Mengovirus-induced cytopathic effect in L-cells: protective effect of interferon

Inhibition of host cell protein synthesis by UV-inactivated poliovirus

The ability of poliovirus that was irradiated with UV light at energies up to 2,160 ergs/mm2 to subsequently inhibit host cell protein synthesis was measured. The inactivation of the host cell shutoff function followed one-hit kinetics. Increasing irradiation did not affect the rate of inhibition until the multiplicity of infection after irradiation was reduced to approximately 1 PFU/cell. At higher functional multiplicities, the rate was unchanged, but an increasing lag before the onset of inhibition was observed with increasing irradiation. The energy levels required to inactivate virus-induced inhibition of host cell protein synthesis suggest that damage to virus RNA rather than to virus capsid proteins is responsible for the loss of function. When the inactivation of host cell shutoff was compared with the inactivation of other viral functions by UV irradiation, it correlated exactly with the loss of infectivity but not with other viral functions measured. Guanidine treatment, which prevents detectable viral RNA and protein synthesis, completely inhibited host cell shutoff by low multiplicities of unirradiated virus infection but not higher multiplicities. When a high multiplicity of virus was first reduced to a low titer by irradiation, host cell shutoff was still evident in the presence of guanidine. The results demonstrate that the complete inhibition of host cell protein synthesis can be accomplished by one infectious viral genome per cell.

Cell killing by simian virus 40: variation in the pattern of lysosomal enzyme release, cellular enzyme release, and cell death during productive infection of normal and simian virus 40-transformed simian cell lines

Simian virus 40 (SV40) growth on rhesus kidney cells and on the T-22 line of SV40-transformed green monkey kidney (GMK) cells is largely limited by the low plating efficiency of SV40 on these cells. In addition, a fraction of the rhesus kidney and T-22 cells are resistant to infection by SV40. Nevertheless, 72-h viral yields per infected rhesus kidney and T-22 cell are nearly equivalent to that obtained on normal GMK cells and are independent of the multiplicity of infection. Despite the production of high viral yields, infected rhesus kidney and T-22 cells are killed slowly by SV40. Monolayers of these cells are also refractory to plaque formation by SV40. SV40 induces the release of lysosomal N-acetyl-beta-glucosaminidase into the cytoplasmic fractions of rhesus kidney and T-22 cells to an extent equal to that observed during infection of rapidly killed normal GMK cells. In contrast, damage to the plasma membrane, as indicated by the release of the cellular enzymes lactic dehydrogenase and glutamic oxaloacetic transaminase into the overlay media, occurred to a much greater extent in the normal GMK cells than in the rhesus kidney or T-22 cells. Neither a lysosomal hydrolase mechanism nor viral release appear to be responsible for this phenomenon. The different rates and extents of the SV40 cytocidal process on these cells do not result from the differences in the viral plating efficiency on them.

Fate of interferon-treated cells

Interferon-treated cultures of Ly cells survived initial infection with high multiplicities of vesicular stomatitis (VSV) or herpes simplex virus (HSV). In the case of HSV, infectious virus and intracellular viral antigen were rapidly eliminated from the interferon-treated cultures, and the cells grew out to form apparently normal monolayers that could be cultured indefinitely. In the VSV-infected Ly cultures, virus titers remained at low levels in interferon-treated cells but after about 14 days rapidly rose and the culture was destroyed. If interferon was added to the medium on days 4 and 6 after infection, virus titers rapidly declined but again recovered and the cells were destroyed. If, however, interferon treatment was resumed 9 days after initial infection, detectable infectious VSV was eliminated from the medium. Several methods, including cocultivation and molecular hybridization, failed to demonstrate persistence of a significant portion of the VSV genome in these cultures.

Viral infection and host defense

Double-stranded RNA, made as an intermediary substance in the replication of most, if not all, viruses, may play a much more important role in the pathogenesis and the recovery from virus infections than has hitherto been suspected. Apparently, dsRNA is used by both the challenge virus and the host cell in an attempt to gain "molecular control." Double-stranded RNA exerts a set of effects, which may be well balanced, not only at the level of the individual cell but also at the complex assemblage of these cells termed the organism (Fig. 1). In the cell, interferon synthesis is triggered, although interferon mRNA translation may not occur if dsRNA shuts off protein synthesis too quickly. In the whole organism, the disease severity will depend on how certain toxic reactions evoked by infection (such as cell necrosis and fever) are counterbalanced by an increase in the host defense mechanisms (for example, immune responsiveness and interferon production). Many aspects of the response, relating to either progress of, or recovery from, the disease, can be explained on the basis of a dsRNA. In addition to drawing attention to the biodynamic role of dsRNA, our hypothesis suggests specific experimental vectors designed to enhance our information on the molecular basis of the morbid process which occurs with viral infection. Finally, we suggest that, although the dsRNA molecule may be viewed as a rather simple unit structure, the opportunity for further diversity in the biological activity of a given dsRNA molecule always exists. Namely, each deviation from a perfectly double-helical arrangement introduces the possibility for emphasizing one biological reactivity at the expense of another. This latter structure-activity property may partially account for the extreme apparent diversity, commonly encountered, in the presentations of virologic illness. Appendix note added in proof. Subsequent to submission of this text, we have found that the potent mitogen effect of dsRNA for lymphocytes (murine and human) is also exquisitively sensitive to the fidelity in base pairing of the input polymer pair (59). For example, infrequent "loops" (one nucleotide per 20 base pairs) in an otherwise perfectly helical rI(n) (.) rC(n) molecule [for example, rI(n) (.) r(C(19,)U)(n)] strongly changes its mitogenic properties. This observation, which supports our thesis that a "fine structure" term can be developed for other reactions triggered by dsRNA's in biological systems, emphasizes that diverse biological effects may be encountered with an ostensibly uniform family of dsRNA's.

Correlation between the antiviral effect of interferon treatment and the inhibition of in vitro mRNA translation in noninfected L cells

When noninfected L-cell suspension cultures are treated with interferon (specific activities superior to 10(6) reference units per mg of protein), the cell-free cytoplasmic extracts obtained are inactive for the translation of exogenous natural mRNAs. The dose-response curve shows that comparable amounts of interferon are required to produce a 50% reduction of Mengo virus multiplication in vivo and Mengo RNA translation in vitro. With higher doses of interferon, Mengo RNA translation is completely abolished, while poly U translation and endogenous protein synthesis are only slightly affected. The inactivation of Mengo RNA translation is reversible; after removal of interferon, normal translation activity is regained together with the ability to support Mengo virus multiplication. Fractionation of the cell-free extracts shows that the effect is localized in the fraction which can be washed off the ribosomes by high salt. These results establish that interferon induces a block in genetic translation in noninfected L cells.

Mechanism of Mengo virus-induced cell injury in L cells: use of inhibitors of protein synthesis to dissociate virus-specific events

L cells were infected with Mengo virus in the presence of varying concentrations of protein synthesis inhibitors (azetidine-2-carboxylic acid, p-fluorophenylalanine, puromycin), and examined with respect to the effects of the inhibitors on several features of virus-induced cell injury. The virus-specific events in the cells could be dissociated into three groups, based on their sensitivity to the inhibitors: (i) viral ribonucleic acid (RNA) synthesis, bulk viral protein synthesis, and infectious particle production, all of which were prevented by low inhibitor concentrations; (ii) the cytopathic effect (CPE) and stimulation of phosphatidylcholine synthesis, which were sensitive to intermediate concentrations of the inhibitors; and (iii) the virus-induced inhibitions of host RNA and protein synthesis, which were resistart to the inhibitors of protein synthesis except at very high concentrations. It is concluded from this that the virus-induced CPE and stimulation of phosphatidylcholine synthesis are not consequences of the inhibition of cellular RNA or protein synthesis. Analysis of the virus-specific protein and RNA synthesized at several concentrations of azetidine and puromycin suggests that the CPE may be induced by a viral protein precursor. Virus-induced inhibition of host RNA and protein synthesis occurred at azetidine concentrations which blocked the synthesis of over 99.7% of the total viral RNA and over 99% of the viral double-stranded RNA (dsRNA). Calculations show that this would correspond to less than 150 dsRNA molecules per infected cell, resulting in a dsRNA-polysome ratio of less than 1:1,000; this indicates that host protein synthesis cannot be inhibited by an irreversible binding of dsRNA to polysomes.

Effect of double-stranded viral RNA on mammalian cells in culture

During bovine enterovirus infection of Ehrlich ascites tumor cells, large amounts of double-stranded RNA accumulate. Addition of this double-stranded RNA to uninfected cells leads to rapid cell death. This is not a result of infectious virus production. Neither single-stranded RNA nor heat-denatured double-stranded RNA has this effect. Similar experiments with synthetic double-stranded polymers, poly(I).poly(C) and poly(A).poly(U), show that they are only slightly toxic at the concentrations used. The effect of the double-stranded RNA is nonspecific for cells of different origins. The implications of this finding in relation to the cytopathic effects of picornavirus and to cancer chemotherapy are discussed.

Action of interferon: kinetics and differential effects on viral functions

A highly purified rabbit interferon was tested for its capacity to inhibit various manifestations of infection of primary rabbit kidney (RK) cells with vesicular stomatitis (VS) virus. A kinetic analysis of the actinomycin-sensitive phase of interferon-induced cellular resistance revealed that RK cells could transcribe virtually all of the hypothetical antiviral messenger ribonucleic acid (mRNA) within 3 hr. Similar exposure to interferon reduced virus yield by 95 to 99% and markedly inhibited cytopathic effect on RK cells infected at a multiplicity of 10 or less. Interferon was less effective in blocking cytopathic effects when RK cells were infected at a multiplicity of 100. However, RK cells pretreated with the same amount of interferon and infected at a multiplicity of 100 failed to incorporate (3)H-amino acids into structural or nonstructural proteins of VS virus identified by polyacrylamide gel electrophoresis. Despite this inhibition of viral protein synthesis, interferon did not prevent the switch off by VS virus of cellular protein synthesis. The rapidity with which a high multiplicity of VS virus switched off cellular protein synthesis, even in cells rendered resistant to viral infection by interferon, is further evidence that this reaction is caused by an infecting virion component rather than by a newly synthesized viral product.