Direct binding of radioiodinated monoclonal antibody to tumor cells: significance of antibody purity and affinity for drug targeting or tumor imaging

Enzyme purification and sustained enzyme activity for pharmaceutical biocatalysis by fusion with phase-separating intrinsically disordered protein

In recent decades, biocatalysis has emerged as an important alternative to chemical catalysis in pharmaceutical manufacturing. Biocatalysis is attractive because enzymatic cascades can synthesize complex molecules with incredible selectivity, yield, and in an environmentally benign manner. Enzymes for pharmaceutical biocatalysis are typically used in their unpurified state, since it is time-consuming and cost-prohibitive to purify enzymes using conventional chromatographic processes at scale. However, impurities present in crude enzyme preparations can consume substrate, generate unwanted byproducts, as well as make the isolation of desired products more cumbersome. Hence, a facile, nonchromatographic purification method would greatly benefit pharmaceutical biocatalysis. To address this issue, here we have captured enzymes into membraneless compartments by fusing enzymes with an intrinsically disordered protein region, the RGG domain from LAF-1. The RGG domain can undergo liquid-liquid phase separation, forming liquid condensates triggered by changes in temperature or salt concentration. By centrifuging these liquid condensates, we have successfully purified enzyme-RGG fusions, resulting in significantly enhanced purity compared to cell lysate. Furthermore, we performed enzymatic reactions utilizing purified fusion proteins to assay enzyme activity. Results from the enzyme assays indicate that enzyme-RGG fusions purified by the centrifugation method retain enzymatic activity, with greatly reduced background activity compared to crude enzyme preparations. Our work focused on three different enzymes-a kinase, a phosphorylase, and an ATP-dependent ligase. The kinase and phosphorylase are components of the biocatalytic cascade for manufacturing molnupiravir, and we demonstrated facile co-purification of these two enzymes by co-phase separation. To conclude, enzyme capture by RGG tagging promises to overcome difficulties in bioseparations and biocatalysis for pharmaceutical synthesis.

Biotin-streptavidin-guided two-step pretargeting approach using PLGA for molecular ultrasound imaging and chemotherapy for ovarian cancer

Background

Ovarian cancer seriously threatens the lives and health of women, and early diagnosis and treatment are still challenging. Pre-targeting is a promising strategy to improve the treatment efficacy of ovarian cancer and the results of ultrasound imaging.

Purpose

To explore the effects of a pre-targeting strategy using streptavidin (SA) and paclitaxel (PTX)-loaded phase-shifting poly lactic-co-glycolic acid (PLGA) nanoparticles with perfluoro-n-pentane (PTX-PLGA-SA/PFPs) on the treatment and ultrasound imaging of ovarian cancer.

Methods

PTX-PLGA/PFPs were prepared with a single emulsion (O/W) solvent evaporation method and SA was attached using carbodiimide. The encapsulation efficiency of PTX and the release characteristics were assessed with high performance liquid chromatography. The phase-change characteristics of the PTX-PLGA-SA/PFPs were investigated. The anti-carcinoembryonic antigen (CEA) antibody (Ab) was covalently attached to PTX-PLGA/PFPs via carbodiimide to create PTX-PLGA-Ab/PFPs. The targeting efficiency of the nanoparticles and the viability of ovarian cancer SKOV3 cells were evaluated in each group using a microscope, flow cytometry, and cell counting kit 8 assays.

Results

THE PTX-PLGA-SA/PFPs were spheres with a size of 383.0 ± 75.59 nm. The encapsulation efficiency and loading capability of the nanoparticles for PTX were 71.56 ±  6.51% and 6.57 ± 0.61%, respectively. PTX was burst-released up to 70% in 2–3 d. When irradiated at 7.5 W for 3 min, the PTX-PLGA-SA/PFPs visibly enhanced the ultrasonography images (P < 0.05). At temperatures of 45°C and 60°C the nanoparticles phase-shifted into micro-bubbles and the sizes increased. The binding efficiencies of SA and Ab to the PTX-PLGA/PFPs were 97.16 ±  1.20% and 92.74 ± 5.75%, respectively. Pre-targeting resulted in a high binding efficacy and killing effect on SKOV3 cells (P < 0.05).

Conclusions

The two-step pre-targeting process can significantly enhance the targeting ability of PTX-loaded PLGA nanoparticles for ovarian cancer cells and substantially improve the therapeutic efficacy. This technique provides a new method for ultrasonic imaging and precise chemotherapy for ovarian cancer.

Kinetic analysis of unpurified native antigens available in very low quantities and concentrations

Affinity measurements of antigen-antibody interactions are generally performed using known concentrations of purified or recombinant materials. In addition, many technologies that measure affinity require the interacting components to be present in at least microgram quantities. Specifically, if the antigen is either available only in low quantities or unable to be purified, or if the quantity is unknown, then the measurement of affinity can be very difficult. Using the Kinetic Exclusion Assay (KinExA) technology, here we describe a method that overcomes the requirement for large amounts of purified and known quantities of antigen. We used this method to precisely measure the affinity of fully human anti-human interleukin 13 (IL13) monoclonal antibodies to IL13 produced in native form from primary T cells derived from a variety of species, including human. These antigens were available only in the limited quantities present in the conditioned cell culture medium, and the affinity was measured directly without further purification.

High-affinity binding measurements of antibodies to cell-surface-expressed antigens

A simple method that allows affinity measurements of antibodies to integral membrane proteins is described. Kinetic Exclusion Assay was used to determine the concentration of free antibody that remains in solution after equilibrium has been established between the antibody and the cell-surface-expressed antigen, from which the equilibrium dissociation constant (Kd) was determined. It eliminates the requirement for soluble antigen and modifications such as radio-labeling or fluorescent labeling of the antibody. For one of the cell-surface-expressed antigens, it was determined that the affinity of the antibody to the cell-surface-expressed antigen was similar to that of the purified, soluble form of the antigen. In addition to the simplicity of the approach, the method provides a true measure of the affinity/avidity of the antibody to the native form of cell-surface-expressed targets, including antigens that cannot be produced in soluble forms, and to unknown cell surface antigens.

Measurement of the functional affinity constant of a monoclonal antibody for cell surface receptors using kinetic exclusion fluorescence immunoassay

Measuring a protein-ligand interaction in solution, away from the ligand's cellular environment, may not provide an affinity value applicable in vivo. Here, we present a simple, accurate and highly sensitive method for determining the antibody affinity to cell surface receptor, hIGFR, and compare this data to affinity determined for the soluble receptor. Measurements were performed on both full-length bivalent IgG and the monovalent Fab fragments to assess possible differences in apparent affinity introduced by avidity of the bivalent IgG. Affinities determined for soluble hIGFR were 4 x 10(-12) M for the bivalent IgG and monovalent Fab. Comparable affinities of 6 x 10(-12) M and 1 x 10(-11) M for the bivalent IgG and Fab, respectively, were also determined for full-length hIGFR on cell surface. The method described allows estimation of reactant concentrations (anti-IGFR antibody) relative to one known reference concentration (the concentration of soluble hIGFR in our case) allowing us to estimate the average receptor density on the cell surface. Taken together, we believe these data can provide valuable insight into antibody behavior in vivo, especially in the case of insoluble or difficult to purify transmembrane receptors.

Therapy of rat tracheal carcinoma IC-12 in SCID mice: vascular targeting with [213Bi]-MAb TES-23

In previous work, we have demonstrated that vascular targeting of [213Bi], an alpha-emitter, to lung blood vessels could efficiently destroy tumour colonies growing in the lung. In order to expand this approach to treatment of tumours growing in other sites, we employed the monoclonal antibody (MAb) TES-23, which reacts with CD44H, preferentially expressed on new blood vessels in tumours. Biodistribution studies of N-succinimidyl [125I] 3-iodobenzoate (SIB)-radiolabelled MAb TES-23 in ICR-severe combined immunodeficient (SCID) mice bearing subcutaneous (s.c.) and intramuscular (i.m.) IC-12 tumours, demonstrated efficient tumour uptake. At 24 h, accumulation in small tumours was 45%ID/g for s.c. tumours, and 58%ID/g for i.m. tumours and in large tumours it was 25%ID/g for s.c. tumours and 17%ID/g for i.m. tumours. Micro-autoradiography data confirmed that radiolabel accumulated in or near tumour blood vessels. Normal tissues had very low levels of radioactivity. Treatment of mice bearing small IC-12 tumours with [213Bi] MAb TES-23 retarded tumour growth relative to animals treated with cold MAb TES-23. Biodistribution and therapy experiments were also performed in BALB/c mice bearing both s.c. and i.m. syngeneic, lung carcinoma (line 498) tumours. [I(125)] SIB MAb TES-23 accumulated efficiently in both s.c. and i.m. tumours (14%ID/g and 15%ID/g, respectively, at 4 h); however, no therapeutic effect of [213Bi] MAb TES-23 treatment could be demonstrated in this model system. The data demonstrate that the timing of vascularisation of the tumours and the delivery kinetics of MAb relative to the half-life of the therapeutic radionuclide are critical for effective therapy.

Actinium-225 conjugates of MAb CC49 and humanized delta CH2CC49

Radioisotopes with moderate half-lives are essential for conventional radioimmunotherapy using tumor-selective MAbs which require days for localization. Actinium-225, with a half-life of 10 days and a yield of 4 alpha particles in its decay chain, may be an ideal choice for tumor-targeted radioimmunotherapy. Release of daughter radioisotopes from the primary chelator after the first decay has been a complication with the use of 225Ac. It has been reported that the domain-deleted product of MAb CC49, Hu-delta CH2 CC49, is able to extravasate and penetrate more deeply into tumors than the parent IgG molecule. We reasoned that once the 225Ac-chelate-MAb had penetrated into the tumor, the daughter radioisotopes would remain trapped even if they had been released from the primary chelator. Actinium-225 HEHA MAb CC49 conjugates were tested for distribution, micro-distribution and therapy in immunocompromised mice which had LS174T tumors growing at subcutaneous or intramuscular sites. Both 125I and 225Ac CC49 and Hu-delta CH2 CC49 were efficient in delivery of the radioisotopes to tumor sites. Tissue micro-autoradiography for the two antibody forms did not demonstrate any differences in micro-distribution of either 125I or 225Ac in the tumor. Furthermore, there was no detectable difference for the two carriers in the tumor retention of daughter radioisotopes from 225Ac. Therapy experiments with 225Ac were complicated by radiotoxicity of the conjugates. The lethal dose was about 0.5 microCi in two strains of mice regardless of the carrier. At injected doses of 0.5 and 0.25 microCi, CC49 was slightly active in tumor stasis, whereas no consistent significant effect of 225Ac-Hu-delta CH2 CC49 on growth of tumors was observed. The potential of 225Ac in radioimmunotherapy is limited by the radiotoxicity of its daughter radioisotopes. Its potential will only be realized if stable conjugates, capable of daughter radioisotope retention, can be devised.

Antigen protection of monoclonal antibodies undergoing labelling

The effectiveness of a methodology designed to protect the antigen binding capacity of monoclonal antibodies undergoing labelling with a number of reagents was examined. The antigen binding sites of monoclonal antibodies were protected by complexing them with their antigen. Chemical modification with 6 mM of the water soluble Bolton-Hunter reagent of site protected monoclonal antibodies to glucoamylase resulted in antibodies that could tolerate a four-fold increase in reagent incorporation, without any loss of antigen binding capacity. Iodination of these antibodies (modified under site protected conditions) yielded over 70% increase in radioactivity incorporated in the active antibody fraction, compared with the incorporation into unprotected antibodies. Site protected labeling was found to be effective in retaining the antigen binding capacity of monoclonal antibodies modified with all reagents tested with the exception of chloramine-T.

Inhibition of hematopoietic progenitor colony growth by a monoclonal antibody against the transferrin receptor: comparison of unconjugated antibody with an immunotoxin containing recombinant ricin A chain

We studied an immunotoxin consisting of recombinant ricin A chain (rRA) conjugated to 454A12 MoAb, a monoclonal antibody which recognizes an epitope on the human transferrin receptor, and compared the ability of 454A12 MoAb-rRA immunotoxin to inhibit the growth of erythroid burst-forming units (BFU-e) and myeloid colony-forming units (CFU-c) with unconjugated 454A12 MoAb. A significant reduction in BFU-e colony growth was observed at 0.001 microgram/ml of 454A12 MoAb-rRA versus 0.1 microgram/ml of unconjugated 454A12 MoAb (p = 0.005). Comparison of the effects of 454A12 MoAb-rRA and 454A12 MoAb on myeloid colony development gave markedly different results. Unconjugated antibody had no effect on CFU-c colony growth; in contrast, 0.01 microgram/ml of 454A12 MoAb-rRA reduced the number of colonies from 139 per 1 X 10(5) to 75 per 1 X 10(5) cells plated (p = 0.0005). No myeloid progenitor colonies developed at 0.1 microgram/ml of immunotoxin. These observations suggest that 454A12 MoAb-rRA inhibits growth by a potent, ricin A chain-mediated toxic effect on any proliferating cells expressing transferrin receptors, whereas the 454A12 MoAb exerts a selective inhibitory effect primarily on erythroid progenitors by perturbing the transferrin cycle. While growth factor receptors expressed on hematopoietic cells represent promising targets for immunotoxin therapy, our data indicate that an immunotoxin could inhibit cellular proliferation by a different mechanism than the corresponding unconjugated MoAb. Depending on the antibody used, these differences may be important in trials using immunotoxins for in vivo treatment or in vitro purging of malignant hematopoietic cells.

Rat monoclonal antibody distribution in mice: an epitope inside the lung vascular space mediates very efficient localization

MoAb to an epitope on lung endothelial cells accumulates rapidly in the lung resulting in localization ratios of over 100. MoAb to a macrophage antigen found in spleen and lung has maximum localization ratios of 24 and 5, respectively, while MoAb specific for a human tumor grown in nude mice has a maximum ratio of about 4. Epitope concentrations in target organs (300-600 ng/mg protein) are comparable in all three systems, indicating that the MoAb to endothelium is efficient in localization to the lung.

An optimized antibody-chelator conjugate for imaging of carcinoembryonic antigen with indium-111

A monoclonal antibody to carcinoembryonic antigen showing minimal cross-reactivity with blood cells and normal tissues was derivatized with benzylisothiocyanate derivatives of EDTA and DTPA. Seven chelators per immunoglobulin could be incorporated without loss of immunoreactivity. The resulting conjugates, labeled with indium-111, showed low liver uptake in animals. A cold kit, comprising the DTPA conjugate at a molarity of antibody bound chelator exceeding 1 x 10(-4) M, gave radiochemical yields of indium labeled antibody of greater than or equal to 95% and was stable for 1 yr.

Measurement of antibody affinity for cell surface antigens using an enzyme-linked immunosorbent assay

We present a fast, simple, and accurate method to determine the affinity constants of antibodies that bind to cell surface antigens. This procedure utilizes intact cells and native, unmodified antibody in a conventional enzyme-linked immunosorbent assay. Target cells are incubated with serial dilutions of antibody and allowed to reach equilibrium. Cells are then pelleted by centrifugation, and aliquots of unbound antibody in the supernatant are added to a microtiter plate precoated with capture antibody and measured in a conventional enzyme-linked immunosorbent assay (ELISA). We measured the affinity constant of murine monoclonal antibody CLB-1H-gran2, which binds to K562 cells (a human erythroleukemia line), and compared the ELISA-based results to those obtained by flow cytometric determination of antibody affinity. The affinity constants obtained by the two methods are in good agreement. The affinity constant is calculated utilizing only the concentrations of bound and free antibody, so that the actual antigen concentration (or number of antigenic sites per cell) need not be known. However, the number of antibody molecules bound per cell can be estimated from the results.

A perspective of monoclonal antibodies: past, present, and future

In 1975, the development of the technique to produce monoclonal antibodies revolutionized the approach to cancer detection and therapy. Hundreds of monoclonal antibodies to the epitopes of tumor cells have been produced, providing more specific tools for probing the cellular elements of cancer. At the same time, these tools have disclosed greater complexity in the character of these cells and stimulated further investigation. Although there are antibodies to specific epitopes of neoplastic cells, this purity has not provided the improved detection and therapy of cancer first expected. Technical manipulations have provided limited improvement in results, but more sophisticated techniques, such as biologic response modifiers, may be required to attain clinical results that can be universally applied. The intense research in monoclonal antibodies and their application does offer promise that the goal of improved cancer detection and therapy will be forthcoming.

The localisation of radiolabelled murine monoclonal antibody 81C6 and its Fab fragment in human glioma xenografts in athymic mice

The localisation of the radioiodinated Fab fragment of monoclonal antibody (Mab) 81C6, reactive with a glioma-associated extracellular matrix antigen, was studied in athymic mice bearing subcutaneous and intracranial xenografts of D-54 MG glioma cells. In vitro 81C6 Fab showed a marked loss of immunoreactivity and affinity for antigen compared to intact Mab 81C6. In vivo, the plasma half-life of 81C6 Fab was 7.0 hours compared to 2.1 days for 81C6. 81C6 Fab levels in tumours peaked at 2.6-3.8% injected dose/g in 2-6 h; Mab 81C6 reached 33.9% dose/g at 48 h. Localisation indices and tumour:tissue ratios were superior for Mab 81C6. Estimated radiation doses to tumour and normal tissues were lower for 131I-81C6 Fab than 131I-81C6. To realise the theoretical benefits of fragments as localising agents, Fab fragments of higher immunoreactivity and affinity, or bivalent F(ab')2 fragments are required.

Selective localisation of two radiolabelled anti-sarcoma monoclonal antibodies in human osteosarcoma xenografts

Two mouse monoclonal antibodies (MoAbs), TP-1 and TP-3, previously shown in immunohistochemical studies to react with osteosarcomas, were labelled with 125I or 131I and evaluated for their ability to localise to human osteogenic sarcoma xenografts after intravenous injection. The radiolabelled TP-1 and TP-3 MoAbs had immunoreactive fractions of 70% and 67%, respectively, and bound to target cells with binding constants of 8.5 X 10(8) M-1 and 4.0 X 10(9) M-1, respectively. After injection of labelled TP-3 IgG, approximately 16% of the dose X g-1 tissue was found in the tumour after 24 hours. Maximum tumour/blood radioactivity ratios of 6-7 were achieved 3-4 days after antibody injection, while the ratios for the normal tissues were less than 1. The tumours could be clearly visualised by whole-body gamma scintigraphy without the need for subtraction techniques. The TP-1 IgG accumulated to a large extent also in the spleen. Hence, with this antibody the tumour was less well delineated from the adjacent normal tissues. However, the F(ab')2 fragments, derived from the TP-1 IgG, gave tumour/blood ratios up to approximately 40 after 3-4 days and yielded sharp gamma scintigrams of the tumour. Specificity of the antibody localisation was indicated by the lack of accumulation in a contralateral melanoma xenograft and the failure of 2 isotype-matched irrelevant MoAbs to localise to the sarcomas. With the F(ab')2 fragments satisfactory images could be obtained already after 16 hours. The results suggest that this preparation may be useful in clinical radioimmunodetection of osteogenic sarcomas.

Theoretical aspects of screening for high affinity monoclonal antibody to cell surface antigens

The relative affinity of the cell-antibody interaction can be determined by finding the highest titer of one reactant which binds effectively to a fixed, dilute concentration of the other. There are two ways to perform these experiments: the antibody dilution method and the cell dilution method. The antibody dilution approach requires measuring the highest antibody dilution which can react with a small, fixed concentration of cells. If the endpoint antibody concentration is small, the antibody has a high binding constant. In the cell dilution method the investigator must find the highest cell dilution which can bind a small, fixed antibody concentration. If the antibody has a high affinity constant, the endpoint cell concentration will be small. When the antibody and cell receptor concentrations are known, functional binding constants can be calculated directly from binding data using equations presented in this paper. Binding constants measured by these methods will help predict the behavior of monoclonal antibodies in immunoassays and in experiments where monoclonal antibody is injected in vivo for imaging or pharmacologic delivery.

Tumor detection using radiolabeled monoclonal antibodies

Radioisotope conjugated to monoclonal antibody products has been used for imaging tumors targeted by the antibody. As imaging progresses, new sets of procedural and technical questions arise. In this chapter, we discuss several current problems in imaging tumor with radiolabeled monoclonal antibody. These include (1) methods for selection of specific antibody and, once the particular antibody is selected, which fragment form is to be used; (2) imaging procedures: what are the optimum imaging parameters, such as optimum time for imaging after administration of tracer and considerations regarding background subtraction; and (3) noninvasive quantitative techniques: quantitation of localization of antibody indirectly from quantitative information in the images.

Criteria for selecting monoclonal antibodies with respect to accumulation in melanoma tissue

Immunohistology provides a necessary but insufficient criterion for selecting monoclonal antibodies (MAbs) capable of tumour targeting in vivo. Additional selection procedures have been evaluated using a panel of anti-melanoma MAbs, including immunoreactivity of (labelled) MAbs, antibody affinity, kinetics of binding and release, apparent antigen density and accumulation in nude mouse transplants. According to these criteria, MAbs M.2.7.6 and M.2.9.4 showed the most favourable properties, i.e. high immunoreactivity and pronounced internalization into melanoma cells. With MAbs M.2.10.15 and KG 6-56, moderate immunoreactivity and a binding pattern characterized by temperature dependence in the absence of internalization was observed. According to the paired label assay, all four MAbs showed specific accumulation into solid melanoma tissue. However, application in the patient still requires evaluation of the side effects of antigen cross-expression on normal human tissues.

Evaluation of protocols for purification of mouse monoclonal antibodies. Yield and purity in two-dimensional gel electrophoresis

Protocols for purification of mouse monoclonal antibodies (MAbs) from nude mice ascites were investigated in order to assess the yield and to compare the purified products in two-dimensional gel electrophoresis (2DGE). Three MAbs (one IgG2 and two IgG1), selected for their differing behaviours towards protein A, were purified by ammonium sulphate precipitation and/or gel filtration, anion exchange (DEAE), hydroxylapatite and affinity (protein A) chromatography, or by a combination of these methods. Protein A constantly provided the highest purity whatever the IgG subclass. The best results in terms of yields and purity were a function of the optimization of the protein A protocol. In our study, they were obtained in a 3 h protocol (IgG2), a 16 h protocol with discontinuous pH gradient method (IgG1 with sufficiently high affinity for protein A) or a multi-step protocol involving DEAE and protein A (IgG1 with low affinity for protein A). DEAE chromatography alone provided a slightly better yield, but only moderate purity. Hydroxylapatite chromatography appeared to be less potent in terms of yield, purity and day-to-day reproducibility. Salt precipitation and gel filtration enabled only relative enrichment of the MAb solution. Some degradation products of both heavy and light chains clearly appeared in the 2DGE patterns of antibodies purified by different protocols, and seem to be partly related to the elution pH and to the duration of the purification procedure. Finally, this work highlights considerable heterogeneity not only between two different MAbs of the IgG1 subclass but also within a monoclonal population of immunoglobulins.

Study of the plasma clearance of antibody--ricin-A-chain immunotoxins. Evidence for specific recognition sites on the A chain that mediate rapid clearance of the immunotoxin

In recent years, antibody--ricin-A-chain immunotoxins have been investigated as anti-neoplastic agents. To achieve in vivo therapy it is necessary that the immunotoxin remains in circulation at a sufficiently high level for a sufficiently long time to allow binding to tumor cells to occur. Therefore, examination of the pharmacology of immunotoxins may elucidate the reasons for the poor in vivo tumoricidal effect of immunotoxin described before. In this study the plasma clearance of antibody--ricin-A-chain immunotoxins, after intravenous injection in animals of different species, has been examined. Sensitive and reproducible techniques were developed to monitor the level of immunotoxin and its constituents in the blood. It is shown that immunotoxins are rapidly eliminated from the bloodstream. Neither the properties of the antibody moiety nor the nature of the linkage binding ricin A-chain to antibody account for the disappearance of immunotoxin from the plasma. On the other hand, we found that the rapid clearance of immunotoxin is due to the mannose residues on the ricin A-chain moiety which are specifically recognized by liver cells. When immunotoxin is administrated together with yeast mannan, which enhances the level of active immunotoxin in circulation by inhibition of liver uptake, the anti-cancer efficacy of immunotoxin in vivo is drastically improved.

Use of the direct linear plot to estimate, in an immunoglobulin solution, the proportion which is specific monoclonal antibody against an unknown cell surface antigen

A Cornish-Bowden direct linear plot was used to assess the level of specific monoclonal antibody in a protein A preparation from mouse ascitic fluid. With this method, experimental observations are plotted directly as lines in parameter space and estimates of kinetic parameters are read directly from the plot without need for further calculation. This method is particularly well suited for the analysis of systems, such as the one outlined here, where conventional kinetic analysis is not possible because the preparation contains both specific and non-specific antibody. Results obtained with the direct linear plot showed that estimates of the level of specific immunoglobulin present in a given protein A preparation are consistent, not only within individual experiments, but also throughout a series of experiments using more than one labelled antibody preparation.

Production and characterization of monoclonal antibodies against human thyroglobulin

Spleen cells of Biozzi-HR mice immunized with human thyroglobulin (hTg) were fused with P3-X63-Ag8.653 mouse myeloma cells. Twenty monoclonal antibodies (MAbs) selected by an enzyme immunoassay (indirect ELISA) were produced, purified and characterized. The equilibrium association constant (Ka) of one of the MAbs, determined by Scatchard analysis of the ELISA data, was found to be 2 X 10(9) M-1; the Ka of the other MAb, estimated from titration curves by comparison with the aforementioned MAb, ranged from 8 X 10(9) M-1 to 6 X 10(7) M-1. The reaction between the MAb and hTg was not inhibited by thyroxin (T4), triiodothyronine (T3) and triiodothyropropionic acid (DT3). Species specificity of the MAb was studied using bovine and porcine Tgb. The topology of the MAb was investigated by competitive inhibition immunoassays. Seven distinct antigenic regions were identified.