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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 PDTRAPAP. 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 antimucin monoclonal antibodies.

The primary in vitro IgM response to the major histocompatibility (MHC) antigens expressed on chicken red blood cells (CRBC) has been shown to critically depend on the cooperation between antigen-specific helper factor from CRBC-primed thymus-derived helper (T) cells, bone marrow-derived (B) cells, and radioresistant (A) cells. A 500- to 1000-fold enrichment for immunocompetent B cells by means of a rosette technique permitted the culturing of as few as 2000 B cells that yielded on the average 500 plaque-forming cells against CRBC within 3 days of culture. Cell reconstitution experiments clearly showed that the presence of A cells can be circumvented by an A cell-derived secretory product. This is in disagreement with the hypothesis that helper factor acts as a mediator for antigen presentation on the A cell surface membrane. The spectrum of helper factor cross-reactivity with respect to certain alleles of the B locus was found comparable to that of antibody.