pIgR Stem Zone-Targeted Nanobodies as Apical-to-Basolateral Carriers for Inhaled Biologic Delivery Across Mucosal Barriers

Background: The mucosal barrier presents a significant challenge for non-invasive delivery of macromolecular therapeutics, often requiring administration with poor bioavailability and increased toxicity risks. The polymeric immunoglobulin receptor (pIgR) contains an extracellular secretory component (SC) for immunoglobulin binding and a membrane-anchored stem domain capable of apical-to-basolateral transcytosis. We hypothesized that targeting the stem domain could enable active drug transport across mucosal barriers. Methods: Using phage display, we identified four high-affinity nanobodies against human and murine pIgR. Two lead candidates (3LTHMP-4 and 3LTHMP-5) demonstrated efficient apical-to-basolateral transport in vitro (Transwell assays) and in vivo (fluorescence imaging). Engineered bispecific antibodies fusing these nanobodies with anti-IL-5 mAb reslizumab were administered via inhalation in a murine asthma model at one-tenth the intraperitoneal reslizumab dose. Resluts: The bispecific antibodies showed significant therapeutic efficacy, while reslizumab alone at equivalent concentrations failed to demonstrate efficacy. Hydrogen–Deuterium Exchange Mass Spectrometry (HDX-MS) revealed that both 3LTHMP-4 and 3LTHMP-5 specifically bind to the pIgR stem domain (residues 578–612), a region distinct from the dimeric IgA binding site. Conclusions: These findings suggest that stem domain-specific binding may facilitate transport across the mucosal barrier while preserving native receptor physiology, offering a potential strategy for effective transmucosal delivery of biologics.