Cytomegalovirus cell tropism, replication, and gene transfer in brain

Passive immunization reduces murine cytomegalovirus-induced brain pathology in newborn mice

Human cytomegalovirus (HCMV) is the most frequent cause of congenital viral infections in humans and frequently leads to long-term central nervous system (CNS) abnormalities that include learning disabilities, microcephaly, and hearing loss. The pathogenesis of the CNS infection has not been fully elucidated and may arise as a result of direct damage of CMV-infected neurons or indirectly secondary to inflammatory response to infection. We used a recently established model of mouse CMV (MCMV) infection in newborn mice to analyze the contribution of humoral immunity to virus clearance from the brain. In brains of MCMV-infected newborn mice treated with immune serum, the titer of infectious virus was reduced below detection limit, whereas in the brains of mice receiving control (nonimmune) serum significant amounts of virus were recovered. Moreover, histopathological and immunohistological analyses revealed significantly less CNS inflammation in mice treated with immune serum. Treatment with MCMV-specific monoclonal antibodies also resulted in the reduction of virus titer in the brain. Recipients of control serum or irrelevant antibodies had more viral foci, marked mononuclear cell infiltrates, and prominent glial nodules in their brains than mice treated with immune serum or MCMV-specific antibodies. In conclusion, our data indicate that virus-specific antibodies have a protective role in the development of CNS pathology in MCMV-infected newborn mice, suggesting that antiviral antibodies may be an important component of protective immunological responses during CMV infection of the developing CNS.

Brain trauma enhances transient cytomegalovirus invasion of the brain only in mice that are immunodeficient

Cytomegalovirus (CMV) is one of the most common viral pathogens leading to neurological dysfunction in individuals with depressed immune systems. How CMV enters the brain remains an open question. The hypothesis that brain injury may enhance the entrance of CMV into the brain was tested. Insertion of a sterile needle into the brain caused a dramatic increase in mouse CMV in the brains of immunodeficient SCID mice inoculated peripherally within an hour of injury and examined 1 week later; peripheral inoculation 48 h after injury and a 1-week survival resulted in only a modest infection at the site of injury. In contrast, uninjured SCID mice, as well as injured immunocompetent control mice, showed little sign of viral infection at the same time intervals. Direct inoculation of the brain resulted in widespread dispersal and enhanced replication of mCMV in SCID brains tested 1 week later but not in parallel control brains. Differential viremia was unlikely to account for the greater viral load in the SCID brain, since increased mCMV in the blood of SCID compared to controls was not detected until a longer interval. These data suggest that brain injury enhances CMV invasion of the brain, but only when the adaptive immune system is compromised, and that the brain's ability to resist viral infection recovers rapidly after injury.

CD8+ T lymphocytes control murine cytomegalovirus replication in the central nervous system of newborn animals

Human CMV infection of the neonatal CNS results in long-term neurologic sequelae. To define the pathogenesis of fetal human CMV CNS infections, we investigated mechanisms of virus clearance from the CNS of neonatal BALB/c mice infected with murine CMV (MCMV). Virus titers peaked in the CNS between postnatal days 10-14 and infectious virus was undetectable by postnatal day 21. Congruent with virus clearance was the recruitment of CD8(+) T cells into the CNS. Depletion of CD8(+) T cells resulted in death by postnatal day 15 in MCMV-infected animals and increased viral loads in the liver, spleen, and the CNS, suggesting an important role for these cells in the control of MCMV replication in the newborn brain. Examination of brain mononuclear cells revealed that CD8(+) T cell infiltrates expressed high levels of CD69, CD44, and CD49d. IE1(168)-specific CD8(+) T cells accumulated in the CNS and produced IFN-gamma and TNF-alpha but not IL-2 following peptide stimulation. Moreover, adoptive transfer of brain mononuclear cells resulted in decreased virus burden in immunodepleted MCMV-infected syngeneic mice. Depletion of the CD8(+) cell population following transfer eliminated control of virus replication. In summary, these results show that functionally mature virus-specific CD8(+) T cells are recruited to the CNS in mice infected with MCMV as neonates.

Cytomegalovirus cell tropism

The human cytomegalovirus (HCMV) can infect a remarkably broad cell range within its host, including parenchymal cells and connective tissue cells of virtually any organ and various hematopoietic cell types. Epithelial cells, endothelial cells, fibroblasts and smooth muscle cells are the predominant targets for virus replication. The pathogenesis of acute HCMV infections is greatly influenced by this broad target cell range. Infection of epithelial cells presumably contributes to inter-host transmission. Infection of endothelial cells and hematopoietic cells facilitates systemic spread within the host. Infection of ubiquitous cell types such as fibroblasts and smooth muscle cells provides the platform for efficient proliferation of the virus. The tropism for endothelial cells, macrophages and dendritic cells varies greatly among different HCMV strains, mostly dependent on alterations within the UL128-131 gene locus. In line with the classification of the respective proteins as structural components of the viral envelope, interstrain differences concerning the infectivity in endothelial cells and macrophages are regulated on the level of viral entry.

Neonatal neural progenitor cells and their neuronal and glial cell derivatives are fully permissive for human cytomegalovirus infection

Congenital human cytomegalovirus (HCMV) infection causes central nervous system structural abnormalities and functional disorders, affecting both astroglia and neurons with a pathogenesis that is only marginally understood. To better understand HCMV's interactions with such clinically important cell types, we utilized neural progenitor cells (NPCs) derived from neonatal autopsy tissue, which can be differentiated down either glial or neuronal pathways. Studies were performed using two viral isolates, Towne (laboratory adapted) and TR (a clinical strain), at a multiplicity of infection of 3. NPCs were fully permissive for both strains, expressing the full range of viral antigens (Ags) and producing relatively large numbers of infectious virions. NPCs infected with TR showed delayed development of cytopathic effects (CPE) and replication centers and shed less virus. This pattern of delay for TR infections held true for all cell types tested. Differentiation of NPCs was carried out for 21 days to obtain either astroglia (>95% GFAP(+)) or a 1:5 mixed neuron/astroglia population (beta-tubulin III(+)/GFAP(+)). We found that both of these differentiated populations were fully permissive for HCMV infection and produced substantial numbers of infectious virions. Utilizing a difference in plating efficiencies, we were able to enrich the neuron population to approximately 80% beta-tubulin III(+) cells. These beta-tubulin III(+)-enriched populations remained fully permissive for infection but were very slow to develop CPE. These infected enriched neurons survived longer than either NPCs or astroglia, and a small proportion were alive until at least 14 days postinfection. These surviving cells were all beta-tubulin III(+) and showed viral Ag expression. Surprisingly, some cells still exhibited extended processes, similar to mock-infected neurons. Our findings strongly suggest neurons as reservoirs for HCMV within the developing brain.

Characterization of murine cytomegalovirus m157 from infected cells and identification of critical residues mediating recognition by the NK cell receptor Ly49H

Activated NK cells mediate potent cytolytic and secretory effector functions and are vital components of the early antiviral immune response. NK cell activities are regulated by the assortment of inhibitory receptors that recognize MHC class I ligands expressed on healthy cells and activating receptors that recognize inducible host ligands or ligands that are not well characterized. The activating Ly49H receptor of mouse NK cells is unique in that it specifically recognizes a virally encoded ligand, the m157 glycoprotein of murine CMV (MCMV). The Ly49H-m157 interaction underlies a potent resistance mechanism (Cmv1) in C57BL/6 mice and serves as an excellent model in which to understand how NK cells are specifically activated in vivo, as similar receptor systems are operative for human NK cells. For transduced cells expressing m157 in isolation and for MCMV-infected cells, we show that m157 is expressed in multiple isoforms with marked differences in abundance between infected fibroblasts (high) and macrophages (low). At the cell surface, m157 is exclusively a glycosylphosphatidylinositol-associated protein in MCMV-infected cells. Through random and site-directed mutagenesis of m157, we identify unique residues that provide for efficient cell surface expression of m157 but fail to activate Ly49H-expressing reporter cells. These m157 mutations are predicted to alter the conformation of a putative m157 interface with Ly49H, one that relies on the position of a critical alpha0 helix of m157. These findings support an emerging model for a novel interaction between this important NK cell receptor and its viral ligand.

Prior infection with murine cytomegalovirus (MCMV) limits the immunocontraceptive effects of an MCMV vector expressing the mouse zona-pellucida-3 protein

We have developed a murine cytomegalovirus (MCMV)-vectored vaccine expressing the mouse zona-pellucida-3 gene (rMCMV-ZP3), which successfully induces infertility in experimentally inoculated laboratory or wild-derived mice. However, the future success of this vector as a fully disseminating vaccine in free-living mice may be compromised by pre-existing immunity since there is a high prevalence of naturally acquired MCMV infection in these mice. To evaluate the effect of prior immunity to MCMV on vaccine efficacy, we constructed two new biologically effective recombinant MCMV vectors expressing the mouse ZP3 protein from two MCMV strains (N1 and G4) derived from free-living mice. In wild mice, mixed MCMV infection is common and could be acquired either by simultaneous coinfection or sequential infection with different MCMV strains. Interestingly, while coinfection with both wild-type and rMCMV-ZP3 via the intraperitoneal route reduced the impact of the rMCMV-ZP3, prior infection with the same wild-type strain as that used to construct the rMCMV-ZP3 abrogated the immunocontraceptive effects of either N1-ZP3 or G4-ZP3. However, prior infection with G4 28 days before the introduction of N1-ZP3 had a reduced influence on the efficacy of the rMCMV-ZP3. Thus, the strain of virus and the timing of prior infection are factors that may influence the efficacy of the rMCMV-ZP3. Given that mixed infection of mice with MCMV is common, it is possible that prior immunity acquired by natural mucosal infection may have less a less inhibitory effect on the immunocontraceptive outcome.

Altered development of the brain after focal herpesvirus infection of the central nervous system

Human cytomegalovirus infection of the developing central nervous system (CNS) is a major cause of neurological damage in newborn infants and children. To investigate the pathogenesis of this human infection, we developed a mouse model of infection in the developing CNS. Intraperitoneal inoculation of newborn animals with murine cytomegalovirus resulted in virus replication in the liver followed by virus spread to the brain. Virus infection of the CNS was associated with the induction of inflammatory responses, including the induction of a large number of interferon-stimulated genes and histological evidence of focal encephalitis with recruitment of mononuclear cells to foci containing virus-infected cells. The morphogenesis of the cerebellum was delayed in infected animals. The defects in cerebellar development in infected animals were generalized and, although correlated temporally with virus replication and CNS inflammation, spatially unrelated to foci of virus-infected cells. Specific defects included decreased granular neuron proliferation and migration, expression of differentiation markers, and activation of neurotrophin receptors. These findings suggested that in the developing CNS, focal virus infection and induction of inflammatory responses in resident and infiltrating mononuclear cells resulted in delayed cerebellar morphogenesis.

Sensitive detection of human cytomegalovirus in tumors and peripheral blood of patients diagnosed with glioblastoma

Human cytomegalovirus (HCMV) has been described to be associated with several human malignancies, though the frequency of detection remains controversial. It is unclear whether HCMV plays an active role in malignant tumor progression or becomes reactivated under pathologic conditions that result in chronic inflammation or immunosuppression. In this study, we report on the investigation of detecting HCMV in the tumors and peripheral blood of patients with newly diagnosed glioblastoma multiforme (GBM). Using immunohistochemistry, in situ hybridization, and polymerase chain reaction amplification of viral DNA, the detection of HCMV was investigated in tumor and blood specimens from patients with GBM as well as in the peripheral blood of normal volunteers and patients undergoing craniotomy for diagnoses other than GBM. We found that a high percentage (>90%) of GBM tumors, not surrounding normal brain, are associated with HCMV nucleic acids and proteins. Furthermore, a significant proportion of patients (80%) with newly diagnosed GBM have detectable HCMV DNA in their peripheral blood, while sero-positive normal donors and other surgical patients did not exhibit detectable virus, suggesting either a systemic reactivation of HCMV within patients with GBM or shedding of viral DNA from infected tumor cells into the periphery. These results confirm the association of HCMV with malignant gliomas and demonstrate that subclinical HCMV viremia (presence of viral DNA in blood without clinical symptoms of infection) is a previously unrecognized disease spectrum in patients with GBM.

Bystander attenuation of neuronal and astrocyte intercellular communication by murine cytomegalovirus infection of glia

Astrocytes are the first cells infected by murine cytomegalovirus (MCMV) in primary cultures of brain. These cells play key roles in intercellular signaling and neuronal development, and they modulate synaptic activity within the nervous system. Using ratiometric fura-2 digital calcium imaging of >8,000 neurons and glia, we found that MCMV-infected astrocytes showed an increase in intracellular basal calcium levels and an enhanced response to neuroactive substances, including glutamate and ATP, and to high potassium levels. Cultured neurons with no sign of MCMV infection showed attenuated synaptic signaling after infection of the underlying astrocyte substrate, and intercellular communication between astrocytes with no sign of infection was reduced by the presence of infected glia. These bystander effects would tend to cause further deterioration of cellular communication in the brain in addition to the problems caused by the loss of directly infected cells.

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.

Late human cytomegalovirus (HCMV) proteins inhibit differentiation of human neural precursor cells into astrocytes

Human cytomegalovirus (HCMV) is the most common cause of congenital infections in developed countries, with an incidence varying between 0.5-2.2%. Such infection may be the consequence of either a primary infection or reactivation of a latent infection in the mother and the outcome may vary from asymptomatic to severe brain disorders. Moreover, infants that are asymptomatic at the time of birth may still develop neurologic sequelae at a later age. Our hypothesis is that infection of stem cells of the central nervous system by HCMV alters the proliferation, differentiation or migration of these cells, and thereby gives rise to the brain abnormalities observed. We show that infection of human neural precursor cells (NPCs) with the laboratory strain Towne or the clinical isolate TB40 of HCMV suppresses the differentiation of these cells into astrocytes even at an multiplicity of infection (MOI) as low as 0.1 (by 33% and 67%, respectively). This inhibition required active viral replication and the expression of late HCMV proteins. Infection as late as 24 hr after the onset of differentiation, but not after 72 hr, also prevented the maturation of infected cultures. Furthermore, in cultures infected with TB40 (at an MOI of 1), approximately 54% of the cells were apoptotic and cell proliferation was significantly attenuated. Clearly, HCMV can reduce the capacity of NPCs to differentiate into astrocytes and this effect may provide part of the explanation for the abnormalities in brain development associated with congenital HCMV infection.

Marking of peripheral T-lymphocytes by retroviral transduction and transplantation of CD34+ cells in a canine X-linked severe combined immunodeficiency model

A retrovirus vector containing an enhanced green fluorescent protein complimentary DNA (EGFP cDNA) was used to mark and dynamically follow vector-expressing cells in the peripheral blood of bone marrow transplanted X-linked severe combined immunodeficient dogs. CD34(+) cells isolated from young normal dogs were transduced, using a 2 day protocol, with an amphotropic retroviral vector that expressed enhanced green fluorescent protein (EGFP) and the canine common gamma chain (gammac) cDNAs. Following transplantation of the transduced cells, normal donor peripheral blood lymphocytes (PBL) appeared by 1 month post-bone marrow transplant (BMT) and rescued three of five treated dogs from their lethal immunodeficiency. PCR and flow cytometric analysis of post-BMT PBL documented the peripheral EGFP expressing cells as CD3(+) T cells, which varied from 0% to 28%. Sorting of EGFP(+) and EGFP(-) peripheral blood T cells from two dogs, followed by vector PCR analysis, showed no evidence of vector shutdown. EGFP expression in B cells or monocytes was not detected. These marking experiments demonstrate that the transduction protocol did not abolish the lymphoid engraftment capability of ex vivo transduced canine CD34(+) cells and supports the potential utility of the MSCV retroviral vector for gene transfer to XSCID affected canine hematopoietic progenitor cells (HPC).

Enhancement of susceptibility of adult mouse brain to cytomegalovirus infection by infusion of epidermal growth factor

Neural precursor cells, including neural stem and progenitor cells, in the subventricular zone (SVZ) are the main targets for cytomegalovirus (CMV) infection in developing brains. The neural precursor cells in the SVZ of the adult brain have been reported to respond by proliferating after infusion with epidermal growth factor (EGF). Here we report the susceptibility of the precursor cells in the adult mouse brain to murine CMV (MCMV) infection. Adult mouse brains from 10-, 25-, and 70-week-old (W) mice were infused with either phosphate-buffered saline or EGF into the brain for 3 days, and then intracerebrally infected with MCMV for 5 days. The susceptibility of the adult brains to MCMV was significantly increased by infusion of EGF in terms of viral titers and viral antigen-positive cells. The susceptibility of the young adult brain from 10-week-old mice to MCMV was higher than that of the adult brains from 25-week-old or 70-week-old mice. Both the ependymal and the SVZ cells were susceptible to MCMV infection. The number of virus-infected cells in the SVZ was significantly increased by infusion of EGF, whereas the number of infected ependymal cells was not significantly increased. Among the virus-infected cells in the SVZ, 73% were positive for nestin, 87% were positive for Musashi, 86% were positive for GFAP, and 96% were positive for PCNA. These results indicate that the susceptibility of the adult brain to MCMV is correlated with the proliferative ability of the neural precursor cells in the SVZ of the adult brain.

Cytomegalovirus induces interferon-stimulated gene expression and is attenuated by interferon in the developing brain

Cytomegalovirus (CMV) is considered the most common infectious agent causing permanent neurological dysfunction in the developing brain. We have previously shown that CMV infects developing brain cells more easily than it infects mature brain cells and that this preference is independent of the host B- and T-cell responses. In the present study, we examined the innate antiviral defenses against mouse (m) and human (h) CMVs in developing and mature brain and brain cells. mCMV infection induced interferon (IFN)-stimulated gene expression by 10- to 100-fold in both glia- and neuron-enriched cultures. Treatment of primary brain cultures with IFN-alpha, -beta, and -gamma or a synthetic RNA, poly(I:C), reduced the number of mCMV-infected cells, both in older cells and in fresh cultures from embryonic mouse brains. When a viral dose that killed almost all unprotected cells was used, IFN-protected cells had a natural appearance, and when they were tested with whole-cell patch clamp recording, they appeared physiologically normal with typical resting membrane potentials and action potentials. mCMV infection increased expression of representative IFN-stimulated genes (IFIT3, OAS, LMP2, TGTP, and USP18) in both neonatal and adult brains to similarly large degrees. The robust upregulation of gene expression in the neonatal brain was associated with a much higher degree of viral replication at this stage of development. In contrast to the case for downstream gene induction, CMV upregulated IFN-alpha/beta expression to a greater degree in the adult brain than in the neonatal brain. Similar to the case with cultured brain cells, IFN treatment of the developing brain in vivo depressed mCMV replication. In parallel work with cultured primary human brain cells, IFN and poly(I:C) treatment reduced hCMV infection and prevented virus-mediated cell death. These results suggest that coupling IFN administration with current treatments may reduce CMV infections in the developing brain.

Human cytomegalovirus inhibits neuronal differentiation and induces apoptosis in human neural precursor cells

Human cytomegalovirus (HCMV) is the most common cause of congenital infections in developed countries, with an incidence varying between 0.5 and 2.2% and consequences varying from asymptomatic infection to lethal conditions for the fetus. Infants that are asymptomatic at birth may still develop neurological sequelae, such as hearing loss and mental retardation, at a later age. Infection of neural stem and precursor cells by HCMV and consequent disruption of the proliferation, differentiation, and/or migration of these cells may be the primary mechanism underlying the development of brain abnormalities. In the present investigation, we demonstrate that human neural precursor cells (NPCs) are permissive for HCMV infection, by both the laboratory strain Towne and the clinical isolate TB40, resulting in 55% and 72% inhibition of induced differentiation of human NPCs into neurons, respectively, when infection occurred at the onset of differentiation. This repression of neuronal differentiation required active viral replication and involved the expression of late HCMV gene products. This capacity of HCMV to prevent neuronal differentiation declined within 24 h after initiation of differentiation. Furthermore, the rate of cell proliferation in infected cultures was attenuated. Surprisingly, HCMV-infected cells exhibited an elevated frequency of apoptosis at 7 days following the onset of differentiation, at which time approximately 50% of the cells were apoptotic at a multiplicity of infection of 10. These findings indicate that HCMV has the capacity to reduce the ability of human NPCs to differentiate into neurons, which may offer one explanation for the abnormalities in brain development associated with congenital HCMV infection.

Double-stranded RNA binding by a heterodimeric complex of murine cytomegalovirus m142 and m143 proteins

In response to viral infection, cells activate a variety of antiviral responses, including several that are triggered by double-stranded (ds) RNA. Among these are the protein kinase R and oligoadenylate synthetase/RNase L pathways, both of which result in the shutoff of protein synthesis. Many viruses, including human cytomegalovirus, encode dsRNA-binding proteins that prevent the activation of these pathways and thereby enable continued protein synthesis and viral replication. We have extended these analyses to another member of the beta subfamily of herpesviruses, murine cytomegalovirus (MCMV), and now report that products of the m142 and m143 genes together bind dsRNA. Coimmunoprecipitation experiments demonstrate that these two proteins interact in infected cells, consistent with their previously reported colocalization. Jointly, but not individually, the proteins rescue replication of a vaccinia virus mutant with a deletion of the dsRNA-binding protein gene E3L (VVDeltaE3L). Like the human cytomegalovirus dsRNA-binding protein genes TRS1 and IRS1, m142 and m143 are members of the US22 gene family. We also found that two other members of the MCMV US22 family, M23 and M24, encode dsRNA-binding proteins, but they do not rescue VVDeltaE3L replication. These results reveal that MCMV, like many other viruses, encodes dsRNA-binding proteins, at least two of which can inhibit dsRNA-activated antiviral pathways. However, unlike other well-studied examples, the MCMV proteins appear to act in a heterodimeric complex.

Experimental confirmation of global murine cytomegalovirus open reading frames by transcriptional detection and partial characterization of newly described gene products

Murine cytomegalovirus (MCMV) and human CMV (HCMV) share many features making the mouse system a potential small-animal model for HCMV. Although the genomic DNA sequence and the predicted open reading frames (ORFs) of MCMV have been determined, experimental evidence that the ORFs are actually transcribed has been lacking. We developed an MCMV global-DNA microarray that includes all previously predicted ORFs and 14 potential ones. A total of 172 ORFs were confirmed to be transcribed, including 7 newly discovered ORFs not previously predicted. No gene products from 10 previously predicted ORFs were detected by either DNA microarray analysis or reverse transcriptase PCR in MCMV-infected mouse fibroblasts, although 2 of those were expressed in a macrophage cell line, suggesting that potential gene products from these open reading frames are silenced in fibroblasts and required in macrophages. Immunohistochemical localization of the six newly described ORF products and three recently identified ones in cells transfected with the respective construct revealed four of the products in the nucleus and five in mitochondria. Analysis of two ORFs using site-directed mutagenesis showed that deletion of one of the mitochondrion-localized gene products led to significantly decreased replication in fibroblasts.

Mouse cytomegalovirus crosses the species barrier with help from a few human cytomegalovirus proteins

Strong species specificity and similar tropisms suggest mouse cytomegalovirus (mCMV) as a potential vector for transgenes into human cells. We reexamined the dogma that mouse cytomegalovirus cannot productively replicate in human cells and found that mouse cytomegalovirus can produce infectious particles albeit at a level that does not sustain an infection. This finding demonstrates that mouse cytomegalovirus can undergo all processes of its life cycle in human cells but may not be well adapted to circumvent the human cell's intrinsic defenses. The suppression of mCMV production in human cells is affected at several levels, which additively or synergistically result in the appearance of species specificity. Hydrolysis of most newly replicated viral DNA and very low capsid protein transcription reduced the potential particle production to insignificant levels. These effects can be ameliorated by adding human cytomegalovirus tegument proteins and immediate-early protein 1. They function synergistically to produce significant amounts of mCMV in human cells. While the possibility that mouse cytomegalovirus might replicate in human cells raises caution in the use of this virus as a transgene vector, manipulation of the mouse cytomegalovirus genome to allow limited spread to other human cells might also provide an advantage for the distribution of certain transgenic products.

Viral infections in the developing and mature brain

A number of different RNA and DNA viruses can invade the brain and cause neurological dysfunction. These range from the tiny polio picornavirus, which has only 7kb of RNA genetic code that preferentially infects motor neurons, to the relatively large cytomegalovirus, which has >100 genes in its 235kb DNA genome and causes various neurological problems in the developing brain but is comparatively harmless to adults. This brief overview of some aspects of neurovirology addresses the complex problems that underlie an appreciation of the contribution of viral infections to brain disease. [This review is part of the INMED/TINS special issue "Nature and nurture in brain development and neurological disorders", based on presentations at the annual INMED/TINS symposium (http://inmednet.com/).]

Differential expression of the immediate-early 2 and 3 proteins in developing mouse brains infected with murine cytomegalovirus

Murine cytomegalovirus (MCMV) immediate-early (IE) 2 protein has been reported to be dispensable for growth and latency in mice. Therefore, its role in viral pathogenesis and tissue tropism is not known. Here we prepared specific antibodies to the IE2 and IE3 proteins by using fusion proteins expressed in Escherichia coli as antigens. Immunostaining of MCMV-infected cultured fibroblasts revealed IE2 protein to be expressed diffusely in the nucleoplasm similar to the IE1 protein. In contrast, expression of the IE3 protein, 88 kDa, exhibited a punctate pattern in the nucleus in the early phase of infection then diminished. In the brain of neonatal mice infected with MCMV, both IE2 and IE3 proteins were detected immunohistochemically in the cells of the ventricular walls early in infection. When the infection was prolonged, the IE2 protein was expressed in neurons of the cortex and hippocampus, while the IE3 protein was preferentially expressed in glial cells in the early phase of infection, and its levels declined during the infection. These results suggest that the IE2 protein may play a role in persistent infection in neurons, whereas the IE3 protein, expressed preferentially in glial cells, may play the main role in acute infection.

Neural precursor cell susceptibility to human cytomegalovirus diverges along glial or neuronal differentiation pathways

Cytomegalovirus (CMV) is a major cause of congenital brain disease, and its neuropathogenesis may be related to viral infection of rapidly dividing, susceptible neural precursor cells (NPCs). In the present study, we evaluated the susceptibility of human fetal brain-derived NPCs (nestin(+), A2B5(+), CD133(+)) to infection with CMV. Data derived from these studies demonstrated that undifferentiated NPCs supported productive viral replication. After differentiation in the presence of serum, a treatment that promotes development of an astroglial cell phenotype (GFAP(+), nestin(-), A2B5(-)), viral expression was retained. However, differentiation of NPCs in medium containing platelet-derived growth factor and brain-derived neurotropic factor, conditions that support the development of neurons (Tuj-1(+), nestin(-), A2B5(-)), resulted in reduced viral expression, with corresponding decreased CMV major immediate-early promoter (MIEP) activity relative to undifferentiated cells. Further experiments showed that cellular differentiation into a neuronal phenotype was associated with elevated levels of various CCAAT/enhancer binding protein beta (C/EBP)-beta isoforms, which suppressed MIEP activity in cotransfected NPCs. Taken together, these data demonstrate that the susceptibility of primary human NPCs to CMV is retained concomitantly with differentiation into glial cells but is actively repressed following differentiation into neurons.

Sequence variability of human cytomegalovirus UL146 and UL147 genes in low-passage clinical isolates

OBJECTIVES

Human cytomegalovirus (HCMV) infects a number of organs and cell types in vivo. The different symptoms and tissue tropisms of HCMV infection perhaps result from the genetic polymorphism. A new region of DNA containing at least 19 open reading frames (ORFs - denoted UL133-151) was found in the low-passage HCMV clinical strain Toledo and several other low-passage clinical isolates, but not present in the HCMV laboratory strain AD169. Two of these genes, UL146 and UL147, encode proteins with sequence characteristics of CXC (alpha) chemokines, suggesting that they might influence the behavior of neutrophils during infection. This research was to study the sequence variability of UL146 and UL147 ORFs in HCMV clinical isolates and examine the possible associations between gene variability and the outcome of HCMV infection.

METHODS

UL146 and UL147 genes from strains obtained from suspected congenitally HCMV-infected infants were PCR amplified and sequenced.

RESULTS

High variability was found in UL146 and UL147 gene among HCMV clinical strains. However, the alpha chemokine motif in UL146 and UL147 genes was conserved in almost all sequences. According to the phylogenetic analysis, all sequences of UL146 in clinical isolates could be divided into three groups. All strains from congenital megacolon infants existed in G2A only, and all from asymptomatic infants existed in G2B peculiarly.

CONCLUSIONS

Sequence variability among HCMV clinical strains may affect the ability of UL146 and UL147 to attract human neutrophils and influence viral dissemination. No obvious linkage was observed between UL146 polymorphisms and outcome of suspected congenital HCMV infection.

Cytomegalovirus MCK-2 controls mobilization and recruitment of myeloid progenitor cells to facilitate dissemination

Murine cytomegalovirus encodes a secreted, pro-inflammatory chemokine-like protein, MCK-2, that recruits leukocytes and facilitates viral dissemination. We have shown that MCK-2-enhanced recruitment of myelomonocytic leukocytes with an immature phenotype occurs early during infection and is associated with efficient viral dissemination. Expression of MCK-2 drives the mobilization of a population of leukocytes from bone marrow that express myeloid marker Mac-1 (CD11b), intermediate levels of Gr-1 (Ly6 G/C), platelet-endothelial-cell adhesion molecule-1 (PECAM-1, CD31), together with heterogeneous levels of stem-cell antigen-1 (Sca-1, Ly-6 A /E). Recombinant MCK-2 mediates recruitment of this population even in the absence of viral infection. Recruitment of this cell population and viral dissemination via the bloodstream to salivary glands proceeds normally in mice that lack CCR2 and MCP-1 (CCL2), suggesting that recruitment of macrophages is not a requisite component of pathogenesis. Thus, a systemic impact of MCK-2 enhances the normal host response and causes a marked increase in myelomonocytic recruitment with an immature phenotype to initial sites of infection. Mobilization influences levels of virus dissemination via the bloodstream to salivary glands and is dependent on a myelomonocytic cell type other than mature macrophages.

Targeting human glioblastoma cells: comparison of nine viruses with oncolytic potential

Brain tumors classified as glioblastomas have proven refractory to treatment and generally result in death within a year of diagnosis. We used seven in vitro tests and one in vivo trial to compare the efficacy of nine different viruses for targeting human glioblastoma. Green fluorescent protein (GFP)-expressing vesicular stomatitis (VSV), Sindbis virus, pseudorabies virus (PRV), adeno-associated virus (AAV), and minute virus of mice i-strain (MVMi) and MVMp all infected glioblastoma cells. Mouse and human cytomegalovirus, and simian virus 40 showed only low levels of infection or GFP expression. VSV and Sindbis virus showed strong cytolytic actions and high rates of replication and spread, leading to an elimination of glioblastoma. PRV and both MVM strains generated more modest lytic effects and replication capacity. VSV showed a similar oncolytic profile on U-87 MG and M059J glioblastoma. In contrast, Sindbis virus showed strong preference for U-87 MG, whereas MVMi and MVMp preferred M059J. Sindbis virus and both MVM strains showed highly tumor-selective actions in glioblastoma plus fibroblast coculture. VSV and Sindbis virus were serially passaged on glioblastoma cells; we isolated a variant, VSV-rp30, that had increased selectivity and lytic capacity in glioblastoma cells. VSV and Sindbis virus were very effective at replicating, spreading within, and selectively killing human glioblastoma in an in vivo mouse model, whereas PRV and AAV remained at the injection site with minimal spread. Together, these data suggest that four (VSV, Sindbis virus, MVMi, and MVMp) of the nine viruses studied merit further analysis for potential therapeutic actions on glioblastoma.

Cytomegalovirus infection inhibits the expression of N-methyl-D-aspartate receptors in the developing mouse hippocampus and primary neuronal cultures

Cytomegalovirus (CMV) is the most significant infectious cause of developmental brain disorders in humans. The infection occasionally persists and causes neurological disorders. The N-methyl-D-aspartate (NMDA) subtype of glutamate receptors is essential for the development and plasticity of synapses, but also is involved in neuronal excitotoxicity during viral infection. Here we investigated the effects of murine CMV (MCMV) infection on the expression of NMDA receptors in the hippocampal neurons of neonatal mice and primary neuronal cultures. Viral antigen was mostly found in hippocampal pyramidal neurons from the CA1 to CA3. Image analysis of immunohistochemistry demonstrated that the expression of NMDA receptor subunit 1 (NMDA-R1) protein in CA1 neurons of MCMV-infected brain was reduced to 40% of that in uninfected brain. The signal of in situ hybridization for NMDA-R1 mRNA was also decreased in CA1 neurons of MCMV-infected brain. In primary neuronal cultures, reduction of NMDA-R1 expression in MCMV-infected neurons was also detected by immunocytochemistry and Western blotting. These results suggest that reduction of NMDA receptor expression by MCMV infection may cause a decrease in the susceptibility of the neurons to excitotoxic cell death, and may be related to the establishment of viral persistence and functional disturbances in MCMV-infected neurons.

Neuropathogenesis in cytomegalovirus infection: indication of the mechanisms using mouse models

Cytomegalovirus (CMV) is the most frequent infectious cause of developmental brain disorders and also causes brain damage in immunocompromised individuals. Although the brain is one of the main targets of CMV infection, little is known about the neuropathogenesis of the brain disorders caused by CMV in humans because of the limitations in studying human subjects. Murine CMV (MCMV) is similar to human CMV (HCMV) in terms of genome structure, pattern of gene expressions, cell tropism and infectious dynamics. In mouse models, it has been shown that neural stem/progenitor cells are the most susceptible to CMV infection in developing brains. During brain development, lytic infection tends to occur in immature glial cells, presumably causing structural disorders of the brain. In the prolonged phase of infection, CMV preferentially infects neuronal cells. Infection of neurons may tend to become persistent by evasion of immune reactions, anti-apoptotic effects and neuron-specific activation of the e1-promoter, presumably causing functional neuronal disorders. It has also been shown that CMV infection in developing brains may become latent in neural immature cells. Brain disorders may occur long after infection by reactivation of the latent infection.

Physiological properties of hypothalamic MCH neurons identified with selective expression of reporter gene after recombinant virus infection

Neurons that synthesize melanin-concentrating hormone (MCH) may modulate arousal and energy homeostasis. The scattered MCH neurons have been difficult to study, as they have no defining morphological characteristics. We have developed a viral approach with AAV for selective long-term reporter gene (GFP) expression in MCH neurons, allowing the study of their cellular physiology in hypothalamic slices. MCH neurons showed distinct membrane properties compared to other neurons infected with the same virus with a cytomegalovirus promoter. Transmitters of extrahypothalamic arousal systems, including norepinephrine, serotonin, and the acetylcholine agonist muscarine, evoked direct inhibitory actions. Orexigenic neuropeptide Y was inhibitory by pre- and postsynaptic mechanisms; an anorexigenic melanocortin agonist had no effect. In contrast, the hypothalamic arousal peptide hypocretin/orexin evoked a direct inward current and increased excitatory synaptic activity and spike frequency in the normally silent MCH neurons. Together, these data support the view that MCH neurons may integrate information within the arousal system in favor of energy conservation.

Systemic immune deficiency necessary for cytomegalovirus invasion of the mature brain

Cytomegalovirus (CMV) is a significant opportunistic pathogen associated with AIDS and immunosuppressive therapy. Infection of the mature central nervous system (CNS) can cause significant pathology with associated neurological deficits, mental disorders, and cognitive impairment and may have potentially fatal consequences. Using genetically immunocompromised mice, we studied mechanisms of CMV invasion into, and behavior within, the CNS. Adult immunodeficient (nude and SCID) and control mice were peripherally infected with recombinant mouse CMV expressing a green fluorescent protein reporter gene. Control mice actively eliminated acute peripheral infection and were resistant to invasion of CMV into the brain. In contrast, virus infected brains of immunodeficient mice but only after a minimum of 21 days postinoculation. After inoculation, CMV was found in circulating leukocytes (MAC-3/CD45(+)) and in leukocytes within the brain, suggesting these cells as a possible source of CMV entry into the CNS. CNS infection was observed in many different cell types, including neurons, glial cells, meninges, ependymal cells, and cells of cerebral vessels. Infection foci progressively expanded locally to adjacent cells, resulting in meningitis, choroiditis, encephalitis, vasculitis, and necrosis; clear indication of axonal transport of CMV was not found. Regional distribution of CMV was unique in each brain, consisting of randomly distributed, unilateral foci. Testing whether CMV gained access to brain through nonspecific vascular disruption, vascular injections of a tracer molecule revealed no obvious disruption of the blood brain barrier in mice with CMV in the brain. Results indicate the importance of host adaptive immunity (particularly T cells) in controlling entry and dissemination of CMV into the brain and are consistent with the view that virus may be carried into the brain by circulating mononuclear cells that traffic through the blood brain barrier.

Neuron-specific activation of murine cytomegalovirus early gene e1 promoter in transgenic mice

The brain is the main target in congenital cytomegalovirus (CMV) infection and immunocompromised patients. No definite evidence that a CMV has special affinity for the central nervous system (CNS) has been published. Here, we generated transgenic mice with an e1 promoter/enhancer region connected to the reporter gene lacZ. Surprisingly, expression of the transgene was completely restricted to the CNS in all lines of transgenic mice. The transgene was expressed in subpopulation of neurons in the cerebral cortex, hippocampus, diencephalon, brainstem, cerebellum, and spinal cord in all of the lines. Non-neuronal cells in the CNS were negative for transgene expression. Activation of the transgene was first observed in neurons of mesencephalon in late gestation, and then the number of positive neurons increased in various parts of the brain as development proceeded. Upon infection of the transgenic mouse brains with MCMV, the location of the activated neurons became more extensive, and the number of such neurons increased. These results suggest that there are host factor(s) that directly activate the MCMV early gene promoter in neurons. This neuron-specific activation may be associated with persistent infection in the brain and may be responsible for the neuronal dysfunction and neuronal cell loss caused by CMV infection.

Dendritic cells and HCMV cross-presentation

The outcome of a viral infection is the result of an endless fight between the organism whose task is to mount an antiviral response and the virus that adapts strategies to circumvent the host response. Human cytomegalovirus (HCMV), a latent herpesvirus, can be considered as a spearhead in exploiting co-existence with the host to develop numerous immuno-evasion mechanisms. The ability of the organism to initiate a primary immune response against viruses such as HCMV is highly dependent on the capacity of professional antigen-presenting cells (APCs), namely dendritic cells (DCs), to prime and activate specific effector T cells. Recent findings emerging from the murine cytomegalovirus (MCMV) animal model demonstrated that infection of murine DCs with MCMV impaired their capacity to prime an effective T cell response. Even though data on interference of HCMV with DC functions are still limited, immunosuppressive effects identical to those reported for MCMV can be suspected and we may then ask how a cytotoxic T lymphocyte (CTL) response is generated in these unfavourable conditions. In response to this question, cross-presentation of HCMV antigens by uninfected DCs to CD8+ T cells could be considered a key process in initiating an immune response. In this chapter we discuss the mechanisms through which DCs could acquire HCMV antigens and how cross-presentation could be modulated throughout infection. Moreover, further knowledge of DC functions is key for the development of DC-based immunotherapy against HCMV.

Murine cytomegalovirus abortively infects human dendritic cells, leading to expression and presentation of virally vectored genes

Dendritic cells (DC) are potent antigen-presenting cells that play a crucial role in antigen-specific immune responses. Thus, the targeting of exogenous antigens to DC has become a popular approach for cancer immunotherapy and vaccine development. In this report, we studied the interplay between murine cytomegalovirus (MCMV) and human monocyte-derived DC. The results showed that an enhanced green fluorescent protein (EGFP)-encoding, replication-competent MCMV vector underwent abortive infection in human DC; this was accompanied by the efficient expression of EGFP. Infection of human DC by this vector resulted in a modest increase in the expression of cell surface proteins associated with DC maturation and has no significant effect on the immunostimulatory function of the cells, as reflected by their ability to support T-cell proliferation in a mixed-lymphocyte reaction. Finally, an MCMV vector encoding the human immunodeficiency virus type 1 (HIV-1) gp120 envelope glycoprotein was constructed and used to infect cultured human DC. The infected DC were shown to be capable of stimulating the expansion of autologous, gp120-specific, class I-restricted T lymphocytes from an HIV-1-negative donor, as determined by tetramer staining and enzyme-linked immunospot analysis. Taken together, these results suggest that MCMV may have potential utility as a vector for human vaccine development.

Inhibition of HLA-DR assembly, transport, and loading by human cytomegalovirus glycoprotein US3: a novel mechanism for evading major histocompatibility complex class II antigen presentation

Human cytomegalovirus (HCMV) establishes persistent lifelong infections and replicates slowly. To withstand robust immunity, HCMV utilizes numerous immune evasion strategies. The HCMV gene cassette encoding US2 to US11 encodes four homologous glycoproteins, US2, US3, US6, and US11, that inhibit the major histocompatibility complex class I (MHC-I) antigen presentation pathway, probably inhibiting recognition by CD8(+) T lymphocytes. US2 also inhibits the MHC-II antigen presentation pathway, causing degradation of human leukocyte antigen (HLA)-DR-alpha and -DM-alpha and preventing recognition by CD4(+) T cells. We investigated the effects of seven of the US2 to US11 glycoproteins on the MHC-II pathway. Each of the glycoproteins was expressed by using replication-defective adenovirus vectors. In addition to US2, US3 inhibited recognition of antigen by CD4(+) T cells by a novel mechanism. US3 bound to class II alpha/beta complexes in the endoplasmic reticulum (ER), reducing their association with Ii. Class II molecules moved normally from the ER to the Golgi apparatus in US3-expressing cells but were not sorted efficiently to the class II loading compartment. As a consequence, formation of peptide-loaded class II complexes was reduced. We concluded that US3 and US2 can collaborate to inhibit class II-mediated presentation of endogenous HCMV antigens to CD4(+) T cells, allowing virus-infected cells to resist recognition by CD4(+) T cells.

Alphaviral gene transfer in neurobiology

Semliki Forest virus (SFV), Sindbis virus (SIN), and Venezuelan equine encephalitis virus are simple, enveloped plus-strand RNA viruses belonging to the Alphavirus genus of the Togaviridae family. They have been developed into expression vectors that infect a wide host cell range and cause rapid and high-level transgene expression. Their easy and fast generation, classification into biosafety levels 1 and 2, and preferential transduction of neurons in cell and tissue cultures makes them an increasingly used gene transfer system. This review summarizes the alphaviral replication and expression, the replicon system, and its application in neurobiology. Alphaviral vectors can introduce multiple transgenes into host cells, and mutants with low or absent cytotoxicity and increased or decreased transgene expression levels are available. Temperature-dependent mutants permit to control the host cell specificity as well as the on- and offset of gene expression. These features, together with the transduction characteristics revealed in a direct comparison of alphaviral and other viral vectors in hippocampal slice cultures, make SFV and SIN vectors a powerful tool for neurobiological studies.

The amount of immature glial cells in organotypic brain slices determines the susceptibility to murine cytomegalovirus infection

Cytomegalovirus (CMV) is the most common infectious cause of congenital anomalies of the brain and also causes brain damage in immunocompromised individuals. We investigated the effects of murine cytomegalovirus (MCMV) infection on the developing mouse brain in terms of susceptible cells and age-related resistance to MCMV in brain slice cultures. Brain slices from BALB/c mice at different developmental stages were infected with recombinant MCMV in which the lacZ gene was inserted into a late gene. The subventricular zone and cortical marginal region were the sites most susceptible to MCMV infection, and the susceptibility declined with the development of the brain. Immunohistochemical staining showed that the virus-susceptible cells were positive for GFAP, nestin, and Musashi-1, and that most of the infected cells were positive for the proliferative cell nuclear antigen and labeled with bromodeoxyuridine. These results suggest that the susceptible cells in the subventricular zone are immature glial cells, including neural progenitor cells. Immature glial cells proliferated when the brain slices were cultured for a prolonged time and furthermore, they showed themselves to be susceptible to virus infection even under serum-free conditions. These results suggest that the amount of immature glial cells, which include neural progenitor cells, in the developing brain or in the damaged brain with neural proliferation may be closely associated with the susceptibility of the brain to CMV infection in humans.

Enhanced cytomegalovirus infection of developing brain independent of the adaptive immune system

Cytomegalovirus (CMV) has been suggested as the most prevalent infectious agent causing neurological dysfunction in the developing brain; in contrast, CMV infections are rare in the adult brain. One explanation generally given for the developmental susceptibility to the virus is that the developing immune system is too immature to protect the central nervous system from viral infection, but as the immune system develops it can protect the brain. We suggest an alternate view: that developing brain cells are inherently more susceptible to CMV infection, independent of the immune system. We used a recombinant mouse CMV that leads to green fluorescent protein expression in infected cells. Control experiments demonstrated a high correlation between the number of cells detected with the viral GFP reporter gene and with immunocytochemical detection of the virus. After intracerebral inoculation, the number of CMV-infected cells in neonatal brains was many times greater than in mature control or mature immunodepressed SCID mice, and the mortality rate of neonates was substantially greater than SCID or control adults. Parallel experiments with live brain slices inoculated in vitro, done in the absence of the systemic immune system, generated similar data, with immature hippocampus, hypothalamus, cortex, striatum, and cerebellum showing substantially greater numbers of infected cells (100-fold) than found in adult slices in these same regions. Interestingly, in the cerebellar cortex, CMV-infected cells were more prevalent in the postmitotic Purkinje cell layer than in the mitotic granule cell layer, suggesting a selective infection of some cell types not dependent on cell division. Together, these data support the view that CMV has an intrinsic preference for infection of developing brain cells, independent, but not mutually exclusive, of the developmental status of the systemic immune system in controlling CMV infection.

Innate immune responses to cytomegalovirus infection in the developing mouse brain and their evasion by virus-infected neurons

Cytomegalovirus (CMV) is the most frequent infectious cause of developmental brain disorders in humans. Here we show the role of innate immune responses caused by natural killer (NK) cells and nitric oxide (NO) derived from brain macrophages during murine CMV (MCMV) infection of the developing brain. Viral replication in the brain of newborn mice was significantly enhanced by administration of anti-asialo-GM1 antibody, specific for NK cells, or L-N6-(1-imminoethyl)-lysine, a specific inhibitor of NO synthase 2 (NOS2). These results suggest that NK cells and NO contribute to the viral clearance from the brain. At 3 days postinfection (dpi) MCMV early antigen (Ag)-positive cells were immunohistochemically detected in the periventricular area, where most of the positive cells were macrophages. At 7 dpi MCMV-Ag was found not only in cells of the periventricular area but also in neurons of the hippocampus and cortex. At 11 dpi MCMV-Ag disappeared from the periventricular area, but persisted in neurons. In the periventricular area, NK cells and NOS2-positive macrophages were associated with MCMV-Ag-positive cells. In contrast, there were very few NK cells and NOS2-positive macrophages around the MCMV-Ag-positive neurons. In situ hybridization for MCMV DNA demonstrated that positive signals were found mostly in the periventricular cells, and rarely in neurons. These results suggest that the innate immune responses are restricted to the virus-replicating cells, and do not affect MCMV-infected neurons. Therefore, evasion of the innate immune responses by MCMV-infected neurons may be an important factor in supporting the viral persistence in the developing brain.

A recombinant rhesus cytomegalovirus expressing enhanced green fluorescent protein retains the wild-type phenotype and pathogenicity in fetal macaques

To facilitate identification of rhesus cytomegalovirus (RhCMV)-infected cells, a recombinant virus expressing enhanced green fluorescent protein (EGFP), designated RhCMV-EGFP, was constructed. An expression cassette for EGFP under the control of the simian virus 40 (SV40) early promoter was inserted into the intergenic region between unique short 1 (US1) and US2 of the RhCMV genome by homologous recombination. RhCMV-EGFP exhibited comparable growth kinetics to that of wild-type virus in rhesus fibroblast cultures and retained its pathogenicity in monkey fetuses. Typical neurologic syndromes caused by CMV infection were observed in all fetuses experimentally inoculated with RhCMV-EGFP, as evidenced by sonographic and gross examinations. Systemic RhCMV infections were established in all fetuses, as viral antigen was detected in multiple organs and virus was isolated from fetal blood samples. The engineered viral genome was stable following rapid serial passages in vitro and multiple rounds of replication in vivo. Infected cells could be readily distinguished by green fluorescence both in tissue cultures and in the fetuses. In addition, EGFP expression was detected in various cell types that were permissive to RhCMV infection, consistent with a broad tissue tropism of the SV40 promoter. These results demonstrate that RhCMV can be successfully engineered without loss of wild-type replication and pathogenic potential. Further, the spectrum of cortical anomalies and the distribution of infected cells in the brain tissues indicated that RhCMV may have preferentially targeted immature neuronal cells. The pattern of RhCMV infection in the central nervous system may offer an explanation for the severe developmental outcomes associated with congenital human CMV infection early in gestation.

Radial glias and radial fibers: what is the function of radial fibers?

Radial glias have been thought to serve as a scaffold to support and direct neurons in their radial migration during the development of the cerebral cortex. However, a recent series of studies on neocortical development showed that these cells behave as multifunctional cells. These studies have provided strong support for the idea that radial glias function as multipotential progenitor cells that give rise to various cell types, including neurons. Proliferation of the radial glia was thought to result in inheritance of the radial fiber by a postmitotic neuron, and it was considered that one role of the radial fiber was to act as a cable to lift the postmitotic neuron. In this review, we propose another function of the radial fiber in the developing neocortex. This novel function is proposed based on the fact that the characteristics of the radial glias are identical to those of neuroepithelial cells. Before neuron production, the neuroepithelial cells proliferate and increase in number, retaining and reproducing radial fibers attached to the surface of the basement membrane (the pia mater). During neuron production, the radial glias also reproduce radial fibers at the time of each cell division. We believe that the radial fiber functions to receive signals for morphogenesis, proliferation and differentiation from the pia mater and acts as a cable to conduct information transduced from such signals to the cell soma.

Relative neurotropism of a recombinant rhabdovirus expressing a green fluorescent envelope glycoprotein

A new recombinant vesicular stomatitis virus (rVSV) that expresses green fluorescent protein (GFP) on the cytoplasmic domain of the VSV glycoprotein (G protein) was used in the mouse as a model for studying brain infections by a member of the Mononegavirales order that can cause permanent changes in behavior. After nasal administration, virus moved down the olfactory nerve, first to periglomerular cells, then past the mitral cell layer to granule cells, and finally to the subventricular zone. Eight days postinoculation, rVSV was eliminated from the olfactory bulb. Little sign of infection could be found outside the olfactory system, suggesting that anterograde or retrograde axonal transport of rVSV was an unlikely mechanism for movement of rVSV out of the bulb. When administered intracerebrally by microinjection, rVSV spread rapidly within the brain, with strong infection at the site of injection and at some specific periventricular regions of the brain, including the dorsal raphe, locus coeruleus, and midline thalamus; the ventricular system may play a key role in rapid rVSV dispersion within the brain. Thus, the lack of VSV movement out of the olfactory system was not due to the absence of potential for infections in other brain regions. In cultures of both mouse and human central nervous system (CNS) cells, rVSV inoculations resulted in productive infection, expression of the G-GFP fusion protein in the dendritic and somatic plasma membrane, and death of all neurons and glia, as detected by ethidium homodimer nuclear staining. Although considered a neurotropic virus, rVSV also infected heart, skin, and kidney cells in dispersed cultures. rVSV showed a preference for immature neurons in vitro, as shown by enhanced viral infection in developing hippocampal cultures and in the outer granule cell layer in slices of developing cerebellum. Together, these data suggest a relative affinity of rVSV for some neuronal types in the CNS, adding to our understanding of the long-lasting changes in rodent behavior found after transient VSV infection.

Cytomegalovirus infection accelerates inflammation in vascular tissue overexpressing monocyte chemoattractant protein-1

Cardiovascular disease is the leading cause of mortality in the United States. Atherosclerosis is responsible for most of this pathology and is an inflammatory disease with multiple cytokines and adhesion molecules expressed during atherogenesis. Cytomegalovirus (CMV), monocytes, and monocyte chemoattractant protein-1 (MCP-1) have all been implicated in human atherogenesis. A transgenic mouse overexpressing MCP-1 in the myocardium and pulmonary arteries develops myocarditis and pulmonary vascular inflammation. We infected MCP-1 transgenic mice with a sublethal dose of murine cytomegalovirus (MCMV) to look for evidence of accelerated inflammation in vascular tissues overexpressing MCP-1 to determine if MCMV could interact with monocytes and MCP-1 in a manner similar to what may occur in atherogenesis. MCMV infection of MCP-1 transgenic mice caused ascites, myocarditis, and pulmonary artery inflammation, which was not present in mock-infected MCP-1 or MCMV-infected wild-type mice. Inflammatory infiltrates in these tissues consisted of macrophages and T lymphocytes similar to the infiltrates seen in atherosclerosis. Virus presence in inflamed tissues was demonstrated by infecting transgenic mice with MCMV recombinant virus containing the gene sequence for the enhanced green fluorescent protein (EGFP). Human CMV could be involved in atherogenesis in a similar manner by interacting with monocytes and MCP-1 specifically expressed in vascular walls.

Murine cytomegalovirus CC chemokine homolog MCK-2 (m131-129) is a determinant of dissemination that increases inflammation at initial sites of infection

The murine cytomegalovirus CC chemokine homolog MCK-2 (m131-129) is an important determinant of dissemination during primary infection. Reduced peak levels of viremia at day 5 were followed by reduced levels of virus in salivary glands starting at day 7 when mck insertion (RM461) and point (RM4511) mutants were compared to mck-expressing viruses. A dramatic MCK-2-enhanced inflammation occurred at the inoculation site over the first few days of infection, preceding viremia. The data further reinforce the role of MCK-2 as a proinflammatory signal that recruits leukocytes to increase the efficiency of viral dissemination in the host.

Activation of murine cytomegalovirus immediate-early promoter in cerebral ventricular zone and glial progenitor cells in transgenic mice

Cytomegalovirus (CMV) is the most common infectious cause of congenital anomalies of the CNS in humans. We recently reported that the murine cytomegalovirus (MCMV) immediate-early (IE) gene promoter directs astrocyte-specific expression in adult transgenic mice. In the present study, we analyzed the activation of the MCMV IE promoter in developing transgenic mouse brains and compared the activation with that of the Musashi 1 (Msi1) gene, which is expressed in neural progenitor cells, including neural stem cells. During the early phase of neurogenesis, the transgene was expressed predominantly in endothelial cells of the vessels, but not in neuroepithelial cells in which Msi1 was expressed. During later stages of gestation, expression of the transgene was largely restricted to the ventricular zone (VZ) in the CNS, similar to the expression of Msi1. In neurosphere cultures from transgenic embryos in the late phase of neurogenesis, the transgene was expressed in some cells of neurospheres expressing Msi1 and nestin. In neural precursor cells induced to differentiate from stem cells, expression of the transgene was detected in glial progenitor cells, expressing GFAP, nestin, and Msi1, but not in cells expressing MAP2 or MAG. In postnatal development, persistent expression of the transgene was observed in astrocyte lineage cells as was Msi1. These spatiotemporal changes of the MCMV IE promoter activity during development of transgenic mice correlated with susceptible sites in congenital HCMV infection. Moreover, this transgenic mouse model may provide useful model for analysis of the regulation of the switching of neuronal and astrocyte differentiation, and the maintenance of the astrocyte lineage.

Mouse cytomegalovirus in developing brain tissue: analysis of 11 species with GFP-expressing recombinant virus

Cytomegaloviruses (CMVs) are species-specific large double-stranded DNA viruses. Mouse and human CMVs have a similar morphology, similar gene sequence, and exert similar cellular effects, but the replication of the virus outside its primary host species is limited. This may confer upon CMV certain advantages for expression of foreign genes or cellular labels in brain cells of nonhost species. We examined the ability of recombinant mouse (m)CMV expressing green fluorescent protein (GFP) to serve as a vector for transgene expression in developing neurons and glia outside the normal host species. For comparative purposes, 11 species were examined. Mouse CMV reporter gene expression was particularly strong in the developing brain of its normal host species, mouse, where it replicated in cultures and brain slices, leading to cell death. All mammalian species tested (human, rat, gerbil, hamster, mouse) showed reporter gene expression after mCMV infection. High levels of mCMV infection were also found in chicken central nervous system cells in vitro, and a low level of mCMV expression was found after an initial delay in turtle neurons and glia. No mCMV reporter gene expression was found in frog cells or aplysia neurons or glia or in drosophila or fungal cells. Infection of nonmouse neurons by low concentrations of mCMV led to strong expression of GFP in dendrites and axons with normal morphology. Despite the lack of replication, high doses of mCMV induced morphologic changes in neurons and glia from hamster and rat brain slices, leading to cells rounding up, and to the formation of giant cells consisting of an aggregate of many cells fused together into a syncytium. In contrast, in human hippocampal slices, GFP-expressing cells infected with mCMV had a relatively normal appearance 12 days after inoculation. To determine whether a CMV from another species could serve as a vector for gene transfer, a recombinant human CMV-expressing GFP was used for transgene expression in rat brain cells in vitro. Cytomegaloviruses thus have potential as useful vectors for gene transfer and labeling central nervous system cells, with the actions of CMV being dependent on a number of factors.

Neurons in Golgi-stain-like images revealed by GFP-adenovirus infection in vivo

Neurons in the adult brain have a very complex morphology with many processes, including tremendously long axons. Since dendrites and axons play key roles in the input and output of neural information, respectively, the visualization of complete images of these processes is necessary to reveal the mechanism of neural information processing. Here we made a recombinant adenovirus vector which encodes green fluorescent protein (GFP) tagged with a palmitoylation site, a membrane-targeting signal, produced specific antibodies to GFP, and used them as probes for staining the nervous system. In the neocortex, after injection of the recombinant virus and immunoperoxidase staining with the antibodies, many different types of cells were labeled in a Golgi stain-like fashion. Although the number of labeled cells varied depending on the amount of virus injected, the recombinant virus was considered to be infectious to cortical neurons of all cell types without selectivity. In contrast, the viral infection in the cerebellar cortex and superior cervical ganglion showed some selectivity toward the cell type. It is expected that this recombinant virus will be a useful tool for the morphological analysis of neuronal connections, especially the analysis of microcircuitry in the cerebral cortex.

Enhanced green fluorescent protein as a marker for localizing murine cytomegalovirus in acute and latent infection

A recombinant murine cytomegalovirus (mCMV) that expresses enhanced green fluorescent protein (EGFP) under control of the native immediate-early 1/3 promoter was constructed to detect directly sites of viral activity in latent and reactivated infections. The recombinant virus had acute and latent infection characteristics similar to those of wild-type mCMV. Rare green-fluorescing foci were observed in paraffin sections from lungs and spleens infected latently. Positive immunoperoxidase staining for EGFP in sections of the same lung tissues suggests that these cells may be sites of restricted viral gene expression. EGFP was detected easily in tissue explants reactivating from latent infection in vitro. Morphology and adhesion characteristics of fluorescing cells suggest that viral reactivation occurs in tissue macrophages in explant cultures. The observations presented in this study demonstrate the usefulness of EGFP-expressing recombinants as tools for direct tracking of mCMV activity in vivo and in vitro.

Cytomegalovirus infection of the central nervous system stem cells from mouse embryo: a model for developmental brain disorders induced by cytomegalovirus

Cytomegalovirus (CMV) is the most frequent infectious cause of developmental disorders of the central nervous system (CNS) in humans. Infection of the CNS stem cells seems to be primarily responsible for the generation of the brain abnormalities. In this study, we evaluated the infectivity of murine CMV (MCMV) in epidermal growth factor (EGF)-responsive CNS stem cells prepared from fetal mouse brains, and studied the effect of infection on growth and differentiation of the stem cells. The CNS stem cells were permissive for MCMV infection, although MCMV replication was slower than in mouse embryonic fibroblasts. MCMV infection inhibited the growth and DNA replication of the stem cells. A clonogenic assay revealed that MCMV infection suppressed generation of colonies from single stem cells. When uninfected stem cells were induced to differentiate, a decrease in expression of the primitive neuroepidermal marker nestin was observed by immunocytochemistry and flow cytometry, whereas expression of neurofilament and glial fibrillary acidic protein (GFAP) were induced. In virus-infected CNS stem cells, nestin expression was retained, whereas the expression of neurofilament was more severely inhibited than that of GFAP in these cells. Two-color flow cytometry showed that differentiated glial precursor cells were preferentially susceptible to MCMV infection. MCMV-infected and uninfected CNS stem cells were transplanted into the neonatal rat brains. The reduced number of infected stem cells were engulfed into the subventricular zone and expressed GFAP, but did not migrate further, in contrast to the uninfected stem cells. These results suggest that suppression of the growth of the CNS stem cells and inhibition of the neuronal differentiation by CMV infection may be primary causes of disorders of brain development in congenital CMV infection.