Different blocking agents cause variation in the immunologic detection of proteins transferred to nitrocellulose membranes

Reducing Unspecific Protein Adsorption in Microfluidic Papers Using Fiber-Attached Polymer Hydrogels

Microfluidic paper combines pump-free water transport at low cost with a high degree of sustainability, as well as good availability of the paper-forming cellulosic material, thus making it an attractive candidate for point-of-care (POC) analytics and diagnostics. Although a number of interesting demonstrators for such paper devices have been reported to date, a number of challenges still exist, which limit a successful transfer into marketable applications. A strong limitation in this respect is the (unspecific) adsorption of protein analytes to the paper fibers during the lateral flow assay. This interaction may significantly reduce the amount of analyte that reaches the detection zone of the microfluidic paper-based analytical device (µPAD), thereby reducing its overall sensitivity. Here, we introduce a novel approach on reducing the nonspecific adsorption of proteins to lab-made paper sheets for the use in µPADs. To this, cotton linter fibers in lab-formed additive-free paper sheets are modified with a surrounding thin hydrogel layer generated from photo-crosslinked, benzophenone functionalized copolymers based on poly-(oligo-ethylene glycol methacrylate) (POEGMA) and poly-dimethyl acrylamide (PDMAA). This, as we show in tests similar to lateral flow assays, significantly reduces unspecific binding of model proteins. Furthermore, by evaporating the transport fluid during the microfluidic run at the end of the paper strip through local heating, model proteins can almost quantitatively be accumulated in that zone. The possibility of complete, almost quantitative protein transport in a µPAD opens up new opportunities to significantly improve the signal-to-noise (S/N) ratio of paper-based lateral flow assays.

Association of Intestinal Alkaline Phosphatase With Necrotizing Enterocolitis Among Premature Infants

Importance

Necrotizing enterocolitis (NEC) in preterm infants is an often-fatal gastrointestinal tract emergency. A robust NEC biomarker that is not confounded by sepsis could improve bedside management, lead to lower morbidity and mortality, and permit patient selection in randomized clinical trials of possible therapeutic approaches.

Objective

To evaluate whether aberrant intestinal alkaline phosphatase (IAP) biochemistry in infant stool is a molecular biomarker for NEC and not associated with sepsis.

Design, Setting, and Participants

This multicenter diagnostic study enrolled 136 premature infants (gestational age, <37 weeks) in 2 hospitals in Louisiana and 1 hospital in Missouri. Data were collected and analyzed from May 2015 to November 2018.

Exposures

Infant stool samples were collected between 24 and 40 or more weeks postconceptual age. Enrolled infants underwent abdominal radiography at physician and hospital site discretion.

Main Outcomes and Measures

Enzyme activity and relative abundance of IAP were measured using fluorometric detection and immunoassays, respectively. After measurements were performed, biochemical data were evaluated against clinical entries from infants' hospital stay.

Results

Of 136 infants, 68 (50.0%) were male infants, median (interquartile range [IQR]) birth weight was 1050 (790-1350) g, and median (IQR) gestational age was 28.4 (26.0-30.9) weeks. A total of 25 infants (18.4%) were diagnosed with severe NEC, 19 (14.0%) were suspected of having NEC, and 92 (66.9%) did not have NEC; 26 patients (19.1%) were diagnosed with late-onset sepsis, and 14 (10.3%) had other non-gastrointestinal tract infections. For severe NEC, suspected NEC, and no NEC samples, median (IQR) fecal IAP content, relative to the amount of IAP in human small intestinal lysate, was 99.0% (51.0%-187.8%) (95% CI, 54.0%-163.0%), 123.0% (31.0%-224.0%) (95% CI, 31.0%-224.0%), and 4.8% (2.4%-9.8%) (95% CI, 3.4%-5.9%), respectively. For severe NEC, suspected NEC, and no NEC samples, median (IQR) enzyme activity was 183 (56-507) μmol/min/g (95% CI, 63-478 μmol/min/g) of stool protein, 355 (172-608) μmol/min/g (95% CI, 172-608 μmol/min/g) of stool protein, and 613 (210-1465) μmol/min/g (95% CI, 386-723 μmol/min/g) of stool protein, respectively. Mean (SE) area under the receiver operating characteristic curve values for IAP content measurements were 0.97 (0.02) (95% CI, 0.93-1.00; P < .001) at time of severe NEC, 0.97 (0.02) (95% CI, 0.93-1.00; P < .001) at time of suspected NEC, 0.52 (0.07) (95% CI, 0.38-0.66; P = .75) at time of sepsis, and 0.58 (0.08) (95% CI, 0.42-0.75; P = .06) at time of other non-gastrointestinal tract infections. Mean (SE) area under the receiver operating characteristic curve values for IAP activity were 0.76 (0.06) (95% CI, 0.64-0.86; P < .001), 0.62 (0.07) (95% CI, 0.48-0.77; P = .13), 0.52 (0.07) (95% CI, 0.39-0.67; P = .68), and 0.57 (0.08) (95% CI, 0.39-0.69; P = .66), respectively.

Conclusions and Relevance

In this diagnostic study, high amounts of IAP protein in stool and low IAP enzyme activity were associated with diagnosis of NEC and may serve as useful biomarkers for NEC. Our findings indicated that IAP biochemistry was uniquely able to distinguish NEC from sepsis.

An overview of technical considerations for Western blotting applications to physiological research

The applications of Western/immunoblotting (WB) techniques have reached multiple layers of the scientific community and are now considered routine procedures in the field of physiology. This is none more so than in relation to skeletal muscle physiology (i.e., resolving the mechanisms underpinning adaptations to exercise). Indeed, the inclusion of WB data is now considered an essential aspect of many such physiological publications to provide mechanistic insight into regulatory processes. Despite this popularity, and due to the ubiquitous and relatively inexpensive availability of WB equipment, the quality of WB in publications and subsequent analysis and interpretation of the data can be variable, perhaps resulting in spurious conclusions. This may be due to poor laboratory technique and/or lack of comprehension of the critical steps involved in WB and what quality control procedures should be in place to ensure robust data generation. The present review aims to provide a detailed description and critique of WB procedures and technicalities, from sample collection through preparation, blotting and detection, to analysis of the data collected. We aim to provide the reader with improved expertise to critically conduct, evaluate, and troubleshoot the WB process, to produce reproducible and reliable blots.

Other Notable Methods of Membrane Protein Detection: A Brief Review

Several techniques have been employed to detect proteins on membranes. These include the use of quantum dot luminescent labels, oxyblot immunochemical detection, polymer immunocomplexes, "coupled" probing approach, in situ renaturation of proteins for detecting enzyme activities in crude or purified preparations, immunochromatographic assay, western-phosphatase assay, and use of Congo red dye (a cosmetic color named Alta), Pro-Q Emerald 488 dye, or amine-reactive dye in combination with alkaline phosphatase- and horseradish peroxidase-antibody conjugates for the simultaneous trichromatic fluorescence detection of proteins. Several methods have been used to improve the detection of proteins on membranes, including glutaraldehyde treatment of nitrocellulose blots, elimination of keratin artifacts in immunoblots probed with polyclonal antibodies, and washing of immunoblots with excessive water and manipulation of Tween-20 in wash buffer. These methods are briefly reviewed in this chapter.

Co-Application of Corticosterone and Growth Hormone Upregulates NR2B Protein and Increases the NR2B:NR2A Ratio and Synaptic Transmission in the Hippocampus

OBJECTIVES

This in vitro study aimed to investigate the possible mechanism underlying the protective effect of growth hormone (GH) on hippocampal function during periods of heightened glucocorticoid exposure.

METHODS

This study was conducted between January and June 2005 at the Joan C. Edwards School of Medicine, Marshall University, in Huntington, West Virginia, USA. The effects of the co-application of GH and corticosterone (CORT) were tested at different concentrations on the field excitatory postsynaptic potentials (fEPSPs) of the hippocampal slices of rats in two different age groups. Changes in the protein expression of N-methyl-D-aspartate receptor (NMDAR) subunits NR1, NR2B and NR2A were measured in hippocampal brain slices treated with either artificial cerebrospinal fluid (ACSF), low doses of CORT alone or both CORT and GH for three hours.

RESULTS

The co-application of CORT and GH was found to have an additive effect on hippocampal synaptic transmission compared to either drug alone. Furthermore, the combined use of low concentrations of GH and CORT was found to have significantly higher effects on the enhancement of fEPSPs in older rats compared to young ones. Both GH and CORT enhanced the protein expression of the NR2A subunit. Simultaneous exposure to low concentrations of GH and CORT significantly enhanced NR2B expression and increased the NR2B:NR2A ratio. In contrast, perfusion with CORT alone caused significant suppression in the NR1 and NR2B protein expression and a decrease in the NR2B:NR2A ratio.

CONCLUSION

These results suggest that NMDARs provide a potential target for mediating the GH potential protective effect against stress and age-related memory and cognitive impairment.

The necessity of and strategies for improving confidence in the accuracy of western blots

Western blotting is one of the most commonly used laboratory techniques for identifying proteins and semi-quantifying protein amounts; however, several recent findings suggest that western blots may not be as reliable as previously assumed. This is not surprising since many labs are unaware of the limitations of western blotting. In this manuscript, we review essential strategies for improving confidence in the accuracy of western blots. These strategies include selecting the best normalization standard, proper sample preparation, determining the linear range for antibodies and protein stains relevant to the sample of interest, confirming the quality of the primary antibody, preventing signal saturation and accurately quantifying the signal intensity of the target protein. Although western blotting is a powerful and indispensable scientific technique that can be used to accurately quantify relative protein levels, it is necessary that proper experimental techniques and strategies are employed.

Fabrication of carboxylated silicon nitride sensor chips for detection of antigen-antibody reaction using microfluidic reflectometric interference spectroscopy

In this study, we report label-free detection of alpha-fetoprotein (AFP), which has been used as a biomarker for hepatocellular carcinoma, by a microfluidic reflectometric interference spectroscopy (RIfS) system adopting a simple halogen light source and an inexpensive silicon-based sensor chip. Introduction of carboxy groups on a silicon nitride sensor chip to immobilize anti-AFP monoclonal antibody (anti-AFP) was carried out simply by immersion in aqueous solution containing triethoxysilylpropylmaleamic acid bearing a carboxy group and a silanol group. The RIfS system with the anti-AFP-immobilized sensor chip was found to give a reversible response through 100 on/off cycles using a regeneration buffer with high reproducibility (coefficient of variation (CV) = 5.7%). The limit of detection (LOD) of AFP was 100 ng mL(-1), and the measurement range spanned 3 orders of magnitude. Furthermore, the sensor chip showed no cross-reactivity with human serum albumin, Immunoglobulin G, transferrin, or fibrinogen at 100 μg mL(-1) without the use of blocking reagents such as bovine serum albumin. Consequently, the proposed RIfS system is a potentially effective tool for biomarker detection and in vitro diagnostics.

A brief review of other notable protein detection methods on blots

Several methods have been used for detecting proteins on membranes. These include the use of quantum dot luminescent labels, oxyblot immunochemical detection, polymer immunocomplexes, "coupled" probing approach, in situ renaturation of proteins for detecting enzyme activities in crude or purified preparations, immunochromatographic assay, western-phosphatase assay, and the use of Congo red dye, a cosmetic color named Alta, Pro-Q Emerald 488 dye, or amine-reactive dye in combination with alkaline phosphatase and horseradish peroxidase-antibody conjugates for the simultaneous trichromatic fluourescence detection of proteins. Several methods have been used to improve the detection of proteins on membranes, including glutaraldehyde treatment of nitrocellulose blots, elimination of keratin artifacts in immunoblots probed with polyclonal antibodies, and the washing of immunoblots with excessive water and manipulation of Tween-20 in wash buffer. These methods are briefly reviewed in this chapter.

Performing and optimizing Western blots with an emphasis on chemiluminescent detection

Immunodetection refers to any detection method that exploits the interaction of an antibody and antigen. The choice of detection method, such as enzyme-linked immunosorbent assays (ELISAs) or Western blotting, depends on the researcher's preferences and requirements. If a researcher wants to quantify a low-abundance target protein then a chemiluminescent ELISA is used. If a researcher wants to identify a protein that is in high abundance, a colorimetric Western blot will suffice. If there are multiple targets within an assay, then multiplex fluorescence is typically used. This article focuses on Western blotting. Although colorimetric and fluorescent detection methods are discussed, chemiluminescent detection is used most often and is, therefore, discussed in great detail. Included is specific information about the chemiluminescent signal and factors that affect its intensity and longevity. We also describe types of blotting and present data and suggestions for obtaining semiquantitative data. Although classical Western blotting is typically used for qualitative purposes, we present information about effective quantitative analysis using specific controls. Common occurrences within the methodology and their possible explanations are also detailed. One frequent result is the appearance of ghost bands, which, based on our research, can be caused by high amounts of target or antibody cross-reactivity. Also included are the basic Western blot protocol and protocols for troubleshooting common problems and optimizing many of the specific factors that influence results.

Binding of nontarget microorganisms from food washes to anti-Salmonella and anti-E. coli O157 immunomagnetic beads: most probable composition of background Eubacteria

We present herein the composition of bacterial communities occurring in ground chicken and the changes which arise in these populations based upon nonselective partitioning by commercially-available Dynal anti-Salmonella and anti-E. coli O157 immunomagnetic beads (IMB). Our enumeration and colony selection protocol was based upon a 6 × 6 drop plate method (n = 18 for each 25-g sub-sampling) using a dilution which resulted in ca. 4-8 colonies per drop. An average of 82 ± 13 colonies were selected from three 25-g ground chicken subsamplings per batch, each of which was repeated seasonally for one year. DNA was extracted from each colony and the composition of Eubacteria in each of these harvests was determined by sequence-based identification of 16S rDNA amplicons. The Gram-positive bacteria Brochothrix thermosphacta and Carnobacterium maltaromticum were the most commonly found organisms in both the total chicken wash (PBS) and in the IMB-bound (PBS-washed) fractions. The remaining background organisms which also adhered to varying degrees to commercial IMBs were: Pseudomonas oleovorans, Acinetobacter lwoffi, Serratia spp., and one Rahnella spp. A large number of the organisms were also cladistically evaluated based on rDNA basepair disparities: all Brochothrices were monophyletic; twelve different Pseudomonads were found along with eight Carnobacteria, seven Acinetobacteres, four Serratiae, and two Rahnellae. Carnobacterium alone showed an IMB-based concentration enhancement (ca. two to sixfold).

Autocrine effects of IGF-I-induced VEGF and IGFBP-3 secretion in retinal pigment epithelial cell line ARPE-19

Hypoxia-induced physiological stress plays a central role in various neovascular diseases of the eye. Increased expression of hypoxia-inducible factor 1α (HIF-1α) and subsequent formation of HIF-1 dimers active at the vascular endothelial growth factor (VEGF) promoter lead to expression of this potent angiogenic factor in the retina, including retinal pigment epithelial (RPE) cells. We previously demonstrated that insulin-like growth factor I (IGF-I) stimulates VEGF and IGF binding protein (IGFBP)-3 secretion in RPE cells. In this study we examined IGF-I-induced HIF-1α expression, VEGF and IGFBP-3 secretion, and the autocrine actions of VEGF and IGFBP-3 on these processes in the spontaneously transformed RPE cell line ARPE-19. Cells were treated with CoCl2, IGF-I, recombinant human (rh)IGFBP-3, and rhVEGF. Immunoblot analysis revealed IGF-I-induced upregulation of total HIF-1α protein, whereas luciferase reporter assays of HIF-1 transcriptional activity demonstrated accumulation of HIF-1α correlated with the formation of functional HIF-1 heterodimers. Western and ligand blot analyses of RPE cell conditioned medium confirmed that IGF-I stimulated VEGF and IGFBP-3 secretion. rhVEGF stimulated IGFBP-3 secretion in an IGF-I- and HIF-1α-independent manner, whereas rhIGFBP-3 attenuated IGF-I-induced VEGF secretion. These findings demonstrate the multifaceted autocrine regulation of IGF-I-induced VEGF secretion by IGFBP-3 secreted in response to both IGF-I and, to a lesser extent, VEGF. These results provide evidence for HIF-1-dependent and -independent mechanisms by which IGF-I regulates VEGF and IGFBP-3 secretion.

Blocking nonspecific adsorption of native food-borne microorganisms by immunomagnetic beads with ι-carrageenan

We present herein the partitioning characteristics of anti-Salmonella and anti-Escherichia coli O157 immunomagnetic beads (IMB) with respect to the nonspecific adsorption of several nontarget food-borne organisms with and without an assortment of well-known blocking agents, such as casein, which have been shown to be useful in other immunochemical applications. We found several common food-borne organisms that strongly interacted with both types of IMB, especially with anti-Salmonella form (av DeltaG0=-20 +/- 4 kJ mol(-1)) even in the presence of casein [1% (w/v): DeltaG0=-18 +/- 3 kJ mol(-1); DeltaDeltaG0 approximately -2 kJ mol(-1)]. However, when one of the most problematic organisms (a native K12-like E. coli isolate; DeltaG0=-19 +/- 2 kJ mol(-1)) was tested for nonspecific binding in the presence of iota-carrageenan (0.03-0.05%), there was an average decline of ca. 90% in the equilibrium capture efficiency xi (DeltaG0=-11 +/- 4 kJ mol(-1); DeltaDeltaG0 approximately -8 kJ mol(-1)). Other anionic polysaccharides (0.1% kappa-carrageenan and polygalacturonic acid) had no significant effect (av DeltaG0=-19 +/- 1 kJ mol(-1); DeltaDeltaG0 approximately 0 kJ mol(-1)). Varying iota-carrageenan from 0% to 0.02% resulted in xi significantly diminishing from 0.69 (e.g., 69% of the cells captured; DeltaG0=-19 +/- 3 kJ mol(-1)) to 0.05 (DeltaG0=-11 +/- 2 kJ mol(-1); DeltaDeltaG0 approximately -9 kJ mol(-1)) at about 0.03% iota-carrageenan where xi leveled off. An optimum blocking ability was achieved with 0.04% iota-carrageenan suspended in 100 mM phosphate buffer. We also demonstrated that the utilization of iota-carrageenan as a blocking agent causes no great loss in the IMBs capture efficiency with respect to the capture of its target organisms, various salmonellae.

A solid-phase method for evaluation of gold conjugate used in quantitative detection of antigen by immunogold-labeling electron microscopy

Rapid and sensitive screening for confirming the reactivity of reagents, before proceeding for electron microscopy, is highly desirable. ELISA-based methods have been shown to be highly efficient and successful for rapid prescreening and optimization of immunological as well as sample-processing reagents for the sensitive detection and quantitation of antigen by electron microscopy. The drawback of these methods lies in their inability to provide any information regarding the gold conjugate used for the final observed and measured signal. In this work, we demonstrate a simple and rapid, solid-phase method in ELISA format that is also suitable for evaluation and optimization of the gold conjugate. We have demonstrated the utility of this technique by screening for Vitreoscilla hemoglobin (VHb) antigen in cell lysates and confirming the results directly with immunogold-labeling transmission electron microscopy (TEM) of cell sections. The sensitivity of detection and quantitation of antigens by immuno-electron microscopy depends upon the assay procedure being optimized to obtain the best possible signal. Our study indicates that evaluation of gold conjugate by the solid-phase assay could help in the rapid optimization of this reagent for immunogold localization and quantification of antigens by TEM.

Optimization of immunogold labeling TEM: an ELISA-based method for evaluation of blocking agents for quantitative detection of antigen

We developed an ELISA-based method for rapid selection of optimal blocking agents to be used in antigen quantification by immunogold labeling electron microscopy. Casein, skim milk, BSA from two sources, acetylated BSA, fish skin gelatin, horse serum, and goat serum were tested for their ability to block nonspecific binding of antibody to recombinant Vitreoscilla hemoglobin (VHb) antigen expressed in Escherichia coli cells by ELISA and the results were confirmed by quantitative immunogold labeling transmission electron microscopy (TEM). Ability to minimize NSB was also evaluated by dot-blot and Western blotting methods. The results demonstrated that ELISA was most accurate in predicting the most efficient blocking agent for TEM. Existing methods could not provide an accurate picture of the ability of various reagents to suppress background labeling. The sensitivity of detection of antigens by immunoelectron microscopy depends on the assay procedure being optimized to obtain the highest possible signal along with as low a background (noise) as possible. Our study indicated that an ELISA-based evaluation of various blocking agents could help in the rapid selection and optimization of a suitable protocol for immunogold localization and quantification of antigens by TEM.

The use of Tween 20 in immunoblotting assays for the detection of autoantibodies in connective tissue diseases

Autoantibodies directed against intracellular antigens can be detected by immunoblotting (IB). Due to its high sensitivity this technique has many advantages, but it can give misleading results when the specific bands are weak or blurred against the background staining. To decrease background staining, non-ionic detergents (Tween 20, Triton X-100, Nonidet P-40) are generally used as blocking agents. Moreover, these agents appear to have a renaturating action towards proteins and antigens. Tween 20 has a more pronounced renaturating effect on proteins than other detergents and thereby improves antigen-antibody binding. To evaluate the effect of Tween 20 on specific autoantibody detection by IB, we tested the sera of 162 patients with connective tissue diseases (CTDs) by adding this detergent at certain steps of the IB assay. We found that the use of Tween 20 in the IB procedure significantly improved the binding of autoantibodies to Jo-1, Scl70, (U1)RNP 68 kDa and C, Sm B/B' and D. Moreover, it increased the sensitivity for the detection of anti-Sm D peptide in systemic lupus erythematosus (SLE) sera with no decrease in specificity. In contrast, the addition of Tween 20 significantly decreased the binding of autoantibodies specific for ribosomal P proteins, La/SSB, Ro/SSA, but not the overall sensitivity and specificity of the method. We conclude that the addition of Tween 20 to standard IB is advantageous for anti-nuclear antigen antibody detection and improves the sensitivity of the method in revealing anti-Sm-positive sera in SLE. However, Tween 20 is not recommended for the detection of anti-cytoplasmic antibodies.

Capture-ELISA: a new assay for the detection of immunoglobulin M isotype antibodies using Chlamydia trachomatis antigen

We present here a new method of IgM antibody-capture enzyme-linked immunosorbent assay (IgM-Capture-ELISA) for the diagnosis of recently acquired infections with Chlamydia trachomatis. For this analysis, plates were coated with goat IgG anti-human Fc mu. The capture of serum IgM antibodies was revealed indirectly by the sequential addition of biotinylated chlamydial proteins and peroxidase-conjugated streptavidin. In chlamydial extracts, cysteine-rich proteins are preferential antigenic targets for the humoral response. 3-(N-maleimidopropionyl)-biocytin (MPB), which binds biotinylated moieties to sulfhydryl groups, was used for the labeling procedure. The preservation of the antigenic specificity of labeled proteins was controlled by a blotting of these proteins, which were, respectively, probed either with specific IgM antibodies or with streptavidin. This analysis revealed that, after labeling, recognized epitopes are more particularly present on the major outer membrane protein (MOMP) of Chlamydia trachomatis. The validation of IgM-Capture-ELISA was assessed by using 170 selected sera from patients suspected of being infected by Chlamydia. Results were respectively compared to conventional indirect immunofluorescence assays (MIF-IgM assays) and to Western blotting. Sixteen sera were found to possess IgM antibodies against Chlamydia trachomatis with IgM-Capture-ELISA. Among these 16 sera, 14 and 15 were, respectively, positive with MIF-IgM assays and in Western blotting. Data obtained with IgM-Capture-ELISA reveal the absence of false-positive results in sera containing rheumatoid factor, which has been shown to interfere in the two other methods. IgM-Capture-ELISA value was then confirmed using sera from patients consulting for genital or pulmonary diseases, from patients with confirmed chlamydial infections, and from patients with other pathologies. IgM-Capture-ELISA appears as an alternative simple semi-quantitative assay for the detection of early chlamydial infection.

Optimisation of a peptide-based indirect ELISA for the detection of antibody in the serum of HIV-1 seropositive patients

A modified peptide-based indirect ELISA technique for the detection of HIV-1 specific antibodies in the sera of HIV-1 seropositive individuals is described. We found that the reduction of non-specific binding of HIV-1 seropositive sera to the ELISA plate was essential for the reliable detection of serum antibodies in the peptide based indirect ELISA. Optimal results were obtained using Immulon microtitre plates, different concentrations of denatured. purified grade of casein in the blocking (1%) and washing (0.25%) solutions and by diluting HIV-1 seropositive sera 1 in 1600. These conditions reduced non-specific binding and improved assay sensitivity. We show that the inclusion of a control peptide is essential to reducing the incidence of false positive and false negative results. Taken together, the modifications described in this report improve reliability of the peptide-based indirect ELISA without compromising its sensitivity and have particular relevance for those wishing to apply the peptide-based indirect ELISA technique to serum samples which exhibit high levels of non-specific binding. To illustrate this, levels of antibody in the sera of HIV-1 seropositive and seronegative donors that are specific for peptides derived from a conserved region of HIV-1 gp120 sharing homology with the FAS apoptosis antigen were analysed using this technique.

Evaluation of different methods for the detection of outer membrane proteins and lipopolysaccharides of Pasteurella haemolytica by immunoblotting

The optimal conditions for the detection of outer membrane proteins (OMPs) and lipopolysaccharide (LPS) of Pasteurella haemolytica by immunoblotting were evaluated. The variables examined included the equilibration time of the gels before transfer, composition of the transfer buffer, type of blotting membrane, blocking agent, effect of the zwitterionic detergent Empigen BB on protein renaturation, and the development reagent. The composition of the transfer buffer and time of gel equilibration significantly affected the efficiency of transfer of both OMPs and LPS. However, the optimal conditions for the transfer of OMPs were not the same as those for LPS. Thus, optimal transfer of OMPs occurred in Tris-glycine buffer, with prior equilibration of the gels to allow for expansion, whereas optimal transfer of LPS was achieved in Tris-glycine-methanol buffer with no equilibration of the gels. In Tris-glycine-methanol buffer, gel equilibration resulted in a significantly reduced transfer of both OMPs and LPS, probably due to the removal of SDS from these components. The use of Zeta-Probe blotting membrane which, unlike nitrocellulose, does not require methanol for optimal protein binding, did not result in improved binding of OMPs or LPS in the absence of methanol and, even after prolonged blocking (> 2 h), gave higher background staining than did nitrocellulose. Effective blocking of nitrocellulose was achieved with 3% (w/v) gelatin, 2.5% (w/v) skimmed milk or 0.3% (v/v) Tween 20, whereas increased background staining occurred with 1% (w/v) bovine serum albumin or 1% (w/v) ovalbumin. The incorporation of Empigen BB in the primary antibody buffer did not improve antibody recognition of proteins as a result of their renaturation. For the horseradish-peroxidase enzyme development system, the substrate 3,3'-diaminobenzidine tetrahydrochloride was more sensitive, and developed more quickly, than 4-chloro-1-naphthol, but faded more rapidly after drying of the membrane. 4-chloro-1-naphthol was more suitable for identifying OMPs because less background staining occurred, whereas 3,3'-diaminobenzidine tetrahydrochloride was more suitable for the detection of LPS due to its greater sensitivity.

Antibodies bound to nitrocellulose in acidic buffers retain biological activity

This report compares the binding of proteins to nitrocellulose membranes in acidic buffers (pH 2 and 3) with binding in neutral buffer (pH 7). Initially, similar amounts of antibodies and other proteins bound to the nitrocellulose membrane in both acidic and neutral buffers. However, the susceptibility of individual proteins to displacement (stripping) from the membrane by the milk blocking agent depended on the pH of the buffer used to bind the proteins to the membrane. Most proteins that bound to nitrocellulose in acidic buffers were relatively resistant to milk-stripping compared to proteins bound in pH 7 buffer. Acid-binding of proteins to nitrocellulose also decreased the amount of protein that was stripped from the nitrocellulose membrane when Tween 20 was included in the washing buffer. After correcting for the amount of antibody remaining on the membrane after the milk block, it was found that acid-bound antibodies were unchanged in biological activity when compared with the same antibodies bound at neutral pH. These results suggest that acid-binding of proteins could increase the sensitivity of nitrocellulose membrane assays that use milk and/or Tween 20.

Mapping of the immunoreactive domains of a small nuclear ribonucleoprotein-associated Sm-D autoantigen

The Sm-D(D1) small nuclear ribonucleoprotein (snRNP) polypeptide is a major target of autoantibodies diagnostic for systemic lupus erythematosus. The cDNA encoding the protein from Raji cells was expressed in Escherichia coli as a fusion protein with anthranilate synthase (TrpE-Sm-D). When tested by protein blot, the recombinant polypeptide was strongly immunoreactive under defined blotting conditions, which appear to facilitate the refolding of the polypeptide into a native conformation. Multiple translational fusions between the trpE gene and fragments encompassing the length of the Sm-D coding sequence were constructed for epitope mapping. The results describe two general patterns of anti-Sm reactivity: (i) antibodies that recognize only the full-length antigen and are presumably directed against discontinuous epitopes, and (ii) antibodies that recognize the carboxy terminus of the antigen which embodies an extended/charged structure.

Binding of antibodies and other proteins to nitrocellulose in acidic, basic, and chaotropic buffers

This report compares the binding of proteins to nitrocellulose membranes in acidic buffers (pH 2 and 3) with binding in neutral buffer (pH 7), basic buffers (pH 12 and 13), 8 M urea (pH 2, 3, and 7), and 6 M guanidine hydrochloride (pH unadjusted). Initially, similar amounts of antibodies and other proteins bound to the nitrocellulose membrane in all of these buffers and solvents. However, the susceptibility of individual proteins to displacement (stripping) from the membrane by the milk blocking agent depended on both the pH and the type of buffer or solvent used to bind the proteins to the membrane. Most proteins that were bound to nitrocellulose in acidic buffers were relatively resistant to milk stripping compared to proteins bound in pH 7 buffer. After correction for the amount of antibody remaining on the membrane after the milk block, it was found that acid-bound antibodies were unchanged in biological activity when compared with the same antibodies bound at neutral pH. These results suggest that acid binding of proteins could increase the sensitivity of nitrocellulose membrane assays using a milk block.

An immunoblot technique for identification of allergens of the Australian paralysis tick Ixodes holocyclus

The blocking conditions appropriate for Western blot identification of allergens of the Australian paralysis tick, Ixodes holocyclus, have been investigated. The results suggest that human serum is a potent blocking agent which may have wide application in the immunodetection of antigens.

Inhibition of the streptavidin-biotin interaction by milk

Nonfat dried milk is routinely used as a blocking agent and diluent in immunoassays. However, the data presented in this paper demonstrate that milk contains an inhibitor of the biotin-streptavidin interaction. It is recommended that milk be dialyzed or used at lower concentrations when it is employed as a diluent of streptavidin.

Loss of antigens from immunoblotting membranes

Immunoblotting of PAGE-separated proteins is widely used. After electrophoretically transferring proteins from PAGE gels, sites on the membrane which are unoccupied are blocked or quenched with excess protein. The sensitivity of the immunoblotting technique is in part dependent on the amount of antigen associated with the membrane. We have investigated the loss of blotted proteins during blocking of various membranes with milk. Transblotted proteins are lost from nitrocellulose membranes (NC) (0.45 microns) in proportion to the amount of milk in the blocking solution and loss increases with time. When 5% milk was used as the blocking solution the amount lost was about 10% within 1 h and approached 25% at 24 h. This loss was nonselective and involved blotted antigens of a wide range of Mr. This time- and concentration-dependent relationship for the loss of transblotted proteins was tested with four immunoblotting membranes: Immobilon-P (Millipore), NitroPlus-2000 (MSI), and NC (0.2 microns and 0.45 microns pore size). NC (0.2 microns) gave the best results with the lowest percentage loss at 1 h (less than 5%). We recommend using NC (0.2 microns) and blocking with 0.5% milk for 30 min to minimize the loss of antigens from immunoblotting membranes.

Enhanced immunodetection of blotted house dust mite protein allergens on nitrocellulose following blocking with Tween 20

The effect of blocking nitrocellulose membranes with the nonionic detergent Tween 20 on the detection, by protein blotting, of IgE-binding to house dust mite Dermatophagoides pteronyssinus allergens has been investigated. Tween blocking led to enhanced immunodetection of allergens despite removal of proteins from the membrane when compared to protein blocking agents which did not displace transferred components. The enhancement varied with the different mite components and, for one in particular, antigen Der p II, an increase of more than 100-fold in IgE antibody binding occurred despite a concurrent loss of more than 90% of Der p II from the membrane. Both the enhancement of binding and loss of components from the membrane were dependent upon the time course of blocking and the concentration of Tween used.

Western immunoblotting: temperature-dependent reduction in background staining

The effect of incubation temperature on the background staining of Western blots with monoclonal antibodies to a human milk protein, alpha-lactalbumin (Mr 14,500), is presented. Human milk proteins were electrophoretically separated and transferred to nitrocellulose membranes which were then blocked with bovine serum albumin, "BLOTTO", casein, or Tween 20. They were subsequently incubated with mouse monoclonal antibody to human alpha-lactalbumin, biotinylated anti-mouse antibody, strepavidin-biotinylated horseradish peroxidase complexes and a substrate containing diaminobenzidine and nickel chloride. Reduction of incubation temperature from 37 degrees C to 22 degrees C and 4 degrees C was found to decrease the extent of non-specific background staining independent of the type of blocking reagent used. Good specific staining with minimal background was found using 0.1% Tween 20 in phosphate-buffered saline, pH 7.2, as blocking agent and incubation temperatures of 4 degrees C.

Visualization of multiple protein bands on the same nitrocellulose membrane by double immunoblotting

A method has been developed which allows the simultaneous immunodetection of more than one type of protein on the same nitrocellulose membrane. This procedure does not require special labeling of samples or elution of antibodies from the membrane as do the alternatives cited in the literature (1,2). Proteins are separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and electrophoretically transferred to the membrane before specific immunostaining with either peroxidase/4-chloro-1-naphthol or immunogold/silver staining. Antigen identity is visually determined by the formation of different-colored precipitates on the membrane. This innovation in protein blotting offers a savings in time and reagents as well as permitting identification of closely spaced bands with certainty.

A comparison of immunological methods for the detection of Trichinella spiralis antigen

Eight immunological methods all using the same monoclonal antibody reagent were compared for the detection of Trichinella spiralis antigen. These were based on: (1) the direct adsorption of the antigen to the immunoadsorbent (nitrocellulose membrane, polyvinyl chloride strip or microplate); (2) capture of the antigen by antibodies pre-sensitized on the immunoadsorbent; and (3) latex agglutination. The methods found suitable were: (a) capture radioimmunoassay (capture-RIA) (sensitivity: less than 0.5 microgram/ml antigen); (b) direct enzyme immunoassay (direct-ELISA) (less than 0.5 microgram/ml); (c) tube latex agglutination test (2.2 micrograms/ml); and (d) direct immunodot assay (8.8 micrograms/ml). However, the performance of the direct-ELISA was greatly affected by the presence of each of three extraneous substances (bovine serum albumin (BSA), lipopolysaccharide (LPS), normal swine muscle homogenate (NSM) added to the antigen sample. The direct immunodot assay was also affected by the presence of BSA or LPS, whereas both the capture-RIA and the tube latex agglutination methods were affected by the presence of NSM only. The findings are discussed with a view of detecting T. spiralis larvae directly from pork samples by immunological means.

Restoration of antibody binding to blotted meningococcal outer membrane proteins using various detergents

Restoration of IgG antibody binding to heat-denatured meningococcal outer membrane proteins has been studied on immunoblots with a series of 14 detergents. Nitrocellulose strips with the blotted proteins were incubated with the detergents and sera from human volunteers vaccinated with meningococcal membrane proteins. Zwitterionic and ionic detergents, containing substituted quarternary ammonium or amino groups with a minimum of 10 C atoms in the alkyl chain, restored the antigenicity of the serotype-specific class 2 porin protein. The concentrations of the Zwittergent detergents necessary for activation decreased with increasing alkyl chain length of the homologues. Only zwitterionic detergents renatured the class 1 protein. Both proteins were weakly antigenic in the presence of the nonionic detergents Triton X-100 and Tween 20. Meningococcal lipopolysaccharide restored antibody binding to the porin, but not to the class 1 protein. Similar concentrations of lipopolysaccharides from two other gram-negative bacteria had no effect.

Immunoblot analysis of humoral immune responses following infection with Bordetella pertussis or immunization with diphtheria-tetanus-pertussis vaccine

To help develop better diagnostic tests for pertussis, we examined the serologic response to whole-cell proteins of Bordetella pertussis after natural infection or vaccination with diphtheria-tetanus-pertussis vaccine. Serum specimens collected during a pertussis outbreak investigation and from uninfected persons were used in Western blot (immunoblot) analyses to determine the presence of immunoglobulin G (IgG) and IgA antibodies to specific B. pertussis proteins. IgG antibodies to proteins of molecular masses 220 and 210 kilodaltons (kDa) were detected in 14 of 18 serum samples obtained from patients with culture-confirmed pertussis greater than or equal to 40 days after the onset of coughing. IgA antibodies were detected in 15 of the 18 samples. Of 19 serum samples obtained from patients who had not been ill with pertussis, 6 contained IgG antibodies to these proteins and 1 contained IgA antibodies. The two proteins bound antiserum specific for filamentous hemagglutinin and comigrated with purified filamentous hemagglutinin. IgG antibodies to two additional protein bands of molecular masses 84 and 75 kDa were associated with previous vaccination. Antibody to the 84-kDa protein was detected in 15 of 17 vaccinated, never-infected persons, and antibody to the 75-kDa protein was detected in 16 of the 17. None of 11 nonvaccinated, never-infected persons tested had antibodies to either protein. All seven fully vaccinated persons with culture-documented infection had antibodies to both proteins. Antibodies to the 84-kDa protein were detected in 6 of 22 nonvaccinated and infected persons, and antibodies to the 75-kDa protein were detected in 8 of the 22. Use of Western blot analysis in this study allowed us to distinguish antibody responses to infection and immunization.

The use of Tween 20 alone as a blocking agent for immunoblotting can cause artefactual results

The use of Tween 20 as a suitable blocking agent in immunoblotting studies was evaluated by screening a panel of monoclonal antibodies (MoAbs) against a selection of blotted proteins which were unrelated to the antigens used to raise the MoAbs. Using Tween 20 alone to block the nitrocellulose membranes clear reactions were observed between the panel of MoAbs and several components of the blotted protein mixture. In contrast, when haemoglobin was used to block the membranes such reactions were not observed. In the absence of added protein the use of Tween 20 alone as a blocking agent for immunoblotting appears to lead to false positive reactions by non-specific antigen-antibody complexes.

The porin protein of the outer membrane of Escherichia coli: reactivity in immunoblotting, antibody-binding by the native protein, and cross-reactivity with other enteric bacteria

The experimental conditions for antibody-binding by the 38.5 kD porin protein of an E. coli 055 strain in immunoblotting were investigated. A non-ionic detergent in the buffer which contained the primary antibody was required for antibody-binding by electroblots of the SDS-denatured protein. Immunoblotting, using antiserum absorbed with bacteria or the outer membrane (OM) of the E. coli 055 strain, showed results concordant with inaccessibility to antibodies of the 38.5 kD porin protein in its native configuration in the bacterial cells, but immunoreactivity when contained in the OM. OM from strains of different genera of the Enterobacteriaceae and antisera against these strains when used in immunoblot analyses showed that the E. coli 055 porin protein harboured antigenic determinants which are common to the various genera of the enteric bacilli. Cross-reactivity with non-enteric Gram-negative bacteria was not observed.

Improvements of Western blotting to detect monoclonal antibodies

A comparison of the effects of different factors on the sensitivity of Western blotting technique to detect monoclonal antibodies is described. The major improvements were obtained by: A) renaturating the antigen in the gel before transferring it in carbonate buffer at pH 10 onto nitrocellulose and B) using alkaline-phosphatase-conjugated second antibody instead of peroxidase-conjugated second antibody.

Quantitative differences among various proteins as blocking agents for ELISA microtiter plates

We tested instantized dry milk, casein, gelatins from pig and fish skin, serum albumin and several other proteins for their abilities to block non-specific binding (NSB) of a peroxidase-conjugated immunoglobulin to polystyrene microtiter plate wells. Each blocking protein was tested across a million-fold concentration range, both in simultaneous incubation with the peroxidase conjugate and as a pretreatment agent where excess protein was washed away before incubation with the conjugate. Overall, instantized milk and casein were the most effective proteins tested: they inhibited NSB by over 90% in both the simultaneous and pretreatment modes at far lower concentrations than most of eight other proteins. Enzymatically hydrolyzed porcine skin gelatin was the least effective protein tested: it did not reduce NSB by more than 90% even at its highest concentrations; its blocking ability fell rapidly upon dilution; and it was almost useless as a pretreatment agent. Fish skin gelatin showed much better blocking activity than hydrolyzed porcine gelatin, and it still had the practical advantage of remaining fluid even under refrigeration. Our results suggest that some proteins (such as casein) block NSB to plastic primarily through protein-plastic interactions, while others (such as porcine skin gelatin) block primarily through protein-protein interactions. Although the optimal blocking agent for any particular ELISA system must be determined by empirical testing, these results should be helpful in selecting the best possible candidate proteins for further evaluation.

Identification and isolation of soluble histones from bovine milk and serum

An immunoassay for soluble histones as trace components of biological fluids was developed on the basis of the dual capacity of histones to bind solid-phase DNA and monoclonal anti-histone antibody. Application of this histone-capture assay to bovine milk resulted in a positive signal, and DNA-cellulose chromatography was used to isolate histone-like material in microgram quantities. Western-blot analysis using a panel of anti-histone antibodies demonstrated the presence of histones H2A, H2B and H4 in apparently intact form. DNAase digestion experiments indicated that at least a portion of milk histone was complexed to DNA. Bovine serum was analysed in the same manner on serial DNA-cellulose columns, and H4 and partially degraded H2A were detected by Western-blot analysis. The finding of soluble histones in bovine milk and serum may account for unexpected results when these biological fluids are used as blocking reagents in Western blots and other immunoassays and may have ramifications in the origin and significance of anti-histone antibodies in human disease.

Tween 20 removes antibodies and other proteins from nitrocellulose

It has generally been assumed that the binding of most proteins to nitrocellulose is stable in the non-ionic detergent Tween 20. However, the following paper demonstrates that in immunoassays where antibodies or other proteins are bound directly to the nitrocellulose, 0.05% Tween 20 may dissociate these proteins from the membrane. The degree of dissociation appears to be dependent on the individual protein studied. Some antibodies and other proteins bind tightly to nitrocellulose and dissociation of these proteins by Tween 20 is barely detectable. In contrast, other proteins are nearly completely stripped from the nitrocellulose by the same detergent. Therefore, unless it is known from control experiments what proteins will or will not be dissociated from nitrocellulose by Tween 20, the routine use of Tween 20 in the development of Western blots, native blots and dot-blots should be discontinued.

Effects of the blocking agents bovine serum albumin and Tween 20 in different buffers on immunoblotting of brain proteins and marker proteins

The effects of the blocking agents bovine serum albumin and Tween 20 in buffers at pH values 7.2 and 10.2 were compared in immunoblotting with 2 different antisera. The antisera were raised against a purified brain-specific protein fraction from human brain, soluble in perchloric acid, and phosphate-activated glutaminase from pig brain, respectively. The antigens were a crude perchloric acid-soluble brain extract, a crude brain phosphate-activated glutaminase fraction, and proteins commonly used as molecular weight markers. The binding patterns of the 2 antisera to the respective brain antigen preparations changed, depending on the blocking agent and the pH of the blocking buffer. Also, antibody binding to the molecular weight marker proteins was observed with some of the blocking buffers. Immunoblotting with Tris-saline, pH 10.2, containing 3% bovine serum albumin as blocking agent and diluting buffer for the antisera, showed negligible antibody binding to the marker proteins and most specific binding to the brain antigens.