Unilateral Phenotypic Mixing of Envelope Antigens between Togaviruses and Vesicular Stomatitis Virus or Avian RNA Tumour Virus

Abstract

SUMMARY Vesicular stomatitis virus (VSV) in mixed infection with Sindbis virus (SbV) produces a proportion of phenotypically mixed particles (pseudotypes) containing VSV genomes and neutralization antigen(s) provided by SbV. This was demonstrated by heat-stabilization of the thermolabile tlB17 mutant of VSV and by neutralization with corresponding antisera. Phenotypic mixing is apparently unilateral because no SbV(VSV) pseudotypes could be found. Similarly, avian RNA tumour virus (ATV) in mixed infection with Sindbis virus produces a proportion of phenotypically mixed particles containing ATV genomes and SbV antigens, but no detectable particles containing SbV genomes and ATV envelope antigens. SbV acts as a helper virus for envelope-defective Rous sarcoma virus (RSV). When an avian helper virus is also present in the mixed infection, more than 90% of the RSV particles bearing SbV envelope antigens also bear ATV envelope antigens and are doubly neutralizable by antisera specific to either parent virus. In mixed infection of Langat virus and VSV, a proportion of doubly neutralizable particles containing VSV genomes were produced, but no pure pseudotypes. These results indicate that in mixed infections between enveloped animal viruses, VSV and ATV readily assemble foreign envelope glycoproteins, but that SbV does not. In certain phenotypically mixed virus stocks, only doubly neutralizable particles are found and are presumed to bear a mosaic of envelope antigens; in other stocks, particles can also be detected which are resistant to neutralization by antiserum specific to the envelope antigen encoded by their genomes, and these are presumed to represent pure pseudotypes.