B-lymphocyte-associated mouse IgM: Discrimination between surface and ‘intracellular’ FCμ antigens

Various affinity-purified anti-IgM antibodies were used as probes to study mouse B-lymphocyte-associated IgM. A quantitative assay, using lymph node cells attached to plastic vials, showed that a small fraction (30–35%) of anti-Fcμ antibodies bound to the surface of live lymphocytes. Most of the binding (65–70%) was intracellular, and occurred only after cell fixation with ethanol-ether. The binding to surface Fcμ was pronase-sensitive, whereas the binding to intracellular Fcμ was not. An inverse ratio of surface binding vs intracellular was observed with anti-μ N-terminal antibodies. The relatively large amounts of intracellular Fcμ antigens raise the possibility of the transmembrane nature of surface IgM.