Expression of interferon-α and Mx1 protein in pigs acutely infected with porcine reproductive and respiratory syndrome virus (PRRSV)

The expression of mRNA encoding interferon-alpha (IFN-alpha) and Mx1 protein was studied, by reverse transcription-polymerase chain reaction and by in-situ hybridization with a non-radioactive digoxigenin-labelled cDNA probe, in formalin-fixed, paraffin wax-embedded lung tissue from pigs experimentally infected with a Korean isolate (North American genotype) of porcine reproductive and respiratory syndrome virus (PRRSV). The animals were examined over a period of 10 days after intranasal inoculation. IFN-alpha and Mx1 protein was detected in the lung at 1 day post inoculation (dpi), the number of positive cells increasing at 7 dpi, and rapidly decreasing thereafter. Hybridization signals for IFN-alpha and Mx1 protein were usually associated with inflammation, and in particular with macrophages. Expression of IFN-alpha and Mx1 protein was negative in non-lesional lung of PRRSV-infected pigs and in normal lung from control pigs. There was a good statistical correlation between the number of cells positive for mRNA encoding IFN-alpha and Mx1 protein in the infected lungs (r = 0.95, P < 0.05). The results suggest that the expression of IFN-α and Mx1 protein plays a role in the early host defence against PRRSV infection. (C) 2004 Elsevier Ltd. All rights reserved.