IL-18 Expression in Pigs Following Infection with Mycoplasma hyopneumoniae

Little is known about the detail of the immune response during infection of pigs with Mycoplasma hyopneumoniae(Mhp). To further understand this important porcine pathogen, we examined the interleukin-18 (IL- 18) response in experimentally infected piglets. We found that large amounts of IL-18 were produced in the bronchoalveolar lavage fluids (BALF) of pigs experimentally infected with Mhp. However, the concentration of interferon-γ (IFN-γ) in the same BALF was negatively correlated with that of IL-18. The antibody response against Mhp was found to be associated with the IL-18 concentration in the BALF. Immunohistochemical staining revealed that both IL-18 and IL-18 receptor α chain (IL-18Rα) were present in macrophages and plasma cells in the lungs of Mhp-infected pigs. Lung mononuclear cells isolated from pneumonic lesions secreted IL-18 and prostaglandin E₂ (PGE₂) in vitro, and PGE₂ production was enhanced by stimulation with IL-18. These results indicate that IL-18 produced in the pig lung contributes to the development of innate and acquired immune responses against Mhp as a proinflammatory cytokine rather than as an IFN-γ-inducing factor and may be involved in immunomodulation in pigs.