Constitutive immune mechanisms: mediators of host defence and immune regulation

The immune system enables organisms to combat infections and to eliminate endogenous challenges. Immune responses can be evoked through diverse inducible pathways. However, various constitutive mechanisms are also required for immunocompetence. The inducible responses of pattern recognition receptors of the innate immune system and antigen-specific receptors of the adaptive immune system are highly effective, but they also have the potential to cause extensive immunopathology and tissue damage, as seen in many infectious and autoinflammatory diseases. By contrast, constitutive innate immune mechanisms, including restriction factors, basal autophagy and proteasomal degradation, tend to limit immune responses, with loss-of-function mutations in these pathways leading to inflammation. Although they function through a broad and heterogeneous set of mechanisms, the constitutive immune responses all function as early barriers to infection and aim to minimize any disruption of homeostasis. Supported by recent human and mouse data, in this Review we compare and contrast the inducible and constitutive mechanisms of immunosurveillance

Legionella pneumophila strain 130b evades macrophage cell death independent of the effector SidF in the absence of flagellin

International audienceThe human pathogen Legionella pneumophila must evade host cell death signaling to enable replication in lung macrophages and to cause disease. After bacterial growth, however, L. pneumophila is thought to induce apoptosis during egress from macrophages. The bacterial effector protein, SidF, has been shown to control host cell survival and death by inhibiting pro-apoptotic BNIP3 and BCL-RAMBO signaling. Using live-cell imaging to follow the L. pneumophila-macrophage interaction, we now demonstrate that L. pneumophila evades host cell apoptosis independent of SidF. In the absence of SidF, L. pneumophila was able to replicate, cause loss of mitochondria membrane potential, kill macrophages, and establish infections in lungs of mice. Consistent with this, deletion of BNIP3 and BCL-RAMBO did not affect intracellular L. pneumophila replication, macrophage death rates, and in vivo bacterial virulence. Abrogating mitochondrial cell death by genetic deletion of the effectors of intrinsic apoptosis, BAX, and BAK, or the regulator of mitochondrial permeability transition pore formation, cyclophilin-D, did not affect bacterial growth or the initial killing of macrophages. Loss of BAX and BAK only marginally limited the ability of L. pneumophila to efficiently kill all macrophages over extended periods. L. pneumophila induced killing of macrophages was delayed in the absence of capsase-11 mediated pyroptosis. Together, our data demonstrate that L. pneumophila evades host cell death responses independently of SidF during replication and can induce pyroptosis to kill macrophages in a timely manner

Interleukin-1 receptor-associated kinase 4 (IRAK4) plays a dual role in myddosome formation and Toll-like receptor signaling

Toll-like receptors (TLRs) form part of the host innate immune system, in which they act as sensors of microbial and endogenous danger signals. Upon TLR activation, the intracellular Toll/interleukin-1 receptor domains of TLR dimers initiate oligomerization of a multiprotein signaling platform comprising myeloid differentiation primary response 88 (MyD88) and members of the interleukin-1 receptor-associated kinase (IRAK) family. Formation of this myddosome complex initiates signal transduction pathways, leading to the activation of transcription factors and the production of inflammatory cytokines. To date, little is known about the assembly and disassembly of the myddosome and about the mechanisms by which these complexes mediate multiple downstream signaling pathways. Here, we isolated myddosome complexes from whole-cell lysates of TLR-activated primary mouse macrophages and from IRAK reporter macrophages to examine the kinetics of myddosome assembly and disassembly. Using a selective inhibitor of IRAK4\u27s kinase activity, we found that whereas TLR cytokine responses were ablated, myddosome formation was stabilized in the absence of IRAK4\u27s kinase activity. Of note, IRAK4 inhibition had only a minimal effect on NF-kappaB and mitogen-activated protein kinase (MAPK) signaling. In summary, our results indicate that IRAK4 has a critical scaffold function in myddosome formation and that its kinase activity is dispensable for myddosome assembly and activation of the NF-kappaB and MAPK pathways but is essential for MyD88-dependent production of inflammatory cytokines. Our findings suggest that the scaffold function of IRAK4 may be an attractive target for treating inflammatory and autoimmune diseases

Inflammasome priming in sterile inflammatory disease

The inflammasome is a cytoplasmic protein complex that processes interleukins (IL)-1β and IL-18, and drives a form of cell death known as pyroptosis. Oligomerization of this complex is actually the second step of activation, and a priming step must occur first. This involves transcriptional upregulation of pro-IL-1β, inflammasome sensor NLRP3, or the non-canonical inflammasome sensor caspase-11. An additional aspect of priming is the post-translational modification of particular inflammasome constituents. Priming is typically accomplished in vitro using a microbial Toll-like receptor (TLR) ligand. However, it is now clear that inflammasomes are activated during the progression of sterile inflammatory diseases such as atherosclerosis, metabolic disease, and neuroinflammatory disorders. Therefore, it is time to consider the endogenous factors and mechanisms that may prime the inflammasome in these conditions

Dual Role for Inflammasome Sensors NLRP1 and NLRP3 in Murine Resistance to Toxoplasma gondii

Induction of immunity that limits Toxoplasma gondii infection in mice is critically dependent on the activation of the innate immune response. In this study, we investigated the role of cytoplasmic nucleotide-binding domain and leucine-rich repeat containing a pyrin domain (NLRP) inflammasome sensors during acute toxoplasmosis in mice. We show that in vitro Toxoplasma infection of murine bone marrow-derived macrophages activates the NLRP3 inflammasome, resulting in the rapid production and cleavage of interleukin-1β (IL-1β), with no measurable cleavage of IL-18 and no pyroptosis. Paradoxically, Toxoplasma-infected mice produced large quantities of IL-18 but had no measurable IL-1β in their serum. Infection of mice deficient in NLRP3, caspase-1/11, IL-1R, or the inflammasome adaptor protein ASC led to decreased levels of circulating IL-18, increased parasite replication, and death. Interestingly, mice deficient in NLRP1 also displayed increased parasite loads and acute mortality. Using mice deficient in IL-18 and IL-18R, we show that this cytokine plays an important role in limiting parasite replication to promote murine survival. Our findings reveal T. gondii as a novel activator of the NLRP1 and NLRP3 inflammasomes in vivo and establish a role for these sensors in host resistance to toxoplasmosis. IMPORTANCE Inflammasomes are multiprotein complexes that are a major component of the innate immune system. They contain “sensor” proteins that are responsible for detecting various microbial and environmental danger signals and function by activating caspase-1, an enzyme that mediates cleavage and release of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18. Toxoplasma gondii is a highly successful protozoan parasite capable of infecting a wide range of host species that have variable levels of resistance. We report here that T. gondii is a novel activator of the NLRP1 and NLRP3 inflammasomes in vivo and establish a role for these sensors in host resistance to toxoplasmosis. Using mice deficient in IL-18 and IL-18R, we show that the IL-18 cytokine plays a pivotal role by limiting parasite replication to promote murine survival.National Institutes of Health (U.S.) (Intramural Research Program of the NIH and NIAID)Crohn's and Colitis Foundation of America (Research Fellowship)Crohn's and Colitis Foundation of America (CCFA Helmsley Scholar)National Institutes of Health (U.S.) (NIH grant AI104170)National Institutes of Health (U.S.) (R01-AI080621)Pew Charitable Trusts (Pew Scholars Program in the Biomedical Sciences)Canadian Institute for Advanced Research (CIFAR Program for Integrated Microbial Biodiversity

Searching for nuclear stellar discs in simulations of star cluster mergers

The nuclei of galaxies often host small stellar discs with scalelengths of a few tens of parsecs and luminosities up to 107 L�. To investigate the formation and properties of nuclear stellar discs (NSDs), we look for their presence in a set of N-body simulations studying the dissipationless merging of multiple star clusters in galactic nuclei. A few tens of star clusters with sizes and masses comparable to those of globular clusters observed in the Milky Way are accreted on to a pre-existing nuclear stellar component: either a massive super star cluster or a rapidly rotating, compact disc with a scalelength of a few parsecs, mimicking the variety of observed nuclear structures. Images and kinematic maps of the simulation time-steps are then built and analysed as if they were real and at the distance of the Virgo cluster. We use the Scorza–Bender method to search for the presence of disc structures via photometric decomposition. In one case, the merger remnant has all the observed photometric and kinematic properties of NSDs observed in real galaxies. This shows that current observations are consistent with most of the NSD mass being assembled from the migration and accretion of star clusters into the galactic centre. In the other simulation instead, we detect an elongated structure from the unsharp masked image, that does not develop the photometric or kinematic signature of an NSD. Thus, in the context of searches for a disc structure, the Scorza–Bender method is a robust and necessary tool

The ACS Fornax Cluster Survey. VI. The Nuclei of Early-Type Galaxies in the Fornax Cluster

The Advanced Camera for Surveys (ACS) Fornax Cluster Survey is a Hubble Space Telescope program to image 43 early-type galaxies in the Fornax cluster, using the F475W and F850LP bandpasses of the ACS. We employ both 1D and 2D techniques to characterize the properties of the stellar nuclei in these galaxies, defined as the central "luminosity excesses" relative to a Sersic model fitted to the underlying host. We find 72+/-13% of our sample (31 galaxies) to be nucleated, with only three of the nuclei offset by more than 0.5" from their galaxy photocenter, and with the majority of nuclei having colors bluer than their hosts. The nuclei are observed to be larger, and brighter, than typical Fornax globular clusters, and to follow different structural scaling relations. A comparison of our results to those from the ACS Virgo Cluster Survey reveals striking similarities in the properties of the nuclei belonging to these different environments. We briefly review a variety of proposed formation models and conclude that, for the low-mass galaxies in our sample, the most important mechanism for nucleus growth is probably infall of star clusters through dynamical friction, while for higher mass galaxies, gas accretion triggered by mergers, accretions and tidal torques is likely to dominate, with the relative importance of these two processes varying smoothly as a function of galaxy mass. Some intermediate-mass galaxies in our sample show a complexity in their inner structure that may be the signature of "hybrid nuclei" that arose through parallel formation channels.Comment: 34 pages, 27 figures, accepted for publication in ApJ

Pyrin Modulates the Intracellular Distribution of PSTPIP1

PSTPIP1 is a cytoskeleton-associated adaptor protein that links PEST-type phosphatases to their substrates. Mutations in PSTPIP1 cause PAPA syndrome (Pyogenic sterile Arthritis, Pyoderma gangrenosum, and Acne), an autoinflammatory disease. PSTPIP1 binds to pyrin and mutations in pyrin result in familial Mediterranean fever (FMF), a related autoinflammatory disorder. Since disease-associated mutations in PSTPIP1 enhance pyrin binding, PAPA syndrome and FMF are thought to share a common pathoetiology. The studies outlined here describe several new aspects of PSTPIP1 and pyrin biology. We document that PSTPIP1, which has homology to membrane-deforming BAR proteins, forms homodimers and generates membrane-associated filaments in native and transfected cells. An extended FCH (Fes-Cip4 homology) domain in PSTPIP1 is necessary and sufficient for its self-aggregation. We further show that the PSTPIP1 filament network is dependent upon an intact tubulin cytoskeleton and that the distribution of this network can be modulated by pyrin, indicating that this is a dynamic structure. Finally, we demonstrate that pyrin can recruit PSTPIP1 into aggregations (specks) of ASC, another pyrin binding protein. ASC specks are associated with inflammasome activity. PSTPIP1 molecules with PAPA-associated mutations are recruited by pyrin to ASC specks with particularly high efficiency, suggesting a unique mechanism underlying the robust inflammatory phenotype of PAPA syndrome