Ultrastructural Studies of Arenaviruses

Summary

The morphology and morphogenesis of all arenaviruses are remarkably similar: 1) virus particles are round or pleomorphic and vary widely in size (60 to 350 nm diameter), 2) they have a membranous envelope and surface projections, and 3) they have characteristic ribosome-like granules in varying numbers within an u~structured interior. Viral morphogenesis occurs via budding upon plasma membranes. Despite the distinctiveness of these virion characteristics, ultrastructural study of in vivo arenavirus infections proved difficult. In salivary gland cells of Calomys callosus, Machupo virus clearly matured upon plasma membranes and yielded salivary secretions containing large numbers of particles. However, in other tissues, such as lymph nodes, thymus, spleen, and central nervous system, where widespread infection of particular host species was anticipated from immunofluorescence, only modest numbers of virus particles were found. Inclusion bodies, easily identified in infected cells in culture, were impossible to discern in vivo. Some cytopathology was spatially associated with virus particles in lymphoid tissues. Ultrastructural study of in vivo arenavirus infections has proven excessively difficult without an immuno-electron microscopic contribution.