A comparison of the impact of Shimen and C strains of classical swine fever virus on Toll-like receptor expression

Classical swine fever is one of the most important swine diseases worldwide and has tremendous socioeconomic impact. In this study, we focused on the signalling pathways of Toll-like receptors (TLRs) because of their roles in the detection and response to viral infections. To this end, two classical swine fever virus (CSFV) strains, namely the highly virulent CSFV Shimen strain and the avirulent C strain (a vaccine strain), were employed, and the expression of 19 immune effector genes was analysed by real-time PCR, Western blot analyses, ELISA and flow cytometry analyses. In vitro experiments were conducted with porcine monocyte-derived macrophages (pMDMs). The results showed that the mRNA and protein levels of TLR2, TLR4 and TLR7 were upregulated in response to CSFV infection, but TLR3 remained unchanged, and was downregulated after infection with the C strain and the Shimen virus, respectively. Furthermore, TLR3-mediated innate immune responses were inhibited in Shimen-strain-infected pMDMs by stimulation with poly(I:C). Accordingly, comprehensive analyses were performed to detect TLR-dependent cytokine responses and the activation of TLR signalling elements. CSFV infection induced mitogen-activated protein kinase activation, but did not elicit NF kappa B activation, thereby affecting the production of pro-inflammatory cytokines. The Shimen strain infection resulted in a significant activation of IFN regulatory factor IRF7 and suppression of IRF3. These data provided clues for understanding the effect of CSFV infection on the TLR-mediated innate immune response and associated pathological changes.