Correlations between Lung Proinflammatory Cytokine Levels, Virus Replication, and Disease after Swine Influenza Virus Challenge of Vaccination-Immune Pigs

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Abstract
During experimental infection of pigs with swine influenza virus (SIV), there is a strong temporal correlation between peak virus titers in the lungs, levels of different proinflammatory cytokines in bronchoalveolar lavage (BAL) fluids, and disease. Vaccination against SIV can greatly reduce or prevent virus replication after challenge and the resulting disease. Here, we took advantage of pigs from vaccination-challenge experiments, with different degrees of virological and clinical protection, to further correlate SIV replication with cytokines and disease. Forty-nine pigs were vaccinated twice with a commercial inactivated SIV vaccine or with experimental vaccines, and 35 control pigs were not vaccinated. Between 2 and 4 weeks after the last vaccination, all pigs were challenged intratracheally with SIV. Twenty-four hours after the challenge, we determined body temperatures, respiratory scores, lung virus titers, and neutrophils and cytokines in BAL fluids. Interferon-α (IFN-α), tumor necrosis factor (TNF-α), interleukin-1 (IL-1), and -6 (IL-6) were determined by bioassay, and IL-8 by a commercial ELISA. The results were analyzed for three comparison groups. The unvaccinated control pigs (group 1, n = 35) were positive for all or most parameters examined. Vaccinated pigs with challenge virus replication in the lungs (group 2, n = 28) had slightly lower virus titers than the challenge control pigs, and clear reductions in disease severity and mean titers of all five cytokines, but neutrophil numbers were not affected. Vaccinated pigs without detectable virus replication (group 3, n = 21) were largely protected against clinical signs and neutrophil infiltration. Mean levels of IFN-α, TNF-α, and IL-6, but not IL-1 or IL-8, were lower than in both other groups. Virus titers in the lungs of individual pigs showed highly significant correlations with IFN-α and IL-6, and lower correlations with TNF-α and IL-8. Clinical signs were most closely associated with IFN-α, IL-6, and TNF-α. The relationship between disease and IL-8 or IL-1 was much weaker. Our data provide further evidence for a role of IFN-α, TNF-α, and IL-6 in the pathogenesis of SIV. The similarities with cytokine profiles during human influenza virus infection are discussed.