The researchers, led by Mami Amaki and Yasutomo Imai, hypothesized that caspase-1 might control IL-33 release from keratinocytes in vivo. Their reasoning drew on a parallel case: IL-1α, like IL-33, is a nuclear cytokine stored under steady-state conditions, and its extracellular release requires caspase-1. Previous studies, however, had suggested that IL-33 release is independent of caspase-1, but those conclusions came largely from non-cutaneous tissues or in vitro systems. In the airway epithelium, for example, allergen protease-activated stress granule assembly and gasdermin D fragmentation have been shown to control IL-33 secretion without caspase-1. The Japanese team suspected that the skin might operate by different rules, and their new findings support that idea.

The first technical step was to establish where caspase-1 actually resides in skin. Using immunofluorescence staining of mouse skin sections with a Cy3-conjugated anti-caspase-1 P10 polyclonal antibody, the researchers detected caspase-1 immunoreactivity in the nuclei of epidermal keratinocytes. No specific staining appeared in caspase-1/11-deficient mice, which served as negative controls. This was a significant observation because earlier studies had concluded that caspase-1 expression was restricted to hematopoietic cells, conclusions based largely on the absence of detectable interleukin-1 beta-processing activity and the limited sensitivity of the detection methods available at the time. Subsequent work with improved methodologies has revealed low-level but functional caspase-1 expression in epithelial cells, including keratinocytes. For caspase-1 to be involved in the release of a nuclear alarmin like IL-33, the protease would need to be present in keratinocyte nuclei, and the new staining data confirm exactly that.

Next, the team examined what happens to nuclear IL-33 when the skin is stimulated. They applied 20 microliters of 0.5 percent 2,4-dinitrofluorobenzene (DNFB), a contact-sensitizing hapten dissolved in acetone and olive oil, to the left ear of mice while the right ear received vehicle alone. Six hours later, they processed the ear skin for immunofluorescence staining for IL-33, using an affinity-purified rabbit anti-mouse IL-33 polyclonal antibody detected with a biotinylated goat anti-rabbit IgG secondary antibody and streptavidin-conjugated Alexa Fluor 594, with nuclei counterstained using DAPI. The disappearance of nuclear IL-33 staining in keratinocytes was interpreted as evidence that IL-33 had been released into the extracellular space, an established surrogate readout that parallels findings in experimental allergic rhinitis, where allergen challenge induces a rapid loss of IL-33 staining in nasal epithelial cells alongside increased IL-33 levels in nasal lavage fluids.

The result was striking. In wild-type mice, IL-33 fluorescence in the epidermis was substantially reduced six hours after DNFB treatment, consistent with the team’s previous report that cutaneous stimulation triggers extracellular IL-33 release in vivo. In caspase-1/11-deficient mice, however, IL-33 immunoreactivity remained clearly detectable in the nuclei of epidermal keratinocytes at the same time point. In other words, deleting caspase-1/11 prevented the DNFB-induced loss of keratinocyte IL-33. Notably, quantitative real-time PCR showed that DNFB application increased Il33 mRNA expression in both wild-type and caspase-1/11-deficient mice, even though nuclear IL-33 staining persisted in the knockout animals. This indicates that Il33 transcription is induced by DNFB independently of IL-33 release, meaning the protease is specifically required for the release step rather than for the transcriptional response.

To test whether this release mechanism matters for actual disease, the researchers crossed their caspase-1/11-deficient mice with the hK14mIL33tg transgenic line, which overexpresses murine IL-33 in keratinocytes under the control of the human keratin-14 promoter. These IL33tg mice spontaneously develop atopic dermatitis-like skin inflammation. When caspase-1/11 was deleted in this background, the resulting IL33tg Casp1/11KO mice exhibited markedly attenuated skin inflammation compared with IL33tg mice. Histological examination of hematoxylin and eosin-stained skin sections showed pronounced leukocyte and eosinophil infiltration accompanied by epidermal hyperplasia in IL33tg mice, while these pathological features were minimal in the double-deficient animals, indicating a substantially reduced inflammatory response in the absence of caspase-1/11.

The molecular signature of the type 2 immune response told the same story. Quantitative PCR analysis of skin tissue, performed with a TaqMan RNA-to-Ct Kit and an ABI7900HT sequence detection system, revealed significant reductions in the expression of type 2 cytokines including Il4, Il5, and Il13, as well as Prg2, which encodes eosinophil peroxidase and serves as a specific marker of murine eosinophils. The reduction in Prg2 expression was consistent with the histological findings showing markedly reduced eosinophil infiltration in IL33tg Casp1/11KO mice. Together, these data indicate that caspase-1/11 deficiency suppresses IL-33-driven type 2 immune responses in the skin, dampening the entire downstream allergic cascade that IL-33 normally sets in motion through ILC2 activation.

The findings carry important caveats that the authors address candidly. The knockout mice used in the study have historically been described as caspase-1-deficient, but they are now recognized to carry a combined deficiency of both caspase-1 and caspase-11, because the Casp1 and Casp11 genes lie in close genomic proximity within the same gene cluster, resulting in simultaneous disruption of both loci in this strain. The potential contribution of caspase-11 therefore cannot be formally excluded. However, previous studies indicate that epithelial inflammasome-related responses, including IL-33 regulation, are mainly mediated by caspase-1 rather than caspase-11. Caspase-11 is primarily implicated in noncanonical inflammasome signaling in myeloid cells and localizes predominantly in the cytoplasm, making its direct involvement in the nuclear export of IL-33 from keratinocytes less likely. The authors also note that loss of nuclear staining does not directly quantify extracellular IL-33 levels, so future studies using biochemical measurements will be needed to confirm the release mechanism.

There is another puzzle worth noting. Several studies have suggested a role for caspase-1 in IL-33 inactivation rather than release, since IL-33 can be inactivated after maturation by caspase-1. If caspase-1-mediated inactivation were essential in vivo, IL-33-transgenic mice lacking caspase-1/11 would be expected to show exacerbated skin inflammation, yet the opposite was observed. The authors also point out that pharmacological inhibition of caspase-1 may not fully reflect its role in alarmin release, because for IL-1α the presence of caspase-1 protein, rather than its enzymatic activity, can be required for secretion. Because caspase-1/11 was deleted in all cells in the mice used, the possibility that caspase-1 deficiency in immune cells influences IL-33-induced skin inflammation cannot be excluded, though the marked reduction in inflammation suggests that caspase-1/11 deficiency affects an early step in IL-33-driven skin inflammation, most likely IL-33 release from keratinocytes.

The broader implication is that alarmin biology may be tissue-specific in ways the field has not fully appreciated. IL-33 can be released independently of caspase-1 in other tissues, notably the airway epithelium, where gasdermin D-mediated pathways dominate. But in the skin, the new evidence suggests that caspase-1 is involved in the pathogenic release of IL-33 from keratinocytes in vivo, and that this release step is a critical control point for atopic dermatitis-like inflammation. If the findings hold up in biochemical assays and in human tissue, they could reshape how researchers think about targeting IL-33 in atopic dermatitis, pointing to the release machinery within keratinocytes as a potential therapeutic frontier alongside the IL-33-blocking antibodies already in clinical development.

Subject of Research: Immunology

Article Title: Deficiency of inflammatory caspases attenuates IL-33–mediated dermatitis

Article References: Amaki, M., Murata, T., Kanazawa, N., & Imai, Y. (2026). Deficiency of inflammatory caspases attenuates IL-33–mediated dermatitis. Journal of Cutaneous Immunology and Allergy, 9, Article 16358. https://doi.org/10.3389/jcia.2026.16358

Image Credits: AI Generated

DOI: 10.3389/jcia.2026.16358

Keywords: atopic dermatitis, IL-33, caspase-1, keratinocytes, alarmin, skin inflammation, type 2 cytokines, innate lymphoid cells, Deficiency, inflammatory, caspases, attenuates