Experiments were performed to determine the contribution of TLR9 to the generation of protective immunity against the intracellular respiratory bacterial pathogen Legionella pneumophila. In initial studies, we found that the intratracheal (i.t.) administration of L. pneumophila to mice deficient in TLR9 (TLR9

Sepsis results in a profound state of immunosuppression, which is temporally associated with impaired leukocyte function. The mechanism of leukocyte reprogramming in sepsis is incompletely understood. In this study, we explored mechanisms contributing to dysregulated inflammatory cytokine expression by pulmonary macrophages during experimental sepsis. Pulmonary macrophages (PM) recovered from the lungs of mice undergoing cecal ligation and puncture (CLP) display transiently reduced expression of some, but not all innate genes in response to LPS. Impaired expression of TNF-α and iNOS was associated with reduced acetylation and methylation of specific histones (AcH4 and H3K4me3) and reduced binding of RNA polymerase II to the promoters of these genes. Transient impairment in LPS-induced cytokine responses in septic PM temporally correlated with induction of IRAK-M mRNA and protein, which occurred in a MyD88-dependent fashion. PM isolated from IRAK-M−/− mice were largely refractory to CLP-induced impairment in cytokine expression, chromatin remodeling, recruitment of RNA polymerase II, and induction of histone deacetylase-2 observed during sepsis. Our findings indicate that systemic sepsis induces epigenetic silencing of cytokine gene expression in lung macrophages, and IRAK-M appears to be a critical mediator of this response.

The often fatal sepsis syndrome is characterized by the systemic release of inflammatory mediators, which is regulated and counterbalanced by the coordinated expression of anti-inflammatory molecules. The magnitude of sepsis-induced tissue injury and subsequent risk of infectious complications is dictated by the balance between the expression of pro- and anti-inflammatory mediators. As our understanding of the pathophysiology of sepsis continues to evolve, we have gained a greater appreciation for the profound effects that sepsis and similar states of overwhelming stress have on host innate and adaptive immunity. Impaired leukocyte function in sepsis has important clinical consequences, as high mortality rates have been observed in patients who display evidence of sepsis-induced immune dysregulation. Functional defects in leukocytes isolated from patients with sepsis include diminished expression of important cell surface molecules, dysregulated cytokine production, alterations in antigen-presenting ability, and accelerated apoptosis. In this article, we review the current literature supporting the notion that dysregulation of host immunity occurs during sepsis syndrome, and describe novel therapeutic interventions directed at augmenting host immunity during sepsis.

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