Production and characterization of antibody against aflatoxin M1

Efficient Conjugation of Aflatoxin M1 With Bovine Serum Albumin through Aflatoxin M1-(O-carboxymethyl) Oxime and Production of Anti-aflatoxin M1 Antibodies

BACKGROUND

Aflatoxins are the most extensively studied group of mycotoxins produced by molds, especially the Aspergillus group, which are highly toxic to animals and humans.

OBJECTIVES

Since immunoassay is a simple and rapid method for the analysis of many toxic substances in comparison to the chromatographic methods, it is necessary to produce specific and sensitive antibodies for detection of Aflatoxin M1 (AFM1). The current study was conducted to produce bioconjugate of Aflatoxin M1 (AFM1) with Bovine Serum Albumin (BSA) as well as to generate specific antibodies against AFM1 for immunoassay of the mycotoxin.

MATERIALS AND METHODS

First, AFM1 was converted to AFM1-(O-carboxymethyl) oxime derivative. Then, AFM1-oxime was coupled with BSA and the product was assessed by UV-VIS spectrophotometry. In order to generate polyclonal antibodies against AFM1, rabbits were immunized with BSA-AFM1 conjugate. Produced antibodies were purified using ion exchange chromatography and BSA-Sepharose 4B affinity chromatography. The titers and specificity of the produced antibodies were determined by Enzyme-Linked Immunosorbent Assay (ELISA).

RESULTS

The results indicated that coupling of AFM1 with O-(Carboxymethyl) hydroxylamine hemihydrochloride was suitable and 12 moles of AFM1-oxime were successfully coupled to each mole of BSA. In addition, the titers and specificity of the prepared antibody were considerable compared to standard anti-AFM1 antibodies. The relative cross-reactivity of each toxin (relative to AFM1) with purified anti-AFM1 antibodies, as determined by the amount of aflatoxin necessary to cause 50% inhibition of enzyme activity, was 70, 105, 240, and 2500 ng/mL for AFB1, AFB2, AFG1, and AFG2, respectively.

CONCLUSIONS

The prepared antibody can be used for the development of an ELISA kit to assay AFM1 in milk and other biological fluids.

Development and application of an indirect competitive enzyme-linked immunoassay for aflatoxin m(1) in milk and milk-based confectionery

High-titer rabbit polyclonal antibodies to aflatoxin M(1) (AFM1) were produced by utilizing AFM1-bovine serum albumin (BSA) conjugate as an immunogen. An indirect competitive enzyme-linked immunosorbent assay was standardized for estimating AFM1 in milk and milk products. To avoid the influence of interfering substances present in the milk samples, it was necessary to prepare AFM1 standards in methanol extracts of certified reference material (CRM) not containing detectable AFM1 (< 0.05 ng/g). The reliability of the procedure was assessed by using CRM with AFM1 concentrations of < 0.5 and 0.76 ng/g. Also, assays of milk samples mixed with AFM1 ranging in concentration between 0.5 and 50 ng/L gave recoveries of > 93%. The relative cross-reactivity with aflatoxins (AF) and ochratoxin A, assessed as the amount of AFM1 necessary to cause 50% inhibition of binding, was 5% for AFB1 and much less for AFB2, AFG1, and AFG2; there was no reaction with ochratoxin A. AFM1 contamination was measured in retail milk and milk products collected from rural and periurban areas in Andhra Pradesh, India. Of 280 milk samples tested, 146 were found to contain < 0.5 ng/mL of AFM1; in 80 samples it varied from 0.6 to 15 ng/mL, in 42 samples from 16 to 30 ng/mL, and in 12 samples from 31 to 48 ng/mL. Most of the milk samples that contained high AFM1 concentrations were obtained from periurban locations. The results revealed a significant exposure of humans to AFM1 levels in India and thus highlight the need for awareness of risk among milk producers and consumers.

Selection and characterisation of recombinant single-chain antibodies to the hapten Aflatoxin-B1 from naive recombinant antibody libraries

Selection of antibodies from large repertoire phage display libraries has become a common technique for isolation of specific antibodies to antigens. Many of these libraries are shown to contain antibodies specific to haptens, but only when these haptens are derivatised or conjugated to an immobilising molecule, such as bovine serum albumin (BSA). There has been little demonstration of the suitability of naive recombinant antibody libraries for isolating antibodies that bind low molecular weight haptens in the absence of a carrier molecule and few have addressed the problems associated with selecting antibodies that only recognize the combination of hapten and the carrier molecule. We have panned two-phage antibody libraries against AflatoxinB1-BSA and screened single-chain antibody fragments for binding to AflatoxinB1-BSA and Aflatoxin-B1. Many of the antibodies isolated specifically bound AflatoxinB1-BSA, but not soluble Aflatoxin-B1 or BSA. Modification of the protocol led to isolation of single-chain fragment variable antibody domain (scFv) antibodies that specifically bound soluble Aflatoxin-B1 with an affinity of 6x10(-9) M.

[Evaluation of enzyme-linked immunosorbent assay (ELISA) in milk powder contaminated with known concentrations of aflatoxin M1]

The evaluation of commercially available test systems of competitive enzyme-linked immunosorbent assay (ELISA), for Aflatoxin M1 (AFM1) was performed in experimental conditions, through repeated analysis, in samples of milk powder contaminated with known concentrations of the toxin. Recoveries of AFM1 added to milk at levels of 0.10, 0.20, 0.50 and 1.00 ng/ml were 83.0%, 87.5%, 103.0% and 111.8% respectively. Relative standard deviations for the above mentioned concentrations were 65.5%, 31.8%, 10.9% and 13.6%, respectively (n = 10, per spiking level). According to these results the ELISA is appropriate for AFM1 research and/or surveying, mainly for concentrations between 0.20-1.00 ng/ml.

Production and characterization of antibodies cross-reactive with major aflatoxins

Antibodies cross-reactive with 4 major aflatoxins were demonstrated three weeks after immunization of rabbits with an immunogen which was prepared by conjugating aflatoxin B3 to bovine serum albumin. Aflatoxin B3 was first converted to its hemisuccinate before conjugation to the protein. Tritiated aflatoxin B1 (AFB1) was used as the marker ligand both for antibody titer determination as well as for analysis of antibody specificity. Competitive RIA revealed that the antibodies have good cross-reactivity with aflatoxins B1, B2, G1, and G2 when tritiated AFB1 was used as the marker ligand. The concentrations causing 50% inhibition of binding of 3H-AFB1 to the antibodies by unlabeled aflatoxins B1, B2, G1, G2 and B3 were found to be 0.25, 3.34, 0.32, 4.0 and 0.53 ng/assay, respectively. The antibodies could be used for simultaneous analysis of aflatoxins B1 and G1, two of the most important toxic metabolites produced by Aspergillus flavus and A. parasiticus.

Production and characterization of monoclonal antibodies against aflatoxin M1

By using an indirect enzyme-linked immunosorbent assay, four monoclonal antibodies were selected after fusion of mouse P3-NS1-Ag4-1 myeloma cells with spleen cells isolated from BALB/c mice that had been immunized with aflatoxin M1 (AFM1) conjugated to bovine serum albumin. Two of these antibodies were found to be specific for AFM1 and were designated AMW-1 and AMW-4. The specificities of AMW-1, which had higher affinity to AFM1, were determined by a competitive direct enzyme-linked immunosorbent assay with peroxidase-AFM1 as the marker. The relative cross-reactivity of each toxin (relative to AFM1) with AMW-1, as determined by the amount of aflatoxin necessary to cause 50% inhibition of enzyme activity, was 12, greater than 40, 12, and greater than 40 for B1, B2, G1, and G2, respectively.

Production and characterization of antibody against aflatoxin Q1

Antibodies against aflatoxin Q1 (AFQ1) were obtained from rabbits after immunization of either AFQ1-hemisuccinate or AFQ2a conjugated to bovine serum albumin. Both radioimmunoassay and enzyme-linked immunosorbent assaY (ELISA) were used for the determination of antibody titers and specificities. Antibodies obtained from rabbits after immunization with AFQ1-hemisuccinate-bovine serum albumin had the highest affinity to aflatoxin B1, whereas antibodies obtained from rabbits after immunization with AFQ2a-bovine serum albumin bound most effectively with AFQ2a. AFQ2a antibody was selected for the subsequent direct and indirect ELISA for the detection of AFQ1 in biological fluids. When AFQ2a-peroxidase and AFQ2a antibody were used, direct ELISA was able to detect as low as 2 ppb (ng/ml) of AFQ1 spiked in the urine samples that had been subjected to a Sep-Pak cleanup treatment. In indirect ELISA in which the antigen (AFQ2a-bovine serum albumin) was coated to the solid phase followed by reaction with rabbit antibody and goat anti-rabbit immunoglobulin G-peroxidase conjugate, 50-fold less antibody was used without sacrificing sensitivity. Recoveries of AFQ1 added to urine samples (2 to 40 ppb) were 46.3 to 73% and 65.8 to 85.8% for direct and indirect ELISA, respectively.

Kinetics of transformation of aflatoxin B1 into aflatoxin M1 in lactating mouse: an ELISA analysis

A new enzyme-linked immunosorbent assay (ELISA) was used to study the kinetics of transformation of aflatoxin B1 into aflatoxin M1 in lactating mice. Aflatoxin M1 concentration in the milk samples reached a maximum 30 min after injection of aflatoxin B1 and decreased thereafter. At the maximum time, the levels of aflatoxin M1 in the samples were proportional to the dosages administered. Aflatoxin B1 was also detected in the milk samples but at a lower concentration.

Production and characterization of aflatoxin B2a antiserum

The specificity and sensitivity of antiserum elicited from rabbits against aflatoxin B2a-bovine serum albumin conjugates were characterized with a radioimmunoassay (RIA) and an enzyme-linked immunosorbent assay (ELISA). Aflatoxin B1 was first converted to aflatoxin B2a and then conjugated to bovine serum albumin and horseradish peroxidase by a reductive alkylation method. The antiserum was developed in New Zealand white rabbits by multiple-site injection with the aflatoxin B2a-bovine serum albumin conjugate. Antibody titers were determined by both RIA and ELISA. Competitive RIAs with various aflatoxin analogs indicated that the antiserum was most reactive with aflatoxin B1 and slightly cross-reactive with aflatoxins B2a, B2, and M1. Competitive ELISAs showed the antiserum to be equally specific for aflatoxins B2a and B12 and less reactive with aflatoxins B2 and M1. The relative sensitivities of RIA and ELISA for aflatoxin B1 quantitation were 100 and 10 pg per assay, respectively.

Quantitation of aflatoxin B1 and aflatoxin B1 antibody by an enzyme-linked immunosorbent microassay

A specific microtest plate enzyme immunoassay has been developed for the rapid quantitation of aflatoxin B1 at levels as low as 25 pg per assay. Multiple-site injection of rabbits with an aflatoxin B1 carboxymethyloxime-bovine serum albumin conjugate was used for the production of hyperimmune sera. Dilutions of the purified antibody were air dried onto microplates previously treated with bovine serum albumin and glutaraldehyde and then incubated with an aflatoxin B1 carboxymethyloxime-horseradish peroxidase conjugate. The amount of enzyme bound to antibody was determined by monitoring the change in absorbance at 414 nm after the addition of a substrate solution consisting of hydrogen peroxide and 2,2'-azino-di-3-ethyl-benzthiazoline-6-sulfonate. Antibody titers determined in this manner closely correlated with those determined by radioimmunoassay. Competition assays as performed by incubation of different aflatoxin analogs with the peroxidase conjugate showed that aflatoxins B1 and B2 and aflatoxicol caused the most inhibition of conjugate binding to antibody. Aflatoxins G1 and G2 inhibited the conjugate binding to a lesser degree, whereas aflatoxins M1 and B2a had no effect of the assay.

Comparison of antibody production against aflatoxin B1 in goats and rabbits

Antibody production against aflatoxin B1 was compared in three rabbits and one goat. Titers obtained were 20 times higher in the rabbits than in the goat. The goat antiserum appeared to have a higher degree of cross-reactivity for other aflatoxins and related metabolites than did the rabbit antiserum.