Foot-and-mouth disease virus (FMDV) leader proteinase negatively regulates the porcine interferon-λ1 pathway

► Porcine IFN-λ1 significantly inhibits FMDV replication. ► FMDV infection cannot induce IFN-λ1 mRNA expression. ► FMDV Lpro inhibits poly(I:C)-induced IFN-λ1 mRNA expression. ► Lpro inhibits RIG-I/MDA5-mediated IFN-λ1 activation. ► The catalytic activity and a SAP domain of Lpro are necessary to inhibit IFN-λ1.