Interleukin-3-treated non-B, non-T cells switch activated B cells to IgG1/IgE synthesis

The switch of activated B cells to IgE synthesis is an interleukin (IL)-3-dependent process. It is currently thought that specific T cells activated by antigen presented in the context of class II major histocompatibility complex are the major source of IL-4. Recently it has been demonstrated that a splenic non-T non-B cell population (termed NBNT) has the capacity to produce IL-4 following IgE and IgG receptor cross-linkage. In this study we demonstrate that IL-4 producing NBNT cells can induce the switch of lipopolysaccharide-activated B cells to the synthesis of IgG1 and IgE antibodies. Furthermore, it was found that not only IgE receptor cross-linkage but IL-3 was able to stimulate NBNT cells to produce IL-4 and induce the switch of B cells to IgE synthesis. NBNT cells derived from the spleen and bone marrow of SCID mice were able to produce IL-4 on exposure to IL-3. This suggested that the ability of IL-3 to stimulate IL-4 production was not dependent on prior exposure of the NBNT cells to antibody complexes in vivo. Taken together these findings represent the first observation that enough IL-4 is produced by NBNT cells to actually influence a B cell IgG/Ig response. The findings also clearly demonstrate that B cells do not need high concentrations of IL-4 to be directed to switch to IgG1 and IgE synthesis.