Endocytosis of antigen, anti-idiotype, and anti-immunoglobulin antibodies and receptor re-expression by murine B cells.

The endocytosis of Ag mediated by membrane-associated Ig (mIg) molecules has been spectrophotometrically monitored using a cell line (2C3) specific for the hapten phthalate (Xmp) and employing conjugates of Xmp and horseradish peroxidase (HRP) as the labeled ligand. Approximately 50% of both Xmp-HRP, or the larger ligand, Xmp-keyhole limpet hemocyanin-HRP, are internalized rapidly, reaching an initial plateau by 30 min. The rate of endocytosis of anti-idiotype-HRP is similar to the rates that were observed for the hapten-bearing ligands, while a slower rate of endocytosis of anti-Ig-HRP was observed. The percent of ligand bound that is internalized and the rate of endocytosis appear to be largely independent of the size and amount of ligand bound per cell. However, mIg-mediated endocytosis is markedly reduced when mIg-ligand complexes are more extensively cross-linked by the binding of a second antibody. In addition to the initial rapid phase of endocytosis, there is a prolonged phase during which more of the bound ligand is internalized, and up to 90% of the internalized ligand is degraded. Re-expression of Ag-binding receptors by the 2C3 cells is independent of new protein synthesis and is accomplished in part by the translocation of a presynthesized pool of mIg molecules from the cytoplasm to the plasma membrane of the cell. The kinetics of endocytosis of HRP-labeled anti-Ig antibodies by BALB/c splenic B-lymphocytes and other B-lymphocyte cell lines is very similar to the endocytosis of Ag and anti-idiotype observed with the 2C3 cell line. Light and electron microscopy are also performed to visually confirm that the HRP-labeled ligands are being internalized and to determine the percentage of cells involved in this process. Finally it was determined that the transmembrane and cytoplasmic domains of the mIg molecules are required for endocytosis since the secreted form of the molecule (which lacks these domains) fails to mediate the internalization of bound ligand.