Expression of the human cytomegalovirus genome in mouse cells and in human-mouse heterokaryons

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.

Molecular biology and immunology of cytomegalovirus

The application of modern biochemical techniques has led to a rapid improvement in our knowledge of the molecular biology of CMV. Several coding regions of the DNA genome have been identified with certainty and major virus-coded proteins have been given provisional names. The cascade expression of the CMV genome has been shown to be controlled by mechanisms similar to those found in other herpes viruses, together with novel post-transcriptional controls which remain to be defined. The control of CMV replication by the host involves both non-specific and specific defence mechanisms. The induction of natural killer cells and interferon early after CMV infection appears to be the most important aspects of the non-specific host defence against the virus. The cell-mediated immune response, in particular the generation of Tc cells against CMV early antigens, is probably the most important facet of the specific immune defence against CMV. When intact these defence mechanisms appear to be efficient in restricting viral replication; however, when such immunity is compromised, the balance rapidly swings in favour of the virus. As our understanding of the interaction between the host and the virus increases, it may be possible to redress the balance in such cases in favour of the host.

Human cytomegalovirus: demonstration of permissive epithelial cells and nonpermissive fibroblastic cells in a survey of human cell lines

To more clearly define the characteristics which render a cell permissive for human cytomegalovirus (HCMV), we screened a panel of human cell lines differing in morphology, ploidy, and extent of differentiation for the ability to sustain productive HCMV replication. Cells were exposed to HCMV at 5 to 20 PFU per cell and examined at 4 to 14 days postinfection to detect the production of infectious virus by a plaque assay and the assembly of progeny virions by electron microscopy. By these criteria, high-titer HCMV replication (10(6) to 10(7) PFU/ml) occurred in a well-differentiated, diploid, epithelial cell line, HCMC, which had been derived from normal human colonic mucosa. In contrast, all aneuploid human cell types proved to be nonpermissive, including a fibroblastic cell line designated HT-144. These results indicate that HCMV replication in cultures is not strictly limited to fibroblasts and conversely that not all human fibroblastic cells are permissive for HCMV. Nonpermissive cell types were further investigated by attempts to chemically induce HCMV replication. Treatment of nonpermissive cell types with 25 to 500 micrograms of 5-iodo-2'-deoxyuridine per ml prior to infection did not convert them to the permissive state. The implications of these findings for the possible mechanisms maintaining the nonpermissive state are discussed.

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.

Characterization of guinea pig cytomegalovirus DNA

The genome of guinea pig cytomegalovirus (GPCMV) was analyzed and compared with that of human cytomegalovirus (HCMV). GPCMV and HCMV DNAs were isolated from virions and further purified by CsCl centrifugation. Purified GPCMV DNA sedimented as a single peak in a neutral sucrose gradient and was infectious when transfected into guinea pig embryo fibroblast cells. The cytopathology was characteristic of that seen after infection with GPCMV. Virus DNA purified from virions isolated from infected GPEF or 104C1 cells had a CsCl buoyant density of 1.713 g/cm3, which corresponds to a guanine plus cytosine content of 54.1%. The CsCl buoyant density of GPCMV DNA was slightly less than that of HCMV DNA (1.716 g/cm3), but sufficiently different so that the two virus DNA peaks did not coincide. GPCMV DNA cosedimented with T4 DNA in a neutral sucrose gradient. Restriction endonuclease cleavage of GPCMV or HCMV DNAs with HindIII, XbaI, or EcoRI yielded fragments easily separable by agarose gel electrophoresis and ranging from 1.0 X 10(6) to 25.8 X 10(6) daltons. The number, size, and molarity of GPCMV DNA fragments generated by restriction enzymes were determined. Hybridization of restriction endonuclease-cleaved GPCMV DNA with radioactively labeled HCMV DNA and, conversely, hybridization of restriction endonuclease-cleaved HCMV DNA with radioactively labeled GPCMV DNA indicated sequence homology between the two virus DNAs.

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

Certain functions normally expressed during the early phase of productive human cytomegalovirus (HCMV) infection in human embryonic lung (HEL) cells are not expressed in abortively infected rabbit kidney (RK) cells. To determine the stage at which infection is blocked in RK cells, HCMV transcripts synthesized by these abortively infected cultures were compared to those synthesized during early stages of infection in productively infected HEL cells. The rate of accumulation of HCMV-specific RNA was approximately six- to eightfold lower in RK than in HEL cells during the first 24 hr postinfection. Although the rate of RNA accumulation did not decrease thereafter in RK cells, it did increase significantly in HEL cells and was dependent on the replication of virus DNA. Transcripts obtained from HEL cells 24 hr postinfection (either in the absence or presence of phosphonoacetic acid) and from abortively infected RK cells were analyzed by the Southern technique. Most of the "early" transcripts which accumulated in infected permissive cells were also present in the infected nonpermissive cells at 24 hr postinfection. However, some of the early transcripts (complementary to approximately 7% of the genome) were significantly underrepresented in RK cells. Furthermore, transcripts originating from one region of the genome, which were abundantly represented as cytoplasmic RNA both in productively and nonproductively infected cells, and which were polysome-associated in HEL cells, were found associated with polysomes in RK cells in very low amounts only. Northern blot analysis of the total and polysomal RNA from infected HEL cells and RK cells confirmed these findings. These results show that differences between productively infected HEL and nonproductively infected RK cells exist at the level of accumulation of some early transcripts, as well as at the level of association of some of these transcripts with polysomes.

Post-transcriptional control of human cytomegalovirus gene expression

During the immediate-early, early, and late phases of human cytomegalovirus infection in human fibroblasts, transcripts accumulated, respectively, from approximately 20, 75, and 90% of the sequences on the genome. Not all of the sequences which accumulated during the immediate-early and early phases were represented on polysomes, however. Four transcripts synthesized in cycloheximide-treated cells were studied in detail. A 2.2-kb transcript (0.713-0.733 map units) represented 95% of the polysome-associated RNA in cycloheximide-treated cells and was the first to be detected on polysomes at 2 hr postinfection in untreated cells. A second, less abundant, transcript of 5.2 kb (0.670-0.733 map units) was also found on polysomes in cycloheximide-treated cells, and preliminary evidence suggested that this transcript may be spliced during processing. A 3.25-kb transcript (0.190-0.217 map units) was identified also as a minor polysome-associated species of RNA. One transcript of 4.8 kb (0.630-0.670) remained associated with the nucleus and was not processed into mRNA in cycloheximide-treated cells. Differential stability between the various transcripts was observed, the 2.2-kb transcript being the most stable. The results showed that in human cytomegalovirus-infected cells controls exist at the level of transcript accumulation, transport into the cytoplasm, preferential association with polysomes, and relative stability of RNAs.

Characterization of human cells persistently infected with cytomegalovirus and exposed to a chemical carcinogen

A non-productive, persistent human cytomegalovirus (CMV) infection was developed in human embryo lung (LU) cells by initially using supraoptimal temperatures to restrict the replication of a recent CMB isolate (76-24). Although CMV replication was observed in some cells within a few weeks of shifting the cultures to 37 degrees C and subculturing, CMV cytopathic effects and infectious CMV were not detected with further subculturing during the next 5 weeks. CMV-specific nuclear antigens were, however, observed in most cells (designated LU-76-24) for 38 additional culture passages made during the following 12 months. Although the LU-76-24 cells were morphologically distinct from LU cell cultures carried in parallel as controls, the LU-76-24 were contact-inhibited in spite of a somewhat disoriented growth pattern. Treatment of LU-76-24 cells with 0.05 microgram/ml of the chemical carcinogen 4-nitroquinoline 1 oxide (NQO) induced a lytic CMV infection from which a clone of cells (designated LUC-NQO) was isolated. No effect was seen when LU cells were treated with the same dose of NQO. LUC-NQO cells were morphologically altered from LU-76-24 cells and had lost contact inhibition. Even though CMV-specific nuclear antigens were observed in LUC-NQO cells, infectious CMV was not detected in lysates of these cells, nor was CMV rescued by co-cultivation of these cells with susceptible cells or by induction with 5-iodo-2'-deoxyuridine. LUC-NQO, but not LU or LU-76-24 cells, plated with good efficiency in semi-solid medium. The results indicate that long-term, non-productive, persistent CMV infections can be established in vitro and suggest that such cells may be more susceptible than non-infected cells to the action of chemical carcinogens.

Lack of association of cytomegalovirus with adenocarcinoma of the colon

Tumor specimens from patients with adenocarcinoma of the colon or rectum were examined for the presence of cytomegalovirus (CMV), and specimens of normal mucosa from the same patients were studied in parallel. Frozen sections of 14 specimens were made and the presence of CMV mRNA assayed by in situ hybridisation using 3H-labelled CMV-DNA as a probe. Nine of these sections were also tested for cytomegalovirus antigens by immunofluorescence. No viral nucleic acids or antigens were detected. In addition to these direct approaches, the specimens were disaggregated and 19 were successfully cultured in various media over several months without yielding virus on any occasion. Areas containing epithelial cells were found in some cultures, foci of bipolar cells in others, while, in several, fibroblastic cells predominated. To ensure that any virus-containing cells were not lost by this method, the disaggregated tumour and normal intestinal cells were directly co-cultivated and also fused with human embryo lung cells, which are permissive for cytomegalovirus replication. The resulting cultures were examined over two to three months for the presence of cytomegalovirus, and in no instance was virus found, despite attempted induction by iododeoxyuridine. Two fusion cultures became contaminated with cytomegalovirus, strain AD-169, which was being handled in the laboratory at the same time. The strain was identified by the pattern of viral DNA fragments produced by restriction endonuclease cleavage. Thus the accidental passage of virus in the heterokaryons did not alter its DNA and would further indicate the absence of any cytomegalovirus genomes in the adenocarcinoma cells.

Vesicular stomatitis virus pseudotypes produced by cells abortively infected or transformed by human cytomegalovirus

Pseudotypes of vesicular stomatitis virus (VSV) and human cytomegalovirus (HCMV) were produced by normal hamster cells abortively infected with HCMV and superinfected by VSV at a certain stage of abortive HCMV infection. Hamster cells transformed in vitro by HCMV (87-TRH-5 and CX-90-3B cells) also produce VSV (HCMV) pseudotypes after infection of the cells by VSV, but the same cells after passage in vivo (TSC-1, TSC-2 cells) do not.

Patterns of transcription of human cytomegalovirus in permissively infected cells

The rate of accumulation of cytomegalovirus transcripts in permissively infected human embryonic lung (HEL) cells was analyzed at various times after infection by hybridization of infected cell RNA to undigested or restriction endonuclease-digested cytomegalovirus DNA fixed to nitrocellulose filters. Differences in patterns of transcript accumulation were determined by measuring the abundance levels of RNA which hybridized to different HindIII-, XbaI-, or EcoRI-generated fragments of cytomegalovirus DNA. The results showed that a small but significant amount of cytomegalovirus RNA was detectable within the first 3 h after infection and that the rate of accumulation of these transcripts was static during the first 24 h, but increased thereafter. In general, the viral transcripts accumulating in infected cells could be divided into three classes. Immediate-early RNA (synthesized in the absence of protein synthesis in infected cells) hybridizes predominantly to a very restricted part of the genome and can be identified during the first 2 to 4 h postinfection. Early RNA (synthesized up to about 24 h after infection) originates from most regions of the genome but is characterized by the presence of transcripts which hybridize in great abundance to certain fragments. Late RNA (synthesized after 24 h, i.e., after the onset of viral DNA synthesis) hybridizes in approximately equal abundance to most regions of the viral genome. These results showed that a block in the transition from immediate-early to early RNA did not account for the extended period of time that elapses between the time of infection and the initiation of viral DNA synthesis. Interestingly, despite rapid adsorption and penetration and a static level of accumulation of transcripts in the cultures during the first 24 h, the number of cells that synthesized detectable amounts of viral antigens increased steadily during this time.

Human cytomegalovirus induced changes in rabbit cells. Brief report

After infection with human cytomegalovirus, rabbit lung fibroblasts showed, during the four week period of the experiment, cytopathic changes and virus-specific antigens demonstrable by fluorescent antibody. Infectious virus could be recovered from the infected cells by co-cultivation with human lung fibroblasts.