Constitutive expression of interferons in swine leukocytes

Interferon (IFN)-alpha and IFN-gamma positive cells were revealed by flow cytometry in peripheral blood mononuclear cells (PBMC) of Specific Pathogen Free (SPF) pigs. A low prevalence of IFN-gamma positive cells was also detected in PBMC of some Porcine Reproductive and Respiratory Syndrome virus-infected pigs and uninfected, control pigs. IFN-alpha positive cells showed phenotypes of both monocytes and plasmacytoid dendritic cells. The presence of IFN-alpha in PBMC was also confirmed by Western blotting. By immunoprecipitation, IFN-alpha was detected as 32 and 55-57 kDa bands in PBMC of healthy SPF piglets. These samples were also IFN-gamma positive; the cytokine was revealed as 24, 37 and 54 kDa bands. The unusual molecular mass values of intracellular interferons were probably due to oligomerization, as previously described for human IFN-alpha. Swine intracellular IFN-alpha displayed the expected antiviral activity on bovine MDBK cells. The results indicate that interferons are constitutively expressed in swine leukocytes with peculiar molecular features.