IL-4 induces Fas resistance in B cells

Activated B cells express Fas (CD95) and are targets for apoptosis induced by CD4+ Th1 effector cells that kill in a Fas-dependent fashion. We report here that IL-4 reverses the susceptibility to Fas-mediated apoptosis that characterizes CD40-stimulated primary B cells. IL-4-induced Fas resistance is not associated with an alteration in the elevated level of Fas expression produced by CD40 ligand and does not depend on additional receptor-mediated signals. However, IL-4-induced resistance to Th1 cell-mediated cytotoxicity (Th1-CMC) develops more slowly than resistance mediated by surface Ig and is not affected by protein kinase C depletion, unlike anti-Ig-induced Fas resistance. By these two criteria, IL-4-and anti-Ig-induced resistance to Th1-CMC appear to be driven through distinct mechanisms; in keeping with this, suboptimal doses of IL-4 and anti-Ig act in synergy to induce marked protection against Th1-CMC. An important role for IL-4-induced Fas resistance is suggested by the observation that sera from IL-4-overexpressing transgenic mice contain autoreactive Abs.