Virulence and attenuation of murine cytomegalovirus

Immune surveillance of cytomegalovirus in tissues

Cytomegalovirus (CMV), a representative member of the Betaherpesvirinae subfamily of herpesviruses, is common in the human population, but immunocompetent individuals are generally asymptomatic when infected with this virus. However, in immunocompromised individuals and immunologically immature fetuses and newborns, CMV can cause a wide range of often long-lasting morbidities and even death. CMV is not only widespread throughout the population but it is also widespread in its hosts, infecting and establishing latency in nearly all tissues and organs. Thus, understanding the pathogenesis of and immune responses to this virus is a prerequisite for developing effective prevention and treatment strategies. Multiple arms of the immune system are engaged to contain the infection, and general concepts of immune control of CMV are now reasonably well understood. Nonetheless, in recent years, tissue-specific immune responses have emerged as an essential factor for resolving CMV infection. As tissues differ in biology and function, so do immune responses to CMV and pathological processes during infection. This review discusses state-of-the-art knowledge of the immune response to CMV infection in tissues, with particular emphasis on several well-studied and most commonly affected organs.

Construction and Characterization of a High-Capacity Replication-Competent Murine Cytomegalovirus Vector for Gene Delivery

We investigated the basic characteristics of a new murine cytomegalovirus (MCMV) vector platform. Using BAC technology, we engineered replication-competent recombinant MCMVs with deletions of up to 26% of the wild-type genome. To this end, we targeted five gene blocks (m01-m17, m106-m109, m129-m141, m144-m158, and m159-m170). BACs featuring deletions from 18% to 26% of the wild-type genome exhibited delayed virus reconstitution, while smaller deletions (up to 16%) demonstrated reconstitution kinetics similar to those of the wild type. Utilizing an innovative methodology, we introduced large genomic DNA segments, up to 35 kbp, along with reporter genes into a newly designed vector with a potential cloning capacity of 46 kbp (Q4). Surprisingly, the insertion of diverse foreign DNAs alleviated the delayed plaque formation phenotype of Q4, and these large inserts remained stable through serial in vitro passages. With reporter-gene-expressing recombinant MCMVs, we successfully transduced not only mouse cell lines but also non-rodent mammalian cells, including those of human, monkey, bovine, and bat origin. Remarkably, even non-mammalian cell lines derived from chickens exhibited successful transduction.

Maternal antibodies induced by a live attenuated vaccine protect neonatal mice from cytomegalovirus

Human cytomegalovirus (HCMV) frequently causes congenital infections, resulting in birth defects and developmental disorders. A vaccine is needed, but unavailable. We analyzed the potential of CMV mutants, lacking their STAT2 antagonists to serve as live attenuated vaccine viruses in mice. Infections with attenuated viruses elicited strong ELISA-reactive binding IgG responses and induced neutralizing antibodies as well as antibodies stimulating cellular Fcγ receptors, including the antibody-dependent cellular cytotoxicity (ADCC)-eliciting receptors FcγRIII/CD16 and FcγRIV. Accordingly, vaccinated mice were fully protected against challenge infections. Female mice vaccinated prior to gestation transmitted CMV-specific IgG to their offspring, which protected the progeny from perinatal infections in a mouse model for congenital CMV disease. To define the role of maternal antibodies, female mice either capable or incapable of producing antibodies were vaccinated and subsequently bred to males of the opposite genotype. Challenge infections of the genotypically identical F1 generation revealed the indispensability of maternal antibodies for vaccine-induced protection against cytomegaloviruses.

Mouse Models for Cytomegalovirus Infections in Newborns and Adults

This article describes procedures for infecting adult mice with murine cytomegalovirus (MCMV) and for infecting newborn mice to model congenital CMV infection. Methods are included for propagating MCMV in cell cultures and preparing a more virulent form of MCMV from the salivary glands of infected mice. A plaque assay is provided for determining MCMV titers of infected tissues or virus stocks. Also, methods are described for preparing the murine embryonic fibroblasts used for propagating MCMV, and for the plaque assay. © 2022 Wiley Periodicals LLC.

MCMV Centrifugal Enhancement: A New Spin on an Old Topic

Human cytomegalovirus (HCMV) is a ubiquitous pathogen infecting a majority of people worldwide, with diseases ranging from mild to life-threatening. Its clinical relevance in immunocompromised people and congenital infections have made treatment and vaccine development a top priority. Because of cytomegaloviruses' species specificity, murine cytomegalovirus (MCMV) models have historically informed and advanced translational CMV therapies. Using the phenomenon of centrifugal enhancement, we explored differences between MCMVs derived in vitro and in vivo. We found centrifugal enhancement on tissue culture-derived virus (TCV) was ~3× greater compared with salivary gland derived virus (SGV). Using novel "flow virometry", we found that TCV contained a distinct submicron particle composition compared to SGV. Using an inhibitor of exosome production, we show these submicron particles are not extracellular vesicles that contribute to centrifugal enhancement. We examined how these differences in submicron particles potentially contribute to differing centrifugal enhancement phenotypes, as well as broader in vivo vs. in vitro MCMV differences.

Rodent Models of Congenital Cytomegalovirus Infection

Human cytomegalovirus (HCMV) is a leading viral cause of congenital infections in the central nervous system (CNS) and may result in severe long-term sequelae. High rates of sequelae following congenital HCMV infection and insufficient antiviral therapy in the perinatal period makes the development of an HCMV-specific vaccine a high priority of modern medicine. Due to the species specificity of HCMV, animal models are frequently used to study CMV pathogenesis. Studies of murine cytomegalovirus (MCMV) infections of adult mice have played a significant role as a model of CMV biology and pathogenesis, while MCMV infection of newborn mice has been successfully used as a model of perinatal CMV infection. Newborn mice infected with MCMV have high levels of viremia during which the virus establishes a productive infection in most organs, coupled with a robust inflammatory response. Productive infection in the brain parenchyma during early postnatal period leads to an extensive nonnecrotizing multifocal widespread encephalitis characterized by infiltration of components of both innate and adaptive immunity. As a result, impairment in postnatal development of mouse cerebellum leads to long-term motor and sensor disabilities. This chapter summarizes current findings of rodent models of perinatal CMV infection and describes methods for analysis of perinatal MCMV infection in newborn mice.

A Review of Murine Cytomegalovirus as a Model for Human Cytomegalovirus Disease-Do Mice Lie?

Since murine cytomegalovirus (MCMV) was first described in 1954, it has been used to model human cytomegalovirus (HCMV) diseases. MCMV is a natural pathogen of mice that is present in wild mice populations and has been associated with diseases such as myocarditis. The species-specific nature of HCMV restricts most research to cell culture-based studies or to the investigation of non-invasive clinical samples, which may not be ideal for the study of disseminated disease. Initial MCMV research used a salivary gland-propagated virus administered via different routes of inoculation into a variety of mouse strains. This revealed that the genetic background of the laboratory mice affected the severity of disease and altered the extent of subsequent pathology. The advent of genetically modified mice and viruses has allowed new aspects of disease to be modeled and the opportunistic nature of HCMV infection to be confirmed. This review describes the different ways that MCMV has been used to model HCMV diseases and explores the continuing difficulty faced by researchers attempting to model HCMV congenital cytomegalovirus disease using the mouse model.

Anticytomegalovirus Peptides Point to New Insights for CMV Entry Mechanisms and the Limitations of In Vitro Screenings

In the absence of an effective vaccine to prevent HCMV infections, alternative interventions must be developed. Prevention of viral entry into susceptible cells is an attractive alternative strategy. Here we report that heparan sulfate-binding peptides effectively inhibit entry into fibroblasts of in vitro-derived CMVs and partially inhibit in vivo-derived CMVs. This includes the inhibition of urine-derived HCMV (uCMV), which is highly resistant to antibody neutralization. While these antiviral peptides are highly effective at inhibiting cell-free virus, they do not inhibit MCMV cell-to-cell spread. This underscores the need to understand the mechanism of cell-to-cell spread and differences between in vivo-derived versus in vitro-derived CMV entry to effectively prevent CMV’s spread.

Human cytomegalovirus (HCMV) is a ubiquitous betaherpesvirus that can cause severe disease following in utero exposure, during primary infection, or latent virus reactivation in immunocompromised populations. These complications lead to a 1- to 2-billion-dollar economic burden, making vaccine development and/or alternative treatments a high priority. Current treatments for HCMV include nucleoside analogues such as ganciclovir (GCV), foscarnet, and cidofovir. Recently, letermovir, a terminase complex inhibitor, was approved for prophylaxis after stem cell transplantation. These treatments have unwanted side effects, and HCMV is becoming resistant to them. Therefore, we sought to develop an alternative treatment that targets a different stage in viral infection. Currently, small antiviral peptides are being investigated as anti-influenza and anti-HIV treatments. We have developed heparan sulfate-binding peptides as tools for preventing CMV infections. These peptides are highly effective at stopping infection of fibroblasts with in vitro-derived HCMV and murine cytomegalovirus (MCMV). However, they do not prevent MCMV infection in vivo. Interestingly, these peptides inhibit infectivity of in vivo-derived CMVs, albeit not as well as tissue culture-grown CMVs. We further demonstrate that this class of heparan sulfate-binding peptides is incapable of inhibiting MCMV cell-to-cell spread, which is independent of heparan sulfate usage. These data indicate that inhibition of CMV infection can be achieved using synthetic polybasic peptides, but cell-to-cell spread and in vivo-grown CMVs require further investigation to design appropriate anti-CMV peptides.

IMPORTANCE In the absence of an effective vaccine to prevent HCMV infections, alternative interventions must be developed. Prevention of viral entry into susceptible cells is an attractive alternative strategy. Here we report that heparan sulfate-binding peptides effectively inhibit entry into fibroblasts of in vitro-derived CMVs and partially inhibit in vivo-derived CMVs. This includes the inhibition of urine-derived HCMV (uCMV), which is highly resistant to antibody neutralization. While these antiviral peptides are highly effective at inhibiting cell-free virus, they do not inhibit MCMV cell-to-cell spread. This underscores the need to understand the mechanism of cell-to-cell spread and differences between in vivo-derived versus in vitro-derived CMV entry to effectively prevent CMV’s spread.

Cytomegalovirus Infection: Mouse Model

This unit describes procedures for infecting newborn and adult mice with murine cytomegalovirus (MCMV). Methods are included for propagating MCMV in cell cultures and for preparing a more virulent form of MCMV from salivary glands of infected mice. A plaque assay is provided for determining MCMV titers of infected tissues or virus stocks. Also, a method is described for preparing the murine embryonic fibroblasts used for propagating MCMV and for the plaque assay. © 2018 by John Wiley & Sons, Inc.

Murine cytomegalovirus infection of mouse macrophages stimulates early expression of suppressor of cytokine signaling (SOCS)1 and SOCS3

Human cytomegalovirus (HCMV) is a species-specific β-herpesvirus that infects for life up to 80% of the world’s population and causes severe morbidity in at-risk immunocompromised populations. Suppressors of cytokine signaling (SOCS)1 and SOCS3 are host proteins that act as inducible negative feedback regulators of cytokine signaling and have been implicated in several ocular diseases and viral infections. We recently found in our mouse model of experimental cytomegalovirus retinitis that subretinally-injected murine cytomegalovirus (MCMV) stimulates ocular SOCS1 and SOCS3 during retrovirus-induced immune suppression of murine AIDS (MAIDS), and that infiltrating macrophages are prominent cellular sources of retinal SOCS1 and SOCS3 expression. Herein we investigate possible virologic mechanisms whereby MCMV infection may stimulate SOCS1 and/or SOCS3 expression in cell culture. We report that infection of IC-21 mouse macrophages with MCMV propagated through the salivary glands of BALB/c mice, but not from tissue culture in C57BL/6 fibroblasts, transiently stimulates SOCS1 and SOCS3 mRNA transcripts, but not SOCS5 mRNA. Viral tegument proteins are insufficient for this stimulation, as replication-deficient UV-inactivated MCMV fails to stimulate SOCS1 or SOCS3 in IC-21 macrophages. By contrast, infection of murine embryonic fibroblasts (MEFs) with either productive MCMV or UV-inactivated MCMV significantly stimulates SOCS1 and SOCS3 mRNA expression early after infection. Treatment of MCMV-infected IC-21 mouse macrophages with the antiviral drug ganciclovir significantly decreases MCMV-stimulated SOCS3 expression at 3 days post-infection. These data suggest cell type-specific, different roles for viral immediate early or early gene expression and/or viral tegument proteins in the early stimulation of SOCS1 and SOCS3 during MCMV infection. Furthermore, putative biphasic stimulation of SOCS3 during late MCMV infection of IC-21 mouse macrophages may occur by divergent virologic mechanisms.

Repair of a Mutation Disrupting the Guinea Pig Cytomegalovirus Pentameric Complex Acquired during Fibroblast Passage Restores Pathogenesis in Immune-Suppressed Guinea Pigs and in the Context of Congenital Infection

Guinea pig cytomegalovirus (GPCMV) provides a valuable model for congenital cytomegalovirus transmission. Salivary gland (SG)-passaged stocks of GPCMV are pathogenic, while tissue culture (TC) passage in fibroblasts results in attenuation. Nonpathogenic TC-derived virus N13R10 (cloned as a bacterial artificial chromosome [BAC]) has a 4-bp deletion that disrupts GP129, which encodes a subunit of the GPCMV pentameric complex (PC) believed to govern viral entry into select cell types, and GP130, an overlapping open reading frame (ORF) of unknown function. To determine if this deletion contributes to attenuation of N13R10, markerless gene transfer in Escherichia coli was used to construct virus r129, a variant of N13R10 in which the 4-bp deletion is repaired. Virions from r129 were found to contain GP129 as well as two other PC subunit proteins, GP131 and GP133, whereas these three PC subunits were absent from N13R10 virions. Replication of r129 in fibroblasts appeared unaltered compared to that of N13R10. However, following experimental challenge of immunocompromised guinea pigs, r129 induced significant weight loss, longer duration of viremia, and dramatically higher (up to 1.5 × 10(6)-fold) viral loads in blood and end organs compared to N13R10. In pregnant guinea pigs, challenge with doses of r129 virus of ≥5 × 10(6) PFU resulted in levels of maternal viremia, congenital transmission, pup viral loads, intrauterine growth restriction, and pup mortality comparable to that induced by pathogenic SG virus, although higher doses of r129 were required. These results suggest that the GP129-GP130 mutation is a significant contributor to attenuation of N13R10, likely by abrogating expression of a functional PC.

IMPORTANCE

Tissue culture adaptation of cytomegaloviruses rapidly selects for mutations, deletions, and rearrangements in the genome, particularly for viruses passaged in fibroblast cells. Some of these mutations are focused in the region of the genome encoding components of the pentameric complex (PC), in particular homologs of human cytomegalovirus (HCMV) proteins UL128, UL130, and UL131A. These mutations can attenuate the course of infection when the virus is reintroduced into animals for vaccine and pathogenesis studies. This study demonstrates that a deletion that arose during the process of tissue culture passage can be repaired, with subsequent restoration of pathogenicity, using BAC-based mutagenesis. Restoration of pathogenicity by repair of a frameshift mutation in GPCMV gene GP129 using this approach provides a valuable genetic platform for future studies using the guinea pig model of congenital CMV infection.

Comparison of the pathogenesis of the highly passaged MCMV Smith strain with that of the low passaged MCMV HaNa1 isolate in BALB/c mice upon oronasal inoculation

Murine cytomegalovirus (MCMV) Smith strain is widely used in mouse models to study HCMV infections. Due to high serial passages, MCMV Smith has acquired genetic and biological changes. Therefore, a low passaged strain would be more relevant to develop mouse models. Here, the pathogenesis of an infection with MCMV Smith was compared with that of an infection with a low passaged Belgian MCMV isolate HaNa1 in BALB/c adult mice following oronasal inoculation with either a low (10⁴ TCID₅₀/mouse) or high (10⁶ TCID₅₀/mouse) inoculation dose. Both strains were mainly replicating in nasal mucosa and submandibular glands for one to two months. In nasal mucosa, MCMV was detected earlier and longer (1–49 days post inoculation (dpi)) and reached higher titers with the high inoculation dose compared to the low inoculation dose (14–35 dpi). In submandibular glands, a similar finding was observed (high dose: 7–49 dpi; low dose: 14–42 dpi). In lungs, both strains showed a restricted replication. In spleen, liver and kidneys, only the Smith strain established a productive infection. The infected cells were identified as olfactory neurons and sustentacular cells in olfactory epithelium, macrophages and dendritic cells in NALT, acinar cells in submandibular glands, and macrophages and epithelial cells in lungs for both strains. Antibody analysis demonstrated for both strains that IgG_2a was the main detectable antibody subclass. Overall, our results show that significant phenotypic differences exist between the two strains. MCMV HaNa1 has been shown to be interesting for use in mouse models in order to get better insights for HCMV infections in immunocompetent humans.

A novel murine cytomegalovirus vaccine vector protects against Mycobacterium tuberculosis

Tuberculosis remains a global health problem so that a more effective vaccine than bacillus Calmette–Guérin is urgently needed. Cytomegaloviruses persist lifelong in vivo and induce powerful immune and increasing (“inflationary”) responses, making them attractive vaccine vectors. We have used an m1–m16-deleted recombinant murine CMV (MCMV) expressing Mycobacterium tuberculosis Ag 85A to show that infection of mice with this recombinant significantly reduces the mycobacterial load after challenge with M. tuberculosis, whereas control empty virus has a lesser effect. Both viruses induce immune responses to H-2^d–restricted epitopes of MCMV pp89 and M18 Ags characteristic of infection with other MCMVs. A low frequency of 85A-specific memory cells could be revealed by in vivo or in vitro boosting or after challenge with M. tuberculosis. Kinetic analysis of M. tuberculosis growth in the lungs of CMV-infected mice shows early inhibition of M. tuberculosis growth abolished by treatment with NK-depleting anti–asialo ganglio-N-tetraosylceramide Ab. Microarray analysis of the lungs of naive and CMV-infected mice shows increased IL-21 mRNA in infected mice, whereas in vitro NK assays indicate increased levels of NK activity. These data indicate that activation of NK cells by MCMV provides early nonspecific protection against M. tuberculosis, potentiated by a weak 85A-specific T cell response, and they reinforce the view that the innate immune system plays an important role in both natural and vaccine-induced protection against M. tuberculosis.

Gene products of the embedded m41/m41.1 locus of murine cytomegalovirus differentially influence replication and pathogenesis

Cytomegaloviruses utilize overlapping and embedded reading frames as a way to efficiently package and express all genes necessary to carry out a complex lifecycle. Murine cytomegalovirus encodes a mitochondrial-localized inhibitor of Bak oligomerization (vIBO) from m41.1, a reading frame that is embedded within the m41 gene. The m41.1-encoded mitochondrial protein and m41-encoded Golgi-localized protein have both been implicated in cell death suppression; however, their contribution to viral infection within the host has not been investigated. Here, we report that mitochondrial-localized m41.1 (vIBO) is required for optimal viral replication in macrophages and has a modest impact on dissemination in infected mice. In contrast, Golgi-localized m41 protein is dispensable during acute infection and dissemination as well as for latency. All together, these data indicate that the primary evolutionary focus of this locus is to maintain mitochondrial function through inhibition of Bak-mediated death pathways in support of viral pathogenesis.

Antiviral T cell response triggers cytomegalovirus hepatitis in mice

One common sign of human cytomegalovirus infection is altered liver function. Murine cytomegalovirus strain v70 induces a rapid and severe hepatitis in immunocompetent mice that requires the presence of T cells in order to develop. v70 exhibits approximately 10-fold-greater virulence than the commonly used strain K181, resulting in a more severe, sustained, and lethal hepatitis but not dramatically higher viral replication levels. Hepatitis and death are markedly delayed in immunodeficient SCID compared to immunocompetent BALB/c mice. Transfer of BALB/c splenocytes to SCID mice conferred rapid disease following infection, and depletion of either CD4 or CD8 T cells in BALB/c mice reduced virus-induced hepatitis. The frequency of CD8 T cells producing gamma interferon and tumor necrosis factor in response to viral antigen was higher in settings where more severe disease occurred. Thus, virus-specific effector CD8 T cells appear to contribute to lethal virus-induced hepatitis, contrasting their protective role during sublethal infection. This study reveals how protection and disease during cytomegalovirus infection depend on viral strain and dose, as well as the quality of the T cell response.

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.

Virus progeny of murine cytomegalovirus bacterial artificial chromosome pSM3fr show reduced growth in salivary Glands due to a fixed mutation of MCK-2

Murine cytomegalovirus (MCMV) Smith strain has been cloned as a bacterial artificial chromosome (BAC) named pSM3fr and used for analysis of virus gene functions in vitro and in vivo. When sequencing the complete BAC genome, we identified a frameshift mutation within the open reading frame (ORF) encoding MCMV chemokine homologue MCK-2. This mutation would result in a truncated MCK-2 protein. When mice were infected with pSM3fr-derived virus, we observed reduced virus production in salivary glands, which could be reverted by repair of the frameshift mutation. When looking for the source of the mutation, we consistently found that virus stocks of cell culture-passaged MCMV Smith strain are mixtures of viruses with or without the MCK-2 mutation. We conclude that the MCK-2 mutation in the pSM3fr BAC is the result of clonal selection during the BAC cloning procedure.

Cytomegalovirus microRNAs facilitate persistent virus infection in salivary glands

Micro (mi)RNAs are small non-coding RNAs that regulate the expression of their targets' messenger RNAs through both translational inhibition and regulation of target RNA stability. Recently, a number of viruses, particularly of the herpesvirus family, have been shown to express their own miRNAs to control both viral and cellular transcripts. Although some targets of viral miRNAs are known, their function in a physiologically relevant infection remains to be elucidated. As such, no in vivo phenotype of a viral miRNA knock-out mutant has been described so far. Here, we report on the first functional phenotype of a miRNA knock-out virus in vivo. During subacute infection of a mutant mouse cytomegalovirus lacking two viral miRNAs, virus production is selectively reduced in salivary glands, an organ essential for virus persistence and horizontal transmission. This phenotype depends on several parameters including viral load and mouse genetic background, and is abolished by combined but not single depletion of natural killer (NK) and CD4+ T cells. Together, our results point towards a miRNA-based immunoevasion mechanism important for long-term virus persistence.

Dissection of the antiviral NK cell response by MCMV mutants

Our understanding of virus control by natural killer (NK) cells relies mainly on in vitro observations. The significance of these findings for virus control in vivo is not yet fully understood. Complexity is added by the fact that many viruses, particularly herpesviruses, are equipped with sets of genes that, dependent on the genetic background of the host, modify the NK cell response. The advent of recombinant DNA technology and mutagenesis procedures for BAC-cloned viral genomes has made it possible not only to screen for viral proteins with such functions but also to assess their biological relevance. Mutant viruses with gene defects reveal the efficacy and complexity of NK cell control. Here, we describe procedures to assess the NK cell response to mouse cytomegalovirus (MCMV), a prominent virus model for studying NK cell functions in vivo.

A mouse model for cytomegalovirus infection

This unit describes procedures for infecting newborn and adult mice with murine cytomegalovirus (mCMV). Methods are included for propagating mCMV in cell cultures and for preparing a more virulent form of mCMV from salivary glands of infected mice. A plaque-forming cell (PFC) assay is provided for measuring mCMV titers of infected tissues or virus stocks. In addition, a method is described for preparing the murine embryonic fibroblasts used for propagating mCMV and for the PFC assay.

Surgical removal of mouse salivary glands

Surgical removal of the salivary glands is a simple procedure aimed at providing glandular tissues for studies in histopathology, immunohistology, DNA and RNA analysis, cytokine production, and virus detection and isolation. This unit describes the surgical removal of the salivary glands in mice, but a similar protocol is applicable to rats and other rodents.

Human cytomegalovirus uses two distinct pathways to enter retinal pigmented epithelial cells

Human cytomegalovirus infects multiple cell types, including fibroblasts and epithelial cells. It penetrates fibroblasts by fusion at the cell surface but is endocytosed into epithelial cells. In this report, we demonstrate by electron microscopy that the virus uses two different routes to enter retinal pigmented epithelial cells, depending on the cell type in which the infecting virus was produced. Virus produced in epithelial cells preferentially fuses with the plasma membrane, whereas fibroblast-derived virus mostly enters by receptor-mediated endocytosis. Treatment of epithelial cells with agents that block endosome acidification inhibited infection by virus produced in fibroblasts but had only a modest effect on infection by virus from epithelial cells. Epithelial cell-generated virions had higher intrinsic "fusion-from-without" activity than fibroblast-generated particles, and the two virus preparations triggered different cellular signaling responses, as evidenced by markedly different transcriptional profiles. We propose that the cell type in which a human cytomegalovirus particle is produced likely influences its subsequent spread and its contribution to pathogenesis.

A focused salivary gland infection with attenuated MCMV: an animal model with prevention of pathology associated with systemic MCMV infection

While the salivary gland has been recognized as an important effector site of the common mucosal immune system, a useful model for studying anti-viral salivary gland immune responses in vivo and for exploring the role of the salivary gland within the common mucosal system has been lacking. Murine cytomegalovirus (MCMV) is a beta-herpesvirus that displays a strong tropism for the salivary gland and produces significant morbidity in susceptible mice when introduced by intraperitoneal (i.p.) inoculation. This study tested the hypothesis that MCMV morbidity and pathology could be reduced by injecting the virus directly the submandibular salivary gland (intraglandular (i.g.)), using either in vivo derived MCMV or the less virulent, tissue-culture-derived MCMV (tcMCMV). Peak salivary gland viral titers were completely unaffected by infection route (i.p vs. i.g.) after inoculation with either MCMV or tcMCMV. However, i.g. tcMCMV inoculation reduced viremia in all systemic tissues tested compared to i.p. inoculation. Furthermore, systemic organ pathology observed in the liver and spleen after i.p. inoculation with either MCMV or tcMCMV was completely eliminated by i.g. inoculation with tcMCMV. Cellular infiltrates in the salivary glands, after i.p. or i.g. inoculation were composed of both B and T cells, indicating the potential for a local immune response to occur in the salivary gland. These results demonstrate that a focused MCMV infection of the salivary gland without systemic organ pathology is possible using i.g. delivery of tcMCMV.

Severe focal sialadenitis and dacryoadenitis in NZM2328 mice induced by MCMV: a novel model for human Sjögren's syndrome

The genetic and environmental factors that control the development of Sjögren's syndrome, an autoimmune disease mainly involving the salivary and lacrimal glands, are poorly understood. Viruses which infect the glands may act as a trigger for disease. The ability of sialotropic murine CMV (MCMV) to induce acute and chronic glandular disease was characterized in an autoimmune-prone mouse strain, NZM2328. MCMV levels were detectable in the salivary and lacrimal glands 14-28 days after i.p. infection and correlated with acute inflammation in the submandibular gland. After latency, virus was undetectable in the glands by PCR. At this stage, NZM2328 female mice developed severe chronic periductal inflammation in both submandibular and lacrimal glands in contrast to the much milder infiltrates found in female B6-lpr and male NZM2328. The focal infiltrates consisted of CD4+ and B220+ cells as opposed to diffuse CD4+, CD8+, and B220+ cells during acute infection. Salivary gland functional studies revealed a gender-specific progressive loss of secretory function between days 90 and 125 postinfection. Latent MCMV infection did not significantly affect the low incidence of autoantibodies to Ro/SSA and La/SSB Ags in NZM2328 mice. However, reactivities to other salivary and lacrimal gland proteins were readily detected. MCMV infection did not significantly alter the spontaneous onset of kidney disease in NZM2328. Thus, chronic inflammation induced by MCMV with decreased secretory function in NZM2328 mice resembles the disease manifestations of human Sjögren's syndrome.

Mucosal and parenteral vaccination against acute and latent murine cytomegalovirus (MCMV) infection by using an attenuated MCMV mutant

We used a live attenuated murine cytomegalovirus (MCMV) mutant to analyze mechanisms of vaccination against acute and latent CMV infection. We selected MCMV mutant RV7 as a vaccine candidate since this virus grows well in tissue culture but is profoundly attenuated for growth in normal and severe combined immunodeficient (SCID) mice (V. J. Cavanaugh et al., J. Virol. 70:1365-1374, 1996). BALB/c mice were immunized twice (0 and 14 days) subcutaneously (s.c.) with tissue culture-passaged RV7 and then challenged with salivary gland-passaged wild-type MCMV (sgMCMV) intraperitoneally (i.p.) on day 28. RV7 vaccination protected mice against challenge with 10(5) PFU of sgMCMV, a dose that killed 100% of mock-vaccinated mice. RV7 vaccination reduced MCMV replication 100- to 500-fold in the spleen between 1 and 8 days after challenge. We used the capacity to control replication of MCMV in the spleen 4 days after challenge as a surrogate for protection. Protection was antigen specific and required both live RV7 and antigen-specific lymphocytes. Interestingly, RV7 was effective when administered s.c., i.p., perorally, intranasally, and intragastrically, demonstrating that attenuated CMV applied to mucosal surfaces can elicit protection against parenteral virus challenge. B cells and immunoglobulin G were not essential for RV7-induced immunity since B-cell-deficient mice were effectively vaccinated by RV7. CD8 T cells, but not CD4 T cells, were critical for RV7-induced protection. Depletion of CD8 T cells by passive transfer of monoclonal anti-CD8 (but not anti-CD4) antibody abrogated RV7-mediated protection, and RV7 vaccination was less efficient in CD8 T-cell-deficient mice with a targeted mutation in the beta2-microglobulin gene. Although gamma interferon is important for innate resistance to MCMV, it was not essential for RV7 vaccination since gamma interferon receptor-deficient mice were protected by RV7 vaccination. Establishment of and/or reactivation from latency by sgMCMV was decreased by RV7 vaccination, as measured by diminished reactivation of MCMV from splenic explants. We found no evidence for establishment of splenic latency by RV7 after s.c. vaccination. We conclude that RV7 administered through both systemic and mucosal routes is an effective vaccine against MCMV infection. It may be possible to design human CMV vaccines with similar properties.

The location and sequence composition of the murine cytomegalovirus replicator (oriLyt)

DNA replication during human or simian cytomegalovirus (CMV) infection has been shown to be under control of a replicator region referred to as oriLyt. The murine CMV oriLyt has been mapped to a region of the genome located upstream of the gene encoding the herpesvirus-conserved single-stranded DNA binding protein, analogous to human and simian CMV oriLyts. A minimal oriLyt of approximately 1.7 kbp has been identified using a transient replication system. Like occurs with human and simian CMV counterparts, addition of flanking sequences to this minimal origin-stimulated replication efficiency. Analysis of the DNA sequence in this region shows that murine CMV oriLyt is complex and exhibits an asymmetric distribution of nucleotides as well as many repeat sequence elements, including distinct AT- and GC-rich regions and region with arrays of closely spaced direct repeats. Despite similarities in organization of all three CMV oriLyts, no sequence identity and only limited DNA sequence similarity was detectable. Consistent with this sequence divergence, the human and murine CMV oriLyts were unable to substitute for one another in transient replication assays.

Murine cytomegalovirus with a deletion of genes spanning HindIII-J and -I displays altered cell and tissue tropism

Murine cytomegalovirus (MCMV) gene products dispensable for growth in cell culture are likely to have important functions within the infected host, influencing tissue tropism, dissemination, or immunological responses against the virus. To identify such genes, our strategy was to delete large regions of the MCMV genome likely to contain genes nonessential for virus replication in NIH 3T3 cells. Mutant virus RV7 contained a deletion of 7.7 kb spanning portions of MCMV HindIII-J and -I. This virus grew comparably to wild-type (WT) virus in NIH 3T3 fibroblasts, primary embryo fibroblasts, and bone marrow macrophages. However, RV7 failed to replicate in target organs of immunocompetent BALB/c mice and severe combined immunodeficient mice, which are exquisitely sensitive to MCMV infection. This defect in vivo growth may be related to the observation that RV7 grew poorly in the peritoneal macrophage cell line IC-21, which is highly permissive for growth of WT MCMV. Two other mutant viruses with an insertion or smaller deletion in the region common to the RV7 deletion grew comparably to WT virus in the macrophage cell line and replicated in salivary gland tissue. The poor growth of RV7 in IC-21 cells was due to a block in immediate-early gene expression, as levels of RNA from immediate-early gene IE1 were reduced eightfold compared with levels for WT virus in macrophages infected with RV7. Consequently, levels of RNA from early and late genes were also reduced. The lower expression of IE1 in RV7-infected IC-21 macrophages was not due to defective entry of virus into the cells, as equal amounts of viral DNA were present in cells 3 h after infection with RV7 or WT MCMV. These studies demonstrate that deletion of sequences in HindIII-J and -I confer altered cell and tissue tropism.

Structure and function of the murine cytomegalovirus sgg1 gene: a determinant of viral growth in salivary gland acinar cells

The salivary gland has long been recognized as an important target organ for cytomegalovirus replication in the infected host. A viral gene, denoted sgg1, plays an important role for replication in the salivary gland even though it is dispensable for growth in other organs or in cultured cells. The nucleotide sequence of this gene and of cDNA clones representing two spliced transcripts (1.5 and 1.8 kb in size) has been determined. The more abundant 1.5-kb transcript contains a 312-amino-acid (aa) open reading frame (ORF) and encodes the corresponding 37-kDa protein (Sgg1) when expressed in transfected COS-7 cells. The 1.8-kb transcript initiates upstream of the 1.5-kb transcript and contains a 108-aa ORF in addition to the 312-aa ORF. This longer cDNA also encodes the 37-kDa protein Sgg1, although at lower abundance than the 1.5-kb cDNA. Sgg1 localizes to the cytoplasm of COS-7 cells, which is consistent with the predicted structural characteristics of the 312-aa ORF: a type 1 integral membrane protein. During viral infection, expression of both sgg1 transcripts is highest at early times (8 to 12 h) after infection; only the 1.5-kb transcript is present, at low levels, late in infection. A recombinant virus, RM868, carrying a lacZ-gpt insertion within sgg1, fails to express Sgg1 protein and exhibits reduced growth in the salivary gland. RM868 retains the capacity to disseminate in the infected mouse and to enter serous acinar cells, although it fails to replicate efficiently in this cell type. These results suggest that sgg1 is critical for high levels of viral replication in the salivary gland.

Molecular and biological characterization of new strains of murine cytomegalovirus isolated from wild mice

Studies of the prevalence of antibody to murine cytomegalovirus (MCMV) in free-living wild mice (Mus domesticus) trapped in diverse regions of Australia and on a sub-Antarctic island indicated that 90% of 468 mice had serum antibody to MCMV. Twenty-six field isolates of MCMV were plaque-purified from salivary gland extracts of representative seropositive mice. These isolates varied considerably in their ability to replicate in the salivary glands of weanling BALB/c mice with 9 of 15 failing to reach significant titres in this organ and the titres of the remaining 6 strains varying by at least 100-fold. The high frequency of restriction fragment length polymorphisms observed suggests widespread genetic heterogeneity exists among the strains. This observation was mirrored at the polypeptide level by Western blot analyses with polyclonal antisera to MCMV. The isolation in this study of four genetically distinct strains of MCMV from a single wild mouse and several strains from other individual mice demonstrates that multiple infections with MCMV may be commonplace in wild mice.

Pathogenicity of murine cytomegalovirus for newborn mice: analysis with the attenuated mutants

Mutants (ts21, rev21, and rec21w) of murine cytomegalovirus (MCMV) with reduced ability to kill newborn mice have been isolated from the Smith strain (wild type; wt). The postulated mutations for these mutants are as follows; a mutation(s) responsible for temperature sensitivity (ts); a mutation(s) responsible for attenuation (att); a mutation(s) which modifies temperature sensitivity (mts); and a mutation(s) responsible for modified growth characteristics (mgc). Genotypes of the mutants have been proposed from their phenotypes; ts21 (ts, att, mts+, mgc+); rev21 (ts, att, mts, mgc+) or (ts+, att, mts+, mgc); and rec21w (ts+, att, mts+, mgc+). Inoculation of 2.0 x 10(4) plaque forming units (PFU) of wt intraperitoneally into mice resulted in lethal infection accompanying intraperitoneal hemorrhage and pathognomonic changes in the target organs. Such changes were less in rec21w-infected mice than in wt-infected mice, and almost absent in ts21- and rev21-infected mice, on the sixth day after inoculation. The lesser damage to the organs correlated with poor growth of these viruses in the corresponding organs. On the other hand, in the case of rec21w-infected mice, virus titer in the target organs was almost equivalent to that of wt-infected mice. The growth characteristic of rec21w in the liver was also similar to that of wt. Nevertheless, damage to the organs by rec21w was less than that by wt. Comparison of the phenotypic characteristics between rec21w and wt demonstrated that att affects the ability of MCMV to induce the organ damage without affecting viral growth. Similar comparison between ts21 and rec21w revealed that ts affects the growth of MCMV in most organs.

Cytomegalovirus determinant of replication in salivary glands

Murine cytomegalovirus carrying a deletion mutation disrupting the expression of a gene dispensable for growth in cultured cells was found to disseminate poorly in the mouse. The mutation resulted in a dramatic decrease in the expression of a 1.5-kb major and a 1.8-kb minor beta transcript from a region adjacent to the ie2 gene in the viral genome. Nucleotide sequence determination indicated that 323 bp, including a predicted polyadenylation signal, was deleted from this beta gene. In cultured cells, the plaque morphology and growth characteristics of the mutant were similar to those of parental or rescued wild-type viruses. Following intraperitoneal inoculation of BALB/c mice, growth of the mutant in the salivary gland was dramatically reduced 10,000-fold, while growth in the liver and spleen was not dramatically affected. The beta gene was thus denoted sgg1 (salivary gland growth gene 1). Neither intranasal infection nor direct inoculation into the salivary glands completely overcame the restriction of growth in this organ, suggesting that the sgg1 gene encoded a determinant of tissue tropism. To investigate the impact of the sgg1 mutation on virus dissemination via the blood, the virus titer in peripheral blood leukocytes was determined. No difference was found between the sgg1 mutant and rescued wild-type virus. Thus, murine cytomegalovirus sgg1 gene products appear to be involved in entry or replication of virus in salivary gland cells.

Rescue of myeloid lineage-committed preprogenitor cells from cytomegalovirus-infected bone marrow stroma

The effect of murine cytomegalovirus on myelopoiesis was studied in long-term bone marrow culture to find an in vitro correlate for the lethal virus interference with bone marrow reconstitution (W. Mutter, M. J. Reddehase, F. W. Busch, H.-J. Bühring, and U. H. Koszinowski, J. Exp. Med. 167:1645-1658, 1988). The in vitro generation of granulocyte-monocyte progenitors (CFU-GM) discontinued after infection of the stromal cell layer, whereas the proliferation and differentiation of CFU-GM to granulocyte-monocyte colonies remained unaffected. A protocol was established to probe the functional integrity of earlier hematopoietic cells. Pre-CFU-GM (the progenitors of the CFU-GM) could be recovered from an infected bone marrow donor culture by transfer onto an inductive recipient stromal cell layer. Thus, at least in vitro, infection of bone marrow stroma appears to be the only cause of the defect in myelopoiesis.

Attenuated murine cytomegalovirus binds to N-acetylglucosamine, and shift to virulence may involve recognition of sialic acids

Treatment of cells with lectins specific for N-acetylglucosamine (GlcNAc) blocked infection by mouse cytomegalovirus (MCMV), and GlcNAc pretreatment of the lectin blocked this effect. MCMV failed to infect N-acetylglucosaminidase (GlcNAcase)-treated mouse embryo fibroblasts (MEF). GlcNAc and GlcNAc-containing synthetic oligosaccharides directly inhibited viral infectivity. Ulex lectin inhibition of infection was shown to be due to inhibition of surface adsorption of 35S-labeled virus. Also, GlcNAcase eluted 35S-labeled virus adsorbed to MEF at 4 degrees C and inhibited plaque formation if added after adsorption at this temperature. These findings indicate that GlcNAc binding is involved in attachment rather than in some later step in infection. High-performance thin-layer chromatography overlay of [35S]MCMV indicated that it binds to a GlcNAc-containing asialoglycolipid. Analogous experiments indicated that MCMV made virulent by in vivo salivary gland passage binds to sialic acids in addition to GlcNAc. Treatment of MEF with sialic acid-binding lectins blocked infectivity. Incubation of virus with sialic acids also prevented infection. N-acetylneuraminic acid was 10(3)-fold more potent than N-glycolylneuraminic acid. Sialidase-treated target cells were not efficiently infected by the virus. Thus, MCMV binds to GlcNAc on the cell surface, and the shift to virulence (by in vivo salivary gland passage) correlates with viral recognition of sialic acids.

The prevention of cytomegalovirus disease in renal transplantation

Primary cytomegalovirus (CMV) disease can be prevented in renal transplant recipients with the use of either CMV hyperimmune globulin (CMVIg) or acyclovir. Started within 72 hours of transplantation and continued for 16 weeks posttransplant, CMVIg decreases the incidence of primary CMV disease from 60% to 21%. Acyclovir administered preoperatively and for 3 months thereafter decreases the incidence of CMV disease from 29% to 8% and is the most cost-effective therapy. The effectiveness of these preparations in preventing CMV reinfection or reactivation has not been established. The utility of therapies other than CMVIg or acyclovir for the prevention of CMV disease has not been proven; CMV vaccination is ineffective, polyvalent immunoglobulins require further study, and interferon alpha (IFN-alpha) has been associated with frequent irreversible rejection reactions and therefore should not be used for CMV prophylaxis. Although further investigation is necessary, patients at risk for primary CMV disease should receive prophylactic therapy with either CMVIg or acyclovir.

Animal cytomegaloviruses

Cytomegaloviruses are agents that infect a variety of animals. Human cytomegalovirus is associated with infections that may be inapparent or may result in severe body malformation. More recently, human cytomegalovirus infections have been recognized as causing severe complications in immunosuppressed individuals. In other animals, cytomegaloviruses are often associated with infections having relatively mild sequelae. Many of these sequelae parallel symptoms associated with human cytomegalovirus infections. Recent advances in biotechnology have permitted the study of many of the animal cytomegaloviruses in vitro. Consequently, animal cytomegaloviruses can be used as model systems for studying the pathogenesis, immunobiology, and molecular biology of cytomegalovirus-host and cytomegalovirus-cell interactions.

Replication of murine cytomegalovirus in mast cells

Mast cells purified from the peritoneal cell population and mast cells derived in culture from bone marrow cells were examined for their sensitivity to murine cytomegalovirus (MCMV) infection in vitro. While up to 70% of mast cells expressed viral antigens, less than 12% of the cells produced infectious virus. Transmission electron microscopy demonstrated nucleocapsids in the nuclei and in association with the cisternal elements of the Golgi apparatus. Some complete virions were found within small cytoplasmic vacuoles. In contrast with previous studies of macrophages and fibroblasts, the susceptibility of mast cells to MCMV infection in vitro was not influenced by the H-2 or non-H-2 genotype of the donor.

Murine cytomegalovirus-induced suppression of antigen-specific cytotoxic T lymphocyte maturation

Antigen-specific cytotoxic T lymphocyte (CTL) maturation was inhibited in mice acutely infected with murine cytomegalovirus (MCMV). When immunization with Simian virus 40 (SV40) either preceded or followed infection with MCMV by 1 day, the frequency of SV40-specific CTL precursors among lymph node cells (LNC) was significantly reduced compared to noninfected mice. Replication of the herpesvirus in LNC could not be detected; however, MCMV rendered noninfectious by heat treatment was not suppressive to CTL development. Lymph node cells from nonimmunized, MCMV-infected mice contained a cell(s) present in low frequency which suppressed in vitro maturation of SV40 CTL in immune lymph nodes from mice not infected with MCMV. These suppressor cells did not affect the antigen- and interleukin-2-dependent growth nor cytotoxic activity of a mature, SV40-specific CTL clone. These results indicate that MCMV interferes with immunoregulatory functions required for development of antigen-specific CTL precursors to mature effector CTL. The immunosuppression is mediated at least in part by the MCMV-induced suppressor cell(s).

Increased susceptibility to pentobarbital following mouse cytomegalovirus infection: relative roles of viral-induced interferon and viral infection of the liver

The purpose of this study was to determine the relative roles of viral-induced interferon (IFN) and viral infection of the liver in mouse cytomegalovirus (MCMV)-induced depression of cytochrome P-450 (cyt P-450) levels and enhancement of pentobarbital-induced sleeping time (PEN-ST). This was done by establishing the temporal relationship among the IFN response, viral infection of the liver, suppression of cyt P-450 levels, and enhancement of PEN-ST, by determining the effect of anti-IFN antibody treatment on all of these responses, and by manipulating factors known to influence viral pathogenesis and host response to virus such as animal age, virulence of the virus, and dose of virus. In general, manipulation of these factors toward increased stimulation of host immune responses resulted in greater depression of cyt P-450. The data are consistent with the hypothesis that some IFN-dependent mechanism may have contributed to the effects of MCMV infection on both cyt P-450 levels and PEN-ST; however, the temporal relationship among the various responses measured following viral infection suggested that the effect of the IFN response may be indirect and due to modulation of other host defense mechanisms. Use of anti-IFN antisera to definitively establish a role for IFN in the effects observed here proved unsuccessful. Effects on PEN-ST and cyt P-450 levels did not appear to be related to the magnitude of infection in the liver.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of recombinant murine interferon-beta on the replication of murine cytomegalovirus

Pretreatment of mouse embryo fibroblasts (MEF) with recombinant murine interferon-beta (rMuIFN-beta) induced a high level of intracellular 2',5'-oligoadenylate (2-5A) synthetase activity. However, murine cytomegalovirus (MCMV) replicated under such condition, indicating that MCMV is relatively insensitive in vitro to rMuIFN-beta. Thus, there was a dissociation of 2-5A synthetase activity and antiviral activity against MCMV. In contrast to MCMV, the two parameters were closely associated in the case of vesicular stomatitis virus (VSV).

A nonstructural viral protein expressed by a recombinant vaccinia virus protects against lethal cytomegalovirus infection

The nonstructural immediate-early protein pp89 of murine cytomegalovirus (MCMV) is the first viral protein synthesized after infection and has a regulatory function in viral gene expression. Despite its localization in the nucleus of infected cells, pp89 is also the dominant antigen recognized by MCMV-specific cytolytic T lymphocytes. The recombinant vaccinia virus MCMV-ieI-VAC, which expresses pp89, was used to study the capacity of this protein to induce protective immunity in BALB/c mice. Vaccination with MCMV-ieI-VAC induced a long-lasting immunity that protected mice against challenge with a lethal dose of MCMV but did not prevent infection and morbidity. In vivo depletion of CD8+ T lymphocytes before challenge completely abrogated the protective immunity. CD8+ T lymphocytes derived from MCMV-ieI-VAC-primed donors and adoptively transferred into sublethally irradiated and MCMV-infected recipients were found to limit viral replication in host tissues, whereas CD4+ T lymphocytes and pp89-specific antiserum had no protective effect. The data demonstrate for the first time that a single nonstructural viral protein can confer protection against a lethal cytolytic infection and that this immunity is entirely mediated by the CD8+ subpopulation of T lymphocytes.

Murine cytomegalovirus infection of mouse testes

With the aim of illustrating a mechanism of cytomegalovirus (CMV) venereal transmission, we induced murine CMV infection in the mouse testes of immunologically competent mice. Using in situ cytohybridization, we were able to show that murine CMV-specific DNA was associated with spermatocytes and mature sperm. Electron microscopy studies also supported sperm infection. The virus could be reisolated from infected epididymal sperm by cocultivation with mouse embryo fibroblasts. We found no difference in either the sexual performance or the fertilization efficiency of the sperm between infected and uninfected males.

The role of NK cell activity in age-dependent resistance of mice to murine cytomegalovirus infection

The resistance of mice to cell culture passaged murine cytomegalovirus (CC-MCMV) infection developed with age. In parallel with this finding, augmentation of the splenic NK cell activity in older mice was always higher than that of younger mice. The splenic NK cell activity reached the maximum level at 6 day post infection (PI) in 2-4-week-old mice while in 6-8-week-old mice it peaked at 4 days PI. When the dose of CC-MCMV was increased, the NK cell activity was potentiated accordingly. However, it was decreased on the infection with increased doses of the salivary gland passaged MCMV (SG-MCMV). NK cells augmented by MCMV infection actually inhibited in vitro replication of MCMV when they were added to mouse embryonic fibroblast (MEF) monolayers infected with CC-MCMV. Splenic and peritoneal macrophages inhibited in vitro replication of MCMV, but their activities were less potent than those of NK cells.

Genetic control of murine cytomegalovirus infection: virus titres in resistant and susceptible strains of mice

The course of MCMV infection was studied in a number of inbred strains of mice which differ in their resistance to the development of fatal disease induced by MCMV. Both H-2 and non-H-2 associated genes control this form of resistance and were found to influence the extent of virus replication during sublethal and severe infection. However, for a given strain the summated virus titre of the 5 organs studied was not always proportional to resistance strains. Peak titres of virus were found in the liver and spleen of each strain on days 2 to 3 during high dose infection and in resistant mice during low dose infection. Thereafter titres declined rapidly. When the proportion of the summated virus titre which was present in the spleen and liver was compared for a number of strains, variations in the growth of MCMV in the spleen were noted with 13-84 per cent of virus being found in the spleen of BALB/c, BALB.B, BALB.K and A/WySn mice and usually less than 3 per cent in the spleen of C57BL/6, C57BL/10, B10.BR, C3H and CBA mice.