The role of intra-domain disulfide bonds in heat-induced irreversible denaturation of camelid single domain VHH antibodies

A novel sustainable immunoassay for sensitive detection of atrazine based on the anti-idiotypic nanobody and recombinant full-length antibody

Immunoassays have been widely used to determine small-molecule compounds in food and the environment, meeting the challenge of obtaining false positive or negative results because of the variance in the batches of antibodies and antigens. To resolve this problem, atrazine (ATR) was used as a target, and anti-idiotypic nanobodies for ATR (AI-Nbs) and a recombinant full-length antibody against ATR (ATR-rAb) were prepared for the development of a sustainable enzyme-linked immunosorbent assay (ELISA). AI-Nb-7, AI-Nb-58, and AI-Nb-66 were selected from an immune phage display library. ATR-rAb was produced in mammalian HEK293 (F) cells. Among the four detection methods explored, the assay using AI-Nb-66 as a coating antigen and ATR-rAb as a detection reagent yielded a half maximal inhibitory concentration (IC50) of 1.66 ng mL-1 for ATR and a linear range of 0.35-8.73 ng mL-1. The cross-reactivity of the assay to ametryn was 64.24%, whereas that to terbutylazine was 38.20%. Surface plasmon resonance (SPR) analysis illustrated that these cross-reactive triazine compounds can bind to ATR-rAb to varying degrees at high concentrations; however, the binding/dissociation kinetic curves and the response values at the same concentration are different, which results in differences in cross-reactivity. Homology modeling and molecular docking revealed that the triazine ring is vital in recognizing triazine compounds. The proposed immunoassay exhibited acceptable recoveries of 84.40-105.36% for detecting fruit, vegetables, and black tea. In conclusion, this study highlights a new strategy for developing sustainable immunoassays for detecting trace pesticide contaminants.

Anti-CTLA-4 nanobody as a promising approach in cancer immunotherapy

Cancer is one of the most common diseases and causes of death worldwide. Since common treatment approaches do not yield acceptable results in many patients, developing innovative strategies for effective treatment is necessary. Immunotherapy is one of the promising approaches that has been highly regarded for preventing tumor recurrence and new metastases. Meanwhile, inhibiting immune checkpoints is one of the most attractive methods of cancer immunotherapy. Cytotoxic T lymphocyte-associated protein-4 (CTLA-4) is an essential immune molecule that plays a vital role in cell cycle modulation, regulation of T cell proliferation, and cytokine production. This molecule is classically expressed by stimulated T cells. Inhibition of overexpression of immune checkpoints such as CTLA-4 receptors has been confirmed as an effective strategy. In cancer immunotherapy, immune checkpoint-blocking drugs can be enhanced with nanobodies that target immune checkpoint molecules. Nanobodies are derived from the variable domain of heavy antibody chains. These small protein fragments have evolved entirely without a light chain and can be used as a powerful tool in imaging and treating diseases with their unique structure. They have a low molecular weight, which makes them smaller than conventional antibodies while still being able to bind to specific antigens. In addition to low molecular weight, specific binding to targets, resistance to temperature, pH, and enzymes, high ability to penetrate tumor tissues, and low toxicity make nanobodies an ideal approach to overcome the disadvantages of monoclonal antibody-based immunotherapy. In this article, while reviewing the cellular and molecular functions of CTLA-4, the structure and mechanisms of nanobodies' activity, and their delivery methods, we will explain the advantages and challenges of using nanobodies, emphasizing immunotherapy treatments based on anti-CTLA-4 nanobodies.

Generation of Synthetic Acinetobacter baumannii-Specific Nanobodies

The bacterial pathogen Acinetobacter baumannii is a leading cause of drug-resistant infections. Here, we investigated the potential of developing nanobodies that can recognize A. baumannii over other Gram-negative bacteria. Through generation and panning of a synthetic nanobody library, we identified several potential lead candidates. We demonstrate how incorporation of next-generation sequencing analysis can aid in the selection of lead candidate nanobodies. Using monoclonal phage display, we validated the binding of lead nanobodies to A. baumannii. Subsequent purification and biochemical characterization revealed one particularly robust nanobody that specifically bound select A. baumannii strains compared to other common drug-resistant pathogens. These findings support the potential for nanobodies to selectively target A. baumannii and the identification of lead candidates for future investigation.

Increasing the melting temperature of VHH with the in silico free energy score

VHH, the antigen-binding fragment of a heavy chain-only antibody, is a useful component of antibody-based therapeutics. Thermal stability, represented by the melting temperature (Tm), is one of the key factors affecting the developability of antibody-based therapeutics. In this study, we examined whether the in silico free energy score dStability can be used to design mutants with improved Tm compared to the anti-lysozyme VHH, D3-L11. After verifying that exhaustive mutagenesis was inefficient for improving Tm, we performed a two-round rational approach that combined dStability calculations with a small number of experiments. This method improved the Tm by more than 5 °C in several single mutants including A79I. It reduced the affinity for the antigen by less than 1.6-fold. We speculate that stabilization of A79I required exquisite compatibility among neighboring residues to fill in the internal cavity in the protein. Given that we identified only one mutation that could simultaneously improve Tm and almost maintain affinity, we concluded that achieving both is extremely difficult, even with single mutations that are not located in the paratope. Therefore, we recommend using a variety of approaches when trying to achieve such a feat. Our method will be a useful complementary approach to other existing methods.

Electrospun Mucoadhesive Zein/PVP Fibroporous Membrane for Transepithelial Delivery of Propranolol Hydrochloride

Mucoadhesive drug delivery systems have been extensively studied to effectively reduce the limitations of conventional drug delivery systems. Zein and polyvinyl pyrrolidone (PVP) are appraised for mucoadhesive properties. This study focuses on developing a mechanically stable zein/PVP electrospun membrane for propranolol hydrochloride (PL) transport. Fourier transform infrared, Raman spectra, and swelling studies gave evidence for PVP crosslinking, whereas circular dichroism spectroscopy revealed crosslinking of zein owing to the conformational change from α-helix to β-sheet. A 10 h thermal treatment of zein/PVP imparted 3.92 ± 0.13 MPa tensile strength to the matrix. Thermally crosslinked electrospun zein/PVP matrix showed 22.1 ± 0.1 g mm work of adhesion in porcine buccal mucosa tissue. Qualitative and quantitative evaluation of cytotoxicity in RPMI 2650 has been carried out. The in vitro drug release profile of PL from thermally crosslinked zein/PVP best fitted with the Korsmeyer-Peppas model. Immunostaining of β-catenin adherens junctional protein confirmed the absence of paracellular transport through the junctional opening. Still, drug permeation was observed through the porcine buccal mucosa, attributed to the transcellular transport of PL owing to its lipophilicity. The ex vivo permeation of PL through porcine buccal mucosa was also evaluated.

Intracellular VHHs to monitor and modulate GPCR signaling

Single-domain antibody fragments, also known as VHHs or nanobodies, have opened promising avenues in therapeutics and in exploration of intracellular processes. Because of their unique structural properties, they can reach cryptic regions in their cognate antigen. Intracellular VHHs/antibodies primarily directed against cytosolic proteins or transcription factors have been described. In contrast, few of them target membrane proteins and even less recognize G protein-coupled receptors. These receptors are major therapeutic targets, which reflects their involvement in a plethora of physiological responses. Hence, they elicit a tremendous interest in the scientific community and in the industry. Comprehension of their pharmacology has been obscured by their conformational complexity, that has precluded deciphering their structural properties until the early 2010's. To that respect, intracellular VHHs have been instrumental in stabilizing G protein-coupled receptors in active conformations in order to solve their structure, possibly bound to their primary transducers, G proteins or β-arrestins. In contrast, the modulatory properties of VHHs recognizing the intracellular regions of G protein-coupled receptors on the induced signaling network have been poorly studied. In this review, we will present the advances that the intracellular VHHs have permitted in the field of GPCR signaling and trafficking. We will also discuss the methodological hurdles that linger the discovery of modulatory intracellular VHHs directed against GPCRs, as well as the opportunities they open in drug discovery.

A Cell-free Expression Pipeline for the Generation and Functional Characterization of Nanobodies

Cell-free systems are well-established platforms for the rapid synthesis, screening, engineering and modification of all kinds of recombinant proteins ranging from membrane proteins to soluble proteins, enzymes and even toxins. Also within the antibody field the cell-free technology has gained considerable attention with respect to the clinical research pipeline including antibody discovery and production. Besides the classical full-length monoclonal antibodies (mAbs), so-called "nanobodies" (Nbs) have come into focus. A Nb is the smallest naturally-derived functional antibody fragment known and represents the variable domain (V_HH, ∼15 kDa) of a camelid heavy-chain-only antibody (HCAb). Based on their nanoscale and their special structure, Nbs display striking advantages concerning their production, but also their characteristics as binders, such as high stability, diversity, improved tissue penetration and reaching of cavity-like epitopes. The classical way to produce Nbs depends on the use of living cells as production host. Though cell-based production is well-established, it is still time-consuming, laborious and hardly amenable for high-throughput applications. Here, we present for the first time to our knowledge the synthesis of functional Nbs in a standardized mammalian cell-free system based on Chinese hamster ovary (CHO) cell lysates. Cell-free reactions were shown to be time-efficient and easy-to-handle allowing for the "on demand" synthesis of Nbs. Taken together, we complement available methods and demonstrate a promising new system for Nb selection and validation.

Kinetic and Thermodynamic Study of Plantaricin IIA-1A5, a Bacteriocin Produced by Indonesian Probiotic Lactobacillus plantarum IIA-1A5

BACKGROUND

Plantaricin IIA-1A5 is a bacteriocin produced by Lactobacillus plantarum IIA-1A5, a locally isolated probiotic from Indonesia. Plantaricin IIA-1A5 exhibits antibacterial activity against wide spectrum of pathogenic bacteria, thus promising to be applied in various food products. Nevertheless, thermal stability of this bacteriocin remains to be fully investigated.

OBJECTIVE

This study aims to determine thermal stability of plantaricin IIA-1A5 through kinetic and thermodynamic parameters.

METHOD

To address, plantaricin IIA-1A5 was purified from Lactobacillus plantarum IIA-1A5, which was growth under whey media, using ammonium sulfate precipitation followed by ionexchange chromatography. Purified plantaricin IIA-IA5 was then subjected to analysis of its bacteriocin activity. The thermal inactivation of bacteriocin from L. plantarum IIA-1A5 was calculated by incubating the bacteriocin at different temperatures ranging from 60-80 °C for 30 to 90 min, which was then used to calculate its kinetic and thermodynamic parameters.

RESULTS

The result showed the inactivation rates (k-value) were ranging from 0.008 to 0.013 min-1. Heat resistance of plantaricin IIA-1A5 (D-value) at constant heating temperature of 60, 65, 70, 75, and 80 °C were 311.6, 305.9, 294.5, 198.9, and 180.2 min, which indicated a faster inactivation at higher temperatures. D-value sensitivity for temperature changes (z-value) was calculated to be 75.76 °C. Further, thermodynamic analysis suggested that plantaricin IIA-1A5 is thermostable, with activation energy (Ea) of 29.02 kJ mol-1.

CONCLUSION

This result showed that plantaricin IIA-1A5 is considerably more heat-stable than plantaricin members and promises to be applied in food industries where heat treatments are applied. Furthermore, a possible mechanism by which plantaricin IIA-1A5 maintains its stability was also discussed by referring to its thermodynamic parameters.

Nanobody-Based Indirect Competitive ELISA for Sensitive Detection of 19-Nortestosterone in Animal Urine

Nanobody (Nb), a new type of biorecognition element generally from Camelidae, has the characteristics of small molecular weight, high stability, great solubility and high expression level in E. coli. In this study, with 19-nortestosterone (19-NT), an anabolic androgenic steroid as target drug, three specific Nbs against 19-NT were selected from camel immune library by phage display technology. The obtained Nbs showed excellent thermostability and organic solvent tolerance. The nanobody Nb2F7 with the best performance was used to develop a sensitive indirect competitive enzyme-linked immunosorbent assay (ic-ELISA) for 19-NT detection. Under optimized conditions, the standard curve of ic-ELISA was fitted with a half-maximal inhibitory concentration (IC₅₀) of 1.03 ng/mL and a detection limit (LOD) of 0.10 ng/mL for 19-NT. Meanwhile, the developed assay had low cross- reactivity with analogs and the recoveries of 19-NT ranged from 82.61% to 99.24% in spiked samples. The correlation coefficient between ic-ELISA and the ultra-performance liquid chromatography/mass spectrometry (UPLC-MS/MS) method was 0.9975, which indicated that the nanobody-based ic-ELISA could be a useful tool for a rapid analysis of 19-NT in animal urine samples.

Nanobodies targeting immune checkpoint molecules for tumor immunotherapy and immunoimaging (Review)

The immune checkpoint blockade is an effective strategy to enhance the anti-tumor T cell effector activity, thus becoming one of the most promising immunotherapeutic strategies in the history of cancer treatment. Several immune checkpoint inhibitor have been approved by the FDA, such as anti-CTLA-4, anti-PD-1, anti-PD-L1 monoclonal antibodies. Most tumor patients benefitted from these antibodies, but some of the patients did not respond to them. To increase the effectiveness of immunotherapy, including immune checkpoint blockade therapies, miniaturization of antibodies has been introduced. A single-domain antibody, also known as nanobody, is an attractive reagent for immunotherapy and immunoimaging thanks to its unique structural characteristic consisting of a variable region of a single heavy chain antibody. This structure confers to the nanobody a light molecular weight, making it smaller than conventional antibodies, although remaining able to bind to a specific antigen. Therefore, this review summarizes the production of nanobodies targeting immune checkpoint molecules and the application of nanobodies targeting immune checkpoint molecules in immunotherapy and immunoimaging.

Antibody stability: A key to performance - Analysis, influences and improvement

An antibody's stability greatly influences its performance (i.e. its specificity and affinity). Thus, stability is a major issue for researchers and manufacturers, especially with the increasing use of antibodies in therapeutics, diagnostics and rapid analytical platforms. Here we review antibody stability under five headings: (i) measurement techniques; (ii) stability issues in expression and production (expression, proteolysis, aggregation); (iii) effects of antibody format and engineering on stability and (iv) formulation, drying and storage conditions. We consider more than 100 sources, including patents, and conclude with (v) recommendations to promote antibody stability.

PCR-based approach for site-specific conjugation of long double-stranded DNA to a single-domain VHH antibody

Site-specific conjugation of double-stranded DNA using antibodies enables the development of unique applications for antibody-drug conjugates utilizing recent advances in nucleic acid medicines. Here, we describe a novel method to conjugate a camelid-derived single-domain VHH (variable domain of a heavy chain antibody) antibody with arbitrarily sized double-stranded DNA by PCR. Cysteine in anti-human epidermal growth factor receptor (EGFR) VHH was replaced by alanine, and an unpaired cysteine was introduced at the carboxyl terminus. These modifications enabled site-specific labelling with a maleimide-modified DNA oligo via thioether bond formation; the ensuing product-single-stranded DNA conjugated at the carboxyl terminus of VHH-retained its affinity for EGFR. To investigate whether this VHH-single-stranded DNA conjugate might be used as a forward primer, we subjected it to PCR, producing 100-500 bp DNA. We confirmed the amplification of the VHH-double-stranded DNA conjugate by examining its mobility on acrylamide gel; retention of the binding affinity of the conjugate for EGFR was identified by immuno-PCR.

Development of a Simple Pretreatment Immunoassay Based on an Organic Solvent-Tolerant Nanobody for the Detection of Carbofuran in Vegetable and Fruit Samples

Nanobodies are one-tenth the size of conventional antibodies and are naturally obtained from the atypical heavy-chain-only antibodies present in camelids. Their small size, high solubility, high stability, and strong resilience to organic solvents facilitate their use as novel analytical reagents in immunochemistry. In this study, specific nanobodies against pesticide carbofuran were isolated and characterized from an immunized library via phage display platform. We further established an indirect competitive enzyme-linked immunosorbent assay (ELISA) using nanobody Nb316 to detect carbofuran in vegetable and fruit samples. The results showed a half-maximal inhibitory concentration (IC₅₀) of 7.27 ng/mL and a detection limit of 0.65 ng/mL. A simplified sample pretreatment procedure omitting the evaporation of organic solvent was used. The averaged recovery rate of spiked samples ranged between 82.3% and 103.9%, which correlated with that of standard UPLC–MS/MS method. In conclusion, a nanobody with high specificity for carbofuran was characterized, and a nanobody-based sensitive immunoassay for simple and rapid detection of carbofuran in real samples was validated.

Removal of a Conserved Disulfide Bond Does Not Compromise Mechanical Stability of a VHH Antibody Complex

Single-domain VHH antibodies are promising reagents for medical therapy. A conserved disulfide bond within the VHH framework region is known to be critical for thermal stability, however, no prior studies have investigated its influence on the stability of VHH antibody-antigen complexes under mechanical load. Here, we used single-molecule force spectroscopy to test the influence of a VHH domain's conserved disulfide bond on the mechanical strength of the interaction with its antigen mCherry. We found that although removal of the disulfide bond through cysteine-to-alanine mutagenesis significantly lowered VHH domain denaturation temperature, it had no significant impact on the mechanical strength of the VHH:mCherry interaction with complex rupture occurring at ∼60 pN at 103-104 pN/sec regardless of disulfide bond state. These results demonstrate that mechanostable binding interactions can be built on molecular scaffolds that may be thermodynamically compromised at equilibrium.

High Expression Achievement of Active and Robust Anti-β2 microglobulin Nanobodies via E.coli Hosts Selection

Nanobodies (VHHs) overcome many of the drawbacks of conventional antibodies, and the related technologies represent state-of-the-art and advanced applications in scientific research, pharmaceuticals, and therapies. In terms of productivity and economic cost, the cytoplasmic expression of VHHs in Escherichia coli (E. coli) is a good process for their recombinant production. The cytoplasmic environment of the host is critical to the affinity and stability of the recombinant VHHs in soluble form, yet the effects have not been studied. For this purpose, recombinant anti-β2 microglobulin VHHs were constructed and expressed in four commercialized E. coli hosts, including BL21 (DE3), Rosetta-gami B (DE3) pLysS, Origami 2 (DE3) and SHuffle T7 Express. The results showed that anti-β2 microglobulin (β2MG) VHHs expressed in different hosts exhibited distinctive differences in the affinity and structural characteristics. The VHHs expressed in Rosetta-gami B (DE3) pLysS possessed not only the greatest affinity of (equilibrium dissociation constant) K_D = 4.68 × 10⁻⁸ M but also the highest yields compared with the VHHs expressed in BL21 (DE3), Origami 2 (DE3) and SHuffle T7 Express. In addition, the VHHs expressed in Rosetta-gami B (DE3) pLysS were more stable than the VHHs expressed in the rest three hosts. Thus far, we have successfully realized the high expression of the active and robust anti-β2MG VHHs in Rosetta-gami B (DE3) pLysS. The underlying principle of our study is able to guide the expression strategies of nanobodies on the context of industrial large-scale production.

Isolation of Bactrian Camel Single Domain Antibody for Parathion and Development of One-Step dc-FEIA Method Using VHH-Alkaline Phosphatase Fusion Protein

A heavy chain variable fragment of heavy chain only antibodies derived from camelids termed VHH shows beneficial characteristics for immunoassay in terms of high sensitivity, outstanding stability and ease in expression. In the present study, we isolated six VHHs from phage display library against parathion, which is a widely used organophosphorus pesticide with high toxicity and persistence. One of six selected VHHs named VHH9, showed highest specificity and superior thermo-stability. A VHH9-alkaline phosphatase (AP) fusion was constructed and used to establish a one-step direct competitive fluorescence enzyme immunoassay (dc-FEIA) with a half maximal inhibitory concentration (IC50) of 1.6 ng/mL and a limit of detection of 0.2 ng/mL which was 4-fold or 3-fold higher sensitivity than direct competitive enzyme-linked immunoassay (dc-ELISA) and indirect competitive enzyme-linked immunoassay (ic-ELISA) for parathion. Furthermore, our assay indicated a 50% reduction on operation time compared with the ic-ELISA method. The presented immunoassay was validated with spiked Chinese cabbage, cucumber, and lettuce samples, and confirmed by UPLC-MS/MS. The results indicated that the VHH-AP-based dc-FEIA is a reproducible detection assay for parathion residues in vegetable samples.

The structural basis of nanobody unfolding reversibility and thermoresistance

Nanobodies represent the variable binding domain of camelid heavy-chain antibodies and are employed in a rapidly growing range of applications in biotechnology and biomedicine. Their success is based on unique properties including their reported ability to reversibly refold after heat-induced denaturation. This view, however, is contrasted by studies which involve irreversibly aggregating nanobodies, asking for a quantitative analysis that clearly defines nanobody thermoresistance and reveals the determinants of unfolding reversibility and aggregation propensity. By characterizing nearly 70 nanobodies, we show that irreversible aggregation does occur upon heat denaturation for the large majority of binders, potentially affecting application-relevant parameters like stability and immunogenicity. However, by deriving aggregation propensities from apparent melting temperatures, we show that an optional disulfide bond suppresses nanobody aggregation. This effect is further enhanced by increasing the length of a complementarity determining loop which, although expected to destabilize, contributes to nanobody stability. The effect of such variations depends on environmental conditions, however. Nanobodies with two disulfide bonds, for example, are prone to lose their functionality in the cytosol. Our study suggests strategies to engineer nanobodies that exhibit optimal performance parameters and gives insights into general mechanisms which evolved to prevent protein aggregation.

Heat denaturation of the antibody, a multi-domain protein

The antibody is one of the most well-studied multi-domain proteins because of its abundance and physiological importance. In this article, we describe the effect of the complex, multi-domain structure of the antibody on its denaturation by heat. Natural antibodies are composed of 6 to 70 immunoglobulin fold domains, and are irreversibly denatured at high temperatures. Although the separated single immunoglobulin fold domain can be refolded after heat denaturation, denaturation of pairs of such domains is irreversible. Each antibody subclass exhibits a distinct heat tolerance, and IgE is especially known to be heat-labile. IgE starts unfolding at a lower temperature compared to other antibodies, because of the low stability of its CH3 domain. Each immunoglobulin domain starts unfolding at different temperatures. For instance, the CH3 domain of IgG unfolds at a higher temperature than its CH2 domain. Thus, the antibody has a mixture of folded and unfolded structures at a certain temperature. Co-existence of these folded and unfolded domains in a single polypeptide chain may increase the tendency to aggregate which causes the inactivation of the antibody.

Evaluation of irreversible protein thermal inactivation caused by breakage of disulphide bonds using methanethiosulphonate

Many extracellular globular proteins have evolved to possess disulphide bonds in their native conformations, which aids in thermodynamic stabilisation. However, disulphide bond breakage by heating leads to irreversible protein denaturation through disulphide-thiol exchange reactions. In this study, we demonstrate that methanethiosulphonate (MTS) specifically suppresses the heat-induced disulphide-thiol exchange reaction, thus improving the heat-resistance of proteins. In the presence of MTS, small globular proteins that contain disulphides can spontaneously refold from heat-denatured states, maintaining wild-type disulphide pairing. Because the disulphide-thiol exchange reaction is triggered by the generation of catalytic amounts of perthiol or thiol, rapid and specific perthiol/thiol protection by MTS reagents prevents irreversible denaturation. Combining MTS reagents with another additive that suppresses chemical modifications, glycinamide, further enhanced protein stabilisation. In the presence of these additives, reliable remnant activities were observed even after autoclaving. However, immunoglobulin G and biotin-binding protein, which are both composed of tetrameric quaternary structures, failed to refold from heat-denatured states, presumably due to chaperon requirements. Elucidation of the chemical modifications involved in irreversible thermoinactivation is useful for the development of preservation buffers with optimum constitutions for specific proteins. In addition, the impact of disulphide bond breakage on the thermoinactivation of proteins can be evaluated using MTS reagents.

Single-Domain Antibodies As Versatile Affinity Reagents for Analytical and Diagnostic Applications

With just three CDRs in their variable domains, the antigen-binding site of camelid heavy-chain-only antibodies (HcAbs) has a more limited structural diversity than that of conventional antibodies. Even so, this does not seem to limit their specificity and high affinity as HcAbs against a broad range of structurally diverse antigens have been reported. The recombinant form of their variable domain [nanobody (Nb)] has outstanding properties that make Nbs, not just an alternative option to conventional antibodies, but in many cases, these properties allow them to reach analytical or diagnostic performances that cannot be accomplished with conventional antibodies. These attributes include comprehensive representation of the immune specificity in display libraries, easy adaptation to high-throughput screening, exceptional stability, minimal size, and versatility as affinity building block. Here, we critically reviewed each of these properties and highlight their relevance with regard to recent developments in different fields of immunosensing applications.

Enhancing Stability of Camelid and Shark Single Domain Antibodies: An Overview

Single domain antibodies (sdAbs) are gaining a reputation as superior recognition elements as they combine the advantages of the specificity and affinity found in conventional antibodies with high stability and solubility. Melting temperatures (T_ms) of sdAbs cover a wide range from below 50 to over 80°C. Many sdAbs have been engineered to increase their T_m, making them stable until exposed to extreme temperatures. SdAbs derived from the variable heavy chains of camelid and shark heavy chain-only antibodies are termed VHH and VNAR, respectively, and generally exhibit some ability to refold and bind antigen after heat denaturation. This ability to refold varies from 0 to 100% and is a property dependent on both intrinsic factors of the sdAb and extrinsic conditions such as the sample buffer ionic strength, pH, and sdAb concentration. SdAbs have also been engineered to increase their solubility and refolding ability, which enable them to function even after exposure to temperatures that exceed their melting point. In addition, efforts to improve their stability at extreme pH and in the presence of chemical denaturants or proteases have been undertaken. Multiple routes have been employed to engineer sdAbs with these enhanced stabilities. The methods utilized to achieve these goals include grafting complementarity-determining regions onto stable frameworks, introduction of non-canonical disulfide bonds, random mutagenesis combined with stringent selection, point mutations such as inclusion of negative charges, and genetic fusions. Increases of up to 20°C have been realized, pushing the T_m of some sdAbs to over 90°C. Herein, we present an overview of the work done to stabilize sdAbs derived from camelids and sharks. Utilizing these various strategies sdAbs have been stabilized without significantly compromising their affinity, thereby providing superior reagents for detection, diagnostic, and therapeutic applications.

Selection and characterization of specific nanobody against bovine virus diarrhea virus (BVDV) E2 protein

Bovine viral diarrhea-mucosal disease (BVD-MD) is caused by bovine viral diarrhea virus (BVDV), and results in abortion, stillbirth, and fetal malformation in cows. Here, we constructed the phage display vector pCANTAB 5E-VHH and then transformed it into Escherichia coli TG1-competent cells, to construct an initial anti-BVDV nanobody gene library. We obtained a BVDV-E2 antigen epitope bait protein by prokaryotic expression using the nucleotide sequence of the E2 gene of the BVDV-NADL strain published in GenBank. Phage display was used to screen the anti-BVDV nanobody gene library. We successfully constructed a high quality phage display nanobody library, with an initial library capacity of 4.32×105. After the rescue of helper phage, the titer of the phage display nanobody library was 1.3×1011. The BVDV-E2 protein was then expressed in Escherichia coli (DE3), and a 49.5 kDa band was observed with SDS-PAGE analysis that was consistent with the expected nanobody size. Thus, we were able to isolate one nanobody that exhibits high affinity and specificity against BVDV using phage display techniques. This isolated nanobody was then used in Enzyme Linked Immunosorbent Assay and qRT-PCR, and ELISA analyses of BVDV infection of MDBK cells indicated that the nanobodies exhibited good antiviral effect.

VHH antibodies: emerging reagents for the analysis of environmental chemicals

A VHH antibody (or nanobody) is the antigen binding fragment of heavy chain only antibodies. Discovered nearly 25 years ago, they have been investigated for their use in clinical therapeutics and immunodiagnostics, and more recently for environmental monitoring applications. A new and valuable immunoreagent for the analysis of small molecular weight environmental chemicals, VHH will overcome many pitfalls encountered with conventional reagents. In the work so far, VHH antibodies often perform comparably to conventional antibodies for small molecule analysis, are amenable to numerous genetic engineering techniques, and show ease of adaption to other immunodiagnostic platforms for use in environmental monitoring. Recent reviews cover the structure and production of VHH antibodies as well as their use in clinical settings. However, no report focuses on the use of these VHH antibodies to detect small environmental chemicals (MW < 1500 Da). This review article summarizes the efforts made to produce VHHs to various environmental targets, compares the VHH-based assays with conventional antibody assays, and discusses the advantages and limitations in developing these new antibody reagents particularly to small molecule targets. Graphical Abstract Overview of the production of VHHs to small environmental chemicals and highlights of the utility of these new emerging reagents.