Diphtheria Cell Models for Research

Diphtheria is a vaccine-preventable disease caused by toxigenic strains of Corynebacterium diphtheriae. Despite widespread vaccination, outbreaks persist where immunization coverage is low. According to WHO, in 2022 there were 8,622 reported cases globally, a 60% increase from 2021, with a case fatality rate of 5–10% even with treatment. The disease primarily affects children under 5 and unvaccinated adults. Risk factors include incomplete vaccination, crowded living conditions, and travel to endemic regions (e.g., parts of Africa, Asia, and Eastern Europe).

Clinical impact: Respiratory diphtheria causes severe pharyngitis, pseudomembrane formation, and myocarditis or neuropathy due to toxin dissemination. Cutaneous diphtheria is less severe but contributes to transmission. Without antitoxin, mortality can reach 20% in young children. Survivors may suffer long-term cardiac or neurological sequelae.

Vaccination with diphtheria toxoid has reduced incidence dramatically in high-income countries, but waning immunity in adults and vaccine hesitancy fuel outbreaks. The 2022–2023 outbreaks in Europe and Africa highlight the need for continued research into toxin biology, host susceptibility, and novel therapeutics.

Diphtheria is an ideal model for studying bacterial toxin-host interactions, protein synthesis inhibition, and vaccine development. The diphtheria toxin (DT) is one of the most well-characterized A-B toxins, and its receptor, heparin-binding epidermal growth factor-like growth factor (HBEGF), is a key mediator of entry. Research using diphtheria focuses on:

• • Toxin structure-function relationships: DT consists of catalytic (A) and binding/translocation (B) domains; mutations in these domains inform vaccine design and targeted therapies.
• • Host cell susceptibility: Genetic variations in HBEGF and other host factors influence toxin sensitivity, making it a model for gene-environment interactions.
• • Vaccine and therapeutic development: Diphtheria toxoid is a classic vaccine antigen; new antitoxins and small-molecule inhibitors are needed for outbreak response.
• • Cancer research: DT is used in targeted immunotoxins (e.g., denileukin diftitox) to kill cancer cells overexpressing HBEGF or IL-2 receptors, linking diphtheria research to oncology.

Public datasets such as NCBI Gene, UniProt, and ClinVar provide sequence and variant information for DT and host genes, enabling functional genomics studies. The availability of CRISPR-engineered cell models allows precise dissection of toxin entry, trafficking, and cellular responses.

Diphtheria is not a carcinogenic disease, but its toxin exploits fundamental cellular pathways that are also relevant to cancer and other diseases. The major pathways involved in DT pathogenesis include:

1. Receptor-mediated endocytosis and retrograde transport:

• • DT binds HBEGF on the cell surface.
• • The toxin-receptor complex is internalized via clathrin-dependent endocytosis.
• • Retrograde transport through the Golgi and endoplasmic reticulum (ER) delivers the catalytic domain to the cytosol.

2. Protein synthesis inhibition:

• • The catalytic domain ADP-ribosylates elongation factor 2 (EF-2) using NAD+ as a substrate.
• • This modification inactivates EF-2, halting translation and causing cell death.

3. Inflammatory and stress responses:

• • DT triggers NF-κB and MAPK signaling, leading to cytokine release.
• • In sensitive tissues (e.g., myocardium, nerves), this contributes to tissue damage.

4. Immune evasion and vaccine targets:

• • The toxin's B domain is immunodominant; antibodies against it neutralize toxin activity.
• • Mutations in the toxin gene (tox) can alter virulence and immune escape.

Diphtheria is caused by a bacterial toxin, so genetic alterations refer to variations in the pathogen's toxin gene and host susceptibility genes. The table below summarizes key alterations from public databases (NCBI, UniProt, ClinVar).

Note: Frequencies are approximate and based on population studies and database entries. Host genetic variants are rare but can influence susceptibility.

DT hijacks several signaling networks to promote its entry and cytotoxicity. Key nodes include:

• • HBEGF/EGFR signaling:
• • HBEGF is a ligand for EGFR; DT binding competes with EGFR activation.
• • HBEGF shedding by metalloproteases (ADAM17) modulates toxin sensitivity.
• • Endocytic trafficking machinery:
• • Clathrin, dynamin, and Rab GTPases mediate DT uptake.
• • Retrograde transport requires COPI and COPII vesicles, and the ER-resident protein derlin-1.
• • Translational control:
• • EF-2 ADP-ribosylation activates stress kinases (e.g., GCN2, PERK) and inhibits mTOR signaling.
• • This leads to apoptosis via BAX/BAK and caspase activation.
• • Inflammatory pathways:
• • NF-κB and p38 MAPK are activated by DT, inducing IL-6, TNF-α, and other cytokines.
• • In cardiac cells, this contributes to myocarditis.
• • Immune signaling:
• • Antibodies against DT neutralize toxin by blocking receptor binding or translocation.
• • T-cell responses to toxoid vaccine involve MHC class II presentation.

Several cell lines are used to study DT biology, including those sensitive and resistant to the toxin. The table below lists common models.

Organoids: Human airway or cardiac organoids derived from induced pluripotent stem cells (iPSCs) can model DT-induced tissue damage. They offer 3D architecture and multicellular complexity, improving translation to human disease. Organoids from HBEGF-knockout iPSCs can serve as isogenic controls.

Animal models are essential for studying DT pathogenesis and vaccine efficacy. Examples include:

• • Guinea pig: Classic model for diphtheria toxin lethality and antitoxin testing.
• • Mouse: Not naturally sensitive to DT; transgenic mice expressing human HBEGF (HBEGF-Tg) are used.
• • Non-human primates: Used for vaccine immunogenicity and challenge studies.
• • Zebrafish: Larvae are sensitive to DT and allow live imaging of toxin effects.
• • Induced models: Localized DT administration (e.g., intradermal) mimics cutaneous diphtheria.
• • PDX: Not applicable since diphtheria is not a cancer; however, DT-based immunotoxins are tested in cancer PDX models.

These models help evaluate vaccines, antitoxins, and small-molecule inhibitors.

CRISPR-Cas9 gene editing has enabled the creation of isogenic cell lines with precise modifications to study DT-host interactions. Examples include:

• • HBEGF knockout lines: Generated in HeLa or HEK293 cells, these lines are resistant to DT and serve as negative controls for toxin binding.
• • HBEGF knock-in lines: Introduction of human HBEGF into CHO cells confers DT sensitivity.
• • EF-2 mutant knock-in lines: Cells expressing ADP-ribosylation-resistant EF-2 (e.g., EF-2 G717R) survive DT treatment, allowing dissection of downstream effects.
• • Reporter lines: Luciferase or fluorescent reporters under the control of DT-responsive promoters (e.g., NF-κB) enable high-throughput screening.
• • Isogenic pairs: Wild-type and HBEGF-knockout lines from the same parental background control for genetic drift.

Commercially available, sequence-verified gene-edited cell models accelerate research by providing ready-to-use tools for target validation and drug screening. These models are engineered using CRISPR and validated by sequencing and functional assays, ensuring reproducibility.

Gene-edited cell lines enable functional validation of genes involved in DT susceptibility and response. Examples:

• • Genome-wide CRISPR screens: Identify host factors required for DT entry or toxicity. Known hits include HBEGF, EF-2, and trafficking proteins.
• • Knockout validation: HBEGF knockout cells confirm the receptor's necessity for toxin binding.
• • Knock-in of variants: Introducing SNPs in HBEGF (e.g., rs200342067) tests their impact on toxin sensitivity.
• • Epigenetic editing: CRISPR interference (CRISPRi) or activation (CRISPRa) modulates gene expression to study dose-dependent effects.

These approaches link genotype to phenotype and uncover novel therapeutic targets.

Isogenic cell pairs (e.g., HBEGF wild-type vs. knockout) are powerful tools for drug screening:

• • Toxin inhibitors: Screen small molecules that protect wild-type cells but not knockouts, indicating specificity for toxin entry or activity.
• • Antitoxin antibodies: Evaluate neutralizing antibodies in cell-based assays.
• • Resistance modeling: Generate EF-2 mutant lines to study resistance mechanisms and potential cross-resistance to other ADP-ribosylating toxins.
• • Combination therapies: Test synergy between antitoxin and antibiotics (e.g., penicillin) in killing C. diphtheriae.

High-throughput screening with gene-edited reporter lines accelerates hit identification.

CRISPR synthetic lethality screens can identify biomarkers of DT sensitivity or resistance:

• • Synthetic lethal interactions: Genes that are essential only in HBEGF-expressing cells may serve as biomarkers for toxin susceptibility.
• • Resistance biomarkers: EF-2 mutations or loss of HBEGF predict resistance; these can be monitored in clinical isolates.
• • Immune biomarkers: Gene-edited immune cells (e.g., macrophages) help identify cytokine signatures of severe diphtheria.
• • Vaccine response biomarkers: HLA types and T-cell receptor repertoires can be studied using edited cell models.

These biomarkers inform patient stratification and therapeutic decision-making.