Tilmicosin inhibits the infections of currently prevalent porcine reproductive and respiratory syndrome viruses via the downregulation of CD163 expression.

Porcine reproductive and respiratory syndrome virus (PRRSV) has a disastrous impact on the global swine industry. Even though PRRS commercial vaccines can provide homologous protection, broadly effective anti-PRRSV strategies are still in urgent need. Tilmicosin is a semi-synthetic macrolide antibiotic that was noticed to have anti-PRRSV capacity in the field. However, the antiviral activities of tilmicosin against currently prevalent PRRSV isolates have not been systematically evaluated. More importantly, the anti-PRRSV mechanisms of tilmicosin are not clarified. In this study, in vitro experiments showed that tilmicosin dose-dependently inhibits the replications of prevalent PRRSV (NADC34-like PRRSV-2, NADC30-like PRRSV-2, HP-PRRSV-2, and PRRSV-1) isolates in different target cells (PAMs and Marc-145). In vivo studies supported that tilmicosin inhibits NADC34-like PRRSV-2 replication and reduces pathogenic lesions in weaned piglets. Remarkably, the transcriptomic analysis provided the first clue that PRRSV key receptor CD163 was significantly downregulated by tilmicosin, which was further confirmed by qPCR and WB detections. In addition, anti-inflammatory factors (such as IL-10) were downregulated, while pro-inflammatory factors (such as IL-1β) were upregulated after tilmicosin treatment. Flow cytometric analysis indicated that anti-inflammatory may reverse the downregulation effect of tilmicosin on CD163, while pro-inflammatory mediators can enhance the downregulation effect of tilmicosin on CD163, which leads to increased and decreased replication of PRRSV, respectively. These findings demonstrate that tilmicosin can inhibit PRRSV replication via the reduction of CD163 expression, which provides new insights into anti-PRRSV mechanisms of tilmicosin.