Comparative structure, morphogenesis and biological characteristics of the respiratory syncytial (RS) virus and the pneumonia virus of mice (PVM)

Abstract

A comparative study of the respiratory syncytial (RS) virus and the pneumonia virus of mice (PVM) discloses very few dissimilar details of structure, morphogenesis and biological characteristics. Peak titer of the RS virus on HEp-2 cells at an input multiplicity of 5∶1 was obtained between 24 and 72 hours whereas that of PVM on Vero cells at an input multiplicity of 1.5∶1 was reached between 48 and 96 hours. The replication of PVM is followed by a definite but still incomplete cytopathic effect at 11 days while that of RS virus results in a complete cell degeneration after 3 days. These two viruses show the same kind of eosinophilic cytoplasmic inclusion bodies with hematoxylin-eosin. These inclusions are stained yellow-green with acridine orange. Indirect immunofluorescence reveals the presence of viral antigens only in the cytoplasm of the cells, detectable as early as 8 hours in the case of RS virus and at 24 hours in the case of PVM. By negative staining, the diameter of the RS virus spherical particle is 100 to 350 nm and that of PVM 100 to 200 nm. In addition to these spherical forms, pleomorphic and filamentous forms are frequently encountered, the latter predominating in PVM preparations. The diameter of both RS virus and PVM nucleocapsids averages 13.5 nm and the pitch of the helix is 6.5 nm compared to 17.5 nm and 5.0 nm for parainfluenza type 2 used as an internal control. The RS virus and PVM envelopes are studded with projections of 12 nm in length, 10 nm apart in the case of RS virus but only 6 nm apart in the case of PVM. In ultrathin sections, RS virus and PVM appear to bud from the cytoplasmic membrane mainly into filamentous forms, diameter 100 nm including a fringe of projections, 12 nm in length. Nucleocapsids within the filamentous forms appear as dots in cross sections or parallel fibers in longitudinal sections, diameter 12 nm in both cases. Cytoplasmic inclusions containing the same 12 nm structures are seen in both cases, occasionally contiguous to the cytoplasmic membrane with budding viral particles. The difference in surface structure of RS virus and PVM may explain their different hemagglutinating properties. These two viruses could be classified in a subgroup for which the name metamyxovirus, previously proposed, would seen appropriate.