Ultrasensitive enzyme-linked immunosorbent assay (ELISA) for the detection of picogram quantities of IgE

Detection of Allergen-Specific IgE Antibody Responses

Allergen-specific IgE production is the central event in the pathogenesis of atopic disorders and increases in specific IgE serum antibodies are an indicator of immediate hypersensitivity responses in humans and in animal models of allergy. Consequently, accurate and user-friendly methods are needed to measure serum levels of allergen-specific IgE. This review examines historical and recent developments in in vivo and in vitro methods for the detection of allergen-specific IgE in humans and in animal models. Routinely, in vitro methods such as enzyme-linked immunosorbant assays or radioallergosorbant tests and in vivo methods such as the skin prick test (SPT) for humans and the passive cutaneous anaphylaxis assay (PCA) used in animals are utilized to detect allergen-specific IgE. While in vivo assays are usually more accurate than in vitro assays since they provide a functional readout of IgE activity, they are relatively costly and require considerable expertise. On the other hand in vitro assays are limited by the fact that the amount of allergen-specific serum IgG exceeds IgE antibody by several orders of magnitude, resulting in competition for allergen binding. Consequently, methods that use allergen as a direct capture step are limited by the availability of free allergen binding sites for IgE. In order to circumvent this problem, in vitro methods usually require prior depletion of IgG or use high amounts of allergen in order to facilitate availability of free binding sites for IgE detection. Clearly, these approaches are limited for small sample volumes and allergens that are in short supply. New methods such as protein microarray could potentially overcome this problem by providing high allergen concentrations in a relatively small reaction volume. Currently, in vitro methods are rarely used in isolation for prognosis but are used primarily to complement the information obtained from in vivo assays. With the emergence of new technologies it is conceivable that in vitro assays may in the future replace in vivo assays, however until then in vivo assays remain the gold standard of allergen-specific IgE detection.

Detection of femtomolar proteins by nonfluorescent ZnS nanocrystal clusters

Cation exchange (CX) in the nonfluorescent ZnS nanocrystal clusters (NCCs) was employed to detect trace biomolecules with immunoassays. The NCCs were porous and allowed fast cation exchange reaction to release an ultralarge number of Zn(2+) from each cluster that turned on the Zn-responsive dyes for fluorescence detection. The ZnS NCCs were highly stable in biological buffers and more biocompatible than quantum dots. Zn(2+) release efficiency and target binding by NCCs with average diameters of 44 nm, 86 nm, and 144 nm were investigated. The smallest NCCs exhibited the highest CX efficiency because of its larger surface area and bigger pores inside the cluster structure, and 71.0% of the enclosed Zn(2+) were freed by CX with 2-min microwave irradiation. They also experienced the least space hindrance and the fastest rate when binding to target molecules immobilized on surface. When the 44-nm NCCs were used to detect IgE in a sandwich assay, the limit of detection (LOD) was 5 pg/mL (33 fM), 1,000 times better than that of ELISA. Our results well demonstrate that CX in the ZnS NCCs is superior to the conventional signaling strategies in its high amplification efficiency, robustness, and biocompatibility.

Pressure: a novel tool for enzyme-linked immunosorbent assay procedure

The enzyme-linked immunosorbent assay (ELISA) technique is the backbone of the diagnostic assay for detection of infectious and allergic diseases. Here, we demonstrate a unique ELISA method for the detection of an antigen or antibody, in which ELISA steps are carried out under pressure instead of conventional thermal incubation. Pressure-mediated ELISA (PELISA), carried out in 1 h shows more than a 2-fold increase in absorbance value than the control experiment carried out at the same time and temperature without applying pressure. Estimation of total IgE by the 1-h PELISA method gives similar absorbance value (1.081±0.031, 823.12 IU) to that obtained by 3-h heat-mediated ELISA on an activated surface (HELISA) (1.165±0.037, 810.96 IU). Since PELISA is sensitive, specific, and reproducible (intra- and interassay CVs were 6.47% and 9.65%, respectively), it could be an excellent alternative to HELISA or conventional ELISA procedures.

Heat-mediated enzyme-linked immunosorbent assay procedure on a photoactivated surface

The human burden of infectious and allergic disease has necessitated rapid screening and measurement of antibodies in sera. Here, we report an enhancement of speed and sensitivity of an enzyme-linked immunosorbent assay (ELISA) technique by performing it at elevated temperature on an activated surface. The activated polymer was able to bind covalently anti-human IgG at 50 degrees C over 40 min to form a solid phase. The covalently bound solid phase was stable enough to withstand subsequent ELISA steps at elevated temperature. Thus, when blocking, human IgG and antibody-enzyme conjugate binding were performed on this solid phase at 40 degrees C in 40 min, 50 degrees C in 45 min and 50 degrees C in 40 min, respectively. The ELISA readings obtained were 1.5-fold higher than those obtained at 37 degrees C over similar incubation times. Total IgE was also determined by the heat-mediated ELISA (HELISA) technique in less than 3 h and gave similar ELISA values to those obtained by the conventional procedure carried out for 18 h on an untreated surface. A stable covalently bound solid phase is a prerequisite for the HELISA technique and was further verified when a solid phase prepared through adsorption onto an untreated surface showed less than half the ELISA absorbance values obtained with the activated surface at elevated temperatures. As surface activation can be achieved by application of a simple technique, the HELISA procedure could be a powerful alternative to conventional ELISA.

Improvement of supersensitive immunohistochemistry with an autostainer: a simplified catalysed signal amplification system

The ImmunoMax/catalysed signal amplification (CSA) system is a supersensitive method of paraffin immunohistochemistry. It incorporates antigen retrieval, the streptavidin-biotin complex (sABC) method, and the catalysing reporter deposition/catalysing biotinylated tyramide reaction. Strong, non-specific cytoplasmic reaction in the ImmunoMax/CSA is due to endogenous biotin unmasked in the antigen retrieval step. We examined procedures to diminish this non-specific immunoreaction and improved the ImmunoMax/CSA. Antigen retrieval in a hot water bath yielded a smaller endogenous biotin immunoreaction than antigen unmasking in an autoclave. Post-antigen retrieval fixation in buffered 10% formalin solution suppressed the biotin immunoreaction but masked the target antigen, Ki67. Post-reaction washing with 0.1% Tween 20 in Tris-HCl buffer at 35 degrees C did not diminish the endogenous biotin immunoreaction. Animal serum also did not suppress the non-specific immunoreactivity of biotin and antibodies. Because endogenous biotin is detected by duplicated biotin-streptavidin reactions in the ImmunoMax/CSA, we replaced the sABC step with a labelled polymer secondary antibody (the EnVision system)--a simplified CSA system--because the sensitivity of the EnVision system was the same as that of the sABC method. The non-specific immunoreaction induced by the EnVision system was masked competitively by blocking protein. By using an antibody against Ki67 antigen that can react only with the nucleus, we were able to evaluate the non-specific cytoplasmic immunoreaction induced by the detection system. We believe that the simplified CSA system will open up the field of supersensitive paraffin immunohistochemistry.

Comparison between inhibitory indirect ELISA and HPLC methods to quantify free and adducted aflatoxins in human urine

HPLC and Inhibitory Indirect ELISA (I.I. ELISA) methods for quantitation of aflatoxins (AF) in human urine were compared in terms of specificity, sensitivity, easiness and cost. I.I. ELISA was optimized in kind of antibody in use, type of plastic plate, adduct synthesis technique, peroxidase and antibody dilutions, etc. Both polyclonal (Cuban) and monoclonal (British) anti-AF antibodies were statistically studied and the process was standardized. HPLC and electrophoresis were performed while synthetizing AFB(1)-DNA and AFB(1)-Cl-Ovalbumin (AFB(1)-Cl-Ov) adducts. Costar polystyrene plate had the best adherence. Optimum coating dilution was 10 ng of AFB(1)-Cl-Ov per well. Dilutions of 1:1000 of monoclonal antibody from purified culture or 1:300 from monoclonal antibody from tissue culture and 1:1000 of peroxidase anti-mouse conjugate were the best. Optimum separation with HPLC was obtained isocratically with 60% MeOH and 40% distilled water mobile phase. ELISA had a sensitivity of 1 pg mL(-1) AFB(1) and HPLC sensitivity was 0.1 ng mL(-1) AFB(1) with fluorescence detector and 4.5 ng mL(-1) with UV detector. Monoclonal antibody gave more accurate results for determination of free and adducted AFB(1) in urine analysis.

Evaluation of a high IgE-responder mouse model of allergy to bovine beta-lactoglobulin (BLG): development of sandwich immunoassays for total and allergen-specific IgE, IgG1 and IgG2a in BLG-sensitized mice

An animal model of food allergy represents an important tool for studying the mechanisms of induction and repression of an allergic reaction, as well as for the development of an immunotherapy to prevent or minimize such an adverse reaction. IgE and IgG1 (Th2 response) vs. IgG2a (Th1 response) are good markers for the induction of an allergic response in mice. Nevertheless, while the total serum concentrations of these isotypes are easy to measure using classical sandwich immunoassays, this is not the case for allergen-specific isotypes. To develop an animal model of allergy to bovine beta-lactoglobulin (BLG), we set up quantitative assays for total and for allergen-specific IgE, IgG1 and IgG2a. Microtiter plates coated either with anti-isotype antibodies (Abs) or with allergen were used for Ab capture, while anti-isotype Fab' fragments coupled to acetylcholinesterase were used for visualization. These assays of anti-BLG specific Abs are original in two ways. First, assay calibration is performed using anti-BLG specific mAbs, thus allowing good quantification of the different isotypes and subclasses of serum antibodies. Second, the detection of all anti-BLG specific Abs, i.e., those recognizing both the native and denatured forms of the protein, is achieved through indirect coating of BLG using biotin-streptavidin binding. The present assays are quantitative, specific to the isotype (cross-reactivity <0.5%), very sensitive (detection limit in the 10 pg/ml range), and reproducible (coefficient of variation less than 10%). Applied to the humoral response in mice sensitized with BLG adsorbed on alum, these assays proved to be a very useful tool for monitoring high IgE-responder mice following BLG immunization, and for an immunotherapy directed at polarizing the immune response.

Nonatopic wheezy children have reduced interferon-gamma

Viruses cause asthmatic exacerbations in schoolchildren. We tested the hypothesis that children who wheezed with viral respiratory tract infections secrete higher levels of the type 1 cytokine interferon-gamma (IFN-gamma) in the peripheral circulation than children who had never wheezed. Blood was taken from 13 children (eight atopic) with episodic wheeze and 11 controls. CD4 and CD8 cells were separated from peripheral blood mononuclear cells and stimulated with phorbol 12-myrisate 13-acetate (PMA) and ionomycin for 24 h. IFN-gamma, IL-4, and IL-5 were measured in the supernatant by ELISA. IFN-gamma production by CD4 and CD8 cells was lower in children with a history of wheeze (CD4, P = 0.046; CD8, P = 0.037). These children were then analysed according to atopic status. CD4 and CD8 IFN-gamma production in nonatopic wheezy children was reduced (CD4, P=0.009; CD8, P=0.003). IFN-gamma production by atopic wheezy children was lower than by controls, but the differences were not significant (CD4, P = 0.2831; CD8, P = 0.1372). CD8 IL-5 was lower in children who wheezed (P=0.012). Release of IL-4 and IL-5 by CD4 cells did not differ between the three groups. We propose that defective IFN-gamma secretion by CD4 and CD8 cells may contribute to viral-induced wheeze in nonatopic children.

A comparison of performances of four enzymes used in ELISA with special reference to beta-lactamase

Horse radish peroxidase, alkaline phaosphatase and beta-D-galactosidase are widely used as labels in the development of enzyme immunoassays (EIAs). Enzyme beta-lactamase, though introduced as a label in late seventies has not yet become very popular inspite of having the necessary features of an enzyme to be used in EIAs. The present article reviews assays developed with this enzyme, highlights its salient features and brings out an argument in favour of its wide spread use in EIAs.

Highly sensitive time-resolved fluoroimmunoassay of human immunoglobulin E by using a new europium fluorescent chelate as a label

A new europium fluorescent chelate, 4,4'-bis(1",1",1",2",2",3",3"-heptafluoro-4",6"-hexanedione-6"-yl) - chlorosulfo-o-terphenyl (BHHCT)-Eu3+, was used as a label for highly sensitive time-resolved fluoroimmunoassay of human IgE. Two assay formats were employed in the analysis. In the first format, an immunoconjugate of rabbit anti-human IgE antibody-human IgE-biotinylated goat anti-human IgE antibody-BHHCT-Eu(3+)-labeled SA (or BHHCT-Eu(3+)-labeled BSA-SA; BSA, bovine serum albumin; SA, streptavidin) was used for measurement. The method gives the detection limits of 3.6 x 10(-2) IU/ml (labeled SA) and 1.1 x 10(-2) IU/ml (labeled SA-BSA). In the second format, an immunoconjugate of goat anti-human IgE antibody-human IgE-rabbit anti-human IgE antibody-biotinylated goat anti-rabbit IgG antibody-BHHCT-Eu(3+)-labeled SA (or BHHCT-Eu(3+)-labeled BSA-SA) was used for measurement. The detection limits of these methods are 3.0 x 10(-3) IU/ml (labeled SA) and 1.5 x 10(-3) IU/ml (labeled BSA-SA). The above detection limits are one to two orders of magnitude lower than those of the conventional radioimmunoassay and enzyme immunoassay. The CV of the present method is less than 7%, and the recovery is in the range of 85-105% for serum samples.

Eosinophilia in patients infected with the human immunodeficiency virus

The prevalence and significance of peripheral blood eosinophilia in patients infected with the human immunodeficiency virus (HIV) were evaluated. Fifteen of 119 consecutive patients had absolute eosinophil counts of > 450/mm3. During a mean follow-up period of 419 days eosinophilia could be identified as secondary to a parasitic infection in only one patient. Correlation with disease stage showed a higher rate of advanced disease in patients with absolute eosinophilia. In a multivariate regression analysis, only low CD4+ cell counts, not the CDC disease stage or the use of antiretroviral therapy or primary prophylaxis, contributed significantly to the prevalence of eosinophilia. It is concluded that expensive laboratory investigations in asymptomatic patients with advanced-stage HIV disease are neither necessary nor cost effective.

Enzyme-linked DNA oligotyping. A practical method for clinical HLA-DNA typing

The use of PCR and oligonucleotide hybridization has increased the accuracy and resolution of typing for HLA class II alleles, but current procedures, performed in batches, take too long and are not suited for testing single samples. We have developed a typing method using enzyme-linked oligonucleotides and PCR products immobilized in 96-well trays. Trays preloaded with typing probes, covalently linked with alkaline phosphatase, have been kept for weeks at 4 degrees C without loss of enzyme-probe activity. Bound alkaline phosphatase was detected using a color reaction with enzymatic amplification which produces readings in 30 minutes. Coupled with a quick DNA preparation method, results can be obtained in about 4 hours. This method can be easily performed in small laboratories. It is accurate, reproducible, and sensitive, and will make oligotyping for HLA alleles more convenient for testing clinical samples.

Influence of stress on IgE production

Intense acute stress, consequent to restraint, leads to a diminished production of IgE antiovalbumin antibodies in mice. The IgE content of sera from experimental or control animals was assayed by the enzyme-linked immunosorbent assay (ELISA) at days 8, 16, and 20 after immunization. The statistics revealed significant differences in the IgE level of the animals submitted to acute stress and home cage control animals on days 16 and 20 after immunization, but not on day 8.

Detection of circulating histamine releasing autoantibodies with functional properties of anti-IgE in chronic urticaria

Circulating histamine releasing factor(s) have been demonstrated previously in chronic urticaria by an immediate weal-and-flare response to intradermal autologous serum injection. We have studied 25 chronic urticaria patients by in vivo skin testing with autologous sera and an in vitro histamine release assay using mixed leukocytes of healthy donors, to define the nature and functional properties of the serum factor(s). Twenty showed a weal response to autologous serum (mean +/- s.e.m., 37.3 +/- 6.8 mm3). Weal formation was confined to ultrafiltered serum fractions greater than 100 kD in nine of nine patients. There was no response in 10 healthy controls of five patients with symptomatic dermographism. Fourteen chronic urticaria sera elicited histamine release greater than 10% (mean +/- s.e.m., 44.3% +/- 6.7) above basal levels from leukocytes of at least one of seven healthy donors. This in vitro response was also confined to ultrafiltered serum fractions greater than 100 kD in seven of seven sera and was present in IgG fractions of six of seven chronic urticaria sera that showed histamine releasing activity. Functional studies indicated that this histamine releasing autoantibody had the properties of anti-IgE: chronic urticaria sera 'desensitized' basophil leukocytes to subsequent challenge with other chronic urticaria sera and to goat anti-human IgE antibody; human myeloma IgE inhibited histamine release from leukocytes in response to chronic urticaria sera; removal of surface-bound IgE by lactic acid 'stripping' reduced histamine release in response to chronic urticaria sera and anti-IgE and subsequent passive sensitization with IgE myeloma serum partially restored it. Stainable peripheral blood basophils/mm3 in chronic urticaria patients were significantly reduced (mean +/- s.e.m, 7.9 +/- 2.0) when compared to healthy controls (39.6 +/- 4.4), P less than 0.001. These results suggest that histamine releasing autoantibodies are important in the pathogenesis of chronic urticaria by stimulating or facilitating degranulation of basophils and cutaneous mast cells through cross-linking cell surface IgE receptors.

Neonatal IgE: a poor screen for atopic disease

Screening for atopic disease using neonatal serum IgE has been advocated on the basis of the predictive value of elevated levels. However, this is only one measure of validity. The test was validated fully in 92 infants with a bi-parental history of atopy using 0.7 IU/ml as the cut-off. All infants were assessed prospectively for evidence of atopic disease (eczema, recurrent wheezing or food reactions) and skin-prick test positivity in the first year. Total serum IgE was measured by ultrasensitive ELISA on 61 cord blood samples and 92 samples taken at 7 days. All cord samples were re-analysed by PRIST and the first 33 by ultrasensitive RIA giving, respectively, 82% and 94% concordance (regarding undetectable, detectable and elevated levels) with ELISA. Maternal contamination was indicated in 7% of cord samples by high serum IgA. Ninety-five per cent of cord/7-day IgE pairs showed no change or minor rises at 7 days. Forty-nine per cent of the infants had evidence of atopic disease. Only 5% had elevated 7-day IgE. The positive and negative predictive values of the 7-day test were 60% and 52%, respectively, and specificity 96% but the sensitivity was only 7%. High levels did not distinguish the infants with the most unequivocal evidence of disease, i.e. eczema with a positive skin test. In conclusion IgE at 7 days is comparable to and more reliable than cord IgE. However, neonatal IgE screening is too insensitive to have clinical application.

The IgE and IgG subclass antibody response to foods in babies during the first year of life and their relationship to feeding regimen and the development of food allergy

This follow-up study of 191 babies investigated the development of food allergy in an unselected population and its relationship to total and antigen-specific IgE and IgG subclass levels. Sensitization to egg, as indicated by a positive skin test or RAST, was found in 5% of 1-year-old babies, but none of the babies in this series fulfilled the clinical criteria for immediate-type milk allergy. For both bovine casein (CAS) and egg albumin, the IgG response was largely restricted to IgG1 in contrast to the predominant IgG4 response to these antigens that is found in adults. The level of IgG4, but not IgG1, antibody to CAS and ovalbumin (OV) was lower in some of the babies compared with that of their mothers (N = 166; p less than 0.05, Student's paired t test). However, there was no difference in the total serum IgG subclass levels between mothers and babies. These results demonstrate that, in the population of babies studied, (1) type I hypersensitivity to egg occurred in 5% of 1-year-old babies, (2) the predominant IgG subclass of antibodies to CAS and OV in babies is IgG1, and (3) in the 22% of babies, there was substantially (greater than 1000-fold) less IgG4 antibody to CAS and OV than in their mothers, suggesting specific exclusion of some IgG4 antibodies.