Development of a new monoclonal antibody based direct competitive enzyme-linked immunosorbent assay for detection of brevetoxins in food samples

Identification of three conserved linear B cell epitopes on the SARS-CoV-2 spike protein

Spike (S) glycoproteins is the most significant structural protein of SARS-CoV-2 and a key target for neutralizing antibodies. In light of the ongoing SARS-CoV-2 pandemic, identification and screening of epitopes of spike glycoproteins will provide vital progress in the development of sensitive and specific diagnostic tools. In the present study, NTD, RBD and S2 gene were inserted to the pcDNA3.1(+) vector and designed with N-terminal 6×His-tag for fusion expression in HEK293F cells by transient transfection. Six monoclonal antibodies (4G, 9E, 4B, 7D, 8F, 3D) were prepared using the expressed proteins by cell fusion technique. The characterization of mAbs were performed by indirect -ELISA, western blot and IFA. We designed 49 overlapping synthetized peptides cover the extracellular region of S protein which 6 amino acid residues were offset between adjacent (S1-S49). Peptides S12, S19 and S49 were identified as the immunodominant epitopes regions by the mAbs. These regions were further truncated and the peptides S12.2 ²⁸⁶TDAVDCALDPLS²⁹⁷, S19.2 ⁴⁶⁴FERDISTEIYQA⁴⁷⁵ and S49.4 ¹²⁰²ELGKYEQYIKWP¹²¹³ were identified as B- cell linear epitopes for the first time. Alanine scans showed that, the ^D467, ^I468, ^E471, ^Q474, ^A475 of the epitope S19.2 and ^K1205, ^Q1208, ^Y1209 of the epitope S49.4 were the core sites involved in the mAbs binding. Multiple sequence alignment analysis showed that these three epitopes were highly conserved among the variants of concern (VOCs) and variants of interest (VOIs). Taken together, the findings provide a potential material for rapid diagnosis methods of COVID-19.

Screening and identification of B cell epitope of the nucleocapsid protein in SARS-CoV-2 using the monoclonal antibodies

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the virus that causes the coronavirus disease (COVID-19). It is confirmed that nucleocapsid (N) protein is closely related to viral pathogenesis, modulation of host immune response, RNA transcription, and replication and virus packaging. Therefore, the N protein is a preponderant antigen target for virus detection. The codon-optimized N gene was designed according to the encoding characteristics of insect cells and inserted into pFastBac^TM1 vector with 6 × His-tag-fused N protein for expression in insect sf21 cells. Six anti-N mAbs (4G3, 5B3, 12B6, 18C7-A2, 21H10-A3, 21H10-E9) were prepared by recombinant N protein. The mAbs showed high titers, antibody affinity, and reactivity with the SARS-CoV-2 N protein. Then, fourteen overlapped peptides that covered the intact N protein were synthesized (N1-N14). Peptide N14 was identified as the main linear B-cell epitope region via peptide-ELISA and dot-blot assay, and this region was truncated gradually until mapping the peptide 401-DFSKQLQQ-408. Simultaneously, compared with the sequence of variants of concern (VOCs) and variants of interest (VOIs) strains among the several countries, epitope 401-DFSKQLQQ-408 is very conservative among them. The findings provide new guidance for the design and detection of COVID-19 targets. KEY POINTS: • The N protein was optimized according to the insect cell codon preference and was highly expressed. • The monoclonal antibodies prepared in this study were shown high antibody titers and high affinity. • Monoclonal antibodies were used to map the epitope 401-408 amino acids of N protein for the first time in this study.

Spectroscopic ellipsometry methods for brevetoxin detection

The spectroscopic ellipsometry (SE), and attenuated internal reflection spectroscopic ellipsometry (TIRE) are promising methods in label-free biosensing applications. An ellipsometer running under surface plasmon resonance (SPR) conditions has unique advantages over other SPR-based methods in terms of sensitivity and real-time/label-free measurement capability. In this study, both SE and TIRE-based brevetoxin B (BTX) sensors were developed using two anti-BTX aptamers reported before. A new aptamer sequence was also derived from these two antiBTX aptamers using predictive modeling tools and an exclusion method. All three antiBTX aptamers' analytical performances were quite competitive in terms of both detecting range and detection limits. However, the selectivity of the previously reported aptamers against analogs of BTX was poor at low detection ranges, especially for okadaic acid. Furthermore, the selectivity of the derived aptamer was lower than its predecessors. The sensors were capable of detecting BTX in the range of 0.05 nM-1600 nM in the TIRE and 0.5 nM-2000 nM in the SE configuration. The detection limits of the sensors were 1.48 nM (1.32 ng/mL) and 0.80 nM (0.72 ng/mL) for SE and TIRE configurations, respectively. Both configurations have been used successfully to detect BTX standards spiked into real fish and shrimp samples.

Development of a New Monoclonal Antibody against Brevetoxins in Oyster Samples Based on the Indirect Competitive Enzyme-Linked Immunosorbent Assay

The consumption of shellfish contaminated with brevetoxins, a family of ladder-frame polyether toxins formed during blooms of the marine dinoflagellate Karenia brevis, can cause neurotoxic poisoning, leading to gastroenteritis and neurotoxic effects. To rapidly monitor brevetoxin levels in oysters, we generated a broad-spectrum antibody against brevetoxin 2 (PbTx-2), 1 (PbTx-1), and 3 (PbTx-3) and developed a rapid indirect competitive enzyme-linked immunosorbent assay (icELISA). PbTx-2 was reacted with carboxymethoxylamine hemihydrochloride (CMO) to generate a PbTx-2-CMO hapten and reacted with succinic anhydride (HS) to generate the PbTx-2-HS hapten. These haptens were conjugated to keyhole limpet hemocyanin (KLH) and bovine serum albumin (BSA) to prepare immunogen and coating antigen reagents, respectively, using the active ester method. After immunization and cell fusion, a broad-spectrum monoclonal antibody (mAb) termed mAb 1D3 was prepared. The 50% inhibitory concentration (IC₅₀) values of the icELISA for PbTx-2, PbTx-1, and PbTx-3 were 60.71, 52.61, and 51.83 μg/kg, respectively. Based on the broad-spectrum mAb 1D3, an icELISA was developed to determine brevetoxin levels. Using this approach, the limit of detection (LOD) for brevetoxin was 124.22 μg/kg and recoveries ranged between 89.08% and 115.00%, with a coefficient of variation below 4.25% in oyster samples. These results suggest that our icELISA is a useful tool for the rapid monitoring of brevetoxins in oyster samples.

Characterization of a Novel Interleukin-1 Receptor Antagonist from Sheep (Ovis aries)

Interleukin-1 receptor antagonist (IL-1Ra) is an antagonist of IL-1β binding IL-1β receptors but does not induce intracellular responses or signal transduction. In this study, the full-length complementary DNA (cDNA) of the IL-1Ra gene (OaIL-1Ra) was identified from sheep (Ovis aries) using rapid amplification of cDNA ends PCR and submitted to GenBank with the accession number KC425613. The OaIL-1Ra cDNA comprised an open reading frame of 525 bp encoding a protein of 19765.8 Da, a 5'-untranslated region (UTR) of 27 bp, and a 3'-UTR of 676 bp with a poly(A) tail. Recombinant OaIL-1Ra with bioactivity was expressed in a prokaryotic expression system, and a monoclonal antibody against native OaIL-1Ra was prepared. Through Western blot analyses, the OaIL-1Ra protein was widely expressed in lung, heart, spleen, liver, kidney, muscle, intestine, lymphonodi, rumen, and white blood cells, with the highest levels in liver and spleen. The expression of OaIL-1Ra in primary cultured white blood cells of sheep were highly induced in a time-dependent manner when challenged with different bacteria. These results implied that OaIL-1Ra is associated with immune responses during bacterial infections.

Preparation of a Specific ssDNA Aptamer for Brevetoxin-2 Using SELEX

The existing assays for detecting brevetoxin (BTX) depend on expensive equipment with a professional operator or on an antibody with limited stability, which requires complex processes, a high cost, and a considerable amount of time. The development of an alternative detection probe is another promising research direction. This paper reports the use of aptamers binding to BTX-2 in an analytical assay using the systematic evolution of ligands by exponential enrichment (SELEX). After 12 rounds of selection, the secondary structures of 25 sequences were predicted. Compared to other aptamers, Bap5 has relatively high affinity with the lowest dissociation constant of 4.83 μM, and IC₅₀ is 73.81 ng mL^-1. A good linear regression formula of y = 30.688x - 7.329 with a coefficient correlation of R² = 0.9798 was obtained using a biotin-avidin ELISA. Moreover, there is no cross-reaction with the detected marine toxins, except for BTX-2. Thus, Bap5 has potential to detect BTX-2 in shellfish in the future as a substitute for the recognition probe.

Generation of Internal-Image Functional Aptamers of Okadaic Acid via Magnetic-Bead SELEX

Okadaic acid (OA) is produced by Dinophysis and Prorocentrum dinoflagellates and primarily accumulates in bivalves, and this toxin has harmful effects on consumers and operators. In this work, we first report the use of aptamers as novel non-toxic probes capable of binding to a monoclonal antibody against OA (OA-mAb). Aptamers that mimic the OA toxin with high affinity and selectivity were generated by the magnetic bead-assisted systematic evolution of ligands by exponential enrichment (SELEX) strategy. After 12 selection rounds, cloning, sequencing and enzyme-linked immunosorbent assay (ELISA) analysis, four candidate aptamers (O24, O31, O39, O40) were selected that showed high affinity and specificity for OA-mAb. The affinity constants of O24, O31, O39 and O40 were 8.3 × 10⁸ M⁻¹, 1.47 × 10⁹ M⁻¹, 1.23 × 10⁹ M⁻¹ and 1.05 × 10⁹ M⁻¹, respectively. Indirect competitive ELISA was employed to determine the internal-image function of the aptamers. The results reveal that O31 has a similar competitive function as free OA toxin, whereas the other three aptamers did not bear the necessary internal-image function. Based on the derivation of the curvilinear equation for OA/O31, the equation that defined the relationship between the OA toxin content and O31 was Y = 2.185X − 1.78. The IC₅₀ of O31 was 3.39 ng·mL⁻¹, which was close to the value predicted by the OA ELISA (IC₅₀ = 4.4 ng·mL⁻¹); the IC₁₀ was 0.33 ng·mL⁻¹. The above data provides strong evidence that internal-image functional aptamers could be applicable as novel probes in a non-toxic assay.

Aptamer-based competitive electrochemical biosensor for brevetoxin-2

Brevetoxins (BTXs) are very potent marine neurotoxins that increased in geographical distribution in the past decade causing the illness clinically described as neurological shellfish poisoning (NSP). The ethical problems as well as the technical difficulties associated with the currently employed analysis methods for marine toxins are encouraging the research for suitable alternatives to be applied in a regulatory monitoring regime. Here, we report an electrochemical biosensor platform for BTX-2 detection utilising aptamer as specific receptor. Using in vitro selection, high affinity DNA aptamers to BTX-2 were successfully selected for the first time from a large pool of random sequences. The binding of BTX-2 to aptamer pools/clones was monitored using fluorescence and electrochemical impedance spectroscopy (EIS). The aptamer BT10 exhibited the highest binding affinity to BTX-2, with a dissociation constant of 42nM. The effects of the incubation time, pH and metal ions concentrations on the aptamer-toxin binding were studied. The aptamer BT10 was used to construct a label-free competitive impedimetric biosensor for BTX-2 achieving a detection limit of 106pg/ml. We observed a high degree of cross reactivity of the selected aptamer to the two similar congeners, BTX-2 and -3, whereas no cross reactivity to other marine toxins was obtained. Moreover, the aptasensor was applied for the detection of BTX-2 in spiked shellfish extract showing a very high recovery percentage. We believe that the proposed aptasensor will facilitate the routine detection of BTX-2 in food samples.

Displacement-type quartz crystal microbalance immunosensing platform for ultrasensitive monitoring of small molecular toxins

A novel displacement-type quartz crystal microbalance (QCM) immunosensing strategy, based on glucose and its analogue dextran for concanavalin A (ConA) binding sites, was designed for ultrasensitive monitoring of small molecular biotoxins (brevetoxin B, PbTx-2, used as a model) with signal amplification on a graphene-functionalized sensing interface. To construct such a QCM immunosensing platform, phenoxy-functionalized dextran (DexP) was initially assembled onto the surface of graphene-coated QCM probe via the π-stacking interaction, and ConA-labeled monoclonal mouse anti-PbTx-2 capture antibody was then immobilized on the DexP-modified probe by dextran-ConA binding. Gold nanoparticle heavily functionalized with glucoamylase and bovine serum albumin-PbTx-2 (PbTx-2-BSA) conjugate was employed as the trace tag. A competitive-type immunoassay format was adopted for the online monitoring of PbTx-2 between anti-PbTx-2 antibody immobilized on the QCM probe and PbTx-2-BSA labeled on the gold nanoparticle. Accompanying the gold nanoparticle, the carried glucoamylase could hydrolyze amylopectin in glucose. The produced glucose competed with dextran for ConA and displaced the ConA-streptavidin-anti-PbTx-2 complex from the QCM probe, resulting in the frequency change. Under optimal conditions, the frequency of the QCM immunosensor was indirectly proportional to the concentration of target PbTx-2 in the sample and exhibited a dynamic range from 1.0 pg·mL(-1) to 10 ng·mL(-1) with a detection limit (LOD) of 0.6 pg·mL(-1) at the 3Sblank level. Intra- and interassay coefficients of variation were below 7.5% and 9.5%, respectively. In addition, the methodology was evaluated for analysis of PbTx-2 in 15 spiked seafood samples and showed good accordance between results obtained by the displacement-type QCM immunosensor and a commercialized enzyme-linked immunosorbent assay (ELISA) method.

Direct competitive immunosorbent assay for detection of MEHP in human urine

Di-(2-ethylhexyl) phthalate (DEHP) is the most commonly used plasticizer for flexible polyvinyl chloride (PVC), which is also known as one of the environmental endocrine disruptors with the reproductive, developmental and embryonic toxicity after entering human body. Mono-2-ethylhexyl phthalate (MEHP) is one of the most complicate metabolites from DEHP in vivo and responsible for many toxic effects of DEHP. In order to evaluate human exposure to DEHP, a direct competitive enzyme-linked immunosorbent (dcELISA) based on monoclonal antibody (mAb) was developed to detect MEHP. A hybridoma cell line 4B9 secreting mAb against MEHP was prepared, and the horseradish peroxidase (HRP) labeled antigen as a probe in the dcELISA was made. After optimization of ELISA reaction conditions, the standard curve with a linear range from 0.56 to 1000 ng mL(-1) and a detection limit of 0.39 ng mL(-1) was established. The cross-reactivities of anti-MEHP mAb to other ten phthalate esters were less than 5% except for mono-methylphthalate (MME). The average recoveries of MEHP from distilled water and negative human urine were both between 87.4% and 94.72% with coefficient of variation (CV) less than 5%. Here, the ELISA method on detecting MEHP was successfully established and applied to real urine sample analyses and the results were confirmed by HPLC. Furthermore, it was indicated that the immunoassay was reliable and suitable for monitoring MEHP.

Simultaneous multiplexed stripping voltammetric monitoring of marine toxins in seafood based on distinguishable metal nanocluster-labeled molecular tags

Marine toxins from microscopic algae can accumulate through the food chain and cause various neurological and gastrointestinal illnesses for human health. Herein, we designed a new ultrasensitive multiplexed immunoassay protocol for simultaneous electrochemical determination of brevetoxin B (BTX-2) and dinophysistoxin-1 (DTX-1) in seafood using distinguishable metal nanocluster-labeled molecular tags as traces on bifunctionalized magnetic capture probes. To construct such a bifunctionalized probe, monoclonal mouse anti-BTX-2 (mAb(1)) and anti-DTX-1 (mAb(2)) antibodies were co-immobilized on a magnetic bead (MB-mAb(1,2)). The distinguishable metal nanoclusters including cadmium nanoclusters (CdNC) and copper nanoclusters (CuNC) were synthesized using the artificial peptides with amino acid sequence CCCYYY, which were used as distinguishable signal tags for the label of the corresponding bovine serum albumin-BTX-2 and bovine serum albumin-DTX-1 conjugates. A competitive-type immunoassay format was adopted for the online simultaneous monitoring of BTX-2 and DTX-1 on a homemade flow-through magnetic detection cell. The assay was based on the stripping voltammetric behaviors of the labeled CdNC and CuNC at the various peak potentials in pH 2.5 HCl containing 0.01 M KCl using square wave anodic stripping voltammetry (SWASV). Under optimal conditions, the multiplexed immunoassays enabled simultaneous detection of BTX-2 and DTX-1 in a single run with wide working ranges of 0.005-5 ng mL(-1) for two marine toxins. The limit of detection (LOD) and limit of quantification (LOQ) were 1.8 and 6.0 pg mL(-1) for BTX-2, while those for DTX-1 were 2.2 and 7.3 pg mL(-1), respectively. No non-specific adsorption and electrochemical cross-talk between neighboring sites were observed during a series of procedures to detect target analytes. The covalent conjugation of biomolecules onto the nanoclusters and magnetic beads resulted in good repeatability and intermediate precision down to 9.5%. The method featured unbiased identification of negative (blank) and positive samples. No significant differences at the 0.05 significance level were encountered in the analysis of 12 spiked samples, including Sinonovacula constricta , Musculista senhousia , and Tegillarca granosa , between the multiplexed immunoassay and commercially available enzyme-linked immunosorbent assay (ELISA) for analysis of BTX-2 and DTX-1.

Development of a monoclonal antibody-based sandwich-type enzyme-linked immunosorbent assay (ELISA) for detection of abrin in food samples

Abrin is a plant toxin, which can be easily isolated from the seeds of Abrus precatorius. It may be used as a biological warfare agent. In order to detect abrin in food samples, a two-layer sandwich format enzyme-linked immunosorbent assay based on the monoclonal antibody (mAb) (as capture antibody) and rabbit polyclonal serum (as detecting antibody) was developed and applied for the determination of abrin in some food matrices. The linear range of the mAb was 1-100 μg L(-1) with a detection limit of 0.5 μg L(-1) for abrin in phosphate buffered saline (PBS). The recoveries of abrin from sausage, beer and milk samples ranged 97.5-98.6%, 95.8-98.4% and 94.8-9.6%, respectively, with a coefficient of variation (CV) of 3.7% or less. The newly developed sandwich ELISA using the mAb appears to be a reliable and useful method for detection of abrin in sausage, beer and milk.