Internalized membrane immunoglobulin meets intracytoplasmic DR antigen in human B lymphoblastoid cells

The important role of type II antigens of the major histocompatibility complex (Ia and DR) in the antigen presentation and in the antigen-specific cooperation between T and B cells is by now firmly established. Human B lymphoblastoid lines MWE and BL were stained with different fluoresceinated anti-DR antibodies. Not only surface but intracytoplasmic DR structures were brightly stained. The intracellular stain accumulated in a cluster close to the nucleus. Different specificity controls were performed. MWE and BL cells quickly internalize membrane IgM if cross-linked by corresponding antibodies. The intracellular location of the vesicles containing these complexes showed substantial coincidence with DR-positive structures in the paranuclear location. It is speculated that IgM-ligand complexes meet internal DR molecules and are both reexpressed on the plasma membrane to be recognized by primed T cells.