Sequence of spontaneous Epstein-Barr virus activation and selective DNA synthesis in activated cells in the presence of hydroxyurea

Autonomic and glucocorticoid associations with the steady-state expression of latent Epstein-Barr virus

Previous studies have demonstrated the impact of psychological stress on the steady-state expression/reactivation of latent Epstein-Barr virus (EBV). Stress-induced decrements in the cellular immune response result in less control over the expression of the latent virus, resulting in increases in antibody to the virus. In Study 1, we investigated whether the steady-state expression of latent EBV in vivo differed between high and low stress reactors, as defined by sympathetic cardiac reactivity. Autonomic activity and antibody titers to Epstein-Barr virus capsid antigen (VCA) were measured in 50 elderly women latently infected with EBV. Results revealed that women who were high stress reactors were characterized by higher antibody titers to the latent virus than low stress reactors. High reactors tended to show larger stress-related increases in cortisol than low reactors, but the differences were not significant. Daily stressors can activate the autonomic nervous system and promote the release of pituitary and adrenal hormones, especially in high reactors. Glucocorticoid hormones have been shown to reactivate EBV in vitro from cells latently infected with the virus. We hypothesized that absolute levels of plasma cortisol may not be the only explanation for stress-induced reactivation of latent EBV and that the diurnal changes in the production of cortisol may be an important factor in these interactions. To examine the feasibility of this hypothesis, an in vitro study was conducted (Study 2) to determine whether changing glucocorticoid concentrations in the medium, in which EBV latently infected cells were cultured, to mimic diurnal changes in plasma cortisol concentrations would enhance the reactivation of the latent virus. Cells latently infected with EBV were exposed to either constant or varying concentrations of the synthetic glucocorticoid hormone dexamethasone (Dex), for 72 h. Results revealed a three- to eightfold enhancement of reactivation of latent EBV in cells pulsed with varying Dex concentrations when compared with cells exposed to a constant and/or a higher mean level of one Dex concentration. Together, these studies raise the possibility that differences in the kinetics of glucocorticoid concentrations may contribute to differences in the reactivation of latent EBV.

Effects of DNA synthesis inhibitors on early antigen expression following primary infection or superinfection by Epstein-Barr virus

Seven lymphoid cell lines previously characterized with respect to their resident Epstein-Barr virus (EBV) genome content were infected or superinfected with concentrated EBV from supernatant of the P3HR-1 cell line. Immunofluorescence assays were conducted on smears 48 hours after infection, using human sera containing antibodies to EBV early antigen (EA). Two EBV nuclear antigen (EBNA) negative cell lines containing no detectable resident EBV DNA and five EBNA positive cell lines containing EBV genomes were tested. The cell lines did not spontaneously express EBV EA (i.e., they were non-producers). All cell lines responded to infection or superinfection with EBV by expressing EA. Treatment of the cell lines with arabinosylcytosine (Ara-C) 10 micrograms/ml, at the time of infection resulted in significant decreases in the number of cells expressing detectable EA after drug treatment in all cell lines (72 +/- 5 percent inhibition of EA expression). Experiments were also conducted with hydroxyurea (HU) and phosphonoacetic acid (PAA). It was found that treatment with HU (100 micrograms/ml) inhibited EA production in cell lines containing EBV genome copies by 81 percent as compared to the superinfected cultures receiving no drug. In primary infection of EBNA negative cell lines, HU had minimal effects. PAA (100 micrograms/ml), on the other hand, had very little effect on EA expression following superinfection of cell lines harboring the EBV genome, but reduced the EA expression after primary infection of EBNA negative cell lines by 70 to 80 percent. All drugs were used at concentrations having little effect on RNA and protein synthesis. However, HU and Ara-C significantly reduced DNA synthesis and cell division in the treated cultures.

Sedimentation characteristics of newly synthesized Epstein-Barr viral DNA in superinfected cells

Replicating Epstein-Barr virus (EBV) DNA molecules isolated from superinfected Raji cells were shown to consist of 80S to 65S and 58S (mature) molecules Pulse-chase experiments showed that radioactive label of DNAS molecules with the larger sedimentation coefficients was partially chased into 58S labeled forms. Formation of large concatemers of viral DNA could not be detected at any time after superinfection. The continuous presence of the 65S viral DNA intermediate throughout the replicative cycle combined with the observed inhibition of EBV DNA synthesis by addition of nontoxic levels of ethidium bromide to the superinfected cell culture led us to propose that EBV replication proceeds via a relaxed circular DNA intermediate.

The selective inhibition of viral DNA synthesis by chemotherapeutic agents: an indicator of clinical usefulness?

A cell culture system has been utilized to measure the effects of drugs on DNA synthesis in uninfected and HSV-(herpes simplex virus)-infected KB cells. DNA from HSV-infected cells was separated into viral and cellular components by isopycnic centrifugation in CsCl gradients. The amount of [3H]thymidine incorporated into acid-insoluble material was measured in the absence and presence of drugs. Dose-response relationships were established by linearly regressing the probit value of the percent inhibition DNA synthesis against the logarithm of drug concentration. Fifty percent inhibitory (I50) concentrations were interpolated from the corresponding regression lines for inhibition of the following: (i) DNA synthesis is uninfected KB cells, (ii) total DNA synthesis in HSV-infected KB cells (iii) cellular DNA synthesis in HSV-infected cells, and (iv) viral DNA synthesis in HSV-infected cells. We have derived an index (SI, selective index) that quantifies the preferential inhibition of viral or uninfected cellular DNA synthesis. This index can be expressed as SI = log10 I50 concentration for DNA synthesis in uninfected cells divided by I50 concentration for viral DNA synthesis in HSV-infected cells. The SI is positive if viral DNA synthesis is inhibited preferentially and negative if uninfected cellular DNA synthesis is more strongly inhibited. A positive SI value of 0.5 was obtained for the clinically useful antiviral drug arabinosyladenine (ara-A) and a value of 0.4 for its metabolite, arabinosylhypoxanthine (ara-H). Although the adenosine deaminase inhibitor coformycin greatly increased the potency of ara-A, the inhibitor did not increase the selectivity of the drug (SI = 0.3). Stallimycin (distimycin A) (SI = 0.3) and phosphonoacetic acid (SI = 0.3) were similarly effective in preferentially inhibiting the synthesis of HSV DNA. In contrast, arabinosylcytosine (ara-C) and ribavirin inhibited DNA synthesis in uninfected cells to a greater degree than viral DNA synthesis (SI = -0.5 and -1.9, respectively). An analysis of the advantages and limitations of this experimental procedure is made and the suggestion is offered that the in vitro determination of a drug's selective index may be a valid predictor of clinical usefulness.

Inducibility of the Epstein-Barr virus (EBV) cycle and surface marker properties of EBV-negative lymphoma lines and their in vitro EBV-converted sublines

Two EBV-negative lymphoma lines of human B-cell origin, BJAB and Ramos, were compared with altogether six of their in vitro EBV-converted, EBNA- and EBV-DNA-carrying sublines (four of Ramos and two of BJAB derivation). All converted lines closely resembled the parental line with regard to karyotype and HL-A and B antigen typing. Induction of EBV antigens (EA and VCA) by P3HR-1 virus superinfection was either similar in the converted and the negative lines, or somewhat increased in certain converted lines. These findings argue against a simple, virally determined repressor model and emphasize the role or cellular controls in restricting the EBV cycle in virus-carrying B-lymphocyte lines of human origin. IUdR inducibility varied in the different converted lines. There was a possible relationship between average number of EBV-genome equivalents per cell and inducibility. Converted sublines did not differ from the original negative lines with regard to surface immunoglobulin and Fc receptors. There was a dramatic increase in complement-consuming ability, however, following EBV conversion. Among the EBV-positive lines, there was a linear relationship between complement-consuming and EBV-receptor activity, the latter measured by a quantitative absorption test.

Inhibition of Epstein-Barr virus DNA synthesis and late gene expression by phosphonoacetic acid

Growth of lymphoblastoid cells (B95-8, Raji) is not inhibited by the presence of 0.4 mM phosphonoacetic acid. The synthesis of Epstein-Barr virus (EBV) in the producer line B95-8 is completely inhibited, as shown by the total inhibition of viral capsid antigen synthesis. Early viral antigens are made normally in the presence of phosphonoacetic acid, but EBV DNA synthesis is blocked in cells entering the productive cycle. Nonproducer cells in the population replicate the resident EBV DNA by a mechanism that is resistant to phosphonoacetic acid. These results are consistant with the hypotheses that EBV DNA is replicated by two mechanisms, one in the noninduced cell and a different mechanism in the producer cell, and that prior replication of EBV DNA, probably by the second mode, is a prerequisite for late gene expression.

Antiviral activity of arabinosyladenine and arabinosylhypoxanthine in herpes simplex virus-infected KB cells: selective inhibition of viral deoxyribonucleic acid synthesis in synchronized suspension cultures

The drug 9-beta-d-arabinofuranosyladenine (ara-A) significantly suppressed the formation of herpes simplex virus type 1-induced syncytia in BHK-21/4 cells at concentrations as low as 0.1 mug/ml. Optimal activity was noted when the drug was added before initiation of viral deoxyribonucleic acid (DNA) synthesis (3.5 h postinfection). The deaminated derivative of ara-A, 9-beta-d-arabinofuranosylhypoxanthine (ara-H), was at least 10 times less effective in suppressing the development of herpes simplex virus-induced syncytia. The replication of herpes simplex virus was measured by assaying fluids and cells from infected drug-treated cultures by using a plaque production technique. Ara-A at drug levels of >10 < 32 mug/ml completely blocked the replication of infectious virus particles. Ara-H was less effective than ara-A in reducing the replication of virions. Rates of host and viral DNA synthesis were monitored by pulse labeling herpes simplex virus-infected synchronized KB cells with [(3)H]thymidine and subsequently separating viral from cellular DNA in CsCl density gradients. During synthetic (S) phase, ara-A or ara-H at concentrations ranging from 3.2 to 32 mug/ml selectively inhibited viral DNA synthesis. At 3.2 mug of ara-A per ml, viral DNA synthesis was reduced 74% although total cellular DNA synthesis was unaffected. Increasing concentrations of ara-A produced increasing temporal delays in the maximal rate of host DNA synthesis. This time shift was not observed in cells treated with ara-H.

Amounts of Epstein-Barr virus DNA in somatic cell hybrids between burkitt lymphoma-derived cell lines

The amounts of Epstein-Barr virus DNA in somatic cell hybrids between human lymphoid cell lines were found to be higher than in the parental cell lines, although the hybrids were not induced to spontaneous virus production.

Cycloheximide induction of xenotropic type C virus from synchronized mouse cells: metabolic requirements for virus activation

The information for type C RNA viruses is genetically transmitted within the cellular DNA of the normal mouse cell. These viruses can be induced after exposure of cells to two classes of chemicals, inhibitors of protein synthesis and halogenated pyrimidines. The metabolic requirements for activation of one endogenous virus of BALB/c mouse cells by representatives of each class of drugs were studies. Cycloheximide and iododeoxyuridine each induce virus efficiently from cultures in exponential growth but are inactive on cells in stationary phase. However, cells are maximally sensitive to the actions of each drug at different times within the cell cycle. Further, virus induction in response to each is differentially inhibited under conditions of simultaneous cell exposure to inhibitors of DNA or RNA synthesis. The results provide support for the concept that inhibitors of protein synthesis and halogenated pyrimidines act by different mechanisms to induce type C virus release.

Expression of Epstein-Barr viral capsid, complement fixing, and nuclear antigens in stationary and exponential phase cultures

Three continuous lymphoblastoid cell lines, 2 productive of nucleocapsids and 1 nonproductive line, were studied for their content of Epstein-Barr viral (EBV) antigens during transition from stationary to logarithmic phase growth. As a preliminary step, viable cells were separated from degenerating ones in discontinuous gradients of serum albumin. Viral capsid antigens were found in both living and dead cells of the 2 producer lines; however, complement fixing (CF) antigens and nuclear antigen were detected only in viable cell subpopulations. The content of antigen detectable in extracts of viable cells by complement fixation remained constant in replicating and resting cultures; further, all viable cells of the 3 lines demonstrated intranuclear antigen by anticomplement immunofluorescence in all stages of growth. In contrast, the proportion of cells with viral capsid antigen in the producer lines increased 7- to 24-fold following entry of resting populations into the phase of exponential growth.The results suggest that expression of viral capsid antigens is discontinuous and is initiated in response to events in log phase, possibly DNA synthesis or mitosis. Expression of the complement fixing and nuclear antigens in continuous in viable cells. These findings emphasize the intimate relationship of the CF and nuclear antigen to the transformed state and suggest that study of this antigen complex will shed light on the mechanisms of lymphocyte transformation by EBV.

Replication of the resident repressed Epstein-Barr virus genome during the early S phase (S-1 period) of nonproducer Raji cells

Replication of the resident repressed Epstein-Barr virus genome in synchronized nonproducer Raji cells was shown to occur during the early S phase (S-1 period) by hybridization of cell DNA with virus-specific complementary RNA (cRNA). The S-1 period was previously identified as the critical period for virus activation induced by thymidine analogues. The findings reported here and elsewhere are consistent with the proposal that: (i) virus activation is initiated at or near the site of association of the resident viral genome with cell DNA, (ii) replication of the resident virus genome in nonactivated cells is under cell control mechanisms, and (iii) the resident virus genome is physically associated with early replicating cell DNA.