Interleukin 18 participates in the early inflammatory response and bacterial clearance during pneumonia caused by nontypeable Haemophilus influenzae

Abstract

Nontypeable Haemophilus influenzae (NTHi) is a common gram-negative respiratory pathogen. To determine the role of the proinflammatory cytokine interleukin 18 (IL-18) during NTHi pneumonia, normal wild-type (WT) and IL-18 knockout (KO) mice were intranasally infected with NTHi. IL-18 KO mice displayed a delayed clearance of NTHi from the respiratory tract, resulting in >20-fold higher bacterial loads in their lungs at 24 h after infection, preceded by a strongly attenuated pulmonary innate immune response as determined by cytokine and chemokine induction and histopathology. These data identify IL-18 as part of an adequate innate immune response during NTHi pneumonia.

FIG. 1.

(A) Bacterial clearance. WT and IL-18 KO mice were infected i.n. with 1 × 10⁷ CFU of NTHi. Mice were sacrificed at 6, 24, and 44 h postinfection, and bacterial loads were determined in lung homogenates. (B) Levels of IL-1β, IL-1α, IL-6, TNF, KC and MIP-1α measured in lung homogenates of WT and IL-18 KO mice obtained after 6 h of infection, when bacterial loads were equal. Dotted lines represent baseline lung levels in uninfected mice. Data are means ± standard errors of the means for seven or eight mice per group. CFU data were analyzed after log transformation using two-way ANOVA followed by a Bonferroni posttest, and cytokine/chemokine data were compared using a Mann-Whitney U test. * and **, P < 0.05 and P < 0.01 (KO versus WT) at the indicated time points.

FIG. 2.

Representative lung histology of WT (A and C) and IL-18 KO (B and D) mice 6 (A and B) and 24 (C and D) h after i.n. infection with 1 × 10⁷ CFU of NTHi. The lung sections (hematoxylin and eosin stained) are representative of seven or eight animals per group per time point. Histopathological scores (given as mean ± standard error under the corresponding image) were analyzed by Mann-Whitney U tests. Original magnification, ×10. (Insets) Ly6 staining for neutrophils (original magnification, ×20). **, P < 0.01 (KO versus WT).

FIG. 3.

Schematic overview. We previously demonstrated that recognition of NTHi is mediated by CD14 and TLR4 (17). Subsequently, it was reported that the MyD88-dependent route of TLR4 signaling enhances the inflammatory response and bacterial clearance (17). Moreover, we demonstrated that host defense against NTHi was not dependent on TNF or the platelet-activating factor (3, 9). Here, we found that the MyD88-dependent IL-18 is important for the inflammatory response and antibacterial host defense. It will be of interest to determine the role of IL-1, whose signaling is also dependent on MyD88, in this experimental model.