Recombinant Human Claudin-18 (CLDN18)

What is Claudin-18 and what are its main structural features?

Claudin-18 (CLDN18) is a transmembrane protein belonging to the claudin family, which comprises essential components of tight junctions in epithelial and endothelial tissues. The human CLDN18 protein consists of 261 amino acids with a molecular weight of approximately 27-30 kDa . Structurally, CLDN18 contains four transmembrane domains with two extracellular loops and intracellular N- and C-termini. The first extracellular loop contains characteristic claudin family motifs that participate in homophilic and heterophilic interactions with other claudins, forming the backbone of tight junction strands. The second extracellular loop provides additional stability and specificity to these interactions.

The protein contains multiple functional domains, including a PDZ-binding motif in its C-terminal tail that facilitates interactions with cytoplasmic scaffold proteins such as ZO-1, ZO-2, and ZO-3. This organization enables CLDN18 to serve as a crucial link between the tight junction and the actin cytoskeleton, contributing to cellular polarity and barrier function.

What are the different isoforms of CLDN18 and how do they differ functionally?

CLDN18 exists in at least two major splice variants or isoforms, designated as CLDN18.1 (isoform A1) and CLDN18.2 (isoform A2), which differ in their tissue distribution and function:

CLDN18.1 is primarily expressed in lung tissue and is involved in the maintenance of the alveolar microenvironment through regulation of pH and subsequent T-cell activation in the alveolar space, indirectly limiting C. neoformans infection . In contrast, CLDN18.2 is predominantly expressed in normal gastric mucosa and is required for the formation of the gastric paracellular barrier through its role in tight junction formation . Importantly, CLDN18.2 has gained significant attention as a therapeutic target due to its expression in various cancers, particularly gastric and gastroesophageal junction (GEJ) adenocarcinomas.

How is recombinant CLDN18 produced for research applications?

Recombinant CLDN18 can be produced using multiple expression systems, each offering distinct advantages for different research applications:

For E. coli-based expression, recombinant human CLDN18.2 typically includes specific domains (e.g., Asp28-Leu76) with an N-terminal 8His tag . This approach is suitable for applications requiring partial protein domains, particularly extracellular regions for interaction studies.

Mammalian cell expression systems provide full-length CLDN18 with appropriate post-translational modifications, ensuring proper protein folding and biological activity. These preparations are ideal for functional assays, as they can bind anti-CLDN18.2 recombinant antibodies with high specificity. For example, mouse CLDN18 produced in mammalian cells demonstrates binding activity in functional ELISAs with EC50 values of 6.115-11.01 ng/mL .

Wheat germ expression systems offer another alternative for producing full-length human CLDN18 (1-261 amino acids), suitable for applications such as ELISA and Western blotting .

What methods are effective for detecting CLDN18.2 expression in tissue samples?

Detection of CLDN18.2 expression in tissue samples is crucial for both research and clinical applications. Several methodological approaches can be employed:

Immunohistochemistry (IHC): This is the most widely used method for CLDN18.2 detection in fixed tissue samples. Key considerations include:

• Antibody selection: Use of validated antibodies specific to the CLDN18.2 isoform is essential to prevent cross-reactivity with CLDN18.1

• Scoring system: A standardized scoring system based on membrane staining intensity and percentage of positive tumor cells should be employed

• Controls: Include both positive controls (normal gastric mucosa) and negative controls

RNA-based methods:

• RT-PCR: Can distinguish between CLDN18.1 and CLDN18.2 transcripts based on isoform-specific primers

• RNA-Seq: Provides quantitative expression data and can identify novel splice variants

• In situ hybridization: Allows visualization of transcript expression within tissue architecture

What functional assays are recommended for studying CLDN18 activity?

Several functional assays can be employed to study CLDN18 activity in research settings:

Barrier Function Assays:

• Transepithelial/endothelial electrical resistance (TEER) measurements to assess barrier integrity

• Paracellular permeability assays using labeled tracers (e.g., FITC-dextran)

• Calcium switch assays to study dynamic tight junction assembly and disassembly

Binding and Interaction Assays:

• ELISA-based binding assays: Recombinant mouse CLDN18 immobilized at 5 μg/mL can bind anti-CLDN18.2 recombinant antibody with EC50 values of 6.115-11.01 ng/mL

• Co-immunoprecipitation studies to identify protein-protein interactions

• Proximity ligation assays to visualize protein interactions in situ

Functional Cellular Assays:

• Cell migration and invasion assays to assess CLDN18's role in cancer progression

• Cell proliferation assays following CLDN18 manipulation

• Apoptosis assays to evaluate the effects of CLDN18-targeting agents

How does CLDN18.2 function as a therapeutic target in gastric cancer?

CLDN18.2 has emerged as a significant therapeutic target in gastric cancer based on several biological properties:

Expression Pattern and Accessibility:

• CLDN18.2 is normally restricted to gastric mucosa but becomes exposed in gastric tumors due to loss of cell polarity

• The extracellular loops of CLDN18.2 are accessible to antibody binding in the tumor microenvironment

• CLDN18.2 expression is maintained in metastatic lesions, making it a consistent target

Therapeutic Approaches:

• Monoclonal antibodies: Zolbetuximab, a chimeric IgG1 CLDN18.2 monoclonal antibody, binds specifically to CLDN18.2 on tumor cell surfaces, eliciting antibody-dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), apoptosis, and suppressing cell proliferation

• Antibody-drug conjugates: Coupling cytotoxic payloads to CLDN18.2-targeting antibodies

• CAR-T cell therapy: Engineering T cells to recognize CLDN18.2

Clinical Evidence:

• Zolbetuximab received FDA approval in October 2024 for use in combination with chemotherapy in CLDN18.2-positive metastatic gastric and GEJ cancers

• Treatment decisions currently vary among oncologists: When asked about approaches for HER2-negative, CLDN18.2-positive patients with a combined positive score of 10, 58.1% preferred first-line chemotherapy plus immunotherapy, while 40.4% favored chemotherapy plus zolbetuximab

• More specifically, 52.8% selected mFOLFOX6 plus immunotherapy, 5.3% opted for CAPOX plus immunotherapy, 33.9% chose mFOLFOX6 plus zolbetuximab, and 6.5% recommended CAPOX plus zolbetuximab

What are the challenges in producing and purifying recombinant CLDN18?

Producing functional recombinant CLDN18 presents several technical challenges:

Expression System Selection:

• Transmembrane proteins like claudins are notoriously difficult to express and purify in functional forms

• E. coli systems often yield inclusion bodies requiring refolding, while mammalian systems are more costly but provide proper folding and post-translational modifications

• Wheat germ cell-free systems offer an alternative for producing full-length protein without membrane insertion challenges

Purification Considerations:

• Affinity chromatography methods using tags (N-6His, N-8His) are commonly employed

• Endotoxin removal is critical for downstream applications, with levels typically maintained below 1.0 EU/μg as determined by the LAL method

• Detergent selection is crucial for maintaining protein solubility and native conformation

Stability and Storage:

• Recombinant CLDN18 is typically provided as a lyophilized powder

• Reconstitution requires careful handling, with protocols recommending brief centrifugation prior to opening

• Proteins should be reconstituted in deionized sterile water to a concentration of 0.1-1.0 mg/mL

• Addition of 5-50% glycerol (final concentration) is recommended for long-term storage

• Storage at -20°C/-80°C with avoidance of repeated freeze-thaw cycles is advised

How can researchers optimize CLDN18.2 antibody development and validation?

Development and validation of CLDN18.2-specific antibodies require careful consideration of several factors:

Epitope Selection:

• Target extracellular domains that are uniquely exposed in CLDN18.2

• Consider isoform-specific regions to minimize cross-reactivity with CLDN18.1

• Humanized IgG1 recombinant antibodies specifically targeting CLDN18.2 have demonstrated effectiveness

Validation Approaches:

• Flow cytometry (FC): Validated at dilutions of 1:50-1:200

• ELISA: Confirm binding specificity and affinity

• Immunohistochemistry: Evaluate membrane localization and specificity

• Functional assays: Assess ability to induce ADCC, CDC, and apoptosis

Quality Control Metrics:

• Purity assessment through affinity chromatography

• Testing for reactivity with human CLDN18.2 protein

• Evaluation of potential cross-reactivity with other claudin family members

• Confirmation of functional activity in tumor cell killing assays

What are the current knowledge gaps and emerging research directions for CLDN18?

Despite significant advances, several knowledge gaps and emerging research directions remain:

Structural Biology:

• Detailed structural characterization of CLDN18.2 in tight junction complexes

• Molecular mechanisms underlying CLDN18.2's role in maintaining epithelial barriers

• Structural changes in CLDN18.2 during cancer progression

Signaling Pathways:

• CLDN18's role in YAP1 localization and regulation of epithelial progenitor cell proliferation

• Involvement in β2-adrenergic signaling and actin cytoskeleton organization

• Potential role in RANKL-induced osteoclast differentiation through relocation of TJP2/ZO-2

Clinical Applications:

• Predictive biomarkers for response to CLDN18.2-targeted therapies

• Mechanisms of resistance to CLDN18.2-targeted treatments

• Combination therapy approaches to enhance efficacy

What is the current awareness and implementation status of CLDN18.2 testing in oncology?

Despite the approval of zolbetuximab, significant knowledge gaps exist regarding CLDN18.2 testing in clinical practice:

Awareness Metrics:

• According to the REFLECT study presented at the 2025 American Society of Clinical Oncology Gastrointestinal Cancers Symposium, 47% of community oncologists were unaware of CLDN18.2 as a therapeutic target in gastric and GEJ cancers

• 79% of oncologists had not seen phase 3 data on zolbetuximab, despite its potential impact on treatment decisions

Testing Accessibility:

• Only 40% of oncologists reported having access to CLDN18.2 testing

• 52% of those practicing in the central United States were uncertain about testing availability

• This gap in testing access could hinder the timely integration of zolbetuximab into treatment regimens

Education and Implementation:

• Education on patient identification and correct utilization of CLDN18.2 testing may accelerate adoption of new regimens

• Academic centers that participated in claudin trials tend to have greater familiarity with testing procedures

• Testing is considered technically easier than some other biomarkers: "The truth about claudin is that it's a much easier test to do and to read compared to PD-L1, for example. There's a lot less heterogeneity: if it's positive, it's positive"

How does CLDN18.2 testing fit into the broader landscape of biomarker-driven gastric cancer therapy?

CLDN18.2 testing represents an important addition to the evolving landscape of biomarker-driven gastric cancer therapy:

Evolution of Biomarker Testing:

• Gastric cancer treatment has changed significantly over the past five years with biomarker-driven therapies reshaping the landscape

• Initially, HER2 was the only routinely tested biomarker, followed by PD-L1 and mismatch repair deficiency

• CLDN18.2 represents the newest addition to this testing panel

Comparison with Other Biomarkers:

• PD-L1 testing has been particularly challenging, with recent FDA Oncologic Drugs Advisory Committee meetings debating the benefits in certain patients

• CLDN18.2 testing appears to offer more clear-cut results compared to PD-L1 testing

• The integration of multiple biomarkers (HER2, PD-L1, MMR, CLDN18.2) creates a more complex but potentially more personalized treatment approach

Implementation Challenges:

• The lag between positive trial results for zolbetuximab and the need for CLDN18.2 testing contributed to low familiarity with the biomarker

• Without claudin testing, patients may miss out on more effective therapy and receive only chemotherapy

• Testing and uptake are expected to increase as educational efforts continue