Cyclophilin inhibitors block arterivirus replication by interfering with viral RNA synthesis

Virus replication strongly depends on cellular factors, in particular, on host proteins. Here we report that the replication of the arteriviruses equine arteritis virus (EAV) and porcine reproductive and respiratory syndrome virus (PRRSV) is strongly affected by low-micromolar concentrations of cyclosporine A (CsA), an inhibitor of members of the cyclophilin (Cyp) family. In infected cells, the expression of a green fluorescent protein (GFP) reporter gene inserted into the PRRSV genome was inhibited with a half-maximal inhibitory concentration (IC₅₀) of 5.2 μM, whereas the GFP expression of an EAV-GFP reporter virus was inhibited with an IC50 of 0.95 μM. Debio-064, a CsA analog that lacks its undesirable immunosuppressive properties, inhibited EAV replication with an IC₅₀ that was 3-fold lower than that of CsA, whereas PRRSV-GFP replication was inhibited with an IC₅₀ similar to that of CsA. The addition of 4 μMCsA after infection prevented viral RNA and protein synthesis in EAV-infected cells, and CsA treatment resulted in a 2.5-to 4-log-unit reduction of PRRSV or EAV infectious progeny. A complete block of EAV RNA synthesis was also observed in an in vitro assay using isolated viral replication structures. The small interfering RNA-mediated knockdown of Cyp family members revealed that EAV replication strongly depends on the expression of CypA but not CypB. Furthermore, upon fractionation of intracellular membranes in density gradients, CypA was found to cosediment with membranous EAV replication structures, which could be prevented by CsA treatment. This suggests that CypA is an essential component of the viral RNA-synthesizing machinery.