Cell density-dependent expression of viral antigens during persistence of foot-and-mouth disease virus in cell culture

Immunofluorescence analyses of FMDV persistently infected BHK-21 cells showed that in cultures from early stages of the persistence (passage 15) only about 10% of cells displayed viral antigens, while at late stages (passage 100) no FMDV antigen-positive cells were found. Positive cells at passage 15 displayed a number of structural alterations that did not differ from those observed in lytically infected cells. In these monolayers, and remarkably, clusters of cells that exceeded confluence were associated with an enhancement of cells positive for FMDV antigens, suggesting cell density-dependent expression of viral antigens. Inhibition of virus spread by blocking endosomal acidification, or addition of neutralizing antibodies to the culture medium reduced the number of FMDV antigen-positive cells within the monolayers. These results suggest that extracellular virus transmission plays an important role during FMDV persistence in cell culture and that this process fits the characteristics of a carrier culture model.