Expression of a fusion protein of scFv-biotin mimetic peptide for immunoassay

Antifouling surface layers for improved signal-to-noise of particle-based immunoassays

A 10-fold improvement in the signal-to-noise (S/N) ratio of an optically encoded silica particle-based immunoassay was achieved through incorporating a protein resistant poly(ethylene glycol) (PEG) surface layer and optimizing antibody immobilization conditions. PEG was activated using 2,2,2-trifluoroethanesulfonyl chloride (tresyl) and required a minimum reaction time of 1.5 h. The activated PEG had a reactive half-life of approximately 5 h when stored in acidified dimethyl sulfoxide (DMSO). By increasing the protein incubation time and concentration, a maximum antibody loading on the particle surface of 1.6 x 10(-2) molecules per nm(2) was achieved. The assay S/N ratio was assessed using a multiplexed multicomponent optically encoded species-specific immunoassay. Encoded particles were covalently grafted or nonspecifically coated with either bovine or mouse IgG for the simultaneous detection of complementary anti-IgG "target" or uncomplementary anti-IgG "noise". The versatility and potential as a serum-based assay platform was demonstrated by immobilizing either a polyclonal antibody or an engineered single-chain variable fragment (scFv) capture probe on particles for the detection of the ovarian cancer biomarker, mesothelin (MSLN). The MLSN antigen was spiked into PBS buffer or 50% human serum. Both capture probe orientations, and media conditions showed similar low level detection limits of 5 ng/mL; however, a 40% decrease in maximum signal intensity was observed for assays run in 50% serum.

Antibody production with yeasts and filamentous fungi: on the road to large scale?

Yeasts and filamentous fungi have gained significant interest for the production of recombinant antibodies and antibody fragments. The opportunities and constraints of antibody (fragment) production in these hosts are highlighted as well as cell engineering strategies to overcome the constraints. Following aspects are addressed: folding, assembly and secretion of antibody related proteins, process optimization to improve productivity and quality, proteolysis, and, as a major point of interest, glycosylation.

Engineering of Pichia pastoris for improved production of antibody fragments

The methylotrophic yeast Pichia pastoris has been used for the expression of many proteins, including antibody fragments. However, limitations became obvious especially when secreting heterodimeric Fab fragments. Up-to-date, antibody fragments have only been expressed under control of the strong inducible alcohol oxidase 1 (AOX1) promoter, which may stress the cells by excessive transcription. Here, we examined the secretion characteristics of single chain and Fab fragments of two different monoclonal anti-HIV1 antibodies (2F5 and 2G12) with both the AOX1 and the glyceraldehyde-3-phosphate dehydrogenase (GAP) promoter. Also, the influences of different secretion leaders and strains were evaluated. Interestingly, secretion was only achieved when using the GAP promoter and the Saccharomyces cerevisiae mating factor alpha (MFalpha leader), whereas there was no difference between the two P. pastoris strains. During fed batch fermentation of a 2F5 Fab expressing strain, intracellular retention of Fab heavy chains was observed, while both intact Fab and single light chain molecules were only detected in the supernatants. This led to the conclusion that protein folding and heterodimer assembly in the ER are rate limiting steps in Fab secretion. To alleviate this limitation, S. cerevisiae protein disulfide isomerase (PDI) and the unfolded protein response (UPR) transcription factor HAC1 were constitutively overexpressed in P. pastoris. While the overexpression of HAC1 led to a moderate increase of Fab secretion of 1.3-fold, PDI enabled an increase of the Fab level by 1.9-fold. Hence, the formation of interchain disulfide bonds can be seen as a major rate limiting factor to Fab assembly and subsequent secretion.

Development of a biotin mimic tagged ScFv antibody against western equine encephalitis virus: bacterial expression and refolding

Single chain antibodies (ScFvs) are heavy and light chain variable domains connected by an artificial linker. Because of their smaller size, ScFvs show improved tissue penetration in vivo and reduced immunogenicity, making them ideal for therapeutic applications. We have cloned a ScFv against western equine encephalitis (WEE) using rDNA technology. The ScFv was generated from a hybridoma cell line (11D2) specific to the WEE virus E1 glycoprotein and is arranged in the V(L)-V(H) orientation with a (gly(4)ser)(3) linker. This ScFv was engineered successfully with a biotin mimic tag (11 amino acid peptide) and cloned in the pET22b+ expression vector. The ScFv was expressed as a approximately 32kDa protein in Escherichia coli as inclusion bodies, with an estimated yield of 20-40 mg/l. Different refolding protocols were used to solubilise the inclusion bodies. Most of the functional ScFv was generated when the inclusion bodies were solubilized in a detergent, air oxidised in the presence of CuSO(4) and then denatured in urea buffer in comparison to other protocols. The product was renatured finally in Tris arginine buffer (pH 8.0). Refolded protein was dialysed against phosphate buffer saline (PBS) (pH 7.3) to remove the Tris and arginine. Our refolding protocol generated up to a 50% yield of soluble protein, which retained antigen-binding activity with whole inactivated WEE virus as demonstrated by ELISA and Western blot analysis. This 11D2-biotin mimic ScFv complexed with streptavidin horseradish peroxidase (St-HRPO) will be useful as a detector reagent in the ultrasensitive ELISA detection of WEE virus antigen.

Development of immunofiltration assay by light addressable potentiometric sensor with genetically biotinylated recombinant antibody for rapid identification of Venezuelan equine encephalitis virus

A genetically biotinylated single chain fragment variable antibody (scFv) against Venezuelan equine encephalitis virus (VEE) was applied in a system consisting of an immunofiltration enzyme assay (IFA) with a light addressable potentiometric sensor (LAPS) for the rapid identification of VEE. The IFA involved formation of an immunocomplex sandwich consisting of VEE, biotinylated antibody, fluoresceinated antibody and streptavidin, capture of the sandwich by filtration on biotinylated membrane, and labeling of the sandwich by anti-fluorescein urease conjugate. The concentration ratio of biotinylated to fluoresceinated antibodies was investigated and optimized. By the IFA/LAPS assay, the limit of detection (LOD) of VEE was approximately 30 ng/ml, similar to that achieved when chemically biotinylated monoclonal antibody (mAb) was applied. Total assay variance of the IFA/LAPS assay for both intra- and inter-assay precision was less than 20%. Assay accuracy was measured by comparing VEE concentrations estimated by IFA/LAPS standard curve to those obtained by conventional protein assay. VEE concentrations were found to differ by no more than 10%. The IFA/LAPS assay sensitivity was approximately equal to that of a conventional enzyme-linked immunosorbent assay (ELISA) utilizing polystyrene plates and a chromogenic substrate; however, less time and effort were required for performance of the IFA/LAPS assay. More importantly, use of genetically biotinylated scFv in the IFA/LAPS assay obviates the need for chemical biotinylation of antibody with resultant possible impairment of the antigen-binding site. Furthermore, the potential for batch-to-batch variability resulting from inequality in the number of biotin molecules labeled per antibody molecule is eliminated.

Molecular engineering of biotin-glutamic acid decarboxylase 65 fusion protein (Biotin-GAD65) for non-radioactive GAD65 antibody assay

We constructed biotinylated fusion proteins that linked to three detection tags to GAD65 at the N-terminus, and expressed them in an E. coli expression system. The Biotin14-GAD65 protein exhibited the strongest binding to both the GAD65 antibody and the streptavidin among the three constructs. We describe the optimal conditions using a Biotin14-GAD65-based immunoassay for the detection of GAD65 antibody.

Genetic engineering of streptavidin-binding peptide tagged single-chain variable fragment antibody to Venezuelan equine encephalitis virus

A recombinant gene encoding a single-chain variable fragment (scFv) antibody against Venezuelan equine encephalitis virus (VEE) was cloned into a prokaryotic T7 RNA polymerase-regulated expression vector. A streptavidin-binding peptide gene fused to a 6His tag was attached downstream to the scFv gene. The recombinant fusion protein was expressed in bacteria as inclusion bodies that were subsequently solubilized with 8 M urea and renatured by an arginine system. Purification of the fusion protein was achieved by immobilized metal affinity chromatography. Enzyme-linked immunosorbent assay (ELISA) and Western blotting results revealed that the fusion protein not only retained VEE antigen binding and specificity properties similar to those of its parent native monoclonal antibody (MAb), but also possessed streptavidin-binding activity. This experimental approach can eliminate the need for chemical biotinylation of antibodies and the risk associated of antibody denaturation and can provide a stable and reproducible reagent for rapid and efficient immunoassay of VEE when detected by horseradish peroxidase (HRP)-conjugated streptavidin.

Single-chain Fv fragments derived from an anti-11-deoxycortisol antibody. Affinity, specificity, and idiotype analysis

Single-chain Fv fragments (scFvs) against a corticosteroid, 11-deoxycortisol (11-DC), have been generated as a template antibody fragment from which a comprehensive mutated antibody library containing various anti-steroid antibodies could be constructed. The cDNAs encoding variable heavy (V(H)) and light (V(L)) domains of a mouse anti-11-DC antibody (CET-M8), were amplified by RT-PCR, combined via a common linker to construct the sequence of 5'-V(H)-(Gly(4)Ser)(3)-V(L)-3', and cloned into a phagemid vector, pEXmide 5. The phage clones exhibiting binding activity to 11-DC were isolated after single panning against a hapten-immobilizing immunotube. The scFv gene in one of these clones was reamplified to introduce the ochre codons, and then expressed in the bacterial periplasm as the soluble antibody fragment. Two different scFvs (#6 and #12) were cloned, whose binding characteristics were examined by a radioimmunoassay using a tritium-labeled 11-DC. Both of them showed high affinity (K(a)=1.3x10(10)M(-1)) and practical specificity (cross-reactivity: cortisol, <0.2%; cortisone, <0.3%) to 11-DC, and furthermore, strong reactivity with an anti-idiotype antibody which recognizes the paratope of CET-M8. These results suggest that the present scFvs retain the three-dimensional structure of the paratope of the original monoclonal antibody.

Secretion of active anti-Ras single-chain Fv antibody by the yeasts Yarrowia lipolytica and Kluyveromyces lactis

Yarrowia lipolytica and Kluyveromyces lactis secretion vectors were constructed and assessed for the expression of heterologous proteins. An anti-Ras single-chain antibody fragment (scFv) coding sequence was fused in-frame to different pre- or prepro-regions, or downstream from a reporter secretory gene (Arxula adeninivorans glucoamylase), separated by a Kex2 protease (Kex2p)-like processing sequence. Both organisms are able to secrete soluble scFv, with yields depending on the nature of the expression cassette, up to levels ranging from 10 to 20 mg l(-1). N-terminal sequence analysis of the purified scFv showed that fusions are correctly processed to the mature scFv by a signal peptidase or a Kex2p-type endoprotease present in Y. lipolytica and K. lactis. The scFv protein also retains the capacity to bind to a glutathioneS-transferase (GST)-Harvey-Ras(Val12) fusion, indicating that the antibody is functional. These results indicate that the yeasts Y. lipolytica and K. lactis have potential for industrial production of soluble and active scFv.

A recombinant scFv/streptavidin-binding peptide fusion protein for the quantitative determination of the scorpion venom neurotoxin AahI

We created a construct encoding a peptide known to mimic the binding properties of biotin fused to the carboxy-terminus of a scFv fragment that binds a scorpion toxin (AahI). This fusion protein was produced in the periplasm of bacteria and purified to homogeneity by single-step affinity chromatography on streptavidin-agarose with a yield close to 1 mg/l. DNA sequencing, dot blot and mass spectrometric analyses demonstrated the integrity of the soluble immunoconjugate. Fusion to the streptavidin-binding peptide did not affect the ability of the scFv to recognize its antigen with a high affinity (Kd = 2.3 x 10(-10) M). Similarly, the streptavidin-binding property was not impaired in the fusion protein. Thus, the immunoconjugate was bifunctional and had a low molecular mass of 28 kDa. This enabled us to develop rapid and sensitive immunoassays for the specific detection of the toxin AahI accurately to 0.6 ng/ml, opening up new perspectives for the diagnosis of envenomations.

High-yield expression of the recombinant, atrazine-specific Fab fragment K411B by the methylotrophic yeast Pichia pastoris

In this report, we describe the high-yield secretory expression ( approximately 40 mg x l(-1)) of pure, atrazine-specific Fab fragments (K411B) from Pichia pastoris that was achieved by co-integration of the genes encoding the heavy and light chains (both under the control of the alcohol oxidase promoter) into the genome of the yeast cells. Antibody-expressing clones were selected by SDS-PAGE and ELISA and fed-batch fermentations were carried out in a 5-l scale. Both chains of the Fab were successfully expressed upon methanol induction and almost no other proteins were secreted into the media. Approximately 30% of the two chains formed the active Fab fragment containing the intermolecular disulphide bond, as determined by Western blot analysis under non-reducing conditions. Crude culture supernatant was used to study the binding properties of the Fab fragment toward different s-triazines by means of competitive ELISA: the IC50 value for the detection of atrazine was determined from the standard curve as 3 microg x l(-1), which is one magnitude higher than the value obtained with the parental mAb K4E7 but equals that obtained when the same Fab fragment was expressed in Escherichia coli cells. In addition, the cross-reactivity pattern of the Fab from Pichia is comparable to that of E. coli and to the parental mAb K4E7.

Recombinant protein expression in Pichia pastoris

The methylotrophic yeast Pichia pastoris is now one of the standard tools used in molecular biology for the generation of recombinant protein. P. pastoris has demonstrated its most powerful success as a large-scale (fermentation) recombinant protein production tool. What began more than 20 years ago as a program to convert abundant methanol to a protein source for animal feed has been developed into what is today two important biological tools: a model eukaryote used in cell biology research and a recombinant protein production system. To date well over 200 heterologous proteins have been expressed in P. pastoris. Significant advances in the development of new strains and vectors, improved techniques, and the commercial availability of these tools coupled with a better understanding of the biology of Pichia species have led to this microbe's value and power in commercial and research labs alike.

Heterologous protein expression in the methylotrophic yeast Pichia pastoris

During the past 15 years, the methylotrophic yeast Pichia pastoris has developed into a highly successful system for the production of a variety of heterologous proteins. The increasing popularity of this particular expression system can be attributed to several factors, most importantly: (1) the simplicity of techniques needed for the molecular genetic manipulation of P. pastoris and their similarity to those of Saccharomyces cerevisiae, one of the most well-characterized experimental systems in modern biology; (2) the ability of P. pastoris to produce foreign proteins at high levels, either intracellularly or extracellularly; (3) the capability of performing many eukaryotic post-translational modifications, such as glycosylation, disulfide bond formation and proteolytic processing; and (4) the availability of the expression system as a commercially available kit. In this paper, we review the P. pastoris expression system: how it was developed, how it works, and what proteins have been produced. We also describe new promoters and auxotrophic marker/host strain combinations which extend the usefulness of the system.