Antibody‐induced changes in expression of an immunoglobulin surface antigen

An IgG 1 ‐secreting myeloma possesses Ig‐related surface antigen, Ig‐mem, detected both by a cytotoxic and a 125 I‐labeled anti‐Ig binding assay. The properties of Ig‐mem were studied by examining the fate of 125 I‐labeled rabbit anti‐Ig and the complement sensitivity of cells sensitized by the binding of labeled antibody at 0 °C. Shifting the temperature to 24 °C or 37 °C leads to a rapid loss of sensitivity to lysis by complement, as well as a loss in the capacity to detect the rabbit antibody on the cell surface. These reactions occur without any appreciable loss of radioactivity from cells at 24 °C. The 125 I‐labeled antibody is subsequently degraded to amino acids (detected as iodotyrosine) more rapidly at 37 °C than at 24 °C. We interpret these results as an antibody‐induced interiorization of the IG‐mem, together with its bound antibody. The interiorization reaction does not occur if 125 I‐labeled, univalent Fab fragment of the rabbit antibody is used. Instead, most of this reagent remains bound to cells for at least an hour at 24 °C or 37 °C. These latter findings suggest that the bulk of Ig‐mem may not be an immediate precursor of the secreted IgG 1 , in transit through the plasma membrane.