Title: microRNA Regulation of Endotoxin Tolerance
Description: Sepsis affects hundreds of thousands each year in the United States alone, with an estimated 20-30% mortality rate in spite of current treatment regimens. Sepsis mortality was originally understood to be the caused by overproduction of inflammatory cytokines in response to pathogen detection by the host. However, recent studies suggest that with modern treatments, secondary infection, rather than inflammatory shock, may be of greater concern. In either case, the failure of a large number of anti-inflammatory agents to produce beneficial outcomes in sepsis treatment during clinical trial suggests that the development of a new class of immunomodulatory agents may be required for effective treatment. In experimental models, pre-treatment with sub-lethal doses of lipopolysaccharide (LPS, previously referred to as endotoxin) induces a state of "LPS tolerance" that reduces septic shock lethality. Paradoxically, LPS tolerance also results in increased antimicrobial gene expression and resistance to secondary infection in some models.

Further exploration of this process may provide drug targets capable of limiting inflammation without dampening antimicrobial immunity, which could be of great benefit in the treatment of sepsis and chronic inflammatory disease. Many groups have studied signaling changes that occur during LPS tolerance. However, mediators of tolerance that can account for the changes in LPS-induced gene expression that result in increased microbial resistance are not well described. This has prevented proper testing of the physiological effects of tolerance on disease, and it remains unclear if this process could be artificially induced or is of any benefit to sepsis patients. Recent in vitro work suggests that tolerant gene expression patterns are the result of large scale changes in chromatin organization that occur in macrophages after prolonged LPS stimulation. Because microRNAs (miRNAs), a new class of gene regulator, have been found to regulate chromatin modifying complexes in other systems, LPS-induced miRNAs were screened to identify potential mediators of tolerance that could cause changes in gene expression patterns without necessarily impacting LPS signaling itself.

Several tolerance-associated miRNAs were identified. One miRNA in particular, miR-222, was found to repress tumor necrosis factor (Tnf) and Brahma-related gene one (Brg1) expression. This attenuates expression of genes dependent on nucleosome remodeling, primarily affecting inflammatory genes. Consequently, miR-222 expression effectively limits septic shock lethality. However, low-level responses, as well as NF-κB signaling and the expression of a subset of antimicrobial and antiviral genes, are left intact. Thus, although miR-222 does not entirely recapitulate the tolerance response, by directing the LPS response into a less damaging expression profile, miR-222 may accelerate the onset of tolerance and be a promising target for therapeutics aiming to treat inflammatory disease without compromising host immunity.