Nature of the block in the expression of some early virus genes in cells abortively infected with human cytomegalovirus

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

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

Effects of cytomegalovirus infection in human neural precursor cells depend on their differentiation state

Cytomegalovirus (CMV) is the most common cause of congenital infection in developed countries and a major cause of neurological disability in children. Although CMV can affect multiple organs, the most important sequelae of intrauterine infection are related to lesions of the central nervous system. However, little is known about the pathogenesis and the cellular events responsible for neuronal damage in infants with congenital infection. Some studies have demonstrated that neural precursor cells (NPCs) show the greatest susceptibility to CMV infection in the developing brain. We sought to establish an in vitro model of CMV infection of the developing brain in order to analyze the cellular events associated with invasion by this virus. To this end, we employed two cell lines as a permanent source of NPC, avoiding the continuous use of human fetal tissue, the human SK-N-MC neuroblastoma cell line, and an immortalized cell line of human fetal neural origin, hNS-1. We also investigated the effect of the differentiation stage in relation to the susceptibility of these cell lines by comparing the neuroblastoma cell line with the multipotent cell line hNS-1. We found that the effects of the virus were more severe in the neuroblastoma cell line. Additionally, we induced hNS-1 to differentiate and evaluated the effect of CMV in these differentiated cells. Like SK-N-MC cells, hNS-1-differentiated cells were also susceptible to infection. Viability of differentiated hNS-1 cells decreased after CMV infection in contrast to undifferentiated cells. In addition, differentiated hNS-1 cells showed an extensive cytopathic effect whereas the effect was scarce in undifferentiated cells. We describe some of the effects of CMV in neural stem cells, and our observations suggest that the degree of differentiation is important in the acquisition of susceptibility.

Dominance of virus over host factors in cross-species activation of human cytomegalovirus early gene expression

Human cytomegalovirus (HCMV) exhibits a highly restricted host range. In this study, we sought to examine the relative significance of host and viral factors in activating early gene expression of the HCMV UL54 (DNA polymerase) promoter in murine cells. Appropriate activation of the UL54 promoter at early times is essential for viral DNA replication. To study how the HCMV UL54 promoter is activated in murine cells, a transgenesis system based on yeast artificial chromosomes (YACs) was established for HCMV. A 178-kb YAC, containing a subgenomic fragment of HCMV encompassing the majority of the unique long (UL) region, was constructed by homologous recombination in yeast. This HCMV YAC backbone is defective for viral growth and lacks the major immediate-early (IE) gene region, thus permitting the analysis of essential cis-acting sequences when complemented in trans. To quantitatively measure the level of gene expression, we generated HCMV YACs containing a luciferase reporter gene inserted downstream of either the UL54 promoter or, as a control for late gene expression, the UL86 promoter, which directs expression of the major capsid protein. To determine the early gene activation pathway, point mutations were introduced into the inverted repeat 1 (IR1) element of the UL54 promoter of the HCMV YAC. In the transgenesis experiments, HCMV YACs and derivatives generated in yeast were introduced into NIH 3T3 murine cells by polyethylene glycol-mediated fusion. We found that infection of YAC, but not plasmid, transgenic lines with HCMV was sufficient to fully recapitulate the UL54 expression program at early times of infection, indicating the importance of remote regulatory elements in influencing regulation of the UL54 promoter. Moreover, YACs containing a mutant IR1 in the UL54 promoter led to reduced ( approximately 30-fold) reporter gene expression levels, indicating that HCMV major IE gene activation of the UL54 promoter is fully permissive in murine cells. In comparison with HCMV, infection of YAC transgenic NIH 3T3 lines with murine cytomegalovirus (MCMV) resulted in lower (more than one order of magnitude) efficiency in activating UL54 early gene expression. MCMV is therefore not able to fully activate HCMV early gene expression, indicating the significance of virus over host determinants in the cross-species activation of key early gene promoters. Finally, these studies show that YAC transgenesis can be a useful tool in functional analysis of viral proteins and control of gene expression for large viral genomes.

Detection of the Human Cytomegalovirus 2.0-kb Immediate Early Gene 1 Transcripts in Permissive and Nonpermissive Infections by RNA in situ Hybridization

The immediate early gene 1 (IE1) is the first gene to be expressed following the entry of the human cytomegalovirus (HCMV) into the cell and it does not require prior protein synthesis for its expression. Therefore, the IE1 gene is a potential candidate for the development of probes to detect HCMV in various states of infection. Using strand-specific (32)P- or digoxigenin-labeled riboprobes derived from an exon-specific subgenomic fragment of the HCMV Towne IE1 gene, we performed Northern blot analysis and RNA in situ hybridization on HCMV-infected human (permissive cells) and mouse (nonpermissive cells) fibroblasts and on 10 formalin-fixed paraffin-embedded sections of human tissue. By Northern blot analysis and by in situ hybridization, expression of the 2.0-kb IE1 gene was found in permissive as well as in nonpermissive infections. Specific nuclear and cytoplasmic hybridization was found at 5, 10, 24 and 72 h after infection in human fibroblasts. In comparison, hybridization was first detected at 10 h after infection in mouse fibroblasts. Hybridization with the IE1 probe was detected in cells with and without cytopathic changes in the formalin-fixed paraffin-embedded HCMV-infected human tissues. Hybridization patterns of the IE1 riboprobe were compared to those of the HCMV 2.7-kb major early beta-riboprobe which we have previously described [Am J Pathol 141:1247-1254;1992]. Although both riboprobes hybridize to their respective target sequences in the consecutive tissue sections, the patterns of hybridization are different. On occasion, sections of HCMV-infected human tissue showing no specific hybridization for the 2.7-kb riboprobe will show specific in situ hybridization when using the IE1 riboprobe. Our results suggest that RNA in situ hybridization with a probe directed at the IE1 transcripts is an effective method of detecting early and late stages of both permissive and nonpermissive HCMV infections. Copyright 1997 S. Karger AG, Basel

Cell types involved in replication and distribution of human cytomegalovirus

As the number of patients suffering from severe HCMV infections has steadily increased, there is a growing need to understand the molecular mechanisms by which the virus causes disease. The factors that control infection at one time and the events leading to virus multiplication at another time are only beginning to be understood. The interaction of HCMV with different host cells is one key for elucidating these processes. Through modern techniques, much has been learned about the biology of HCMV infections in culture systems. In addition to endothelial cells, epithelial cells, and smooth muscle cells, fibroblasts are one cell population preferentially infected in solid tissues in vivo. From these sites of multiplication, the virus may be carried by peripheral monocytes and circulating endothelial cells to reach distant sites of the body. This would explain the multiorgan involvement in acute HCMV infection and the modes of viral transmission. From what has been learned mainly from human fibroblast culture systems, future studies will focus on how HCMV regulates the expression of its putative 200 genes in different host cells at different stages of cell differentiation and activation to result in viral latency and pathogenesis.

Anti-epithelial (anti-A549) antibodies: their nature, specificity and relevance to transplantation

Several studies have addressed the possible importance of anti-epithelial cell antibodies in kidney transplantation using the A549 cell line as an in vitro model. In this paper we report our results using for the first time an enzyme-linked immunosorbent assay (ELISA) to detect the anti-A549 cell antibodies. Sera from 129 kidney transplant patients were tested for IgM anti-epithelial cell antibodies directed against the A549 cell line prior to transplantation; only three sera were positive (2.3%). 101 of these patients were then followed-up post-transplantation; sera were collected routinely at 2, 6 and 12 weeks and at the time of rejection episodes. All samples were also tested for cytomegalovirus (CMV) IgM antibodies. Sixteen patients developed anti-A549 IgM antibodies, and there was no correlation with acute graft rejection. Anti-epithelial antibodies showed no binding to sections of normal kidney or biopsies of rejected kidneys. Eleven patients were positive for anti-CMV IgM antibodies. In nine cases both IgM anti-A549 and IgM anti-CMV antibodies were found, which was a highly significant association (p < 0.001). Analysis of A549 cellular proteins by immunoblotting gave evidence for the presence of CMV polypeptides in the cell lysate. Electron-microscopic examination of A549 cell preparations revealed intracellular particles which were compatible in size with CMV. Polymerase chain reaction analysis confirmed the presence of a specific CMV DNA sequence in A549 cells of several batches from different sources. Our data strongly suggest that the A549 cell line used in several published reports is infected with CMV and that in the majority of cases the anti-A549 'anti-epithelial' antibodies found in renal transplant patients are anti-CMV antibodies.

Viral and host cellular transcription in Autographa californica nuclear polyhedrosis virus-infected gypsy moth cell lines

Infection of two gypsy moth cell lines (IPLB-Ld652Y and IPLB-LdFB) by the Autographa californica multiple-enveloped nuclear polyhedrosis virus (AcMNPV) is characterized by extremely attenuated viral protein synthesis followed by a total arrest of all viral and cellular protein production. In this study, AcMNPV- and host cell-specific transcription were examined. Overall levels of viral RNAs in infected gypsy moth cells were, at most measured times, comparable to RNA levels from an infected cell line (TN-368) permissive for AcMNPV replication. Northern blot (RNA) analyses using viral and host gene-specific probes revealed predominantly normal-length virus- and cell-specific transcripts postinfection. Transport of viral RNAs from the nucleus to the cytoplasm and transcript stability in infected gypsy moth cells also appeared normal compared with similar parameters for AcMNPV-infected TN-368 cells. Host cellular and viral mRNAs extracted from gypsy moth and TN-368 cells at various times postinfection and translated in vitro yielded similar spectra of host and viral proteins. Treatment of infected gypsy moth cells with the DNA synthesis inhibitor aphidicolin eliminated the total protein synthesis shutoff in infected IPLB-LdFB cells but had no effect on protein synthesis inhibition in infected IPLB-Ld652Y cells. The apparent selective block in the translation of viral transcripts early in infection and the absence of normal translation or transcription of host cellular genes at later times is discussed.

Human cytomegalovirus UL36-38 and US3 immediate-early genes: temporally regulated expression of nuclear, cytoplasmic, and polysome-associated transcripts during infection

During permissive in vitro infection, the human cytomegalovirus (HCMV) UL36-38 and US3 immediate-early (IE) regions give rise to multiple distinct species of RNA in a temporally regulated manner. We have compared the temporally regulated expression of the UL36-38 and US3 regions with that of the well-characterized major IE (MIE) region. Northern (RNA) blot hybridizations with antisense RNA probes were used to examine RNA isolated from infected cells at IE, early, and late times after infection and from cells infected in the presence of anisomycin (used to block de novo viral protein synthesis) or in the presence of phosphonoformate (used to block HCMV DNA synthesis). Different US3 region transcripts were expressed in the cytoplasm during the IE and late phases of infection, with kinetics similar to those of the MIE region. In contrast, various cytoplasmic transcripts from the UL36-38 region were expressed during each of the IE, early, and late phases of infection, including some expressed from IE through late times. The levels of steady-state RNA from the US3 and MIE regions were increased dramatically by infection in the presence of anisomycin, predominantly because of an increase in multiply spliced transcripts. Two of the three UL36-38 IE transcripts were largely unaffected by anisomycin and were expressed abundantly throughout infection, but a third, multiply spliced UL36-38 IE transcript was abundant only during infection in the presence of anisomycin. Nuclear, cytoplasmic, and polysome-associated transcripts from the three IE regions were not significantly different qualitatively or quantitatively. These results suggest that posttranscriptional controls at the levels of nuclear retention or polysome exclusion of transcripts are not operative for the IE region genes. Overall, these results indicate common features of expression of US3, MIE, and UL36-38, in addition to distinctive expression of the UL36-38 region during all temporal phases of expression.

Differential detection of viral DNA and RNA in situ in cells infected with human cytomegalovirus

We have analyzed the ability to use in situ cytohybridization to distinguish between human cytomegalovirus (HCMV) DNA and RNA in human cells infected in vitro. Two different viral-specific probes were used, one for an abundantly expressed late gene, and one which includes at least two genes coding for immediate early (IE) proteins. In productively infected cells, hybridization of the late gene probe extended over both the nucleus and cytoplasm and was RNase sensitive, whereas hybridization of the IE probe was restricted to the nucleus and was DNase-sensitive. In nonproductively infected cells hybridization of the IE probe was localized to the cytoplasm and was RNase-sensitive. The specific nuclease sensitivities indicate that a cytoplasmic hybridization pattern correlates with detection of viral RNA sequences, whereas a nuclear pattern represents detection of viral DNA. These results demonstrate that in situ cytohybridization can potentially be used to determine the extent of HCMV infection in a particular tissue or cell type by distinguishing between transcription and replication of specific viral genes.

Further characterization of the murine cytomegalovirus induced early proteins in permissive and nonpermissive cells

Some of the properties of the immediate-early (IE) and early proteins induced by the murine cytomegalovirus (Smith strain) were examined in permissively infected 3 T 3-L 1 cells, and in non-permissively infected J 774A.1 (mouse macrophage) and human fibroblast cells, in order to determine differences that could account for the restriction in virus replication in the latter two cell lines. The different virus induced proteins had distinctive partitioning characteristics between nuclear and cytoplasmic fractions. The 96 K major IE protein had an exclusively nuclear association, as did the most abundant early proteins of 39 K and 36 K. The other viral proteins however were evenly distributed between nucleus and cytoplasm. In general these patterns were also seen in the infected non-permissive cells. Several proteins showed more than one charge isomer on two-dimension gels, and in addition five IE proteins and two early proteins were phosphorylated. Only two differences between the permissive and nonpermissive infections were observed; the IE proteins of 100 K and 89 K when synthesized in the human cells had a stronger affinity for the nuclear fraction; also a phosphorylated form of the 30 K IE protein was not detected in MCMV infected J 774 A.1 cells.

Constitutive and retinoic acid-inducible expression of cytomegalovirus immediate-early genes in human teratocarcinoma cells

Human teratocarcinoma stem cells are nonpermissive for human cytomegalovirus (HCMV) but become permissive after being induced to differentiate by treatment with retinoic acid. We show that in uninduced teratocarcinoma stem cells, and also in transformed human 293 cells expressing adenovirus E1a gene products, the HCMV immediate-early (IE) 68,000-molecular-weight polypeptide (68K polypeptide) was not expressed, and consequently input viral genomes were not replicated. However, after differentiation of the teratocarcinoma cells, synthesis of the HCMV IE 68K polypeptide was induced, and viral DNA replication occurred. In contrast to our observations for HCMV, simian cytomegalovirus (SCMV) displayed constitutive expression of its analogous IE 94K polypeptide, and the input SCMV genomes were replicated in both uninduced stem cells and 293 cells. Since little, if any, HCMV IE RNA was detectable in human teratocarcinoma or 293 cells after infection under IE conditions, we suggest that a direct transcriptional block to permissivity occurs in these cells. The presence of tandemly repeated sequences which bind nuclear factor I protein in the promoter for the SCMV IE 94K polypeptide gene but not in the promoter for the HCMV IE 68K polypeptide gene may allow the expression of the simian but not of the human IE gene product in transformed cells.

Rabbit kidney cells abortively infected with human cytomegalovirus are arrested in mitotic phase

In rabbit kidney epithelial cells (RK13) abortively infected with human cytomegalovirus (HCMV), DNA synthesis at 1 or 2 days post-infection was enhanced 4 to 5 fold, compared to mock-infected cells. DNA analysis by isopycnic centrifugation revealed that the DNA newly synthesized in the virus infected RK13 cells was of cellular origin. HCMV infection also caused a marked increase in the mitotic activity of RK13 cells. When semi-confluent RK13 cells were infected more than 20 per cent of cells demonstrated mitosis at 72 hours post-infection although the rate of cell growth was considerably reduced compared to that of uninfected cells. The most frequent chromosomal change observed was fragmentation although other aberrations, gap, break, deletion etc. occurred also. Two immediate-early viral polypeptides with apparent molecular weights 72,000 (72K) and 76,000 (76K) daltons were produced in both RK13 cells and human embryonic lung cells (HEL) by 3 hours post-infection. Synthesis of the 76K polypeptide was greater than that of the 72K polypeptide in non-permissive RK13 cells whereas the reverse occurred in permissive HEL cells. Furthermore, of three early polypeptides which were expressed in productively infected HEL cells two, 88K and 80K, were not detected in abortively infected RK13 cells. These results suggest that the arrest in mitosis of the abortively infected RK13 cells and the subsequent chromosomal changes are associated with the altered expression of immediate-early or early virus functions in these cells.

Detection of human cytomegalovirus in peripheral blood lymphocytes in a natural infection

In situ hybridization was used to detect human cytomegalovirus (HCMV) in the peripheral blood mononuclear cells of some naturally infected (seropositive) individuals. A subpopulation of cells hybridized specifically to a portion of the HCMV genome that is heavily transcribed during the immediate-early period of infection. The hybridization signal was markedly reduced by base hydrolysis and ribonuclease, and therefore the probe appears to be detecting viral RNA. A fluorescence-activated cell sorter was used to select lymphocytes bearing the OKT4 and OKT8 markers. Hybridization with the HCMV probe revealed a higher proportion of positive cells in the OKT4 than in the OKT8 subset. This observation specifically identifies lymphocytes as a cell population involved in natural HCMV infection and suggests that lymphocytes may be a reservoir for maintaining infection and may also serve as a vehicle for its spread by blood transfusion.

Effect of dimethyl sulfoxide on interaction of human cytomegalovirus with host cell: conversion of a nonproductive state of cell to a productive state for virus replication

The effect of dimethyl sulfoxide (DMSO) on the interaction of human cytomegalovirus (HCMV) with host cell was studied. Confluent state of a human rhabdomyosarcoma cell line (A204) showed a much lower susceptibility to HCMV infection when compared to that in subconfluent actively growing cell cultures. Treatment of confluent cultures with DMSO, however, converted many nonproductive cells in these cultures to a productive state for virus replication. Infectious center assay revealed that approximately 100-fold more cells in the compound-treated cultures are able to produce infectious virus. The amount of infectious virus produced in DMSO-treated confluent cultures was enhanced by approximately 10,000-fold over production in untreated cultures and recovered to the level of that produced in subconfluent cultures productive state for virus replication. This cell physiology-dependent inhibition of HCMV replication and enhancement of virus growth by DMSO did not occur with herpes simplex virus type 2. Immunofluorescence staining, gel electrophoresis, and DNA analyses indicate that block of HCMV replication in confluent cultures probably occurs at the level of early transcription or translation of the viral genome. In contrast, in DMSO-treated confluent cultures appreciable amounts of HCMV DNA polymerase (an early virus function), viral DNA, and late antigens were synthesized. Pretreatment of confluent cultures with DMSO enabled the cells to support HCMV replication. In addition, the most effective enhancement by DMSO was found in cultures that had been treated with the compound up to 5 hr after infection. These results suggest that the enhancing effect by DMSO is primarily expressed through some host cellular function(s) and the early stages in virus growth cycle are most likely under control by DMSO action.

Abortive infection with human cytomegalovirus induces an alteration of growth pattern: morphological changes with cytocidal effect in rabbit kidney epithelial cells. Brief report

Rabbit kidney epithelial cells (RK13) exhibited a cytopathic effect (CPE) characterized by cell rounding after infection with human cytomegalovirus (HCMV). Although HCMV-specific immediate early and early antigens were detected by indirect immunofluorescence techniques, neither late antigens nor infectious progeny virus could be observed in virus-infected RK13 cells. HCMV-infected RK13 cells showed a prolonged doubling time and a decreased saturation density in cell growth compared to uninfected control cells. Moreover, colony forming ability (CFA) of virus-infected cells decreased by approximately 70 per cent compared to that of uninfected cells during the first 24 hours after infection. These results indicate that an abortive infection of RK13 cells with HCMV induces an alteration of growth pattern including morphological changes with cytocidal effect.

Analysis of the major transcripts encoded by the long repeat of human cytomegalovirus strain AD169

In this report, we describe the size and kinetics of appearance of RNAs from the long repeat of human cytomegalovirus. The most abundant RNA from this region was a 2.7-kilobase (kb) species that was detected throughout the infection and was most abundant at 27 and 72 h after infection. The 2.7-kb RNA was the only major species detected with a probe that included the terminus of the long repeat and the heterogeneous L-S junction region. Other transcripts were detected with probes from the internal portion of the long repeat, including an immediate-early RNA of 1.3 kb, early and late RNAs of 1.2 kb, and minor late transcripts of 4.4, 3.6, 3.3, and 1.8 kb. S1 nuclease and exonuclease VII protection analyses of RNA from immediate-early, early, midpoint, and late times in the infection indicated that the major 2.7-kb RNA was not spliced and that the RNA mapped within the long repeat, 1.6 kb from the heterogeneous region. No evidence for temporally regulated changes in transcription initiation, splicing, or choice of 3' end of this RNA was observed. Nuclease protection analysis also demonstrated that the second most abundant late RNA from this region, the 1.2-kb species, was not spliced and had the same polarity as the 2.7-kb RNA. The 1.2-kb also mapped entirely within the long repeat, with its 3' terminus 1.7 kb upstream from the 5' terminus of the 2.7-kb RNA.

Cytomegalovirus infects human lymphocytes and monocytes: virus expression is restricted to immediate-early gene products

In this investigation, we studied the ability of human cytomegalovirus to infect peripheral blood mononuclear cells. With monoclonal antibody technology, we demonstrated that cytomegalovirus could infect human lymphocytes of T- and B-cell lineage, natural killer cells, and monocytes. Furthermore, virus expression was limited to the synthesis of immediate-early cytomegalovirus polypeptides. These peripheral blood mononuclear cells did not produce infectious virus, nor were mature virions visualized by electron microscopy. This abortive infection of mononuclear cells was most convincingly shown with stocks of cytomegalovirus that had been recently isolated from infected patients and passaged minimally in fibroblasts. This argues for an increased lymphotropic effect of some isolates of cytomegalovirus, compared to strains of virus that are extensively adapted to growth in fibroblasts. Furthermore, immunocompetent cells that were shown to be abortively infected with cytomegalovirus lost selected differentiated functions.