IL-33 is a chromatin-associated cytokine of the IL-1 family that has recently been linked to many diseases, including asthma, rheumatoid arthritis, atherosclerosis, and cardiovascular diseases. IL-33 signals through the IL-1 receptor-related protein ST2 and drives production of pro-inflammatory and T helper type 2-associated cytokines in mast cells, T helper type 2 lymphocytes, basophils, eosinophils, invariant natural killer T cells, and natural killer cells. It is currently believed that IL-33, like IL-1␤ and IL-18, requires processing by caspase-1 to a mature form (IL-33 112-270) for biological activity. Contrary to the current belief, we report here that full-length IL-33 1-270 is active and that processing by caspase-1 results in IL-33 inactivation, rather than activation. We show that full-length IL-33 1-270 binds and activates ST2, similarly to IL-33112-270, and that cleavage by caspase-1 does not occur at the site initially proposed (Ser 111), but rather after residue Asp178 between the fourth and fifth predicted ␤-strands of the IL-1-like domain. Surprisingly, the caspase-1 cleavage site (DGVD 178G) is similar to the consensus site of cleavage by caspase-3, and IL-33 is also a substrate for this apoptotic caspase. Interestingly, we found that full-length IL-33, which is constitutively expressed to high levels by endothelial cells in most normal human tissues, can be released in the extracellular space after endothelial cell damage or mechanical injury. We speculate that IL-33 may function, similarly to the prototypical alarmins HMGB1 and IL-1␣, as an endogenous danger signal to alert cells of the innate immune system of tissue damage during trauma or infection.inflammation ͉ Interleukin ͉ endothelial cell C ytokines of the IL-1 family play a major role in a wide range of inflammatory, infectious, and autoimmune diseases (1, 2). IL-33 [previously known as nuclear factor from high endothelial venule, or NF-HEV (3)] is the most recent addition to the IL-1 family (4, 5). Based on animal model studies and analyses of diseased tissues from patients, IL-33 has been proposed to represent a promising therapeutic target for several important diseases, including asthma and other allergic diseases (4), rheumatoid arthritis (5, 6), atherosclerosis (7), and cardiovascular diseases (8, 9). IL-33 has been shown to signal through the IL-1 receptor-related protein ST2 (4) and to drive production of cytokines [both pro-inflammatory and T helper type 2 (Th2)-associated cytokines] and chemokines in mast cells, Th2 lymphocytes, basophils, eosinophils, invariant natural killer (NK) T cells, and NK cells (4,(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). IL-33 signaling has also been shown to require the IL-1 receptor (IL-1R) accessory protein, IL-1RAcP, indicating that IL-33 shares with IL-1 not only structural homology but also signaling pathways (11,14).We initially discovered IL-33 as a nuclear factor abundantly expressed in endothelial cells of high endothelial venules in lymphoid organs (3, 5), but we (20) and others (21) have...

SummaryInterleukin-33 (IL-33) is a tissue-derived nuclear cytokine from the IL-1 family abun- K E Y W O R D Sallergy, cytokine, IL-33, ILC2, inflammation, innate immunity

IL-33 is a nuclear cytokine from the IL-1 family constitutively expressed in epithelial barrier tissues and lymphoid organs, which plays important roles in type-2 innate immunity and human asthma. Recent studies indicate that IL-33 induces production of large amounts of IL-5 and IL-13 by group 2 innate lymphoid cells (ILC2s), for initiation of allergic inflammation shortly after exposure to allergens or infection with parasites or viruses. IL-33 appears to function as an alarmin (alarm signal) rapidly released from producing cells upon cellular damage or cellular stress. In this review, we discuss the cellular sources, mode of action and regulation of IL-33, and we highlight its crucial roles in vivo with particular emphasis on results obtained using IL33-deficient mice.

Interleukin-33 (IL-33) (NF-HEV) is a chromatin-associated nuclear cytokine from the IL-1 family, which has been linked to important diseases, including asthma, rheumatoid arthritis, ulcerative colitis, and cardiovascular diseases. IL-33 signals through the ST2 receptor and drives cytokine production in type 2 innate lymphoid cells (ILCs) (natural helper cells, nuocytes), T-helper (Th)2 lymphocytes, mast cells, basophils, eosinophils, invariant natural killer T (iNKT), and natural killer (NK) cells. We and others recently reported that, unlike IL-1β and IL-18, full-length IL-33 is biologically active independently of caspase-1 cleavage and that processing by caspases results in IL-33 inactivation. We suggested that IL-33, which is released upon cellular damage, may function as an endogenous danger signal or alarmin, similar to IL-1α or high-mobility group box 1 protein (HMGB1). Here, we investigated the possibility that IL-33 activity may be regulated by proteases released during inflammation. Using a combination of in vitro and in vivo approaches, we demonstrate that neutrophil serine proteases cathepsin G and elastase can cleave full-length human IL-33 1-270 and generate mature forms IL-33 , and IL-33 . These forms are produced by activated human neutrophils ex vivo, are biologically active in vivo, and have a ∼10-fold higher activity than full-length IL-33 in cellular assays. Murine IL-33 is also cleaved by neutrophil cathepsin G and elastase, and both fulllength and cleaved endogenous IL-33 could be detected in the bronchoalveolar lavage fluid in an in vivo model of acute lung injury associated with neutrophil infiltration. We propose that the inflammatory microenvironment may exacerbate disease-associated functions of IL-33 through the generation of highly active mature forms.innate immunity | inflammatory protease | serine protease inhibitor | alveolar epithelium C ytokines of the IL-1 family (IL-1α, IL-1β, IL-18) play a major role in inflammatory, infectious, and autoimmune diseases (1-3). IL-33 [previously known as nuclear factor from high endothelial venule or NF-HEV (4, 5)], is a chromatin-associated nuclear cytokine from the IL-1 family (6, 7), which has been linked to important diseases (8-10), including asthma (11), rheumatoid arthritis (12, 13), ulcerative colitis (14), and cardiovascular diseases (15).IL-33 signals through the ST2 receptor (4), a member of the IL-1 receptor family, which is expressed (or induced) on various immune cell types, including mast cells, basophils, eosinophils, Thelper (Th)2 lymphocytes, invariant natural killer T (iNKT) and natural killer (NK) cells, macrophages, dendritic cells, and neutrophils (8-10). IL-33 stimulation of ST2 on Th2 cells induces secretion of the Th2 cytokines IL-5 and IL-13 (4, 16). Recently, IL-33 has been shown to drive production of extremely high amounts of these Th2 cytokines by type 2 innate lymphoid cells (ILCs) (natural helper cells, nuocytes, innate helper 2 cells), which play important roles in innate immune responses, after helminth infec...

Interleukin-33 (IL-33) is an alarmin cytokine from the IL-1 family. IL-33 activates many immune cell types expressing the interleukin 1 receptor-like 1 (IL1RL1) receptor ST2, including group-2 innate lymphoid cells (ILC2s, natural helper cells, nuocytes), the major producers of IL-5 and IL-13 during type-2 innate immune responses and allergic airway inflammation. IL-33 is likely to play a critical role in asthma because the IL33 and ST2/IL1RL1 genes have been reproducibly identified as major susceptibility loci in large-scale genome-wide association studies. A better understanding of the mechanisms regulating IL-33 activity is thus urgently needed. Here, we investigated the role of mast cells, critical effector cells in allergic disorders, known to interact with ILC2s in vivo. We found that serine proteases secreted by activated mast cells (chymase and tryptase) generate mature forms of IL-33 with potent activity on ILC2s. The major forms produced by mast cell proteases, IL-33 95-270 , IL-33 107-270 , and IL-33 109-270 , were 30-fold more potent than full-length human IL-33 1-270 for activation of ILC2s ex vivo. They induced a strong expansion of ILC2s and eosinophils in vivo, associated with elevated concentrations of IL-5 and IL-13. Murine IL-33 is also cleaved by mast cell tryptase, and a tryptase inhibitor reduced IL-33-dependent allergic airway inflammation in vivo. Our study identifies the central cleavage/activation domain of IL-33 (amino acids 66-111) as an important functional domain of the protein and suggests that interference with IL-33 cleavage and activation by mast cell and other inflammatory proteases could be useful to reduce IL-33-mediated responses in allergic asthma and other inflammatory diseases.cytokine | IL-33 | allergic inflammation | mast cell protease | innate lymphoid cells

Interleukin-33 (IL-33) is an IL-1-like ligand for the ST2 receptor that stimulates the production of Th2-associated cytokines. Recently, we showed that IL-33 is a chromatin-associated factor in the nucleus of endothelial cells in vivo. Here, we report the identification of a short IL-33 chromatin-binding peptide that shares striking similarities with a motif found in Kaposi sarcoma herpesvirus LANA (latency-associated nuclear antigen), which is responsible for the attachment of viral genomes to mitotic chromosomes. Similar to LANA, the IL-33 peptide docks into the acidic pocket formed by the H2A-H2B dimer at the nucleosomal surface and regulates chromatin compaction by promoting nucleosome-nucleosome interactions. Taken together, our data provide important new insights into the nuclear roles of IL-33, and show a unique example of molecular mimicry of a chromatinassociated cytokine by a DNA tumour virus. In addition, the data provide, to the best of our knowledge, the first demonstration of the existence of non-histone cellular factors that bind to the acidic pocket of the nucleosome.

A unique feature of p21 that distinguishes it from the other cyclin-dependent kinase (CDK) inhibitors is its ability to associate with the proliferating cell nuclear antigen (PCNA), an auxiliary factor for DNA polymerases d and e. While it is now well established that inhibition of cyclin/CDK complexes by p21 can result in G1 cell cycle arrest, the consequences of p21/PCNA interaction on cell cycle progression have not yet been determined. Here, we show, using a tetracyclineregulated system, that expression of wild-type p21 in p53-de®cient DLD1 human colon cancer cells inhibits DNA synthesis and causes G1 and G2 cell cycle arrest. Similar eects are observed in cells expressing p21 CDK7 , a mutant impaired in the interaction with CDKs, but not in cells expressing p21 PCNA7 , a mutant de®cient for the interaction with PCNA. Analysis of cells treated with a p21-derived PCNA-binding peptide provides additional evidence that the growth inhibitory eects of p21 and p21 CDK7 result from their ability to bind to PCNA. Our results suggest that p21 might inhibit cell cycle progression by two independent mechanisms, inhibition of cyclin/CDK complexes, and inhibition of PCNA function resulting in both G1 and G2 arrest.

Allergic inflammation has crucial roles in allergic diseases such as asthma. It is therefore important to understand why and how the immune system responds to allergens. Here we found that full-length interleukin 33 (IL-33), an alarmin cytokine with critical roles in type 2 immunity and asthma, functioned as a protease sensor that detected proteolytic activities associated with various environmental allergens across four kingdoms, including fungi, house dust mites, bacteria and pollens. When exposed to allergen proteases, IL-33 was rapidly cleaved in its central 'sensor' domain, which led to activation of the production of type 2 cytokines in group 2 innate lymphoid cells. Preventing cleavage of IL-33 reduced allergic airway inflammation. Our findings reveal a molecular mechanism for the rapid induction of allergic type 2 inflammation following allergen exposure, with important implications for allergic diseases.