- Volume 28, Issue 2, 1975

Using an MLg cell line originating from a single newborn ddY mouse, N-tropic C-type virus able to infect the cells of origin was induced by 5-iodo-2′-deoxyuridine (IUdR). All the clones isolated from the MLg cell line released the virus after IUdR-treatment. There was more than a 100-fold difference in the virus inducibility between the highly and lowly inducible clones. The [125I]-IUdR incorporation was, however, the same for the both types of clones.

When the induced virus was titrated in the S + L - C182 cells, the XC plaque titre was about tenfold higher than murine sarcoma virus rescue focus titre.

At concentrations that inhibit bacterial growth, some antibiotics including gentamicin completely inhibited virus multiplication in protoplasts, and other antibiotics partially inhibited virus multiplication. The inhibition caused by each antibiotic was largely prevented by adding a divalent metal; MnCl2 was more effective than CaCl2 and other salts of divalent metals when added at 10 mm to the incubation medium. When added immediately after infection, 1 µg/ml of gentamicin halved the final virus concentration and 3 µg/ml completely inhibited virus multiplication, although 10 µg/ml was required to stop bacterial growth. Gentamicin inhibited virus multiplication even when added 24 h after virus inoculation. Also, when protoplasts were exposed to gentamicin for only 1 or 2 h, either immediately after inoculation or 2 h later, the virus concentration was considerably decreased.

Gentamicin seemed not to affect virus multiplication in whole plants. Sap from Dianthus barbatus also strongly inhibited virus multiplication in protoplasts but, unlike gentamicin, it acted in the presence of MnCl2. By contrast, chelating agents such as 1 mm-EDTA or 5 mm-potassium citrate were strong inhibitors of virus multiplication that were inactive in the presence of MnCl2. It is suggested that gentamicin and other antibiotics may chelate metals from the protoplast membranes, thus disorganizing their function and affecting virus multiplication.

The technique of virus RNA-cellular DNA hybridization in solution with DNA excess was used to compare the nucleotide sequences of the 70S RNA genome of the Kirsten mouse sarcoma virus (Ki-MSV) with that of mouse erythroblastosis virus (MEV) which gave rise to Ki-MSV after in vivo propagation in rat. It is suggested that a loss of about 30% of the genomic sequences of MEV with a concomitant gain of roughly equal amounts of rat-specific sequences in a genetically stable recombinant state led to the formation of Ki-MSV.

A guinea pig herpes-like virus (GPHLV) has been isolated from randomly selected guinea pigs. This virus is serologically related, if not identical, to the guinea pig herpes-like virus isolated by Hsiung-Kaplow. The frequency of latent infection in guinea pigs was found to be highly variable among animals of the same commercial origin. Ultrastructural studies have shown that the morphological development of this virus was similar to that reported for other herpes viruses in the early stages, but frequently differed at the envelopment stage. In the cytoplasm, virus particles were associated with electron-dense zones from which they acquired a thick and rough envelope.

The effect of interferon on expression of Epstein–Barr virus (EBV) early gene functions was investigated. The ‘early antigen’ synthesis which follows either EBV superinfection of established lymphoid cell lines or 5′-iododeoxyuridine activation of the intrinsic EBV genomes harboured by these cells could be suppressed with interferon. In contrast, the spontaneous early antigen expression that occurs in a few per cent of the cells in the producer cell lines could not be blocked with interferon.

The lymphoid cell lines tested differed in their ability to acquire an antiviral state after exposure to interferon. Several cell lines were also growth inhibited by the interferon preparations. The antiviral and growth inhibitory activities of different interferon preparations could not be separated by a number of criteria.

Established human lymphoid cell lines, many of which spontaneously produce interferon, differ in the efficiency by which they allow expression of Epstein–Barr virus (EBV) lytic functions. Six EBV carrying lymphoid cell lines, selected to either be extremely susceptible or very refractory to EBV superinfection, were tested for spontaneous interferon production. Only the three cell lines which were poorly superinfectable with EBV were found to produce interferon. These same three lines could not be induced to express EBV-specific early antigens from intrinsic EBV genomes. It is suggested that interferon acts as a negative control factor affecting a cell’s susceptibility to EBV.

Mice were infected i.c. or i.p. by an avirulent clone of Semliki Forest virus at various times before or after the i.p. administration of a single dose of cyclophosphamide. Consequent changes in the patterns of viraemia and antibody production are interpreted in terms of the potentiation of the initial infection or the decreased resistance to subsequent lethal challenge.

These changes demonstrate two distinct modes of immunomodification determined by the time of cyclophosphamide administration in relation to virus infection, but not correlated with the death or protection of individual mice.

These results are discussed in relation to the early regulatory functions of T and B cells which may influence the expression of virulence but escape conventional assay during the critical first 3 days following infection.

Studies in mice of the modes of immunomodification imposed by cyclophosphamide, have been extended to comparative studies with Myocrisin, l-asparaginase and interferon. It has been shown for mice infected i.p. or i.c. by an avirulent clone of SFV, that the potentiation of disease may be marked by many distinct changes in the type and rate of response.

For low i.p. doses of virus, enhancement (Myocrisin, l-asparaginase) or impairment (interferon) of the efficiency of infection may be associated with death (potentiation by Myocrisin) or protection (immunoenhancement by l-asparaginase).

For higher doses of virus the increased mortality after infection (primary potentiation) is determined within 2 or 3 days and appears to be due to inhibition of phagocytosis (Myocrisin and l-asparaginase) and of T cell functions (l-asparaginase and cyclophosphamide). The increased incidence of death after challenge (secondary potentiation) appears to be due to inhibition of B cell functions (cyclophosphamide) associated with suppression of antibody synthesis and persistence of viraemia.

These results are discussed in relation to the expression of virulence by a heterogeneous and replicating antigen. The critical cellular and humoral changes which occur within 2 or 3 days of infection are emphasized.

A series of ω-aminoalkyl-agaroses differing in the number of carbon atoms in the alkyl chains was prepared and used for chromatography of hepatitis B surface antigen (HBsAg). HBsAg was separated from the major part of serum proteins by adsorption to columns of 1,9-diaminononane or 1,10-diaminodecane linked to agarose beads (Sepharose 4B or 2B) followed by elution with 4 m-NaSCN.

Tobacco plants grown for 12 h at 25 °C in light of 10800 lux alternating with 12 h at 20 °C in darkness were reliable sources of mesophyll protoplasts, which were very susceptible to infection with tobacco rattle virus. Increasing the photoperiod or the light intensity decreased susceptibility to infection, and increasing or decreasing the photoperiod increased the fragility of the protoplasts. Ease of infection with raspberry ringspot virus seemed to parallel that with tobacco rattle virus. The optimum defined environment gave protoplasts that were at least as suitable for infection experiments as the best from glasshouse grown plants.

We have already discussed the presence and functioning of a interferon system in primary cultures of tortoise kidney cells and described some physico-chemical properties of the induced interferon elsewhere (Galabov, Savov & Vassileva, 1973; Galabov, Petrunova & Savov, 1973; Galabov & Savov, 1973). In order to more completely characterize the interferon producing system in such poikilothermic vertebrates, a study was carried out using tortoise (Testudo graeca) peritoneal cells.

Incubation with antiserum to chick embryo (CE) cells, in the presence of complement, inactivates Rous sarcoma virus (RSV) produced by heterokaryons formed by non-permissive RSV-transformed hamster cells and CE cells as well as RSV produced by CE cells. No inactivation of RSV produced by heterokaryons is observed following incubation with antiserum to the transformed hamster cells, plus complement. Hence, the envelope of RSV activated in heterokaryons, as that of RSV produced by CE cells, must contain a surface antigen of the CE cell, and the virus must mature only, or preferentially, at chicken-specific sites of the heterokaryon surface.