by electrophoresis on a 5 to 20% gradient polyacrylamide gel slab. After gel electrophoresis,, Coomassie brilliant blue staining profiles and autoradiograms of the purified virus preparations were compared. At least 33 virus structural protein bands were present. The molecular weights of these proteins ranged from 11,500 to 255,000. The sum of the molecular weights of the virus structural proteins was 2,462,000. Autoradiograms obtained from electrophoresis of purified [14C]glucosamine-labeled virus showed that at lease 6 of the 33 viral structural proteins were glycoproteins.

Comparative studies of the Smith and Raynaud strains of murine cytomegalovirus

A number of the biological, biophysical, and immunological parameters of the Smith and Raynaud strains of murine cytomegalovirus were compared. The two strains were found to be indistinguishable.

Influence of complement on the neutralization of murine cytomegalovirus by rabbit antibody

Antiserum against murine cytomegalovirus produced in the rabbit contained complement (C')-requiring neutralizing (CRN) antibody. The proportion of CRN was extremely high (up to 98%) during the early portion of an immunization procedure, whereas the antisera produced late had a much lower proportion that required C'. The antiserum produced was specific for MCMV with or without C'.

Nonproductive infection of guinea pig cells with human cytomegalovirus

Human cytomegalovirus was capable of adsorbing to and penetrating guinea pig cells, but was unable to replicate new virus. Cultures infected with virus inoculum of high titer showed a cytopathic effect (CPE) characterized by cell rounding. This CPE depended upon the presence of infectious virus, and its extent was directly related to the multiplicity of infection. Staining by indirect immunofluorescence by using human convalescent sera was positive as early as 4 h postinfection. Maximal fluorescence was observed 24 h postinfection when 50% of the cells contained fluorescent antigens both in nuclei and cytoplasm. No evidence for viral replication was found, and no defective particles were detected by electron microscopy. Treatment with actinomycin D or with cycloheximide strongly inhibited both the fluorescent antigens and the CPE, whereas 5-fluorodeoxyuridine and bromodeoxyuridine were ineffective.

Growth characteristics of cytomegalovirus in human fibroblasts with demonstration of protein synthesis early in viral replication

In high-multiplicity infection of human fibroblasts, human cytomegalovirus of WI-38 human diploid cells produced early cell rounding 6 to 24 h after inoculation. This early cell rounding was caused only by inoculation with infectious virions. Inhibitors of protein synthesis, but not DNA inhibitors, prevented this cytopathic effect. Apparently, a new protein is synthesized in infected fibroblasts from about 2 h postinoculation. Infectivity of cell-associated and supernatant infectious virus reached maximal levels at 5 to 7 and 10 days postinoculation, respectively. Synthesis of DNA, infectious virus, complement-fixing antigen, and precipitin antigen all began between 24 and 48 h, with the bulk of synthesis occurring 48 to 96 h postinoculation.