Overview of Single Domain Antibody (sdAb) & Derivatives Expression

Bacteria is among the most commonly used expression system for expressing antibody fragments. Due to the lack of the Fc domain with their N-linked oligosaccharide, sdAbs are preferably produced in bacterial systems, such as Escherichia coli. There are two basic strategies for obtaining sdAbs from E. coli, the periplasm expression and cytoplasm expression. E. coli system is advantageous because it is fast and convenient, structure-simple, easy of genetic manipulations, and shows relatively low capital costs. E. coli is a gold standard of the bacterial expression system and the essential expression platform for sdAbs in laboratory environments.

The yeast system in especial Pichia pastoris and Saccharomyces cerevisiae combines some of the advantages of eukaryotic expression systems, e.g., efficient folding of correctly antibody fragments with secretion machinery, the high speed and cost-efficiency approach. Yeasts can be rapidly grown in simple growth media, and the production of sdAbs in yeast is relatively straightforward with a high level of yield. Yeasts do not produce endotoxins, so they are safer, simplify the processes of downstream processing. All these advantages make yeast systems the most valuable host for large-scale production of sdAbs in industry.

The production levels for functional antibodies in mammalian cell expression systems have improved dramatically because of the effectively promoting correct posttranslational modifications. Transient or stable transfection of different cell lines can be used more and more widely to produce large quantities of sdAbs in a timely and cost-effective way. Chinese hamster ovary (CHO) cells and human embryonic kidney 293 (HEK293) cells have been rapidly growing as the most popular mammalian cell expression systems for the expression of sdAbs. Expression of sdAbs in mammalian cells is a desirable strategy to ensure the production of biologically functional sdAbs.

Recently, the plant-based expression system is considered to be a promising alternative to the conventional mammalian cell lines to produce antibodies. Transient and stable expression strategies have been used successfully for the production of sdAbs in plants. These plant-produced sdAbs have the same antigen-binding activity and affinity as those produced in other production systems. Although high yields are attainable in bacterial and yeast cell culture, the highly scalable nature of this alternative system offers significant promise for the future.