GERMLINE AND PRODUCTIVE C-EPSILON-GENE EXPRESSION DURING IN-VIVO IGE RESPONSES

In vitro studies have established that Ig isotype switching typically involves deletion of C(H) genes that are located between VDJ and the C (H) gene that will be expressed, and is preceded by transcription of a germline (g) form of that C(H) gene. Increases in gepsilon transcript levels are induced by the cytokine IL-4, and always precede switching to IgE. To evaluate whether a similar relationship occurs in vivo, we examined IL-4 mRNA, gepsilon RNA, productive (p)epsilon mRNA, and ser um IgE levels in two in vivo systems: one in which the injection of an ti-IgD antibody induces mIgD+ B cells to switch to the expression of I gE and to secrete this isotype, and a second in which the injection of anti-IgE antibody stimulates IgE secretion by B cells that had been i nduced to express membrane IgE by earlier treatment with anti-IgD anti body. Increases in IL-4 transcript levels in anti-IgD-injected mice we re followed within 24 h by increases in gepsilon RNA, and, one to two days later, by increased pepsilon mRNA and serum IgE levels. IL-4 anta gonists blocked the gepsilon and pepsilon RNA and serum IgE responses in these mice, whereas the injection of otherwise untreated mice with IL-4 stimulated, within 24 h, a large increase in gepsilon RNA levels, followed 1-2 days later by a small increase in pepsilon mRNA. Injecti on of anti-IgD-primed mice with anti-IgE antibody also stimulated incr eases in IL-4, gepsilon, and pepsilon RNA levels; however, the increas es in IL-4 and gepsilon RNA were considerably smaller, and the increas es in pepsilon mRNA and serum IgE considerably larger, than those obse rved in anti-IgD antibody-injected mice. IL-4 antagonists blocked the anti-IgE antibody-induced gepsilon RNA response, but not the pepsilon mRNA or serum IgE responses. Thus, IL-4 is required for the induction of gepsilon RNA in at least two in vivo systems, increased gepsilon RN A levels precede increases in pepsilon RNA levels in vivo as in vitro, and neither IL-4 nor gepsilon RNA is required to induce B cells that have already switched to IgE expression to differentiate into IgE-secr eting cells.