ASSEMBLY OF AFRICAN SWINE FEVER VIRUS - QUANTITATIVE ULTRASTRUCTURAL ANALYSIS IN-VITRO AND IN-VIVO

African swine fever virus is an icosahedral double-stranded DNA virus which replicates in the cytoplasm of porcine monocytic cells. Progeny virus particles, like poxviruses and iridoviruses, are assembled in cy toplasmic inclusions, termed virus factories. The formation of these s tructures in susceptible cells infected in vitro with pathogenic and a ttenuated ASFV isolates has been studied by semiquantitative and quali tative electron microscopy, Recognizable virus factory elements were d etected by 8 hr postinfection (hpi) and increased significantly in pro file area between 12 and 18 hpi, The number of virus particles associa ted with the virus factories also increased significantly between 12 a nd 24 hpi. These included particles with (''full'') and without (''emp ty'') a nucleo-protein core. Similar results were obtained for both pa thogenic (Malawi) and attenuated (Uganda) virus isolates, but the repl ication of pathogenic virus in the macrophage was more efficient than that of the attenuated virus in a porcine kidney cell line, where a re latively higher percentage of defective ''empty'' particles were obser ved. Analysis of virus factory formation was also done directly on lun g and liver tissue from a pig infected with the highly pathogenic Mala wi virus isolate. The in vive data for virus factory area and particle number in both pulmonary intravascular macrophages and liver Kupffer cells at Day 4 postinfection were similar to those observed in vitro, As expected, using postembedding immunoelectron microscopy, DNA (both cellular and viral) was detected in the cell nucleus, cytoplasmic viru s, and mature extracellular virus. More interestingly, DNA was absent in the cytoplasm, but readily observed in virus factories. This DNA, w hich we presume to be viral, was found in close association with membr ane-like material and ''empty'' particles, suggesting that the virus D NA may enter a partially formed capsid which is then sealed in order t o develope into a fully assembled particle. According to this hypothes is, ASFV morphogenesis involves the initial formation of ''empty'' par ticles within the virus factory. The adjacently formed nucleo-protein material is then inserted into the ''empty'' particles, as has been de scribed for poxviruses. These particles then mature in to the ''full'' particles and leave the virus factory as a prelude to release from th e cell by budding. (C) 1996 Academic Press, Inc.