IL-3 AND GRANULOCYTE-MACROGHAGE COLONY-STIMULATING FACTOR STRONGLY INDUCE IG SECRETION BY SORT-PURIFIED MURINE B-CELLS ACTIVATED THROUGH THE MEMBRANE IG, BUT NOT THE CD40, SIGNALING PATHWAY

Sort-purified resting murine B cells proliferate in response to dextra n-conjugated anti-IgD Abs (alpha delta-dex) but fail to secrete signif icant amounts of Ig even after the addition of IL-1 + IL-2. We show th at either IL-3 or granulocyte-macrophage CSF (GM-CSF) stimulates 10- t o 50-fold enhancements in IgM secretion by sort-purified B cells treat ed with alpha delta-dex + IL-1 + IL-2, and that the combined actions o f IL-3 and GM-CSF are typically greater than additive. Both IL-3 and G M-CSF act primarily as B cell differentiation factors, although IL-3 i nduces a modest enhancement in cellular outgrowth. The enhancing effec ts of IL-3 and CM-CSF require multivalent Ag receptor cross-linkage, m ediated by alpha delta-dex, as neither cytokine induces IgM secretion in the presence of unconjugated anti-IgD Abs, Although both alpha delt a-dex and IL-1 + IL-2 are required for optimal IL-3- and CM-CSF-mediat ed IgM secretion, both IL-3 and GM-CSF stimulate a modest IgM secretor y response by cells activated with alpha delta-dex alone. In this rega rd, supernatant from either an activated CD4(+) Th1 or Th2 clone poten tly induces IgM secretion by alpha delta-dex + IL-1 + IL-2-activated B cells and this is due, in large part, to the presence in these supern atants of either IL-3 and/or GM-CSF. Neither IL-3 nor GM-CSF stimulate s significant IgM secretion by B cells activated through the CD40 sign aling pathway alone, although the combination of CD40 and membrane Ig signaling leads to a strong enhancement of the IL-3 + CM-CSF-mediated IgM synthesis above that obtained with membrane Ig signaling alone. Th e demonstration that IL-3 and GM-CSF act directly as differentiation f actors for B cells activated through their Ag receptor establishes a n ovel cytokine pathway for induction of humoral immunity.