Innate immunity against Francisella tularensis is dependent on the ASC/caspase-1 axis

Francisella tularensis is a highly infectious gram-negative coccobacillus that causes the zoonosis tularemia. This bacterial pathogen causes a plague-like disease in humans after exposure to as few as 10 cells. Many of the mechanisms by which the innate immune system fights Francisella are unknown. Here we show that wild-type Francisella, which reach the cytosol, but not Francisella mutants that remain localized to the vacuole, induced a host defense response in macrophages, which is dependent on caspase-1 and the death-fold containing adaptor protein ASC. Caspase-1 and ASC signaling resulted in host cell death and the release of the proinflammatory cytokines interleukin (IL)-1β and IL-18. F. tularensis–infected caspase-1– and ASC-deficient mice showed markedly increased bacterial burdens and mortality as compared with wild-type mice, demonstrating a key role for caspase-1 and ASC in innate defense against infection by this pathogen

Vav Regulates Peptide-specific Apoptosis in Thymocytes

The protooncogene Vav functions as a GDP/GTP exchange factor (GEF) for Rho-like small GTPases involved in cytoskeletal reorganization and cytokine production in T cells. Gene-targeted mice lacking Vav have a severe defect in positive and negative selection of T cell antigen receptor transgenic thymocytes in vivo, and vav−/− thymocytes are completely resistant to peptide-specific and anti-CD3/anti-CD28–mediated apoptosis. Vav acts upstream of mitochondrial pore opening and caspase activation. Biochemically, Vav regulates peptide-specific Ca2+ mobilization and actin polymerization. Peptide-specific cell death was blocked both by cytochalasin D inhibition of actin polymerization and by inhibition of protein kinase C (PKC). Activation of PKC with phorbol ester restored peptide-specific apoptosis in vav−/− thymocytes. Vav was found to bind constitutively to PKC-θ in thymocytes. Our results indicate that peptide-triggered thymocyte apoptosis is mediated via Vav activation, changes in the actin cytoskeleton, and subsequent activation of a PKC isoform

Polyene Macrolide Antifungal Drugs Trigger Interleukin-1β Secretion by Activating the NLRP3 Inflammasome

The use of antimycotic drugs in fungal infections is based on the concept that they suppress fungal growth by a direct killing effect. However, amphotericin and nystatin have been reported to also trigger interleukin-1β (IL-1β) secretion in monocytes but the molecular mechanism is unknown. Here we report that only the polyene macrolides amphotericin B, nystatin, and natamycin but none of the tested azole antimycotic drugs induce significant IL-1β secretion in-vitro in dendritic cells isolated from C57BL/6 mouse bone marrow. IL-1β release depended on Toll-like receptor-mediated induction of pro-IL-1β as well as the NLRP3 inflammasome, its adaptor ASC, and caspase-1 for enzymatic cleavage of pro-IL-1β into its mature form. All three drugs induced potassium efflux from the cells as a known mechanism for NLRP3 activation but the P2X7 receptor was not required for this process. Natamycin-induced IL-1β secretion also involved phagocytosis, as cathepsin activation as described for crystal-induced IL-1β release. Together, the polyene macrolides amphotericin B, nystatin, and natamycin trigger IL-1β secretion by causing potassium efflux from which activates the NLRP3-ASC-caspase-1. We conclude that beyond their effects on fungal growth, these antifungal drugs directly activate the host's innate immunity

High-pressure x-ray diffraction study of SrMoO4 and pressure-induced structural changes

SrMoO4 was studied under compression up to 25 GPa by angle-dispersive x-ray diffraction. A phase transition was observed from the scheelite-structured ambient phase to a monoclinic fergusonite phase at 12.2(9) GPa with cell parameters a = 5.265(9) A, b = 11.191(9) A, c = 5.195 (5) A, and beta = 90.9, Z = 4 at 13.1 GPa. There is no significant volume collapse at the phase transition. No additional phase transitions were observed and on release of pressure the initial phase is recovered, implying that the observed structural modifications are reversible. The reported transition appeared to be a ferroelastic second-order transformation producing a structure that is a monoclinic distortion of the low-pressure phase and was previously observed in compounds isostructural to SrMoO4. A possible mechanism for the transition is proposed and its character is discussed in terms of the present data and the Landau theory. Finally, the EOS is reported and the anisotropic compressibility of the studied crystal is discussed in terms of the compression of the Sr-O and Mo-O bonds.Comment: 36 pages, 9 figures, 1 tabl

GM-CSF Signalling Boosts Dramatically IL-1Production

GM-CSF is mostly known for its capacity to promote bone marrow progenitor differentiation, to mobilize and mature myeloid cells as well as to enhance host immune responses. However the molecular actions of GM-CSF are still poorly characterized. Here we describe a new surprising facet of this “old” growth factor as a key regulator involved in IL-1βsecretion. We found that IL-1β release, a pivotal component of the triggered innate system, is heavily dependent on the signaling induced by GM-CSF in such an extent that in its absence IL-1β is only weakly secreted. GM-CSF synergizes with LPS for IL-1β secretion mainly at the level of pro-IL-1β production via strengthening the NF-κB signaling. In addition, we show that expression of Rab39a, a GTPase required for caspase-1 dependent IL-1β secretion is greatly augmented by LPS and GM-CSF co-stimulation suggesting a potential GM-CSF contribution in enhancing IL-1β exocytosis. The role of GM-CSF in regulating IL-1β secretion is extended also in vivo, since GM-CSF R−/− mice are more resistant to LPS-mediated septic shock. These results identify GM-CSF as a key regulator of IL-1β production and indicate GM-CSF as a previously underestimated target for therapeutic intervention

The nuclear receptor LXR modulates interleukin-18 levels through multiple mechanisms

IL-18 is a member of the IL-1 family involved in innate immunity and inflammation. Deregulated levels of IL-18 are involved in the pathogenesis of multiple disorders including inflammatory and metabolic diseases, yet relatively little is known regarding its regulation. Liver X receptors or LXRs are key modulators of macrophage cholesterol homeostasis and immune responses. Here we show that LXR ligands negatively regulate LPS-induced mRNA and protein expression of IL-18 in bone marrow-derived macrophages. Consistent with this being an LXR-mediated process, inhibition is abolished in the presence of a specific LXR antagonist and in LXR-deficient macrophages. Additionally, IL-18 processing of its precursor inactive form to its bioactive state is inhibited by LXR through negative regulation of both pro-caspase 1 expression and activation. Finally, LXR ligands further modulate IL-18 levels by inducing the expression of IL-18BP, a potent endogenous inhibitor of IL-18. This regulation occurs via the transcription factor IRF8, thus identifying IL-18BP as a novel LXR and IRF8 target gene. In conclusion, LXR activation inhibits IL-18 production through regulation of its transcription and maturation into an active pro-inflammatory cytokine. This novel regulation of IL-18 by LXR could be applied to modulate the severity of IL-18 driven metabolic and inflammatory disorders

TLR2 Signaling Contributes to Rapid Inflammasome Activation during F. novicida Infection

Early detection of microorganisms by the innate immune system is provided by surface-expressed and endosomal pattern recognition receptors (PRRs) such as Toll-like receptors (TLRs). Detection of microbial components by TLRs initiates a signaling cascade leading to the expression of proinflammatory cytokines including IL-6 and IL-1β. Some intracellular bacteria subvert the TLR response by rapidly escaping the phagosome and entering the cytosol. However, these bacteria may be recognized by the inflammasome, a multi-protein complex comprised of a sensor protein, ASC and the cysteine protease caspase-1. Inflammasome activation leads to release of the proinflammatory cytokines IL-1β and IL-18 and death of the infected cell, an important host defense that eliminates the pathogen's replicative niche. While TLRs and inflammasomes are critical for controlling bacterial infections, it is unknown whether these distinct host pathways cooperate to activate defenses against intracellular bacteria.Using the intracellular bacterium Francisella novicida as a model, we show that TLR2(-/-) macrophages exhibited delayed inflammasome activation compared to wild-type macrophages as measured by inflammasome assembly, caspase-1 activation, cell death and IL-18 release. TLR2 also contributed to inflammasome activation in response to infection by the cytosolic bacterium Listeria monocytogenes. Components of the TLR2 signaling pathway, MyD88 and NF-κB, were required for rapid inflammasome activation. Furthermore, TLR2(-/-) mice exhibited lower levels of cell death, caspase-1 activation, and IL-18 production than wild-type mice upon F. novicida infection.These results show that TLR2 is required for rapid inflammasome activation in response to infection by cytosolic bacterial pathogens. In addition to further characterizing the role of TLR2 in host defense, these findings broaden our understanding of how the host integrates signals from spatiotemporally separated PRRs to coordinate an innate response against intracellular bacteria