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

Dimethyl Sulfoxide Enhances Kaposi’s Sarcoma-Associated Herpesvirus Production During Lytic Replication

Kaposi’s sarcoma-associated herpesvirus (KSHV) is an etiologic agent of Kaposi’s sarcoma, primary effusion lymphoma, and multicentric Castleman disease. In studies of KSHV, efficient virus production and isolation are essential. Reactivation of KSHV can be initiated by treating latently infected cells with chemicals, such as 12-O-tetradecanoyl-phorbol-13-acetate and sodium butyrate. These chemicals have been used as tools to induce lytic replication and viral production in KSHV-producing cell lines. Dimethyl sulfoxide (DMSO) is an organosulfur compound that is frequently used as an aprotic solvent similar to water. In experiments exploring signaling pathways in KSHV-infected cells, DMSO treatment alone as a vehicle affected the lytic gene expression of KSHV. However, to the best of our knowledge, the effects of DMSO on KSHV-producing cells have not yet been reported. Therefore, in this study, we investigated whether DMSO could be used as a reagent to enhance viral production during lytic replication in KSHV-producing cells and assessed the underlying mechanisms. The effects of DMSO on KSHV production were analyzed in iSLK BAC16 cells, which have been widely used for recombinant KSHV production. We found that the production of KSHV virions was significantly increased by treatment with DMSO during the induction of lytic replication. Mechanistically, lytic genes of KSHV were enhanced by DMSO treatment, which was correlated with virion production. Additionally, DMSO induced the phosphorylation of JNK during lytic replication, and inhibition of JNK abolished the effects of DMSO on lytic replication and virion production. Our findings showed that additional treatment with DMSO during the induction of lytic replication significantly improved the yield of KSHV production.

In vitro biology of fibropapilloma-associated turtle herpesvirus and host cells in Hawaiian green turtles (Chelonia mydas)

Fibropapillomatosis (FP) of green turtles has a global distribution and causes debilitating tumours of the skin and internal organs in several species of marine turtles. FP is associated with a presently non-cultivable alphaherpesvirus Chelonid fibropapilloma-associated herpesvirus (CFPHV). Our aims were to employ quantitative PCR targeted to pol DNA of CFPHV to determine (i) if DNA sequesters by tumour size and/or cell type, (ii) whether subculturing of cells is a viable strategy for isolating CFPHV and (iii) whether CFPHV can be induced to a lytic growth cycle in vitro using chemical modulators of replication (CMRs), temperature variation or co-cultivation. Additional objectives included determining whether non-tumour and tumour cells behave differently in vitro and confirming the phenotype of cultured cells using cell-type-specific antigens. CFPHV pol DNA was preferentially concentrated in dermal fibroblasts of skin tumours and the amount of viral DNA per cell was independent of tumour size. Copy number of CFPHV pol DNA per cell rapidly decreased with cell doubling of tumour-derived fibroblasts in culture. Attempts to induce viral replication in known CFPHV-DNA-positive cells using temperature or CMR failed. No significant differences were seen in in vitro morphology or growth characteristics of fibroblasts from tumour cells and paired normal skin, nor from CFPHV pol-DNA-positive intestinal tumour cells. Tumour cells were confirmed as fibroblasts or keratinocytes by positive staining with anti-vimentin and anti-pancytokeratin antibodies, respectively. CFPHV continues to be refractory to in vitro cultivation.

Replication of human cytomegalovirus in a rhabdomyosarcoma cell line depends on the state of differentiation of the cells

The replication of human cytomegalovirus (HCMV) was investigated in a new human rhabdomyosarcoma cell line (KFR) with morphological and biochemical characteristics of fetal striated muscle precursors (rhabdomyoblasts). KFR cells exhibited the unique property for spontaneous morphological transformation from a poorly-differentiated state into well-differentiated (myotube-like) rhabdomyoblasts. The poorly-differentiated rhabdomyoblasts promoted both complete viral gene expression and the production of infectious virus. In contrast, in well-differentiated rhabdomyoblasts HCMV infection was abortive. The results showed that replication of HCMV in this human rhabdomyosarcoma cell line depended on the state of cellular differentiation.

Physical and chemical methods for enhancing rapid detection of viruses and other agents

Viral replication events can be enhanced by physical, chemical, or heat treatment of cells. The centrifugation of cells can stimulate them to proliferate, reduce their generation times, and activate gene expression. Human endothelial cells can be activated to release cyclo-oxygenase metabolites after rocking for 5 min, and mechanical stress can stimulate endothelial cells to proliferate. Centrifugation of virus-infected cultures can increase cytopathic effects (CPE), enhance the number of infected cells, increase viral yields, and reduce viral detection times and may increase viral isolation rates. The rolling of virus-infected cells also has an effect similar to that of centrifugation. The continuous rolling of virus-infected cultures at < or = 2.0 rpm can enhance enterovirus, rhinovirus, reovirus, rotavirus, paramyxovirus, herpesvirus, and vaccinia virus CPE or yields or both. For some viruses, the continuous rolling of infected cell cultures at 96 rpm (1.9 x g) is superior to rolling at 2.0 rpm for viral replication or CPE production. In addition to centrifugation and rolling, the treatment of cells with chemicals or heat can also enhance viral yields or CPE. For example, the treatment of virus-infected cells with dimethyl sulfoxide can enhance viral transformation, increase plaque numbers and plaque size, increase the number of cells producing antigens, and increase viral yields. The infectivity of fowl plague virus is increased by 80-fold when 4% dimethyl sulfoxide is added to culture medium immediately after infection. The heat shocking of virus-infected cells also has been shown to have a stimulatory effect on the replication events of cytomegalovirus, Epstein-Barr virus, and human immunodeficiency virus. The effects of motion, chemicals, or heat treatments on viral replication are not well understood. These treatments apparently activate cells to make them more permissive to viral infection and viral replication. Perhaps heat shock proteins or stress proteins are a common factor for this enhancement phenomenon. The utility of these treatments alone or in combination with other methods for enhancing viral isolation and replication in a diagnostic setting needs further investigation.

Comparison of MRC-5 and continuous cell lines for detection of cytomegalovirus in centrifugation cultures

Continuous cell lines were assessed for use for rapid human cytomegalovirus (HCMV) detection procedures combining tissue culture, centrifugation, and immediate early antigen (IEA) immunostaining. Human cells (MRC-5 embryonic fibroblasts, U-373MG astrocytoma cells, differentiated teratocarcinoma (Tera-2) cells), murine cells (BALB/c-3T3 and Y-1 cells), BHK21 hamster cells, and mink lung (ML) cells were first inoculated with HCMV laboratory strain. IEA synthesizing cells were detected by immunoperoxidase assay using a monoclonal antibody. ML cells and differentiated Tera-2 cells exhibited more positive cells than MRC-5 cells. BHK21, and MRC-5 cells were equivalent in sensitivity whereas U-373MG, BALB/c-3T3, and Y-1 cells had only reduced IEA positive cells. When 63 urine specimens were inoculated onto MRC-5, ML and differentiated Tera-2 cells, 20 (31.7%) were positive in MRC-5 cells versus 18 (28.5%) in ML or Tera-2 cells. Moreover, greater numbers of infected cells were detected in MRC-5 cells than in these two cell lines. MRC-5 cells were superior for detection of HCMV in clinical samples by centrifugation cultures.

Sodium butyrate-inducible replication of human cytomegalovirus in a human epithelial cell line

Replication of human cytomegalovirus (HCMV) in a human epithelial thyroid papillary carcinoma cell line (TPC-1) was restricted. However, pretreatment of these cells with 5 mM sodium butyrate (NaB) for 24 hr before infection enhanced both HCMV yield and infectious center titer to a similar level of that seen in human embryonic lung fibroblast cells. Immunofluorescence staining, gel electrophoresis, and Northern blot analysis revealed that TPC-1 cells are nonpermissive for expression of HCMV major immediate early (IE1) functions, but many of the cells become permissive after being treated with NaB. The presence of cycloheximide during NaB pretreatment of the cells efficiently diminished the stimulatory effect of NaB on expression of the IE1 gene. Therefore, it appeared that NaB induces the synthesis of a cellular protein(s) which apparently plays an important role in the conversion of nonpermissive cells to a permissive state for expression of this critical viral gene. Transient chloramphenicol acetyltransferase (CAT) assay experiments indicated that in TPC-1 cells the HCMV-CAT construct which contains the complete IE1 promoter regulatory region was expressed poorly, whereas a high level of CAT activity was detectable in the NaB-treated cells. Therefore, these results suggest that the enhancing effect of NaB on HCMV replication is expressed through some host cellular factor(s), and the HCMV IE1 promoter regulatory region is most likely to be the primary target of NaB action.

Recovery of a herpesvirus from a roan antelope (Hippotragus equinus)

A herpesvirus was recovered in culture from the cells of a roan antelope (Hippotragus equinus) following cryopreservation in DMSO and it is thought that the DMSO may have been involved in reactivation. The virus was shown to be antigenically related to alcelaphine herpesvirus-1 (AHV-1) of wildebeest and ovine herpesvirus-2 (OHV-2) of domestic sheep (formerly designated the sheep-associated agent of malignant catarrhal fever (MCF]. Cloned DNA fragments of AHV-1 and OHV-2 cross hybridised with DNA prepared from cells infected with the roan antelope virus and the intensity of reaction suggested that this virus was more closely related to AHV-1 than is OHV-2. The virus represents the third gamma herpesvirus isolated from large African antelope and should be provisionally designated hippotragine herpesvirus-1. On inoculation into rabbits the virus induced malignant catarrhal fever indicating that roan antelope should be considered as a possible source of infection.

Human immunodeficiency virus type 1 tat gene enhances human cytomegalovirus gene expression and viral replication

Clonal lines of human rhabdomyosarcoma (RD) cells, constitutively expressing human immunodeficiency virus type 1 (HIV-1) tat gene (RD tat cell lines) showed enhanced expression of human cytomegalovirus (HCMV) immediate-early (IE) and late (L) proteins upon HCMV infection, as compared with control RD cells. One of the RD tat cell lines produced infectious HCMV. The RD-tat cell lines, following transfection with recombinant plasmids containing the full length of the HCMV-IE enhancer/promoter linked to the bacterial chloramphenicol acetyltransferase (CAT) gene, exhibited an increased CAT expression by the tat product. A chronically HIV-1-infected human T-lymphoid cell line, SupT1, superinfected with HCMV, expressed HCMV-IE proteins while the parental SupT1 cells infected with HCMV were negative. Parental SupT1 cells coinfected with HIV-1 and HCMV also expressed HCMV-IE proteins, indicating that HIV-1-encoded proteins exert a positive regulatory effect on HCMV expression.

Cell surface expression of the varicella-zoster virus glycoproteins and Fc receptor

Varicella-zoster virus (VZV) specifies the synthesis of viral glycoproteins which are important antigens for induction of the host immune response. In this report the technology of laser-activated flow cytometry has been employed to measure the membrane expression of VZV glycoproteins gpI, gpII, gpIII, and gpIV. By use of biotinylated monoclonal antibodies as probes, all four glycoproteins were demonstrated on the infected cell surface. The temporal appearance of the viral glycoproteins was defined in a time course experiment and shown to be maximal about 24 hr postinfection. The issue whether VZV induces the cell surface expression of an Fc receptor (FcR) was investigated with biotinylated nonimmune human IgG, followed by streptavidin-phycoerythrin. By this technique a 10-fold increase in fluorescence intensity was seen in the VZV-infected cells as compared to the mock-infected controls. When the experiment was repeated with purified human Fc fragment rather than whole IgG, a similar degree of binding was seen. Both the VZV glycoproteins and the VZV FcR were exquisitely sensitive to trypsin treatment (1 mg/ml); likewise, the cell surface expression of these VZV products was diminished by treatment of the infected cultures with monensin, an inhibitor of glycoprotein transport. In order to prove that VZV infection was not causing the induction of a cellular Fc gamma R, the VZV-infected and mock-infected cells were stained with monoclonal antibodies directed against each of the three human cellular IgG FcR, but no differences were observed. Therefore, the FcR activity seen in the infected culture was not due to one of the known cellular Fc gamma R.

Kaposi's sarcoma in immunosuppression. Possibly the result of a dual viral infection

Kaposi's sarcoma of the gingiva and skin developed in an HIV-negative renal transplant patient while he was receiving cyclosporine therapy. The Kaposi's sarcoma developed shortly after the patient had an acute infection with cytomegalovirus (CMV). Electron microscopy of the tumor's established cell line showed two types of virus-like particles. CMV DNA was identifiable in the cell line whereas infectious CMV could be isolated only after repeated passages (only after 3 months of culture). The other virus could not be identified, but did not appear to be either HIV or papilloma virus. The patient's tumor regressed after the discontinuation of cyclosporine therapy and the recovery from the acute CMV infection.

Detection of human cytomegalovirus early and late antigen and DNA production in cell culture and the effects of dimethyl sulfoxide, dexamethasone, and DNA inhibitors on early antigen induction

Recently a conventional method for the laboratory diagnosis of human cytomegalovirus (HCMV) infection was improved by using centrifugation culture to enhance viral adsorption and by detecting HCMV early antigen and DNA. Comparison of the sensitivity of three rapid methods using commercial diagnostic reagents for the detection of HCMV early antigen (EA), late antigen (LA), and DNA was quantitatively evaluated in centrifugation cultures of human fibroblast cells (MRC-5) infected with HCMV. HCMV-EA was first detected 4 hours after infection, and the number of antigen-positive cells increased rapidly thereafter. Using biotinylated DNA probe, viral DNA was first detected 12 hours postinfection; the number of DNA-positive cells increased slowly. HCMV-LA was first seen 48 hours postinfection, and the number of LA-positive cells also increased thereafter. Thus detection of HCMV-EA was the most rapid and sensitive method for HCMV diagnosis. Several chemical compounds have been used to enhance HCMV replication. The effect of dimethyl sulfoxide (DMSO), dexamethasone (DEX), 5-bromo-2-deoxyuridine (BrdU), 5-fluoro-deoxyuridine (FdU), and cytosine arabinoside (Ara-C) on HCMV-EA induction was evaluated in centrifugation cultures of MRC-5 cells infected with HCMV. Infected cells treated with 1% DMSO alone or with DMSO plus DEX (10(-5) M) have been shown to increase the number of HCMV-EA-positive cells three- to fivefold over the untreated control cultures. The enhancing effects of Ara-C, BrdU, and BrdU plus FdU were demonstrated only occasionally.

Effect of treatment of shell vial cell cultures with dimethyl sulfoxide and dexamethasone and age of MRC-5 monolayers for detection of cytomegalovirus

Pretreatment of MRC-5 cell monolayers in commercially prepared shell vials with 1% dimethyl sulfoxide (DMSO) and 10(-5) mol/L dexamethasone (DEX) was evaluated. Preliminary experiments indicated enhanced infectivity of AD-169 for pretreated MRC-5 cells in shell vials of ages 9 and 16 days. Compared with untreated shell vials, DMSO-DEX increased positivity (day 9, 19 vs. 26 shell vials, p less than 0.03; day 16, 13 vs. 29 shell vials, p less than 0.001) and increased the mean number of fluorescent foci (days 9 and 16, p less than 0.001). Pretreatment of 8-15-day-old monolayers was evaluated clinically using 146 urine specimens. Fifty specimens were positive for cytomegalovirus (CMV) in both treated and untreated shell vials with ten positive in untreated only and three positive in treated only (p = NS). The median number of fluorescent foci was not significantly higher in treated shell vials. Increased toxicity of MRC-5 cells was observed in treated monolayers (p less than 0.0001). Pretreatment with DMSO-DEX did not enhance CMV isolation from clinical specimens and can be toxic to MRC-5 monolayers.

Replication of duck hepatitis B virus in primary duck hepatocytes and its dependence on the state of differentiation of the host cell

Primary duck hepatocytes obtained from Pekin ducks congenitally infected with duck hepatitis B virus were used to monitor expression of viral proteins and replication of viral DNA in cell culture. Duck hepatitis B virus core antigen, duck hepatitis B virus pre-surface antigen and duck hepatitis B virus DNA were detectable for at least 12 days after cell plating. Whereas expression of duck hepatitis B pre-surface antigen was constant during this time, expression of duck hepatitis B core antigen and of viral DNA rapidly declined. This diminished production of viral components in vitro was paralleled by a change of the hepatocytes toward a fibroblast-like morphology. Supplementation of cell culture medium with 2% dimethyl sulfoxide, a solvent known to maintain the differentiated state of cultured cells, retained competence of the cultured hepatocytes to express duck hepatitis B core antigen and duck hepatitis B virus DNA at high levels. In a second set of experiments, duck hepatitis B virus negative hepatocytes were infected with duck hepatitis B virus from serum of congenitally infected ducks. Dimethyl sulfoxide remarkably improved the competence of cultured duck hepatocytes to become productively infected. This function was maintained for at least 12 days postplating.

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.

Increased detection of herpes simplex virus in MRC-5 cells treated with dimethyl sulfoxide and dexamethasone

Treatment of MRC-5 cells with dexamethasone, dimethyl sulfoxide, or a combination of the two enhanced the detection of herpes simplex virus by three- to fourfold in these cells. Fluorescent plaques were noticeably larger on cover slips treated with the enhancing agents. Low-positive clinical specimens were stored and tested in parallel in treated and untreated shell vials, and 6 to 40% of these stained positive in the treated cultures but not in the untreated controls. In standard tube cultures, cytopathic effect began earlier and was more extensive in treated tubes.

Enhanced detection of cytomegalovirus in confluent MRC-5 cells treated with dexamethasone and dimethyl sulfoxide

Optimum growth conditions for human cytomegalovirus (HCMV) include the use of subconfluent, actively growing cultures of human embryonic fibroblasts. Many clinical virology laboratories, however, use tissue culture cells from commercial sources. These cells are usually confluent, static cultures that tend to be less sensitive to viral infection. To determine whether dimethyl sulfoxide (DMSO) or dexamethasone (DEX), which are known enhancers of HCMV, facilitates the detection of the virus in confluent cells, we tested both HCMV AD169 and a number of clinical specimens suspected to contain HCMV on MRC-5 cells in both shell vials and conventional tube cultures. We found that, in the shell vial test, treatment of the cultures with either DMSO or DEX before and after inoculation increased the number of cells staining positive by three- to sixfold compared with untreated controls. Best results were obtained by pretreating the cultures with DEX alone and by treating the cultures with a combination of DEX and 1% DMSO postinfection. In the conventional MRC-5 culture tubes, treatment with the reagents resulted in the more rapid appearance of cytopathic effect and a more extensive infection of the cell sheet. The experimental findings indicate that the enhancing effect of DEX is attributable mainly to the increased production of a cellular mRNA during the period preceding viral infection.

Amplification of Epstein-Barr virus (EBV) DNA by superinfection with a strain of EBV derived from nasopharyngeal carcinoma

Epstein-Barr virus (EBV) from a nasopharyngeal carcinoma (NPC) hybrid cell line (NPC-KT) lacking defective viral DNA molecules superinfected Raji cells and induced EBV early antigens (EA), as did virus from P3HR-1 cells, which contained defective molecules. The EBV polypeptides induced by NPC-KT appeared to be identical to those induced by P3HR-1 virus. The ability of NPC-KT virus to induce EA was enhanced more than 10-fold by treatment of superinfected cells with dimethyl sulfoxide; however, dimethyl sulfoxide treatment did not enhance superinfection by P3HR-1 virus. After infection, DNA synthesis of both the superinfecting NPC-KT virus and the resident Raji viral genome was induced. In addition to amplified Raji EBV episomal DNA, a fused terminal fragment of NPC-KT viral DNA was detected. The detection of fused terminal DNA fragments suggests that the superinfecting virion DNA either circularizes or polymerizes after superinfection and is possibly amplified through circular or concatenated replicative intermediates.

Rapid quantitative assay of cytomegalovirus infectivity

A rapid quantitative assay was developed to determine cytomegalovirus infectious units. The assay yielded results in three days. The rapid quantitation of cytomegalovirus infectivity was developed by evaluating the number of cells that expressed cytomegalovirus-late antigens at the end of a single replication cycle (62 h post-infection). Late antigens were visualized by immunoalkaline phosphatase staining using a monoclonal antibody against the major viral capsid protein (66.8 kDa). Thirty-five cytomegalovirus samples were examined. The values of the infectious units obtained by immunoalkaline phosphatase staining were correlated significantly with the values determined by the classical plaque-forming technique.

Effect of treatment of shell vial cell cultures with dimethyl sulfoxide and dexamethasone for detection of cytomegalovirus

Urine specimens submitted for the diagnosis of cytomegalovirus infection were inoculated into shell vials that had been pretreated with a combination of dimethyl sulfoxide (DMSO) and dexamethasone (DEX). The results were compared with those for inoculated shell vials which had received no drug treatment. Of 664 specimens, 100 (15%) were positive for cytomegalovirus. Of the 100 strains of cytomegalovirus, 88 (88%) were detected in both DMSO-DEX-treated and untreated shell vials. Of the remaining 12 positive specimens, 6 were detected with untreated shell vials exclusively and 6 were detected with DMSO-DEX-treated shell vials alone (not significant by the sign test). The median number of fluorescent foci was not significantly higher in DMSO-DEX-treated shell vials compared with that in untreated cultures (Wilcoxon signed-rank test; P = 0.1). DMSO-DEX-treated monolayers did not enhance the sensitivity detection of cytomegalovirus in shell vial cell cultures.

Inhibitors of prostaglandin synthesis inhibit growth of human cytomegalovirus and reactivation of latent virus in a productively and latently infected human cell line

Indomethacin and tetracaine, inhibitors of prostaglandin synthesis, inhibited production of infectious human cytomegalovirus (HCMV) in a human thyroid papillary carcinoma cell line (TPC-1) by 99.9% when added to cultures at the concentration of 2 x 10(-4) M during the first 24 hr after infection. Although immediate early virus proteins were synthesized at similar molar ratios in mock- and compound-treated cultures, induction of HCMV-specific DNA polymerase (one of the early virus proteins) was inhibited by treatment with these compounds, suggesting that the early stages of the virus growth cycle are most likely to be under the control of indomethacin or tetracaine action. We have previously developed an in vitro HCMV latency model system in TPC-1 cultures. This system was used to study the effect of these compounds on reactivation of the latent virus. When TPC-1 cultures preheated for 48 hr at 40.5 degrees were infected with HCMV and incubated at 40.5 degrees, the cultures could be maintained for 30 days without detection of infectious virus. The latent HCMV was reactivated within 10 days by reducing the incubation temperature from 40.5 to 37 degrees. However, when the latently infected cultures were treated with indomethacin or tetracaine immediately after being shifted to 37 degrees, reactivation of the latent virus was not observed.

Cytomegalovirus replicates efficiently in human kidney mesangial cells

Human cytomegalovirus (HCMV) is a major renal pathogen in congenitally infected infants and renal allograft recipients. We postulated that a specific renal cell type was involved in HCMV infection and reactivation. Human fetal kidney cortex cell cultures were assayed for their ability to support HCMV infection. Infectious center assays indicated that the low level of viral replication observed by virus yield assay occurred from a fraction of the cells in the mixed cultures. Virus-specific immunofluorescence and in situ hybridization documented the presence of HCMV-specific protein and nucleic acid, respectively, in a morphologically distinct cell type. These cells were purified, were identified as kidney mesangial cells, and were observed to support efficient HCMV replication. Our research identifies mesangial cells as a renal cell type that supports HCMV replication and provides evidence to implicate these cells in the pathogenesis of HCMV-induced renal disease.

DMSO induces reactivation of cytomegalovirus in vitro from spleens of latently infected mice. Brief report

Reactivation of murine cytomegalovirus in spleen explant cultures from latently infected CD 1 mice was studied. A significant increase in the incidence of reactivation was observed when 200 mM DMSO was included in the culture medium. Virus could also be reactivated from the bone marrow of some mice.

Establishment and biological characterization of an in vitro human cytomegalovirus latency model

In an attempt to develop an in vitro human cytomegalovirus (HCMV) latency model system, the growth characteristics of HCMV in a human thyroid papillary carcinoma cell line (TPC-1) were examined. When TPC-1 cultures preheated at 40.5 degrees for 48 hr were infected with HCMV and incubated at a supraoptimal temperature (40.5 degrees), the cultures could be maintained for at least 65 days without detection of infectious virus. In contrast, when the infected cultures were incubated at 37 degrees, HCMV persistently infected cultures were established. HCMV was reactivated from the latently infected cultures by decreasing the incubation temperature from 40.5 to 37 degrees, and the cultures subsequently entered into virus persistent infection. Although HCMV-specific polypeptides which comigrate with the immediate early virus polypeptides and nuclear antigens were continuously detectable in the majority (more than 95%) of the cells during the latent period, a detectable level of virus-specified DNA polymerase (one of the early virus proteins) was not induced, suggesting that the blockage of HCMV replication in the latently infected cultures occurs at the early stages of the HCMV replication cycle. Infectious center assay revealed that 0.002 to 0.2% of the cells contain an HCMV genome that can be activated during the latent period. The latently infected cells were susceptible to superinfection with homologous and heterologous strains of HCMV. In persistently infected cultures approximately 38% of the cells were lysed by reaction with HCMV immune serum and complement, whereas complement-mediated immune cytolysis could not be detected in the latently infected cultures. The data presented suggest that a temperature-sensitive cellular function(s) that controls the expression of the HCMV early functions plays an important role in maintenance of the HCMV genome in the latent state and reactivation of HCMV by decreasing the incubation temperature.