In vitro endocytosis of benign and malignant human bone marrow plasma cells

We report ultrastructural evidence of the phagocytic potential of plasma cells and myeloma cells. The incubation of plasma cells and myeloma cells in vitro with horseradish peroxidase (HRP) and cationized ferritin (CF) allows the tracing of fluid-phase and receptor-mediated pathways. Surface-bound ligands (CF) and solutes (HRP) taken up in primary pinocytic vesicles are internalized to the endosomal compartment. After 1 hr of incubation, CF was found not only in plasma cells but also in myeloma cells. Reaction products of HRP were observed only in myeloma cells. In myeloma cells, however, HRP was located only in the lysosomal system, whereas CF was present within membrane cisternae as well as within lysosomes. These myeloma cells morphologically produced interleukin-6 (IL-6).