Isolation and Characterization of “Heavy Chain Antibody” Variable Domain Gene of Indian Desert Camel (Camelus dromedarius, L.)

Abstract

Camelids have “heavy chain antibodies” (HCAbs) of IgG isotype (IgG2 and IgG3 subtypes) and lack light chains and heavy chain constant domain 1 (CH1) in their structure. The molecular biological features of VHH (i.e., antigen binding variable domain of HCAbs) gene isolated from Indian desert camel were examined as a prelude to construct VHHs-phage display library. VHH cDNA was synthesized by reverse transcription-polymerase chain reaction (RT-PCR) from B lymphocyte mRNA, cloned and sequenced (GenBank accession number: EU429319). Like all camelid VHHs sequences, Indian desert camel IgG VHH (Idc-IGVHH) clone 2 also belonged to human VH subgroup III. It was found phylogenetically closer to Arabian camel VHH than to that of llama sequences within family Camelidae. Four out of five camelid VHH signature amino acid positions were present in Idc-IGVHH. CDR1 had structure similar to that of canonical class 1, whereas CDR2 similar to canonical class 4. Idc-IGVHH CDR3 was relatively long (20 residues), like most other VHHs and contained Cys110 in it. Protein 3D model constructed using an anti-lysozyme Arabian camel VHH domain template revealed typical VHH domain structure containing β-strands and loops in Idc-IGVHH. The above features were strongly suggestive of VHH clone having potential of a functional single-domain antibody.

Keywords: heavy chain antibodies; VHH sequence; structural features; Indian desert camel