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Antigenic suppression of individual complement components can be induced when newborn animals or cells in tissue culture are treated for a period of time with antibody directed against that component. The initial exposure to antibody induces a network involving regulatory lymphocytes and soluble factors. Antigenic suppression has been accomplished either in vivo or in vitro with all three members of the evolutionarily related family C3, C4, and C5. In this report we have demonstrated that antigenic suppression is mainly the result of posttranscriptional regulation. After antibody treatment is terminated, during a period in which C3 protein is undetectable by sensitive assays, C3 mRNA is often briefly increased, after which it is modestly reduced. Therefore, the major mechanisms mediating immunologic control of C3 synthesis and secretion are not directly related to steady-state levels of C3 mRNA. The initially increased levels of C3 mRNA prove that there is no simple direct quantitative relationship between C3 mRNA and C3 synthesis and secretion. The later decreases in C3 mRNA levels are not sufficient to explain the quantitatively greater decreases in C3 synthesis. Therefore, the rate determining step in immune regulation of C3 production occurs after transcription. Regulation could be mediated by altered stability of C3 mRNA in the presence of antibody, formation of a defective primary transcript, defective processing of the transcript resulting in an abnormal mRNA, or a defective C3 translational apparatus. Although regulation is multifactorial, interference with translation is most likely to predominate.
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