Study objective: Interleukin (IL)-18 is a proinflammatory cytokine, originally termed interferon (IFN)-γ-inducing factor, which promotes T-helper type 1 (Th1) cytokine responses. We recently reported that serum IL-18 levels were elevated in children with Mycoplasma pneumoniae pneumonia (MP). In this study, we investigated the contribution of IL-18 to the infection and assessed the Th1 cytokine response to pulmonary involvement in adults.
Methods: We investigated the clinical course, pulmonary involvement, and serum levels of IL-18, IFN-γ, IL-12p40, and soluble IL-2 receptor (sIL-2R) in 21 patients with acute-stage MP and in 21 age- and sex-matched control subjects.
Results: Significantly (p < 0.001) increased serum IL-18 (median, 248 pg/mL [range, 89 to 441 pg/mL] vs median, 126 pg/mL [range, 47 to 217 pg/mL]) and sIL-2R (median, 617 U/mL [range, 410 to 1,032 U/mL] vs median, 425 U/mL [range, 268 to 601 U/mL]) were found in patients with MP as compared with healthy control subjects, and there was a tendency toward increased serum IFN-γ and IL-12p40. Circulating IL-18 values had a positive correlation with serum sIL-2R levels (r = 0.62, p = 0.028) and the number of affected pulmonary lobes (ς = 0.61, p = 0.024), but not with the serum levels of antibodies to M pneumoniae, IFN-γ, or IL-12p40. Serum IL-18 and sIL-2R values in severe cases were significantly higher (p < 0.03) than those in mild cases. IFN-γ and sIL-2R levels in four patients with pleural effusion were significantly (p < 0.05) higher than those in the other 17 subjects.
Conclusions: Serum levels of IL-18 were raised during the acute phase of MP. We suggest IL-18 and Th1 cytokines may play a significant role in the immunopathologic responses in MP.