Impact of Immunoglobulin Isotype and Epitope on the Functional Properties of Vibrio cholerae O-Specific Polysaccharide-Specific Monoclonal Antibodies

Vibrio cholerae causes the severe diarrheal disease cholera. Clinical disease and current oral cholera vaccines generate antibody responses associated with protection. Immunity is thought to be largely mediated by lipopolysaccharide (LPS)-specific antibodies, primarily targeting the O-antigen. However, the properties and protective mechanism of functionally relevant antibodies have not been well defined. We previously reported on the early B cell response to cholera in a cohort of Bangladeshi patients, from which we characterized a panel of human monoclonal antibodies (MAbs) isolated from acutely induced plasmablasts. All antibodies in that previous study were expressed in an IgG1 backbone irrespective of their original isotype. To clearly determine the impact of affinity, immunoglobulin isotype and subclass on the functional properties of these MAbs, we re-engineered a subset of low- and high-affinity antibodies in different isotype and subclass immunoglobulin backbones and characterized the impact of these changes on binding, vibriocidal, agglutination, and motility inhibition activity. While the high-affinity antibodies bound similarly to O-antigen, irrespective of isotype, the low-affinity antibodies displayed significant avidity differences. Interestingly, despite exhibiting lower binding properties, variants derived from the low-affinity MAbs had comparable agglutination and motility inhibition properties to the potently binding antibodies, suggesting that how the MAb binds to the O-antigen may be critical to function. In addition, not only pentameric IgM and dimeric IgA, but also monomeric IgA, was remarkably more potent than their IgG counterparts at inhibiting motility. Finally, analyzing highly purified F(ab) versions of these antibodies, we show that LPS cross-linking is essential for motility inhibition.IMPORTANCE Immunity to the severe diarrheal disease cholera is largely mediated by lipopolysaccharide (LPS)-specific antibodies. However, the properties and protective mechanisms of functionally relevant antibodies have not been well defined. Here, we have engineered low and high-affinity LPS-specific antibodies in different immunoglobulin backbones in order to assess the impact of affinity, immunoglobulin isotype, and subclass on binding, vibriocidal, agglutination, and motility inhibition functional properties. Importantly, we found that affinity did not directly dictate functional potency since variants derived from the low-affinity MAbs had comparable agglutination and motility inhibition properties to the potently binding antibodies. This suggests that how the antibody binds sterically may be critical to function. In addition, not only pentameric IgM and dimeric IgA, but also monomeric IgA, was remarkably more potent than their IgG counterparts at inhibiting motility. Finally, analyzing highly purified F(ab) versions of these antibodies, we show that LPS cross-linking is essential for motility inhibition.

Implementation of an antibody characterization procedure and application to the major ALS/FTD disease gene C9ORF72

Antibodies are a key resource in biomedical research yet there are no community-accepted standards to rigorously characterize their quality. Here we develop a procedure to validate pre-existing antibodies. Human cell lines with high expression of a target, determined through a proteomics database, are modified with CRISPR/Cas9 to knockout (KO) the corresponding gene. Commercial antibodies against the target are purchased and tested by immunoblot comparing parental and KO. Validated antibodies are used to definitively identify the most highly expressing cell lines, new KOs are generated if needed, and the lines are screened by immunoprecipitation and immunofluorescence. Selected antibodies are used for more intensive procedures such as immunohistochemistry. The pipeline is easy to implement and scalable. Application to the major ALS disease gene C9ORF72 identified high-quality antibodies revealing C9ORF72 localization to phagosomes/lysosomes. Antibodies that do not recognize C9ORF72 have been used in highly cited papers, raising concern over previously reported C9ORF72 properties.

Impact of PCSK9 monoclonal antibodies on circulating hs-CRP levels: a systematic review and meta-analysis of randomised controlled trials

OBJECTIVE

To evaluate the potential effects of proprotein convertase subtilisin/kexin type 9 monoclonal antibody (PCSK9-mAb) on high-sensitivity C reactive protein (hs-CRP) concentrations.

DESIGN

A systematic review and meta-analysis of randomised controlled trials.

DATA SOURCES

PubMed, MEDLINE, the Cochrane Library databases, ClinicalTrials.gov and recent conferences were searched from inception to May 2018.

ELIGIBILITY CRITERIA FOR SELECTING STUDIES

All randomised controlled trials that reported changes of hs-CRP were included.

RESULTS

Ten studies involving 4198 participants were identified. PCSK9-mAbs showed a slight efficacy in reducing hs-CRP (-0.04 mg/L, 95% CI: -0.17 to 0.01) which was not statistically different. The results did not altered when subgroup analyses were performed including PCSK9-mAb types (alirocumab: 0.12 mg/L, 95% CI: -0.18 to 0.43; evolocumab: 0.00 mg/L, 95% CI: -0.07 to 0.07; LY3015014: -0.48 mg/L, 95% CI: -1.28 to 0.32; RG7652: 0.35 mg/L, 95% CI: -0.26 to 0.96), treatment duration (≤12w: 0.00 mg/L, 95% CI: -0.07 to 0.07; >12w: -0.11 mg/L, 95% CI: -0.45 to -0.23), participant characteristics (familial hypercholesterolaemia: 0.00 mg/L, 95% CI: -0.07 to 0.07; non-familial hypercholesterolaemia: 0.07 mg/L, 95% CI: -0.12 to 0.26; mix: -0.48 mg/L, 95% CI: -1.28 to 0.32) and treatment methods (monotherapy: 0.00 mg/L, -0.08 to 0.07; combination therapy: -0.08 mg/L, -0.37 to 0.21). Meta-regression analyses suggested no significant linear correlation between baseline age (p=0.673), sex (p=0.645) and low-density lipoprotein cholesterol reduction (p=0.339).

CONCLUSIONS

Our updated meta-analysis suggested that PCSK9-mAbs had no significant impact on circulating hs-CRP levels irrespective of PCSK9-mAb types, participant characteristics and treatment duration or methods.

Immunoglobulin G subclass switching impacts sensitivity of an immunoassay targeting Francisella tularensis lipopolysaccharide

The CDC Tier 1 select agent Francisella tularensis is a small, Gram-negative bacterium and the causative agent of tularemia, a potentially life-threatening infection endemic in the United States, Europe and Asia. Currently, there is no licensed vaccine or rapid point-of-care diagnostic test for tularemia. The purpose of this research was to develop monoclonal antibodies (mAbs) specific to the F. tularensis surface-expressed lipopolysaccharide (LPS) for a potential use in a rapid diagnostic test. Our initial antigen capture ELISA was developed using murine IgG3 mAb 1A4. Due to the low sensitivity of the initial assay, IgG subclass switching, which is known to have an effect on the functional affinity of a mAb, was exploited for the purpose of enhancing assay sensitivity. The ELISA developed using the IgG1 or IgG2b mAbs from the subclass-switch family of 1A4 IgG3 yielded improved assay sensitivity. However, surface plasmon resonance (SPR) demonstrated that the functional affinity was decreased as a result of subclass switching. Further investigation using direct ELISA revealed the potential self-association of 1A4 IgG3, which could explain the higher functional affinity and higher assay background seen with this mAb. Additionally, the higher assay background was found to negatively affect assay sensitivity. Thus, enhancement of the assay sensitivity by subclass switching is likely due to the decrease in assay background, simply by avoiding the self-association of IgG3.

Targeted Immunomodulatory Therapy: An Overview

Monoclonal antibodies and other biologic response modifiers have allowed for targeted drug therapy in managing various autoimmune diseases. A number of immune pathways have been exploited in the development of targeted immunomodulatory therapies, including cytokine-directed therapies such as tumor necrosis factor-alpha and interleukins, integrins, B-cells, and co-stimulation modulators. With new targeted therapies in the pipeline, more options are becoming available for treatment of autoimmune diseases. [Full article available at http://rimed.org/rimedicaljournal-2016-12.asp].

B-cell epitopes in GroEL of Francisella tularensis

The chaperonin protein GroEL, also known as heat shock protein 60 (Hsp60), is a prominent antigen in the human and mouse antibody response to the facultative intracellular bacterium Francisella tularensis (Ft), the causative agent of tularemia. In addition to its presumed cytoplasmic location, FtGroEL has been reported to be a potential component of the bacterial surface and to be released from the bacteria. In the current study, 13 IgG2a and one IgG3 mouse monoclonal antibodies (mAbs) specific for FtGroEL were classified into eleven unique groups based on shared VH-VL germline genes, and seven crossblocking profiles revealing at least three non-overlapping epitope areas in competition ELISA. In a mouse model of respiratory tularemia with the highly pathogenic Ft type A strain SchuS4, the Ab64 and N200 IgG2a mAbs, which block each other’s binding to and are sensitive to the same two point mutations in FtGroEL, reduced bacterial burden indicating that they target protective GroEL B-cell epitopes. The Ab64 and N200 epitopes, as well as those of three other mAbs with different crossblocking profiles, Ab53, N3, and N30, were mapped by hydrogen/deuterium exchange–mass spectrometry (DXMS) and visualized on a homology model of FtGroEL. This model was further supported by its experimentally-validated computational docking to the X-ray crystal structures of Ab64 and Ab53 Fabs. The structural analysis and DXMS profiles of the Ab64 and N200 mAbs suggest that their protective effects may be due to induction or stabilization of a conformational change in FtGroEL.

[Preparation and partial characterization of monoclonal antibodies against HA protein of H1 subtype influenza virus]

AIM

Preparation of H1 subtype of influenza virus HA protein monoclonal antibody (mAbs), and to analyse their reactivity.

METHODS

H1N1 influenza virus vaccine (2009) and seasonal A1 influenza virus vaccine as antigen, conventional immunization, fusion, cloning, access to the antigen-specific mAbs. To study the reactivity and specificity of mAbs by ELISA, HI test and Western blot.

RESULTS

We have obtained 97 hybridoma of stable secreting anti-H1 subtype of influenza virus HA protein. According to their different reactivity, these mAbs can be divided into four categories: 39 strains for strain-specific, of which 29 have HI activity; 7 strains for subtype-specific, of which 5 have HI activity; 16 strains for the 2009 pandemic strain and the seasonal strains to common antigens, of which 9 have HI activity; 35 strains common for influenza virus antigens, of which 22 have HI activity.

CONCLUSION

Both vaccines have better immunogenicity and immune protection activity. Preparation of four types antibody against HA protein of H1 subtype of influenza virus, can provide experimental data for preparation subtype-specific diagnostic kits and influenza A and B virus diagnostic kit, and lay a foundation for further studying of H1N1 influenza virus HA epitope.

Therapeutic proteins

Protein-based therapeutics are highly successful in clinic and currently enjoy unprecedented recognition of their potential. More than 100 genuine and similar number of modified therapeutic proteins are approved for clinical use in the European Union and the USA with 2010 sales of US$108 bln; monoclonal antibodies (mAbs) accounted for almost half (48%) of the sales. Based on their pharmacological activity, they can be divided into five groups: (a) replacing a protein that is deficient or abnormal; (b) augmenting an existing pathway; (c) providing a novel function or activity; (d) interfering with a molecule or organism; and (e) delivering other compounds or proteins, such as a radionuclide, cytotoxic drug, or effector proteins. Therapeutic proteins can also be grouped based on their molecular types that include antibody-based drugs, Fc fusion proteins, anticoagulants, blood factors, bone morphogenetic proteins, engineered protein scaffolds, enzymes, growth factors, hormones, interferons, interleukins, and thrombolytics. They can also be classified based on their molecular mechanism of activity as (a) binding non-covalently to target, e.g., mAbs; (b) affecting covalent bonds, e.g., enzymes; and (c) exerting activity without specific interactions, e.g., serum albumin. Most protein therapeutics currently on the market are recombinant and hundreds of them are in clinical trials for therapy of cancers, immune disorders, infections, and other diseases. New engineered proteins, including bispecific mAbs and multispecific fusion proteins, mAbs conjugated with small molecule drugs, and proteins with optimized pharmacokinetics, are currently under development. However, in the last several decades, there are no conceptually new methodological developments comparable, e.g., to genetic engineering leading to the development of recombinant therapeutic proteins. It appears that a paradigm change in methodologies and understanding of mechanisms is needed to overcome major challenges, including resistance to therapy, access to targets, complexity of biological systems, and individual variations.

Novel antibodies against follicular non-Hodgkin's lymphoma

The anti-CD20 monoclonal antibody rituximab has revolutionized the treatment of patients with follicular B-cell lymphoma. With the combination of chemotherapy and rituximab the overall survival rate has increased with approximately 30%. Unfortunately, there is resistance to rituximab with relapse of the disease in about 60% of the patients during the first five years of treatment and eventually in all patients. To this end, there is a need to develop improved anti-CD20 monoclonal antibodies and antibodies that target other attractive molecules expressed on the follicular lymphoma cell. This review describes the development and clinical achievements so far of next generation anti-CD20 and other antibodies in the treatment of follicular B-cell lymphoma.

[Influence of different products of platelet membrane glycoprotein monoclonal antibodies used internationally on tests for monoclonal antibody-specific immobilization of platelet antigens]

This study was aimed to investigate the influence of different platelet membrane glycoprotein monoclonal antibodies (McAb) which are common used in laboratories on the monoclonal antibody-specific immobilization of platelet antigens (MAIPA) technique according to the request of 14th International Society of Blood Transfusion Platelet Immunology Workshop. 30 participant laboratories were provided with 10 known human platelet antigen (HPA) antibodies, 1 normal serum, 9 different McAbs (against GPIIb/IIIa, GPIa/IIa, GPIb/IX and GPIV respectively), and the same protocol. Each participant laboratory carried out the test as the protocol to compare the results of different McAbs against the same glycoprotein and submitted the data to organizer. The results indicated that in McAbs against GPIIb/IIIa, AP2, Gi-5 and PL2-73 showed higher mean S/CO than that of others; in GPIa/IIa, MBC202.2 and 143.1 showed higher mean S/CO than that of others; in GPIb/IX, 142.11 and CLB-MB45 (CD42b) showed higher mean S/CO than that of others; as to GPIV, 131.4 showed higher mean S/CO. In conclusion, capture effects of various McAbs are different, so that different products of McAbs exert influences on the sensitivity of MAIPA. To use a panel of McAbs against the same glycoprotein may avoid the false negative results.

Discrimination among IgG1-kappa monoclonal antibodies produced by two cell lines using charge state distributions in nanoESI-TOF mass spectra

Charge state distributions (CSDs) of proteins in nanoESI mass spectra are affected by the instrumental settings and experimental conditions, in addition to the conformations of the proteins in the analyzed solutions. In the presented study, instrumental and experimental parameters--the desolvation gas flow rate, temperature, pH, buffer (ammonium acetate), and organic modifier (methanol) concentrations--were optimized according to a reduced central composite face experimental design to maximize the separation of CSDs of monoclonal IgG1-kappa antibodies produced by two production systems (CHO and GS-NS0 cell lines). Principal component analysis and Fisher linear discriminant analysis were then used to reduce the dimensions of the acquired dataset and quantify the separation of the protein classes, respectively. The results show that the IgG1-kappa molecules produced by the two production systems can be clearly distinguished using the described approach, which could be readily applied to other proteins and production systems.

[International classification of various types of monoclonal antibodies]

Significant advances in the development of monoclonal antibodies ("mabs") have been acknowledged during the last two decades. Successive developments led to the marketing of murine antibodies ("o-mab" first, followed by chimeric antibodies ("xi-mab"), humanised antibodies ("zu-mab") and, finally, human monoclonal antibodies ("u-mab"). In order to facilitate the distinction between the various monoclonal antibodies used in clinical practice, an international nomenclature has been proposed with the use of a specific suffix corresponding to the origine/source of "mabs" preceded by an infix referring to the medicine's target. The efforts in developing new types of monoclonal antibodies aimed at improving their pharmacokinetics (longer half-life), pharmacodynamics (better efficacy because of stronger affinity to human receptor), and safety profile (less antigenic and immunogenic reactions). These progresses could be obtained thanks to the remarkable development of molecular biotechnology.

Understanding monoclonal antibodies

Elsewhere in this issue, we review new treatments for neovascular age-related macular degeneration, of which, bevacizumab and ranibizumab are monoclonal antibodies. These are two of a growing number of monoclonal antibodies currently available for therapeutic use in the UK. We have reviewed some of the others before: bevacizumab and cetuximab for colorectal cancer; infliximab for rheumatoid arthritis; omalizumab for severe asthma; and trastuzumab for breast cancer. The expanding range of these products and their clinical applications can make it difficult to keep abreast of developments in this field. With this in mind, we describe the key principles underlying the production, use and naming of monoclonal antibodies.

Drug insight: using monoclonal antibodies to treat multiple sclerosis

Multiple sclerosis (MS) is an immunopathological, presumably autoimmune, disease of the CNS. Several immunomodulatory treatments, including various preparations of interferon-beta, glatiramer acetate and mitoxantrone, have been approved for MS therapy. Because these agents are only partially effective, the search for better therapies continues. Therapeutic monoclonal antibodies (mAbs), a class of biotechnological agents, allow the precise targeting of molecules involved in pathological processes. Therapeutic mAbs have shown much promise in the treatment of many disorders, including inflammatory and putative autoimmune diseases such as MS. These agents have intrinsic limitations, however, such as induction of neutralizing 'anti-antibodies', systemic inflammatory reactions and severe adverse effects, some of which remain to be explained. Most notably, natalizumab (Tysabri), a mAb against alpha4 integrin, was very effective in suppressing MS activity, but had to be withdrawn from the market because several treated patients developed progressive multifocal leukoencephalopathy. This article reviews the state of development of various therapeutic mAbs for MS treatment.

Generation and characterization of hybridoma antibodies for immunotherapy of tularemia

Tularemia is caused by the Gram-negative facultative intracellular bacterium Francisella tularensis, which has been classified as a category A select agent-a likely bioweapon. The high virulence of F. tularensis and the threat of engineered antibiotic resistant variants warrant the development of new therapies to combat this disease. We have characterized 14 anti-Francisella hybridoma antibodies derived from mice infected with F. tularensis live vaccine strain (LVS) for potential use as immunotherapy of tularemia. All 14 antibodies cross-reacted with virulent F. tularensis type A clinical isolates, 8 bound to a purified preparation of LVS LPS, and 6 bound to five protein antigens, identified by proteome microarray analysis. An IgG2a antibody, reactive with the LPS preparation, conferred full protection when administered either systemically or intranasally to BALB/c mice post challenge with a lethal dose of intranasal LVS; three other antibodies prolonged survival. These anti-Francisella hybridoma antibodies could be converted to chimeric versions with mouse V regions and human C regions to serve as components of a recombinant polyclonal antibody for clinical testing as immunotherapy of tularemia. The current study is the first to employ proteome microarrays to identify the target antigens of anti-Francisella monoclonal antibodies and the first to demonstrate the systemic and intranasal efficacy of monoclonal antibodies for post-exposure treatment of respiratory tularemia.

Characterization and Application of Monoclonal Antibodies against Turbot (Scophthalmus maximus) Rhabdovirus

Five monoclonal antibodies (mAbs), 1G8, 1H9, 2D2, 2D3, and 2F5, against Scophthalmus maximus rhabdovirus (SMRV) were prepared. Characterization of the mAbs included indirect enzyme-linked immunosorbent assay, isotyping, viral inhibition assay, immunofluorescence staining of virus-infected cell cultures, and Western blot analysis. Isotyping revealed that 1G8 and 1H9 were of the IgG2b subclass and that the other three were IgM. 2D2, 2D3, and 2F5 partially inhibited SMRV infection in epithelioma papulosum cyprinid (EPC) cell culture. Western blotting showed that all five mAbs could react with two SMRV proteins with molecular masses of approximately 30 kDa (P) and 26 kDa (M). These two proteins were localized within the cytoplasm of SMRV-infected EPC cells by immunofluorescence assay. Also, progressive foci of viral replication in cell cultures were monitored from 6 to 24 h, using mAb 2D3 as the primary antibody. A flow cytometry procedure was used to detect and quantify SMRV-infected (0.01 PFU/cell) EPC cells with mAb 2D3, and 10.8% of cells could be distinguished as infected 36 h postinfection. Moreover, mAb 2D3 was successfully applied for the detection of viral antigen in cryosections from flounder tissues by immunohistochemistry tests.

Serological and phylogenetic evidence of monotypic immune responses to different mumps virus strains

The recent resurgence of mumps epidemics in many countries with ongoing vaccination programs along with evidence of antigenic diversity among mumps virus strains have recently challenged the assumption that mumps virus is serologically monotypic. To address this controversy, we sought to identify two mumps virus strains that would best represent different serotypes, should multiple serotypes exist, and assess the ability of human sera to neutralize both strains. The virus strains, Enders and Lo1, were selected based upon a phylogenetic analysis of the major target of neutralizing antibody, the viral hemagglutinin-neuraminidase (HN) protein, along with data reported by others indicating that (1) these viruses are antigenically distinct and (2) genotypically similar strains have been implicated in cases of reinfection. Our results show that of sera capable of neutralizing one of the virus strains, 90% could neutralize the other, although significant differences in neutralization titers were noted. Though the latter confirms the existence of inter-strain antigenic variability, the fact that few sera were unable to neutralize both virus strains argues against the presence of multiple serotypes. Of those sera incapable of co-neutralization, all but one had low neutralization titers (1:8), suggesting that individuals possessing low levels of neutralizing antibody may be at risk for breakthrough infections, thereby providing an explanation for cases of infection in previously infected or vaccinated individuals.

Correlation of molecular characteristics, isotype, and in vitro functional activity of human antipneumococcal monoclonal antibodies

Structure-function correlations of pneumococcal antibodies are important in predicting how changes in the pneumococcus (Pnc)-specific B-cell repertoire will influence humoral immunity against invasive Pnc disease. Using a unique panel of human hybridomas derived from memory B cells after pneumococcal conjugate vaccination, we analyzed the structure-function relationship of nine monoclonal antibodies (MAbs) reactive to Pnc polysaccharides. The avidities of the antibodies correlated with the avidity of donor immune serum (R, 0.7; P < 0.025), and this relationship was particularly strong for immunoglobulin A clones (R, 1; P < 0.0005), suggesting that the MAbs may represent important clones contributing to serological memory. Common heavy-light chain combinations and amino acid replacement mutations were seen for clones with the same serospecificity from different individuals. The two highest-avidity MAbs used Vh3-48, and two MAbs with the same serospecificity, using the same V gene pairings (Vh3-7 and Vk2A17), had similar avidities, suggesting that canonical V gene use makes an important contribution to avidity. Although all clones had mutation levels consistent with their being derived from memory B cells, low levels of replacement mutation were associated with high avidities. This relationship was strongest for Vh genes (R, 0.8; P < 0.01). Opsonophagocytosis was demonstrated for all clones, and there was a trend toward clones using canonical genes with low levels of mutation having high opsonophagocytic activities (R, 0.5). These data suggest that the use of canonical genes in the Pnc antibody response is associated with highly functional antibodies and that most somatic mutations seen in these genes are not antigen selected.

Classifying Antibodies Using Flow Cytometry Data: Class Prediction and Class Discovery

Classifying monoclonal antibodies, based on the similarity of their binding to the proteins (antigens) on the surface of blood cells, is essential for progress in immunology, hematology and clinical medicine. The collaborative efforts of researchers from many countries have led to the classification of thousands of antibodies into 247 clusters of differentiation (CD). Classification is based on flow cytometry and biochemical data. In preliminary classifications of antibodies based on flow cytometry data, the object requiring classification (an antibody) is described by a set of random samples from unknown densities of fluorescence intensity. An individual sample is collected in the experiment, where a population of cells of a certain type is stained by the identical fluorescently marked replicates of the antibody of interest. Samples are collected for multiple cell types. The classification problems of interest include identifying new CDs (class discovery or unsupervised learning) and assigning new antibodies to the known CD clusters (class prediction or supervised learning). These problems have attracted limited attention from statisticians. We recommend a novel approach to the classification process in which a computer algorithm suggests to the analyst the subset of the "most appropriate" classifications of an antibody in class prediction problems or the "most similar" pairs/ groups of antibodies in class discovery problems. The suggested algorithm speeds up the analysis of a flow cytometry data by a factor 10-20. This allows the analyst to focus on the interpretation of the automatically suggested preliminary classification solutions and on planning the subsequent biochemical experiments.

Therapeutic applications of superantibodies

Superantibody technology represents a method to enhance the potency and utility of monoclonal antibodies. The blueprint for superantibody technology is taken from rare naturally occurring superantibodies with unique sequence regions, conferring specific biological functions not detected on most antibodies. In superantibody technology, peptides with specific amino acid sequences are crosslinked to antibodies using affinity-site-specific chemistry. Three types of superantibodies have been engineered: dimerizing superantibodies with enhanced effector potency, superantibodies with the ability to penetrate living cells and superantibodies as vaccines with built-in molecular adjuvant. Collectively, superantibody technology generates a new class of antibodies with higher levels of therapeutic potency.

Development of panel of monoclonal antibodies specific to urochordate cell surface antigens

Monoclonal antibodies are an important tool in the study of botryllid ascidians' immunology and developmental biology. Here we describe the development of a panel of 38 monoclonal antibodies that are specific to Botryllus schlosseri (Ascidiacea; subfamily Botryllinae) cell surface antigens. Many of these hybridomas recognize (by enzyme-linked immunosorbent assay and immunohistochemistry) epitopes of Botrylloides subpopulations (SP) II and III from the Mediterranean coast of Israel and show, on blood cell smear assays, reactions with subsets of Botryllus circulating blood cells. Fluorescence-activated cell sorting analyses using antibodies positive for botryllid tissues revealed up to 3.6% positive cells. ELISA screenings were performed with 64 new monoclonal antibodies on 5 different individual botryllid ascidian colonies (B. schlosseri, Botrylloides). The positive antibodies in this panel identified a large number of different antigenic determinants, some of which distinguish Botryllus versus Botrylloides colonies, and other, different colonies within these two species, or different cell types within tissues, embryos, and buds of individual colonies. Only 21 monoclonal antibodies tested positive with all colonies. Cross-reactivity with at least one Botrylloides colony was recorded in 49 hybridomas that identified Botryllus cells. This wide panel of monoclonal antibodies is the first such detailed set of monoclonals available for studies on botryllid ascidians.

Different ataxin-2 antibodies display different immunoreactive profiles

We have developed a monoclonal antibody (4A7) directed against the C-terminus of the ataxin-2 protein that is involved in the polyglutamine neurodegenerative disorder spinocerebellar ataxia type 2. Comparison with other ataxin-2 antibodies showed that 4A7 specifically recognized ataxin-2. In contrast, a previously reported ataxin-2 antibody (15F6) did not appear to recognize full-length ataxin-2 in our systems. Immunocytochemical and subcellular fractionation studies using 4A7 confirmed previous reports that ataxin-2 is localized to both the cytoplasm and the trans-Golgi network in rat PC12 cells and rat brain tissue. In contrast, 4A7 failed to label the trans-Golgi network in the three primate cell lines examined. Cytoplasmic ataxin-2 was not associated with mitochondria, lysosomes, endoplasmic reticulum, peroxisomes, proteasomes, clathrin-coated pits or vesicles, or F-actin. Ataxin-2 was found to be phosphorylated but not glycosylated, and exhibited an estimated half-life of not less than 21 h. Interestingly, another commercially available ataxin-2 antibody did not detect ataxin-2 localized to the trans-Golgi network. This antibody was also found to immunoprecipitate fewer proteins/protein partners than 4A7. Although cross-reactivity of the 4A7 antibody with other protein(s) cannot be ruled out, it appears likely that the interaction of ataxin-2 with other cell components is dependent on both the host cell type and its subsequent subcellular localization.

Rapid grouping of monoclonal antibodies based on their topographical epitopes by a label-free competitive immunoassay

Topography of epitopes of monoclonal antibodies (MAbs), identified as the mutual competition of the MAbs, can be valuable indicators for the biological functions of MAbs. However, the determination of topographical epitopes is not performed before the functional screening of MAbs, because the requirement for purifying and labeling of MAbs makes the mapping experiment difficult, particularly in the early stage of MAb production. Here we describe a new label-free competitive enzyme-linked immunosorbent assay (LFC-ELISA) for the rapid grouping of MAbs based on the topography of their epitopes. In the LFC-ELISA, the immune complex formed by a competitor, MAb#2, and an antigen is challenged by an indicator, MAb#1 that had been captured on the ELISA plate through a secondary antibody. The MAb#2-antigen immune complex is trapped by MAb#1 only if MAb#1 reacts with an epitope different from that of MAb#2. The immune complex (MAb#2-antigen-MAb#1) is detected with an enzyme-labeled reagent specific to a tag on the antigen. Our experiments using different anti-CD30 MAbs and a CD30-Fc fusion protein as the antigen revealed that the LFC-ELISA performed well with MAbs of different isotypes (IgG1, IgG2a, and IgG2b), and in a practical range of MAb concentrations (0.3-10 microg/ml) and affinities (0.9-13 nM of Kd). We obtained pairwise competition data from all 26 anti-CD30 MAbs. We then utilized a cluster analysis and a bootstrap method to analyze the competition data for grouping of the MAbs. This objective and automated analysis identified eight distinct topographical epitopes on CD30. The reactivity of the anti-CD30 MAbs in immunoblot, and their inhibiting activity on CD30-CD30-ligand binding correlated with the topographical epitopes. The results show that the LFC-ELISA combined with cluster analysis is a useful new method for grouping MAbs based on their topographical epitopes and can be used in the early stage of MAb production. One useful application is to identify MAbs reacting with different epitopes from a large number of MAbs so that the most appropriate MAbs can be selected for therapeutic use.

Unexpected diversity in the fine specificity of monoclonal antibodies that use the same V region gene to glucuronoxylomannan of Cryptococcus neoformans

Most mAbs to the capsular polysaccharide glucuronoxylomannan (GXM) of Cryptococcus neoformans are generated from the same VH and VL gene families. Prior Ab studies have assessed protective efficacy, Id structure and binding to capsular polysaccharides, and peptide mimetics. These data have been interpreted as indicating that most mAbs to GXM have the same specificity. A new approach to Ab specificity analysis was investigated that uses genetic manipulation to generate C. neoformans variants with structurally different capsules. C. neoformans mutants expressing GXM with defective O-acetylation were isolated and complemented by the C. neoformans gene CAS1, which is necessary for the O-acetylation of GXM. The mAbs exhibited differences in their binding to the GXM from these mutant strains, indicating previously unsuspected differences in specificity. Analysis of three closely related IgMs revealed that one (mAb 12A1) bound to an epitope that did not require O-acetylation, another (mAb 21D2) was inhibited by O-acetylation, and the third (mAb 13F1) recognized an O-acetylation-dependent conformational epitope. Furthermore, an IgG Ab (mAb 18B7) in clinical development retained binding to de-O-acetylated polysaccharide; however, greater binding was observed to O-acetylated GXM. Our findings suggest that microbial genetic techniques can provide a new approach for epitope mapping of polysaccharide-binding Abs and suggest that this method may applicable for studying the antigenic complexity of polysaccharide Ags in other capsulated microorganisms.

The clinical application of monoclonal antibody therapies in renal transplantation

Monoclonal antibodies have become valuable tools for the precise clinical manipulation of the immune system. These highly specific proteins have proven their usefulness in both the treatment and prevention of organ transplant rejection. Indeed, they are the centrepieces of many evolving regimens designed to reduce or eliminate the need for chronic immunosuppression. This manuscript will review the monoclonal antibodies that have made their way into the clinic either as experimental therapies or approved drugs. It will provide a general overview of this class of agents and their mechanisms of action. Standard therapies and potential new applications will be described.

Database on monoclonal antibodies to Cytokeratins

Cytokeratins (CK) are being extensively used as diagnostic markers for various malignancies and other diseases, including human oral precancer and cancer, due to their tissue specific expression. CK are epithelia specific intermediate filament (IF) proteins, which are expressed in a differentiation dependent and tissue specific manner. There are about 30 polypeptides of CK expressed by different human epithelia. Each type of epithelium expresses about 4-6 polypeptides. CK polypeptides share many common epitopes, due to which the antibodies developed against CK tend to cross react. Therefore, a large number of monoclonal and polyclonal antibodies have been developed to distinguish among these proteins. Many of these antibodies are not only monospecific but are also epitope specific. These antibodies are being used in pathology laboratories for routine diagnosis using immunohistochemistry. A number of fixatives are used for fixation of tissue sections prior to the use of these antibodies. Sometimes, this leads in epitope masking. Hence, it becomes necessary to use a battery of monoclonal antibodies (MAb) for accurate diagnosis. Apart from the use of these antibodies in diagnostics, they are also being used in basic research for the study of CK function and their interactions with associated proteins and membrane proteins. In the present communication an effort has been made to make a comprehensive list of MAb to CK giving information like cross-reactivity, epitope specificity, various fixatives used, etc. along with the source of the antibodies, which will provide useful information to the users.

Monoclonal antibodies for the prevention and treatment of graft-versus-host disease

Acute and chronic graft-versus-host disease (GvHD) remain major obstacles to successful allogeneic hematopoietic stem cell transplantation, contributing substantially to morbidity and non-relapse mortality. Better understanding of the immunopathophysiology of GvHD has identified a number of targets for intervention. Among newly developed agents suitable for the prevention and treatment of GvHD, monoclonal antibodies hold much promise. Monoclonal antibodies currently available, such as infliximab and anti-interferon-gamma (anti-IFN-gamma), are capable of blocking of the action of initiating and effector cytokines. Antibodies directed against activated T cells, including daclizumab, visilizumab and ABX-CBL, may offer more specificity than the more broadly acting pan-T-cell-depleting agents. Finally, the clinical investigation of antibodies to adhesion molecules (such as LFA-1), or distal effector mechanisms (such as FasL) may offer another level of specificity. Many of these monoclonal antibodies have already undergone clinical testing. Campath-1H has been used for the prevention of acute GvHD with success. Daclizumab, infliximab, visilizumab, and ABX-CBL have shown promising activity in steroid-resistant acute GvHD in early clinical testing. This review summarizes current experience with monoclonal antibodies in the management of acute and chronic GvHD. Over the next decade, however, the challenge will be to define the relative place of these antibodies in the therapeutic armamentarium for GvHD and their impact on long-term survival.

More is not necessarily better: prozone-like effects in passive immunization with IgG

Despite a century of study, the relationship between Ag-specific Ig concentration and protection remains poorly understood for the majority of pathogens. In certain conditions, administration of high Ab doses before challenge with an infectious agent can be less effective than smaller Ab doses, a phenomenon which is consistent with a prozone-like effect. In this study, the relationship between IgG1, IgG2a, IgG2b, and IgG3 dose, infective inocula, and protection was investigated in a mouse model of Cryptococcus neoformans infection. The activity of each IgG subclass ranged from protective to disease-enhancing depending on both the Ab dose and infective inocula used. Enhanced dissemination to the brain was observed in mice given a high IgG2a dose and a relatively low inoculum. Ab administration had immunomodulatory effects, with cytokine expression in lung, brain, and spleen varying as a function of the infective inoculum Ab dose and IgG subclass. In vitro studies did not predict or explain the mechanism of in vivo prozone-like effects, because all isotypes were opsonic and elicited NO release from macrophages. IgG2a was most efficient in inducing a macrophage oxidative burst. These results reveal that an individual Ab can be protective, nonprotective, or disease-enhancing depending on its concentration relative to a challenge inoculum. Our findings have implications for the potential contribution of Ab responses to defense against microbial diseases because Ab-mediated immunity may be protective, nonprotective, or even deleterious to the host.

ECVAM's activities on biologicals

This paper summarises key activities initiated and the progress achieved between April 1993 and June 2002 in implementing the Three Rs in one of ECVAM's priority areas - the production and quality control of biologicals. These have included: organising nine key workshops; financially supporting and/or participating in a number of prevalidation and/or validation studies; financial contributions and sponsorship to relevant international workshops, symposia and conferences; and financial support for the compilation of manuals and expert reports, and training in test methods. The paper complements the papers of Hendriksen et al. and Cussler et al. included in these proceedings.

Monoclonal antibodies as therapeutics in oncology

The specificity of antibodies has been harnessed to target cancer cells and the first therapeutic antibodies for use in oncology are now finding application in the clinic. Studies are currently under way to develop new and improved antibodies. Recent developments have been made in the identification of novel targets, including the use of genomic and proteomic technologies. Several methods are also being developed to enhance antibody efficacy.

[Principal therapeutic uses of monoclonal antibodies in oncology]

In the past few years, new agents based on monoclonal antibodies have been developed in Oncology. Indeed in some case tumor cells express antigenic targets at higher levels than normal cells. There are 2 main types of monoclonal antibodies that can be either conjugated to cytotoxic drugs or radio-active compounds or be non-conjugated. Among the last category, some are currently used in the treatment of patients, including 2 monoclonal antibodies targeting receptors with tyrosine kinase activity (HER2: Herceptin (DCI: trastuzumab), EGFr: Erbitux (DCI: cetuximab). A third monoclonal antibody is commonly used in cancer treatment, which targets CD20, a transmembrane marker of B lymphoma (MabThera (DCI: rituximab). Both Herceptin and MabThera have been associated with improved survival in patients with breast carcinoma and lymphoma, respectively. New promising agents are under investigation such anti EGFr in colon and head and neck carcinoma or new compounds such as anti-VEGF. These examples outline the importance of the recent progress in selectively targeting tumor antigen and the potential impact of these approaches in Oncology.

Modulating targeted adhesion of an ultrasound contrast agent to dysfunctional endothelium

The early stages of atherosclerosis are characterized by increased endothelial cell (EC) surface expression of leukocyte adhesion molecules (LAMs). Ultrasound detection of acoustically active LAM-targeted microbubbles might provide a means to noninvasively assess the functional status of the endothelium. Toward this end, a lipid-based perfluorobutane-filled microbubble was synthesized with various densities of anti-ICAM-1 monoclonal antibodies conjugated to the bubble shell. We hypothesized that modulating the surface antibody density would permit regulation of the adhesion characteristics of the microbubbles, and that microbubble adhesion would be dependent on local wall shear rate. Coverslips of cultured human coronary artery ECs were exposed to microbubbles with various surface antibody densities (1%, 5%, 10%, 50%, 75%, and 100% of maximum coverage) at various wall shear rates (100, 175, 250, 350, and 500 s-1) in a parallel plate perfusion chamber. ECs were either normal or activated by interleukin-1 beta to overexpress ICAM-1. Adhesion was greater to activated vs. normal ECs (p < 0.001), increased with increasing surface antibody density (p < 0.01), and decreased with increasing wall shear rate (p = 0.02). We conclude that shell antibody density and wall shear rate are critical parameters controlling differential microbubble adhesion. This phenomenon might ultimately permit imaging of clinically relevant LAM expression in vivo.

Characterisation of monoclonal antibodies against haemagglutinin associated with Clostridium botulinum type C neurotoxin

Of 11 monoclonal antibodies (MAbs) prepared against the non-toxic component of type C Clostridium botulinum 16S toxin to clarify the function of the non-toxic component, seven recognised HA1, three recognised HA3b and one recognised HA2. Results of epitope mapping indicated that three of the seven anti-HA1 MAbs recognised the region between amino acid residues 121 and 140 and four recognised the three-dimensional structure of HA1. Three anti-HA3b MAbs recognised different regions between (approximately) amino acids 405-430, 180-270 and 275-297. The ability of these MAbs to interfere with binding of 16S toxin or non-toxic component, HA1 or HA3b to erythrocytes and to intestine tissue sections of guinea-pig was observed. MAbs against HA3b and HA2 did not inhibit 16S toxin binding to either erythrocytes or epithelial cells, whereas some MAbs against HA1 did inhibit binding. The seven anti-HA1 MAbs can be classified into four groups based on their binding inhibition activities. The anti-HA1 MAbs that inhibited the binding of 16S toxin to the epithelial cells also neutralised or reduced the oral toxicity in mice, indicating that HA may play an important role in the absorption of the 16S toxin from the small intestine.

Biopharmaceuticals and biotechnology medicines: an issue of nomenclature

Over the past two decades terms such as 'biopharmaceuticals' and 'biotechnology medicines' have crept into the pharmaceutical vocabulary. Such terms often have different meanings for different people and it is perhaps time that they were more formally defined.

Section 1A: Rh serology. Coordinator's report

One hundred forty-two Rh-specific monoclonal antibodies (Mabs) were evaluated by serology in 27 laboratories. Evaluators were asked to test each Mab at three dilutions in specified serological techniques against normal positive and normal negative phenotype cells, and any Rh variant cells that they had available. Raw data was submitted to the coordinator for overall analysis. Results were analysed by expressing the sum of reaction grades for each Mab with each variant cell as a percentage of the sum of reaction grades of that Mab with normal phenotype cells. Anti-D Mabs were sorted into 23 groups which had the same pattern of reactions with different partial D phenotype cells. Eighteen of these corresponded to previously defined patterns; five were new patterns. Combined with data from the previous workshop, this means that 30 different reaction patterns have been defined. A new nomenclature is introduced for numbering the epitopes. Reactions with new variants DNB, DNU and DAR indicated some further subsplits of these patterns. Reactions with Category Va cells indicated that there were five different types of Va cells that could be distinguished serologically with monoclonal antibodies. No patterns of reactivity corresponding to the epitope groups could be observed with the different types of weak D tested. Anti-E Mabs were sorted into 14 groups, and the E variant cells into seven groups.

Production and characterisation of monoclonal antibodies to salmon pancreas disease virus

Six mouse monoclonal antibodies (mAbs) specific to salmon pancreas disease virus (SPDV) were produced following immunisation with purified virus preparations. These mAbs and 2 mAbs resulting from an earlier investigation were characterised. None of the mAbs possessed virus neutralising activity but all reacted with 4 geographically different SPDV isolates as determined by indirect immunofluorescence. Three mAbs produced positive immunostaining with Western blots of SPDV proteins. The 4H1 mAb reacted with the 53 kDa structural E1 glycoprotein present in virus-infected cells and in gradient-purified virus. Two mAbs, 5A5 and 7B2, which exhibited unusual immunofluorescence staining of the nuclear margin, reacted with a 35 kDa protein, which is present in gradient-purified virus and which is considered to be the capsid protein. A sandwich ELISA, based on the use of mAb 2D9 for capture and a biotinylated conjugate of mAb 7A2 for detection, detected SPDV antigen in virus-infected Chinook salmon embryo-214 cells and gradient-purified virus. These mAbs may be of use in pathogenesis studies and in diagnostic test development.

Hepatitis B virus genotypes and HBsAg subtypes in refugees and injection drug users in the United States determined by LiPA and monoclonal EIA

Hepatitis B virus (HBV) genotyping and hepatitis B surface antigen (HBsAg) subtyping were carried out on sera from 196 HBsAg-positive patients, including 151 refugees entering the United States and 45 injection drug users in Seattle. HBsAg subtyping was performed by enzyme immunoassay (EIA) using a panel of monoclonal antibodies and the HBV genotype was determined by polymerase chain reaction (PCR) followed by detection of amplified HBV DNA by a reverse-phase hybridization line probe assay (LiPA) using genotype-specific probes. HBV DNA was detected by PCR in 155 (79%) of the 196 sera and all 155 were genotyped by LiPA. Samples from Southeast Asia were predominantly genotype B/subtype ayw1 and genotype C/adr; samples from the former Soviet Union and eastern Europe were mostly genotype D/ayw2 and genotype D/ayw3; samples from east Africa were mainly genotype A/adw2 and genotype D/ayw2; and samples from injection drug users were mostly genotype D/ayw3 and genotype A/adw2. Some strains of ayw3 gave atypical monoclonal antibody reactivity patterns in the subtyping assay due to a Val/Ala instead of a Thr at amino acid residue 118 and a Thr instead of a Met at residue 125. A strain of ayw2 also gave an atypical monoclonal antibody reactivity pattern due to an Ala instead of a Thr at amino acid residue 123. LiPA genotyping and monoclonal EIA subtyping can provide useful information for epidemiological studies.

Localization of midgut-specific protein antigens from Aedes aegypti (Diptera: Culicidae) using monoclonal antibodies

Most insect transmitted pathogens interact with the midgut of their vectors for infection and, in some cases, development. Therefore, molecules associated with the midgut epithelium in direct contact with pathogens may be potential targets of infection-blocking measures. Here, we identify midgut-specific protein antigens from Aedes aegypti (L.) using monoclonal antibodies. Sixty-four monoclonal antibodies were generated with reactivity to the mosquito midgut, five of which are reported in this article. Three monoclonal antibodies identified protein antigens localized at the midgut epithelial surface (the brush border) and the peritrophic membrane. In addition, two monoclonal antibodies recognized mosquito nucleus-specific proteins by immunofluorescence microscopy. Because potential target antigens for blocking transmission of pathogens most likely are located at the interface of mosquito-pathogen interactions, these monoclonal antibodies could provide valuable tools for studying midgut-specific proteins and interactions of the mosquito midgut with pathogens.

A new approach for rapid detection and typing of serum monoclonal components

When used independently, none of the routine methods to explore serum monoclonal components (MC), including: serum protein electrophoresis (SPE), immunoelectrophoresis (IEP), kappa to lambda ratio (KLR) and immunofixation (IFE), provides a comprehensive quantitative and qualitative identification of the MC. In the past few years the concept of 'protein profile', based on immunonephelometric quantifications of serum proteins, has become widely used. It consists of a qualitative and quantitative graphic representation of numerous serum proteins including immunoglobulins. Aim of study was to develop a multidimensional model based exclusively on protein profiles labeled the protein profile prediction method (PPPM) to improve routine MC detection and typing. Serum samples from 127 hospitalized patients and 99 healthy blood donors were submitted to all of the following: SPE, IFE, KLR and a protein profile (which included IgM, IgA, IgG, kappa and lambda chain detections and quantification). The presence of a MC using IFE was chosen as the gold standard. Healthy donors and patients were randomly divided into two groups defined as testing and validation groups. A logistic model was designed based on the protein profiles of the testing group leading to the determination of a threshold value (called Z(r)) for MC detection. It was then tested to detect MC in the validation group. Using IFE, 73 MC were found in the 127 hospitalized patients. Using the threshold value for MC detection of Z(r)=1.86, the PPPM showed greater sensitivity (94.6%) in detecting a MC compared to either SPE (64.8%) or KLR (89.2%). This result was obtained without diminished specificity (80.8%). The association of SPE or KLR to PPPM did not significantly increase the sensitivity of the PPPM. In the validation group, for samples which had a high predictive probability of a MC using PPPM, the correct MC typing was identified in up to 77% of sera using PPPM only. These results may be interesting in helping to determine when supplementary IFE analysis is required to qualitatively analyze a MC. PPPM allows MC detection with great sensitivity. The immune protein profile dramatically increases the sensitivity of either SPE and/or KLR in detecting MC and may also allow heavy and light chain typing.

Human monoclonal antibodies isolated from type I diabetes patients define multiple epitopes in the protein tyrosine phosphatase-like IA-2 antigen

Protein tyrosine phosphatase-like IA-2 autoantigen is one of the major targets of humoral autoimmunity in patients with insulin-dependant diabetes mellitus (IDDM). In an effort to define the epitopes recognized by autoantibodies against IA-2, we generated five human mAbs (hAbs) from peripheral B lymphocytes isolated from patients most of whom had been recently diagnosed for IDDM. Determination and fine mapping of the critical regions for autoantibody binding was performed by RIA using mutant and chimeric constructs of IA-2- and IA-2beta-regions. Four of the five IgG autoantibodies recognized distinct epitopes within the protein tyrosine phosphatase (PTP)-like domain of IA-2. The minimal region required for binding by three of the PTP-like domain-specific hAbs could be located to aa 777-979. Two of these hAbs cross-reacted with the related IA-2beta PTP-like domain (IA-2beta aa 741-1033). A further PTP-like domain specific hAb required the entire PTP-like domain (aa 687-979) for binding, but critical amino acids clustered in the N-terminal region 687-777. An additional epitope could be localized within the juxtamembrane domain (aa 603-779). In competition experiments, the epitope recognized by one of the hAbs was shown to be targeted by 10 of 14 anti-IA-2-positive sera. Nucleotide sequence analysis of this hAb revealed that it used a V(H) germline gene (DP-71) preferably expressed in autoantibodies associated with IDDM. The presence of somatic mutations in both heavy and light chain genes and the high affinity or this Ab suggest that the immune response to IA-2 is Ag driven.

Western blotting analysis of antibodies to prostate-specific antigen: cross-reactivity with human kallikrein-2

Immunoassays for prostate-specific antigen (PSA) using monoclonal or polyclonal antibodies have clinical applications, such as monitoring and early detection of prostate cancer. However, PSA shares 80% sequence homology with human glandular kallikrein (hK2). In the present study, we have used SDS-PAGE and Western blotting of a recombinant form of hK2 to determine the degree of cross-reactivity in a panel of 83 antibodies submitted to the ISOBM TD-3 PSA Workshop. From this panel, 24 of the 83 antibodies showed cross-reactivity with hK2. The majority of these antibodies showed binding to conformationally independent or linear epitopes. Fourteen of these antibodies appear to map to the same epitope group, indicating the existence of a dominant and linear immunogenic domain shared by PSA and hK2.

Immunological grouping of 53 antibodies against prostate-specific antigen

Microtiter immunoassays were used to determine whether a panel of 53 monoclonal antibodies submitted to the ISOBM TD-3 PSA Workshop could form assay combinations with free prostate-specific antigen (PSA) and the PSA complex with alpha1-antichymotrypsin. A panel of 6 known anti-PSA antibodies (H117, H50, H179, H164, 2E9 and 5A10) was used as labelled tracers. Epitope groups were proposed based on the ability of the Workshop antibodies to form good assay combinations with these 6 known anti-PSA antibodies. Nine of the TD-3 Workshop antibodies were found to react only with free PSA. Two additional epitope clusters were identified with 8 antibodies showing similar reactivity to antibody H117, while 11 antibodies formed a different cluster showing similar reactivity to antibody H50. Defining the nature of these immunodominant regions will be valuable in the development of more appropriate immunoassays for PSA.