Matrix Metalloproteinase-9-Dependent Release of IL-1ß by Human Eosinophils.

Asthma is often associated with airway eosinophilia, and therapies targeting eosinophils are now available to treat severe eosinophilic asthma. Eosinophilic asthma is often due to a type-2 immune response and production of IL-5, which leads to eosinophilopiesis and recruitment of mature eosinophils in the airways. A concomitant type-2 and type-17 response has been reported in some individuals. IL-17 may be enhanced by IL-1ß production and can lead to neutrophilic inflammation. In fact, both eosinophilic and neutrophilic (mixed granulocytic) inflammation are simultaneously present in a large population of patients with asthma. In monocyte/macrophage cell populations, release of mature IL-1ß occurs via toll-like receptor ligand-induced activation of the inflammasome. Within the inflammasome, a cascade of events leads to the activation of caspase-1, which cleaves pro-IL-1ß protein into a mature, releasable, and active form. We have demonstrated that eosinophils can release IL-1ß in a Toll-like receptor ligand-independent fashion. The objective of this study was to determine the mechanisms underlying the production and maturation of IL-1ß in cytokine-activated eosinophils. Using eosinophils from circulating blood and from bronchoalveolar lavage fluid after an airway allergen challenge, the present study demonstrates that cytokine-activated eosinophils express and release a bioactive form of IL-1ß with an apparent size less than the typical 17 kDa mature form produced by macrophages. Using a zymography approach and pharmacological inhibitors, we identified matrix metalloproteinase-9 (MMP-9) as a protease that cleaves pro-IL-1ß into a ~15 kDa form and allows the release of IL-1ß from cytokine-activated eosinophils. Therefore, we conclude that activated eosinophils produce MMP-9, which causes the release of IL-1ß in an inflammasome/caspase-1-independent manner. The production of IL-1ß by eosinophils may be a link between the eosinophilic/type-2 immune response and the neutrophilic/type-17 immune response that is often associated with a more severe and treatment-refractory type of asthma.

Human eosinophil activin A synthesis and mRNA stabilization are induced by the combination of IL-3 plus TNF.

Eosinophils contribute to immune regulation and wound healing/fibrosis in various diseases, including asthma. Growing appreciation for the role of activin A in such processes led us to hypothesize that eosinophils are a source of this transforming growth factor-ß superfamily member. Tumor necrosis factor-α (TNF) induces activin A by other cell types and is often present at the site of allergic inflammation along with the eosinophil-activating common ß (ßc) chain-signaling cytokines (interleukin (IL)-5, IL-3, granulocyte-macrophages colony-stimulating factor (GM-CSF)). Previously, we established that the combination of TNF plus a ßc chain-signaling cytokine synergistically induces eosinophil synthesis of the remodeling enzyme matrix metalloproteinase-9. Therefore, eosinophils were stimulated ex vivo by these cytokines and in vivo through an allergen-induced airway inflammatory response. In contrast to IL-5+TNF or GM-CSF+TNF, the combination of IL-3+TNF synergistically induced activin A synthesis and release by human blood eosinophils. IL-3+TNF enhanced activin A mRNA stability, which required sustained signaling of pathways downstream of p38 and extracellular signal-regulated kinase mitogen-activated protein kinases. In vivo, following segmental airway allergen challenge of subjects with mild allergic asthma, activin A mRNA was upregulated in airway eosinophils compared with circulating eosinophils, and ex vivo, circulating eosinophils tended to release more activin A in response to IL-3+TNF. These data provide evidence that eosinophils release activin A and that this function is enhanced when eosinophils are present in an allergen-induced inflammatory environment. Moreover, these data provide the first evidence for posttranscriptional control of activin A mRNA. We propose that an environment rich in IL-3+TNF will lead to eosinophil-derived activin A, which has an important role in regulating inflammation and/or fibrosis.