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Bispecific antibodies (BsAbs) are engineered to simultaneously bind two different antigens, and offer promising clinical outcomes for various diseases. The dual binding properties of BsAbs may enable superior efficacies and/or potencies compared to standard monoclonal antibodies (mAbs) or combination mAb therapies. Characterizing BsAb binding properties is critical during biotherapeutic development, where data is leveraged to predict efficacy and potency, assess critical quality attributes and improve antibody design. Traditional single-target, single-readout approaches (e.g., ELISA) have limited usefulness for interpreting complex bispecific binding, and double the benchwork. To address these deficiencies, we developed and implemented a new dual-target/readout binding assay that accurately dissects the affinities of both BsAb binding domains directly and simultaneously. This new assay uses AlphaPlex® technology, which eliminates traditional ELISA wash steps and can be miniaturized for automated workflows. The optimized BsAb AlphaPlex assay demonstrates 99-107% accuracy within a 50-150% linear range, and detected >50% binding degradation from photo- and thermal stress conditions. To the best of our knowledge, this is the first instance of a dual-target/readout BsAb AlphaPlex assay with GMP-suitable linear range, accuracy, specificity, and stability-indicating properties. As a highly customizable and efficient assay, BsAb AlphaPlex may be applicable to numerous bispecific formats and/or co-formulations against a variety of antigens beyond the clinical therapeutic space.

The aim of this study was to develop an assay to analyze the serum profile of Mannose-binding lectin (MBL) through a simple and “in-house” method (called “dot-N-man”). Furthermore, the study attempted to associate molecular masses of MBL to the profile of MBL gene polymorphisms in patients with hepatitis C. Heterogeneity in molecular masses of MBL is due to the impairment of oligomers formation, which is linked to genetic polymorphisms in the MBL gene. Individuals with AA genotype (wild-type) produce high-molecular-mass proteins, whereas AO and OO individuals produce intermediate and low-molecular-mass proteins, respectively. Sera of thirty patients carrying the hepatitis C virus (HCV) were investigated using MBL binding assay with mannan-coated nitrocellulose (dot-N-man). Purified MBL was evaluated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting. Dot-N-Man assay yielded MBL with molecular masses ranging between 55 and 320 kDa, comparable to low and high molecular mass forms of MBL. Nonreducing SDS-PAGE showed high molecular mass bands in all AA individuals while bands of 270 and 205 kDa were observed in sera for a number of patients with AO and OO genotypes, respectively. Immunoblotting confirmed the MBL samples obtained from the dot-N-man. These results provide new insights to understand the MBL molecular forms profile in patients infected with HCV- which could be useful in future investigations on the influence of the MBL structure/genotype on both the progression of infection and the response to hepatitis C therapy.

Biomarker quantitation with ligand binding assays has matured greatly in recent years. This maturation has been partly in response to demands for more data points from fewer samples or less available sample volume. Multiplexing offers opportunities to acquire data for multiple analytes from single sample assay iterations, but has its own unique challenges and limitations. ProteinSimple has developed Simple Plex™, an automated immunoassay platform consisting of microfluidic cartridge-based assays run on the Ella instrument. Ella subverts traditional multiplexing challenges by rapidly performing triplicate measurements of up to four different analytes simultaneously, each in their own respective assay vessels and all from a single sample. Here we describe a comparison of the Simple Plex platform versus colorimetric ELISA and their respective abilities to quantitate four common biomarkers (MCP-1/CCL2, VEGF-A, TNF-α, and IL-6) from twenty-eight healthy individual donor plasma samples. Each biomarker was tested on the two platforms on each of two days. Ella analysis required significantly reduced sample volume, manual steps, and total time. Overall, Ella was able to quantify results for all twenty-eight samples for each of the four biomarkers. In contrast, ELISA was able to measure quantifiable results within respective calibration curve ranges for MCP-1/CCL2 (96% of samples) and VEGFA (7% of samples). For TNF-α and IL-6, ELISA was not sensitive enough to quantify any samples in the assay ranges. This stark difference in quantitative results underscores Ella's ability to multiplex without compromising sensitivity, and has far reaching potential for biomarker panel measurement in support of diagnosis, prognosis, and monitoring of disease.

The use of T-cells expressing Chimeric Antigen Receptors (CARs) offers new opportunities for cancer treatment, as well as new challenges for the bioanalysis of this new class of drugs. The analysis of humoral immunogenicity (anti-drug antibodies) against CARs could be performed with a bridging ELISA, using labeled CAR fragments. However, outside of its native cell membrane environment and without potential interaction partners on the cell surface, a labeled or coated recombinant CAR fragment may structurally differ from the membrane-bound CAR expressed on CAR-T cells. Consequently, immunogenicity to CARs may be missed due to the artificial nature of a ligand binding assay setup. T-cell lines expressing the CAR offer the opportunity to measure anti-drug antibodies to the CAR in its natural cell environment, as an alternative to ligand-binding assays. Here we describe a novel, flow cytometry-based humoral immunogenicity assay for tisagenlecleucel (CTL019, Kymriah®) using a human T-cell line that expresses murine CAR19. The assay described here was fully validated according to health authority guidelines for the development and validation of immunogenicity assays and has a sensitivity of 100 ng/mL. A good correlation of screening assay signal strengths to titer assay results was observed while exploring options to increase titration assay throughput. Pre-existing antibodies against the cell line used in the assay as well as against the CAR itself complicate the assay and data interpretation.

A comprehensive cross-platform and cross-assay evaluation using nine technology platforms and four cytokine immunoassays (IL-6, TNFα, IL-17a, IL-2) was performed by comparing assay precision, sensitivity, parallelism and data correlation between platforms. The precision was acceptable for most evaluated assays. In addition to comparing the analytical assay sensitivity using a spiked recombinant analyte in buffer, forty serum samples from both normal controls and multiple sclerosis patients were used to measure the frequency of endogenous analyte detection (FEAD) as a parameter of each assay's ability to detect the endogenous analyte. The highest FEAD measurements were observed on the Simoa™, Erenna®, Milliplex® and Imperacer® platforms. However, only Simoa and Erenna results showed a high correlation across all evaluated cytokine assays, followed by a more moderate correlation of results across platforms for the V-plex™, high sensitivity ELISA and the Ella™ IL-6 and TNFα assays. In contrast, results from the evaluated cytokine assays on the Milliplex, AMMP™ ViBE® and Imperacer platforms did not correlate to each other nor to other evaluated assays. Acceptable parallelism was observed for the Simoa, Erenna, V-plex and Ella assays but not for the Milliplex, AMMP ViBE and Imperacer assays. In conclusion, the Simoa, Erenna,V-plex and Ella platforms performed well in one or more evaluated cytokine assays. Among those, the Simoa and Erenna assays had the highest sensitivity for detection of cytokines present at sub-pg/mL levels in human serum. In addition, the cross-platform and cross-assay comparisons demonstrated that different immunoassays may yield different results, which underscores the importance of performing such comparative evaluations, especially in the absence of reliable reference standards for the quantitative assessments of biomarkers in immunoassays.

Biological matrix interference in detection and quantitation immunoassays remains a major challenge in the field of bioanalysis. For example, circulating drug may interfere with the detection of anti-drug antibodies (ADA) and drug target, or ADA may interfere with quantitation of drug levels in PK/TK analysis. Monoclonal antibody drug interference, especially for human IgG4 drugs, presents an additional challenge for ADA analysis due to its longer half-life and higher dose. Assay tolerance to such interference may depend on assay platform and reagents. Various approaches have been used to improve drug tolerance in ADA analysis but limited success was observed. We have developed a breakthrough novel method that uses Precipitation and Acid dissociation (PandA) to overcome drug interference in the ADA assay. The method principle is based on four components for detection of total ADA (free ADA and drug bound ADA) in the presence of drug in patient samples: (1) use excess drug to saturate free ADA to form drug bound ADA as drug:ADA complexes, (2) precipitate the complex using an agent such as PEG, (3) acid dissociate ADA from drug and immobilize (capture) free ADA (and free drug) under acidic conditions (without neutralization) onto a large capacity surface, and (4) detect free ADA (not the captured drug) using specific anti-human Ig detection reagent. In this manuscript, we are describing case studies for three humanized monoclonal antibodies (an IgG1 and two IgG4 drugs). The three drug specific PandA ADA assays resulted in complete recovery of ADA in samples containing drug levels in excess of those expected in patients, in contrast to the commonly used acid dissociation approach in ECL bridging assays. This breakthrough novel method shows significant improvement over the current approaches. In fact, the drug interference or under detecting of ADA in all three cases was eliminated. This assay principle could be used not only for ADA assays but also PK and biomarker (drug target) analysis in the presence of interference factors.

Bioanalytical data from early human studies conducted in normal volunteers are often used for building pharmacokinetic/pharmacodynamic models that can predict outcomes of future studies in diseased patients. Thus, it is important to develop and validate reliable and accurate bioanalytical assays that instill confidence that the intended therapeutic species (total or free) are being measured. Assays quantifying the free therapeutic species, the partially bound (for multivalent therapeutics) and unbound species, require much more characterization than assays that quantify the total therapeutic species. We have developed an immunoassay to measure free BMS-962476, an Adnectin protein therapeutic against soluble proprotein convertase subtilisin kexin (PCSK)-9, and performed an in-depth characterization of the accuracy of this assay with the assistance of modeling. The experimental data correlates with modeled data within 15% at all clinically relevant levels of PCSK9 in normal and diseased populations.

Response surface methods (RSM) combined with a steepest ascent approach is a powerful technique to optimize assay performance. In this case, a ligand-binding assay (LBA) to quantitate a peptide biotherapeutic was optimized for improved sensitivity using this technique. Conditions were elucidated to enable pg/mL quantitation of the peptide in human plasma using steepest ascent to efficiently optimize assay factors. Instead of relying solely on assay development experience and intuition to improve assay sensitivity, this systematic approach takes advantage of a predictive mathematical model generated through response surface methods that defines a specific path towards greater predicted assay sensitivity. The actual response observed along the steepest ascent path was in good agreement with the model for several steps, until reagent concentrations moved beyond the physical limits of the system, and model breakdown occurred. RSM combined with steepest ascent method proved a useful tool for sensitivity optimization in three ways: (1) The required LBA sensitivity performance (approximately 200 pg/mL), measured as a signal-to-noise ratio (SNR) at the targeted lower limit of quantitation (LLOQ), was efficiently achieved in only two optimization experiments; (2) Steepest ascent confirmed that the desired sensitivity was found within the initial RSM design space, and no further gain in sensitivity was found venturing beyond this design space along the steepest ascent path; (3) The desired assay sensitivity was maintained over a reasonable range of reagent concentrations along the steepest ascent path, indicating assay robustness for this parameter.

Dual specific antibodies and bispecific antibodies that recognize two different antigen targets are currently being regarded as very effective therapeutics for complex human diseases. While effective, designing and developing a bioassay strategy for dual specific antibodies that is reflective of the mechanism of action (MoA) and also measures the dual activities of antibodies pose unique and exciting challenges. An important question asked while developing a bioassay for dual specific antibodies is, "How many bioassays will be needed, one bioassay or two separate bioassays?" Here we present an approach of using one bioassay for a dual specific antibody that targets two receptors in signaling pathways. The presented assay is able to measure the antibody effects on both target bindings, which would not be achievable using two separate assays. Furthermore, this assay can detect changes in the binding of either target, which impact overall efficacy of the antibody. Its improved sensitivity enables substituting two binding assays with this one bioassay for lot release and stability testing to measure any changes on either target binding, ensuring consistency between lots. This is a single-bioassay approach for a dual specific antibody that is MoA reflective of the intended therapeutic indication. The demonstrated assay development and bridging study strategy for this bioassay for a dual specific mAb1 could be applicable to the other dual specific, bispecific antibodies, and antibodies used for combination therapy.

Several detection platforms are available for ligand binding assays (LBA), each claiming superiority in sensitivity and dynamic range. However, little information exists in the literature directly comparing the various LBA platforms for quantitation. We have tested four common platforms to evaluate and compare the interchangeability of detection platforms by comparing sensitivity and dynamic range to a colorimetric LBA. The detection platforms compared are: colorimetric, chemiluminescence, time-resolved fluorescence (TRF) and electrochemiluminescence (ECL). Five different LBA protocols were tested with each of the detection endpoints. The assay protocols include the following ligand binding assay formats: direct binding, sandwich ELISA, competitive and cell based ELISA. We found that no detection platform consistently performed better than all the others and it was not possible to predict which platform would perform best for a given assay protocol. We also found surprising differences in assays (plate coating efficiency, low signal) which add to difficulty in choosing the best platform ad hoc. We propose here that in developing new assay protocols for detection of biotherapeutic agents, multiple detection platforms should be tested in order to forward the best assays possible and for the right reasons.

Soluble targets represent a special challenge when employing ligand binding assays to support pharmacokinetic analysis of monoclonal therapeutics. Target-engaged antibody is not available for binding in immunoassays employing anti-idiotype-specific antibodies or target for capture. We investigated several formats of total antibody assays that show reduced interference of soluble targets: direct target capture, indirect target capture and acid dissociation. While indirect target capture worked well for a regular affinity antibody against DKK1, a high affinity antibody against PCSK9 required an additional acid dissociation step. The choice of a suitable format was antibody and target dependent. Our results offer several choices to approach immunoassay development for soluble targets.

IgG antibodies are important pharmaceutical molecules that successfully treat a variety of human diseases. The neonatal Fc receptor (FcRn) interacts with IgG Fc in the CH2–CH3 domain and plays a key role in IgG antibody homeostasis and affects its pharmacokinetic properties. An in vitro FcRn binding assay could be a highly valuable complementary tool to assess IgG antibody pharmacokinetics in IgG engineering and screening during the early optimization stage. In addition, it could be useful in biological characterization studies for antibody minor variants, process optimization, and comparability study at later stages of antibody development.Here we developed a homogeneous AlphaScreen-based FcRn assay to assess the binding of FcRn to IgG antibody in vitro. The assay is found to be accurate, precise, specific, and simple: donor beads loaded with FcRn and acceptor beads loaded with IgG1 mAb1 are mixed together with sample IgG at various dilutions and incubated for 1h before acquiring data with a fluorescence reader. This assay can run up to four samples per plate in 2h, which is time and cost effective compared with other FcRn binding methods such as cell-based fluorescent-activated cell scan and surface plasma resonance. Our data demonstrated that this assay is suitable for assessing the FcRn binding in vitro and provides a platform approach that can be readily applied to various antibodies.

Human leukocyte antigen (HLA)-bound peptides are central for recognition of infected/transformed cells by T cells, and have formed the basis for many immunotherapy strategies. Epitopes from a given protein sequence (e.g. from viral proteins or oncoproteins) can be predicted by algorithms, as individual HLA receptors bind peptides through defined binding motifs. Peptides with the highest predicted binding score are then normally tested for their binding ability in binding assays. However, with the assays already established, only one peptide can be tested for binding per assay. This is certainly not a reflection of the in vivo situation, where several peptides generated via the major histocompatability complex (MHC)-class I processing pathway compete for HLA-receptor binding. Here, we describe the development of a method that can mimic the competition between multiple peptides for binding to a single HLA receptor molecule. We used silica nanoparticles with immobilised HLA-A2 complexes to screen HLA-A2 binder-peptides out of a known peptide mixture. The washed beads were analysed for selectively bound peptides by matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry. The advantage of the system is that the bound peptides can be unambiguously identified without any prior modification (e.g. radioactive or fluorescence labelling), even from complex peptide mixtures.

We have produced soluble T cell receptor (TCR) derived from a human CD8(+) cytotoxic T lymphocyte (CTL) clone D3 that recognizes the immunodominant HIV Gag peptide SLYNTVATL (SL9) in association with major histocompatibility complex (MHC) class I protein HLA-A2. Drosophila Schneider cells (S2) were used to express genes coding the TCR alpha and beta chains under an inducible promoter. Both chains were labeled with two different tags: a (His)(6) was introduced at the C-terminal end of alpha chain, while beta chain was terminated by c-myc. Since an isolated alpha chain is unstable unless it is associated with a beta chain, this design permits rapid separation of alpha,beta-heterodimer from unpaired beta chain in a single step of Ni-NTA Agarose chromatography yielding 90% pure alpha,beta-TCR. Introduction of the c-myc epitope to the beta chain allows capture of soluble D3 from the culture supernatant by immobilized anti-c-myc antibody, without the need for receptor purification. The TCR specificity was then examined by analyzing the binding of peptide-HLA-A2/tetramer in an ELISA assay. Using this assay, we have also evaluated the binding of monomeric SL9-HLA-A2 complex to the immobilized D3 TCR and determined that the affinity measurement of the D3-SL9-HLA-A2 reaction is similar to that obtained by a biosensor instrument. We propose that the approach described here is generally useful for purification of other soluble TCRs and will allow rapid analysis of their specificity.

In this report, we review a novel method for probing the membrane organization of T cells using dimeric major histocompatibility complexes (MHC), MHC-Ig. MHC-Ig complexes are useful reagents for quantitative analysis of binding data since their valency is controlled. These complexes can be easily labeled and loaded with a variety of peptides. A binding assay using these dimers and quantitative analysis of the MHC-Ig dimer-T cell binding curves is described in detail. Using this approach, we show that the organization of TCR on activated T cells is different from TCR organization on nai;ve T cells. The implications of these findings are discussed with regards to current models of T cell recognition. This analysis offers insight into how T cell controls their biological range of responsiveness. Specifically, these findings reveal the biophysical basis of the ability of activated T cells to recognize low amounts of antigen independent of costimulation.

The p53 tumour suppressor protein is one of the most important topics in cancer research. Its function is associated with the ability to bind DNA in a sequence-specific manner and to operate as a transcription factor. In the present study, we have developed a rapid and reliable method for analysing sequence-specific binding of p53 protein to DNA using a modified enzyme-linked immunosorbent assay (ELISA). In this p53/DNA-ELISA, we use streptavidin-coated microplates to capture biotinylated oligonucleotides containing p53 consensus sequences (p53CON). This newly developed nonradioactive assay allows the detection of p53/DNA complexes using different monoclonal antibodies recognising p53 and has comparable or higher sensitivity to more complicated radioactive methods. Using this method, we can detect binding of endogenous p53 to p53CON and activation of p53 protein for sequence-specific DNA binding. Variations of the basic protocol have also been developed to perform competition experiments and to study p53 binding to natural binding sequences. This modified DNA-ELISA is applicable for screening p53 binding properties from various sources in a short time.

The very low affinity of the T-cell receptor (TCR) for the peptide-major histocompatibility complex (pMHC) has made it very challenging to design assays for testing the functionality of these molecules on small scales, which in turn has severely hampered the progress in developing expression and refolding methodologies for the TCR. We have now developed an ELISA assay for detecting pMHC binding to functional recombinant TCRs. It uses tetramers of biotinylated pMHCs bound to a neutravidin-horseradish peroxidase conjugate and detects the presence of functional TCR, bound in a productive orientation to an immobilized anti-Cb antibody. Specificity can be stringently demonstrated by inhibition with monomeric pMHCs. The assay is very sensitive and specific, and requires only very small amounts of protein. It has allowed us to study the unstable recombinant TCR P14, which we expressed and refolded from Escherichia coli. The TCR P14 is directed against the most abundant epitope of LCMV. We have confirmed the specificity of the interaction by BIAcore, and were able to determine the dissociation constant of the interaction of the P14 TCR and of the gp33-pMHC as 6 mM. This affinity ranks it among the tighter ones of TCR-pMHC interactions, and unusually low affinity thus does not seem to be the cause of the modest protective power of these T-cells, compared to others elicited in the anti-LCMV response. This strategy of multimerizing one partner and immobilizing the other in both a native form and productive orientation should be generally useful for characterizing the weak interactions of cell-surface molecules. © 2000 Elsevier Science B.V. All rights reserved.

Certain monoclonal antibodies are capable of inhibiting the biological binding reactions of their target proteins. At the molecular level, this type of effect may be brought about by completely different mechanisms, such as competition for common binding determinants, steric hindrance or interference with conformational properties of the receptor critical for ligand binding. This distinction is central when employing the antibodies as tools in the elucidation of the structure-function relationship of the protein in question. We have studied the effect of monoclonal antibodies against the urokinase plasminogen activator receptor (uPAR), a protein located on the surface of various types of malignant and normal cells which is involved in the direction of proteolytic degradation reactions in the extracellular matrix. We show that surface plasmon resonance/biomolecular interaction analysis (BIA) can be employed as a highly useful tool to characterize the inhibitory mechanism of specific antagonist antibodies. Two inhibitory antibodies against uPAR, mAb R3 and mAb R5, were shown to exhibit competitive and non-competitive inhibition, respectively, of ligand binding to the receptor. The former antibody efficiently blocked the receptor against subsequent ligand binding but was unable to promote the dissociation of a preformed receptor-ligand complex. The latter antibody was capable of binding the preformed complex, forming a transient trimolecular assembly, and promoting the dissociation of the uPA/uPAR complex. The continuous recording of binding and dissociation, obtained in BIA, is central in characterizing these phenomena. The identification of a non-competitive inhibitory mechanism against this receptor reveals the presence of a determinant which influences the binding properties of a remote site in the molecular structure and which could be an important target for a putative synthetic antagonist.

Current standard methods for the measurement of cell-mediated cytotoxicity rely on radioactive tracers, which either detect the release of cytoplasmic contents after plasma membrane disintegration by dying cells (51Cr release), or retained DNA by living cells (the JAM test). In this study, the annexin V binding assay of early apoptosis was applied to measure cell-mediated cytotoxicity. Primed human lymphocytes were examined for their ability to lyse either xenogeneic pig endothelial or allogeneic human PBMC target cells by assaying annexin V binding and the results compared with those obtained by the JAM test. Assaying annexin V binding by indirect immunofluorescence was demonstrated to be more sensitive and faster than the JAM test, which is a well-described, sensitive and simple assay for DNA fragmentation and cell death. However, the annexin V binding method was considered a more accurate measurement of absolute cytotoxicity as individual cell lysis was detected directly. In other methods, cytotoxic activity was calculated indirectly as a percentage of retained or released radioactive label. In addition, the apoptosis induced by the cell-mediated cytotoxicity can be visualized by this method thereby allowing a more accurate and sensitive quantitation of the number of apoptotic cells present when low effector to target ratios are used. These advantages make the annexin V binding method superior to other conventional cytotoxicity assays, particularly in situations where effector cells can be easily distinguished or separated from target cells.

GAD65Ab and ICA512Ab are strongly associated with insulin-dependent (Type 1) diabetes mellitus. A novel, simple radio-antigen binding assay with microSepharose conjugated with monoclonal antibodies specific for human immunoglobulin light chains was developed to provide diagnostic sensitivity and specificity of GAD65Ab and ICA512Ab for Type 1 diabetes. The Receiver Operating Characteristic (ROC) curve was used to determine the upper level of Normal in 583 new onset Type 1 diabetic patients and in 829 matched controls. The sensitivity of GAD65Ab and ICA512Ab was 66% (384/583) and 41% (211/520), respectively, and the diagnostic specificity was 96% for both autoantibodies. Levels, but not frequency, of GAD65Ab were higher among female Type 1 diabetes patients, whereas ICA512Ab levels did not differ between males and females. Positivity for GAD65Ab, ICA512Ab or both showed a sensitivity of 74% and a specificity of 92% for Type 1 diabetes. This simple, one-step centrifugation, high-capacity radio-antigen binding assay has a high precision and reproducibility to accurately detect both GAD65Ab and ICA512Ab. This assay should also prove useful in other autoantibody assays against conformation-sensitive autoantigens.

A novel cell-free, highly automated peptide-HLA binding assay has been designed during which a mixture of unfolded recombinant HLA heavy chain molecules, beta 2-microglobulin and a fluorescent labeled standard peptide is allowed to form peptide-HLA complexes. The binding of a peptide of interest is monitored as the ability to inhibit the formation of fluorescent peptide-HLA complexes. The assay was validated using published, known HLA-A* 0201 and HLA-A* 0301 binding peptides. In addition a selected set of HIV-1LAI reverse transcriptase derived 10-mer peptides, that had been selected on the basis of HLA-A* 0201 or HLA-A* 0301 binding motifs, were tested for HLA-A* 0201/A* 0301 binding. In that set we identified 8 peptides which bound with high affinity to HLA-A* 0201 and 5 peptides which bound with high affinity to HLA-A* 0301. The major advantage of the use of denatured heavy chain is the improved economy and efficiency, as unfolded protein material is in principle easily accessible by recombinant technology.

In our recent studies, we identified HLA-DRB1*0901-binding peptides by affinity-based selection of a phage random peptide library and showed that two major anchors (WxxS, where x is any amino acid) play an essential role in binding to DR9, determined using a radioactive peptide in combination with column chromatography. In the current study, we established an ELISA-based peptide-HLA binding assay system, with a new index (relative binding affinity; r.b.a.) for quantitation of peptide-HLA binding, using standard curves of competitive inhibition, an approach which enabled handling of a larger number of samples simultaneously. Quantitation of binding between HLA-DR9 molecules and 39 synthetic peptides showed that: (a) this system yields results which correlate with those obtained using the previous assay system (Spearman's rank correlation coefficient; the p value of the putative first anchor

There are a wide variety of ligand binding assay platforms available for implementation in present day bioanalytical laboratories. Selecting the platform that best suits a particular project's needs is highly dependent upon multiple assay characteristics. The active form of glucagon-like protein (GLP-1) is a biomarker of interest for type 2 diabetes (T2DM), and therefore a common target for quantitation. Previous projects requiring active GLP-1 measurements involved the use of a labor intensive ELISA, spurring an investigation towards other potential assay platforms. To that end, four separate ligand binding assay formats (standard ELISA, electrochemiluminescence, Gyrolab, and Singulex) were evaluated. The platforms were compared for numerous assay parameters including dynamic range, sample volume requirements, throughput, and cost. Additionally, thirty individual donor plasmas were run with each assay as representative study samples. Although our evaluation did not show any platform that was better than others in all assay characteristics, there was one that was best in sensitivity (Singulex) and one that was best in throughput and sample volume requirements (Gyrolab). The lack of a technology that was best in all categories underscores the importance of due diligence when selecting an assay platform; there are no silver bullets, and one must take into account what is necessary for project needs and the intended use of the data.

The Fc region of IgG-based molecules plays an important role in determining their in vivo pharmacokinetic profile by its pH-dependent binding to the neonatal Fc receptor (FcRn) which is expressed on the endothelial cells lining blood vessels. By virtue of this pH-specific interaction with IgG-Fc, FcRn mediates IgG homeostasis in human adults by maintaining serum IgG levels, and also transfers maternal IgGs from mother to fetus via the placenta. The Fc-FcRn interaction is also critical for keeping IgG-based therapeutic molecules in circulation thereby enhancing their serum half life. A homogeneous cell-based flow cytometric FcRn binding assay was established to characterize the Fc-FcRn interaction of therapeutic IgG-based molecules. It is a competition-based assay, wherein the IgG-Fc containing test molecule competes with a fixed concentration of fluorescently-labeled IgG-Fc moiety in solution for binding to the cell-expressed FcRn. The cell-bound fluorescence is read on a flow cytometer. Response of the test sample is analyzed relative to the standard sample and the results are reported as % relative binding. The assay is robust and meets the qualification criteria for specificity, method linearity, accuracy and precision over the relative binding range of 60%-160%. This assay was shown to effectively characterize altered Fc-FcRn interactions for photo-stressed, heat-stressed, oxidized, and Fc mutant samples. It was observed that the relative binding of the IgG-Fc to the cell-surface-expressed FcRn in the assay varies across different molecules, even within the same IgG subclass. This indicates that the Fc-FcRn binding can be influenced by the antigen-binding region of the molecules in addition to the IgG subclass. Overall, this assay is reflective of the in vivo mechanism of immunoglobulin binding to membrane-bound FcRn, and can be used as an analytical tool for assessing lot-to-lot consistency and stability testing across different batches of the same molecule. Additionally, the assay can be used as an effective tool for elucidating the amino acids in the IgG-Fc domain that are critical for FcRn binding and also for comparing the binding of different IgG-Fc containing molecules to FcRn.

A solid phase ELISA conglutinin-binding assay (KgBa) was evaluated for the detection of circulating immune complexes. ELISA wells were coated with purified bovine conglutinin and incubated with test sera. Bound IgG was detected with enzyme labelled anti-immunoglobulin. Heat aggregated IgG which had been “solubilized” (i.e., complement treated by incubation with serum) was employed as a reference. The binding of the complement-reacted IgG to solid phase conglutinin was found to be calcium-dependent and inhibitable with N- acetyl - D - glucosamine (GlcNAc). Prolonged incubation (4 days) of aggregated IgG with serum at 37°C abolished the binding to conglutinin, a finding consistent with the complete degradation of deposited C3b to C3c and C3d. The solubilized IgG that bound to solid phase conglutinin was found by gel chromatography to be of high molecular weight (>600 kDa). Binding of IgG to solid phase bovine conglutinin was also observed to a variable degree in normal and pathological sera. However, in this situation the IgG binding was largely calcium-independent, was not inhibited by GlcNAc and did not decrease after prolonged incubation of the serum at 37°C. The reactive IgG eluted on gel chromatography at the position of monomeric IgG suggesting binding via the antigen binding sites. Binding of this IgG was inhibited by both collagen type II and purified conglutinin. These observations suggest that the assay detects cross-reacting autoantibodies against collagen epitopes, or, alternatively, antibodies against the dietary antigen, bovine conglutinin.

An ELISA measuring total Ig antibodies to the capsular polysaccharide of Haemophilus influenzae type b (HbPs) in human sera using an antigen composed of Haemophilus b oligosaccharides conjugated to human serum albumin (HbO-HA) was shown to have an excellent correlation to the radioantigen binding assay (RABA). When 214 sera with different anti-HbPs levels were assayed for total Ig by HbO-HA ELISA and by RABA the correlation coefficient was 0.917 and the paired t test p value was 0.575. Use of competitive ELISA employing soluble HbPs, HbO-HA and human albumin as competitors, showed that the HbO-HA ELISA was specific for antibodies to HbPs. The HbO-HA ELISA yielded reproducible results both within and between laboratories. The HbO-HA ELISA can also be used to determine the isotype of anti-HbPs antibodies by using isotype specific enzyme conjugates. The sum of the IgG, IgA and IgM HbO-HA ELISA results had excellent correlation to the RABA results (correlation coefficient = 0.976). Thus, the HbO-HA ELISA can be substituted for the classical RABA and also be utilized for quantitating the isotype of the anti-HbPs antibodies.

Two IgA immune complex assays, namely the anti-IgA inhibition of binding assay (a-IgA-InhBA) and the IgA polyethylene glycol assay (IgA-PEG assay) were evaluated using IgA aggregates (AIgA) of well defined sizes. AIgA was prepared by heat treatment. After ultracentrifugation the size of AIgA was found to be in a range between 7 S and 64 S. Five AIgA pools (greater than 64 S, 44-64 S, 24-43 S, 20-24 S and 9-19 S) and a pool of monomeric IgA were prepared to test the efficiency of the two assays. The IgA-PEG assay exclusively detected very large sized AIgA (greater than 64 S), whereas the a-IgA-InhBA detected also AIgA of intermediate size (9-19 S). The sensitivity of the latter assay was found to be size dependent. After ultracentrifugation of serum samples from patients with IgA nephropathy, IgA of large and intermediate macromolecular size was detected. It is concluded that the a-IgA-InhBA is useful for the detection of circulating IgA-containing immune complexes in IgA nephropathy.

Two murine IgM monoclonal antibodies (MAs) directed against the lipid A portion of bacterial lipopolysaccharide (LPS) were compared in their binding to Re LPS and lipid A and their idiotypic make-up with rabbit anti-idiotypic sera. Horseradish peroxidase (HRPO)-labelled MAs 8-2 and 26-20 bound equally well to Re LPS. The binding of HRPO-labelled MA 8-2 to synthetic lipid A was low compared to the relatively strong binding of labelled 26-20. The MAs proved to be competitive in a competition binding assay (CBA) with Re LPS as coating antigen. Rabbit immune sera were raised against individual MAs. Anti-idiotypic antibodies (anti-id Abs) were detected with two sensitive enzyme immunoassays (EIA): a solid-phase EIA and an inhibition EIA. The rabbit antisera proved to be idiotype specific, indicating that both MAs recognize separate epitopes. We expect that anti-id Abs will prove to be of value for the differentiation of panels of LPS specific MAs.

We have set up a method to predict peptide binding to HLA-DP4 molecules. These HLA II molecules are the most frequent worldwide and hence are an interesting target for epitope-based vaccines. The prediction is based on quantitative matrices built with binding data for peptides substituted at anchoring positions for HLA-DP4. A set of 98 peptides of various origins was used to compare the prediction with binding activity. At different prediction thresholds, the positive predictive value and the sensitivity of the prediction ranged from 50% to 80%, demonstrating its efficiency. This prediction method can be applied to the entire genomes of pathogens and large peptide sequences derived from tumor antigens.

Dynabeads TALON are uniform superparamagnetic polystyrene beads of 1 microm diameter that bind 6-His-tagged recombinant proteins through Co++-affinity binding, and are normally used for protein purification. We have used these beads to bind 6-His-rNKp30 and 6-His-rNKp46 to use as a matrix for evaluating NKp30 and NKp46 mAb submitted to the 8th International Human Leukocyte Differentiation Antigen Workshop. We show that recombinant protein coated beads are an effective tool to evaluate the specificity and epitope reactivity of mAb.

The association of chemokines with endothelial cells (EC) and extracellular matrices is required for the prototypical pro-emigratory and pro-migratory in vivo activity of these molecules, respectively. In order to investigate chemokine binding to intact microanatomical structures, e.g. venular EC, we have developed an in situ binding assay. This is an autoradiographic morphological method in which the saturable binding of radiolabeled chemokines is studied in vitro in pieces of viable tissues. This article discusses the general applicability, advantages and shortcomings of the in situ binding assay in comparison with the other techniques available for visualizing chemokine receptor binding by cells in the tissues. We used this assay to demonstrate: (a) selective specific binding of CXC and CC chemokines to the EC of postcapillary venules but not capillaries or arteries; (b) selective specific binding of CC chemokines to the EC of afferent lymphatic vessels; and (c) selective specific binding of inflammatory chemokines to the EC lining high endothelial venules (HEV) in lymph nodes. The assessment of ligand cross-competition provided a fingerprint of chemokine-binding specificity of the EC in each of these microanatomical sites. This fingerprint could be paralleled with the chemokine-binding profiles of two non-signaling chemokine-binding molecules, Duffy antigen receptor for chemokines (DARC) and D6, present in venular and lymphatic EC, respectively. These observations allowed us to put forward the hypotheses regarding the involvement of EC DARC and D6 in chemokine transport and presentation by the EC.

The importance of the interaction between lymphocyte function-associated antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1) in the progression of inflammatory responses in vivo has been demonstrated mainly in rats. The present study was undertaken to develop binding assays suitable for measuring the rat ICAM-1/LFA-1 interaction in vitro. We first examined binding of rat T lymphoma FTL43 cells, which express LFA-1, to immobilized rat ICAM-1. Although FTL43 cells bound avidly to immobilized ICAM-1 and the binding was abolished with anti-LFA-1 monoclonal antibodies (mAbs), the binding was not completely inhibited by most anti-ICAM-1 mAbs. We next purified rat LFA-1 from FTL43 cells and constructed a cell-free binding assay. By using a newly developed anti-rat LFA-1 mAb RL14/9, which does not inhibit ICAM-1/LFA-1 interactions, binding of purified rat LFA-1 to immobilized ICAM-1 was successfully detected, whereas only a low signal to noise ratio was observed when binding of ICAM-1 to immobilized LFA-1 was examined. Moreover, we found that simultaneous addition of purified LFA-1 and biotinylated RL14/9 to ICAM-1-coated wells resulted in more sensitive detection of rat ICAM-1/LFA-1 binding. The binding was completely blocked with both anti-LFA-1 and anti-ICAM-1 mAbs and was much more sensitive to inhibition by the ICAM-1-IgG chimera, as compared with the cell-based assay. These results indicate that the cell-free binding assay provides a rapid and sensitive method for screening rat ICAM-1/LFA-1 antagonists, whose therapeutic effect on inflammatory diseases can further be evaluated in vivo.

Vascular endothelial growth factor (VEGF) is an important factor for endothelial cell proliferation and a key regulator of blood vessel development in embryos and angiogenesis in adult tissues. Its biological activity is mediated by two receptor tyrosine kinases, VEGFR-1 (Flt-1) and VEGFR-2 (KDR). In contrast to VEGFR-2, a naturally occurring soluble form of the VEGFR-1 (sVEGFR-1) is produced by endothelial cells by differential splicing of the flt-1 gene, and it is a secreted gene product. In order to develop a specific enzyme-linked immunosorbent assay (ELISA) for the measurement of sVEGFR-1, we established five anti-human receptor antibodies and characterized them in detail. These antibodies recognize different epitopes located within the seven Ig-like domains of the extracellular receptor protein but have no neutralizing activity in ligand binding assays. Together with a polyclonal antiserum, a specific human sVEGFR-1 ELISA was developed using the mAb #190.11. The ELISA can detect human sVEGFR-1 with a minimum detection limit of 1 ng/ml. The ELISA does not show any cross-reactivity with other related soluble receptors. Using this assay, human sVEGFR-1 was measured in the supernatant of different VEGFR-1 expressing cell types. No sVEGFR-1 protein was detectable after heparin Sepharose treatment or size-exclusion filtration (30 kDa). The ELISA assay for sVEGFR-1 was also used to measure the amount of the soluble receptor in amniotic fluid samples of patients undergoing amniocentesis during the course of normal pregnancies. The concentration of the samples was in the range of 5-35 ng/ml. This ELISA could be useful powerful tool for investigations concerning the physiological function of the soluble receptor under normal and pathophysiological conditions.Furthermore, it may facilitate studies of the mechanisms of receptor production.

It is important in the laboratory to develop techniques to preserve leucocyte viability in blood specimens for subsequent flow cytometric analysis. This article describes a new simple whole blood lysis method using Annexin V FITC staining which can be used to define both early and late apoptosis of granulocytes in heterogeneous cell populations, without the need for additional stains or to purify the cells (which may result in loss of the cells of interest). The differential Annexin V binding assay was in good agreement with the light microscopy reference method and showed excellent correlation with 7-aminoactinomycin D (7-AAD) staining. It was not affected by problems of morphological interpretation and artifactal changes of granulocyte deformability noted using light microscopy, or the technical difficulties encountered due to red cell contamination using the 7-AAD method. Using this new differential Annexin V staining method, we determined the optimum conditions that maintain granulocyte viability for subsequent flow cytometric analysis and are now employed in our laboratory. These conditions were lithium heparin (Hep) anticoagulated whole blood specimens kept at 4 degrees C with the addition of nutrient medium. Specimens that are anticoagulated with acid citrate dextrose (ACD) or ethyl-diacetyl-tetraacetic acid (EDTA) should also be treated similarly to preserve granulocyte viability and to overcome problems associated with identification of cell populations by flow cytometry.

A new immunoassay for alpha-fetoprotein based upon liquid-phase binding reactions is described. In this procedure, a sample that contains alpha-fetoprotein is mixed with a solution of two anti-alpha-fetoprotein monoclonal antibodies complexed with peroxidase and sulfated peptide, respectively. After incubation, immune complex is separated from other components by anion exchange column chromatography. Immune complex is quantified using fluorometric detection by peroxidase enzymatic activity. Peroxidase activity correlated with a alpha-fetoprotein with a 1:1 relationship. The stoichiometric immunoreaction allowed a large analytical range (0.4-7500 ng/ml) with a linear dose-response relationship, high sensitivity and good precision. Endogenous substances did not interfere with assay performance. Assay results showed good correlation with other established methods. These results indicate that the method is useful for clinical alpha-fetoprotein determinations.

CD23, a low-affinity receptor for IgE (Fc epsilonRII), is a type II membrane-bound glycoprotein expressed on many hematopoietic cells, particularly activated B-cells. CD23 binds to IgE at a domain homologous to Ca2+-dependent (C-type) animal lectin. This paper describes a binding assay by which only the specific binding of IgE to CD23 expressed on Epstein-Barr virus (EBV)-transformed B-cell line, L-KT9 cells, can be detected. This assay is useful in the search for CD23 ligands among many chemical compounds, because it is easily carried out and does not require the use of any radiolabeled reagents. Using the assay, we investigated the inhibition of IgE binding to CD23 by fucose-1-phosphate which has been reported to inhibit the binding of sCD23 to IgE [Delespesse, G., Sarfati, M., Wu, C.Y., Fournier, S., Letellier, M., 1992. The low affinity receptor for IgE. Immunol. Rev. 125, 77.] and the binding of CD23 to CD21 [Pochon, S., Graber, P., Yeager, M., Jansen, K., Bernard, A.R., Aubry, T.-P., Bonnefoy, J.-Y., 1992. Demonstration of second ligand for the low affinity receptor for immunoglobulin E (CD23) using recombinant CD23 reconstituted into flourescent liposomes. J. Exp. Med. 176, 389.]. Although both alpha- and beta-L-fucose-l-phosphate/di(cyclohexylammonium) salt decreased the extent of IgE binding to CD23, the inhibitory effects were not due to alpha- or beta-L-fucose-1-phosphate but to cyclohexylamine. The inhibitory effect of cyclohexylamine was dose dependent and the effect was decreased when inhibition tests were carried out in the presence of a 10-fold excess of IgE. These results suggest that cyclohexylamine specifically interacts with the binding of CD23 and IgE.

A simple and reproducible radioreceptor assay (RRA) has been developed using membranes from CHO cells which can stably express human platelet-activating factor (PAF) receptor. The CHO cells expressing the PAF receptor, termed CHO · 1F8, showed a significant intracellular Ca 2+ response to PAF, and the same binding properties to [ 3 H]WEB 2086, a PAF antagonist, as reported ( K d , 13.6 ± 1.9 nM; B max , 2.5 ± 0.4 pmol/mg protein ( n = 6)). A competitive binding assay was done using the CHO · 1F8 cell membranes and [ 3 H]WEB 2086. The minimum detectable dose of PAF was 0.3 nM (≈ 30 pg per well) and the assay was highly specific for PAF. This method makes it possible to handle large numbers of samples rapidly and simultaneously, since the receptor membrane is prepared in advance and the binding assay can be completed within 3 h. Using this method, we have determined the production and cell association of PAF in human neutrophils.

Haptoglobin (Hp) is recognised as a major acute phase protein in the bovidae and its presence in serum is used as an indicator of inflammation. A mouse monoclonal antibody (1D9) specific for bovine Hp was labelled with a lanthanide (Eu) chelate and used to develop a competitive immunoassay. This competitive immunoassay allowed direct measurement of Hp in serum and was validated for intra- and interassay coefficients of variation (below 8%). Cross-reactivity with other serum proteins was measured (less than 0.1%) and limits of detection for Hp in serum were established for adult male (0.344 microgram/ml) and adult female cattle (1.589 micrograms/ml). The immunoassay was compared with an established haptoglobin-haemoglobin binding assay.

A new immunometric sequential binding assay has been developed in which the sample is first reacted with a solid phase binding partner in low concentration, and subsequently with a second binding partner at a higher concentration. The amounts of analyte bound to the two solid phase binding partners are separately measured, thus establishing a double standard curve. There is a shift between the two standard curves along the concentration axis. Thus an unambiguous determination of analyte concentration is obtained, even in the descending region of the curves where the ‘hook’ effect causes decreasing signal with increasing analyte concentration. A two-particle immunofluorometric assay for AFP based on this principle measured by flow cytometry, resulted in an assay with rapid binding (∼ 2 h), a detection limit of 0.1 kIU/l and a working range (0.3 to > 3 × 106 kIU/l) in excess of 7 log10 orders. Assay results compared well with those of an immunoradiometric assay.

Affinity capillary electrophoresis was performed to quantitate the binding of Ca2+ and phosphorylcholine to human C-reactive protein (CRP). The assay requires no modifications of any of the molecules involved, uses minuscule amounts of protein (8.5 x 10(-15) mol per analysis, i.e., less than 1 pmol for 15 triplicate data points), and the binding could be examined under conditions of physiological ionic strength and pH. The values for the dissociation constants obtained here (KD = 59 microM for Ca(2+)-CRP and 18 microM for the phosphorylcholine-CRP interaction) were in close agreement with previous studies using gel filtration and equilibrium dialysis. As long as one of the reactants can be detected and recovered quantitatively in the capillary electrophoresis system, the method is generally useful to study interactions where complexed molecules display an electrophoretic mobility that is different from that of unbound molecules and where the rates of association and dissociation are sufficiently fast.

The author regrets that an error was printed in the Materials and methods section of the above paper. The last section of the first paragraph, in the Materials and methods section should read as follows: D2.9A5 and 461.G1 (A. Porgador, Ben Gurion University of the Negev, Beer-Sheva, Israel, and O. Mandelboim, The Lautenberg Center for General and Tumor Immunology, Hebrew University of Jerusalem, Israel).

The parameters of a sandwich assay to analyze syngeneic polyclonal and monoclonal anti-idiotypic antibodies to murine anti-HLA and anti-human melanoma associated antigen monoclonal antibodies have been characterized. Furthermore, the sensitivity of the sandwich assay has been compared with that of assays widely used to characterize anti-idiotypic antibodies, i.e., the binding assay to F(ab')2 fragments of monoclonal antibodies and the radioimmunoassay. The latter is more sensitive than the sandwich assay when monoclonal anti-idiotypic antibodies are tested, but less sensitive when anti-idiotypic antisera are analyzed. Both assays are less sensitive than the binding assay to F(ab')2 fragments of monoclonal antibodies.

A solid-phase enzyme-linked binding assay is described for screening monoclonal antibodies to cell surface antigens. E. coli β-galactosidase was coupled to rabbit anti-rat Ig and used to detect the binding of rat monoclonal antibodies to cells which had been fixed to the wells of microtitre plates using a combination of poly-l-lysine and glutaraldehyde. This method was found to be advantageous for the large scale screening of monoclonal antibodies with a panel of cell types, and has been useful in the selection of antibodies which would be candidates for differentiation markers within the human and mouse haemopoietic systems.

A quick and simple method for determining whether monoclonal antibodies react with the same or different antigens is described which utilises an indirect radioactive binding assay against cells. Six antibodies were selected, BK19.45, BK20.20, GenOx4.17, GenOx4.21, W6-34 and W6-46, which detected antigens present either only on leukocytes (BK19.45 and BK20.20) or on a wider range of cell types including fibroblasts, liver cells and neuroblastoma cells. The saturation binding levels for each antibody, in terms of the amount of 125I-anti-mouse immunoglobulin bound, were determined with respect to a fixed number of cells. The addition of two antibodies with different specities (BK19.45 or BK20.20 to either W6-34 or W6-46) resulted in a higher plateau value of 125I-anti-mouse immunoglobulin bound per fixed number of cells than for either antibody singly. The increase in the amount of antibody bound is equivalent to the sum of the two individual saturation binding levels. In contrast, the addition of BK20.20 to BK19.45 or W6-45 produced no detectable rise in the saturation level. From these data it is concluded that BK20.20 and BK19.45, and W6-34 and W6-46 are bound to either identical epitopes which are in very close spatial relationship. On the other hand 19.45 and W6-34, as expected, detect unrelated antigens. Observations from autoradiograph binding studies on the semi-quantitative distribution on bone marrow cells of the antigens recognised by 19.45 and 20.20 supported the conclusion that the antibodies were recognising identical antigens. A study of the antibodies to HLA (2A1) and β2 microglobulin (M3) showed an increase in the saturation level, when both antibodies were added together, which was less than the sum of the two individual saturation binding levels. The close association of β2 microglobulin and HLA on the cell surface may to some extent prevent independent antibody binding. These data suggest that the above approach will differentiate between monoclonal antibodies which detect antigenic determinants that are located on closely associated surface molecules.

An indirect binding assay, the fluorometric immunoassay (FIAX), was established for the detection of anti-M2 antibodies which are specific markers for primary biliary cirrhosis (PBC). Submitochondrial particles (SMP) from beef heart and rat liver and the ATPase-associated antigen (M2) were used. The antigens were fixed to a cellulose acetate surface, SMP at a concentration of 2 mg/ml, ATPase at a concentration of 0.2 mg/ml. Sera were used at 1:60 and 1:120 and bound antimitochondrial antibodies (AMA) were demonstrated by fluorescent isothiocyanate labelled monospecific anti-human IgG, IgM and IgA antibodies. The fluorescent signals were proportional to the AMA titre in the serum samples and were measured in a fluorometer (FIAX 100). Of 94 patients with PBC, 92 had AMA against SMP from beef heart compared with 76 in the complement fixation test (CFT) and 84 in the immunofluorescence test (IFL). Ninety reacted with the ATPase-associated M2 antigen. Sera from patients known to have AMA of different specificities (anti-M1, anti-M3, anti-M5, anti-M6) reacted with SMP from beef heart and/or rat liver but not with M2.

A newly developed assay using microtitre plates with removable wells permits rapid measurements of IL-2 receptors. Triplicate or quadruplicate samples of several ligand dilutions are easily handled, thus giving more reliable results. In addition, measurement of binding parameters may be accomplished simultaneously on several cell lines. Results are presented which show that the binding assay may also be used for measuring binding constants of antibodies.

A cell binding assay (CBA) was developed in which plant lectins are used as binding agents between microtiter plates and human cells. After binding, the cells were fixed by mild glutaraldehyde treatment. Their antigenic activity was investigated by enzyme-linked immunosorbent assay (ELISA) with several monoclonal antibodies. The binding method resulted in cell layers that remained firmly attached to the plates during the washing and incubation procedures of the ELISA. A comparative phenotype analysis, performed by indirect membrane fluorescence, showed that cells bound by this method, do not lose their antigenic activity. This binding assay can be used as a rapid, large scale screening test for monoclonal antibodies to membrane antigens of malignant and normal cells.

A simple assay for circulating immune complexes has been developed and the optimal conditions for reaction defined. Partially purified radiolabelled polyclonal or monoclonal rheumatoid factor was incubated with aggregated IgG or soluble immune complexes and the resultant macromolecule precipitated with 3% polyethylene glycol. Monoclonal rheumatoid factor was much the better reagent, allowing the detection of approx. 0.35 μg/ml or more of aggregated IgG. Soluble immune complexes formed in vitro at between X2–X20 antigen excess were detectable over a wide range of molecular size. Clinical studies indicated that the assay is a useful addition to the currently available techniques for measuring circulating immune complexes.

A new convenient method for the measurement of platelet-activating factor produced in vitro has been developed. This method involves incubation of neutrophils with stimuli, lipid extraction, purification of lipid extracts by normal phase high performance liquid chromatography (HPLC) and quantification of platelet-activating factor (PAF) by a competitive radioreceptor binding assay. The recovery of PAF from the extraction and purification procedures is 94.5 ± 0.9%. The sensitivity of the assay is 10 pg/tube. Human neutrophils stimulated with 0, 0.25, 0.5 1 and 2 μM A 23187 will produce ND, ND, 720, 840 and 900 pg of PAF, respectively (ND = not detectable). The values obtained for PAF produced by human neutrophils in the present assay are comparable to those obtained with the rabbit platelet aggregation assay.

The inhibition of binding of 125 I-labeled MOPC 104E IgM anti-idiotype to the controlled pore glass-bound IgM by various oligo- and polysaccharides was investigated as a model for the screening of myeloma proteins with unknown hapten binding specificity. The inhibitory efficiencies of the different haptens in this system were found to correlate well with their known abilities to bind to MOPC 104E IgM. Thus B1355S dextran, nigerosyl-α(1–3)-nigerose, and nigerosyl-α(1–3)-glucose, all known for specific binding to MOPC 104E IgM, were efficient inhibitors in the assay, in contrast to a series of more or less unrelated carbohydrate compounds, which showed only weak inhibitory capacity. According to these results there seems to be no obstacle to applying the anti-idiotype assay system to screening myeloma proteins whose binding specificity has not yet been determined.