Previous studies demonstrated that both cellular and viral mutants are selected during maintenance of
persistent infections established in murine L cells with high-passage stocks of mammalian reoviruses. In particular, when one culture was cured of
persistent infection, the resulting cells were found to support the growth of viruses isolated from persistently infected cultures (termed PI viruses here) better than that of wild-type (wt) viruses (R. Ahmed, W. M. Canning, R. S. Kauffman, A. H. Sharpe, J. V. Hallum, and B. N. Fields, Cell 25:325-332, 1981). To address the nature of cellular and viral mutations selected during maintenance of persistent
reovirus infections, we established independent, persistently infected cultures with L cells and high-passage stocks of wt reovirus. These cultures served as sources of new PI viruses and cured cells for study. We found that although wt viruses grew poorly in cured cells when
infection was initiated with intact virions, they grew well in cured cells when
infection was initiated with infectious subvirion particles generated from virions by in vitro treatment with
chymotrypsin. This finding indicates that the block to growth of wt viruses in cured cells involves an early step that is unique to
infection by virions, such as proteolytic processing in an endocytic compartment. We also found that PI viruses grew better than wt viruses in L cells treated with
ammonium chloride, a weak base that inhibits the pH decrease in endosomes and lysosomes. Because
ammonium chloride blocks an early step in
infection by intact virions, probably the proteolytic processing of viral outer
capsid proteins by
acid-dependent cellular
proteases in late endosomes or lysosomes, this finding indicates that PI viruses differ from wt viruses with respect to viral entry into cells. Therefore, these results indicate that both cells and viruses evolve mutations that affect one or more early steps in the viral growth cycle during maintenance of L-cell cultures persistently infected with reoviruses.