Induction of human IgE synthesis in B cells by a basophilic cell line, KU812

Induction of human IgE synthesis in B cells requires, in addition to IL-4 or IL-13, a second signal provided by CD40 ligand (CD40L) on activated Th2-type CD4⁺ T cells that do not or weakly express Fas ligand (FasL). Mast cells and basophils also produce IL-4 or IL-13 and express CD40L after immunologic or pharmacologic stimulation, although it is unknown whether these cells express FasL. This study investigated the capacity of KU812 cells, a human basophilic cell line, to produce IL-4 and IL-13, to express CD40L and FasL, and to induce IgE and IgG4 synthesis in human normal B cells. Upon stimulation of KU812 cells with either phorbol myristate acetate (PMA) or ionomycin (Iono), IL-4, but not IL-13, was produced in response to Iono, while IL-13, but not IL-4, was inducible by PMA. Moreover, both the time courses of IL-4 and IL-13 production and their amounts secreted were different; IL-4 production was transient, IL-13 production gradually increased, and IL-13 was heavily secreted as compared with IL-4. The combination of PMA and Iono (PMA/Iono) induced higher production of IL-4 or IL-13 than did Iono or PMA alone. KU812 cell-derived IL-4 and IL-13 had the ability to cause CD23 expression on B cells. PMA/Iono also up-regulated CD40L expression and induced a very low level expression of FasL. KU812 cells that had been activated by PMA/Iono followed by fixation could induce IgE and IgG4 synthesis in B cells in the presence of recombinant IL-4 or IL-13. This contact-dependent induction of IgE was completely abrogated by adding anti-CD40L MoAb or soluble CD40, whereas anti-FasL antibody did not significantly affect IgE production. These results indicate that activated KU812 cells produce biologically active IL-4 and IL-13, express functional CD40L, and exhibit weak induction of FasL, thereby supporting sufficient IgE production by B cells.