The disclosure relates to a protein composed of a first polypeptide or polypeptide domain having a first specific binding activity for Cytotoxic T-lymphocyte Antigen 4 (CTLA-4) expressed on a T-cell cell surface and a second specific binding activity for Glucose Transporter 2 (GLUT2) or an extracellular ectodomain thereof expressed on a pancreatic β-cell surface, wherein binding of the first polypeptide or polypeptide domain to CTLA-4 induces a CTLA-4 specific agonist response in the T-cell, and binding of the second polypeptide or polypeptide domain to GLUT2 or an ectodomain thereof does not inhibit GLUT2 glucose transporter function, wherein said agonist response in the T-cell induces a response that reduces immunoreactivity against pancreatic β-cells.