Innovation through imitation: IL-33 decoys show promise in pulmonary fibrosis

Interleukin (IL)-33 is a member of the IL-1 family of cytokines, which includes IL-1α, IL-1β, and IL-18. These cytokines, which induce their extracellular activities at low concentrations, direct both innate and adaptive immune responses required for host defense (Dinarello, 2018). IL-33 is notable for being highly expressed by structural cells, particularly at barrier surfaces, both at baseline and during injury. Among the highest expressers are epithelial cells of the lung (Moussion et al, 2008; Pichery et al, 2012; Hardman et al, 2013; Gordon et al, 2016). The constitutive pattern of expression has led to a model in which IL-33, like IL-1α (Di Paolo and Shayakhmetov, 2016), acts as an “alarmin,” which is released upon tissue injury or stress to facilitate protective immune activation. Moreover, in addition to its extracellular activity, IL-33 has nuclear functions as a transcriptional regulator (Choi et al, 2012; Gatti et al, 2021; Park et al, 2021; Wang et al, 2023a). In this issue of the Journal of Pharmacology and Experimental Therapeutics, Atamas et al (2025) report on a novel approach to therapeutically inhibit IL-33. In contrast to current IL-33 therapeutics, which bind to IL-33 or its receptor in the extracellular space, Atamas et al (2025) targeted nuclear IL-33 with the aim of reducing both intracellular and extracellular pools.

Similar to IL-1α but in contrast to IL-1β and IL-18, IL-33 is largely found within the cell nucleus under homeostatic conditions (Carriere et al, 2007). Its N-terminus contains a histone-binding domain located in exon 3 (aa 40–58) (Roussel et al, 2008), while its cytokine signaling activity resides largely in the C-terminus and critically depends on exon 5 (aa 114-156) (Gordon et al, 2016). Reported mechanisms of extracellular release include caspase-3–dependent cell death and/or alternative splicing to remove the histone-binding domain in response to inflammatory signaling (Gordon et al, 2016; Brusilovsky et al, 2021; Raphael et al, 2024). Perhaps unsurprisingly, given its predominantly nuclear localization as well as constitutive expression in many cells, IL-33 has also been found (in addition to its extracellular signaling function) to participate in the regulation of gene transcription through interactions with histone deacetylases and transcription factors, and even by direct transcription factor activity (Choi et al, 2012; Gatti et al, 2021; Park et al, 2021; Wang et al, 2023a).

Although other IL-1 family members are predominantly implicated in type 1 immune responses, the best established function of secreted IL-33 is the induction of type 2 immunity. Extracellular IL-33 potently acts to initiate type 2 immunity by activating type 2 innate lymphoid cells (ILC2), mast cells, basophils, and T helper–type 2 cells (Coyle et al, 1999; Schmitz et al, 2005; Oboki et al, 2010). Consistent with this function, monoclonal antibodies directed at IL-33 have proved beneficial in the treatment of a subset of asthmatic patients with high type 2 immune activity (Wechsler et al, 2021). However, IL-33 has also been shown to have effects on other arms of immunity, including type 1 interferon-driven responses (Smithgall et al, 2008; Clark et al, 2021; Georgakis et al, 2021). Given its high expression in the lung, a great deal of interest surrounds the possibility that IL-33 might mediate other lung diseases characterized by non–type 2 dependent immune activity, such as viral or bacterial infections, chronic obstructive lung disease, and autoimmune lung disease, both via its extracellular cytokine activity and its intranuclear transcription factor function. It is in this context that scientists have become interested in IL-33 as a therapeutic target in lung fibrosis.

Pulmonary fibrosis is a common and devastating syndrome affecting roughly 1–5 per 10,000 people in the United States and abroad (Nalysnyk et al, 2012). Fibrosis is the result of many different disease mechanisms that converge on progressive loss of alveolar type 1 cells and replacement with fibrotic scars, leaving the lung stiff and with poor gas exchange capacity. In the last decade, new antifibrotic drugs have proved effective in slowing the progression of lung decline, but they are limited in their widespread use because of often severe side effects (King et al, 2014; Richeldi et al, 2014). These drugs cannot reverse fibrotic lung disease; thus, novel therapeutics are needed.

Preclinical studies using the bleomycin animal model have proved to be IL-33 dependent. Specifically, extracellular IL-33 activates type 2 cytokine-producing cells, such as basophils, mast cells, and ILC2s, which in turn promote M2 macrophage polarization and myofibroblast differentiation (Li et al, 2014a; Di Carmine et al, 2022; Wang et al, 2023b). Further supporting the role of type 2 cytokines in pulmonary fibrosis, the loss of counterregulatory interferon gamma signaling on ILC2s results in spontaneous pulmonary fibrosis in mice (Otaki et al, 2023), reminiscent of the pulmonary fibrosis observed in the IL-13 transgenic mouse (Lee et al, 2001). During viral infection, nuclear IL-33 promotes basal cell proliferation, which perhaps surprisingly promotes inflammation and lung fibrosis independent of its receptor ST2 (Wu et al, 2021). Animal studies such as these support further exploration of IL-33 as a therapeutic target in pulmonary fibrosis (Luzina et al, 2013).

In an attempt to reduce lung IL-33 levels, Atamas et al (2025) sought to exploit an earlier described mechanism of intracellular IL-33 degradation. The authors previously showed that the nuclear transport protein, importin-5, interacts with IL-33 via amino acids 46–56, which is also the location of the histone-binding domain (Clerman et al, 2017). Knockdown of importin-5 did not alter nuclear localization of IL-33, but it did reduce total IL-33 levels by about half, likely by augmenting its proteosomal degradation. Building on this data, Atamas et al (2025) hypothesized that the introduction of a cell-penetrating peptide (CPP) mimetic of the IL-33 N-terminal domain that binds both importin-5 and histones would compete with full-length IL-33 and promote its degradation.

In the current work, the authors generated several CPP mimetics of the N-terminal histone-binding domain of IL-33 and control CPPs targeting non–histone-binding amino acids. They then administered the peptides by intraperitoneal injection after generating 2 bleomycin mouse models of lung fibrosis using a single dose of intratracheal bleomycin or serial doses of intraperitoneal bleomycin over 5 weeks. In both cases, mice treated with the CPP–IL-33 histone-binding mimetics but not control CPPs showed resolution of bleomycin-induced weight loss, reduction in lung collagen content, and markedly reduced total lung IL-33 levels (Fig. 1). This was accompanied by modest changes in inflammatory cell and cytokine profiles in the lungs. Of importance, the authors reported that the administration of CPPs alone produced no observed toxicity. This represents a remarkable improvement in pathology in this mouse model of fibrosis.

Fig. 1.

Pulmonary fibrosis is a devastating lung disease in which the gas exchanging units of the lung are replaced with collagen-rich scar tissue. In the Journal of Pharmacology and Experimental Therapeutics, Atamas et al (2025) report on the efficacy of a novel drug in an animal model of pulmonary fibrosis. Using rational drug design to mimic the histone-binding domain of the type 2 cytokine IL-33, the investigators find that total lung IL-33 levels are markedly reduced, and animals are protected from bleomycin-induced lung fibrosis. Graphics created with BioRender.com.

The beneficial mechanism of the CPPs was not explored in this study, and further studies will be informative. While collagen content correlated with IL-33 levels, confirmation that the benefits do depend on IL-33 remains to be explored. Furthermore, if the mechanism of action is indeed via degradation of full-length IL-33, one would expect that both the intranuclear functions of IL-33 as well as the caspase-3–dependent cleavage and secretion of IL-33 would be reduced. Conversely, alternatively spliced variants of IL-33 that do not contain a histone-binding domain would be unaffected. Consequently, nuclear functions of full-length IL-33 are likely to be greatly impaired by IL-33-targeting CPPs, whereas the effects on the ST2-dependent functions are less predictable. Further investigations into which forms of IL-33 (nuclear, extracellular cleaved, full-length, or spliced forms) are affected by targeting CPPs and how IL-33-targeted CPPs differ from conventional monoclonal antibodies in their biological activity are needed.

An exploration of anti-IL-33 CPPs will be of great interest in other models of organ fibrosis as well as models of type 2 immunity, in which ST2-dependent extracellular signaling plays a prominent role. In several models of liver fibrosis, including biliary cirrhosis and chemically induced hepatitis, ST2 deficiency attenuates hepatitis fibrosis (McHedlidze et al, 2013; Li et al, 2014b), although clinical trial data for these diseases are not available. IL-33 may also induce fibrosis in other organs such as the pancreas, intestine, kidney, and skin (Kotsiou et al, 2018). In contrast, deficiency of IL-33 or ST2 led to more fibrosis and worse survival in an animal model of cardiac hypertrophy (Sanada et al, 2007; Veeraveedu et al, 2017); however, deficiency in ST2 did not alter survival in a myocardial infarction model (Seki et al, 2009). In models of lung type 2 inflammation, IL-33 and its receptor are critical for disease development (Schmitz et al, 2005; Oboki et al, 2010), and IL-33-blocking monoclonal antibodies benefit patients with severe asthma (Wechsler et al, 2021). These drugs are also being studied in patients with chronic obstructive pulmonary disease (Rabe et al, 2021). Reassuringly, studies in asthma and chronic obstructive pulmonary disease to date reveal no cardiac toxicities (Rabe et al, 2021; Wechsler et al, 2021); however, these drugs are expensive to deploy, and peptide therapeutics may be less costly if found to be equally effective.

In the models studied, the authors found no observable toxicities from the CPP–IL-33 mimetics. However, they do appropriately note that inhibiting beneficial and/or homeostatic effects of IL-33 could lead to undesirable effects. For instance, one might expect that long-term administration might phenocopy the observed hair loss in the inducible deletion of IL-33 in KRT5 basal cells (Wu et al, 2021). Whether such a loss of IL-33 results in further skin abnormalities over time remains unknown and might limit chronic use.

Finally, every discussion of IL-33-directed therapeutics must consider the species differences in IL-33 expression. In mice, IL-33 is expressed in the type 2 alveolar epithelial cell (AT2), and expression is increased upon alveolar lung injury (Moussion et al, 2008; Hardman et al, 2013). In contrast, IL-33 is not expressed in human AT2 cells but instead is expressed constitutively in the basal airway epithelial cell (Gordon et al, 2016). The therapeutic benefit of targeting IL-33 in human pulmonary fibrosis requires further study.

Conflict of interest

The authors declare no conflicts of interest.