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

The effect of porcine reproductive and respiratory syndrome (PRRS) virus in fection on the synthesis and secretion of TNF-alpha and other pro-inflammat ory cytokines by porcine alveolar macrophages (PAM) was investigated as wel l as the effect that TNF-alpha has on the replication of this virus. A clea r reduction of phorbol myristate acetate (PMA)-induced expression of TNF-al pha mRNA was observed in cells incubated with PRRS virus. Moreover, the pre sence of PRRS virus also induced a decrease in IL-1 alpha and MIP-1 beta mR NAs expression with respect to PMA-stimulated uninfected cells. According t o these results, exposure to the PRRS virus led to a reduction of the TNF-a lpha protein in supernatants of PMA-stimulated PAM. On the other hand, addi tion of recombinant porcine TNF-alpha to cultures clearly reduced virus rep lication; however the addition of TNF-alpha to cultures containing IFN-a di d not result in a further reduction of the produced by IFN-alpha alone. Thi s indicates the lack of synergy in the effect of these cytokines on viral r eplication. (C) 2000 Published by Elsevier Science B.V.