A MONOCLONAL ANTIBODY DIRECTED AGAINST A HUMAN B LYMPHOID CELL ACTIVATION ANTIGEN
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The potential effects of free circulating antigen on the ability of monoclonal antibodies to target tumors in vivo were investigated. Tumor models consisted of HCC, NuE and PLC cell lines producing AFP xenografted in nude mice, and the NuE-treated mouse designated as the NuE-bearing mouse injected with AFP prior to the administration of antibody. Immunoscintigraphy and biodistribution were evaluated by using 125I-labeled monoclonal antibody 19F12 raised against AFP. Gel chromatography analysis of plasma from the PLC-bearing mouse which excreted 400 ng AFP/ml in blood injected with 125I-19F12 indicated that all injected antibody 19F12 formed an immune complex in plasma. No immune complex was present in plasma from the NuE-bearing mice, where blood AFP levels were 7 ng/ml, while the intact antibody was found to remain partly in plasma from the NuE-treated mouse. Radioactivities in the whole body of NuE-bearing and NuE-treated mice eventually cleared at the same rate. Our experimental results indicated that the endogeneous circulating antigen retained the antibody in the whole body for a longer period. The ability of monoclonal antibodies to target tumors was influenced not only by how much antigen was present but also by how rapid the antigen was cleared in the blood.
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Immune complex
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Antibodies against human CD36 are responsible for several immune-mediated disorders. The detection of anti-CD36 antibodies using the standard monoclonal antibody (mAb) immobilization of platelet antigens (MAIPA) assay is hampered by a high frequency of false-negative results, most likely due to competitive inhibition of the mAb used as the capture antibody. We generated a panel of mouse mAbs against CD36 and seven hybridomas (GZ-3, GZ-13, GZ-70, GZ-143, GZ-413, GZ-507, and GZ-608), which were selected for MAIPA assays, as they reacted with mouse and human CD36. Fourteen anti-CD36 sera were assayed; all of which showed a positive reaction in a PakPlus (Immucor GTI Diagnostics, Inc., Waukesha, WI, USA) ELISA-based screening (optical density: 0.257-2.292). When the reference anti-CD36 mAb FA6-152 was used in the MAIPA assay, only 6/14 (42.9%) sera displayed a positive reaction. In contrast, anti-CD36 antibodies were detected in 13/14 (92.9%) sera when GZ-70 and GZ-608 mAbs were used. This significant improvement resulted in the identification of anti-CD36 antibodies by an antigen capture assay. Since patient's platelets possibly carrying rare native antigens are used, this method will facilitate the identification of new platelet antibodies against CD36 that are involved in immune-mediated thrombocytopenia and other diseases, such as transfusion-related acute lung injury.
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The 30 workshop monoclonal antibodies identify a number of distinct cell surface antigens of human melanoma. Indirect 125I-protein A binding assays showed that virtually all of the antibodies recognized antigens at the surface of SK-MEL-28 melanoma cells. Immunoprecipitation from detergent lysates of surface-radioiodinated cells showed that 16 of the antibodies recognized cell surface proteins. Antibodies 96.5, 118.1, 133.2 and two antibodies from the Sloan–Kettering group, I12 and L10, recognized a 97,000 MW protein, p97, but none of the other antibodies did so. Many of the antigens appear to have sufficient specificity for melanoma to be of interest as potential diagnostic markers and therapeutic targets and to merit structural and functional studies.
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Hybridomas producing anti-creatine kinase (CK) and anti-lactate dehydrogenase (LDH) antibodies were screened by enzyme linked immunosorbant assay (ELISA) with antigen coated onto plastic wells. Out of seven antibodies positive for each isoenzyme of CK, four antibodies failed to bind to the radiolabeled antigen in solution phase radioimmunoassay (RIA) or native antigen in competitive ELISA. Moreover, out of nine antibodies shown reactive with LDH-1 and seven antibodies binding to LDH-5 by ELISA, not a single antibody bound to either radiolabeled or native antigen in solution. Our results along with other recent studies strongly suggest that antigen conformation on solid phase may frequently be different from in solution and that screening by ELISA in which antigen is attached to solid phase is often inappropriate for determining the presence of useful monoclonal antibodies.
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Antibodies to CA 125 have been used to predict relapse of ovarian cancer, but have performed poorly as therapeutic agents. One rationale for this is antibody binding to circulating shed antigen. Our aim in this study was to develop antibodies to human CA 125 that have enhanced selectivity for the cell-associated form of the antigen.Monoclonal antibodies were raised to a recombinant fragment of CA 125 that included sequence proximal to the putative membrane attachment site. Antibodies were characterized in terms of their binding site, affinity and selectivity for cell-associated CA 125.In assays using patient-derived CA 125, a subset of high-affinity (KD <5 nM) monoclonal antibodies demonstrated a 10- to greater than 200-fold increase in selectivity for cell-associated CA 125 when compared with controls. Based on mapping of the various monoclonal antibodies obtained, it was determined that shedding of CA 125 most likely occurs in the most C-terminal repeat domain.Results from competition analysis using patient-derived shed antigen predict that the antibodies described in this study may have significantly enhanced tumor-targeting properties when compared with existing antibodies to CA 125 in a tumor environment having high concentrations (>10,000 CA 125 units) of shed CA 125.
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Use of monoclonal antibodies for quantitative analysis of antigens in normal and neoplastic tissues.
We describe two methods of analyzing tissues for cell-surface antigens identified by monoclonal antibodies. The first method is a binding assay, in which a membrane fraction prepared by differential centrifugation of a tissue homogenate is incubated with 125I-labeled antibody. The specificity of the observed binding is established by competition with unlabeled antibody. The second method, double-determinant immunoassay, involves two monoclonal antibodies that recognize two distinct epitopes of the antigen molecule. An immunoabsorbent prepared from one of the antibodies is incubated with a detergent lysate of the tissue. The immunoabsorbent is then washed and incubated with the 125I-labeled second antibody. In both assays the amount of radiolabeled antibody bound is proportional to the amount of antigen in the test sample, and the limit of detection is approximately 10 pg of antibody, corresponding to less than 1 fmol of antigen.
Differential centrifugation
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