L'interleukine-33, cytokine de la famille IL-1 : régulation par les protéases inflammatoires

L'interleukine-33 (IL-33) est le dernier membre identifié de la famille des cytokines IL-1. L'intérêt qui lui est porté est grandissant, notamment depuis la découverte de ses cellules cibles majeures, les cellules lymphoïdes innées de type 2 (ILC2), impliquées dans la pathogénie des infections parasitaires et des maladies allergiques comme l'asthme. L'IL-33 se lie au récepteur ST2 via son domaine C-terminal et induit des réponses inflammatoires et de type 2, caractérisées par la sécrétion de l'IL-6, l'IL-5, et l'IL-13. L'IL-33 est une cytokine atypique puisqu'on la retrouve associée à la chromatine via sa partie N-terminale dans le noyau des cellules endothéliales et épithéliales de tissus exposés à l'environnement. A l'inverse des autres membres de la famille IL-1, comme l'IL-1beta ou l'IL-18, l'IL-33 n'a pas besoin d'être clivée par la caspase 1 pour être activée et libérée dans le milieu extracellulaire. Au contraire, l'IL-33 est inactivée par les caspases au cours d'une mort cellulaire programmée par apoptose. En fait, l'IL-33 est libérée sous sa forme de pleine taille et active, par les cellules nécrotiques lors de dommages cellulaires, agissant ainsi comme un signal d'alarme ou alarmine. Toutefois, il n'était pas exclu que d'autres protéases puissent la cliver et réguler son activité. En effet, nos travaux montrent que l'IL-33 peut être clivée et suractivée par les protéases du microenvironnement inflammatoire, issues des neutrophiles (la cathepsine G et l'élastase) ou des mastocytes (la chymase, la tryptase et le granzyme B). Nous avons identifié précisément les sites de clivage par ces diverses protéases et montré que les fragments ainsi générés, contenant le domaine C-terminal de type IL-1, ont une activité biologique augmentée d'un facteur 10 par rapport à la forme de pleine taille. Enfin, nous avons pu mettre en évidence l'existence de telles formes in vivo, dans un modèle murin de lésion pulmonaire aïgue. Ces nouvelles formes physiologiques de l'IL-33 induisent de profonds changements morphologiques in vivo, associés à une forte sécrétion de cytokines pro-inflammatoires et de type 2. Cette découverte apporte de nouveaux éléments quant à l'importance du microenvironnement inflammatoire dans la régulation de l'activité de l'IL-33. Le recrutement et l'activation de neutrophiles et/ou de mastocytes sur le site du dommage entraîneraient une amplification du signal d'alerte par la génération de fragments "super-actifs" de l'IL-33 et pourraient jouer un rôle particulièrement important dans les pathologies infectieuses et inflammatoires dans lesquelles la voie IL-33/ST2 est impliquée.Interleukin-33 (IL-33) is the latest member of the IL-1 family which has attracted much attention since the recent discovery of its major target cells, the Innate lymphoid Cells type 2 (ILC2), involved in the initiation of inflammatory and type 2 immune responses, characterised by secretion of IL-6, IL-5 and IL13, during parasitic infection and allergic diseases such as asthma. IL-33 is a chromatin associated cytokine which possesses an N-terminal chromatin binding motif and is constitutively expressed in the nuclei of endothelial and epithelial cells of tissues exposed to the environment. Extracellularly, IL-33 binds to its receptor ST2 through the C-terminal part of the protein to induce inflammatory and type 2 responses. It has been shown that the full length form of IL-33 is released upon cell injury, acting as an endogenous alarm signal or alarmin. It was initially believed that IL-33, like IL-1ß and IL-18, requires processing by caspase-1 for biological activity. On the contrary, it has been shown that full length IL-33 is biologically active, and that processing by apoptotic caspases results in IL-33 inactivation. However, it was as yet unclear whether other proteases could be involved in the regulation of IL-33. We demonstrate here that the bioactivity of IL-33 can be increased around 10 fold, due to the cleavage by inflammatory proteases secreted in the microenvironment. Immune cells recruited to the site of injury, neutrophils and mastocytes, can regulate IL-33 by releasing elastase and cathepsin G, or chymase, tryptase and granzyme B, respectively. We precisely mapped the different cleavage sites and we show that each of these proteases generate "superactive" forms of IL-33, containing the IL-1 domain. We also demonstrate that these forms do exist in vivo, using a murine model of acute lung injury. We speculate that resident mastocytes and/or recruited neutrophils activated on the site of tissue injury can amplify the IL-33/ST2 pathway and the disease-associated function of the alarmin IL-33

Maladaptive role of neutrophil extracellular traps in pathogen-induced lung injury

Neutrophils dominate the early immune response in pathogen-induced acute lung injury, but efforts to harness their responses have not led to therapeutic advancements. Neutrophil extracellular traps (NETs) have been proposed as an innate defense mechanism responsible for pathogen clearance, but there are concerns that NETs may induce collateral damage to host tissues. Here, we detected NETs in abundance in mouse models of severe bacterial pneumonia/acute lung injury and in human subjects with acute respiratory distress syndrome (ARDS) from pneumonia or sepsis. Decreasing NETs reduced lung injury and improved survival after DNase I treatment or with partial protein arginine deiminase 4 deficiency (PAD4+/-). Complete PAD4 deficiency (PAD4-/-) reduced NETs and lung injury but was counterbalanced by increased bacterial load and inflammation. Importantly, we discovered that the lipoxin pathway could be a potent modulator of NET formation, and that mice deficient in the lipoxin receptor (Fpr2-/-) produced excess NETs leading to increased lung injury and mortality. Lastly, we observed in humans that increased plasma NETs were associated with ARDS severity and mortality, and lower plasma DNase I levels were associated with the development of sepsis-induced ARDS. We conclude that a critical balance of NETs is necessary to prevent lung injury and to maintain microbial control, which has important therapeutic implications

LPS-Induced Lung Platelet Recruitment Occurs Independently from Neutrophils, PSGL-1, and P-selectin

Platelets are recruited to inflammatory foci and contribute to host defense and inflammatory responses. Compared with platelet recruitment in hemostasis and thrombosis, the mechanisms of platelet recruitment in inflammation and host defense are poorly understood. Neutrophil recruitment to lung airspaces after inhalation of bacterial LPS requires platelets and PSGL-1 in mice. Given this association between platelets and neutrophils, we investigated whether recruitment of platelets to lungs of mice after LPS inhalation was dependent on PSGL-1, P-selectin, or interaction with neutrophils. BALB/c mice were administered intranasal LPS (O55:B5, 5 mg/kg) and, 48 hours later, lungs were collected and platelets and neutrophils quantified in tissue sections by immunohistochemistry. The effects of functional blocking antibody treatments targeting the platelet-neutrophil adhesion molecules, P-selectin or PSGL-1, or treatment with a neutrophil-depleting antibody targeting Ly6G, were tested on the extent of LPS-induced lung platelet recruitment. Separately in Pf4-Cre × mTmG mice, two-photon intravital microscopy was used to image platelet adhesion in live lungs. Inhalation of LPS caused both platelet and neutrophil recruitment to the lung vasculature. However, decreasing lung neutrophil recruitment by blocking PSGL-1, P-selectin, or depleting blood neutrophils had no effect on lung platelet recruitment. Lung intravital imaging revealed increased adhesion of platelets in the lung microvasculature which was not associated with thrombus formation. In conclusion, platelet recruitment to lungs in response to LPS occurs through mechanisms distinct from those mediating neutrophil recruitment, or the occurrence of pulmonary emboli

The lung is a site of platelet biogenesis and a reservoir for haematopoietic progenitors

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Metabolic regulation of ILC2 differentiation into ILC1-like cells during Mycobacterium tuberculosis infection

Tissue-resident innate lymphoid cells (ILCs) regulate tissue homeostasis, protect against pathogens at mucosal surfaces and are key players at the interface of innate and adaptive immunity. How ILCs adapt their phenotype and function to environmental cues within tissues remains to be fully understood. Here, we show that Mycobacterium tuberculosis infection alters the phenotype and function of immature lung ILC2 toward a protective interferon-γ-producing ILC1-like population. This differentiation is controlled by type 1 cytokines and is associated with a glycolytic program involving the transcription factor HIF1α. Collectively, our data reveal how tissue-resident ILCs adapt to type 1 inflammation toward a pathogen tailored immune response

The lung is a site of platelet biogenesis and a reservoir for haematopoietic progenitors

Platelets are critical for haemostasis, thrombosis, and inflammatory responses, but the events that lead to mature platelet production remain incompletely understood. The bone marrow has been proposed to be a major site of platelet production, although there is indirect evidence that the lungs might also contribute to platelet biogenesis. Here, by directly imaging the lung microcirculation in mice, we show that a large number of megakaryocytes circulate through the lungs, where they dynamically release platelets. Megakaryocytes that release platelets in the lungs originate from extrapulmonary sites such as the bone marrow; we observed large megakaryocytes migrating out of the bone marrow space. The contribution of the lungs to platelet biogenesis is substantial, accounting for approximately 50% of total platelet production or 10 million platelets per hour. Furthermore, we identified populations of mature and immature megakaryocytes along with haematopoietic progenitors in the extravascular spaces of the lungs. Under conditions of thrombocytopenia and relative stem cell deficiency in the bone marrow, these progenitors can migrate out of the lungs, repopulate the bone marrow, completely reconstitute blood platelet counts, and contribute to multiple haematopoietic lineages. These results identify the lungs as a primary site of terminal platelet production and an organ with considerable haematopoietic potential