Mapping of monoclonal antibody- and receptor-binding domains on human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) using a surface plasmon resonance-based biosensor

Deciphering IgM interference in IgG anti-HLA antibody detection with flow beads assays

In flow beads assays, the interference of IgM for IgG anti-HLA antibodies detection is not precisely understood. Using the screening flow beads assay for class I HLA antibodies, we analyzed the binding of two IgG mAbs, the anti-class I HLA W6/32 and an anti-beta-2-microglobulin, in the presence of an anti-beta-2-microglobulin IgM mAb. In neat serum, the IgM mAb impaired the detection of both IgG. In EDTA-treated serum, the interference was stronger for the anti-beta-2-microglobulin IgG than for W6/32, in agreement with the finding in surface plasmon resonance that this IgM competed with the anti-beta-2-microglobulin IgG but not with W6/32. The IgM interference was higher in neat than in EDTA-treated serum for both IgG mAbs. The IgM interference was also analyzed with class II single antigen flow beads and sera from two kidney recipients containing IgG and IgM donor specific antibodies. Anti-HLA IgG detection was partially corrected by EDTA, and restored by IgM inactivation with DTT, confirming the results observed with the mAbs. Therefore, three mechanisms can explain the IgM interference for IgG anti-HLA antibodies in flow beads assays: direct competition for antigen, steric hindrance and complement activation.

Granulocyte-macrophage colony-stimulating factor as an autocrine survival-growth factor in human gliomas

We studied the expression of granulocyte-macrophage colony-stimulating factor (GM-CSF) and its receptors (GM-CSF.R) in 20 human brain gliomas with different tumor gradings and demonstrated constitutive high levels of both mRNA gene expression and protein production exclusively in the highest-grade tumors (WHO, III-IV grade). Five astrocytic cell lines were isolated in vitro from glioma cells, which had selectively adhered to plates pre-coated with rhGM-CSF. These cells were tumorigenic when xenografted to athymic mice, and produced GM-CSF constitutively in culture. Two lines, particularly lines AS1 and PG1, each from a patient with glioblastoma multiforme, constitutively over-expressed both GM-CSF and GM-CSF.R genes and secreted into their culture media biologically active GM-CSF. Different clones of the AS1 line, isolated after subsequent passages in vitro and then transplanted to athymic mice, demonstrated higher tumorigenic capacity with increasing passages in vivo. Cell proliferation was stimulated by rhGM-CSF in late-stage malignant clones, whereas apoptosis occurred at high frequency in the presence of blocking anti-GM-CSF antibodies. In contrast, rhGM-CSF did not induce any apparent effect in early-stage clones expressing neither GM-CSF nor GM-CSF.R. The addition of rhGM-CSF or rhIL-1β, to cultures induced the overproduction of both GM-CSF and its receptors and increased gene activation for several functional proteins (e.g. NGF, VEGF, VEGF.R1, G-CSF, MHC-II), indicating that these cells may undergo dynamic changes in response to environmental stimuli. These findings thus revealed: (1) that the co-expression of both autocrine GM-CSF and GM-CSF.R correlates with the advanced tumor stage; (2) that an important contribution of GM-CSF in malignant glioma cells is the prevention of apoptosis. These results imply that GM-CSF has an effective role in the evolution and pathogenesis of gliomas.

Poly(ethylene glycol) gradient for biochip development

A novel method of producing a poly(ethylene glycol) (PEG)-based gradient matrix that varies gradually in thickness from 0 to 500 A over a distance of 5-20 mm is presented. The gradient matrix is graft copolymerized from a mixture of PEG methacrylates onto organic thin films providing free radical polymerization sites initiated by UV irradiation at 254 nm. The films used as grafting platforms consist of either a spin-coated cycloolefin polymer or a self-assembled monolayer on planar gold. The thickness/irradiation gradient is realized by means of a moving shutter that slowly uncovers the modified gold substrate. The structural and functional characteristics of the gradient matrix are investigated with respect to thickness profile, degree of carboxylation, and subsequent immobilization of two model proteins of different sizes and shapes. These characteristics are studied with ellipsometry and infrared reflection-absorption microscopy using a grazing angle objective. It is revealed that the relatively small carboxylation agent used offers homogeneous activation throughout the gradient, even in the thick areas, whereas the diffusion/interpenetration and subsequent immobilization of large proteins is partially hindered. This is crucial information in biosensor design that can be easily obtained from a gradient experiment on a single sample. Moreover, the partially hindered protein interpenetration, the marginal swelling upon hydration, and the unspecific nature of the graft polymerization suggest a matrix growth mechanism that favors the formation of a bushlike polymer structure with a certain degree of cross linking.

Dynamics of complex formation between biological and luminescent conjugated polyelectrolytes—a surface plasmon resonance study

A water-soluble polythiophene, POWT, with zwitterionic peptide like side chains possess good characteristics for biosensor applications. The zwitterionic side chains of the polymer can couple to biomolecules via electrostatic and hydrogen bonding. This creates possibilities to imprint biomolecules to spin-coated polymer films with maintained functionality, and use the resulting matrix as a biosensor. Polymer-biomolecular interaction studies done with surface plasmon resonance (SPR) reveal a well performing sensor matrix with high affinity for DNA hybridizations as well as for protein detection. The responses are distinct and very specific. A directional dependence of antibodies binding to POWT layer has also been observed. The polymer films have also been characterized by optical methods. Emission and absorption measurements in different buffer systems confirm that the polymer matrix can undergo structural and conformational changes on surfaces. The dielectric function in the interval 300-800 nm of POWT is reported, based on variable angle spectroscopic ellipsometry. This modeling reveals that a considerable amount of water is included in the material. The polymer layer possesses the characteristics needed for biochip applications and micro array techniques.

Antibodies binding granulocyte-macrophage colony stimulating factor produced by cord blood-derived B cell lines immortalized by Epstein-Barr virus in vitro

We detected natural antibodies (auto-Abs) binding human granulocyte-macrophage colony stimulating factor (GM-CSF) in umbilical cord blood (CB) (23 of 94 samples screened) and peripheral blood of women at the end of pregnancy (6 of 42 samples tested). To demonstrate that Abs detected in CB were produced by the fetus, CB mononuclear cells were infected with Epstein-Barr virus in vitro. Ten cell lines producing constitutively anti-recombinant human GM-CSF (rhGM-CSF) Abs were isolated and characterized. These cells displayed a male karyotype, an early activated B cell phenotype, coexpressed surface IgM and IgD, and secreted only IgM with prevailing lambda clonal restriction. Specific cell surface binding of biotinylated rhGM-CSF and high-level anti-rhGM-CSF IgM Ab production were typical features of early cell cultures. In late cell passages the frequency of more undifferentiated B cells increased. Serum Abs of either maternal or fetal origin or Abs produced in culture did not affect the granulocyte and macrophage colony stimulating activity of rhGM-CSF from bone marrow progenitors in soft agar, suggesting that the Abs produced were nonneutralizing.

Human GM-CSF interaction with the alpha-chain of its receptor studied using surface plasmon resonance

A surface plasmon resonance (SPR) based biosensor has been used for studying the interaction of recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) with genetically engineered alpha-chain subunits of its specific receptor (GM-Ralpha). Western blot analysis of GM-Ralpha confirmed the correct size (80 kDa) and reactivity of these proteins against anti-GM-Ralpha polyclonal or monoclonal antibodies. GM-CSF was immobilized, using standard amine coupling methods, to the dextran-modified gold biosensor surface in order to capture GM-Ralpha subsequently injected over the sensing layer. GM-Ralpha were shown to specifically form complexes with the immobilized ligand. Pre-incubation of constant amounts of GM-Ralpha with dilutions of soluble GM-CSF before injection of the mixture over the GM-CSF matrix, prevented ligand binding in a dose dependent manner. In contrast, unrelated soluble cytokines or serum proteins (e.g. G-CSF, albumin, etc.) were found to exert no inhibition. Complexes formation blockage by pre-incubation of constant amounts of GM-Ralpha with dilutions of neutralizing anti-GM-Ralpha antibodies was concentration dependent, further assessing the specificity of the interaction. To investigate the possibility of relating the effect on binding affinity of critical conformational changes at the contact site, experiments of multisite binding were performed, flowing a set of neutralizing monoclonal antibodies reacting to different epitopes on GM-CSF over the GM-CSF matrix, before injecting GM-Ralpha. The results indicated that antibody interaction with helix D and helix A of GM-CSF markedly inhibited GM-CSF binding to GM-Ralpha. Comparable results were obtained using the biosensor technology and enzyme-linked immunoassays, in representative experiments performed with the same reagents. These experiments demonstrate that SPR can be successfully used for studying complementary interactions between GM-CSF and its receptor alpha-chains in solution without using labels or secondary tracers and, compared with conventional immunoanalysis methods, significantly saving time.

Detection of digitalis compounds using a surface plasmon resonance-based biosensor

An automated surface plasmon resonance-based biosensor system has been used to detect endogenous and exogenous digitalis-like factors (EDLF) in the pmolar range in real time. EDLF was purified from umbilical cord blood. EDLF has been suggested to play a role in hypertension and in perinatal adaptation. Highly specific polyclonal anti-ouabain antibodies showed a high affinity binding capacity for ouabain, ouabagenin and strophantidin with an IC50 value of 5 x 10(-10) M, 7.0 x 10(-10) M and 2 x 10(-8) M, respectively. EDLF cross-reacted with antibodies and its concentration in plasma at IC50 was around 50 pmol ouabain equivalent. This study shows the potential usefulness of the biosensor technology for biomolecular interaction analysis. The features of this technology (fully automated, measured in real time, sharpened response) offer several advantages compared with a traditional immunoassay like radioimmunoassay (RIA) in the detection of digitalis compounds in human fluids.

Biosensors in immunology: the story so far

In conclusion, biosensors are versatile tools with a range of applications. With a thorough knowledge of possible artefacts and limitations, it possible to perform assays that were heretofore not practicable in immunology.