Identification of Variable Lymphocyte Receptors That Target the Human Blood–Brain Barrier

Background/Objectives: Receptor-mediated transcytosis utilizing the native transporters at the blood–brain barrier (BBB) is a growing strategy for the delivery of therapeutics to the brain. One of the major challenges in identifying appropriate human transcytosis targets is that there is a species-specific transporter expression profile at the BBB, complicating translation of successful preclinical candidates into humans. In an effort to overcome this obstacle and identify proteins capable of binding human-relevant BBB ligands, we generated and screened a BBB-targeting library against human-induced pluripotent stem cell-derived brain microvascular endothelial-like cells (iPSC-derived BMEC-like cells). As targeting molecules, we used lamprey antibodies, known as variable lymphocyte receptors (VLRs), and generated a VLR library by immunizing lamprey with iPSC-derived BMEC-like cells, and inserting the resultant VLR repertoire into the yeast surface display system. Methods: The yeast displayed VLR library was then panned against human iPSC-derived BMEC-like cells and lead VLRs were validated using human in vitro models and mouse and human ex vivo brain tissue sections. Results: Finally, brain uptake for a set of VLRs was validated in mice. Of the 15 lead VLR candidates, 14 bound to human BBB antigens, and 10 bound to the murine BBB. Pharmacodynamic testing using the neuroactive peptide neurotensin indicated that the lead candidate, VLR2G, could cross the mouse BBB after intravenous injection and deliver sufficient neurotensin payload to generate a pharmacological response and lower systemic body temperature. Conclusions: Together, these results demonstrate the application of a novel screening technique capable of identifying a VLR with human relevance that can cross the BBB and deliver a payload.