The disclosure provides OX40L huIgG4 fusion polypeptide subunits comprising a human IgG4 Fc domain, a trimerization domain, and the receptor binding domain of Ox40 ligand, where the fusion polypeptide subunits can self-assemble into hexameric proteins. Also provided are methods of making fusion polypeptide subunits and hexameric proteins, and methods of use, e.g., treatment of cancer.