Cultured basophils but not cultured mast cells induce human IgE synthesis in B cells after immunologic stimulation

By generating human mast cells and basophils from umbilical cord blood mononuclear cells cultured in the presence of appropriate cytokines, we investigated whether these two cultured cells could provide the cytokine and cell contact signals that are required to induce IgE synthesis in B cells. To activate cultured mast cells and basophils, cross-linking of cell surface high-affinity IgE receptor (Fc epsilonRI) was performed with specific antigen after sensitization with murine IgE. Upon Fc epsilonRI stimulation, basophils, but not mast cells, secreted significant amounts of immunoreactive IL-4 and IL-13 and expressed detectable CD40 ligand (CD40L) and a very low level of Fas ligand (FasL). These observations at the protein level were consistent with the data obtained at the gene transcriptional level, except for the faint expression of only IL-13 mRNA in mast cells. When added to normal human B cells, activated basophils induced IgE and IgG4 synthesis as well as soluble CD23 release. In contrast, neither IgE nor IgG4 synthesis could be induced by the interaction of B cells with activated mast cells, even in the presence of recombinant IL-4. The induction of IgE synthesis by activated basophils was completely abrogated by two neutralizing MoAbs against IL-4 and IL-13 and by a soluble form of CD40. This abrogation was accompanied by abolished mature C epsilon transcription in both cases. Addition of anti-FasL MoAb, however, did not significantly affect IgE induction mediated by activated basophils. These results demonstrate that unlike cultured mast cells, cultured basophils produce biologically active IL-4 and IL-13 and express functional CD40L after Fc epsilonRI stimulation, thereby contributing to IgE production by B cells, and suggest that relatively weak expression of FasL by cultured basophils is not involved in IgE regulation.

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.