Significance: The circadian clock allows an organism to anticipate daily changes imposed by the environment. The response to LPS is altered depending on time of day; however, the molecular mechanisms underlying this are unclear. We find that the clock in myeloid cells plays a role in LPS-induced sepsis by altering NF-κB activity and the induction of the microRNA miR-155. LPS causes the repression of BMAL1 via the targeting of miR-155 to two seed sequences in the 3′-untranslated region of Bmal1 . Lack of miR-155 has profound effects on circadian function and circadian induction of cytokines by LPS. Thus, the molecular clock is using miR-155 as an important regulatory component to control inflammation variably across the circadian day in myeloid cells.

The ability of an individual to sense pain is fundamental for its capacity to adapt to its environment and to avoid damage. The sensation of pain can be enhanced by acute or chronic inflammation. In the present study, we have investigated whether inflammatory pain, as measured by hypernociceptive responses, was modified in the absence of the microbiota. To this end, we evaluated mechanical nociceptive responses induced by a range of inflammatory stimuli in germ-free and conventional mice. Our experiments show that inflammatory hypernociception induced by carrageenan, lipopolysaccharide, TNF-α, IL-1β, and the chemokine CXCL1 was reduced in germ-free mice. In contrast, hypernociception induced by prostaglandins and dopamine was similar in germ-free or conventional mice. Reduction of hypernociception induced by carrageenan was associated with reduced tissue inflammation and could be reversed by reposition of the microbiota or systemic administration of lipopolysaccharide. Significantly, decreased hypernociception in germ-free mice was accompanied by enhanced IL-10 expression upon stimulation and could be reversed by treatment with an anti-IL-10 antibody. Therefore, these results show that contact with commensal microbiota is necessary for mice to develop inflammatory hypernociception. These findings implicate an important role of the interaction between the commensal microbiota and the host in favoring adaptation to environmental stresses, including those that cause pain.

Objective: Deposition of monosodium urate monohydrate (MSU) crystals in the joints promotes an intense inflammatory response and joint dysfunction. This study evaluated the role of the NLRP3 inflammasome and 5‐lipoxygenase (5‐LOX)–derived leukotriene B₄(LTB₄) in driving tissue inflammation and hypernociception in a murine model of gout.Methods: Gout was induced by injecting MSU crystals into the joints of mice. Wild‐type mice and mice deficient in NLRP3, ASC, caspase 1, interleukin‐1β (IL‐1β), IL‐1 receptor type I (IL‐1RI), IL‐18R, myeloid differentiation factor 88 (MyD88), or 5‐LOX were used. Evaluations were performed to assess neutrophil influx, LTB₄activity, cytokine (IL‐1β, CXCL1) production (by enzyme‐linked immunosorbent assay), synovial microvasculature cell adhesion (by intravital microscopy), and hypernociception. Cleaved caspase 1 and production of reactive oxygen species (ROS) were analyzed in macrophages by Western blotting and fluorometric assay, respectively.Results: Injection of MSU crystals into the knee joints of mice induced neutrophil influx and neutrophil‐dependent hypernociception. MSU crystal–induced neutrophil influx was CXCR2‐dependent and relied on the induction of CXCL1 in an NLRP3/ASC/caspase 1/IL‐1β/MyD88–dependent manner. LTB₄was produced rapidly after injection of MSU crystals, and this was necessary for caspase 1–dependent IL‐1β production and consequent release of CXCR2‐acting chemokines in vivo. In vitro, macrophages produced LTB₄after MSU crystal injection, and LTB₄was relevant in the MSU crystal–induced maturation of IL‐1β. Mechanistically, LTB₄drove MSU crystal–induced production of ROS and ROS‐dependent activation of the NLRP3 inflammasome.Conclusion: These results reveal the role of the NLRP3 inflammasome in mediating MSU crystal–induced inflammation and dysfunction of the joints, and highlight a previously unrecognized role of LTB₄in driving NLRP3 inflammasome activation in response to MSU crystals, both in vitro and in vivo.

Mammals are colonized by an astronomical number of commensal microorganisms on their environmental exposed surfaces. These symbiotic species build up a complex community that aids their hosts in several physiological activities. We have shown that lack of intestinal microbiota is accompanied by a state of active IL-10–mediated inflammatory hyporesponsiveness. The present study investigated whether the germfree state and its hyporesponsive phenotype alter host resistance to an infectious bacterial insult. Experiments performed in germfree mice infected with Klebsiella pneumoniae showed that these animals are drastically susceptible to bacterial infection in an IL-10–dependent manner. In germfree mice, IL-10 restrains proinflammatory mediator production and neutrophil recruitment and favors pathogen growth and dissemination. Germfree mice were resistant to LPS treatment. However, priming of these animals with several TLR agonists recovered their inflammatory responsiveness to sterile injury. LPS pretreatment also rendered germfree mice resistant to pulmonary K. pneumoniae infection, abrogated IL-10 production, and restored TNF-α and CXCL1 production and neutrophil mobilization into lungs of infected germfree mice. This effective inflammatory response mounted by LPS-treated germfree mice resulted in bacterial clearance and enhanced survival upon infection. Therefore, host colonization by indigenous microbiota alters the way the host reacts to environmental infectious stimuli, probably through activation of TLR-dependent pathways. Symbiotic gut colonization enables proper inflammatory response to harmful insults to the host, and increases resilience of the entire mammal-microbiota consortium to environmental pressures.

Dengue disease is a mosquito-borne viral disease of expanding geographical range and incidence. Infection by one of the four serotypes of dengue virus induces a spectrum of disease manifestations, ranging from asymptomatic to life-threatening Dengue hemorrhagic fever/dengue shock syndrome. Many efforts have been made to elucidate several aspects of dengue virus-induced disease, but the pathogenesis of disease is complex and remains unclear. Understanding the mechanisms involved in the early stages of infection is crucial to determine and develop safe therapeutics to prevent the severe outcomes of disease without interfering with control of infection. In this review, we discuss the dual role of the innate and inflammatory pathways activated during dengue disease in mediating both protection and exacerbation of disease. We show that some mediators involved in each of these responses differ substantially, suggesting that interfering in disease-associated immune pathways may represent a potential therapeutic opportunity for the treatment of severe dengue.

Dengue fever is an emerging viral disease transmitted by arthropods to humans in tropical countries. Dengue hemorrhagic fever (DHF) is escalating in frequency and mortality rates. Here we studied the involvement of macrophage migration inhibitory factor (MIF) in dengue virus (DENV) infection and its pathogenesis. Patients with DHF had elevated plasma concentrations of MIF. Both leukocytes from these patients and macrophages from healthy donors infected in vitro with DENV showed a substantial amount of MIF within lipid droplets. The secretion of MIF by macrophages and hepatocytes required a productive infection and occurred without an increase in gene transcription or cell death, thus indicating active secretion from preformed stocks. In vivo infection of wild‐type and miF deficient (Mif ^−/− ) mice demonstrated a role of MIF in dengue pathogenesis. Clinical disease was less severe in Mif ^−/− mice, and they exhibited a significant delay in lethality, lower viremia, and lower viral load in the spleen than wild‐type mice. This reduction in all parameters of severity on DENV infection in Mif ^−/− mice correlated with reduced proinflam‐matory cytokine concentrations. These results demon‐strated the contribution of MIF to the pathogenesis of dengue and pointed to a possible beneficial role of neutralizing MIF as an adjunctive therapeutic approach to treat the severe forms of the disease.—Assuncäo‐Miranda, I., Amaral, F. A., Bozza, F. A., Fagundes, C. T., Sousa, L. P., Souza, D. G., Pacheco, P., Barbosa‐Lima, G., Gomes, R. N., Bozza, P. T., Da Poian, A. T., Teixeira, M. M., Bozza, M. T. Contribution of macro‐phage migration inhibitory factor to the pathogenesis of dengue virus infection. FASEB J . 24, 218–228 (2010). www.fasebj.org