Camel heavy‐chain antibodies: diverse germline V H H and specific mechanisms enlarge the antigen‐binding repertoire

The antigen‐binding site of the camel heavy‐chain antibodies devoid of light chain consists of a single variable domain (V H H) that obviously lacks the V H –V L combinatorial diversity. To evaluate the extent of the V H H antigen‐binding repertoire, a germline database was constructed from PCR‐amplified V H H/V H segments of a single specimen of Camelus dromedarius . A total of 33 V H H and 39 VH unique sequences were identified, encoded by 42 and 50 different genes, respectively. Sequence comparison indicates that the V H H s evolved within the V H subgroup III. Nevertheless, the V H H germline segments are highly diverse, leading to a broad structural repertoire of the antigen‐binding loops. Seven V H H subfamilies were recognized, of which five were confirmed to be expressed in vivo . Comparison of germline and cDNA sequences demonstrates that the rearranged V H H s are extensively diversified by somatic mutation processes, leading to an additional hypervariable region and a high incidence of nucleotide insertions or deletions. These diversification processes are driven by hypermutation and recombination hotspots embedded in the V H H germline genes at the regions affecting the structure of the antigen‐binding loops.