INTERLEUKIN-5 INDUCES S-MU-S-GAMMA-1 DNA REARRANGEMENT IN B-CELLS ACTIVATED WITH DEXTRAN-ANTI-IGD ANTIBODIES AND INTERLEUKIN-4 - A 3-COMPONENT MODEL FOR IG CLASS SWITCHING

The cellular signals required for induction of immunoglobulin (Ig) class switching are only partially understood. Two processes that are considered to be necessary for such induction are DNA synthesis and germline constant heavy (C(H)) gene transcription. We now show that an additional signal, as mediated by interleukin 5 (IL-5), is also required. To induce proliferation of resting B cells, but not Ig secretion, we utilized anti-IgD antibodies conjugated to dextran (alphadelta-dex). The addition of IL-4, a well-established switch factor for the IgG1 subclass, to alphadelta-dex-activated cell cultures failed to induce IgG1 secretion or mIgG1+ cells unless IL-5 was also present. While IL-4 stimulated an increase in germline gamma1 RNA in alphadelta-dex-activated cells, this effect could neither be induced nor enhanced by IL-5. By contrast, IL-5 strongly enhanced steady-state levels of productive gamma1 RNA induced by alphadelta-dex and IL-4, suggesting that IL-5 stimulated IgG1 switch rearrangement. To test this possibility we measured switch (S)mu-Sgamma1 DNA recombination events using a newly developed assay, digestion circularization polymerase chain reaction (DC-PCR). We demonstrated that IL-5 was necessary for induction of Smu-S-gamma1 DNA rearrangement in alphadelta-dex plus IL-4-activated cells but that it had little effect on rearrangement in the absence of IL-4. Our data strongly suggest, therefore, a three-component model for induction of Ig class switching. This model includes germline C(H) gene transcription, DNA synthesis, and a third component that is necessary for recombination.