Porcine reproductive and respiratory syndrome (PRRS) virus down-modulates TNF-α production in infected macrophages

The effect of porcine reproductive and respiratory syndrome (PRRS) virus infection on the synthesis and secretion of TNF-α and other pro-inflammatory cytokines by porcine alveolar macrophages (PAM) was investigated as well as the effect that TNF-α has on the replication of this virus. A clear reduction of phorbolmyristate acetate (PMA)-induced expression of TNF–α mRNA was observed in cells incubated with PRRS virus. Moreover, the presence of PRRS virus also induced a decrease in IL-1α and MIP-1β mRNAs expression with respect to PMA-stimulated uninfected cells. According to these results, exposure to the PRRS virus led to a reduction of the TNF-α protein in supernatants of PMA-stimulated PAM. On the other hand, addition of recombinant porcine TNF-α to cultures clearly reduced virus replication; however the addition of TNF-α to cultures containing IFN-α did not result in a further reduction of the produced by IFN-α alone. This indicates the lack of synergy in the effect of these cytokines on viral replication.