Ability of foot-and-mouth disease virus to form plaques in cell culture is associated with suppression of alpha/beta interferon.

A genetic variant of foot-and-mouth disease virus lacking the leader proteinase coding region (A12-LLV2) is attenuated in both cattle and swine and, in contrast to wild-type virus (A12-IC), does not spread from the initial site of infection after aerosol exposure of bovines. We have identified secondary cells from susceptible animals, i.e., bovine, ovine, and porcine animals, in which infection with A12-LLV2, in contrast to A12-IC infection, does not produce plaques; this result indicates that this virus cannot spread from the site of initial infection to neighboring cells. Nevertheless, A12-LLV2 can infect these cells, but cytopathic effects and virus yields are significantly reduced compared to those seen with A12-IC infection. Reverse transcription-PCR analysis demonstrates that both A12-LLV2 and A12-IC induce the production of alpha/beta interferon (IFN-alpha/beta) mRNA in host cells. However, only supernatants from A12-LLV2-infected cells have significant antiviral activity. The antiviral activity in supernatants from A12-LLV2-infected embryonic bovine kidney cells is IFN-alpha/beta specific, as assayed with mouse embryonic fibroblast cells with or without IFN-alpha/beta receptors. The results obtained with cell cultures demonstrate that the ability of A12-IC to form plaques is associated with the suppression of IFN-alpha/beta expression and suggest a role for this host factor in the inability of A12-LLV2 to spread and cause disease in susceptible animals.