SPONTANEOUS AND LIGAND-INDUCED ENDOCYTOSIS OF CD23 (FC-EPSILON RECEPTOR-II) FROM THE SURFACE OF LYMPHOCYTES-B GENERATES A 16-KDA INTRACELLULAR FRAGMENT

It has been reported that the 45-kDa low-affinity Fc-epsilon receptor (Fc-epsilon-RII) on B cells is cleaved spontaneously from the cell surface to release soluble fragments. This study demonstrates an additional fate of the Fc-epsilon-RII. I-125-labeled CD23+ B cells were cultured for 24 h at 37-degrees-C. After lysis, cell extracts were immunoprecipitated with CD23 monoclonal antibodies. Using this methodology, we demonstrated that an increasing amount of the labeled Fc-epsilon-RII becomes progressively resistant to externally applied trypsin, indicating that a fraction of the cell surface receptors are internalized. In parallel, a labeled 16-kDa material, recognized by CD23 monoclonal antibodies directed to the lectin-like domain of the Fc-epsilon-RII appears inside the cells. Chloroquine does not affect internalization of the Fc-epsilon-RII, but completely abolishes the formation of the intracellular fragment, suggesting that the receptor is processed by proteolytic cleavage in acidic organelle. In addition, the internalization is enhanced in the presence of CD23 monoclonal antibodies. These data demonstrate that Fc-epsilon-RII can be internalized by ligand-induced endocytosis and subsequently cleaved in an intracellular compartment, These results also support the view that the Fc-epsilon-RII is involved in antigen focusing and antigen presentation.