Enhancement of reactive oxygen species production and chlamydial infection by the mitochondrial Nod-like family member NLRX1

Porphyromonas gingivalis-nucleoside-diphosphate-kinase inhibits ATP-induced reactive-oxygen-species via P2X7 receptor/NADPH-oxidase signalling and contributes to persistence

Ligation of P2X7 receptors with a 'danger signal', extracellular ATP (eATP), has recently been shown to result in production of intracellular reactive-oxygen-species (ROS) in macrophages. We show that primary gingival epithelial cells (GECs) produce sustained, robust cellular ROS upon stimulation by eATP. The induction of ROS was mediated by P2X7 receptor signalling coupled with NADPH-oxidase activation, as determined by pharmacological inhibition and RNA interference. Furthermore, Porphyromonas gingivalis, an oral opportunistic pathogen, upregulated the antioxidant glutathione response, modulated eATP-induced cytosolic and mitochondrial ROS generated through P2X7 /NADPH-oxidase interactome, and subsequently blocked oxidative stress in GECs via temporal secretion of a P. gingivalis effector, nucleoside-diphosphate-kinase (Ndk). An ndk-deficient P. gingivalis mutant lacked the ability to inhibit ROS production and persist intracellularly following eATP stimulation. Treatment with recombinant Ndk significantly diminished eATP-evoked ROS production. P. gingivalis infection elicited a strong, time-dependent increase in anti-oxidativemitochondrial UCP2 levels, whereas ndk-deficient mutant did not cause any change. The results reveal a novel signalling cascade that is tightly coupled with eATP signalling and ROS regulation. Ndk by P. gingivalis counteracts these antimicrobial signalling activities by secreting Ndk, thus contributing to successful persistence of the pathogen.

Imbalanced oxidative stress causes chlamydial persistence during non-productive human herpes virus co-infection

Both human herpes viruses and Chlamydia are highly prevalent in the human population and are detected together in different human disorders. Here, we demonstrate that co-infection with human herpes virus 6 (HHV6) interferes with the developmental cycle of C. trachomatis and induces persistence. Induction of chlamydial persistence by HHV6 is independent of productive virus infection, but requires the interaction and uptake of the virus by the host cell. On the other hand, viral uptake is strongly promoted under co-infection conditions. Host cell glutathione reductase activity was suppressed by HHV6 causing NADPH accumulation, decreased formation of reduced glutathione and increased oxidative stress. Prevention of oxidative stress restored infectivity of Chlamydia after HHV6-induced persistence. We show that co-infection with Herpes simplex virus 1 or human Cytomegalovirus also induces chlamydial persistence by a similar mechanism suggesting that Chlamydia -human herpes virus co-infections are evolutionary shaped interactions with a thus far unrecognized broad significance.

Innate barriers to viral infection

Innate immunity represents the foremost barrier to viral infection. In order to infect a cell efficiently, viruses need to evade innate immune effectors such as interferons and inflammatory cytokines. Pattern recognition receptors can detect viral components or pathogen-associated molecular patterns. These receptors then elicit innate immune responses that result in the generation of type I interferons and proinflammatory cytokines. Organized by the Society for General Microbiology, one session of this conference focused on the current state-of-the-art knowledge on innate barriers to infection of different RNA and DNA viruses. Experts working on innate immunity in the context of viral infection provided insight into different aspects of innate immune recognition and also discussed areas for future research. Here, we provide an overview of the session on innate barriers to infection.

Multifaceted roles of purinergic receptors in viral infection

Extracellular nucleotides and purinergic receptors participate in numerous cellular processes during viral infection. Despite their positive role in the immune response, purinergic signals can also favor the infection of cells by viruses and the pathogeny of viral diseases. Here, we highlight the multiple ambiguous roles of purinergic receptors in viral infections.

NLRX1 has a tail to tell

In this issue of Immunity, Hong et al. (2012) report the first structural analysis of the C-terminal fragment of an NLR (nucleotide-binding domain [NBD] and leucine-rich repeat [LRR]-containing) protein, NLRX1. This fragment forms a hexamer and binds RNA.

NLRs, inflammasomes, and viral infection

NLR proteins are innate immune sensors that respond to microbial infection. Upon pathogen infection, some NLR proteins form large complexes, called inflammasomes, which activate caspase-1 and induce the production of active IL-1β and IL-18. Activation of inflammasomes can also lead to an inflammatory cell death program, named pyroptosis. In this review, we will discuss the role of various NLR proteins in sensing different viral infections, as well as the strategies used by several RNA and DNA viruses to counteract the antiviral effects of NLR-dependent inflammasomes.

Activation of an NLRP3 inflammasome restricts Mycobacterium kansasii infection

Mycobacterium kansasii has emerged as an important nontuberculous mycobacterium pathogen, whose incidence and prevalence have been increasing in the last decade. M. kansasii can cause pulmonary tuberculosis clinically and radiographically indistinguishable from that caused by Mycobacterium tuberculosis infection. Unlike the widely-studied M. tuberculosis, little is known about the innate immune response against M. kansasii infection. Although inflammasome activation plays an important role in host defense against bacterial infection, its role against atypical mycobacteria remains poorly understood. In this report, the role of inflammasome activity in THP-1 macrophages against M. kansasii infection was studied. Results indicated that viable, but not heat-killed, M. kansasii induced caspase-1-dependent IL-1β secretion in macrophages. The underlying mechanism was found to be through activation of an inflammasome containing the NLR (Nod-like receptor) family member NLRP3 and the adaptor protein ASC (apoptosis-associated speck-like protein containing a CARD). Further, potassium efflux, lysosomal acidification, ROS production and cathepsin B release played a role in M. kansasii-induced inflammasome activation. Finally, the secreted IL-1β derived from caspase-1 activation was shown to restrict intracellular M. kansasii. These findings demonstrate a biological role for the NLRP3 inflammasome in host defense against M. kansasii.

Rac1 regulates the NLRP3 inflammasome which mediates IL-1beta production in Chlamydophila pneumoniae infected human mononuclear cells

Chlamydophila pneumoniae causes acute respiratory tract infections and has been associated with development of asthma and atherosclerosis. The production of IL-1β, a key mediator of acute and chronic inflammation, is regulated on a transcriptional level and additionally on a posttranslational level by inflammasomes. In the present study we show that C. pneumoniae-infected human mononuclear cells produce IL-1β protein depending on an inflammasome consisting of NLRP3, the adapter protein ASC and caspase-1. We further found that the small GTPase Rac1 is activated in C. pneumoniae-infected cells. Importantly, studies with specific inhibitors as well as siRNA show that Rac1 regulates inflammasome activation in C. pneumoniae-infected cells. In conclusion, C. pneumoniae infection of mononuclear cells stimulates IL-1β production dependent on a NLRP3 inflammasome-mediated processing of proIL-1β which is controlled by Rac1.

Mitochondria in innate immunity

Mitochondria are cellular organelles involved in host-cell metabolic processes and the control of programmed cell death. A direct link between mitochondria and innate immune signalling was first highlighted with the identification of MAVS-a crucial adaptor for RIGI-like receptor signalling-as a mitochondria-anchored protein. Recently, other innate immune molecules, such as NLRX1, TRAF6, NLRP3 and IRGM have been functionally associated with mitochondria. Furthermore, mitochondrial alarmins-such as mitochondrial DNA and formyl peptides-can be released by damaged mitochondria and trigger inflammation. Therefore, mitochondria emerge as a fundamental hub for innate immune signalling.

Innate immune response to influenza virus

PURPOSE OF REVIEW

The recent pandemic of a novel H1N1 influenza virus has stressed the importance of effective approaches to prevent viral infection. The innate immune system is our first line of defense against invading viruses. This review aims to give a brief summary of recent findings on the response of the innate immune system to influenza virus.

RECENT FINDINGS

Three families of pattern recognition receptors, toll-like receptors (TLRs), retinoic acid-inducible gene 1 protein like helicases (RLRs) and nucleotide-binding domain and leucine-rich-repeat-containing proteins (NLRs), are involved in recognition of influenza virus and they cooperatively operate to respond to the virus in cell culture or mouse models. Influenza virus mainly induces two types of innate immune cytokine responses: a proinflammatory response and an antiviral response. Recently, the NLRP3 inflammasome has proved to be an essential component in the host defense against influenza infection. The mitochondrion, traditionally recognized for its key role in respiration, metabolism and apoptosis, is becoming recognized as an important organelle for regulation of innate immune responses to influenza virus.

SUMMARY

The NLRP3 inflammasome is an essential component in the host defense against influenza infection. Further investigations are required to elucidate whether NLRP3 is associated with the adaptive response and to identify the components of influenza virus that activate this important mediator. The role of mitochondria as a potential central platform of innate response is becoming appreciated.

Regulation of the antimicrobial response by NLR proteins

Nucleotide-binding, oligomerization domain (NOD)-like receptor (NLR) proteins are a family of innate immune receptors that play a pivotal role in microbial sensing, leading to the initiation of antimicrobial immune responses. Dysregulation of the function of multiple NLR family members has been linked, both in mice and humans, to a propensity for infection and autoinflammatory disease. Despite our increased understanding of NLR function and interactions, many aspects related to mechanisms of sensing, downstream signaling, and in vivo functions remain elusive. In this review, we focus on key members of the NLR family, describing their activation by diverse microbes, downstream effector functions, and interactions with each other and with other innate sensor protein families. Also discussed is the role of microbial sensing by NLR receptors leading to activation of the adaptive immune arm that collaborates in the antimicrobial defense.

NLRX1 protein attenuates inflammatory responses to infection by interfering with the RIG-I-MAVS and TRAF6-NF-κB signaling pathways

The nucleotide-binding domain and leucine-rich-repeat-containing (NLR) proteins regulate innate immunity. Although the positive regulatory impact of NLRs is clear, their inhibitory roles are not well defined. We showed that Nlrx1(-/-) mice exhibited increased expression of antiviral signaling molecules IFN-β, STAT2, OAS1, and IL-6 after influenza virus infection. Consistent with increased inflammation, Nlrx1(-/-) mice exhibited marked morbidity and histopathology. Infection of these mice with an influenza strain that carries a mutated NS-1 protein, which normally prevents IFN induction by interaction with RNA and the intracellular RNA sensor RIG-I, further exacerbated IL-6 and type I IFN signaling. NLRX1 also weakened cytokine responses to the 2009 H1N1 pandemic influenza virus in human cells. Mechanistically, Nlrx1 deletion led to constitutive interaction of MAVS and RIG-I. Additionally, an inhibitory function is identified for NLRX1 during LPS activation of macrophages where the MAVS-RIG-I pathway was not involved. NLRX1 interacts with TRAF6 and inhibits NF-κB activation. Thus, NLRX1 functions as a checkpoint of overzealous inflammation.

TLRs, NLRs and RLRs: innate sensors and their impact on allergic diseases--a current view

Charles Janeway first wrote 1989 about how important recognition of "certain characteristics or patterns common on infectious agents but absent from the host" would be for our immune response [1]. Surprisingly, it almost took 10 years before his ideas would lead to the revolutionary findings that fundamentally changed the view of the innate immune system over the past decade. Recognition of invading microorganisms belongs to the primary tasks of the innate immune system and is achieved through different families of innate immune sensors. Among these, Toll-like receptors (TLRs), nucleotide-binding domain and leucine-rich repeat containing receptors (NLRs) and Rig-I-like receptors (RLRs) have drawn major interests over the last decade. These receptor families are targeted by overlapping classes of pathogens and share functional domains and signal transduction pathways (see Fig. 1 and Table 1 for an overview of their structural organization, ligands, adaptors and activated pathways). This current view describes our present knowledge about these three main innate immune receptor families and their importance for adaptive immune responses such as asthma and allergy.

The role of reactive-oxygen-species in microbial persistence and inflammation

The mechanisms of chronic infections caused by opportunistic pathogens are of keen interest to both researchers and health professionals globally. Typically, chronic infectious disease can be characterized by an elevation in immune response, a process that can often lead to further destruction. Reactive-Oxygen-Species (ROS) have been strongly implicated in the aforementioned detrimental response by host that results in self-damage. Unlike excessive ROS production resulting in robust cellular death typically induced by acute infection or inflammation, lower levels of ROS produced by host cells are increasingly recognized to play a critical physiological role for regulating a variety of homeostatic cellular functions including growth, apoptosis, immune response, and microbial colonization. Sources of cellular ROS stimulation can include "danger-signal-molecules" such as extracellular ATP (eATP) released by stressed, infected, or dying cells. Particularly, eATP-P2X(7) receptor mediated ROS production has been lately found to be a key modulator for controlling chronic infection and inflammation. There is growing evidence that persistent microbes can alter host cell ROS production and modulate eATP-induced ROS for maintaining long-term carriage. Though these processes have yet to be fully understood, exploring potential positive traits of these "injurious" molecules could illuminate how opportunistic pathogens maintain persistence through physiological regulation of ROS signaling.

What is new with Nods?

Over the last few years, much research has focused on determining the function of members of the cytosolic Nod-like receptor (NLR) family in terms of their triggers and the signaling pathways that they control. Members of this family include the NLRP proteins, which play a role in sensing both microbial and danger signals and triggering the caspase-1 dependent inflammasome, and the Nod subfamily characterized by proteins with a caspase-activating and recruitment domain (CARD) or a so-called 'X' domain. Nod1, Nod2, NLRX1 and NLRC5 are all members of this subfamily and in this review, we will focus on recent work that has shown the importance of these molecules in both pathogen sensing and regulation of innate and adaptive immunity.