Through recombinant DNA technology and antibody engineering techniques, antibody genes can now be successfully cloned and expressed as fragments in bacteria, mammalian cells, yeast, plants, and insect cells. A key advantage of these novel technologies is their ability to retain the complete antigen-binding site (paratope) while reducing the size of the antibody molecule. Compared to their parental antibodies, these minimized antibodies offer several benefits in clinical applications, including improved tumor penetration, faster blood clearance, shorter retention time in non-target tissues, and reduced immunogenicity.This approach also enables the expression of functional antibodies and their fusion in bacteria, allowing for their display on filamentous phages. Moreover, combining small antibody molecules with highly efficient microbial production systems could ultimately yield sufficient quantities of homogeneous proteins for diagnostic, therapeutic, and structural research purposes.

In microorganisms such as Escherichia coli, Fab fragments have been successfully produced in the periplasm. However, due to the limited capacity of the bacterial periplasmic space and issues with improper peptide folding and aggregation, product titers are often low.

I. Structure of Single-Chain Antibody (scFv)

The Fv fragment is the smallest unit of an immunoglobulin molecule that retains antigen-binding activity. An antibody in the single-chain variable fragment (scFv) format consists of the variable regions of the heavy chain (VH) and light chain (VL), connected by a flexible peptide linker. This design allows scFv to be efficiently expressed in a functional form in Escherichia coli, enabling protein engineering to enhance its properties—such as increased affinity and altered specificity.

II. Obtaining Single-Chain Antibodies

First, mRNA is isolated from hybridomas (or spleen, lymphocytes, and bone marrow), then reverse-transcribed into cDNA, which serves as a template for antibody gene amplification (PCR). In scFv (single-chain variable fragment) construction, the order of the domains can be VH-linker-VL or VL-linker-VH. The strategies for recombinant antibody expression in E. coli include the following:

(1) One method involves the direct expression of single-chain antibodies in the cytoplasm of E. coli cells. An E. coli signal peptide is used. The results show that the antibody is highly expressed in the reducing environment of the bacterial cytoplasm, subsequently forming insoluble aggregates known as inclusion bodies. These inclusion bodies must be refolded in vitro.

(2) Another method employs a signal peptide to direct the secretion of scFv (single-chain variable fragment) antibodies into the periplasmic space between the inner and outer membranes of Gram-negative bacteria. This periplasmic space is known to contain proteins such as chaperones and disulfide isomerases, which assist in the proper folding of recombinant proteins. During translocation across the inner membrane into the oxidizing environment of the periplasm, the N-terminal signal peptide is cleaved, allowing the chain to fold and assemble, thereby facilitating the formation of intra- and inter-domain disulfide bonds.

III. Detection of Single-Chain Antibodies

The detection of scFv (single-chain variable fragment) can be performed using secondary antibodies that recognize specific tags fused to either the C- or N-terminus of the scFv. Currently, various tags are commonly used for scFv fusion, such as c-myc or E-tag (Pharmacia). Due to improper folding of soluble scFvs (single-chain variable fragments), they are prone to inactivation when coated onto microtiter plates. This instability arises from the absence of the constant domains of the heavy and light chains.

IV. Applications of Single-Chain Antibodies

Antibodies in the form of scFv (single-chain variable fragment) retain full antigen-binding capability, making them promising molecular tools, particularly in cancer therapy. Another significant application of single-chain antibodies is their use as diagnostic reagents. In recent years, custom-engineered recombinant scFvs produced in bacteria have emerged as potential alternatives to "traditional" immunodiagnostic reagents. The functionality of recombinant antibody fragments (scFvs) as immunological reagents has been demonstrated in several different assay systems.

KMD Bioscience has established a comprehensive recombinant antibody preparation and production system based on a mature protein expression platform. We offer multiple expression systems suitable for antibody production, including:Prokaryotic expression systems (including but not limited to BL21, Nissle 1917, Rosetta, and other prokaryotic hosts);Mammalian expression systems (including but not limited to Expi 293F, Freestyle 293F, CHO-S, CHO-K1, 293T, and other mammalian hosts);Yeast expression systems (including but not limited to GS115, X33 Pichia pastoris systems, and Saccharomyces cerevisiae systems).These systems are designed to meet your diverse needs. For more information, please feel free to contact us!