Using monoclonal anti-human IgE antibodies recognizing the D epsilon 1 or D epsilon 2 epitope, we have developed a sandwich radioimmunoassay (RIA) to determine IgE in human cell cultures. With the help of this assay, various methods of measuring an actual de novo IgE synthesis were compared. It was necessary to subtract performed IgE from released IgE in the culture supernatant and the IgE associated to the cultured cells, in order to determine a net IgE synthesis which would reflect de novo synthesized IgE. Using this differential in vitro IgE assessment, no net IgE synthesis could be demonstrated in culture conditions which lead to strong IgG synthesis. Actual net in vitro IgE antibody production was only found in approximately 30% of cell cultures from atopic donors. This spontaneous IgE synthesis did not correlate to the serum IgE levels of the patients. However, correlations were found between serum IgE levels and amount of performed, released or cell-associated IgE of the cultures.
Several pitfalls may affect studies on human IgE synthesis in vitro. In this paper, the requirement for stringent specificity of the anti-IgE antibodies used and for assessment not only of IgE detectable in culture supernatants but also as cell-associated IgE is emphasized. The use of cycloheximide-treated cultures as controls also leaves wishes open. Activated, human T cells and T cell hybridomas produce IgE-binding factors, which may be detected by a sensitive in vitro test and which may apparently also become the endeavour of synthesis by molecular biological techniques. Although the evidence available in rodents for the role of IgE-binding factors in modulating IgE synthesis has not yet been fully reproduced by us in man, the fact that classical IgE-enhancing procedures in rodents (e.g. radiotherapy, T cell suppression) also affect IgE production in man leads to believe that similar immunoregulation mechanisms apply to the various mammalian species studied so far.
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