Increased Production of Proinflammatory Cytokines following Infection with Porcine Reproductive and Respiratory Syndrome Virus andMycoplasma hyopneumoniae

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Abstract
Induction of the proinflammatory cytokines interleukin-1 (IL-1) (α and β), IL-6, IL-8, IL-10, IL-12, and tumor necrosis factor alpha (TNF-α) in pulmonary alveolar macrophages (PAMs) was assessed following experimental infection with porcine reproductive and respiratory syndrome virus (PRRSV) and/orMycoplasma hyopneumoniaeby using in vivo and in vitro models. The in vivo model consisted of pigs infected with PRRSV and/orM. hyopneumoniaeand necropsied at 10, 28, or 42 days postinfection. Pigs infected with both pathogens had a greater percentage of macroscopic lung lesions, increased clinical disease, and slower viral clearance than pigs infected with either pathogen alone. The pigs infected with both PRRSV andM. hyopneumoniaehad significantly increased levels of mRNA for many proinflammatory cytokines in PAMs collected by bronchoalveolar lavage (BAL) at all necropsy dates compared to those in uninfected control pigs. Increased levels of IL-1β, IL-8, IL-10, and TNF-α proteins in BAL fluid, as measured by enzyme-linked immunosorbent assay, confirmed the increased cytokine induction induced by the pathogens. An in vitro model consisted ofM. hyopneumoniae-inoculated tracheal ring explants cultured with PRRSV-infected PAMs. PAMs were harvested at 6 or 15 h postinfection with either or both pathogens. The in vitro study detected increased IL-10 and IL-12 mRNA levels in PAMs infected with PRRSV at all time periods. In addition, IL-10 protein levels were significantly elevated in the culture supernatants in the presence ofM. hyopneumoniae-inoculated tracheal ring explants. The increased production of proinflammatory cytokines in vivo and in vitro associated with concurrentM. hyopneumoniaeand PRRSV infection may play a role in the increased rates of pneumonia associated with PRRSV infection. The increased levels of IL-10 may be a possible mechanism that PRRSV andM. hyopneumoniaeuse to exacerbate the severity and duration of pneumonia induced by PRRSV and modulate the respiratory immune response.

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