Camelid antibodies (or VHHs) represent single-domain heavy chain-only antibodies that are devoid of light chains. This article describes the advantages and disadvantages of camelid antibodies.
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Despite the absence of a light chain, VHHs are capable of producing functional antibodies. More specifically, the single N-terminal domain of camelid antibodies is fully capable of antigen binding, which is why these antibodies are expressed well in microorganisms such as bacteria and fungi.
Advantages of camelid antibodies
VHHs are relatively small in size (approximately 15 kilodaltons), easy to handle, have high affinity, high specificity, high thermostability, and good solubility. They also exhibit strict monomeric behavior.
The aforementioned properties make it easy for researchers to experiment with VHHs by genetic engineering. In addition, the cost of producing camelid VHHs are much lower when compared to conventional antibody production.
Camelid VHHs also offer the advantage of low immunogenicity that many studies attribute to the loss of the variable light chain (VL) domain. The absence of this VL domain provides VHHs paratopes with a higher structural complexity due to the increase in the number of residues involved in antigen binding.
Camelid antibodies exhibit a higher penetration rate into tissues and rapid clearance through the blood-brain-barrier and kidneys. The biological potency of VHHs can be increased by using bispecific or multivalent constructs without producing the occasional side effects that are observed when conventional (or whole) antibodies are used.
Limitations of camelid antibodies
The production of VHHs makes use of biohazardous materials such as bacteriophages, plasmids, antibiotics, recombinant DNA, and so on. Accordingly, these materials should be carefully handled, and stringent mechanisms for waste disposal should be in place (such as decontamination using disinfectants, the addition of bleach to liquid waste, autoclaving of solid waste, as well as handling of materials by trained personnel with adequate protective gears).
Although rapid clearance of VHHs makes them ideal for therapeutic activity with minimal side effects, its short serum half-life reduces its efficacy in parenteral preparations. To overcome the short serum half-life of VHHs, bispecific VHHs can be used. An example of this is the addition of polyethylene glycol to the foot-and-mouth disease virus-neutralizing VHHs increased the serum half-life, as well as the in vitro neutralizing potency of these antibodies.
Challenges are also faced with respect to the housing of the dromedary animals such as camels, llamas, and other camelid species. At the end of an immunization round, these animals are not sacrificed and need to be provided with permanent housing.
However, as an alternative for this, commercial services are available now wherein camelid immunization and care takes place at a site away from the laboratories. The resultant complementary DNAs (cDNAs) and phage libraries are then ordered for further investigation in laboratories.
Studies have demonstrated that the extent of manipulation and engineering that can be tolerated by camelid antibodies may be restricted. This is primarily due to the availability of only a single domain of approximately 110 amino acids that consequently put a lot of weight on every residue in the VHH domain. Evidence also supports the theory that the position of each amino acid on VHHs can have a direct or indirect effect on the structural integrity and stability of the molecule.
Summary and perspectives
Although VHHs comprise only of the binding region, they can be easily manipulated to add scaffolds, tags, and labels for detection and purification. Crystallography and multidimensional nuclear magnetic resonance (NMR) imaging are some powerful tools that are being used to further elucidate the structure of VHHs.
These techniques could prove to be very useful in pinpointing exact mechanisms that help in binding VHH to their target and amino acids involved in binding. This could, in turn, provide the requisite information for further mutagenesis experiments on VHH antibodies.