Caspase-3–Generated Short IL-18 Mobilizes NK Cells Against Tumors

Study Background and Research Question

Interleukin-18 (IL-18) is a pleiotropic cytokine best known for its role in stimulating immune responses against cancer and infection. Traditionally, IL-18 is synthesized as an inactive precursor (pro-IL-18) and is activated by caspase-1 cleavage within inflammasome complexes, producing an 18-kDa mature form that is secreted and binds the IL-18 receptor to promote natural killer (NK) and T cell cytotoxicity. However, alternative proteolytic processing events, and the resulting functional diversity of IL-18 fragments, remain poorly understood. Previous work hinted that caspase-3 could cleave pro-IL-18, but the biological significance of this event was unclear. Given the centrality of caspase-3 in apoptosis and its frequent activation in cancer, the authors of the reference study set out to determine whether a caspase-3–generated IL-18 fragment has a distinct role in tumorigenesis or tumor control.

Key Innovation from the Reference Study

The pivotal discovery of this paper is the identification and functional characterization of a 15-kDa 'short IL-18' fragment, generated when caspase-3—rather than caspase-1—cleaves pro-IL-18. Unlike the mature, secreted form, short IL-18 accumulates in the nucleus of cancer cells and does not engage the IL-18 receptor. Instead, it orchestrates a novel intracellular signaling cascade, activating STAT1 phosphorylation at Ser727 via CDK8, which ultimately drives the expression and secretion of ISG15, a factor known to mobilize NK cell cytotoxicity.

This mechanism defines a previously unrecognized non-canonical, caspase-dependent anti-tumor pathway. Importantly, patients with colorectal cancer whose tumor cells exhibited nuclear accumulation of short IL-18 had a markedly better prognosis, suggesting the clinical significance of this pathway.

Methods and Experimental Design Insights

The authors employed a combination of biochemical, genetic, and in vivo approaches to dissect the mechanism and function of short IL-18. Key methodological steps included:

• In vitro cleavage assays: Recombinant pro-IL-18 was incubated with various caspases. Only caspase-3 generated the 15-kDa fragment, as confirmed by immunoblotting.
• Site-directed mutagenesis: Aspartic acid residues at predicted caspase-3 cleavage sites (D69 in mouse, D71/D76 in human) were mutated to alanine, rendering IL-18 resistant to cleavage by caspase-3 but not by caspase-1.
• Pharmacological caspase activation: Cancer cell lines (HEK293T, B16-F10, H292) were treated with raptinal or cisplatin to activate caspase-3, resulting in the production of short IL-18.
• Genetic knockout: CRISPR-mediated knockout of caspase-3 (but not caspase-7) abolished short IL-18 formation, confirming specificity.
• Cellular localization and signaling: The nuclear translocation of short IL-18 was visualized, and its effect on STAT1 phosphorylation and ISG15 expression was quantified.
• In vivo tumor models: Mouse models, including syngeneic tumor challenges and colitis-associated colorectal cancer, were used to assess the impact of short IL-18 on NK cell recruitment and tumor progression.
• Clinical association: Patient samples were analyzed for nuclear short IL-18, correlating abundance with clinical outcomes.

This multifaceted experimental design enabled a rigorous interrogation of both the mechanistic and functional dimensions of short IL-18 in cancer biology.

Core Findings and Why They Matter

The study’s most consequential finding is that caspase-3 cleavage of IL-18 in cancer cells generates a nuclear, non-secreted 15-kDa fragment that drives NK cell-mediated tumor suppression. Specifically:

• Only caspase-3 (not -7, -8, or -9) cleaves pro-IL-18 to produce short IL-18 (reference).
• Short IL-18 is retained in the nucleus, where it enhances STAT1 phosphorylation at Ser727 via CDK8, upregulating ISG15 expression and secretion.
• Secreted ISG15 promotes the recruitment and activation of NK cells, boosting their cytotoxicity against tumor cells.
• Short IL-18 does not bind or activate the IL-18 receptor, distinguishing its function from mature, caspase-1–derived IL-18.
• In mouse models, upregulation of short IL-18 suppressed syngeneic tumors and colitis-associated colorectal cancer.
• Colorectal cancer patients with high nuclear short IL-18 in tumors had a more favorable prognosis.

These findings reveal a distinct apoptosis-linked signaling axis in which caspase-3–driven IL-18 processing mobilizes innate immunity against cancer, independent of classical extracellular cytokine signaling. This expands our understanding of how apoptotic pathway components can interface with immune surveillance, suggesting new avenues for cancer immunotherapy and biomarker development.

Comparison with Existing Internal Articles

Existing resources, such as “Z-VAD-FMK (SKU A1902): Precision Pan-Caspase Inhibition for Sensitive Apoptosis Research” and “Z-VAD-FMK: The Gold Standard Caspase Inhibitor for Apoptosis Pathways”, primarily focus on the application of Z-VAD-FMK as a pan-caspase inhibitor for dissecting apoptotic pathways in cancer and immunology research. These articles provide practical guidance on optimizing workflows and troubleshooting experimental challenges in apoptosis inhibition and caspase activity measurement. While these resources underscore the utility of pharmacological inhibitors such as Z-VAD-FMK in parsing apoptotic pathway research, the reference study breaks new ground by demonstrating a non-canonical role for caspase-3—not just in cell death, but as a driver of anti-tumor immunity via generation of a functional cytokine fragment. This highlights the importance of distinguishing between apoptotic executioner functions and additional, non-lethal signaling roles of caspases in cancer biology.

Moreover, while internal articles address how Z-VAD-FMK can modulate T cell proliferation and apoptosis in established models (e.g., THP-1 and Jurkat T cells), the reference study provides mechanistic detail on how inhibiting caspase-3 could potentially affect non-apoptotic immunoregulatory processes—an important consideration when designing experiments using caspase inhibitors.

Limitations and Transferability

Despite its robust findings, the study has several limitations. The precise molecular mechanism by which nuclear short IL-18 facilitates STAT1–CDK8 interplay remains to be fully elucidated. Additionally, while mouse models and human colorectal cancer samples were examined, the generalizability of findings to other cancer types or to in vivo human immune contexts warrants further investigation. The potential for off-target effects or compensatory pathways in response to caspase-3 activity modulation also remains to be explored.

Finally, while pharmacological inhibition of caspase-3 (for example, using Z-VAD-FMK) is a common strategy in apoptosis inhibition, the impact of such inhibition on non-apoptotic, anti-tumor immune pathways, as revealed here, must be carefully considered in experimental design and interpretation.

Protocol Parameters

• IL-18 cleavage assays: Incubate recombinant pro-IL-18 with active recombinant caspase-3 at 37°C for 1–2 hours; confirm fragment generation by immunoblotting.
• Mutagenesis controls: Employ site-directed mutagenesis to substitute predicted cleavage aspartates with alanine to verify specificity of protease cleavage.
• Caspase activation in cells: Treat cell lines with raptinal (10–20 μM, 1–2 hours) or cisplatin (5–10 μM, 12–24 hours) to induce caspase-3–dependent cleavage.
• Caspase inhibition: Pre-treat cells with Z-VAD-FMK (typically 20–50 μM, 1–2 hours prior to stimulus) to block caspase activity, as suggested by internal protocol guidance.
• In vivo tumor models: Administer cancer cells (e.g., B16-F10) subcutaneously in mice; assess tumor growth and NK cell infiltration following genetic or pharmacological manipulation of IL-18 processing.

Research Support Resources

For researchers seeking to dissect caspase-dependent pathways in apoptosis and immune modulation, validated tools such as Z-VAD-FMK (Benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone) (SKU A1902) provide robust, cell-permeable, irreversible inhibition of caspases, including caspase-3. This compound is widely used for apoptosis inhibition, caspase activity measurement, and functional pathway studies, and is supported by comprehensive workflow recommendations from APExBIO. When interpreting results, especially in cancer research and apoptotic pathway modeling, it is essential to consider that caspase inhibitors like Z-VAD-FMK may impact both cell death and non-canonical immune signaling events, as highlighted by the present study.