Transgenic non-human vertebrate for the in vivo production of dual specificity immunoglobulins or hypermutated heavy chain only immunoglobulins

A mouse cell, the mouse cell genome comprising

• (i) an antibody heavy chain locus comprising (in 5′ to 3′ direction) a rearranged or unrearranged variable region, a first switch, a mu constant region, a second switch and a non-mu constant region;
• wherein the heavy chain locus of each cell is capable of undergoing IgM to a non-mu isotype switching;
• and
• wherein
• (ii) the non-mu constant region comprises (in 5′ to 3′ direction) a nucleotide sequence encoding a predetermined epitope binding moiety comprising a CH3 of an antigen binding Fc fragment (Fcab) , for expressing a multivalent polypeptide chain comprising (in N- to C-terminal direction) an antibody variable domain encoded by said variable region of the heavy chain locus, and an epitope binding moiety comprising a CH3 of an antigen binding Fc fragment (Fcab), wherein the variable domain and the epitope binding moiety specifically bind respective first and second epitopes.

A method of producing in a mouse multivalent polypeptide chains comprising (in N- to C-terminal direction) an antibody variable domain, an and an epitope binding moiety comprising a CH3 of an antigen binding Fc fragment (Fcab), wherein the variable domain and the epitope binding moiety specifically bind respective first and second epitopes, the method comprising:

• (a) immunising the mouse with a first predetermined antigen bearing the first epitope, obtaining isotype switching to expression from a non-mu constant region in at least some of the heavy chain loci of the mouse and expression of variable regions that have undergone somatic hypermutation;
• and
• (b) selecting one or more polypeptide chains from said immunised mouse by selection for variable regions that bind said first epitope, wherein each selected chain comprises a somatically hypermutated variable region that specifically binds the epitope and wherein each selected chain binds the first and second epitopes;
• and
• (c) providing the nucleotide sequence of the epitope binding moiety in the non-mu constant region of heavy chain loci in the genome of the mouse, wherein the polypeptide chains selected in step (b) bind the first and second epitopes.