Identification and Fc-Formatting of Alpaca Single-Domain Antibodies Targeting the Nipah Virus Attachment Glycoprotein

Nipah virus (NiV) is a WHO priority pathogen with case-fatality rates of 40–75%, and no antiviral therapeutic is licensed for human use. The attachment glycoprotein (G) mediates receptor engagement and is a validated target for neutralizing antibodies. Here, we aimed to isolate alpaca-derived single-domain antibodies (VHHs) against the NiV G head domain. Two alpacas were immunized with a baculovirus-expressed, truncated NiV G ectodomain (residues 170–575); peripheral blood mononuclear cells from the higher-responding animal were used to build an immune phage display library of 3.2 × 108 transformants. Four rounds of biopanning enriched antigen-specific phages approximately 120-fold, and sequencing of 183 reactive clones yielded seven unique VHHs. All seven bound to recombinant NiV G, observed by ELISA, and inhibited entry of a VSV-based NiV pseudovirus. VHH-0G8 and VHH-1B6 were most active, with fitted IC50 values of 1.34 and 1.76 μg/mL (≈89 and 118 nM). Reformatting the two leads as human IgG1 Fc-YTE fusions lowered the fitted IC50 to 0.97 and 1.09 μg/mL (≈12 and 13 nM); because a wild-type Fc comparator was not included, this reflects the Fc-YTE format (M252Y/S254T/T256E, which enhances binding to FcRn to extend serum half-life) as a whole. A tandem 0G8/1B6 construct did not show a statistically significant improvement in neutralization compared to the Fc fusions (IC50 = 1.08 μg/mL, ≈36 nM). Computational docking placed the two VHHs on non-overlapping surfaces of the G head, raising the hypothesis that a tandem format could enhance neutralization; however, this synergy was not experimentally observed and requires further format optimization. These VHHs provide starting points for NiV antibody development and require authentic virus and in vivo evaluation.