A comparative study of the local cytokine response in the lungs of pigs experimentally infected with different PRRSV-1 strains: Upregulation of IL-1? in highly pathogenic strain induced lesions

Porcine reproductive and respiratory syndrome viruses (PRRSV) show high genetic differences both among and within genotypes. Recently, several highly pathogenic PRRSV (HP-PRRSV) strains have been described. This study compares and characterizes the production of cytokines by pulmonary macrophages in pigs experimentally infected with four different PRRSV-1 strains: two low-virulent strains, Lelystad (LV) and a British field strain (215-06); a HP strain (SU1-bel) from Belarus and the attenuated vaccine strain DV (Porcilis® PRRS). Animals were clinically monitored and post-mortem examinations were performed at 3, 7 and 35 days post-infection (dpi). Lung samples were processed for histopathological and immunohistochemical studies by using specific antibodies against PRRSV, IL1-?, IL-6, TNF-?, IL-10 and IFN-?. SU1-bel infected animals presented the highest mean scores for clinical observations, gross and microscopic lesions as well as for PRRSV expression compared with the other infected groups (p ? 0.027). These animals displayed the highest expression of IL1-? at 7 dpi, together with the highest score for lung pathology, whereas LV, 215-06 and DV inoculated animals only showed a transient enhancement in some of these cytokines. SU1-bel-infected pigs showed a positive correlation between the amount of PRRSV antigen and IL-1? expression (r = 0.645, p < 0.001). The highest expression of IL-10 was detected in 215-06-infected animals (p ? 0.004), with a positive correlation with the numbers of virus-infected cells (r = 0.375, p ? 0.013). In conclusion, the HP-PRRSV SU1-bel strain replicated more efficiently in the lung of infected animals and induced a higher expression of IL-1? than the other PRRSV-1-infected groups, which may have played a key role in the onset of the clinical signs and interstitial pneumonia.