Isolation of PCDH1-specific monoclonal antibodies

The panel of PCDH1-specific mAbs was isolated from a phage-displayed synthetic human antigen-binding fragment (Fab) library (Library F)^{36 (link)}. Binding selections, phage ELISAs and Fab protein purification were performed as described^{37 (link)}. In brief, phage particles displaying the Fabs from Library F were cycled through rounds of binding selections with purified PCDH1–Fc fusion proteins (either full-length extracellular domain (EC1–7; residues 58–851) or EC2–7 (residues 190–851)) immobilized on 96-well Maxisorp Immunoplates (Thermo Fisher) (Extended Data Fig. 4). After four rounds of selection, phages were produced from individual clones grown in a 96-well format and phage ELISAs were performed to detect specific binding clones. Clones with positive binding were subjected to DNA sequencing. The DNAs encoding for variable heavy-and light-chain domains of the positive binders were cloned into vectors designed for the production of Fabs or light chain (human Kappa) or heavy chain (human IgG1). Fabs were expressed from bacterial cells and IgGs from Expi293F cells (Thermo Fisher). Fab and IgG proteins were affinity-purified on protein A affinity columns (GE Healthcare) as described^{37 (link)}.