Cancer development is a complex process driven by cell-intrinsic genetic changes that confer survival advantages and increased proliferative capacity. These recently transformed cells acquire the ability to communicate with their microenvironment in a way that enables them to grow, proliferate, and ultimately evade immune recognition and killing. In the initial stages of tumorigenesis, abnormal cell proliferation in tissues results in local accumulation of inflammatory mediators. Of these mediators, cytokines play a central role, orchestrating downstream immune responses and dictating the tumor microenvironment. These mediators can either promote effective cancer immunosurveillance, leading to cytotoxic immune cell-mediated elimination, or sustain cancer cell growth through immunosuppressive mechanisms (Propper & Balkwill, 2022).

Interleukin (IL)-33, a member of the IL-1 cytokine superfamily, influences the early immune response and, through binding to its receptor ST2, participates in immune cell regulation. This ultimately shapes tumor growth and development (Yeoh et al., 2022). In this review, we provide a comprehensive overview of the role and function of IL-33 in maintaining tissue homeostasis, modulating inflammation, and influencing tumor development, progression, and response to anti-cancer treatments. We describe the protein structure of IL-33 and its signalling pathway, the sources of IL-33 in situ, and the impact of IL-33 signalling on immune cell homeostasis and function. Furthermore, we examine the potential contribution of IL-33 in orchestrating the tumor microenvironment and link its RNA level expression to disease outcomes. Finally, we discuss the therapeutic potential of IL-33, alone and in combination with currently approved immunomodulators.

Endogenous IL-33 protein and mRNA were first visualized in human tissues in 2003 using in situ hybridization and immunohistochemistry (Baekkevold et al., 2003). IL-33 was discovered as a nuclear protein which was abundantly expressed in the high endothelial venules (HEV), and therefore was designated as a ‘nuclear factor from high endothelial venules’ (NF-HEV) (Baekkevold et al., 2003). Based on the similarity of its carboxy terminal domain to the IL-1 cytokine family and its ability to induce type 2 immune reactions via its binding to IL-1 family receptor ST2 (IL-1 receptor-like-1, IL1RL1), NF-HEV was renamed IL-33 (Schmitz et al., 2005). It was also demonstrated to be a chromatin associated nuclear factor in vivo (Carriere et al., 2007).

Encoded by the IL33 gene located on the short arm of chromosome 9 at 9p24.1 (Baekkevold et al., 2003), IL-33 protein comprises of a highly basic N-terminal domain and a highly acidic IL-1 like C-terminal domain, separated by a central domain. The N-terminal domain consists of a chromatin-binding motif (CBM) (Roussel et al., 2008) and a nuclear localization signal (NLS) (Baekkevold et al., 2003). Human IL-33 has around 52 % identity over 270 residues with its mouse orthologues (Cayrol & Girard, 2018). At steady-state, IL-33 is constitutively expressed and acts as a transcriptional regulator. It interacts with the acidic domains of histones H2A and H2B via a short chromatin binding peptide, participating in epigenetic regulatory mechanisms (Roussel et al., 2008). Its C-terminal domain forms a β-trefoil, consisting of 12 β strands which are arranged as 3 strands of 4 pseudo repeat units (Lingel et al., 2009). This β-trefoil structure is also observed in IL-1α, IL-1β and IL-18 (Liu et al., 2013). IL-33 is released from cells and binds to its specific receptor ST2 (suppression of tumorigenicity 2; or IL33R) through its C-terminal domain. Upon binding of IL-33, ST2 forms a heterodimer with the IL-1 receptor accessory protein (IL-1RAcP), triggering downstream signalling pathways (Schmitz et al., 2005).

IL-33 is produced by a diverse array of cell types including endothelial cells, epithelial cells, and stromal cells. Its expression can be increased in response to stress, injury, or inflammatory signals. Its production varies among different cell types, each contributing uniquely to its levels and functions in various contexts. IL-33 is a multifaceted cytokine, maintaining homeostasis and modulating the immune system, making it an important player in both protective and pathological processes, including allergy, autoimmunity, and cancer (Cayrol and Girard, 2022, Liew et al., 2016).

Endothelial cells constitute one of the major sources of IL-33 in human tissues (Cayrol and Girard, 2022, Küchler et al., 2008, Moussion et al., 2008). Knockdown studies in primary human endothelial cell culture have confirmed abundant and constitutive intranuclear IL-33 expression. Enhanced expression of IL-33 has been observed in tissues of patients suffering from autoinflammatory diseases such as rheumatoid arthritis and Crohn’s disease (Carriere et al., 2007). Additionally, high IL-33 expression in the peritumoral blood vessels has also been reported (Moussion et al., 2008). Differences in IL-33 expression exist between mouse and human tissues. In contrast to human endothelial cells, IL-33 is not constitutively expressed in murine endothelial cells of the liver, adipose tissue, or ovary vasculature. It is also absent in HEV lining secondary lymphoid organs such as the lymph nodes and spleen of mice (Pichery et al., 2012). However, IL-33 expression can be induced in murine endothelial cells upon inflammatory stimulations or conditions, such as colitis (Pichery et al., 2012, Sedhom et al., 2013) or in cardiac endothelial cells during myocardial pressure (Chen et al., 2015).

High levels of IL-33 have been observed in the nuclei of epithelial cells at barriers sites such as the lung, gut, and skin. These include human and mouse epithelial cells lining the skin and the digestive, respiratory, and female reproductive tracts (Haenuki et al., 2012, Moussion et al., 2008, Pichery et al., 2012, Salker et al., 2012, Sundnes et al., 2015). In human lungs, bronchial epithelial cells are the major producers of IL-33 (Préfontaine et al., 2010). Additionally, in contrast to mouse keratinocytes which express IL-33 constitutively, IL-33 expression in human keratinocytes can be induced following interferon (IFN)-γ stimulation (Meephansan et al., 2012, Sundnes et al., 2015). IL-33 expression has not been observed in the mouse airway epithelium at steady-state but can be induced following inflammatory insults (Byers et al., 2013, Kearley et al., 2015). However, constitutive expression in alveolar type II pneumocytes (Schmitt et al., 2024) has been reported in the lungs of mice which is further increased following inflammation (Hardman et al., 2013).

IL-33 is abundantly produced in the nuclei of fibroblastic reticular cells (FRCs) in humans and mice (Moussion et al., 2008, Pichery et al., 2012). These cells are present in lymphoid organs including the spleen, lymph nodes, appendix, tonsils and Peyer’s patches. Additionally, constitutive production of IL-33 is seen in fibroblast-like cells in adipose tissue and subepithelial myofibroblasts in the intestine and colon (Mahapatro et al., 2016). This constitutive expression appears to be tissue specific since these fibroblasts do not express IL-33 constitutively in other tissues (Moussion et al., 2008). IL-33 expression is mostly induced during inflammatory diseases and inflammatory cytokines such as IL-1β and TNFα are potent inducers of IL-33 in stromal cells (Cayrol and Girard, 2018, Kobori et al., 2010, Nishida et al., 2010, Sponheim et al., 2010). Alternatively, stromal cells have been reported as important sources of IL-33 especially in conditions of tissue fibrosis and mucosal as well as wound healing (Manetti et al., 2010, Marvie et al., 2010, Masamune et al., 2010, Sponheim et al., 2010).

In addition to barrier tissues and lymphoid organs, IL-33 is also expressed in the nervous system. Glial cells and astrocytes present in healthy brains have been reported to produce significant amounts of IL-33 (Pichery et al., 2012). Following injury, post-mitotic OLIG2+ oligodendrocytes rapidly release IL-33 to drive myeloid cell recruitment and promote recovery. Interestingly, IL-33 transcripts can be found in one-third of murine brain cells (astrocytes, oligodendrocyte progenitor cells and microglia), a finding that has not been confirmed in humans (Gadani et al., 2015). In both the human and the murine eye, Sox2+ Muller glial cells present in the retina are a major source of production of IL-33, with the optic nerve, epithelial cells of the ciliary body and conjunctiva being additional producers (Xi et al., 2016).

The expression of IL-33 in hematopoietic cells remains controversial. While some studies have reported an absence of IL-33 expression in immune cells (De Kleer et al., 2016, Préfontaine et al., 2010), others have found circumstantial expression of IL-33 by hematopoietic lineages. These include monocytes and macrophages, regulatory CD4+ T (Treg) cells, pre-pro-B cells and dendritic cells (Hatzioannou et al., 2020, Nile et al., 2010, Ohno et al., 2009, Samuchiwal et al., 2017, Stier et al., 2019). However, it is possible that the detection of IL-33 in dendritic cells and macrophages may be a result of phagocytosed cells and other debris. Single-cell RNA sequencing studies analysing mRNA expression have demonstrated that Il33 is not expressed in most immune cells. Bone marrow pre-pro B cells express IL-33 but at much lower levels in comparison to non-immune cells (Cayrol, 2021, Polumuri et al., 2012, Stier et al., 2019).

The orphan receptor ST2, also referred to as IL-1R4, was the first receptor identified for IL-33 (Schmitz et al., 2005). Like other receptors for the IL-1 family, the IL-33 receptor (IL-33R) is a heterodimeric complex consisting of ST2 and the IL-1R accessory protein (IL-1RAcP) (Fig. 1). IL-1RAcP also acts as a co-receptor for IL-1α, IL-1β, IL-36α, IL-36β, IL-36γ, and IL-38 (Arend et al., 2008, Dinarello, 2009, Mantovani et al., 2019, O’Neill, 2008). Transmembrane ST2 (ST2L), soluble ST2 (sST2), variant ST2 (ST2V), and long variant ST2 (ST2LV) are the four splice variants produced by Il1rl1 (Iwahana et al., 1999, Iwahana et al., 2004, Tago et al., 2001). IL-33-mediated intracellular signalling requires binding to ST2L and IL-1RAcP while sST2 acts as a decoy receptor, sequestrating IL-33 from the environment and preventing its binding to ST2L on target cells (Fig. 1). The binding of IL-33 to ST2L and its subsequent heterodimerization with IL-1RAcP enables the recruitment of Myeloid differentiation primary response 88 (MyD88) via the Toll-interleukin-1 receptor (TIR) domain expressed in the cytoplasmic region of ST2L. Downstream signalling pathways include the recruitment of Interleukin 1 Receptor Associated Kinase 1 (IRAK1) and 4 (IRAK4), Tumor Necrosis Factor Receptor Associated Factor 6 (TRAF6) and mitogen-activated protein (MAP) kinases, culminating in the activation of Nuclear Factor-κB (NF-κB) and Activator Protein 1 (AP1) transcription factors (Schmitz et al., 2005) (Fig. 1). Reports show that IL-33 binds to another receptor comprising of IL1RAcP, ST2L and single Ig IL-1R-related molecule (SIGIRR), seemingly acting as a negative regulator of IL-33 signalling. SIGIRR is also referred to as Toll IL-1R8 (TIR8) (Bulek et al., 2009, Garlanda et al., 2009).

ST2L is expressed on both haematopoietic and non-haematopoietic cells (Cayrol & Girard, 2018). These include epithelial cells, astrocytes, neurons, fibroblasts and endothelial cells, as well as myeloid and lymphoid immune cell populations. Although the role of IL-33/ST2L pathway in regulating non-hematopoietic cell function remains elusive, IL-33/ST2L signalling in immune cells promotes their proliferation, migration, and survival. ST2L is expressed by a wide range of tissue-resident immune cells such as mast cells (Enoksson et al., 2011, Morita et al., 2015), innate lymphoid cells (ILCs) (Mjösberg et al., 2011, Moro et al., 2010, Neill et al., 2010, Price et al., 2010), and Tregs (Matta et al., 2016, Schiering et al., 2014). Other subsets such as neutrophils, basophils, eosinophils (Mitchell et al., 2018), macrophages, invariant Natural Killer (iNKT) T cells, NK cells (Chan et al., 2003), CD4+ T lymphocytes and CD8+ T cells have also been reported to express ST2L (Bonilla et al., 2012, Kearley et al., 2015, Peine et al., 2016). The ubiquitous expression of ST2L on various immune cell subsets highlights the crucial role played by IL-33 in promoting immune responses and the pleiotropic functions that this cytokine plays at both steady-state and during inflammation (Fig. 2).

Widespread expression of IL-33 and ST2L highlights the importance of this pathway in contributing to tissue homeostasis and inducing potent innate and adaptive immune responses upon tissue damage or infection.

IL-33 production is increased during inflammation along with a subsequent release from dying cells (Cayrol, 2021). Increased IL-33 expression has been observed in various inflammatory states, such as in the airway epithelial cells of patients diagnosed with chronic obstructive pulmonary disease (Byers et al., 2013, Kearley et al., 2015), the intestinal epithelium of bone marrow transplant patients suffering from graft-versus-host disease (Reichenbach et al., 2015), and in the blood vessels and keratinocytes of patients suffering from atopic dermatitis (Savinko et al., 2012). Studies in mouse models have also found a significant increase in IL-33 expression in alveolar type II pneumocytes following nematode or viral infections (Yasuda et al., 2012), and after exposure to cigarette smoke or allergens (ragweed pollen or Alternaria spp.) (Hardman et al., 2013).

Mechanistically, IL-33 expression is induced by inflammatory molecules such as IFN-γ, IL-1β, and TNFα (Kunisch et al., 2012, Meephansan et al., 2012, Nishida et al., 2010, Préfontaine et al., 2009, Saidi et al., 2011, Sponheim et al., 2010). IL-33 expression can also be induced by Notch signalling in endothelial cells (Sundlisaeter et al., 2012). In regard to post-transcriptional regulation, IL-33 expression can also be modulated by miRNA, particularly, miR-200b and miR-200c (Tang et al., 2018). Inhibition of these miRNAs increases IL-33 levels in lung epithelial cells in vitro. In parallel, in vivo studies of allergic inflammation in mice demonstrated decreased IL-33 expression and resolution of inflammation after intranasal inoculation of miR-200b (Tang et al., 2018). Additional studies in this context have also explored an RNA-binding protein Mex-3B, that post-transcriptionally activates IL-33 by inhibiting miRNA-mediated suppressive functions (Yamazumi et al., 2016).

IL-33 plays a crucial role in promoting post-inflammation tissue repair through the activity of ST2L+ Tregs and type 2 innate lymphoid cells (ILC2s). ST2L+ Treg cells have not only been reported to enhance skeletal muscle repair (Burzyn et al., 2013, Kuswanto et al., 2016), but can also control inflammation in the intestinal mucosa. This finding has been substantiated by the inability of ST2-/- Tregs cells to confer protection against experimentally-induced colitis (Schiering et al., 2014) or acute Graft-versus-host disease (Matta et al., 2016). During virus-induced lung inflammation, IL-33 activation has been shown to promote tissue repair by driving Treg-derived amphiregulin (AREG) expression, a mechanism confirmed using conditional deletion of AREG in Foxp3+ cells (Arpaia et al., 2015, Burzyn et al., 2013). IL-33 also potentiates AREG expression in ILC2s to confer protective immunity in intestinal mucosal tissue (Fallon et al., 2015). This has been reported to restore homeostasis and epithelial integrity in the lungs during viral infections (Monticelli et al., 2011), along with promoting cutaneous wound healing (Rak et al., 2016). Specifically, IL-33 deficiency results in delayed cutaneous wound healing while also negatively impacting ILC2 responses (Rak et al., 2016). In contrast, high levels of IL-33 promote tissue fibrosis, an outcome reinforced by the presence of type 2 cytokines. Consequently, IL-33 overexpression during inflammation is associated with fibrotic disease of the liver (Mchedlidze et al., 2013) and lung (Byers et al., 2013, Li et al., 2014, Luzina et al., 2013). However, short-term administration of IL-33 during liver injury can have a protective effect against fibrosis (Li et al., 2014), suggesting a dose-level effect of IL-33 in modulating the delicate balance between tissue repair and fibrosis. Hence, a better understanding of the role of IL-33 in driving tissue repair or fibrosis and the contribution of other effector molecules and cell types involved, is warranted.

Tumors are very dynamic entities that are composed of cancer cells and a diverse network of non-malignant cells, embedded in a vascularized extracellular matrix. These include various immune cells, endothelial cells, cancer associated fibroblasts (CAFs), pericytes, neurons and adipocytes, among others. A continuous bi-directional flow of information between these cells determines cancer pathogenesis and disease outcomes. Importantly, it is these components that come together to influence the functional state of the tumor microenvironment (TME), thereby determining tumor suppression or tumor progression (De Visser & Joyce, 2023). An in-depth study of these molecules also paves the way for designing better therapeutic strategies aiming to stop tumor progression and improve disease outcomes.

The TME is highly heterogeneous and strong intra- and inter-patient variabilities are dictated by tumor-intrinsic and extrinsic factors. These include age, sex, tissue specificity, disease stage, microbiota composition and various genetic and epigenetic factors (Pan & Jia, 2021). Most of the adaptive and innate lymphocytes (T cells, B cells, NKT cells, NK cells and other ILC subsets) as well as myeloid cell subsets (macrophages, neutrophils, monocytes, dendritic cells, mast cells and eosinophils) are present in the TME and various subsets express ST2L. In these subsets, IL-33 stimulation can promote either pro-tumor or anti-tumor responses, depending on its downstream effectors. IL-33 was initially thought to only influence type 2 immune responses by acting directly on eosinophils, basophils, mast cells, dendritic cells, myeloid-derived suppressor cells (MDSCs), Treg, ILC2 and T helper 2 cells (Th2) (Cayrol & Girard, 2014). However, accumulating evidence suggests that IL-33 also elicits type-1 immunity by activating NK cells, NKT cells, CD8+T cells, macrophages, neutrophils and B cells (Bonilla et al., 2012, Smithgall et al., 2008, Varricchi et al., 2018) (Fig. 2). IL-33, therefore, plays a significant role in the TME, by regulating the immune response to influence inflammation and disease outcomes.

The IL-33/ST2L pathway shapes CD4+ T cell homeostasis and function (Alvarez et al., 2019). As most tumor cells are MHC class II negative, CD4+ T cells can elicit potent anti-tumor responses through helper cell functions (Speiser et al., 2023, Xie et al., 2010). In addition, inflammatory cytokines produced by CD4+ T cells such as IFN-γ, can directly act on tumor cells, upregulating MHC class II expression and enhancing the CD4+ T cell mediated anti-tumor response (Accolla et al., 2019, Axelrod et al., 2019, Seliger et al., 2017). Strategies circumventing MHC class-II restrictions by employing an antigen specific recombinant immunoreceptor are being explored to target tumor cells (Hombach et al., 2006, Oh and Fong, 2021). While IL-33 is mainly known for its prominent role in reinforcing a Th2 phenotype, it can also induce type 1 immune responses by working synergistically with IL-12 and mediating CD4+ T helper 1 (Th1) differentiation. IL-33 administration in combination with HPV16 E6/E7-encoded DNA vaccine, elicited antigen specific CD4+ and CD8+ IFN-γ+ cells along with high serum IgG concentration, culminating in tumor regression in mice (Villarreal et al., 2014). However, IL-33 appears to be unable to fully differentiate naïve T cells into Th1 cells (Komai-Koma et al., 2016), since naïve T cells express very low levels or are entirely absent in ST2L but these findings need to be further confirmed. In contrast, IL-33 reinforces T helper cell differentiation programs, with its engagement into these programs being dependent on the composition of the cytokine milieu and the availability of polarizing cytokines, such as IL-12 or IL-4 (Komai-Koma et al., 2016). In addition to stimulating Th1 or Th2 cells, IL-33 can promote Th9 polarization (Blom et al., 2011), a subset involved in anti-tumor immune responses in vivo (Benoit-Lizon and Apetoh, 2021, Lu et al., 2012, Ramadan et al., 2017).

IL-33 plays a pleiotropic role on CD4+ T cells. The ablation of ST2 expression specifically in Tregs impairs their homeostasis, proliferation, and effector function (Ameri et al., 2019, Hatzioannou et al., 2020). IL-33/ST2L-mediated expansion of CD4+Foxp3+GATA3+ Tregs has been observed in vitro and in vivo (Alvarez et al., 2019, Torres et al., 2024, Vasanthakumar et al., 2020). Intestinal tissues are enriched in ST2L+Tregs, where they promote tolerance toward commensal microbiota and dampen inflammation to maintain homeostasis. Mechanistically, IL-33-induced TGF-β1 expression in the intestine promotes the differentiation and accumulation of Tregs (Schiering et al., 2014). In ApcMin/+ colorectal tumor-bearing mice, azoxymethane (AOM)+ dextran sulphate sodium (DSS)-induced colorectal tumor-bearing mice as well as in the context of skin cancers, epithelial cell-derived IL-33 production promotes ST2L+Treg cell expansion, correlating with disease progression and increased tumor burden (He et al., 2017, Meinicke et al., 2017). Poor disease outcomes are associated with increased suppressive Treg function, resulting in decreased immune protection (Ameri et al., 2019, Hatzioannou et al., 2020). Recombinant IL-33 treatment in tumor-bearing mice has been associated with dismal outcomes, while blocking IL-33 decreases the level of ST2L+Tregs, which has been associated with improved anti-tumor immunity and reduced tumor burden (Wang et al., 2017, Zhou et al., 2018).

Collectively, these studies highlight the differential impact of IL-33 stimulation on CD4+ T cells and warrants use of conditional knock out mouse models for specific deletion in the population of interest.

As their name suggests, ILCs are innate cells derived from the common lymphoid progenitor and are characterized by the absence of expression of lineage T cell, B cell, NK and myeloid cell marker expression (Harly et al., 2018, Hoyler et al., 2012, Moro et al., 2010). ILCs are described as the innate counterpart to T cells, mirroring specific T cell subsets through shared expression of major transcription factors and effector molecules. In contrast to T cells however, ILCs lack specific antigen-specific receptors. They are categorized into five subsets, namely NK cells, type 1, type 2 and type 3 ILCs (ILC1s, ILC2s and ILC3s, respectively), as well as lymphoid Tissue Inducer cells (Vivier et al., 2018). ILC2 express high levels of GATA Binding Protein 3 (GATA3) and ST2L, amongst other cytokine receptors. Although, mainly tissue-resident, particularly at barrier sites such as the lungs, skin and the gut, ILC2s can be detected in the secondary lymphoid organs and in circulation (Bajana et al., 2022, Mjösberg et al., 2011). Since they do not express antigen receptors, they respond to various signals from their environment, enabling them to rapidly orchestrate immune responses locally, such as responding to allergen exposure, helminth infection or tissue damage (Fallon et al., 2015, Fallon et al., 2006, Halim et al., 2014). Notably, IL-33 is one of the main alarmins that activate ILC2s (Ricardo-Gonzalez et al., 2018) inducing their production of type 2 cytokines such as IL-5, IL-13 and granulocyte macrophage colony-stimulating factor (GM-CSF) (Jacquelot et al., 2021).

Several studies indicate that presence of ILC2s in the TME are associated with poor cancer prognosis (Bahhar et al., 2023, Jacquelot et al., 2022, Jou et al., 2022, Trabanelli et al., 2015, Trabanelli et al., 2017). ILC2s through their recruitment of Tregs and MDSCs, mediate immunosuppression and result in poor tumor control (Bie et al., 2014). Additionally, ILC2s can produce AREG and participate in tissue repair, further supporting cancer cell proliferation which accelerates tumor growth (Monticelli et al., 2011). However, several reports, including our own work, have described an anti-tumorigenic functions of ILC2s, promoting the recruitment and effector function of antigen-specific CD8+ T cells, eosinophils, and dendritic cells (Jacquelot et al., 2022, Spits and Mjösberg, 2022, Wen et al., 2023). Specifically, IL-33-mediated ILC2 activation is associated with increased anti-tumor responses in melanoma and pancreatic tumor models (Jacquelot et al., 2021, Kim et al., 2016, Moral et al., 2020, Wagner et al., 2020). Despite its potency in ILC2 activation, IL-33 stimulation also results in upregulation of the inhibitory receptor PD-1. Deletion of PD-1 in ILC2s has been shown to improve anti-tumor responses compared to control groups (Jacquelot et al., 2021, Moral et al., 2020). Altogether, these studies underscore the important role that ILC2s play in tumor development and progression, irrespective of their pro- or anti-tumorigenic function. A better understanding of this IL-33/ST2L/ILC2 axis together with other factors that influence ILC2 effector function in tumors is urgently needed.

NK cells are important contributors to host defence against intracellular pathogens. Furthermore, they play a critical anti-cancer role, particularly in their capacity to limit tumor cell spread and metastasis. In addition to their direct killing functions, NK cells promote the recruitment and effector function of other anti-tumor immune subsets. Similar to its effect on CD4+ T cells, IL-33 increases NK cell effector function via cooperation with the other NK cell-stimulating cytokines IL-12 and IL-18 (Smithgall et al., 2008). Increasing IL-33 levels by either overexpression in tumor or host cells or exogenous administration has been associated with enhanced intratumor NK cell infiltration and effector functions, limiting primary tumor growth and preventing progression. This IL-33 mediated anti-tumor protection is abrogated upon NK cell depletion (Gao et al., 2013, Gao et al., 2015, Qi et al., 2020), suggesting a link between IL-33 expression, enhanced NK cell function and tumor control. While both mouse and human NK cells express low levels of ST2L, it is upregulated upon exposure to IL-12 or TNF-α (Ochayon et al., 2020, Smithgall et al., 2008), enhancing their responsiveness to IL-33 stimulation and activity (Eberhardt et al., 2023, Nabekura et al., 2015). Collectively, most studies point towards a beneficial impact of IL-33 stimulation on NK cell effector function. In this context, deletion of ST2 using conditional knock out models should be established to further confirm a NK cell-intrinsic role of ST2 in IL-33-mediated anti-tumor immunity and control of tumor growth.

Similar to NK cells, CD8+ T cells play a critical role in mediating anti-tumor responses as they are key producers of cytotoxic molecules that directly kill cancer cells owing to their antigen specific responses. Therefore, their recruitment and activation is critical to controlling tumor progression. The regulatory role of IL-33 on CD8+ T cells is not well understood, especially in the context of cancer. Naïve CD8+ T cells do not express ST2 but can be induced by IL-12, a mechanisms dependent on T-bet expression (Yang et al., 2011). Together with IL-33, IL-12 promotes IFNγ production in CD8+ T cells to drive potent inflammatory responses (Yang et al., 2011). Studies using tumor cell lines that overexpress IL-33 have shown that IL-33 produced by tumor cells could increase CD8+ T cell proliferation and their production of IFNγ and cytotoxic molecule granzyme B (Chen et al., 2020, Gao et al., 2015). Specifically, tumor-derived IL-33 has been shown to increase the infiltration of tissue-resident memory (CD103+CD8+) T cells into tumors which mediates tumor protection (Chen et al., 2020). Our understanding of the specific role of IL-33 in driving tissue-resident CD8+ T cells infiltration into tumors is still in its infancy. However, this presents an interesting target for immunotherapies as the localisation of immune cells in close proximity to the tumor has a strong influence on tumor prognosis. Therefore, stimulation signals that have the capacity to drive tumour infiltration present as an interesting immunotherapy strategy.

Many studies have highlighted the role of IL-33 in increasing expression of M2 markers on macrophages thereby enhancing the suppressor function of tumor associated macrophages (TAMs). Simultaneously, IL-33 blockade prevented the polarization of these macrophages in lung cancer studies (Wang et al., 2017). Various studies in mouse models of colon cancer (Akimoto et al., 2016, Fang et al., 2017, He et al., 2017, O’Donnell et al., 2016), breast cancer (Jovanovic et al., 2014) and tumor xenograft models have demonstrated the recruitment of M2 macrophages within the cancerous tissues and is associated with cancer progression. IL-33 can induce the production of prostaglandin E2 and MMP-9 in macrophages, supporting macrophage-mediated cancer cell stemness, angiogenesis, and immunosuppression (Fang et al., 2017) while also facilitating angiogenesis and immunosuppression (Andersson et al., 2018, Yang et al., 2016).

Despite low expression levels of ST2, DCs have been reported to respond to IL-33 and up-regulate expression of MHC-II along with CD40, CD80, CD86, OX40L, CCR7 while increasing production of chemokines such as CCL17 and CCL22 and cytokines such as TNF-α, IL-1β, IL-4, IL-5 and IL-13 that can greatly influence the role of these cells in the TME (Besnard et al., 2011, Gabriele et al., 2013, Kurokawa et al., 2013, Rank et al., 2009). They are known for recruiting CD4+ T cells resulting in an atypical Th2-type immune response by polarizing naïve CD4+ T cells to produce IL-5 and IL-13 but not IL-4 (Besnard et al., 2011, Rank et al., 2009). IL-33 is also known to promote DC expansion from bone marrow by triggering basophil-derived GM-CSF secretion. These cells promote a pro-tumorigenic immune response owing to increased expression levels of PD-L1 and PD-L2 and lower levels of MHC-II weakening their capacity to prime naïve T cells (Mayuzumi et al., 2009). Upon IL-33 stimulation, DCs can produce IL-2, which has been reported to support ST2+ Treg expansion (Matta et al., 2016). However, contrasting studies in acute myeloid leukemia model (AML), EG7 lymphoma, B16 and inducible melanoma models have reported anti-tumor role of IL-33 stimulated DCs (Dominguez et al., 2017, Qin et al., 2016). This has been attributed to increased activation and priming of CD8+ T cells via MyD88 and STAT-1 dependent pathway in the tumor microenvironment (Dominguez et al., 2017, Qin et al., 2016). This finding is further contingent on conventional type 1 DCs (cDC1s), where studies have demonstrated both direct injection of IL-33 or immunotherapy with FCGR+CD103+ promotes anti-tumor responses (Wang et al., 2017).

IL-33 has been reported to induce production of IL-5, IL-9, IL-4, and IL-13 by neutrophils. However, studies on its role in the TME are limited. Inflammatory enzymes released by neutrophils are known to cleave IL-33 into more active bioforms (Afferni et al., 2018). A study conducted in ST2-deficient mice demonstrated the ability of IL-33 in preventing mucositis in a mouse model of ectopic colon carcinoma treated with systemic irinotecan, a topoisomerase inhibitor, by enhancing neutrophil recruitment in a IL-33/CXCL1/2/CXCR2-dependent manner (Guabiraba et al., 2014), improving chemotherapy response.

Upon IL-33 stimulation, mast cells and basophils produce vascular endothelial growth factor (VEGF), which plays a crucial role in promoting tumor growth owing to its role in angiogenesis (Marone et al., 2016). Similar to other immune subsets, mast cell activation has been reported in a number of tumor types with non-distinct activity pattern in reference to anti- or pro- tumoral immunity (Varricchi et al., 2017). UV-induced squamous cell carcinomas which evade immune destruction have been found to express high levels of IL-33 associated with mast cells and neutrophils (Byrne et al., 2011). In APC Min/+ model, IL-33 deficiency has been associated with a reduction in tumor burden (Josefowicz et al., 2012, Rothstein and Camirand, 2015) correlating with a decrease in mast cell density and effector functions (Blatner et al., 2010, Cheon et al., 2011, Gounaris et al., 2007).

The effect of IL-33 on basophils is mostly through basophil degranulation, cytokine production and histamine secretion via enhancement in IL-3 and anti-IgE (Marone et al., 2020). However, their role in tumors requires further investigations. Studies in melanoma models have reported the ability of basophils to recruit CD8+ T cells via CCL3 and CCL4 chemokines resulting in Treg cell depletion and thereby contributing to tumor rejection (Sektioglu et al., 2017). Additionally, circulating basophils and eosinophils have been shown to be associated with poor prognosis in CRC patients (Wei et al., 2018) and patients with pancreatic ductal adenocarcinoma (De Monte et al., 2016). In line with human studies, presence of basophils in tumor draining lymph nodes has also been found to correlate with poor prognosis in mouse models due to enhance Th2 responses (De Monte et al., 2016).

IL-33 acts directly on eosinophils resulting in upregulation of the expression of CD11b, CD69, CCR3, cytokines such as IL-6, IL-13 and GM-CSF along with chemokines such as CXCL10, CXCL3, CXCL2 and CCL17 (Johnston & Bryce, 2017). IL-4 and IL-13 production by eosinophils following exposure to IL-33 has been reported to promote M2 macrophage polarization (Willebrand & Voehringer, 2016). IL-33 administration has been reported to support ILC2-mediated eosinophil recruitment and prevent metastasis in melanoma models (Ikutani et al., 2012). Intratumoral expansion of eosinophils is also observed in mice transplanted with IL-33 expressing CT26, EL4, and B16 tumors (Kim et al., 2016). Studies in the B16 melanoma models demonstrated the abrogation of IL-33-induced anti-tumor response in ST2-deficient mice owing to a depletion in eosinophils. Other studies examining the role of IL-33-expanded eosinophils in anti-tumor immunity have highlighted their ability to recruit NK and CD8+T cells in subcutaneous tumors (Carretero et al., 2015). Various studies have reported enhancement of CD11b, CD69, and Granzyme B expression in vitro against melanoma cells resulting in cytotoxicity (Lucarini et al., 2017). Additionally, induction of reactive oxygen species (ROS) production by eosinophils in human as well as mouse cells in vitro, thereby resulting in degranulation (Cherry et al., 2008, Suzukawa et al., 2008), further elaborates on its tumoricidal potential (Caruso et al., 2011, Cormier et al., 2006, Costain et al., 2001, Legrand et al., 2010, Varricchi et al., 2018).

Although absent in healthy individuals, MDSCs are found to expand and proliferate in patients suffering from cancer (Tcyganov et al., 2018). In vivo studies using 4T1 breast cancer model have documented MDSCs expansion following exogenous IL-33 administration leading to an accumulation of TGF-β1 and IL-13α1R producing CD11b+ Gr1+ MDSCs, with a higher frequency of them being monocytic (M-MDSCs) than granulocytic (G-MDSCs) (Jovanovic et al., 2014, Xiao et al., 2016). IL-33 was also reported to upregulate Arg-1 activity and NF-κB and MAPK signalling in vivo (Xiao et al., 2016). Conversely, IL-33 has been reported to inhibit the differentiation of G-MDSCs but not M-MDSCs from BM-progenitor cells and these cells were found to exhibit reduced immunosuppressive ability while also demonstrating reduced ROS expression, IFN-γ production and T-cell proliferation (Lim et al., 2017). Studies in the B16 melanoma models also demonstrated a decrease in MDSCs following IL-33 administration (Lucarini et al., 2017), hinting at the dual role of IL-33 in cancer.