Humanized VHH-hFc Fusion Proteins Targeting the L-HN Fragment of Tetanus Toxin Provided Protection In Vivo

Tetanus toxin, produced by Clostridium tetani, is the second deadliest known toxin. Antibodies capable of neutralizing tetanus toxin (TeNT) are vital for preventing and treating tetanus disease.

Herein, we screened thirty-six single variable domains on a heavy chain (VHHs) binding to the light chain (L) and the translocation domain (HN) (L-HN) fragment of TeNT from a phage-display library. Then, the L-HN-specific clones were identified, humanized, and fused with a human fragment crystallizable region (hFc) to form humanized VHH-hFc fusion proteins.

The humanized VHH-hFc fusion proteins TL-16-h1-hFc, TL-25-h1-hFc, and TL-34-h1-hFc possessed potent efficacy with high binding affinity, specificity, and neutralizing activity. Only 0.3125 μg was required for TL-16-h1-hFc or TL-25-h1-hFc, and 0.625 μg was required for TL-34-h1-hFc to provide full protection against 10 × Lethal Dose 50 (LD₅₀) TeNT. In the prophylactic setting, 125 μg/kg of TL-16-h1-hFc or TL-25-h1-hFc provided full protection even when they were injected 12 days before exposure to 10 × LD₅₀ TeNT, while TL-34-h1-hFc was less effective. In the therapeutic setting, 25 μg/kg of TL-16-h1-hFc or TL-25-h1-hFc could provide complete protection when administered 24 h after exposure to 5 × LD₅₀ TeNT, while TL-34-h1-hFc required 50 μg/kg.

Our results suggest that TL-16-h1-hFc, TL-25-h1-hFc, and TL-34-h1-hFc provide a bright future for the development of anti-TeNT preventive or therapeutic drugs.