Manira Rayamajhi

Induction of IFN-αβ enables Listeria monocytogenes to suppress macrophage activation by IFN-γ

Production of type I interferon (IFN; IFN-αβ) increases host susceptibility to Listeria monocytogenes, whereas type II IFN (IFN-γ) activates macrophages to resist infection. We show that these opposing immunological effects of IFN-αβ and IFN-γ occur because of cross talk between the respective signaling pathways. We found that cultured macrophages infected with L. monocytogenes were refractory to IFN-γ treatment as a result of down-regulation of the IFN-γ receptor (IFNGR). The soluble factor responsible for these effects was identified as host IFN-αβ. Accordingly, macrophages and dendritic cells (DCs) showed reduced IFNGR1 expression and reduced responsiveness to IFN-γ during systemic infection of IFN-αβ–responsive mice. Furthermore, the increased resistance of mice lacking the IFN-αβ receptor (IFNAR−/−) to L. monocytogenes correlated with increased expression of IFN-γ–dependent activation markers by macrophages and DCs and was reversed by depletion of IFN-γ. Thus, IFN-αβ produced in response to bacterial infection and other stimuli antagonizes the host response to IFN-γ by down-regulating the IFNGR. Such cross talk permits prioritization of IFN-αβ–type immune responses and may contribute to the beneficial effects of IFN-β in treatment of inflammatory diseases such as multiple sclerosis.

Cutting Edge: Mouse NAIP1 Detects the Type III Secretion System Needle Protein

The NAIP/NLRC4 inflammasomes activate caspase-1 in response to bacterial type III secretion systems (T3SSs). Inadvertent injection of the T3SS rod protein and flagellin into the cytosol is detected through murine NAIP2 and NAIP5/6, respectively. In this study, we identify the agonist for the orphan murine NAIP1 receptor as the T3SS needle protein. NAIP1 is poorly expressed in resting mouse bone marrow–derived macrophages; however, priming with polyinosinic-polycytidylic acid induces it and confers needle protein sensitivity. Further, overexpression of NAIP1 in immortalized bone marrow–derived macrophages by retroviral transduction enabled needle detection. In contrast, peritoneal cavity macrophages basally express NAIP1 and respond to needle protein robustly, independent of priming. Human macrophages are known to express only one NAIP gene, which detects the needle protein, but not rod or flagellin. Thus, murine NAIP1 is functionally analogous to human NAIP.

Detection of Pyroptosis by Measuring Released Lactate Dehydrogenase Activity

Pyroptosis is a form of programmed, inflammatory cell death that is dependent on the activation of a cysteine protease caspase-1. Following caspase-1 activation via inflammasomes (including NLRP3, NLRC4, Nlrp1b, and AIM2), cells lose membrane integrity and lyse, releasing lactate dehydrogenase (LDH) that is normally maintained within the cell cytosol. Thus, pyroptosis is distinct from apoptosis, which results in cellular contents being enclosed within membrane blebs during cellular demise. LDH is only released from apoptotic blebs after secondary necrosis occurs. Pyroptosis is distinct from necrosis in that it requires the activity of caspase-1. In this chapter, we describe enzymatic assays for the detection of LDH released by pyroptotic cells using a commercially available kit, as well as a simple and cost-effective method adapted from Decker et al. (J Immunol Methods 115:61-69, 1988).

The RIP1-RIP3 complex initiates mitochondrial fission to fuel NLRP3.

Vesicular stomatitis virus, a single-stranded RNA virus, triggers activation of the serine-threonine kinases RIP1 and RIP3, which damages mitochondria by activating the GTPase DRP1. This results in excessive production of reactive oxygen species and subsequent activation of the NLRP3 inflammasome.

The Prostaglandin E2-EP3 Receptor Axis Regulates Anaplasma phagocytophilum-Mediated NLRC4 Inflammasome Activation

Rickettsial agents are sensed by pattern recognition receptors but lack pathogen-associated molecular patterns commonly observed in facultative intracellular bacteria. Due to these molecular features, the order Rickettsiales can be used to uncover broader principles of bacterial immunity. Here, we used the bacterium Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis, to reveal a novel microbial surveillance system. Mechanistically, we discovered that upon A. phagocytophilum infection, cytosolic phospholipase A2 cleaves arachidonic acid from phospholipids, which is converted to the eicosanoid prostaglandin E2 (PGE2) via cyclooxygenase 2 (COX2) and the membrane associated prostaglandin E synthase-1 (mPGES-1). PGE2-EP3 receptor signaling leads to activation of the NLRC4 inflammasome and secretion of interleukin (IL)-1β and IL-18. Importantly, the receptor-interacting serine/threonine-protein kinase 2 (RIPK2) was identified as a major regulator of the immune response against A. phagocytophilum. Accordingly, mice lacking COX2 were more susceptible to A. phagocytophilum, had a defect in IL-18 secretion and exhibited splenomegaly and damage to the splenic architecture. Remarkably, Salmonella-induced NLRC4 inflammasome activation was not affected by either chemical inhibition or genetic ablation of genes associated with PGE2 biosynthesis and signaling. This divergence in immune circuitry was due to reduced levels of the PGE2-EP3 receptor during Salmonella infection when compared to A. phagocytophilum. Collectively, we reveal the existence of a functionally distinct NLRC4 inflammasome illustrated by the rickettsial agent A. phagocytophilum.

Just say NO to NLRP3

Feedback inhibition by interferon-γ blocks the NLRP3 inflammasome by triggering inducible nitric oxide synthase (iNOS), and the resulting nitric oxide (NO) thio-nitrosylates NLRP3, shutting it down. This prevents excessive immunopathology during chronic infection with Mycobacterium tuberculosis.

miniMAVS, You Complete Me!

The functional significance of protein diversification through translational regulation in mammals is largely unexplored. Brubaker et al. now describe the generation of two functionally distinct mammalian proteins, MAVS and miniMAVS, from a single bicistronic mRNA and suggest that noncanonical translation may impact multiple players in innate immune regulation.

Erratum: The RIP1-RIP3 complex initiates mitochondrial fission to fuel NLRP3 (Nature Immunology (2014) 15 (1100-1102))

volume 15 number 12 december 2014 nature immunology and activation of neutrophils but also stimulates the production of chemokines and mucins by lung epithelial cells11. In a clinical setting, airway neutrophilia is correlated with increased asthma severity and steroid resistance11, and increased amounts of the NLRP3 inflammasome and IL-1β have been detected in the sputum of neutrophilic asthmatics12. The association between CLPs and IL-1β could explain their immediate anti-parasitic activity and could also help to elucidate their association with the pathologic sequelae of allergy. Thus, understanding the role of Ym1 and Ym2 in the innate and adaptive immune response should influence how the function of other CLPs in human disease is viewed and should help to determine whether they will provide new therapeutic targets or biomarkers for bacterial and helminth infections, asthma and malignancy2.