With the rapid development of single domain antibodies in antibody medicine and cell therapy, many researchers are engaged in the research and development of Single Domain Antibody (sdAb). Camel, alpaca, and llama are the most frequently mentioned animals in relevant studies. Which one should we choose when conducting research? What are their differences in antibody sequence?

Classification of Camelidae

Camelus: camelus bactrianus, camelus dromedaries

Lama: lama glama (llama), lama guanicoe

Vicugna: vicugna vicugna, vicugna pacos (alpaca or lama pacos)

The Content of Single Domain Antibody in Camelidae

The single domain antibody content in camel is the highest, about 70%, while that of alpaca and llama is about 25% to 45%, as clearly shown in the results of PCR amplification of VHH fragments. The result also reflects that sdAbs can be obtained from camels more easily.

Classification of Camelidae Antibodies

There are both conventional antibodies (IgG1 with two heavy chains and two light chains) and single-domain heavy chain antibodies in the camel family, which only have heavy chains and lack CH1 in the constant region.

Fig.1 Schematic representation of the general structure of camel conventional IgG1 and the nonconventional IgG2 and IgG3. (Ciccarese, 2019)

According to the differences in hinge region, the camelidae antibodies can be divided into the following subtypes:

Among them, IgG2 and IgG3 belong to single domain heavy chain antibodies, and the antibody subtypes in the three animals are as follows.

Alpaca: IgG1a, IgG1b, IgG2b, IgG2C
Llama: IgG1a, IgG1b, IgG2b, IgG2C
Camel: IgG1a, IgG1b, IgG2a, IgG2c, IgG3

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Characteristic Amino Acid

In the FR2 of traditional antibodies, the amino acid residues V42, G49, L50 and W52 are involved in the interaction with VL, while the amino acids of these four positions mutate in VHH, as shown in the table below:

Position and Abundance of Intramolecular Disulfide Bond

In addition to the normal disulfide bonds at positions 23 and 104, an extra pair of disulfide bond often forms between CDR1 or FR2 and CDR3, which can maintain well stability of antibody structure. However, mismatch of that disulfide bond often happens in the process flow, resulting in the loss of antibody activity. There are differences in the position and abundance of the extra disulfide bond in camel, alpaca, and llama.

Homology Between sdAb and Human VH

Camel VHH and human VH have a homology of 67.5%-74.3%, while the homology between VHH of alpaca/llama and human VH is as much as 80%-87.3%.

Classification of sdAb

VHH can be subdivided into 4 subfamilies (from VHH1 to VHH4). In general, the conformation often presented by the hypervariable region main chain is called canonical structure. The main chain specification structure of CDR1 and CDR2 in VHH is different from that of the traditional one, and different VHH subfamilies have different CDR1 and CDR2 specification structures.

The CDR3 length of VHH1 and VHH2 is shorter than the average CDR3 length of traditional VH, but it still maintains high solubility, stability and antigen binding function. Only the CDR3 of VHH3/VHH4 is longer, and the number of amino acid residues reach 16 to 24, which far exceed the number of 7 to 9 amino acid residues of traditional VH. The formed finger convex ring can be embedded in the grooves or fissures of the antigen to increase the interaction area.

VHH3 exists mainly in alpaca, and there is an extra disulfide bond between FR2 and CDR3. VHH4 exists mainly in camels, with an additional disulfide bond between CDR1 and CDR3, and the 11-position leucine (L) mutates to serine (S).

The amino acid sequences of FR1 and FR3 in VHH subfamilies are homologous to each other.

Reference

1. Ciccarese, S.; et al. The camel adaptive immune receptors repertoire as a singular example of structural and functional genomics. Frontiers in genetics. 2019: 997.

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