Summary: Interleukin‐33 (IL‐33) is a tissue‐derived nuclear cytokine from the IL‐1 family abundantly expressed in endothelial cells, epithelial cells and fibroblast‐like cells, both during homeostasis and inflammation. It functions as an alarm signal (alarmin) released upon cell injury or tissue damage to alert immune cells expressing the ST2 receptor (IL‐1RL1). The major targets of IL‐33 in vivo are tissue‐resident immune cells such as mast cells, group 2 innate lymphoid cells (ILC2s) and regulatory T cells (Tregs). Other cellular targets include T helper 2 (Th2) cells, eosinophils, basophils, dendritic cells, Th1 cells, CD8⁺ T cells, NK cells, iNKT cells, B cells, neutrophils and macrophages. IL‐33 is thus emerging as a crucial immune modulator with pleiotropic activities in type‐2, type‐1 and regulatory immune responses, and important roles in allergic, fibrotic, infectious, and chronic inflammatory diseases. The critical function of IL‐33/ST2 signaling in allergic inflammation is illustrated by the fact that IL33 and IL1RL1 are among the most highly replicated susceptibility loci for asthma. In this review, we highlight 15 years of discoveries on IL‐33 protein, including its molecular characteristics, nuclear localization, bioactive forms, cellular sources, mechanisms of release and regulation by proteases. Importantly, we emphasize data that have been validated using IL‐33‐deficient cells.

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.

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-1β 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-33 _112–270 , and that cleavage by caspase-1 does not occur at the site initially proposed (Ser ₁₁₁ ), but rather after residue Asp ₁₇₈ between the fourth and fifth predicted β-strands of the IL-1-like domain. Surprisingly, the caspase-1 cleavage site (DGVD ₁₇₈ G) 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-1α, as an endogenous danger signal to alert cells of the innate immune system of tissue damage during trauma or infection.

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_1–270and generate mature forms IL-33_95–270, IL-33_99–270, and IL-33_109–270. 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 full-length 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.

Significance: Interleukin-33 (IL-33) is an IL-1 family cytokine with important roles in type-2 immunity and human asthma. IL-33 is a key activator of the recently described group-2 innate lymphoid cells (ILC2s), which are involved in the initiation of allergic inflammation. Here, we investigated the mechanisms regulating IL-33 activity. We discovered that mast cells, critical effector cells in allergic disorders, secrete proteases, which cleave IL-33 and generate mature forms with increased biological activity. We demonstrate that these mature forms of IL-33 are 30-fold more potent than full-length IL-33 for activation of ILC2s. Our study suggests that inhibition of IL-33 cleavage by mast cell and other inflammatory proteases could be useful to limit IL-33–mediated responses in allergic asthma and other inflammatory diseases.