the bovine antibody 
 ( blv1h12 ) which has an ultralong heavy chain 
 complementarity determining region 3 ( cdrh3 ) provides a novel scaffold 
 for antibody engineering . by substituting the extended cdrh3 of blv1h12 
 with modified cxcr4 binding peptides that adopt a -hairpin 
 conformation , we generated antibodies specifically targeting the ligand 
 binding pocket of cxcr4 receptor . 
 these engineered antibodies selectively 
 bind to cxcr4 expressing cells with binding affinities in the low 
 nanomolar range . 
 in addition , they inhibit sdf-1-dependent signal 
 transduction and cell migration in a transwell assay . finally , we 
 also demonstrate that a similar strategy can be applied to other cdrs 
 and show that a cdrh2-peptide fusion binds cxcr4 with a kd of 0.9 nm . 
 this work illustrates the versatility of 
 scaffold - based antibody engineering and could greatly expand the antibody 
 functional repertoire in the future .