Venizelos Papayannopoulos

Neutrophils sense microbe size and selectively release neutrophil extracellular traps in response to large pathogens

Neutrophils are critical for antifungal defense, but the mechanisms that clear hyphae and other pathogens that are too large to be phagocytosed remain unknown. We found that neutrophils sensed microbe size and selectively released neutrophil extracellular traps (NETs) in response to large pathogens, such as Candida albicans hyphae and extracellular aggregates of Mycobacterium bovis, but not in response to small yeast or single bacteria. NETs were fundamental in countering large pathogens in vivo. Phagocytosis via dectin-1 acted as a sensor of microbe size and prevented NET release by downregulating the translocation of neutrophil elastase (NE) to the nucleus. Dectin-1 deficiency led to aberrant NET release and NET-mediated tissue damage during infection. Size-tailored neutrophil responses cleared large microbes and minimized pathology when microbes were small enough to be phagocytosed.

Neutrophil extracellular traps license macrophages for cytokine production in atherosclerosis

Neutrophil NETs drive atherosclerosis The buildup of fats, cholesterol, and other substances in arteries causes atherosclerosis, which restricts blood flow and can lead to heart attacks and stroke. Inflammation contributes to the pathogenesis of atherosclerosis, but exactly how is not fully understood. Warnatsch et al. now show that immune cells called neutrophils release NETs (neutrophil extracellular traps) (see the Perspective by Nahrendorf and Swirski). These NETs are composed of DNA and antimicrobial proteins, and in the setting of atherosclerosis they activate innate immune signaling pathways in macrophages. This causes the macrophages to secrete proinflammatory cytokines, exacerbating the disease. Indirectly, NETS also attract a specialized subset of T cells that further amplify the proinflammatory response. Science, this issue p. 316; see also p. 237 Neutrophils release networks of extracellular fibers that contribute to the pathogenesis of atherosclerosis. [Also see Perspective by Nahrendorf and Swirski] Secretion of the cytokine interleukin-1β (IL-1β) by macrophages, a major driver of pathogenesis in atherosclerosis, requires two steps: Priming signals promote transcription of immature IL-1β, and then endogenous “danger” signals activate innate immune signaling complexes called inflammasomes to process IL-1β for secretion. Although cholesterol crystals are known to act as danger signals in atherosclerosis, what primes IL-1β transcription remains elusive. Using a murine model of atherosclerosis, we found that cholesterol crystals acted both as priming and danger signals for IL-1β production. Cholesterol crystals triggered neutrophils to release neutrophil extracellular traps (NETs). NETs primed macrophages for cytokine release, activating T helper 17 (TH17) cells that amplify immune cell recruitment in atherosclerotic plaques. Therefore, danger signals may drive sterile inflammation, such as that seen in atherosclerosis, through their interactions with neutrophils.

Chitinase-like proteins promote IL-17-mediated neutrophilia in a tradeoff between nematode killing and host damage.

Enzymatically inactive chitinase-like proteins (CLPs) such as BRP-39, Ym1 and Ym2 are established markers of immune activation and pathology, yet their functions are essentially unknown. We found that Ym1 and Ym2 induced the accumulation of neutrophils through the expansion of γδ T cell populations that produced interleukin 17 (IL-17). While BRP-39 did not influence neutrophilia, it was required for IL-17 production in γδ T cells, which suggested that regulation of IL-17 is an inherent feature of mouse CLPs. Analysis of a nematode infection model, in which the parasite migrates through the lungs, revealed that the IL-17 and neutrophilic inflammation induced by Ym1 limited parasite survival but at the cost of enhanced lung injury. Our studies describe effector functions of CLPs consistent with innate host defense traits of the chitinase family.

Neutrophil extracellular traps in immunity and disease

Neutrophils are innate immune phagocytes that have a central role in immune defence. Our understanding of the role of neutrophils in pathogen clearance, immune regulation and disease pathology has advanced dramatically in recent years. Web-like chromatin structures known as neutrophil extracellular traps (NETs) have been at the forefront of this renewed interest in neutrophil biology. The identification of molecules that modulate the release of NETs has helped to refine our view of the role of NETs in immune protection, inflammatory and autoimmune diseases and cancer. Here, I discuss the key findings and concepts that have thus far shaped the field of NET biology.

Host DNA released by NETosis promotes rhinovirus-induced type-2 allergic asthma exacerbation

Respiratory viral infections represent the most common cause of allergic asthma exacerbations. Amplification of the type-2 immune response is strongly implicated in asthma exacerbation, but how virus infection boosts type-2 responses is poorly understood. We report a significant correlation between the release of host double-stranded DNA (dsDNA) following rhinovirus infection and the exacerbation of type-2 allergic inflammation in humans. In a mouse model of allergic airway hypersensitivity, we show that rhinovirus infection triggers dsDNA release associated with the formation of neutrophil extracellular traps (NETs), known as NETosis. We further demonstrate that inhibiting NETosis by blocking neutrophil elastase or by degrading NETs with DNase protects mice from type-2 immunopathology. Furthermore, the injection of mouse genomic DNA alone is sufficient to recapitulate many features of rhinovirus-induced type-2 immune responses and asthma pathology. Thus, NETosis and its associated extracellular dsDNA contribute to the pathogenesis and may represent potential therapeutic targets of rhinovirus-induced asthma exacerbations.

TRAIL+ monocytes and monocyte‐related cells cause lung damage and thereby increase susceptibility to influenza–Streptococcus pneumoniae coinfection

Streptococcus pneumoniae coinfection is a major cause of influenza‐associated mortality; however, the mechanisms underlying pathogenesis or protection remain unclear. Using a clinically relevant mouse model, we identify immune‐mediated damage early during coinfection as a new mechanism causing susceptibility. Coinfected CCR2−/− mice lacking monocytes and monocyte‐derived cells control bacterial invasion better, show reduced epithelial damage and are overall more resistant than wild‐type controls. In influenza‐infected wild‐type lungs, monocytes and monocyte‐derived cells are the major cell populations expressing the apoptosis‐inducing ligand TRAIL. Accordingly, anti‐TRAIL treatment reduces bacterial load and protects against coinfection if administered during viral infection, but not following bacterial exposure. Post‐influenza bacterial outgrowth induces a strong proinflammatory cytokine response and massive inflammatory cell infiltrate. Depletion of neutrophils or blockade of TNF‐α facilitate bacterial outgrowth, leading to increased mortality, demonstrating that these factors aid bacterial control. We conclude that inflammatory monocytes recruited early, during the viral phase of coinfection, induce TRAIL‐mediated lung damage, which facilitates bacterial invasion, while TNF‐α and neutrophil responses help control subsequent bacterial outgrowth. We thus identify novel determinants of protection versus pathology in influenza–Streptococcus pneumoniae coinfection.

Reactive Oxygen Species Localization Programs Inflammation to Clear Microbes of Different Size

SUMMARY How the number of immune cells recruited to sites of infection is determined and adjusted to differences in the cellular stoichiometry between host and pathogen is unknown. Here, we have uncovered a role for reactive oxygen species (ROS) as sensors of microbe size. By sensing the differential localization of ROS generated in response to microbes of different size, neutrophils tuned their interleukin (IL)‐1&bgr; expression via the selective oxidation of NF‐&kgr;B, in order to implement distinct inflammatory programs. Small microbes triggered ROS intracellularly, suppressing IL‐1&bgr; expression to limit neutrophil recruitment as each phagocyte eliminated numerous pathogens. In contrast, large microbes triggered ROS extracellularly, amplifying IL‐1&bgr; expression to recruit numerous neutrophils forming cooperative clusters. Defects in ROS‐mediated microbe size sensing resulted in large neutrophil infiltrates and clusters in response to small microbes that contribute to inflammatory disease. These findings highlight the impact of ROS localization on signal transduction. HIGHLIGHTSThe clearance of microbes of different size involves distinct inflammatory programsROS adjusts neutrophil recruitment to the stoichiometry of host‐pathogen interactionsROS localization acts as a sensor of microbe size to tune IL‐1‐&bgr;‐driven inflammationIL‐1&bgr; amplifies neutrophil recruitment and clustering to clear large microbes &NA; Inflammation recruits neutrophils to fight invading pathogens of different size. Warnatsch et al. show that reactive oxygen species localization tunes inflammation to compensate for differences in the number of neutrophils required to clear microbes of different size.

Tumor-associated neutrophils suppress pro-tumoral IL-17+ γδ T cells through induction of oxidative stress

Interleukin 17 (IL-17)–producing γδ T cells (γδ17 T cells) have been recently found to promote tumor growth and metastasis formation. How such γδ17 T-cell responses may be regulated in the tumor microenvironment remains, however, largely unknown. Here, we report that tumor-associated neutrophils can display an overt antitumor role by strongly suppressing γδ17 T cells. Tumor-associated neutrophils inhibited the proliferation of murine CD27− Vγ6+ γδ17 T cells via induction of oxidative stress, thereby preventing them from constituting the major source of pro-tumoral IL-17 in the tumor microenvironment. Mechanistically, we found that low expression of the antioxidant glutathione in CD27− γδ17 T cells renders them particularly susceptible to neutrophil-derived reactive oxygen species (ROS). Consistently, superoxide deficiency, or the administration of a glutathione precursor, rescued CD27− Vγ6+ γδ17 T-cell proliferation in vivo. Moreover, human Vδ1+ γδ T cells, which contain most γδ17 T cells found in cancer patients, also displayed low glutathione levels and were potently inhibited by ROS. This work thus identifies an unanticipated, immunosuppressive yet antitumoral, neutrophil/ROS/γδ17 T-cell axis in the tumor microenvironment.

Activation of the Aryl Hydrocarbon Receptor Interferes with Early Embryonic Development

Summary The transcriptional program of early embryonic development is tightly regulated by a set of well-defined transcription factors that suppress premature expression of differentiation genes and sustain the pluripotent identity. It is generally accepted that this program can be perturbed by environmental factors such as chemical pollutants; however, the precise molecular mechanisms remain unknown. The aryl hydrocarbon receptor (AHR) is a widely expressed nuclear receptor that senses environmental stimuli and modulates target gene expression. Here, we have investigated the AHR interactome in embryonic stem cells by mass spectrometry and show that ectopic activation of AHR during early differentiation disrupts the differentiation program via the chromatin remodeling complex NuRD (nucleosome remodeling and deacetylation). The activated AHR/NuRD complex altered the expression of differentiation-specific genes that control the first two developmental decisions without affecting the pluripotency program. These findings identify a mechanism that allows environmental stimuli to disrupt embryonic development through AHR signaling.

Neutrophil extracellular traps license macrophages and Th17 cells for cytokine production in atherosclerosis

Neutrophil NETs drive atherosclerosis The buildup of fats, cholesterol, and other substances in arteries causes atherosclerosis, which restricts blood flow and can lead to heart attacks and stroke. Inflammation contributes to the pathogenesis of atherosclerosis, but exactly how is not fully understood. Warnatsch et al. now show that immune cells called neutrophils release NETs (neutrophil extracellular traps) (see the Perspective by Nahrendorf and Swirski). These NETs are composed of DNA and antimicrobial proteins, and in the setting of atherosclerosis they activate innate immune signaling pathways in macrophages. This causes the macrophages to secrete proinflammatory cytokines, exacerbating the disease. Indirectly, NETS also attract a specialized subset of T cells that further amplify the proinflammatory response. Science, this issue p. 316; see also p. 237 Neutrophils release networks of extracellular fibers that contribute to the pathogenesis of atherosclerosis. [Also see Perspective by Nahrendorf and Swirski] Secretion of the cytokine interleukin-1β (IL-1β) by macrophages, a major driver of pathogenesis in atherosclerosis, requires two steps: Priming signals promote transcription of immature IL-1β, and then endogenous “danger” signals activate innate immune signaling complexes called inflammasomes to process IL-1β for secretion. Although cholesterol crystals are known to act as danger signals in atherosclerosis, what primes IL-1β transcription remains elusive. Using a murine model of atherosclerosis, we found that cholesterol crystals acted both as priming and danger signals for IL-1β production. Cholesterol crystals triggered neutrophils to release neutrophil extracellular traps (NETs). NETs primed macrophages for cytokine release, activating T helper 17 (TH17) cells that amplify immune cell recruitment in atherosclerotic plaques. Therefore, danger signals may drive sterile inflammation, such as that seen in atherosclerosis, through their interactions with neutrophils.