Immunoglobulins are widely used research tools, applicable to a huge diversity of laboratory techniques. Yet while we are all familiar with the five primary isotypes found in serum (IgG, IgA, IgM, IgE and IgD), the discovery that camelids (camels, llamas and alpacas) produce functional antibodies devoid of light chains has opened the door to many new applications and opportunities.
Camelid immunoglobulins have a molecular weight of approximately 100kDa, which is considerably lower than that of a typical IgG immunoglobulin (150kDa). It has been hypothesised that this allows camelid immunoglobulins to target epitopes which are inaccessible to IgGs, such as the catalytic clefts of enzymes, affording researchers a new and powerful tool with which to investigate proteins that were previously extremely challenging to study. This has, for example, included research designed to elucidate how neurotransmitters such as adrenaline can bind to receptors in the brain.
Each heavy chain of a camelid immunoglobulin is composed of two constant domains and a single variable domain. The latter is commonly referred to as the VHH domain or nanobody and, despite the lack of light chains, contains a complete antigen binding site, making it the smallest naturally-derived functional antigen binding fragment currently known, with a molecular weight of just 15kDa.
The small size of nanobodies means that they offer a wealth of advantages, including exceptional stability in conditions of extreme temperature or pH. This makes them attractive for diagnostic kits such as ELISA or lateral flow assays that do not require refrigeration and provides the opportunity for additional routes of administration such as oral immunotherapy. Nanobodies can also penetrate tissues more easily than their larger counterparts and are suitable for target binding within living cells. The possibility of more rapid accumulation in tissue during imaging studies or therapeutic applications, and potentially lower toxicity due to more rapid clearance are further advantages. Nanobodies can also be coupled to various detection moieties or solid surfaces, allowing their incorporation into existing protocols in place of traditional IgGs.
While it is possible to produce nanobodies via the typical and well-known route of animal immunisation followed by subsequent purification and characterisation, these valuable reagents can be readily expressed in bacteria. This ensures a consistent and reliable supply that is virtually limitless. A recent Nature publication has also reported a fully in vitro platform for nanobody discovery based on yeast surface display. Bacterial and yeast-based approaches represent ethical and cost-effective alternatives to animal-based methodologies.
The range of companies supplying nanobodies for research use is growing steadily, and although commercially available IgG immunoglobulins currently out-number nanobodies significantly it seems highly likely that nanobodies will experience greatly increased usage in the future. Nanobodies have already been documented within clinical trials and pre-clinical testing for indications including oncology, inflammation, pulmonary disease and neurology, and many further programmes are in developmental phases. The research applications and therapeutic potential of nanobodies are extremely promising and will become more so as increasingly efficient methods of producing them continue to evolve.