In vivo regulation of replicative Legionella pneumophila lung infection by endogenous interleukin-12

Abstract

The in vivo role of endogenous interleukin 12 (IL-12) in modulating intrapulmonary growth of Legionella pneumophila was assessed by using a murine model of replicative L. pneumophila lung infection. Intratracheal inoculation of A/J mice with virulent bacteria (10(6) L. pneumophila cells per mouse) resulted in induction of IL-12, which preceded clearance of the bacteria from the lung. Inhibition of endogenous IL-12 activity, via administration of IL-12 neutralizing antiserum, resulted in enhanced intrapulmonary growth of the bacteria within 5 days postinfection (compared to untreated L. pneumophila-infected mice). Because IL-12 has previously been shown to modulate the expression of cytokines, including gamma interferon (IFN-gamma), tumor necrosis factor alpha (TNF-alpha), and IL-10, which regulate L. pneumophila growth, immunomodulatory effects of endogenous IL-12 on intrapulmonary levels of these cytokines during replicative L. pneumophila lung infection were subsequently assessed. Results of these experiments demonstrated that TNF-alpha activity was significantly lower, while protein levels of IFN-gamma and IL-10 in the lung were similar, in L. pneumophila-infected mice administered IL-12 antiserum, compared to similarly infected untreated mice. Together, these results demonstrate that IL-12 is critical for resolution of replicative L. pneumophila lung infection and suggest that regulation of intrapulmonary growth of L. pneumophila by endogenous IL-12 is mediated, at least in part, by TNF-alpha.

FIG. 1

IL-12 activity in BALF and lung homogenates of A/J mice inoculated intratracheally with L. pneumophila. A/J mice were inoculated intratracheally with L. pneumophila as described in Materials and Methods. At specific time points thereafter, the mice were sacrificed and IL-12 was assessed in BALF (A) and whole lung homogenates (B) by ELISA. Results represent the means ± standard errors of the means for three to five animals per time point, ∗, significantly greater than value for uninfected mice (i.e., 125 pg of IL-12 per ml of BALF; 935 pg of IL-12 per lung) (analysis of variance, P < 0.05).

FIG. 2

Effect of IL-12 on endogenous cytokine activity in the lung during replicative L. pneumophila lung infection. A/J mice were administered IL-12 antiserum as described in Materials and Methods prior to intratracheal inoculation of L. pneumophila. At specific time points p.i., the mice were sacrificed. The lungs were excised, homogenized, and filtered. Levels of TNF-α, IFN-γ, and IL-10 were quantified in filtered lung homogenates of L. pneumophila-infected mice depleted of endogenous IL-12 by ELISA or by bioassay and compared to those of similarly infected immunocompetent (i.e., untreated) mice. Results represent the means ± standard errors of the means for five mice per treatment group. ▪, untreated mice; ▤, anti-IL-12-treated mice. ∗, significantly less than value for untreated mice (Student’s t test, P < 0.05).