Sixteen research groups participated in the ISOBM TD-4 Workshop in which the reactivity and specificity of 56 monoclonal antibodies against the MUC1 mucin was investigated using a diverse panel of target antigens and MUC1 mucin- related synthetic peptides and glycopeptides. The majority of antibodies (34/56) defined epitopes located within the 20-amino acid tandem repeat sequence of the MUC1 mucin protein core. Of the remaining 22 antibodies, there was evidence for the involvement of carbohydrate residues in the epitopes for 16 antibodies. There was no obvious relationship between the type of immunogen and the specificity of each antibody. Synthetic peptides and glycopeptides were analyzed for their reactivity with each antibody either by assay of direct binding (e.g. by ELISA or BiaCore) or by determining the capacity of synthetic ligands to inhibit antibody binding interactions. There was good concordance between the research groups in identifying antibodies reactive with peptide epitopes within the MUC1 protein core. Epitope mapping tests were performed using the Pepscan analysis for antibody reactivity against overlapping synthetic peptides, and results were largely consistent between research groups. The dominant feature of epitopes within the MUC1 protein core was the presence, in full or part, of the hydrophilic sequence of PDTRPAP. Carbohydrate epitopes were less easily characterized and the most useful reagents in this respect were defined oligosaccharides, rather than purified mucin preparations enriched in particular carbohydrate moieties. It was evident that carbohydrate residues were involved in many epitopes, by regulating epitope accessibility or masking determinants, or by stabilizing preferred conformations of peptide epitopes within the MUC1 protein core. Overall, the studies highlight concordance between groups rather than exposing inconsistencies which gives added confidence to the results of analyses of the specificity of anti-mucin monoclonal antibodies.
The metastasis-related protein S100A4 is released from tumor cells, and since it is highly expressed in colorectal cancer (CRC), it could be a potential tumor marker in plasma or serum. Monoclonal antibodies (MAbs) were raised against human recombinant S100A4 and shown to detect native and recombinant antigen with high sensitivity and specificity. Using two MAbs, an immunofluorometric assay (IFMA) was established to detect S100A4 in clinical samples with high sensitivity and precision. S100A4 in plasma and serum from patients with CRC was highly influenced by sample hemolysis. Both red blood cells and mononuclear cells were found to contain S100A4, possibly contributing to the measured levels in serum and plasma. Since even very low-level hemolysis influenced the results, a potential contribution from an S100A4-expressing tumor could not be discerned, indicating that S100A4 is not suitable as a plasma or serum tumor marker for CRC. The antibodies and the IFMA may still be useful for research purposes.
To characterize antigenic sites in carcinoembryonic antigen (CEA) further and to investigate whether there are differences between colon tumor CEA and meconium CEA (NCA-2) that can be detected by anti-CEA monoclonal antibodies (MAb), 19 new anti-CEA MAb were analyzed with respect to specificity, epitope reactivity and affinity. Their reactivities were compared with 10 anti-CEA MAb with known CEA-domain binding specificity that have previously been classified into five nonoverlapping epitope groups, GOLD 1–5. Cross-inhibition assays with antigen-coated microtiter plates and immunoradiometric assays were performed in almost all combinations of MAbs, using conventionally purified CEA (domain structure: N-A1B1-A2B2-A3B3-C) from liver metastasis of colorectal carcinomas, recombinant CEA, meconium CEA (NCA-2), truncated forms of CEA and NCA (CEACAM6) as the antigens. The affinity of the MAbs for CEA was also determined. The new MAbs were generally of high affinity and suitable for immunoassays. Three new MAbs were assigned to GOLD epitope group 5 (N-domain binding), 3 MAbs to group 4 (A1B1 domain), 1 to group 3 (A3B3 domain), 3 to group 2 (A2B2 domain) and 3 to group 1 (also the A3B3 domain). Three MAbs formed a separate group related to group 4, they were classified as GOLD 4′ (A1B1 domain binding). The remaining 3 MAbs appear to represent new subspecificities with some relationship to GOLD groups 1, 2 or 4, respectively. Five MAbs, all belonging to epitope group 1 and 3, reacted strongly with tumor CEA but only weakly or not at all with meconium CEA, demonstrating that the two products of the CEA gene differ from each other, probably due to different posttranslational modifications.
Fifty-four anti-MUC1 antibodies submitted to the International Society for Oncodevelopmental Biology and Medicine (ISOBM) Workshop (TD-4) were evaluated in immunoradiometric assays, using sera from carcinoma patients and healthy donors. The carcinoma serum pool contained sera from 30 patients with advanced cancer (10 breast, 10 colon, and 10 ovarian). This serum pool contained 696 kU/l MUC1, 770 µg/l CEA, and 3,700 kU/l CA 125. The reference serum pool was obtained from 10 healthy women combined with 20 sera from pregnant women, of which half had elevated CA 125 (range 82–254 kU/l). The reference serum pool contained 13 kU/l MUC1, 2 µg/l CEA, and 65 kU/l CA 125. The Workshop antibodies were tested both as solid-phase antibodies and as tracer antibodies with the carcinoma serum pool. Twenty-two tracer antibodies and 38 solid-phase antibodies gave at least one combination with >10% binding of the tracer antibody for a total of 836 combinations. These were tested further with the reference serum pool. Antibodies used as tracers could be separated into three categories: Group 1 antibodies, MF06, MF11, B27.29, MF30, and Ma552, gave mainly ‘carcinoma-specific’ assays in combinations with the solid-phase antibodies, i.e. binding ratio between carcinoma MUC1 and reference MUC1 >10. Group 2 antibodies, DF3, 7540MR, A76-A/C7, BC4N154, M38, 7539MR, B12, GP1.4, 232A1, Mc5, and Ma695 gave both ‘specific’ and ‘nonspecific’ binding ratios depending on the solid-phase antibody used. Group 3 antibodies, 214D4, BC4E549, E29, BCP8, BC3, and 3E1.2, gave mainly ‘nonspecific’ combinations, i.e. ratios ≤10. All antibodies used to capture MUC1 on the solid phase gave both ‘specific’ and ‘nonspecific’ combinations depending on the tracer antibody used. Ten antibodies were clearly more efficient as solid-phase capture antibodies; Ma695, B12, M38, GP1.4, 214D4, MF06, B27.29, A76-A/C7, BC3, and KC4. Our findings indicate that the ability to detect ‘carcinoma-specific MUC1’ cannot be deduced from epitope specificity alone.
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