In this study, SV5 infection of Balb/c mouse fibroblast cells has been used as amodel system to investigate the possible mechanisms underlying the establishment andmaintenance of Paramyxovirus persistence.It was found that following entry to these cells, the virus initiated a wave oftranscription and replication, similar to that of a permissive infection, in which normallevels of each of the virus proteins were synthesized. However, by 48-72 hours postinfection (p.i.) there was an almost complete cessation of virus mRNA and proteinsynthesis. Despite the decrease in virus activity, full length viral genome RNA and P andNP, the proteins involved in transcription and replication, could be detected at consistentlyhigh levels up to 5 days p.i., although the levels of HN, M, F and V declined.Immunofluorescence analysis supported these data showing that at later times p.i.although there were some cells positive for all the viral proteins, a high proportion of cellswere strongly positive for NP, L and P, but negative for M, F and HN. In these cells, NP, Land P were often located in discrete cytoplasmic foci. These results suggested that thepersistently infected cell population consisted of some cells in which the virus was activeand other in which it was quiescent within cytoplasmic inclusions.A series of cell lines was established from a monolayer of Balb/c cells that had beeninfected at a high multiplicity. Immunofluorescence studies showed only a minority ofcells in these clones to be infected with virus, indicating that during division, not alldaughter cells became infected. Of the infected cells, some were positive for all the viralproteins, while others were positive for only NP and P. Co-cultivation of the cloned cellswith Vero cells, which are permissive for SV5 replication, rapidly yielded non-defectivevirus, suggesting that the virus was active in some cells. These results suggested that thepersisting virus was in a state of flux, able to reside as inclusions of inactive nucleocapidsfrom which it could reactivate to initiate a new round of infection.Experiments aiming to determine if the persistently infected cells were resistant toimmune attack demonstrated that cells at 5 days p.i., in which the majority of cells werequiescently infected, were less susceptible to immune lysis than cells at 1 day p.i. in whichthere was ongoing protein synthesis.Further experiments were carried out both to try to determine what had induced thepersistent state in mouse cells and also to examine factors which might induce a similarstate in different cell lines.
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