Epidemiological evidence points to the inverse relationship between microbial exposure and the prevalence of allergic asthma and autoimmune diseases in Westernized countries. The molecular basis for this observation has not yet been completely delineated. Here we report that the administration of certain toll-like receptor (TLR) ligands, via the activation of innate immunity, induces high levels of indoleamine 2,3-dioxygenase (IDO), the rate-limiting enzyme of tryptophan catabolism in various organs. TLR9 ligand-induced pulmonary IDO activity inhibits Th2-driven experimental asthma. IDO activity expressed by resident lung cells rather than by pulmonary DCs suppressed lung inflammation and airway hyperreactivity. Our results provide a mechanistic insight into the various formulations of the hygiene hypothesis and underscore the notion that activation of innate immunity can inhibit adaptive Th cell responses.

In the cytochrome bc1 complex, the swivel motion of the ironsulfur protein (ISP) between two redox sites constitutes a key component of the mechanism that achieves the separation of the two electrons in a substrate molecule at the quinol oxidation (Qo) site. The question remaining is how the motion of ISP is controlled so that only one electron enters the thermodynamically favorable chain via ISP. An analysis of eight structures of mitochondrial bc1 with bound Qo site inhibitors revealed that the presence of inhibitors causes a bidirectional repositioning of the cd1 helix in the cytochrome b subunit. As the cd1 helix forms a major part of the ISP binding crater, any positional shift of this helix modulates the ability of cytochrome b to bind ISP. The analysis also suggests a mechanism for reversal of the ISP fixation when the shape complementarity is significantly reduced after a positional reorientation of the reaction product quinone. The importance of shape complementarity in this mechanism was confirmed by functional studies of bc1 mutants and by a structure determination of the bacterial form of bc1. A mechanism for the high fidelity of the bifurcated electron transfer is proposed.crystal structures ͉ electron transfer ͉ inhibitor binding ͉ mechanism W ithin cellular energy-conserving membranes (the mitochondrial inner membrane in eukaryotes and the plasma membrane in prokaryotes), the only mobile carriers of redox equivalents are ubiquinone (Q), ubiquinol (QH 2 ), and their derivatives. In the cytochrome (cyt) bc 1 complex (cyt bc 1 or bc 1 ) of the respiratory chain, QH 2 is oxidized to Q, and concomitantly, protons are pumped against the gradient across the membrane [from the matrix to the intermembrane space (IMS) in mitochondria and from the cytoplasm to the periplasm in prokaryotes], thus contributing to the electrochemical potential that drives ATP synthesis. The proposed mechanism by which bc 1 performs this dual task, the Q-cycle hypothesis (1), has been widely accepted; it suggests that for the oxidation of two molecules of QH 2 , one molecule of Q is reduced at separate catalytic sites (Fig. 1A). According to this mechanism, the Q reduction site is located near the negative side of the membrane (Q i ͞Q n , the mitochondrial matrix or cytoplasmic side) and is electronically linked via two prosthetic groups [high-potential b heme (b H ) and low-potential b heme (b L )] to the quinol oxidation site located close to the positive side of the membrane (Q o ͞Q p , the mitochondrial IMS side or periplasmic side in prokaryotes). At the Q o site, one electron from a substrate enters the energetically favorable path that leads via the iron-sulfur protein (ISP) and cyt c 1 subunit (cyt c 1 ) to the substrate cyt c, whereas the other electron proceeds via the b L and b H heme groups to Q (or semiquinone) bound at the Q i site. The half-reduction of a Q per oxidized quinol is achieved by the bifurcation of the electron pathway at the Q o site. In a complete catalytic cycle, two protons are taken up from the ma...

BackgroundA liver-derived protein, fetuin-A, was first purified from calf fetal serum in 1944, but its potential role in lethal systemic inflammation was previously unknown. This study aims to delineate the molecular mechanisms underlying the regulation of hepatic fetuin-A expression during lethal systemic inflammation (LSI), and investigated whether alterations of fetuin-A levels affect animal survival, and influence systemic accumulation of a late mediator, HMGB1.Methods and FindingsLSI was induced by endotoxemia or cecal ligation and puncture (CLP) in fetuin-A knock-out or wild-type mice, and animal survival rates were compared. Murine peritoneal macrophages were challenged with exogenous (endotoxin) or endogenous (IFN-γ) stimuli in the absence or presence of fetuin-A, and HMGB1 expression and release was assessed. Circulating fetuin-A levels were decreased in a time-dependent manner, starting between 26 h, reaching a nadir around 24–48 h, and returning towards base-line approximately 72 h post onset of endotoxemia or sepsis. These dynamic changes were mirrored by an early cytokine IFN-γ-mediated inhibition (up to 50–70%) of hepatic fetuin-A expression. Disruption of fetuin-A expression rendered animals more susceptible to LSI, whereas supplementation of fetuin-A (20–100 mg/kg) dose-dependently increased animal survival rates. The protection was associated with a significant reduction in systemic HMGB1 accumulation in vivo, and parallel inhibition of IFN-γ- or LPS-induced HMGB1 release in vitro.ConclusionsThese experimental data suggest that fetuin-A is protective against lethal systemic inflammation partly by inhibiting active HMGB1 release.

HAA ͉ indoleamine 2,3-dioxygenase ͉ NF-B ͉ tryptophan

Rhodobacter capsulatus regulates many metabolic processes in response to the level of environmental oxygen and the energy state of the cell. One of the key global redox regulators of the cell's metabolic physiology is the sensor kinase RegB that controls the synthesis of numerous energy generation and utilization processes. In this study, we have succeeded in purifying full-length RegB containing six transmembrane-spanning elements. Exogenous addition of excess oxidized coenzyme Q 1 is capable of inhibiting RegB autophosphorylation ϳ6-fold. However, the addition of reduced coenzyme Q 1 exhibits no inhibitory effect on kinase activity. A ubiquinone-binding site, as defined by azidoquinone photo affinity crosslinking, was determined to lie within a periplasmic loop between transmembrane helices 3 and 4 that contains a fully conserved heptapeptide sequence of GGXXNPF. Mutation of the phenylalanine in this heptapeptide renders RegB constitutively active in vivo, indicating that this domain is responsible for sensing the redox state of the ubiquinone pool and subsequently controlling RegB autophosphorylation.