Differential binding characteristics of protein G and protein A for Fc fragments of papain-digested mouse IgG

Development of immunochromatographic strip assay to detect recent infection of Japanese encephalitis virus in swine population

Japanese Encephalitis (JE) is a mosquito borne re-emerging viral zoonotic disease. Sero-conversion in swine occurs 2-3 weeks before human infection, thus swine act as a suitable sentinel for predicting JE outbreaks in humans. The present study was undertaken with the objective of developing immunochromatographic strip (ICS) assay to detect recent infection of Japanese Encephalitis virus (JEV) in swine population. The two formats of ICS assay were standardized. In the first format, gold nanoparticles (GNP) were conjugated with goat anti-pig IgM (50 μg/ml) followed by spotting of recombinant NS1 protein (1 mg/ml) of JEV on NCM as test line and protein G (1 mg/ml) as control line. In the format-II, GNP were conjugated with rNS1 protein (50 μg/ml) followed by spotting of Goat anti-pig IgM (1 mg/ml) as test line and IgG against rNS1 (1 mg/ml) as control line. To decrease the non- specific binding, blocking of serum and nitrocellulose membrane (NCM) was done using 5% SMP in PBS-T and 1% BSA, respectively. Best reaction conditions for the assay were observed when 10 μl of GNP conjugate and 50 μl of 1:10 SMP blocked sera was reacted on BSA blocked NCM followed by reaction time of 15 mins. Samples showing both test and control line were considered positive whereas samples showing only control line were considered negative. A total of 318 field swine sera samples were screened using indirect IgM ELISA and developed ICS assay. Relative diagnostic sensitivity and specificity of format-I was 81.25% and 93.0% whereas of format-II was 87.50% and 62.93%, respectively. Out of 318 samples tested, 32 were positive through IgM ELISA with sero-positivity of 10.06% while sero-positivity with format-I of ICS was 8.1%. Owing to optimal sensitivity and higher specificity of format-I, it was validated in three different labs and the kappa agreement ranged from 0.80 to 1, which signifies excellent repeatability of the developed assay to test field swine sera samples for detecting recent JEV infection.

A Broad-Range Disposable Electrochemical Biosensor Based on Screen-Printed Carbon Electrodes for Detection of Human Noroviruses

Human noroviruses (HuNoVs) are the major non-bacterial pathogens causing acute gastroenteritis in people of all ages worldwide. No stable culture system in vitro is available for routing the detection of multiple strains of HuNoVs. A simple and rapid method for detection of HuNoVs is of great significance for preventing and controlling this pathogen. In this work, an electrochemical biosensor for sensitive and fast detection of HuNoVs was constructed based on a screen-printed carbon electrode (SPCE). Gold nanoparticles and protein-A were applied on the SPCE surface for enhancement of the electrical signals and the linkage of antibodies with a fixed orientation, respectively. A monoclonal antibody (MAb) against the S domain protein of the viral capsid (VP1) was further immobilized on the SPCE to bind HuNoVs specifically. The binding of VP1 to the coated MAbs resulted in the reduction of conductivity (current) measured by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The reduction in the current was correlated to the concentration of VP1/HuNoVs. The detection limitation of Genogroup I.1 (GI.1) VP1 and Genogroup II.4 (GII.4) VP1 was 0.37 ng/ml (≈1.93×10⁷ HuNoVs/mL) and 0.22 ng/ml (≈1.15×10⁷ HuNoVs/mL), respectively. The detection limitation of both GI and GII HuNoVs in clinical fecal samples was 10⁴ genomic copies/mL. The results could be obtained in 1 h. We demonstrated that this disposable electrochemical biosensor was a good candidate for rapid detection of different genogroup and genotype HuNoVs.

Eriodictyol as a Potential Candidate Inhibitor of Sortase A Protects Mice From Methicillin-Resistant Staphylococcus aureus-Induced Pneumonia

New anti-infective approaches are urgently needed to control multidrug-resistant (MDR) pathogens, such as methicillin-resistant Staphylococcus aureus (MRSA). Sortase A (SrtA) is a membrane-bound cysteine transpeptidase that plays an essential role in the catalysis of covalent anchoring of surface proteins to the cell wall of Staphylococcus aureus (S. aureus). The present study reports identification of a flavonoid, eriodictyol, as a reversible inhibitor of SrtA with an IC50 of 2.229 ± 0.014 μg/mL that can be used as an innovative means to counter both resistance and virulence. The data indicated that eriodictyol inhibited the adhesion of the bacteria to fibrinogen and reduced the formation of biofilms and anchoring of staphylococcal protein A (SpA) on the cell wall. The results of fluorescence quenching experiments demonstrated a strong interaction between eriodictyol and SrtA. Subsequent mechanistic studies revealed that eriodictyol binds to SrtA by interacting with R197 amino acid residue. Importantly, eriodictyol reduced the adhesion-dependent invasion of A549 cells by S. aureus and showed a good therapeutic effect in a model of mouse pneumonia induced by S. aureus. Overall, the results indicated that eriodictyol can attenuate MRSA virulence and prevent the development of resistance by inhibiting SrtA, suggesting that eriodictyol may be a promising lead compound for the control of MRSA infections.

An improved protocol for large scale production of high purity 'Fc' fragment of human immunoglobulin G (IgG-Fc)

We describe a simplified approach for purification and characterization of human 'IgG-Fc' fragment used widely as immunochemical tool and for therapeutic purposes. The 'Fc' fragment was purified from human IgG in a 3-stage column chromatography. The purified 'Fc' fragment appeared as a dimer glycoprotein with an apparent molecular mass of 52,981 Dalton (Ultraflex MALDI TOF/TOF). The Size-exclusion HPLC profile of the purified 'Fc' fragment of human IgG matched that of a commercially procured reference 'Fc' fragment material. The purity of the 'Fc' fragments was >99% by SDS-PAGE and size-exclusion HPLC. The results of Western blotting, immunoelectrophoresis, and mass spectrometry analysis indicate a high purity of the 'Fc' fragment. Peptide mass fingerprint analysis of the purified 'Fc' protein yielded peptides that partially match the known database sequences of FCG3B_HUMAN (Uniprot ID: O75015). This method of purification of the 'Fc' fragment is suitable for achieving high purity level of 'Fc' fragment protein. With this purification approach, the cost of the purified 'Fc' fragment of human IgG is significantly reduced as compared with the current market price of IgG-Fc fragment protein in international market. The purified 'IgG-Fc' fragment protein was found to be negative for major viral markers.

A non-toxic enzyme-linked immunosorbent assay for aflatoxin B1 using anti-idiotypic antibodies as substitutes

BACKGROUND

Immunoassays are widely employed techniques to detect aflatoxins since they are rapid, selective and sensitive. One common disadvantage of them is using aflatoxins as standard substances, which may trigger exposure risks to operators and environmental contamination without proper handling. Anti-idiotypic antibodies (anti-Ids or Ab2s), also named as internal-image anti-Ids, are able to mimic and function as antigens, so a non-toxic enzyme-linked immunosorbent assay (ELISA) for aflatoxin B₁ (AFB₁ ) is developed and validated using anti-Ids as substitutes.

RESULTS

Mouse monoclonal anti-idiotypic antibody (McAb2) to AFB₁ was generated by the hybridoma technique using Fab fragments of rabbit anti-AFB₁ idiotype antibody (Ab1) as immunogen. As indicated by indirect competitive ELISA, McAb2, represented an internal-image of antigen AFB₁ , was able to bind Fab with competition to AFB₁ . Then, analysis of AFB₁ in spiked samples by non-toxic ELISA using anti-Ids as substitutes was developed, and it showed no significant differences with comparison to AFB₁ as competitive antigens.

CONCLUSION

Our work demonstrated that anti-Ids could be used as internal-image mimicry of AFB₁ , and it had potential applications in immunoassays for antigen substitution to reduce operational risk for operators and environmental contamination. © 2016 Society of Chemical Industry.

A new immunoassay of serum antibodies against Peste des petits ruminants virus using quantum dots and a lateral-flow test strip

A fast and ultrasensitive test-strip system combining quantum dots (QDs) with a lateral-flow immunoassay strip (LFIAS) was established for detection of Peste des petits ruminants virus (PPRV) antibody. The highly luminescent water-soluble carboxyl-functionalized QDs were used as the signal output and were conjugated to streptococcal protein G (SPG), which was capable of binding to immunoglobulin G (IgG) from many species through an amide bond to capture the target PPRV IgGs. The PPRV N protein, which was immobilized on the detection zone of the test strip, was expressed by transfecting recombinant Bacmid-PPRV-N with Lipofect into Sf9 insect cells. When exposed to PPRV IgG, QD-SPG bound to PPRV N protein, resulting in the formation of a complex that subsequently produced a bright fluorescent band in response to 365 nm ultraviolet excitation. Sensitivity evaluation showed that the QD-LFIAS limit of detection (LOD) for PPRV antibody was superior to competitive enzyme-linked immunosorbent assay (c-ELISA) and the immunochromatographic strip. No cross reaction was observed when the positive sera of bluetongue virus, canine distemper virus, goat pox virus, and foot-and-mouth disease virus were tested. Further evaluation using field samples indicated that the diagnostic specificity and sensitivity of the QD-LFIAS was 99.47 and 97.67 %, respectively, with excellent agreement between QD-LFIAS and c-ELISA. The simple analysis step and objective results that can be obtained within 15 min indicate that this new method shows great promise for rapid, sensitive detection of PPRV IgG for onsite, point-of-care diagnosis and post vaccination evaluation (PVE). Graphical Abstract Ultrasensitive fluorescent QD immunochromotography in combination with recombinant PPRV N protein could be used to detect PPRV antibody in serum.

Fluorescent Ly6G antibodies determine macrophage phagocytosis of neutrophils and alter the retrieval of neutrophils in mice

Fluorescently labeled Ly6G antibodies enable the tracking of neutrophils in mice, whereas purified anti-Ly6G rapidly depletes neutrophils from the circulation. The mechanisms underlying neutrophil depletion are still under debate. Here, we examined how identical Ly6G antibodies coupled to different fluorochromes affect neutrophil fate in vivo. BM cells stained with Ly6G antibodies were injected into mice. The number of retrieved anti-Ly6G-FITC(+) cells was reduced significantly in comparison with anti-Ly6G-APC(+) or anti-Ly6G-PE(+) cells. Flow cytometry and multispectral imaging flow cytometry analyses revealed that anti-Ly6G-FITC(+) neutrophils were preferentially phagocytosed by BMMs in vitro and by splenic, hepatic, and BM macrophages in vivo. Direct antibody injection of anti-Ly6G-FITC but not anti-Ly6G-PE depleted neutrophils to the same degree as purified anti-Ly6G, indicating that the FITC-coupled antibody eliminates neutrophils by a similar mechanism as the uncoupled antibody. With the use of a protein G-binding assay, we demonstrated that APC and PE but not FITC coupling inhibited access to interaction sites on the anti-Ly6G antibody. We conclude the following: 1) that neutrophil phagocytosis by macrophages is a central mechanism in anti-Ly6G-induced neutrophil depletion and 2) that fluorochrome-coupling can affect functional properties of anti-Ly6G antibodies, thereby modifying macrophage uptake of Ly6G-labeled neutrophils and neutrophil retrieval following adoptive cell transfer or injection of fluorescent anti-Ly6G.

Genetic variations in immunoglobulin G3 and association with staphylococcal intra-mammary infections in cattle and buffaloes

Animals (n = 152) suffering with mastitis were used to study association between immunoglobulin G3 (IgG3) genotypes and staphylococcal mastitis. Thus, animals (affected and unaffected) were evaluated using PCR-RFLP. Restriction digestion of amplicons of IgG3 using BstYI showed allele A and, genotypes AC, AB and AA predominated in Karan Fries, Sahiwal and Murrah, respectively. HphI digestion revealed allele A and, genotypes AC and AB in higher frequency in animals of first group of all the breeds. Additionally, genotypes associated with mastitic infection showed predominance of AB (BstYI) in unaffected animals of Sahiwal and Murrah; whereas AC and AA were observed in affected group only. Genotype AB (HphI) was prevalent in unaffected and AC in affected animals of Karan Fries and Sahiwal. In Murrah, AC was common in affected and unaffected animal; while AB remained in affected category. Identified genotypes associated with determinants of SpA gene of S. aureus strains revealed the significant outcome. For example AB (BstYI) was found to be correalted with SpA ≤ 7R; whereas with SpA > 7R in Karan Fries. Genotypes AA and AB were more favorably associated with SpA ≤ 7R and AB with the SpA > 7R in Sahiwal cattle. The genotype AB seemed influenced (100%) with SpA > 7R and AC in SpA ≤ 7R in cases of Murrah. Similarly, AA (HphI) in Karan Fries was more likely to be correlated with SpA ≤ 7R, while AC with SpA > 7R. Overall, the molecular analysis revealed that IgG3 gene could be use for selection of animals against mastitis. However, further investigations on IgG3 needed to aid in identify disease- resistant animal.

Electrochemical biosensor for rapid and sensitive detection of magnetically extracted bacterial pathogens

Biological defense and security applications demand rapid, sensitive detection of bacterial pathogens. This work presents a novel qualitative electrochemical detection technique which is applied to two representative bacterial pathogens, Bacillus cereus (as a surrogate for B. anthracis) and Escherichia coli O157:H7, resulting in detection limits of 40 CFU/mL and 6 CFU/mL, respectively, from pure culture. Cyclic voltammetry is combined with immunomagnetic separation in a rapid method requiring approximately 1 h for presumptive positive/negative results. An immunofunctionalized magnetic/polyaniline core/shell nano-particle (c/sNP) is employed to extract target cells from the sample solution and magnetically position them on a screen-printed carbon electrode (SPCE) sensor. The presence of target cells significantly inhibits current flow between the electrically active c/sNPs and SPCE. This method has the potential to be adapted for a wide variety of target organisms and sample matrices, and to become a fully portable system for routine monitoring or emergency detection of bacterial pathogens.

3D antibody immobilization on a planar matrix surface

The amount of active capture antibodies immobilized per unit square is crucial to developing effective antibody chips, biosensors, immunoassays and other molecular recognition technologies. In this study, we present a novel yet simple method for oriented antibody immobilization at high density based on the formation of an orderly, organized aggregation of immunoglobulin G (IgG) and staphylococcal protein A (SPA). Following the chelation of His-tag with Ni(2+), antibodies were immobilized on a solid surface in a three-dimensional (3D) manner and exposed the analyte receptor sites well, which resulted in a substantial enhancement of the analytical signal with more than 64-fold increase in detection sensitivity. Pull-down assays confirmed that IgG antibody can only bind to Ni(2+) matrix indirectly via a SPA linkage, where the His-tag is responsible for binding Ni(2+) and homologous domains are responsible for binding IgG Fc. The immobilization approach proposed here may be an attractive strategy for the construction of high performance antibody arrays and biosensors as long as the antibody probe is of mammalian IgG.

Morphological and electrical characteristics of biofunctionalized layers on carbon nanotubes

In this study we have investigated the morphology and electrical characteristics of protein layers non-covalently adsorbed onto an irregular network of carbon nanotubes (CNT). The layer system presents a prototype for an ion-sensitive field-effect transistor based on CNT-networks. The complementary characterization techniques AFM and ellipsometry give the overall morphology of the functionalized layer system and in combination with concentration dependent measurements a detailed image of the adsorption dynamics. The advantage of CNT-based FETs is their huge surface area, which makes them extremely sensitive even to weak adsorption processes. The here-presented comparative investigations clearly show that significant changes in the transport properties of the CNTs occur much below one monolayer. This sensitivity is an important condition for the future development of efficient biodevices with optimal performance parameters for the detection of pathogenic microorganisms.

An ultra-sensitive nanoarray chip based on single-molecule sandwich immunoassay and TIRFM for protein detection in biologic fluids

This paper describes a single-molecule sandwich immunoassay method that utilizes total internal reflection fluorescence microscopy (TIRFM) at the single-molecule level for nanoarray protein chip applications. Nanoarray patterning of a biotin-probe with a spot diameter of 179 +/- 1 nm was performed successfully on a (3-mercaptopropyl)trimethoxysilane (MPTMS)-coated glass substrate by atomic force microscopy (AFM). The formation of biotin patterns was confirmed directly by observing the heights of bound streptavidin and biotin-antibody on glass substrates using an AFM in contact mode. Target protein molecules (or antigen) at the zepto-molar (zM) concentration level (x 10(-21) M) were detected on MPTMS-coated glass nanoarray protein chips by TIRFM. Finally, cytokine clinical samples (i.e. TNF-alpha and IL-1alpha) as cancer marker protein molecules were applied to nanoarray protein chips, and detection limits were at 600 zM.

Intact free prostate-specific antigen and free and total human glandular kallikrein 2. Elimination of assay interference by enzymatic digestion of antibodies to F(ab')2 fragments

Various blood constituents can interfere with immunoassays, usually by binding the Fc portion of antibodies. Our previously developed assays for intact free prostate-specific antigen (PSA), free human kallikrein 2 (hK2), and total hK2 frequently yielded falsely high results despite including an excess of scavenger antibodies. We investigated whether this interference could be eliminated by replacing monoclonal capture or tracer antibodies with F(ab')2 or recombinant Fab fragments. Female heparin plasma samples (n = 1092), which should have negligible PSA and hK2, and male samples (n = 957) were analyzed to identify samples manifesting interference, which then were used to optimize protocols for the immunoassays. We compared original assays (monoclonal antibodies) versus optimized assays (F(ab')2 fragments: denatured mouse IgG added as scavenger) using another set of EDTA plasma (n = 113), heparin plasma (n = 160), and serum samples (n = 171). With the original assays, the frequency of falsely elevated hK2 and intact free PSA was 15 and 13%, respectively. The optimized assays eliminated 70-85% of these falsely elevated results and importantly reduced the magnitude in the remainder. F(ab')2 fragmentation was the most important factor in reducing interference. The optimized intact free PSA, free hK2, and total hK2 assays manifested high accuracy close to the lower limit of detection.

Screening and chromatographic assessing of a novel IgG biomimetic ligand

A novel biomimetic ligand, N-benzyloxycarbonyl-l-tyrosine (N-cbz-l-Tyr), was screened by a combination method of molecular docking and immobilized receptor technique. Then, N-cbz-l-Tyr was immobilized on Sepharose CL-4B to prepare a specific affinity adsorbent for immunoglobulin G (IgG). Scatchard analysis of the binding isotherm for IgG on the adsorbent gave an association constant (K(a)) of 4.91 x 10(6) m(-1) and a theoretical maximum adsorption capacity of 17.3 mg IgG/mL gel. IgG with a purity of 98% was separated from human plasma by this new affinity adsorbent.

Novel immunoassay for carcinoembryonic antigen based on protein A-conjugated immunosensor chip by surface plasmon resonance and cyclic voltammetry

In this study, an immunosensor chip utilizing surface plasmon resonance (SPR) and cyclic voltammetry (CV) was fabricated for detecting carcinoembryonic antigen (CEA). Specifically, we applied in parallel an SPR instrument and a CV device to monitor the assembly of carcinoembryonic antibody (anti-CEA) on a protein A-conjugated surface and the subsequent ligand reaction. The immunosensor chips were constructed by various concentrations of protein A. To determine the surface characteristics of different self-assembly monolayers (SAMs), several quantitative and kinetic measurements were carried out. The extent of immobilization of anti-CEA and the immune response of anti-CEA antibody against CEA were measured using the SPR instrument and CV device. The terminal functional groups of protein A have different effects on the adsorption and covalent binding of immunoprotein depending on the steric hindrance. Through the parallel measurements, we demonstrate that SPR and CV are sensitive to measure the antigen-antibody binding capacity.

Fabrication of an antibody microwell array with self-adhering antibody binding protein

One of the promising methods of preparing antibody arrays is immobilizing antibodies with protein A or protein G, each of which binds specifically to the heavy chain constant (Fc) region of immunoglobulin G (IgG). In this system, antibody immobilization efficiency depends on the number of active Fc binding proteins that need to be immobilized on the surface. Here we have designed and constructed an Fc binding protein with a self-adhering ability that can be immobilized on the hydrophobic surface by simple adsorption. It consists of an Fc binding domain of protein G (G3) and hydrophobic domain of elastin (E72). Direct observation revealed its self-adhering ability on the hydrophobic surface. The enzyme-linked immunosorbent assay (ELISA) showed that it retained antibody binding ability on the surface. The antibody array model was prepared on a hydrophobic microwell glass slide with E72G3, which specifically detect the antigen with a sevenfold greater sensitivity than the G3-treated slide. These results suggest that the E72G3 is useful for simple and effective immobilization of antibodies and can be used to fabricate any immuno devices.

Fabrication of DNA–protein conjugate layer on gold-substrate and its application to immunosensor

The fabrication of antibody thin film using both protein G and oligonucleotide was carried out by self-assembly (SA) technique for immunosensor. A mixture of 11-mercaptoundecanoic acid (MUA) and oligonucleotide with thiol (SH) end group was self-assembled of gold (Au) surface for two-dimensional (2D) configuration. Protein G was chemically adsorbed on the 11-MUA surface, and then the antibody was immobilized on the protein G region. On the immobilized single-stranded DNA, the complementary DNA-antibody conjugate was hybridized for the oriented immobilization of antibody. The formation of self-assembled 11-MUA/oligonucleotide layer, protein G immobilization, antibody layer, and antigen binding was investigated using surface plasmon resonance (SPR). The topographies of the fabricated surfaces were observed by atomic force microscopy (AFM). When compared with the amount of antigen binding on the antibody thin film fabricated by protein G only, the proposed biosurface fabricated with both protein G and oligonucleotide showed better binding capacity, which implicates the improvement of the detection limit.

Development of an enzyme-linked immunoassay for sensitive detection of native and recombinant human interferon-gamma using whole IgG fraction as polyclonal tracer

Monoclonal antibodies (mAbs) and polyclonal antibodies (pAbs) against human interferon gamma (IFN-gamma) were produced and used for development of a sensitive enzyme-linked immunosorbent assay (ELISA) for the detection and quantitation of native and recombinant human IFN-gamma in tissue culture fluid and human sera. The human IFN-gamma ELISA was constructed using mAb CAy-IFNg111 as the capture antibody (Ab) and biotinylated polyclonal mouse immunoglobulin G (IgG) as the tracer Ab. The assay is completed within 4 hr at room temperature (RT). The human IFN-gamma ELISA worked in tissue culture medium and human serum and was capable of detecting both recombinant and native human IFN-gamma. The assay dynamic range extended from 16 to 1000 pg/mL and the sensitivity level was less than 3 pg/mL of human IFN-gamma with averaged intra- and inter-assay variation coefficients less than 8% for both. The results demonstrated that without the need of an antigen-affinity purification, biotinylation of protein G-purified pAb, obtained from 1 mL of mouse blood, was sufficient for constructing the tracer reagent for the establishment of a highly sensitive ELISA (40,000 test) for the quantitative detection of native and recombinant human IFN-gamma in culture supernatant and human sera.