On visna virus: purification and nucleic acid content

[An ELISA test for detection of maedi-visna antibodies. Comparative study with gel immunodiffusion and complement-fixation test]

An indirect microELISA test was performed for detection of maedi-visna antibodies in ovine and caprine species. The antigen consisted in viral particles, highly purified by successive ultracentrifugations. By comparative testing of 934 sera in ELISA and gel immunodiffusion, we found a good correlation between these two tests, and moreover, ELISA revealed another 11.3% of positive samples. The precocity of this ELISA was shown by experimental infection of sheep with different strains of maedi-visna: positive sera were detected 7 weeks post-infection, instead 4-5 months with gel immunodiffusion. The complement fixation test was compared with gel immunodiffusion and was found the less sensitive. This ELISA test appeared to be satisfactory, and may be used for early diagnosis of maedi-visna infection.

Wild-type temperature-sensitive and -resistant visna viruses: isolation and biological comparison

The plaques formed by wild-type (WT) populations of strain K485 visna (V-K485) virus, strain K796 visna (V-K796) virus, and of progressive pneumonia virus in sheep choroid plexus (SCP) cell cultures are heterogeneous in size. Plaque purification procedures showed that this heterogeneity was due to the presence of two biologically different viruses, large plaque-forming (Lpf) virus and small plaque-forming (Spf) virus. The V-K796 Lpf virus, the V-K796 Spf virus, and the V-K796 WT virus are antigenically similar to each other. Although the V-K796 Spf virus is less cytopathic than the V-K796 Lpf virus, both viruses have a 14- to 16-hour latent period in SCP cells and have similar rates of synthesis for the initial 24 to 30 hours after infection. The V-K796 Spf virus is considered a temperature-sensitive visna virus since, unlike the V-K796 Lpf virus, it does not produce plaques at 41 degrees C and it is more thermosensitive than the V-K796 Lpf variant. Population analyses of two strains of wild-type visna virus and one strain of progressive pneumonia virus demonstrated that these populations contain variants with the phenotypes of V-K796 Spf virus and V-K796 Lpf virus, that the Spf virus was not selected against in sheep, and that cultivation of the wild type visna viruses at 37 degrees C selected against the temperature-sensitive variants. The existence of the temperature-sensitive Spf viruses in these wild-type virus populations suggests that the Spf variants maintain the persistent visna virus infection in sheep and delay the development of the host's cell-mediated immune response to the viral proteins.

Immune reactivity of Visna virus-inoculated mice

Visna virus inoculation of mice before immunization with unrelated antigens caused suppression of humoral and cell-mediated immune responses. The immunosuppressive effect was dependent on the virus dose and time of administration. The number of "background" antibody-producing spleen cells increased slightly after inoculation of Visna virus. Heat treatment of Visna virus abolished its immunosuppressive effect. Replication of Visna virus in mice seemed to be a prerequisite for its immunomodulating effect.

Substructures and polypeptides of Visna virus

The protein of Visna virus, disrupted by 8 M guanidine hydrochloride and heating, was resolved into 10 polypeptides by agarose gel column chromatography in 6 M guanidine hydrochloride. Two of the peaks contained glycopolypeptides. Nonidet-disrupted virions were resolved into two fractions by potassium tartrate gradient centrifugation, with densities of 1.08 and 1.24 g/ml, respectively. About 70% of the viral DNA polymerase directed by added template was released into the light fraction, in which very little endogenous enzyme activity was detected. Also released into the light fraction were all of the glycopolypeptides, 50% of the viral RNA, and a part of each of the other viral protein components. The data indicate that extensive degradation of subviral structures occurred, even under mild conditions for virion disruption. The 1.24-g/ml fraction was composed of 50% of the viral RNA, most of the endogenous DNA polymerase activity (80%), and a major internal polypeptide (GuHCl6) with an estimated mol wt of 28,000. Two other polypeptides were also consistently detected in the heavy fraction, but they constituted less than 25% of the ribonucleoprotein complex, compared with 75% for GuHCl6.

Characterization of ribonucleic acid from visna virus

A single-stranded ribonucleic acid(s) has been isolated from purified virions of visna virus. It consists of two major components, namely 63S and "4S," under the conditions employed for ribonucleic acid (RNA) extraction. The 63S component can be converted to subunits by heat and dimethylsulfoxide treatments. Analyses by base composition indicate that the "4S" RNA isolated from visna virus is not a random breakdown product of the 63S component as a result of extraction, nor is it randomly derived from cellular RNA.