The invention provides isolated Pre-Ligand Assembly Domain (PLAD) polypeptides comprising an amino acid sequence of a domain (e.g., a Fibronectin Ill-like domain) of an IL-17 Receptor (IL-17R) family member, wherein the PLAD polypeptide inhibits multimerization of a receptor complex comprising an IL-17R family member. Also provided are isolated PLAD-binding polypeptides, e.g., antibodies and avimers, which specifically bind to a PLAD polypeptide described herein. Related chimeric proteins, conjugates, nucleic acids, vectors, and host cells are provided herein. Further provided are methods of treating an inflammatory or autoimmune disease, methods of inhibiting IL-17-mediated signal transduction, methods of inhibiting IL-17 ligand binding, methods of inhibiting multimerization of IL-17R complexes, and methods of inhibiting the production of at least one cytokine, chemokine, matrix metalloproteinase, or other molecule associated with IL-17 signal transduction are provided.