Effect of lysosomotropic compounds on early events in foot-and-mouth disease virus replication

The effect of three lysosomotropic compounds, chloroquine, monensin and NH4Cl, on the replication of foot-and-mouth disease virus (FMDV) type A12 was studied. Viral replication was almost totally inhibited by 0.5 mM chloroquine, 50 microM monensin, or 25 mM NH4Cl. Monensin and NH4Cl affected replication when added either before or within the first hour of infection. Chloroquine, however, still inhibited viral replication when added up to 2.5 h after infection. Assays of binding of radiolabeled virus to cells showed that these compounds had no effect on viral adsorption. Neither monensin nor NH4Cl had any significant effect on cellular protein synthesis, but there was no evidence of viral protein synthesis in cells infected in the presence of these compounds. In contrast, chloroquine inhibited both cellular and viral protein synthesis. Eclipse assays, performed in the presence of the compounds, showed that while chloroquine and NH4Cl had little effect on cell-induced degradation of incoming virions to 12 S protein subunits, monensin inhibited this reaction. The replication of representative members of all seven serotypes of FMDV was inhibited by monensin although some types were less sensitive to the compound than others. These results are consistent with a model which postulates that viral eclipse is the result of acidification of endocytic vesicles which degrade entrapped virions to 12 S protein subunits resulting in the release of genome RNA.