STUDIES OF THREE VARIANTS OF MENGO ENCEPHALOMYELITIS VIRUS. 3. EFFECT OF OVERLAY AND POLYANIONS OF PLAQUE SIZE

Antiviral activity of carrageenan on hepatitis A virus replication in cell culture

Sulphated polysaccharides such as iota-, lambda- and kappa-carrageenans showed a potent inhibitory effect on the replication of hepatitis A virus (HAV) in the human hepatoma cell line PLC/PRF/5. No cytotoxic effects were detected with concentrations of carrageenans up to 200 micrograms/ml. The selectivity indices of these substances, calculated as the ratio of the dose that reduced the number of viable cells to 50% (CD50) to the effective dose that inhibited 50% of viral antigen expression (ED50), were greater than 400 with iota-carrageenan, greater than 222 with lambda-carrageenan and greater than 10 with kappa-carrageenan. The selectivity index of ribavirin (reference substance) was only 5. The 3 types of carrageenans resulted in concentration-dependent reduction of HAV-antigen expression and HAV infectivity. lota-and lambda-carrageenan emerged, from the present study, as promising candidates for chemotherapy of acute hepatitis A.

Animal viruses promote the entry of polysaccharides with antiviral activity into cells

Charged polysaccharides such as heparin and carrageenan show potent antiviral activity in cultured cells. Labelled [3H]heparin binds to several virion particles as evidenced by Sepharose 6B chromatography. This binding is inhibited by carrageenan. The complex [3H]heparin-HSV1 virions is able to enter cells. Thus, almost no entry of [3H]heparin is observed in control HeLa cells, whereas in the presence of HSV1 or poliovirus the amount of radioactivity internalized is enhanced. This internalization is inhibited by carrageenan, suggesting that these two sulphated polysaccharides bind to the same sites on virion particles and may share a similar antiviral mechanism of action.

Comparison of agar and agarose preparations for mengovirus plaque formation

An agarose overlay yielded mengovirus plaques earlier and in greater size and number than overlays of chemically undefined agars with or without enhancers. Marked variability in plaque-forming efficacy of commercial agarose preparations was noted.

Rapid plaque assay for encephalomyocarditis virus

A liquid overlay plaquing technique is described which offers a rapid and simple plaque assay system for small plaque variants of encephalomyocarditis virus.

Mechanism of enhancement of virus plaques by cationic polymers

It has been assumed that plaque enhancement by cationic polymers is due to their binding of sulfated polysaccharides in agar. However, viruses that are enhanced by cationic polymers, diethylaminoethyl-dextran, and protamine were found not to be inhibited by polyanions in agar under the usual overlay conditions. In the case of adenovirus, enhancement by protamine seems to be due to the protamine serving as a source of arginine; enzymes released from the cultured cells digest the protamine and provide a reservoir of arginine for the cells. Other viruses (herpes and echovirus types 3, 4, 5, and 6) known to be susceptible to agar inhibitors were found to be enhanced by cationic polymers even under starch gel and methylcellulose overlays, which are free of polyanions. Since cationic polymers enhance the diffusion of virus through agar or starch gel, plaque enhancement seems to be the result of the gel becoming positively charged so that viruses can move effectively through them. The observation that starch gel and methylcellulose enhance plaque formation with viruses known to be inhibited under agar was also reinvestigated. When the consistency of the agar gel was reduced to the same viscosity of starch gel and methylcellulose overlays, the same plaque counts and sizes were observed under all three overlays.

Properties of Venezuelan equine encephalomyelitis virus accompanying attenuation in vitro

Virus obtained during serial plaque passage of the virulent parent egg seed (PES) of the Trinidad strain of Venezuelan equine encephalomyelitis (VEE) virus produced only large plaques during either 3 serial plaque passages in chick fibroblasts or 10 plaque passages in L cells, and was lethal for mice by the intraperitoneal route. Virus showing these characteristics was designated the stable large-plaque (Ls) type. In contrast, virus obtained during serial plaque passage of the attenuated 9t strain in chick fibroblasts formed only very small plaques and was not lethal for mice by the intraperitoneal route. Virus showing these properties was designated the stable small-plaque (Ss) type. Under other passage conditions, however, large-plaque virus that yielded about 90% large and 10% small plaques was obtained; this virus was designated the unstable large or Lu type because it differed from the Ls type, which yielded only large plaques. The Lu type continued to yield the same ratio of large to small plaques for several plaque-to-plaque passages. In addition, small-plaque virus that yielded both large and small plaques and that showed a reduced capability to infect mice was also recovered. This virus was designated the unstable small or Su type because it differed from the Ss type in its higher level of virulence and in its plaque-forming properties. Thus, based upon the properties of virulence for mice and plaque size, four viral types could be discerned. The evidence suggests that serial passage in cell culture imposed environmental pressures that sequentially selected the following viral types: Ls, Lu, Su, and Ss.

Isolation of a series of Mengo virus variants differing in sensitivity to polyanions

The isolation of six distinct new plaque variants from pools of S- and M-Mengo (small- and medium-plaque variants respectively) is described. The size of plaques produced in L cell monolayers under nutrient agar by five of the six new variants is strongly inhibited by the addition to the overlay of dextran sulfate of molecular weight 500,000. The sixth, although similar to L-Mengo in its virulence to mice and in having plaques that are almost unaffected by the addition of protamine or dextran sulfate to the overlay, can be distinguished from the latter by the larger and more uniform plaques it produces. The marked heterogeneity in the size of plaques produced by L-Mengo appears to be an inherent property of this variant, since all attempts to isolate other plaque variants from L-Mengo pools have been unsuccessful.The intraperitoneal LD50's of all six new variants in 14- to 16-g mice was determined. There is no complete correlation between the virulence of these variants and their sensitivity to inhibition by dextran sulfate.

Antibiotic activity in the presence of agar

Agar has been shown to interfere with the activity of some antibiotics against Staphylococcus aureus. This interference has been observed as an increase in the minimal inhibitory concentration and in the diameter of the zone of inhibition. Purifying the agar with water extractions substantially reduced this adverse effect.

Enhancement of rhinovirus plaque formation in human heteroploid cell cultures by magnesium and calcium

Fiala, Milan (University of Washington, Seattle), and George E. Kenny. Enhancement of rhinovirus plaque formation in human heteroploid cell cultures by magnesium and calcium. J. Bacteriol. 92:1710-1715. 1966.-A reproducible macroplaque assay for six M and three H strains of rhinoviruses has been developed in several human heteroploid cell lines. Plaques were produced only with suitable solidifying agents: purified agar (Ionagar, Agarose) or methylcellulose. Plaque development was greatly enhanced by increasing Mg(+2) to 30 to 40 mm. Diethylaminoethyl (DEAE) dextran also increased plaque sizes, and the effects of Mg(+2) and DEAE dextran were additive. In addition, Ca(+2) substituted for Mg(+2). The suitability of human heteroploid cell lines for rhinovirus plaque assay varied greatly, ranging from insensitivity through partial to complete sensitivity. This assay was six to seven times more sensitive than an end point tube assay. These results indicate that potentiation of plaque formation by Mg(+2) known for some enteroviruses can also be extended to the rhinovirus group of picornaviruses.

Plaque enhancement of enteroviruses by magnesium chloride, cysteine, and pancreatin

Wallis, Craig (Baylor University College of Medicine, Houston, Tex.), Fred Morales, Joycelyn Powell, and Joseph L. Melnick. Plaque enhancement of enteroviruses by magnesium chloride, cysteine, and pancreatin. J. Bacteriol. 91:1932-1935. 1966.-Plaque formation of 21 echoviruses (types 1-6, 9, 13, 15-19, 23-26, 29-32) and 8 coxsackieviruses (B1-6, A7, and A9) was enhanced by increased concentrations of MgCl(2), l-cysteine, and pancreatin in agar overlay medium. In most cases, cationic and anionic polymers (diethylaminoethyl dextran, dextran sulfate, or protamine sulfate) were ineffective. All strains of poliovirus and group B coxsackieviruses were enhanced under agar by MgCl(2). Five of the eight coxsackieviruses tested were also enhanced by cysteine or pancreatin. Certain enteroviruses, which have been difficult to assay by plaque method, can now be quantified effectively by incorporation of additives such as MgCl(2), cysteine, or pancreatin into the overlay medium.