Mechanism of ligand-specific upregulation of Fc receptors for IgE on murine B cell lines

Several studies have demonstrated that the addition of IgE to mixed lymphocyte populations will induce an increase in the number of B cell Fc receptors for IgE (Fc/sub epsilon/R). Using a Fc/sub epsilon/R+ murine B cell hybridoma, the authors have shown that IgE acts directly on the B cell to upregulate its receptor. The present studies describe the mechanism by which Fc/sub epsilon/R induction occurs. Kinetic studies of 35S-methionine labeled, affinity-purified Fc/sub epsilon/R demonstrate that IgE does not cause an increase in the rate of Fc/sub epsilon/R biosynthesis. Studies which determine the effect of IgE on the decay rates of either surface radiolabeled Fc/sub epsilon/R or Fc/sub epsilon/R on cycloheximide-treated B cells indicate that IgE causes an increase in Fc/sub epsilon/R half-life the degree of which correlates with the degree of upregulation. Thus, the effect of IgE is to slow the rate of Fc/sub epsilon/R degradation. However, the rates determined by the two methods differ, as the t1/2 determined by cycloheximide treatment for both induced and normal cells are 4-fold longer than those determined by surface labeling; this suggests the involvement of an additional protein (blocked by cycloheximide) in receptor degradation. At least in part, this degradation involves the release into the media of a ≅35Kd soluble receptor fragment which can be specifically, isolated by a monoclonal anti-Fc/sub epsilon/R antibody. Thus, it appears that Fc/sub epsilon/R induction by ligand occurs because IgE, in binding to its receptor, inhibits proteolytic cleavage of the Fc/sub epsilon/R and its subsequent release from the cell surface