IL13RA2 Human

What is IL13RA2 and how does it differ from IL13RA1?

IL13RA2 (Interleukin 13 Receptor Alpha 2) is one of two major receptors for the cytokine interleukin 13 (IL-13). Unlike IL13RA1, which has low affinity for IL-13 and is expressed ubiquitously in humans, IL13RA2 demonstrates high binding affinity to IL-13 with expression primarily restricted to the testes in normal tissues . The structural and functional differences between these receptors are significant. IL13RA1 forms a complex with IL-4Rα and activates the JAK/STAT6 pathway upon IL-13 binding, while IL13RA2 has a short cytoplasmic tail with no known signaling motifs, initially leading researchers to classify it as a decoy receptor . This classification has been challenged by recent studies demonstrating that IL-13-mediated IL13RA2 signaling can occur via STAT6-independent pathways . IL13RA1 is essential for allergen-induced airway hyperresponsiveness and mucus production, whereas IL13RA2 was traditionally thought to primarily sequester IL-13, preventing its interaction with the IL13RA1/IL-4Rα complex and thereby regulating IL-13 signaling intensity.

What is the expression pattern of IL13RA2 in normal human tissues?

In normal human tissues, IL13RA2 expression exhibits a highly restricted pattern, predominantly limited to the testes . This contrasts sharply with IL13RA1, which is expressed ubiquitously across human tissues. Immunohistochemistry studies have provided further detail regarding this expression pattern, revealing that in healthy intestinal mucosa, IL13RA2 protein expression is specifically confined to goblet cells . This highly selective expression profile undergoes dramatic changes in pathological conditions. For example, in patients with inflammatory bowel disease (IBD), IL13RA2 protein becomes highly expressed throughout the epithelial cells of the intestinal mucosa rather than remaining restricted to goblet cells . The limited expression of IL13RA2 in normal tissues makes it a particularly attractive target for therapeutic development, especially for conditions where the receptor becomes abnormally overexpressed, as this offers potential for selective targeting with minimal impact on healthy tissues.

What are the species-specific differences in IL13RA2 between humans and mice?

Significant species-specific differences exist in IL13RA2 biology between humans and mice, particularly regarding the generation of soluble IL13RA2 (sIL13RA2). In mice, alternative splicing of the IL13RA2 gene results in a transcript that lacks exon 10, which contains the transmembrane domain, thereby producing a naturally soluble form of the receptor . Consequently, mice have nanogram-per-milliliter levels of sIL13RA2 in their serum. In contrast, humans lack this alternatively spliced transcript entirely. Despite extensive searching, researchers have been unable to identify any human IL13RA2 transcript lacking the transmembrane domain . The serum concentration of sIL13RA2 in mice is approximately 10-fold greater than that of IL-13 on a molar basis, which is sufficiently high to inhibit IL-13 signaling in serum and make sIL13RA2 a major in vivo modifier of IL-13 activity and allergic inflammation in mice . These fundamental differences necessitate caution when translating findings from murine IL-13 studies to humans, as the regulatory mechanisms of IL-13 signaling differ substantially between the two species.

How is soluble IL13RA2 generated in humans?

In humans, soluble IL13RA2 (sIL13RA2) is generated through a mechanism fundamentally different from that in mice. While mice produce sIL13RA2 through alternative splicing, humans generate it exclusively through proteolytic cleavage of membrane-bound IL13RA2 (memIL13RA2) . The primary enzymes responsible for this cleavage are matrix metalloproteinases (MMPs), with MMP-8 playing a particularly crucial role. Experimental evidence supporting this mechanism includes studies showing that siRNA-mediated depletion of full-length IL13RA2 decreased both membrane-bound and soluble forms in human cells, including U87 glioma cells, HaCaT keratinocytes, and human pulmonary artery smooth muscle cells (HPASMC) . Furthermore, inhibition of MMPs, specifically MMP-8, abolished the production of sIL13RA2 from human cells entirely . The direct production of sIL13RA2 from memIL13RA2 in humans suggests a more regulated release process that may respond to specific stimuli, potentially explaining why sIL13RA2 is generally absent in human serum and bronchoalveolar lavage fluid, in contrast to the consistent presence in mouse serum .

What methodologies are most effective for detecting IL13RA2 expression in human samples?

Multiple complementary methodologies have been validated for detecting IL13RA2 expression in human samples, each with specific advantages for particular research questions:

Immunohistochemistry (IHC): This approach has proven valuable for spatial localization of IL13RA2 within tissues. Studies of inflammatory bowel disease (IBD) patients have employed IHC to demonstrate that IL13RA2 protein is highly expressed in epithelial cells throughout the intestinal mucosa, while expression is restricted to goblet cells in healthy controls . IHC allows for assessment of expression patterns within the tissue architecture and can be quantified through analysis of staining intensity and distribution.

Flow Cytometry: This technique effectively quantifies cell surface expression of IL13RA2. Studies have used flow cytometry to assess membrane-bound IL13RA2 on cells such as human pulmonary artery smooth muscle cells and U937 human IL13RA2 transfectants . Antibodies targeting the extracellular domain of IL13RA2 are required, with careful validation to confirm specificity.

RNA-based Methods: RT-PCR and RNA sequencing provide crucial information about IL13RA2 transcription. These approaches have been used to confirm siRNA knockdown efficiency and explore relationships between IL13RA2 mRNA expression and protein levels . Mucosal gene expression analysis has also identified IL13RA2 as a predictive marker for anti-TNF therapy non-responsiveness in IBD .

For optimal detection specificity, researchers should consider including IL13RA1 as a negative control, performing IL-13 competition assays (as research shows IL-13 presence does not interfere with IL13RA2 detection ), and employing multiple detection methods in parallel for comprehensive validation.

How does IL13RA2 contribute to cancer progression and metastasis?

IL13RA2 demonstrates multifaceted roles in cancer progression and metastasis, with effects that can vary depending on cancer type and context:

In glioblastoma multiforme (GBM), IL13RA2 is expressed at high frequency and has emerged as a promising target for cell therapy due to its specific expression in GBM cells, including cancer stem cells . For breast cancer brain metastases (BCBM), high IL13RA2 expression predicts worse survival after brain metastasis diagnosis . The survival of women with brain metastases from breast cancer remains very poor, with over 80% dying within a year of diagnosis, with IL13RA2 apparently contributing to this aggressive phenotype .

Mechanistically, IL13RA2 is essential for cancer cell survival, promoting proliferation while simultaneously repressing invasion . This dual role suggests a complex function in cancer progression. At the molecular level, IL13RA2 knockdown results in focal adhesion kinase (FAK) downregulation, repression of cell cycle and proliferation mediators, and upregulation of Ephrin B1 signaling . Ephrin-B1, in turn, promotes invasion of breast cancer cells in vitro . This opposing relationship between proliferation and invasion suggests that IL13RA2 may regulate a critical balance between these processes in cancer cells.

Additional solid tumors with high IL13RA2 expression include melanoma, renal cell carcinoma, and adrenocortical carcinoma, with expression correlating with poor prognosis across these diverse cancer types . This consistent association across multiple tumor types suggests a fundamental role for IL13RA2 in promoting aggressive cancer phenotypes.

What are the molecular mechanisms underlying IL13RA2's role in resistance to anti-TNF therapy in IBD?

IL13RA2 has emerged as one of the best predictive markers for primary non-responsiveness to infliximab (an anti-TNF therapy) in both ulcerative colitis and Crohn's disease patients . The molecular mechanisms underlying this association involve several interconnected pathways affecting epithelial barrier function and repair.

IL13RA2 expression in IBD demonstrates a distinct spatial pattern, being highly expressed in epithelial cells throughout the mucosa in active IBD patients, while restricted to goblet cells in healthy controls . This abnormal expression pattern appears to negatively influence goblet cell recovery, function, and epithelial restoration after injury. Mucosal IL13RA2 mRNA expression negatively correlates with the expression of several goblet cell-specific and barrier genes, as well as with goblet cell numbers . This relationship suggests that IL13RA2 contributes to persistent epithelial barrier dysfunction in IBD, potentially limiting the efficacy of anti-TNF therapy.

Further research has demonstrated that IL13RA2 expression reflects an increased TNF burden in non-responders to anti-TNF therapy . This suggests that high IL13RA2 expression may be a marker of a particularly severe inflammatory phenotype that requires more aggressive intervention than standard anti-TNF dosing regimens can provide.

Experimental models support these mechanisms, as IL13RA2 knockout mice had similar initial DSS colitis severity to wild-type mice but recovered more rapidly, with faster restoration of goblet cell numbers and mucosal architecture . This accelerated recovery suggests that IL13RA2's inhibition of IL-13 signaling may impair tissue repair processes necessary for resolution of inflammation after anti-TNF therapy.

What experimental approaches are being used to target IL13RA2 in cancer therapy?

Several innovative experimental approaches have been developed to target IL13RA2 in cancer therapy, leveraging its high expression in tumors and limited expression in normal tissues:

CAR-T Cell Therapy:
CAR-T cells targeting IL13RA2 have shown promise for glioblastoma multiforme (GBM). Early approaches using IL-13 Zetakine to target IL13RA2 had limited specificity due to cross-reactivity with IL13RA1 . Recent advances have focused on developing more specific binders through novel approaches:

• Nanobody-based CARs created through immunization, phage display, and deep repertoire mining, followed by humanization and functional screening. This approach specifically used IL13RA1 as a deselection target to ensure high specificity for IL13RA2 .

• Multi-functional CAR-T constructs incorporating multiple elements within a single cassette to address challenges such as limited CAR-T cell persistence and the immunosuppressive nature of the tumor microenvironment .

The development of these targeting approaches involves rigorous specificity testing, including screening against approximately 6,000 native human membrane proteins to ensure selective binding to IL13RA2 . Humanization processes reduce immunogenicity while maintaining binding affinity, and functional screening selects constructs with optimal anti-tumor activity before advancing to in vivo testing in relevant xenograft models.

How do IL13RA2 expression levels correlate with clinical outcomes in different cancers?

IL13RA2 expression levels demonstrate consistent correlations with clinical outcomes across multiple cancer types, generally indicating poorer prognosis in high-expressing tumors:

In breast cancer brain metastases (BCBM), high IL13RA2 expression predicts worse survival after brain metastasis diagnosis . This finding is particularly significant given that the survival of women with brain metastases from breast cancer remains very poor, with over 80% dying within a year of diagnosis . The prognostic value of IL13RA2 in this context suggests it plays a functional role in the aggressive nature of brain metastases.

For glioblastoma multiforme (GBM), IL13RA2 is expressed at high frequency and its presence in cancer stem cells makes it a particularly valuable target, as these cells are often responsible for treatment resistance and recurrence . The specific expression pattern of IL13RA2 in GBM compared to normal brain tissue has made it an attractive target for therapeutic interventions, with ongoing development of targeted approaches including CAR-T cell therapy.

IL13RA2 is also expressed at high frequency in melanoma, renal cell carcinoma, and adrenocortical carcinoma, with its expression correlating with poor prognosis across these diverse tumor types . This consistent association across multiple malignancies suggests a fundamental role for IL13RA2 in promoting aggressive cancer phenotypes regardless of the tissue of origin.

Methodologically, assessing IL13RA2 as a prognostic marker requires standardized immunohistochemistry protocols, correlation with well-defined clinical endpoints, multivariate analyses to assess independence from other prognostic factors, and integration with molecular subtyping to identify patient populations where IL13RA2 has the strongest prognostic significance.

What are the challenges in developing IL13RA2-targeted therapies?

Developing effective IL13RA2-targeted therapies presents several significant challenges that researchers must address through innovative approaches:

Specificity and Cross-Reactivity:
A major challenge is achieving high specificity for IL13RA2 while avoiding cross-reactivity with IL13RA1, which is ubiquitously expressed in normal tissues. Early approaches using IL-13 Zetakine to target IL13RA2 in CAR-T cell therapy had limited specificity due to binding to IL13RA1 . Recent strategies have made significant progress in overcoming this challenge through deselection protocols in phage display to eliminate cross-reactive binders, development of mutated forms with improved selectivity, and creation of humanized nanobodies with enhanced specificity for IL13RA2 .

Heterogeneous Expression:
While IL13RA2 is overexpressed in several tumor types, expression levels can vary significantly between patients and within different regions of the same tumor. This heterogeneity necessitates development of companion diagnostics to identify patients with sufficient IL13RA2 expression, strategies to address regions of the tumor with lower expression levels, and combination approaches that target multiple pathways.

Tumor Microenvironment Barriers:
Particularly in brain tumors and brain metastases, the blood-brain barrier and immunosuppressive tumor microenvironment present additional hurdles. For CAR-T cell therapy, challenges include limited CAR-T cell persistence and the immunosuppressive tumor environment . Addressing these requires enhanced CAR designs with multiple functional domains, strategies to improve cellular persistence and infiltration, and approaches to counteract immunosuppressive signals.

Mechanism of Action Complexities:
The dual role of IL13RA2 in promoting proliferation while repressing invasion suggests that simple blockade or targeting might have complex, potentially contradictory effects. Understanding these nuances requires detailed mechanistic studies of downstream signaling pathways, assessment of effects on both proliferation and invasion in preclinical models, and careful monitoring for unexpected effects in clinical studies.

The complexity of these challenges emphasizes the importance of integrating multiple experimental approaches and perspectives in developing IL13RA2-targeted therapies.

How can researchers distinguish between IL13RA1 and IL13RA2 binding in experimental settings?

Distinguishing between IL13RA1 and IL13RA2 binding is critical for developing specific targeting agents and understanding pathway-specific effects. Several methodological approaches can effectively differentiate between these receptors:

Selective Antibodies and Binding Proteins:
Researchers have successfully developed antibodies that specifically bind to IL13RA2 without cross-reactivity to IL13RA1. For example, HuCl47, a humanized IgG1 antibody used in the ADCT-211 antibody-drug conjugate, demonstrated specific binding to recombinant IL13RA2 without binding to IL13RA1 . Similar specificity was achieved with nanobodies developed through phage display with IL13RA1 deselection . Validation methods include binding assays against recombinant proteins, cell-based assays using cells expressing only one receptor type, and competition assays with known specific ligands.

Comprehensive Specificity Testing:
Advanced screening approaches can test binding against thousands of native human membrane proteins. In one study evaluating nanobodies against IL13RA2, VHH clones were tested against approximately 6,000 native human membrane proteins to confirm their selectivity . This unbiased approach provides robust evidence for specificity beyond what can be achieved with more targeted testing.

Functional Readouts:
IL13RA1 and IL13RA2 activate different signaling pathways, which can be exploited for discrimination. IL13RA1 forms a complex with IL-4Rα and activates the JAK/STAT6 pathway, while IL13RA2 signaling occurs via STAT6-independent pathways . By measuring pathway-specific outputs (e.g., STAT6 phosphorylation versus alternative pathway activation), researchers can distinguish receptor-specific effects.

Genetic Approaches:
Selective knockdown or knockout of each receptor using siRNA, shRNA, or CRISPR-Cas9 allows for assessment of binding and signaling in the absence of one receptor. This approach has been successfully employed in studies of IL13RA2 function and provides clean systems for binding studies when combined with receptor-specific expression models.

What are the current methods for studying IL13RA2 signaling pathways?

Understanding IL13RA2 signaling pathways requires sophisticated methodological approaches due to its non-conventional structure and context-dependent functions:

RNA Interference and Gene Editing:
siRNA and shRNA approaches have been widely used to study IL13RA2 function. Examples include siRNA-mediated depletion of IL13RA2 in U87 glioma cells and HaCaT keratinocytes to demonstrate effects on both membrane-bound and soluble IL13RA2 levels , and inducible shRNAs in breast cancer models to allow temporal control of IL13RA2 expression for in vitro and in vivo studies . CRISPR-Cas9 gene editing provides another approach for complete receptor knockout, offering clean genetic models for signaling studies.

Transcriptomic and Proteomic Analyses:
RNA sequencing following IL13RA2 knockdown has identified changes in focal adhesion kinase (FAK) signaling, cell cycle regulators, and Ephrin B1 pathway components . Similar approaches combined with protein phosphorylation arrays can identify changes in signaling molecule activation states, while pathway enrichment analyses help identify the biological processes most affected by IL13RA2 modulation.

Cell-Based Functional Assays:
Functional readouts connect IL13RA2 to specific cellular processes. Proliferation assays have demonstrated that IL13RA2 is essential for cancer cell survival and promotes proliferation , while invasion assays have shown that IL13RA2 represses invasion, with its knockdown leading to increased invasiveness through Ephrin B1 upregulation . Cell cycle analysis can further reveal how IL13RA2 affects specific phases of the cell cycle.

In vivo Models:
Animal models provide insights into physiological signaling roles. Studies have shown that IL13RA2 knockout mice recover more rapidly from DSS-induced colitis than wild-type animals, with faster restoration of goblet cell numbers and mucosal architecture . Xenograft models with inducible IL13RA2 knockdown help assess its role in tumor growth and metastasis .

Pathway Inhibitor Studies:
Pharmacological inhibitors targeting specific downstream pathways help validate signaling connections. MMP inhibitors have revealed the role of MMPs/MMP-8 in generating soluble IL13RA2 from membrane-bound IL13RA2 in humans , while inhibitors of FAK and Ephrin signaling can confirm the relevance of these pathways downstream of IL13RA2.