Protein complexes for antigen binding and methods of use

A polypeptide complex comprising:

• a first polypeptide comprising from N-terminus to C-terminus:
• a first antigen-binding domain, a first protein monomer, a first thiol residue-containing peptide linker and a second antigen-binding domain, and
• a second polypeptide comprising from N-terminus to C-terminus:
• a third antigen-binding domain, a second protein monomer, a second thiol residue-containing peptide linker and a fourth antigen-binding domain,
• wherein the first protein monomer forms a dimer with the second protein monomer, and each of the first and the second protein monomers comprises a CH3 domain from an immunoglobulin;
• wherein the first peptide linker and the second peptide linker form a disulfide bond, and each of the first thiol peptide linker and the second peptide linker independently comprises a sequence selected from the group consisting of:
• SEQ ID NO:
• 68, SEQ ID NO:
• 69, and SEQ ID NO:
• 70;
• wherein the first antigen-binding domain and the third antigen-binding domain are capable of binding to a first antigen, and the second antigen-binding domain and the fourth antigen-binding domain are capable of binding to a second antigen, and one of the first and the second antigens is human CD3 and the other is human EpCAM or human CD19, and
• wherein each of the first, second, third, and fourth antigen-binding domains is independently selected from the group consisting of VH, VL, Fab, and scFv.