Early Interest in Unusual Antibodies Found in Camels, Buffalo, and Sharks
The original discovery of these unusual antibodies was published in Nature in 1993, emerging from a project on how dromedary camels (the one-humped Arabian kind) and water buffalo keep parasitic infections at bay. In a classic example of chance favoring prepared minds, the researchers observed that antibody tests in the camel blood revealed the presence of simpler antibodies composed of only heavy chains. From a functional standpoint, these antibodies that seemed to lack completely an antigen binding site could bind to target antigens as firmly as normal antibodies, despite having only half as many complementarity-determining regions (CDRs). And unlike Fabs, the heavy-chain-only antibodies do not stick to one another.
Despite the lack of light chains, the investigators noted that the odd antibodies “nevertheless have an extensive antigen-binding repertoire, a finding that calls into question the role of light chains in the camel. Camel heavy-chain IgGs lack CH1, which in one IgG class might be structurally replaced by an extended hinge. Heavy-chain IgGs are a feature of all camelids. These findings open new perspectives in the engineering of antibodies.”
Subsequent studies confirmed the existence of similar immunoglobulins in sharks. The bivalent antibodies targeted antigens though a single immunoblobulin variable domain, displaying two CDR loops. In contrast, conventional antibodies have a variable heavy (VH) + variable light (VL) domain format (~26 kDa) and bind antigens through up to six CDRs. The shark and camel immunoglobulins also encode unusually long and structurally complex CDR3s which display a high degree of variability.
Some features of these antibodies that provoke interest in their use as diagnostic reagents and therapeutic drugs include their small size, high solubility, thermal stability, refolding capacity, and good tissue penetration in vivo. In terms of size, for example, conventional antibodies have a molecular mass of about 150 kDa, while smaller recombinant antibodies have a mass range of about 25–50 kDa. Shark, camel, and llama antibodies, on the other hand, weigh in at 12–13 kDa, or about half the mass of smaller recombinant antibodies and less than a tenth of conventional antibodies. At these small sizes, scientists point out, the antibodies can penetrate further into tissues and tumors.
The antigen-binding site of these heavy-chain antibodies (hcAbs) is designated VNAR in shark hcAbs and VHH in camelid hcAbs. These domains can be and have been produced as recombinant proteins, or single-domain antibodies (sdAbs).
Identification of these molecules as potential diagnostics, research reagents, and therapeutics immediately triggered commercial attention, as Haptogen licensed the rights for the antibody technology developed at the University of Aberdeen and was using shark antibodies to create new therapies for delivering drugs to combat key diseases such as cancer. Wyeth acquired the Scottish biotechnology company in 2007, prior to its own acquisition by Pfizer.