ABSTRACT: BACKGROUND: Porcine reproductive and respiratory syndrome virus (PRRSV) is largely responsible for heavy economic losses in the swine industry worldwide because of its high mutation rate and subsequent emergence of virulent strains. However, the immunological and pathological responses of pigs to PRRSV strains with different virulence have not been completely elucidated. METHODS: Twenty-four piglets were divided into 4 groups (n?=?6 each) and inoculated with highly pathogenic PRRSV isolate BB0907 (HP), low pathogenic PRRSV NT0801 (LP), LP derivative strain NT0801-F70 (LP-der), and DMEM medium (control), respectively. The changes in TLR2, 3, 7, and 8 gene expression and TNF-?, IL-1?, IL-6, IFN-?, and IL-10 secretion were evaluated using real-time PCR and ELISA at 6, 9, and 15 days post inoculation (d.p.i.). The cytokine levels were evaluated in the supernatants of porcine alveolar macrophages (PAMs) and peripheral blood mononuclear cells (PBMCs) following stimulation with LTA, poly(I:C), CL097, and PRRSV individually. RESULTS: HP caused more severe clinical signs and pathological lesions in swine than LP and LP-der had almost no virulence compared with LP. The serum levels of IL-1?, IL-6, TNF-?, and IFN-? were increased in HP-infected piglets, which were greater than in those infected with LP or LP-der. The mRNA levels of TLR3, 7, and 8 were significantly up-regulated in PAMs in HP-infected pigs compared to those in groups LP and LP-der. Furthermore, TNF-? and IL-1? secretion in PAMs from group LP was statistically greater than those from the control group after stimulation with either poly(I:C) or CL097. Meanwhile, TNF-?, IL-1?, and IL-6 levels in CL097-stimulated PBMCs from HP-infected pigs were markedly higher than those from the LP- and LP-der-infected groups. CONCLUSIONS: We found that HP was a stronger inducer of TLR 3, 7, and 8 expression and IL-1?, IL-6, TNF-?, and IFN-? production compared to LP and LP-der. HP enhanced production of TNF-?, IL-1?, and IL-6 in PBMCs following CL097-stimulation more than LP and LP-der, whereas LP enhanced the secretion of TNF-? and IL-1? in poly(I:C)- and CL097-stimulated PAMs. Our data regarding cellular reactivity to different isolates should be useful in the development of more efficacious vaccines.
