Fabs-in-tandem immunoglobulin and uses thereof

A binding protein comprising three polypeptide chains, wherein:

• the first polypeptide chain comprises, from amino to carboxyl terminus, either (i) VLA-CL-VHB-CH1-Fc, wherein CL is fused directly to VHB, or (ii) VHB-CH1-VLA-CL-Fc, wherein CH1 is fused directly to VLA, and wherein there are no linkers inserted between the variable domains and constant domains;
• wherein the second polypeptide chain comprises, from amino to carboxyl terminus, VHA-CH1, wherein there is no linker inserted between VHA and CH1;
• and
• wherein the third polypeptide chain comprises, from amino to carboxyl terminus, VLB-CL, wherein there is no linker inserted between VLB and CL;
• wherein the binding protein is capable of binding two or more epitopes or antigens, wherein VL is a light chain variable domain, CL is a light chain constant domain, VH is a heavy chain variable domain, CH1 is a first constant domain of a heavy chain, A is a first epitope or antigen, and B is a second epitope or antigen, and wherein A and B are different epitopes of the same antigen or are different antigens;
• and
• wherein, on expression in a mammalian host cell, two of said first polypeptide chains, two of said second polypeptide chains, and two of said third polypeptide chains associate to provide a monomeric, tetravalent, and bispecific binding protein comprising six polypeptide chains, having four functional Fab binding regions, and wherein said binding protein binds both epitope or antigen A and epitope or antigen B.