Field observations have suggested that porcine reproductive and respiratory syndrome virus (PRRSV) predispose pigs to secondary infections. The interaction between PRRSV and the secondary invaders has not yet been well elucidated. In this study, we investigated the mRNA expression of Toll-like receptors (TLR) in lymphoid organs and cells, and cytokine
secretions by alveolar macrophages (AMs) and peripheral blood mononuclear cells (PBMCs) in response to pathogen-associated molecular patterns (PAMPs) in PRRSVchallenged pigs. TLR mRNA expressions were measured by semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) and cytokine concentrations were determined using commercially available ELISA kits. PRRSV infection led to significantly increased secretions of IL-1b and IL-6 by AMs of PRRSV-infected pigs. Infection of pigs with PRRSV also resulted in an increased secretion of IL-1b by AMs in response to lipoteichoic acid (LTA) stimulation, and IL-6 by PBMCs in response to lipopolysaccharide (LPS) and LTA
stimulation. Infection of pigs with PRRSV tended to up-regulate the mRNA expression of TLR2, 3, 4, 7 and 8 in at least one of the lymphoid tissues and cells. Further research is required to demonstrate the association between the enhanced expressions of the specific TLRs and the increased susceptibility to secondary agents with more severe clinical outcomes in PRRSV-infected pigs.

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