Preparation of scFv-immobilized quartz crystal microbalance sensor by PS-tag-mediated solid-phase refolding

Development of a novel and broadly applicable sandwich ELISA assay based on rabbit single-chain variable fragments and a modified Ig-binding domain of protein L fused to a polystyrene-binding peptide

Most of currently available sandwich-type enzyme-linked immunosorbent assays (ELISA) require the use of full-length animal-derived antibodies which poses welfare criticisms and are often expensive to produce. There is therefore a strong demand for the development of more affordable and animal-free methods to produce antibodies for sandwich ELISA assay. To address these issues, we propose here the development of a new technology based on two complementary rabbit single-chain variable fragments (scFvs) and an Ig-binding domain of protein L (PpL1) fused to a polystyrene-binding peptide (PS-tag) that can be recombinantly produced in bacteria. Toward this goal, we developed a rabbit scFv capable to bind the antigen via its variable regions while engaging protein L through its constant framework domain. To enhance the density of captured scFv and enable a better solvent exposure, we generated multiple PpL1 variants bearing polystyrene-binding peptides (PS) tags fused to its ends. The tandem trimer of PpL1 variant bearing PS-tags located at the N-terminus (PpL1'-T-PSN) revealed increased antigen-binding signal when immobilized on hydrophilic polystyrene (phi-PS) plates. By CDR-grafting different antigen-binding specificities into our engineered protein L-binding scFv we validated our technology against a different antigen. Finally, to further enhance the sensitivity of our assay, we implemented a protein L-based pretreatment to remove potential inhibitory immunoglobulin often present in the blood samples. The ability to rapidly and cost-effectively generate animal-free recombinant antibody fragments that can be adsorbed and specifically oriented on plates while retaining their antigen-binding properties could lead to the development of innovative and widely applicable sandwich ELISA systems for the efficient, versatile and sensitive detection of different types of antigens.

Design and site-directed immobilization of single-chain Fv antibody to polystyrene latex beads via material-binding peptides and application to latex turbidimetric assay

In this study, immobilization of single-chain Fv (scFv) antibodies on the surfaces of polystyrene (PS) latex beads via material-binding peptides was investigated for sensitive immuno-turbidimetric assay of C-reactive protein (CRP). Anti-CRP scFvs fused with polystyrene-binding peptide (PS-tag) and poly(methylmethacrylate)-binding peptide (PMMA-tag) were over-expressed in Escherichia coli cells and recovered in the active form following refolding. The beads with PMMA-tag-fused scFv (scFv-PM) were successfully suspended with sufficient dispersion at pH 8.0. Three types of alternative scFv-PMs with a penta-asparatic acid tag (D5-tag) introduced at different positions were then designed. All of the D5-tagged scFv-PMs were successfully immobilized on the surfaces of beads with no significant change in the diameter of the latex beads at pH levels ranging from 6.0 to 8.0. According to the results of turbidimetric assay for the detection of CRP, 13 ng/ml of CRP was detectable using beads with D5-tagged scFv-PMs at 400 ng/cm³, and no turbidity change was observed in the absence of antigen. When the density of scFv-PM was 250 ng/cm², which was 63% of the maximum density, the beads were dispersed well and reactive with the antigen at a concentration range comparable to those with D5-tagged scFv-PMs. These results indicate that controlling charge density on the surface of beads after site-directed immobilization is definitely important in order to maintain high levels of dispersion and reactivity. Thus, the usefulness of the scFv-PM as well as D5-tagged scFv-PMs developed in the present study should be significant when used as ligand antibodies in the preparation of immuno-latex beads.

Aptamers, antibody scFv, and antibody Fab' fragments: An overview and comparison of three of the most versatile biosensor biorecognition elements

The choice of biosensing elements is crucial for the development of the optimal biosensor. Three of the most versatile biosensing elements are antibody single-chain Fv fragments (scFv), antibody fragment-antigen binding (Fab') units, and aptamers. This article provides an overview of these three biorecognition elements with respects to their synthesis/engineering, various immobilization techniques, and examples of their use in biosensors. Furthermore, the final section of the review compares and contrasts their characteristics (time/cost of development, ease and variability of immobilization, affinity, stability) illustrating their advantages and disadvantages. Overall, scFv fragments are found to display the highest customizability (i.e. addition of functional groups, immobilizing peptides, etc.) due to recombinant synthesis techniques. If time and cost are an issue in the development of the biosensor, Fab' fragments should be chosen as they are relatively cheap and can be developed quickly from whole antibodies (several days). However, if there are sufficient funds and time is not a factor, aptamers should be utilized as they display the greatest affinity towards their target analytes and are extremely stable (excellent biosensor regenerability).

Efficient refolding and immobilization of PMMA-tag-fused single-chain Fv antibodies for sensitive immunological detection on a PMMA plate

In this study, we investigated the efficient refolding and site-specific immobilization of single-chain variable fragments (scFvs) genetically fused with a poly(methylmethacrylate)-binding peptide (PMMA-tag). According to the results of an aggregation test of a scFv-PM in the presence of 0.5 M urea, aggregation was hardly detectable at a weak-alkaline pH (8.5) with lower concentrations of NaCl. Consequently, more than 93% recovery of the anti-RNase scFv-PM model was attained, when it was refolded by dialysis against 50 mM TAPS (pH8.5). These results suggested that the apparent isoelectric point (pI) of a target scFv was decreased to a great extent by the genetic fusion of a PMMA-tag containing 5 acidic amino acids, and, thus, the solubility of the scFv-PM in its semi-denatured form was considerably improved. We also designed alternative peptide-tags composed of plural aspartic acid residues (D5, D10 and D15-tags) to decrease the apparent pI value of the fusion protein. As a consequence, scFv-D5, scFv-D10 and scFv-D15 were also efficiently refolded with yields of more than 95%. It is noteworthy that even scFv-PS-D15, which had both a positively charged polystyrene-binding peptide (PS-tag) and a negatively charged D15-tag, was serially connected at the C-terminal region of scFvs, and also refolded with a yield of 96.1%. These results clearly indicate that controlling the apparent pI value of scFvs by the fusion of oligo-peptides composed of acidic amino acids at the C-terminus resulted in a high degree of recovery via dialysis refolding. According to the results of a sandwich ELISA using scFv-PMs, scFv-D15 and scFv-PS-D15 as ligands, high antigen-binding signals were detected from both the PMMA and phi-PS plates immobilized with scFv-PMs. Furthermore, the high antigen-binding activity of scFv-PMs was maintained in an adsorption state when it was immobilized on the surface of not only PMMA, but also hydrophilic PS (phi-PS) and polycarbonate (PC). These results strongly suggested that a PMMA-tag introduced at the C-terminus of scFvs preferably recognizes ester and/or carboxyl groups exposed on the surface of plastics. The scFv-PM developed in the present study has advantages such as being a ligand antibody, compared with whole Ab and the conventional PS-tag-fused scFvs (scFv-PS), and, thus, it is considerably useful in a sandwich ELISA as well as in various immuno-detection and immuno-separation systems.