Science Focus (issue 25)

By Helen Wong 王思齊 Figure 1 Structures of (a) a conventional antibody, (b) a camelid heavy chain-only antibody, and (c) a nanobody. (L: light chain, H: heavy chain; gray: variable region, purple: constant region) 15 Alpacas and Nanobodies 羊駝與奈米抗體 When we think of alpacas, the first things that pop up in our minds may be holiday farms or alpaca fleece. However, if you ask a biomedical scientist, they may have thought of nanobody, fragment of a special type of antibody. To appreciate why nanobodies are so special, we must first understand what an antibody looks like (Figure 1). A conventional antibody is composed of two heavy chains and two light chains. These chains are joined by disulfide bonds to form a Y-shaped molecule. The two tips of the Y-shaped antibody are called variable regions, which are responsible for binding to a target antigen. Variable regions of antibodies literally vary greatly and they determine what antigen an antibody will bind to. In addition to conventional antibodies, members of the camelid family, such as alpacas, camels and llamas, also produce a special type of antibody that only consists of two heavy chains [1, 2]. Nanobodies are the variable regions of these special antibodies. It did not take long for scientists to notice the great potential of nanobody after it was first discovered in 1993 [1]. While exhibiting exceptional specificity, stability and solubility, these antigenbinding domains are only one-tenth in size compared to conventional antibodies [3]. All these unique features make nanobodies a promising therapeutic and imaging agent … but wait, how could we generate the nanobodies we want in the first place? You may already have an answer by now. Yes, alpacas (though other camelid family members could also be used)! In a typical screening process [4], scientists would first immunize alpacas with different antigens to induce the production of respective antibodies by their B cells. After extraction of alpaca B cells and their mRNAs, scientists would reverse transcribe the mRNAs to synthesize double-stranded cDNAs. Then, specially designed primers are used to amplify the DNA sequences coding for the variable regions through polymerase chain reaction (PCR). Next, scientists would introduce recombinant plasmids carrying different variable region sequences to phages (a type of virus), causing them to express respective nanobodies on their surface. Phages displaying nanobodies that could bind to target antigens would then be selected, and the DNA sequences coding the nanobodies of interest can be revealed by DNA sequencing. With this piece of information, we can genetically engineer Escherichia coli bacteria to mass-produce the nanobodies we need [5]. Figuring out the distribution of cancerous tissue in a patient’s body is crucial to cancer diagnosis and subsequent treatment. To image a tumor tissue by positron emission tomography (PET), a detectable radioactive tracer (probe) that specifically binds to the target tumor antigen is needed. Undoubtedly, a highly specific nanobody is an ideal probe. To detect