Insight into the Working Mechanism of Quenchbody: Transition of the Dye around Antibody Variable Region That Fluoresces upon Antigen Binding

A breakthrough of immunoassay format for hapten: recent insights into noncompetitive immunoassays to detect small molecules

Immunoassay based on the antibodies specific for targets has advantages of high sensitivity, simplicity and low cost, therefore it has received more attention in recent years, especially for the rapid detection of small molecule chemicals present in foods, diagnostics and environments. However, limited by low molecular weight and only one antigenic determinant existed, immunoassays for these small molecule chemicals, namely hapten substances, were commonly performed in a competitive immunoassay format, whose sensitivities were obviously lower than the sandwich enzyme-linked immunosorbent assay generally adaptable for the protein targets. In order to break through the bottleneck of detection format, researchers have designed and established several novel noncompetitive immunoassays for the haptens in the past few years. In this review, we focused on the four representative types of noncompetitive immunoassay formats and described their characteristics and applications in rapid detection of small molecules. Meanwhile, a systematic discussion on the current technologies challenges and the possible solutions were also summarized. This review aims to provide an updated overview of the current state-of-the-art in noncompetitive immunoassay for small molecules, and inspire the development of novel designs for small molecule detection.

Efficient and rapid linker optimization with heterodimeric coiled coils improves the response of fluorescent biosensors comprising antibodies and protein M

We developed a coiled Q-probe (CQ-probe), a fluorescent probe containing a coiled-coil peptide pair E4/K4, to convert antibodies into biosensors for homogeneous immunoassays. This probe consists of an antibody-binding protein, protein M (PM) with the E4 peptide and the K4 peptide with a fluorescent dye. Compared to PM Q-probes, which are generated by modifying the C-terminus of PM with a fluorescent dye, CQ-probe variants with various linkers are easy to prepare and therefore enable the establishment of biosensors with a significant fluorescence response by localizing the fluorescent dye at the optimal position for quenching and antigen-dependent release. The fluorescence changes of biosensors converted from anti-BGP, anti-cortisol, and anti-testosterone antibodies using the rhodamine 6G (or TAMRA)-labeled CQ-probe upon antigen addition were 13 (or 2.6), 9.7 (or 1.5), and 2.1 (or 1.2) times larger than that of the biosensors converted using the PM Q-probe. Furthermore, the CQ-probe converted anti-digoxin IgG into a functional biosensor, whereas the PM Q-probe/antibody complex showed an insufficient response. This technology exhibits a promising capacity to convert antibodies into high-response biosensors, which are expected to be applied in a wide range of fields, including clinical diagnosis, environmental surveys, food analysis, and biological research.

Production and engineering of nanobody-based quenchbody sensors for detecting recombinant human growth hormone and its isoforms

Due to athletes' misuse of recombinant human growth hormone (rhGH) for performance improvement, the World Anti-Doping Agency has designated rhGH as a prohibited substance. This study focuses on the development and improvement of a simple and fast rhGH detection method using a fluorescence-incorporated antibody sensor "Quenchbody (Q-body)" that activates upon antigen binding. Camelid-derived nanobodies were used to produce stable Q-bodies that withstand high temperatures and pH levels. Notably, pituitary human growth hormone (phGH) comprises two major isoforms, namely 22 and 20 kDa GH, which exist in a specific ratio, and the rhGH variant shares the same sequence as the 22 kDa GH isoform. Therefore, we aimed to discriminate rhGH abuse by analyzing its specific isoform ratio. Two nanobodies, NbPit (recognizing phGH) and NbRec (preferentially recognizing 22 kDa rhGH), were used to develop the Q-bodies. Nanobody production in Escherichia coli involved the utilization of a vector containing 6xHis-tag, and Q-bodies were obtained using a maleimide-thiol reaction between the N-terminal of the cysteine tag and a fluorescent dye. The addition of tryptophan residue through antibody engineering resulted in increased fluorescence intensity (FI) (from 2.58-fold to 3.04-fold). The limit of detection (LOD) was determined using a fluorescence response, with TAMRA-labeled NbRec successfully detecting 6.38 ng/ml of 22 kDa rhGH while unable to detect 20 kDa GH. However, ATTO520-labeled NbPit detected 7.00 ng/ml of 20 kDa GH and 2.20 ng/ml 22 kDa rhGH. Q-bodies successfully detected changes in the GH concentration ratio from 10 to 40 ng/ml in human serum within 10 min without requiring specialized equipment and kits. Overall, these findings have potential applications in the field of anti-doping measures and can contribute to improved monitoring and enforcement of rhGH misuse, ultimately enhancing fairness and integrity in competitive sports.

Quenchbodies That Enable One-Pot Detection of Antigens: A Structural Perspective

Quenchbody (Q-body) is a unique, reagentless, fluorescent antibody whose fluorescent intensity increases in an antigen-concentration-dependent manner. Q-body-based homogeneous immunoassay is superior to conventional immunoassays as it does not require multiple immobilization, reaction, and washing steps. In fact, simply mixing the Q-body and the sample containing the antigen enables the detection of the target antigen. To date, various Q-bodies have been developed to detect biomarkers of interest, including haptens, peptides, proteins, and cells. This review sought to describe the principle of Q-body-based immunoassay and the use of Q-body for various immunoassays. In particular, the Q-bodies were classified from a structural perspective to provide useful information for designing Q-bodies with an appropriate objective.

Generation of a Recombinant scFv against Deoxycholic Acid and Its Conversion to a Quenchbody for One-Step Immunoassay

Development of a rapid detection method for deoxycholic acid (DCA) is crucial for its diagnosis in the early stages of inflammation and cancer. In this study, we expressed a soluble recombinant anti-DCA single-chain variable fragment (scFv) in Escherichia coli. To convert scFv into a Quenchbody (Q-body), we labeled scFv using commercially available maleimide-linked fluorophores. The TAMRA-C5-maleimide-conjugated Q-body showed the highest response within a few minutes of DCA addition, indicating its applicability as a wash-free immunoassay probe for onsite DCA detection.

Immunosensor for realtime monitoring of the expression of recombinant proteins during bioprocess

Recombinant protein expression and purification are crucial in modern life sciences research. A fluorescent immunosensor termed Quenchbody (Q-body) was developed for real-time monitoring of FLAG-fused protein expression. Detection results showed that the limit of detection of the 3 × FLAG peptide detected by the TAMRA-labeled anti-FLAG Q-body was as low as 3.1 nM, with a half-maximal effective concentration of 21.4 nM. Furthermore, the anti-FLAG Q-body was used for detecting different proteins fused with a FLAG-tag at the N- or C-terminal. Subsequently, the constructed Q-body was used to monitor the real-time fermentation process of single-strand DNA-binding protein in Escherichia coli. Unlike previously reported Q-bodies that widely used Fab or scFv, the present study used a full-length anti-FLAG IgG for the first time. Owing to its excellent detection speed and sensitivity, the FLAG Q-body immunosensor has the potential to quantify and monitor target recombinant proteins in multiple biological processes in real-time.

Effect of macrocyclization and tetramethylrhodamine labeling on chemokine binding peptides

Receptor-derived peptides have played an important role in elucidating chemokine-receptor interactions. For the inflammatory chemokine CXC-class chemokine ligand 8 (CXCL8), a site II-mimetic peptide has been derived from parts of extracellular loops 2 and 3 and adjacent transmembrane helices of its receptor CXC-class chemokine receptor 1 (Helmer et al., RSC Adv., 2015, 5, 25657). The peptide sequence with a C-terminal glutamine did not bind to CXCL8, whereas one with a C-terminal glutamate did but with low micromolar affinity. We sought to improve the affinity and protease stability of the latter peptide through cyclization while also cyclizing the former for control purposes. To identify a cyclization strategy that permits a receptor-like interaction, we conducted a molecular dynamics simulation of CXCL8 in complex with full-length CXC-class chemokine receptor 1. We introduced a linker to provide an appropriate spacing between the termini and used an on-resin side-chain-to-tail cyclization strategy. Upon chemokine binding, the fluorescence intensity of the tetramethylrhodamine (TAMRA)-labeled cyclic peptides increased whereas the fluorescence anisotropy decreased. Additional molecular dynamics simulations indicated that the fluorophore interacts with the peptide macrocycle so that chemokine binding leads to its displacement and observed changes in fluorescence. Macrocyclization of both 18-amino acid-long peptides led to the same low micromolar affinity for CXCL8. Likewise, both TAMRA-labeled linear peptides interacted with CXCL8 with similar affinities. Interestingly, the linear TAMRA-labeled peptides were more resistant to tryptic digestion than the unlabeled counterparts, whereas the cyclized peptides were not degraded at all. We conclude that the TAMRA fluorophore tends to interact with peptides altering their protease stability and behavior in fluorescence-based assays.

Creating a Thermostable β-Glucuronidase Switch for Homogeneous Immunoassay by Disruption of Conserved Salt Bridges at Diagonal Interfaces

Escherichia coli β-glucuronidase (GUS) has been used as a reporter enzyme in molecular biology and engineered as an enzyme switch for the development of homogeneous biosensors. In this study, we developed a thermostable GUS enzyme switch based on the thermostable GUS mutant TR3337 by disrupting a conserved salt bridge (H514-E523) between the diagonal subunits of its homotetramer. A combinatorial library (240 variants) was screened using a novel high-throughput strategy, which led to the identification of mutant DLW (H514D/M516L/Y517W) as a functional enzyme switch in a caffeine-recognizing immunosensor. Molecular dynamics simulations were performed to predict the topology change around position 514, and a side-chain flip of D514 (repulsion with E523) was observed in the DLW mutant. Up to 1.8-fold of signal-to-background ratio was confirmed when measured at up to 45 °C, thereby highlighting the DLW mutant as a versatile tool for developing thermostable immunosensors for in vitro and in cellulo applications.

Isolation of a human SARS-CoV-2 neutralizing antibody from a synthetic phage library and its conversion to fluorescent biosensors

Since late 2019, the outbreak of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and the resultant spread of COVID-19 have given rise to a worldwide health crisis that is posing great challenges to public health and clinical treatment, in addition to serving as a formidable threat to the global economy. To obtain an effective tool to prevent and diagnose viral infections, we attempted to obtain human antibody fragments that can effectively neutralize viral infection and be utilized for rapid virus detection. To this end, several human monoclonal antibodies were isolated by bio-panning a phage-displayed human antibody library, Tomlinson I. The selected clones were demonstrated to bind to the S1 domain of the spike glycoprotein of SARS-CoV-2. Moreover, clone A7 in Fab and IgG formats were found to effectively neutralize the binding of S protein to angiotensin-converting enzyme 2 in the low nM range. In addition, this clone was successfully converted to quench-based fluorescent immunosensors (Quenchbodies) that allowed antigen detection within a few minutes, with the help of a handy fluorometer.

Ribosome selectivity and nascent chain context in modulating the incorporation of fluorescent non-canonical amino acid into proteins

Fluorescence reporter groups are important tools to study the structure and dynamics of proteins. Genetic code reprogramming allows for cotranslational incorporation of non-canonical amino acids at any desired position. However, cotranslational incorporation of bulky fluorescence reporter groups is technically challenging and usually inefficient. Here we analyze the bottlenecks for the cotranslational incorporation of NBD-, BodipyFL- and Atto520-labeled Cys-tRNA^Cys into a model protein using a reconstituted in-vitro translation system. We show that the modified Cys-tRNA^Cys can be rejected during decoding due to the reduced ribosome selectivity for the modified aa-tRNA and the competition with native near-cognate aminoacyl-tRNAs. Accommodation of the modified Cys-tRNA^Cys in the A site of the ribosome is also impaired, but can be rescued by one or several Gly residues at the positions −1 to −4 upstream of the incorporation site. The incorporation yield depends on the steric properties of the downstream residue and decreases with the distance from the protein N-terminus to the incorporation site. In addition to the full-length translation product, we find protein fragments corresponding to the truncated N-terminal peptide and the C-terminal fragment starting with a fluorescence-labeled Cys arising from a StopGo-like event due to a defect in peptide bond formation. The results are important for understanding the reasons for inefficient cotranslational protein labeling with bulky reporter groups and for designing new approaches to improve the yield of fluorescence-labeled protein.

Creation of a quick and sensitive fluorescent immunosensor for detecting the mineralocorticoid steroid hormone aldosterone

Aldosterone (ALD) is a steroid hormone secreted by the zona glomerulosa of the adrenal cortex that mainly acts on the kidney to regulate sodium ion and water reabsorption. Detection of ALD plays an important role in the diagnosis of primary aldosteronism in patients with hypertension. For the first time, the gene encoding the anti-ALD antibody, A2E11, was successfully cloned and analyzed using phage display technology. The antibody had an affinity of 2.5 nM against ALD, and after binding to ALD, it reached saturation within 5 s. Using this antibody, a Quenchbody (Q-body) was constructed by labeling the N-termini of heavy and light chains of the antigen-binding fragment of A2E11 with the fluorescent dye ATTO520 to detect ALD based on the principle of photoinduced electron transfer. The sensor detected ALD in 2 min, and the limit of detection was 24.1 pg/mL with a wide detection range from 24.1 pg/mL to 10 µg/mL and a half-maximal effective concentration of 42.3 ng/mL. At the highest concentration of ALD in the assay, the fluorescence intensity increased by 5.0-fold compared to the original fluorescence intensity of the Q-body solution. The Q-body could be applied to analyze 50% of human serum without a significant influence of the matrix. The recoveries of ALD in spiked serum samples with the Q-body assay were confirmed to range from 90.3% to 98.2%, suggesting their potential applications in the diagnosis of diseases, such as essential hypertension.

Isolation of a Monoclonal Antibody and its Derived Immunosensor for Rapid and Sensitive Detection of 17β-Estradiol

Estrogens are effective for stimulating several functions in living organisms and for regulating cancer development by promoting cell proliferation. Estradiol can disrupt the reproductive and endocrine systems, leading to the development of various diseases. In this study, the monoclonal antibody ESC9 was developed by immunizing mice with a 17β-estradiol (E2) conjugate, preparing an antibody phage display library, and screening monoclonal antibodies from the prepared library. An antibody with the same sequence as that of ESC9 has not been reported previously. The equilibrium dissociation constant between ESC9 and E2 was found to be 43.3 nM. Additionally, we generated an ESC9-derived immunosensor named as the ESC9 Quenchbody (Q-body), which can rapidly and sensitively detect E2. The assay can be completed within 2 min with a limit of detection of 3.9 pg/ml and half-maximal effective concentration of 154.0 ng/ml. Serum E2 levels were measured using the ESC9 Q-body without pretreatment with serum and with a high recovery rate of 83.3–126.7%. The Q-body immunosensor shows potential for clinical applications based on its excellent detection speed and sensitivity.

Evaluation and selection of potent fluorescent immunosensors by combining fluorescent peptide and nanobodies displayed on yeast surface

Quenchbody (Q-body) is a quench-based fluorescent immunosensor labeled with fluorescent dye(s) near the antigen-binding site of an antibody. Q-bodies can detect a range of target molecules rapidly and directly. However, because Q-bodies show different antigen responses depending on the antibody used, time-consuming optimization of the Q-body structure is often necessary, and a high-throughput screening method for discriminating and selecting good Q-bodies is required. Here, we aimed to develop a molecular display method of nanobody-based “mini Q-bodies” by combining yeast surface display and coiled-coil forming E4/K4 peptide-based fluorescence labeling. As a result, the yeast-displayed mini Q-body recognizing the anti-cancer agent methotrexate (MTX) showed significant quenching and MTX-dependent dequenching on cells. To demonstrate the applicability of the developed method to select highly responsive mini Q-bodies, a small nanobody library consisting of 30 variants that recognize human serum albumin was used as a model. The best variant, showing a 2.4-fold signal increase, was obtained through selection by flow cytometry. Furthermore, the same nanobody prepared from Escherichia coli also worked as a mini Q-body after dye labeling. The described approach will be applied to quickly obtain well-behaved Q-bodies and other fluorescent biosensors for various targets through directed evolutionary approaches.

Rapid conversion of IgG to biosensor using an antibody-binding protein-based probe

We successfully developed a fluorescent probe that can quickly convert full-length antibodies to Quenchbodies, which represent a type of fluorescent immunosensor with high binding affinity and specificity depending on the reaction of antigens and antibodies. An anti-testosterone IgG was successfully converted to an immunosensor that detects testosterone with a limit of detection (LOD) of 0.76 nM and concentration for 50% of maximal effect (EC₅₀) of 61.5 nM. Another IgG-based immunosensor detected ractopamine with an LOD of 15.5 pM and EC₅₀ of 48.6 nM.

Rapid construction of fluorescence quenching-based immunosensor Q-bodies using α-helical coiled-coil peptides

Here, we report a rapid and efficient method to fabricate Quenchbodies (Q-bodies) that can detect targets with antigen-dependent fluorescence augmentation using a stable coiled-coil peptide pair, E4 and K4 (coiled Q-body, CQ-body). The CQ-body allowed antigen detection not only in buffer but also in 50% plasma. Furthermore, we describe FRET-type CQ-bodies using a dual-coloured K4 peptide, which allowed a more precise antigen quantification. Lastly, successful fabrication of nanobody-based CQ-body shows its applicability to a range of antibody fragments.

BRET Q-Body: A Ratiometric Quench-based Bioluminescent Immunosensor Made of Luciferase-Dye-Antibody Fusion with Enhanced Response

A quenchbody (Q-body) is an immunosensor comprising an antibody fragment containing an antigen-binding site that is site-specifically labeled with a fluorescent dye. The fluorescent dye of a Q-body is quenched in the absence of an antigen; however, its fluorescence recovers in the presence of an antigen, offering simple and rapid systems for antigen detection. In this study, we fused luciferase NanoLuc to a Q-body to construct a new immunosensor termed the "BRET Q-body" that can detect antigens based on the bioluminescence resonance energy transfer (BRET) principle. The resulting BRET Q-bodies for an osteocalcin peptide that emit three different emission colors could detect an antigen without the requirement of an external light source, based on ratiometric detection and color change with two wavelengths for the luciferase and fluorophore. Furthermore, the BRET Q-body produced unexpectedly higher responses up to 12-fold because of the increased BRET efficiency, probably associated with antigen-dependent dye movement. Thus, the BRET Q-body is a useful biosensor as a core of point-of-care tests.

Recent Advances in Quenchbody, a Fluorescent Immunosensor

The detection of viruses, disease biomarkers, physiologically active substances, drugs, and chemicals is of great significance in many areas of our lives. Immunodetection technology is based on the specificity and affinity of antigen–antibody reactions. Compared with other analytical methods such as liquid chromatography coupled with mass spectrometry, which requires a large and expensive instrument, immunodetection has the advantages of simplicity and good selectivity and is thus widely used in disease diagnosis and food/environmental monitoring. Quenchbody (Q-body), a new type of fluorescent immunosensor, is an antibody fragment labeled with fluorescent dyes. When the Q-body binds to its antigen, the fluorescence intensity increases. The detection of antigens by changes in fluorescence intensity is simple, easy to operate, and highly sensitive. This review comprehensively discusses the principle, construction, application, and current progress related to Q-bodies.

Fluorescence Polarization-Based Bioassays: New Horizons

Fluorescence polarization holds considerable promise for bioanalytical systems because it allows the detection of selective interactions in real time and a choice of fluorophores, the detection of which the biosample matrix does not influence; thus, their choice simplifies and accelerates the preparation of samples. For decades, these possibilities were successfully applied in fluorescence polarization immunoassays based on differences in the polarization of fluorophore emissions excited by plane-polarized light, whether in a free state or as part of an immune complex. However, the results of recent studies demonstrate the efficacy of fluorescence polarization as a detected signal in many bioanalytical methods. This review summarizes and comparatively characterizes these developments. It considers the integration of fluorescence polarization with the use of alternative receptor molecules and various fluorophores; different schemes for the formation of detectable complexes and the amplification of the signals generated by them. New techniques for the detection of metal ions, nucleic acids, and enzymatic reactions based on fluorescence polarization are also considered.

Creation of a Nanobody-Based Fluorescent Immunosensor Mini Q-body for Rapid Signal-On Detection of Small Hapten Methotrexate

"Quenchbody (Q-body)" is a quench-based fluorescent biosensor labeled with a fluorescent dye near the antigen-binding site of an antibody. Q-bodies can detect a range of target molecules quickly by simply mixing with a sample. However, the development of Q-bodies using V_HH-nanobodies derived from camelid heavy-chain antibodies has not been reported despite their favorable characteristics. Here, we report a "mini Q-body" that can detect the chemotherapy agent methotrexate (MTX) by using anti-MTX nanobody. Three kinds of constructs each encoding an N-terminal Cys-tag and anti-MTX VHH gene with a different length of linker (GGGS)_n (n = 0, 2, and 4) between them were prepared followed by the expression in Escherichia coli and labeling with several dye maleimides. When the fluorescence intensities in the presence of varied MTX concentrations were measured, TAMRA-labeled nanobodies showed a higher response than ATTO520- or R6G-labeled ones. Especially, TAMRA C6-labeled mini Q-body with no linker showed the highest response of ∼6-fold and a low detection limit of 0.56 nM. When each Trp residue in the mini Q-body was mutated to address the quenching mechanism, the major role of Trp34 at CDR1 in quenching was revealed. Furthermore, the mini Q-body could detect MTX in 50% human serum with a low detection limit of 1.72 nM, showing its applicability to therapeutic drug monitoring. This study is expected to become the basis of the construction of highly responsive mini Q-bodies for sensitive detection of many molecules from small haptens to larger proteins, which will lead to broader applications such as point-of-care tests.

Detection and destruction of HER2-positive cancer cells by Ultra Quenchbody-siRNA complex

Ultra Quenchbody (UQ-body) is a biosensor that utilizes the quenching behavior of the fluorescent dye linked to the antibody V region. When the corresponding antigen is bound to the UQ-body, the fluorescence is restored and allows the detection of target molecules easily and sensitively. In this paper, we constructed UQ-bodies to sensitively detect the human epidermal growth factor receptor 2 (HER2) cancer marker in solution or on cancer cells, which was further used to kill the cancer cells. A synthetic Fab fragment of anti-HER2 antibody Fab37 with many Trp residues at hypervariable region was prepared and labeled with fluorescent dyes to obtain the UQ-bodies. The UQ-body could detect HER2 in solution at concentrations as low as 20 pM with an EC₅₀ of 0.3 nM with a fourfold response. Fluorescence imaging of HER2-positive cells was successfully performed without any washing steps. To deliver small interfering RNA (siRNA) to cancer cells, a modified UQ-body with C-terminal 9R sequence was also prepared. HER2-positive cancer cells were effectively killed by polo-like kinase 1 siRNA intracellularly delivered by the UQ-body-9R. The novel approach employing siRNA-empowered UQ-body could detect and image the HER2 antigen easily and sensitively, and effectively kill the HER2-positive cancer cells.

Development of a fluvoxamine detection system using a Quenchbody, a novel fluorescent biosensor

The misuse of psychotropic drugs intended for medical treatment represents a recent worldwide public health concern. Quenchbody (Q-body) is a novel fluoroimmunosensor that can detect an antigen immediately without additional reagents or washing steps. Here, we describe creating Q-bodies for the detection of the antidepressant fluvoxamine (FLV) and determining optimal conditions to achieve the highest fluorescence intensity (FI). We prepared five Q-bodies with the fluorophore labeled at either the N- or C- terminus and with different linker lengths. Fluorescence was measurable within minutes, indicating the interaction of Q-bodies with FLV. The normalized FI (FI ratio) of the N-terminus labeled Q-body increased approximately 1.5-fold upon FLV addition; Q-bodies labeled at the C-terminus did not significantly increase FI. Among the fluorescence dyes used in this study, Rhodamine 6G labeled Q-body showed the best FI ratio. EC₅₀ values of the N-terminus labeled Q-bodies were similar (23.2-224nM) regardless of linker length or labeling dye. We examined whether the Q-body could be applicable to serum matrix instead of phosphate-buffered saline. The intact serum interfered strongly with the Q-body fluorescence. However, the FI ratios of the Q-body for FLV-spiked serum filtrate, for which proteins were removed by filtration, showed a dose-dependency for detecting FLV levels. Deproteinization, which does not interfere with Q-body fluorescence measurements, is likely necessary to detect serum FLV with high sensitivity. This study demonstrates the potential of Q-body probes as a tool towards developing creative immunoassay applications.

Antigen-responsive fluorescent antibody probes generated by selective N-terminal modification of IgGs

Fluorescent antibody probes showing antigen-dependent fluorescence responses were developed by N-terminal-selective reductive alkylation of IgGs.

Development of a Quenchbody for the Detection and Imaging of the Cancer-Related Tight-Junction-Associated Membrane Protein Claudin

Claudins (CLs) are membrane proteins found in tight junctions and play a major role in establishing the intercellular barrier. However, some CLs are abnormally overexpressed on tumor cells and are valid clinical biomarkers for cancer diagnosis. Here, we constructed antibody Fab fragment-based Quenchbodies (Q-bodies) as effective and reliable fluorescent sensors for detecting and visualizing CLs on live tumor cells. The variable region genes for anti-CL1 and anti-CL4 antibodies were used to express recombinant Fab fragments, and clones recognizing CL4 with high affinity were selected for making Q-bodies. When two fluorescent dyes were conjugated to the N-terminal tags attached to the Fab, the fluorescent signal was significantly increased after adding nanomolar-levels of purified CL4. Moreover, addition of the Q-body to CL4-expressing cells including CL4-positive cancer cells led to a clear fluorescence signal with low background, even without washing steps. Our findings suggested that such Q-bodies would serve as a potent tool for specifically illuminating membrane targets expressed on cancer cells, both in vitro and in vivo.

Flashbody: A Next Generation Fluobody with Fluorescence Intensity Enhanced by Antigen Binding

Fluorescent probes are valuable tools for visualizing the spatiotemporal dynamics of molecules in living cells. Here we developed a genetically encoded antibody probe with antigen-dependent fluorescence intensity called "Flashbody". We first created a fusion of EGFP to the single chain variable region fragment (scFv) of antibody against seven amino acids of the bone Gla protein C-terminus (BGPC7) called BGP Fluobody, which successfully showed the intracellular localization of BGPC7-tagged protein. To generate BGP Flashbody, circularly permuted GFP was inserted in between two variable region fragments, and the linkers were optimized, resulting in fluorescence intensity increase of 300% upon binding with BGPC7 in a dose-dependent manner. Live-cell imaging using BGP Flashbody showed that BGPC7 fused with cell penetrating peptide was able to enter through the plasma membrane by forming a nucleation zone, while it penetrated the nuclear membrane with different mechanism. The construction of Flashbody will be possible for a range of antibody fragments and opens up new possibilities for visualizing a myriad of molecules of interest.