Siglec-1-targeting single domain antibodies block or enhance pathogen binding to dendritic cells.

Siglec-1 (Sialoadhesin/CD169), expressed on myeloid cells, is a receptor for sialic acids that mediates adhesion to host cells and pathogens. However, certain pathogens exploit Siglec-1 binding for further dissemination and interfering with this binding may serve as potential therapeutic interventions. Here we identified high-affinity single domain antibodies, (also known as VHHs or Nanobodies) that allo- or orthosterically modulate ligand binding of Siglec-1. VHH clone 2C2 bound directly to the ligand binding site of Siglec-1 and effectively blocked binding of a diverse set of ligands, such as ganglioside liposomes, Campylobacter jejuni, and SARS-CoV-2, to monocyte-derived dendritic cells (moDCs) and ex vivo Siglec-1+ DCs. In contrast, VHH 1B5 and 1C1 bound outside of the ligand binding site and enhanced Siglec-1 interactions with ganglioside liposomes and Campylobacter jejuni. Mechanistically, VHH 1B5 and 1C1 appear to interfere with the cis-binding sialic acids present on the Siglec-1-expressing cell surface, thereby facilitating increased trans-interactions with ligands. In conclusion, we have isolated VHH that bind to different sites on Siglec-1 and thereby enhance or block ligand binding to a variety of sialylated pathogens. Moreover, unlike conventional blocking antibodies targeting specific pathogens, Siglec-1 binding VHH may serve as broad-spectrum pathogen blocking agents.