Development of a bioassay for human myeloma colony-forming cells.

This chapter outlines our development of an in vitro soft agar assay for detection of human myeloma colony-forming cells. Growth was induced with either 0.02 ml of human type O erythrocytes or 0.25 ml of medium conditioned by the adherent spleen cells of mineral-oil-primed BALB/c mice. A maximum plating efficiency of 0.1% was obtained. The number of myeloma colonies was proportional to the number of cells plated between concentrations of 105--106/ml. Morphological, histochemical, and functional criteria showed the colonies to consist of immature plasmablasts and mature plasma cells. 60%--80% of cells picked from colonies contained intracytoplasmic monoclonal immunoglobulin. Tritiated thymidine suicide studies provided evidence that for most myeloma patients, a very high proportion of myeloma colony-forming cells were actively in transit through the cell cycle. Using biophysical and immunologic studies, we were able to further characterize myeloma stem cells and obtain partial enrichment of the colony-forming cells. Increased numbers of myeloma colonies were seen when the marrow was depleted of CSF, elaborating adherent cells before plating. Antibody to granulocyte colony-stimulating factor, which did inhibit granulocyte colony formation, did not reduce the number or size of the myeloma colonies. This bioassay has subsequently served as the basis for studies of in vitro biological behavior of multiple myeloma, and for measurement of drug sensitivity. The general methodology which we first developed for myeloma appears to have general applicability not only to monoclonal plasma cell disorders, but also to many other tumor types as well. Detailed biological studies and analysis of culture conditions (similar to those we have carried out in myeloma) will no doubt prove important in understanding the biology and drug sensitivity of various forms of human cancer.