Characterization of monoclonal antibodies physically adsorbed onto polystyrene latex particles

Atomic force microscopy analysis of IgG films at hydrophobic surfaces: a promising method to probe IgG orientations and optimize ELISA tests performance

IgG films are widely used in the field of immunoassays, especially in (double) antibody-sandwich ELISA tests where capture antibodies are coated on surfaces like polystyrene or hydrophobic self-assembled monolayers (h-SAMs). It is critical to analyze-at a molecular scale and under liquid conditions-the structure of the deposited IgG film in order to quantitatively address the efficiency of the ELISA test in terms of antigen detection. In this communication, we report an atomic force microscopy (AFM) analysis evidencing a strong relationship between immunological activities of mouse monoclonal anti-human interleukin-2 (IL-2) and 6 (IL-6) antibodies, thickness and roughness of the IgG monolayer adsorbed onto h-SAMs, and surface concentration of IgG molecules. Indirect information may be further obtained on antibody orientation. Collating the results obtained by AFM and those from ELISA tests leads us to conclude that antibodies like anti-IL-6 forming flat monolayers should be more efficient under ELISA detection conditions. In addition, the concentration of IgG in the coating suspension should be optimized to obtain a monolayer heavily populated by "end-on" adsorbed molecules, an orientation that is desirable for enhancing ELISA tests performance.

Hemocompatibility of polymeric nanostructured surfaces

Tissue integration is an important property when inducing transplant tolerance, however, the hemocompatibility of the biomaterial surface also plays an important role in the ultimate success of the implant. Therefore, in order to induce transplant tolerance, it is critical to understand the interaction of blood components with the material surfaces. In this study, we have investigated the adsorption of key blood serum proteins, in vitro adhesion and activation of platelets and clotting kinetics of whole blood on flat polycaprolactone (PCL) surfaces, nanowire (NW) surfaces and nanofiber (NF) surfaces. Previous studies have shown that polymeric nanostructured surfaces improve cell adhesion, proliferation and viability; however it is unclear how these polymeric nanostructured surfaces interact with the blood and its components. Protein adsorption results indicate that while there were no significant differences in total albumin (ALB) adsorption on PCL, NW and NF surfaces, NW surfaces had higher total fibrinogen (FIB) and immunoglobulin-G (IgG) adsorption compared to NF and PCL surfaces. In contrast, NF surfaces had higher surface FIB and IgG adsorption compared to PCL and NW surfaces. Platelet adhesion and viability studies show more adhesion and clustering of platelets on the NF surfaces as compared to PCL and NW surfaces. Platelet activation studies reveal that NW surfaces have the highest percentage of unactivated platelets, whereas NF surfaces have the highest percentage of fully activated platelets. Whole blood clotting results indicate that NW surfaces maintain an increased amount of free hemoglobin during the clotting process compared to PCL and NF surface, indicating less clotting and slower rate of clotting on their surfaces.

Synthesis and characterization of anti-EGFR fluorescent nanoparticles for optical molecular imaging

Molecular imaging, the visualization of molecular and cellular markers, is a promising method for detection of dysplasia and early cancer in the esophagus and can potentially be used to identify regions of interest for biopsy or tumor margins for resection. EGFR is a previously reported cell surface receptor with stepwise increases in expression during the progression from Barrett's metaplasia to adenocarcinoma. In this work, a 200 nm fluorescent nanoparticle contrast agent was synthesized for targeted imaging of EGFR through a series of surface modifications to dye-encapsulated polystyrene particles. Amino-functionalized polystyrene particles were PEGylated using a heterobifunctional PEG linker. Subsequently, thiolated M225 antibodies were conjugated to maleimide functional groups on attached PEGs for EGFR targeting. In vitro binding studies using flow cytometry demonstrated specific binding of M225-PEG-NP to EGFR-expressing cells with minimal nonspecific binding in EGFR(-) cells. Binding was shown to increase proportionally with the number of conjugated M225 antibodies. Adsorbed formulations with unmodified M225 antibodies, M225 + PEG-NP, were synthesized using the same antibody feeds used in M225-PEG-NP synthesis to determine the contribution of adsorbed antibodies to EGFR targeting. Adsorbed antibodies were less efficient at mediated nanoparticle targeting to EGFR than conjugated antibodies. Finally, M225-PEG-NP demonstrated binding to EGFR-expressing regions in human esophageal tissue sections.

Computational design of a carbon nanotube fluorofullerene biosensor

Carbon nanotubes offer exciting opportunities for devising highly-sensitive detectors of specific molecules in biology and the environment. Detection limits as low as 10(-11) M have already been achieved using nanotube-based sensors. We propose the design of a biosensor comprised of functionalized carbon nanotube pores embedded in a silicon-nitride or other membrane, fluorofullerene-Fragment antigen-binding (Fab fragment) conjugates, and polymer beads with complementary Fab fragments. We show by using molecular and stochastic dynamics that conduction through the (9, 9) exohydrogenated carbon nanotubes is 20 times larger than through the Ion Channel Switch ICS(TM) biosensor, and fluorofullerenes block the nanotube entrance with a dissociation constant as low as 37 pM. Under normal operating conditions and in the absence of analyte, fluorofullerenes block the nanotube pores and the polymer beads float around in the reservoir. When analyte is injected into the reservoir the Fab fragments attached to the fluorofullerene and polymer bead crosslink to the analyte. The drag of the much larger polymer bead then acts to pull the fluorofullerene from the nanotube entrance, thereby allowing the flow of monovalent cations across the membrane. Assuming a tight seal is formed between the two reservoirs, such a biosensor would be able to detect one channel opening and thus one molecule of analyte making it a highly sensitive detection design.

Development of a high-performance immunolatex based on "soft landing" antibody immobilization mechanism

Rabbit anti-human ferritin (anti-hFT) polyclonal immunoglobulin G (IgG) and poly(ethylene glycol) (PEG) were sequentially co-immobilized onto polystyrene submicroparticles (sMPs) to construct sMP/anti-hFT/PEG (SAP) immunolatex. Chemical immobilization of anti-hFT was performed at different pH levels to evaluate variations in antigen recognition. Basic pH disfavored conjugation of anti-hFT to sMPs, but remarkably increased its antigen recognition in comparison to that at neutral pH. We investigated this intriguing phenomenon further by assessing the kinetics of antibody binding, including the time-dependency of immobilization, antigen recognition, and orientation of bound anti-hFT. Therefore, we attributed high antigen recognition to significant electrostatic repulsion between sMPs and anti-hFT at basic pH, which predominately prevented anti-hFT access to sMPs and concurrently promoted anti-hFT orientations suitable for antigen recognition. Subsequent PEG modification maintained such anti-hFT orientation, without which antigen-accessible orientations would have decreased with time. Thus, properly oriented antibody and immediate PEGylation after antibody immobilization contributed to the formation of a high-performance SAP immunolatex.

Method for studying immunoglobulin G binding on hydrophobic surfaces

We used a reactant adsorptive membrane bioreactor separator (or RAMBS) system to examine hydrophobic interaction based binding of human immunoglobulin G (HIgG) on synthetic microporous membranes possessing tunable hydrophobicity. Membrane bound HIgG on being pulsed with papain resulted in Fab being obtained in the flowthrough with Fc remaining bound to the membrane. On the other hand, when membrane bound HIgG was pulsed with pepsin, Fc subfragments were obtained in the flowthrough with F(ab')(2) remaining bound to the membrane. These product profiles suggest that HIgG bound to the membrane through its middle region. Enzyme linked immunoadsorbent assay (ELISA), sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), and mass spectrometric analysis of eluate samples obtained from the RAMBS experiments provided evidence that the binding of HIgG took place primarily through the segment consisting of the hinge and C(H)2 domain of Fc. The experimental approach described in this paper could potentially be more widely applicable for studying protein interactions with membrane and surfaces in general.

Relationship between the Structural Conformation of Monoclonal Antibody Layers and Antigen Binding Capacity

Neutron reflection has been used to determine the pH-dependent structural conformation of monoclonal antibody layers adsorbed at the hydrophilic silicon oxide/solution interface, within the pH range 4-8, over which the silicon oxide surface carried weak negative charges and the net charge on the antibody reversed. The depth resolution achieved, by use of D2O as solvent to enhance the neutron contrast of the adsorbed antibody layer, was around 2-3 A. The results have been correlated with the ellipsometric measurement of antigen binding capacity (AgBC). The antibody was a mouse monoclonal anti-hCG (human chorionic gonadotropin) directed against the beta subunit of hCG, with molecular weight of 150 000 and isoelectric point around pH 6.0. At pH 4, the adsorbed antibody could be described as a single layer 40 A thick, consistent with an almost perfect flat-on orientation with all three fragments (Fc, Fab) lying flat on the surface. With increasing pH, the antibody layer swelled (65 A at pH 6, 75 A at pH 8) and could be described as three sublayers of different protein density, consistent with some twisting of molecules so that some fragments became more loosely attached to the surface. At pH 8, the repulsive interaction between protein and surface was reflected in a significantly decreased total adsorbed amount. The dominant effect acting to increase AgBC was decreased surface packing density. The effect of the conformational changes revealed at different pH was less important. The results have shown that within the flat-on orientation adopted by the adsorbed antibody, steric hindrance is the main constraint on binding, restricting the access of the antigen to active sites within the antibody layer.

Orientation of a monoclonal antibody adsorbed at the solid/solution interface: a combined study using atomic force microscopy and neutron reflectivity

Conformational orientations of a mouse monoclonal antibody to the beta unit of human chorionic gonadotrophin (anti-beta-hCG) at the hydrophilic silicon oxide/water interface were investigated using atomic force microscopy (AFM) and neutron reflectivity (NR). The surface structural characterization was conducted with the antibody concentration in solution ranging from 2 to 50 mg.L(-1) with the ionic strength kept at 20 mM and pH = 7.0. It was found that the antibody adopted a predominantly "flat-on" orientation, with the Fc and two Fab fragments lying flat on the surface. The AFM measurement revealed a thickness of 30-33 A of the layer formed in contact with 2 mg.L(-1) antibody in water, but, interestingly, the flat-on antibody molecules formed small nonuniform clusters equivalent to 2-15 antibody molecules. Parallel AFM scanning in air revealed even larger surface clusters, suggesting that surface drying induced further aggregation. The AFM study thus demonstrated that the interaction between protein and the hydrophilic surface is weak and indicated that surface aggregation can be driven by the attraction between neighboring protein molecules. NR measurements at the solid/water interface confirmed the flat-on layer orientation of adsorbed molecules over the entire concentration range studied. Thus, at 2 mg.L(-1), the adsorbed antibody layer was well represented by a uniform layer with a thickness of 40 A. This value is thicker than the 30-33 A observed from AFM, suggesting possible layer compression caused by the tip tapping. An increase in the antibody concentration to 10 mg.L(-1) led to increasing surface adsorption. The corresponding layer structure was well represented by a three-layer model consisting of an inner sublayer of 10 A, a middle sublayer of 30 A, and an outer sublayer of 25 A, with the protein volume fractions in each sublayer being 0.22, 0.42, and 0.10, respectively. The structural transition can be interpreted as a twisting and tilting of segments of the adsorbed molecules, driven by an electrostatic repulsion between them that increases with the surface packing density. Hindrance of antigen access to antibody binding sites, resulting from the change in surface packing, can account for the decrease in antigen binding capacity (AgBC) with increasing surface density of the antibody that is observed.

Adsorption of immunogamma globulin onto various synthetic calcium hydroxyapatite particles

This paper presents data on adsorption of immunogamma globulin (IgG) onto synthetic rodlike calcium hydroxyapatite particles (CaHaps) with various particle lengths and calcium/phosphate (Ca/P) atomic ratios ranging from 1.54 to 1.65 and compares the obtained results to those of acidic (bovine serum albumin, BSA), neutral (myoglobin, MGB), and basic (lysozyme, LSZ) proteins reported before. The effect of electrolyte concentration on IgG adsorption was also examined. The initial rate of IgG adsorption was similar to that of BSA and was slower than that of MGB and LSZ. This fact was interpreted by the difference in the structural stability and molecular weight of these proteins. The isotherms of IgG adsorption onto the CaHap particles were of pseudo-Langmuir type. The saturated amount of adsorbed IgG values (nsIgG) for the particles with mean particle length less than 70 nm decreased with increasing Ca/P ratio. The adsorption behavior of IgG molecules was very similar to that of basic LSZ, though IgG has zero net charge. The nsIgG value was increased with increased mean particle length of CaHaps; the relationship was less significant than that for BSA but similar to those for MGB and LSZ. The similar adsorption behavior of IgG and LSZ suggested that the Fab parts of IgG molecules preferentially adsorb onto CaHap to provide the reversed Y-shaped conformation of IgG. The change of the adsorption mode of IgG molecules from the reversed Y-shaped conformation to side-on by "spreading" the Fc part of IgG molecules onto the particle surface over a longer adsorption time was suggested. The nsIgG value was increased with increasing electrolyte concentration by screening the intra- and intermolecular electrostatic interactions of proteins.

Orientation of a Y-shaped biomolecule adsorbed on a charged surface

The adsorption and orientation properties of a Y-shaped biomolecule, which models an immunoglobulin (Ig), on a charged surface are analyzed mesoscopically by Monte Carlo simulations. The orientation is a consequence of the interplay between van der Waals interactions and electrostatic interactions. For adsorption dominated by van der Waals attraction, the molecule prefers lying flat on the surface. For weak attraction, we observe a depletion zone in the concentration profile, which can result in a negative surface excess. A secondary peak is found for strong adsorption. For electrostatically dominated adsorption, the orientation is mainly determined by electric dipole and a vertically adsorbed molecule can be attained as it possesses strong electric dipole. Our study provides an explanation for experimental observations of preferential orientation.

Structural changes of IgG induced by heat treatment and by adsorption onto a hydrophobic Teflon surface studied by circular dichroism spectroscopy

Thermal denaturation of mouse monoclonal immunoglobulin G (isotype 1), as well as structural rearrangements resulting from adsorption on a hydrophobic Teflon surface, are studied by circular dichroism spectroscopy. Both heat-induced and adsorption-induced denaturation do not lead to complete unfolding into an extended polypeptide chain, but leave a significant part of the IgG molecule in a globular or corpuscular form. Heating dissolved IgG causes a decrease of the fractions of beta-sheet and beta-turn conformations, whereas those of random coil and, to a lesser extent, alpha-helix increase. Adsorption enhances the formation of alpha-helices and random coils, but the beta-sheet content is strongly reduced. Heating adsorbed IgG results in a gradual break-down of the alpha-helix and beta-turn contents, and a concomitant formation of beta-sheet structures. Thus, the structural changes in IgG caused by heating and by adsorption, respectively, are very different. However, after heating, the structure of adsorbed IgG approaches the structure of thermally denatured IgG in solution.

Substitution of carbonate by acetate buffer for IgG coating in sandwich ELISA

The first step of sandwich ELISA, namely adsorption of antibodies to plastic microtiter plates, was studied as a function of the pH of the coating buffer. Coating efficiency was assessed in terms of maximum signal (absorbance) observed in ELISA and also estimated by measuring the amount of functional antibodies adsorbed to the plate. While goat antibodies displayed better results after coating with acetate pH 5 buffer, rabbit IgGs generally worked well at pH 7.4. On average, the classical carbonate pH 9.6 buffer was only 50% as efficient.

Enhancement in antigen binding by a combination of synergy and antibody capture

The effects of orientating pairs of synergistic monoclonal antibodies (mAb) on binding of human chorionic gonadotropin (hCG) was studied by radioimmunoassay (RIA), enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR). Antibody synergy towards hCG required two functionally intact antibodies located adjacent to each other and with different epitope specificities. We investigated whether immobilization procedures avoiding protein denaturation, increasing proper orientation and promoting higher molecular flexibility of the synergistic mAb resulted in significantly enhanced antigen, binding. Synergistic mAb pairs captured through their Fc-region by protein G or a polyclonal serum against the Fc-part of mouse IgG could be used at 10-fold lower coating concentrations to achieve maximal binding of the analyte as compared with the same mAb pairs coated directly onto polystyrene. The synergistic effect observed with protein A used as capture varied greatly with the subclasses of the two synergistic antibodies employed. Scatchard analysis revealed that the number of functionally synergistic antibody sites participating in the binding of hCG for one mAb pair was about 10 times higher for the protein G-captured as compared with the directly coated synergistic pair. Biotinylated synergistic mAb pairs, coated directly or captured by streptavidin, did not display any enhanced antigen binding when tested in SPR or ELISA. With SPR, synergy was only observed when the synergistic mAb had been captured through their Fc-region. Using protein G or a polyclonal rabbit anti-IgG1 serum as capture reagents in SPR, synergistic triple mAb combinations against hCG were demonstrated.

Particle enhanced immunoaggregation of F(ab')2 molecules

The immunological agglutination reactions of physically absorbed F(ab')2 molecules onto anionic and cationic latex particles have been investigated by means of optical absorbance measurements. These measurements have been conducted under different conditions to determine the most influential factors. Surface F(ab')2 and BSA densities, particle concentration in the reaction medium and polyethylene glycol concentration are some of these factors. Sensitized cationic and anionic latexes differ considerably with respect to their colloidal stability and reactivity. As a general rule, the sensitized cationic latex has a relatively higher colloidal stability and hence, it provides reagents with a better optical response. Less than 0.025 microgram/ml of C-reactive protein has been detected using this particle enhanced optical immunoassay.

Evaluation of protein-A linked monoclonal antibody latex agglutination test for diagnosis of nuclear polyhedrosis virus (BmNPV) of silkworm Bombyx mori L

The symptomology of BmNPV infection in Bombyx mori L depends on the stages of infection. Discernible symptoms develop at later stages of infection, which leads to improper diagnosis and poor crop yield with sericulturists. In the present study development of direct and protein-A linked monoclonal antibody latex (PALMAL) agglutination test for the detection of BmNPV infection in silkworm is described. Latex beads were precoated with protein-A and then sensitised with monoclonal antibody MA-231 (125 micrograms/ml). PALMAL test could detect 1 x 10(5) nuclear polyhedra/test and is ten times more sensitive than the direct agglutination test employing purified polyclonal antibodies. No agglutination was observed in presence of B. thuringiensis, S. marcescens, N. bombycis, group-A streptococci, Staphylococcus aureus, E. coli and normal haemolymph protein indicating the specificity of the test. Fifty haemolymph samples collected from the field were evaluated by PALMAL test. Twenty one samples having infection other than BmNPV, failed to show positive agglutination. Twenty five samples having > or = 5 x 10(6) BmNPV/ml showed positive agglutination. However, 4 samples having < 5 x 10(6) BmNPV/ml failed to show positive agglutination thereby indicating the limit of sensitivity of the assay.

The adsorption of F(ab')2 on positively and negatively charged polystyrene beads

An experimental study on the adsorption of F(ab')2 molecules onto positively and negatively charged polystyrene beads is described. Adsorption isotherms at low ionic strength and different pH were performed. In all cases the adsorption isotherms showed well-defined plateau. The positively charged polystyrene beads showed a higher adsorption of F(ab')2 molecules over a range of pH. This latex sample reached a maximum adsorption at pH 7, whereas the negatively charged sample showed a maximum at pH 5. The isoelectric points of the F(ab')2-PS complexes were around 7.5 and 4.5 respectively. The differences in the colloidal stability of the sensitized beads are not explained by the electrophoretic mobility values. The colloidal stability of the F(ab')2-PS complexes is not only determined by the electrical charge of the complexes but also by certain properties of the protein molecules, which can be closely related to the conformational changes undergone by the F(ab')2 molecules in the adsorption process on polymer surfaces.