Human interleukin 4 (IL 4) up‐regulates the expression of CD23 on both resting and “in vivo” activated B cells but induces proliferation and/or differentiation only on “in vitro” activated B lymphocytes. Resting B cells express 360 high‐affinity IL 4 receptors (R) per cell (Kd = 25–75 pM). Activation of resting B cells with anti‐IgM antibody or Staphylococcus aureus Cowan I (SAC) results in a two‐to‐threefold increase of IL 4R number without alteration of IL 4R affinity for IL 4. Flow cytometric analysis of biotinylated IL 4 binding shows that IL 4R expression is up‐regulated on virtually all anti‐IgM‐stimulated B cells, but only on a subpopulation of larger cells among SAC‐activated B lymphocytes. Culturing cells for 40 h with optimal concentrations of IL 4 does not significantly affect IL 4R levels on resting and anti‐IgM‐preactivated B lymphocytes but triples IL 4R levels on SAC‐preactivated B cells. Removal of IL 4 from cell cultures results in a two‐to‐fourfold increase of IL 4R levels 2 h later, suggesting an increase in IL 4R turnover. Resting and activated B cells degrade 1251‐labeled IL 4 at 37 °C. Sodium dodecyl sulfate‐polyacrylamide gel electrophoretic analysis of IL 4 binding molecules on resting, “in vivo” activated and anti‐IgM‐activated B cells reveals the same three species of 130, 80‐75, 70‐65 kDa. Thus, IL 4 displays its different biological activities on resting and activated B cells through IL 4R of the same affinity, gross biochemical structure and ability to mediate IL 4 degradation. Copyright © 1990 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim
