- Volume 66, Issue 4, 1985

The infectivity of tissue culture-passed mouse cytomegalovirus (MCMV) for resident mouse peritoneal macrophages in the presence of serial dilutions of antiviral antibody was studied by fluorescent antibody staining and virus yields. Although MCMV was neutralized by high concentrations of antiserum, there was a twofold enhancement of infectivity by subneutralizing antibody concentrations. On further dilution of antiserum, significant neutralization appeared again. When F(ab′)2 fragments of anti-MCMV IgG were used or when macrophages were pretreated with monoclonal antibody to Fc receptor, there was no enhancement, and no neutralization at high dilutions of antiserum. This suggests that both enhancement and high dilution neutralization are mediated via the Fc portion of IgG and Fc receptors of macrophages. Tissue culture-passed virus whose infectivity for macrophages was reduced by high dilutions of antibody was converted to a more infectious state by addition of anti-mouse immunoglobulin. Similar results were obtained with salivary gland virus, which is less infectious for macrophages and is coated with non-neutralizing antibody. Tissue culture-passed virus is known to be less virulent for suckling mice and more infectious for macrophages than salivary gland-passed virus. When tissue culture-passed virus was coated with appropriately diluted antiviral antibody, not only was its infectivity for macrophages reduced, but it also became more virulent than control virus treated with normal mouse serum. These results are interpreted in terms of the optimal density of Fc on the virus-immunoglobulin complex in relation to the density of Fc receptors on macrophages.

A comparison was made of the virulence in vivo and the infectivity in vitro for macrophages of (i) tissue culture-passed mouse cytomegalovirus (MCMV), (ii) salivary gland virus taken 3 weeks after infection (SGV 3w), and (iii) salivary gland virus taken 1 week after infection (SGV 1w). Salivary gland virus (3w) is known to be coated with non-neutralizing antibody, and is more virulent for newborn suckling mice and less infectious for macrophages than tissue culture-passed virus (TCV). Properties of SGV 1w were similar to those of TCV. Infectivity of SGV 3w for macrophages was significantly enhanced by treatment with trypsin (10 µg/ml) and at the same time virulence was lost. When SGV 1w or TCV were treated with trypsin the infectivity for macrophages was unaltered as long as the inoculum was adjusted to contain the same number of p.f.u. as assayed in MEF. Trypsin-treated SGV 3w was neutralized not by rabbit anti-mouse IgG Fc, but by anti-Fab, whereas untreated virus was neutralized by both of these anti-mouse immunoglobulins. These results are discussed in terms of the association of virulence with virion-bound antibody and Fc receptors on macrophages.

The initial stages of interferon induction in mouse L cells do not require protein synthesis; all steps in the induction process up to and including the transcription of interferon mRNA occurred in the presence of inhibitors of protein synthesis. In contrast, interferon induction in primary chick embryo cells aged in vitro does require a reaction that depends upon protein synthesis, unique in that it is not required for virus replication or the action of interferon.

Recombinant human interferon-γ (HuIFN-γ) injected into rabbits disappeared from the circulation more rapidly than natural IFN-γ. The latter displayed an initial decay curve more rapid than that for natural HuIFN-α although 4 h after injection plasma levels were similar. This result suggests that IFN-γ has pharmacokinetic properties different to those of IFN-α which may be explained by considerable and simultaneous hepatic and renal catabolism. Surprisingly, the hepatic uptake of recombinant (unglycosylated) IFN-γ was more marked than uptake of natural IFN-γ. Moreover, both IFN-γ preparations were cleared by the isolated and perfused kidney and once again the recombinant IFN disappeared more rapidly. This result does not conform with the suggestion that IFN-γ exists as a tetramer which would not be filtered by the glomerulus, but is consistent with the pharmacokinetic behaviour shown in vivo.

The priming activities of human IFN-α, IFN-β and IFN-γ were compared on the same antiviral basis using human buffy coat leukocytes stimulated with Sendai virus or concanavalin A (Con A) to produce IFN-α or IFN-γ, respectively. Pretreatment of leukocytes with any type of IFN enhanced their IFN-α and IFN-γ production, but IFN-α and IFN-β had more potent priming activities than IFN-γ. IFN-α and IFN-γ did not potentiate the priming activity of each other in either the IFN-α- or the IFN-γ-producing system. Pretreatment of leukocytes with relatively high doses of IFN-α or IFN-β (1000 to 3000 IU/ml) resulted in a 40- to 50-fold increase in the IFN-γ production of Con A-stimulated leukocytes. This observation will be of use in producing IFN-γ with a high titre.

The intracellular localization of the major capsid protein (ICP5) of herpes simplex virus was studied during virogenesis. Except for a brief period at the onset of synthesis, this protein was found almost exclusively inside the nucleus. Its localization was not at random since 80% was tightly bound to the nuclear matrix as early as 4 h after infection. Discrete modifications of the fluorescence pattern occurred in an orderly fashion during the progression of the infection. Immunoelectron microscopic studies using Protein A-gold labelling demonstrated that this protein is synthesized on cytoskeleton-bound polyribosomes and accumulates in the central part of the nucleus where formation of viral capsids occurs; no gold particles were found in association with the peripheral chromatin or with the nucleolus.

IgM which neutralized influenza virus infectivity by over 99% prevented the attachment of only half the virus population to BHK cells at 37 °C. However, the half of the population that attached to cells was not internalized. Loss of infectivity brought about by IgM is thus totally different from that caused by neutralizing IgG which does not inhibit attachment or penetration.

A number of cloned viral preparations isolated from Rauscher virus-producing JLS-V5 cells were compared in their competence to induce different types of leukaemias. All preparations were able to induce myeloid leukaemias, but the induction of lymphatic or erythroid leukaemias was also observed. Serial infection of newborn mice with either cell-free extracts or serum from animals suffering from a myeloid leukaemia did not result in the occurrence of relatively more myeloid leukaemias nor did the infection with virus harvested from ascites fluid of permanent myeloid cell lines. It appears that the mechanism by which myeloid leukaemias are induced is not virus-specific.

Actinomycin D (AD) administered to cowpea mosaic virus-infected cowpea protoplasts immediately after inoculation inhibited virus multiplication, whereas late in the incubation period neither virus multiplication nor host DNA transcription were affected. The data obtained suggested that neither virus uncoating nor encapsidation were inhibited by AD. The inhibition of virus multiplication was manifested as a decrease in the level of progeny viral nucleoproteins and (+) and (-) viral RNAs. Cordycepin strongly inhibited coat protein production and (+) and (-) viral RNA synthesis throughout the incubation period.

Gel-separated RNAs of the wild-type (WT), Lab 1 and Lab 2 isolates of soil-borne wheat mosaic virus (SBWMV) were transferred to nitrocellulose filters and hybridized with specific cDNA. RNA I, but not RNA II, from all three SBWMV isolates hybridized to cDNA copied from SBWMV WT RNA I. RNA II from all three SBWMV isolates hybridized to cDNA copied from SBWMV WT RNA II, confirming the hypothesis that the Lab 1 and Lab 2 isolates arose by deletion of part of RNA II of WT virions. RNA II from SBWMV WT (0.5L RNA) or from SBWMV Lab 1 (0.35L RNA) did not bind to oligo(dT)-cellulose in high-salt buffer, or stimulate cDNA synthesis when primed with oligo(dT), suggesting that it has no 3′-poly(A) or oligo(A) sequences. No genome-linked protein was detected at the 5′ ends of either RNA II by iodination, nor did the cap analogue 7-methylguanosine phosphate have an effect on their translation in vitro, indicating no cap structure at their 5′ ends. RNA II was 5′ end-labelled with [32P]ATP and T4 polynucleotide kinase after dephosphorylation with bacterial alkaline phosphatase, but all four bases were labelled, indicating heterogeneity at the 5′ end.

Feeding aphids on plants infected with carnation etched ring virus (CERV) or figwort mosaic virus (FMV) prior to feeding on plants infected with aphid non-transmissible isolates of cauliflower mosaic virus (CaMV) facilitated transmission. This indicates that CERV and FMV code for aphid transmission factors which can substitute for the defective factors in the non-transmissible CaMV isolates.

Virus-specific double-stranded replicative form (RF) RNA was isolated from cowpeas infected with cowpea mosaic virus. The RF was assayed for infectivity in a local lesion host and a systemic host. In neither host was undenatured RF infectious, although infectivity was restored upon denaturation.