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To better understand animal cell plasma membranes, we studied simplified models, namely four-component lipid bilayer mixtures. Here we describe the domain size transition in the region of coexisting liquid-disordered (Ld) + liquid-ordered (Lo) phases. This transition occurs abruptly in composition space with domains increasing in size by two orders of magnitude, from tens of nanometers to microns. We measured the line tension between coexisting Ld and Lo domains close to the domain size transition for a variety of lipid mixtures, finding that in every case the transition occurs at a line tension of ∼0.3 pN. A computational model incorporating line tension and dipole repulsion indicated that even small changes in line tension can result in domains growing in size by several orders of magnitude, consistent with experimental observations. We find that other properties of the coexisting Ld and Lo phases do not change significantly in the vicinity of the abrupt domain size transition.

Freshly explanted human myeloma cells formed colonies of monoclonal plasma cells in soft agar in the presence of medium conditioned by the adherent spleen cells of mineral oil-primed BALB/c mice. The medium showed peak activity at a dilution of 1:4. 2-mercaptoethanol or monothioglycerol was necessary for colony formation. Other thiols tested were ineffective in promoting colony growth. Colony-forming cells adhered to nylon wool, but not glass beads or plastic dishes. The presence of E-rosetting cells was not required for myeloma colony formation. Antibody prepared against a human myeloma cell line, RPMI 8226, reduced colony formation. These studies demonstrate the usefulness of this bioassay for determining functional properties of the myeloma colony-forming cell.

We attempted to define the conditions that promote growth of putative lymphoma stem cell colonies in vitro in a soft agar system. Growth was promoted by feeder layers containing medium conditioned by adherent spleen cells of mineral oil-primed BALB/c mice or by cells from a human B lymphocyte cell line (RPMI-1788). There were 65 patients with all histologic types of non-Hodgkin lymphoma studied. Lymphoid colony growth was obtained in 11 (61%) of 18 bone marrows and in 3 (50%) of 6 lymph nodes histologically involved by lymphocytic lymphoma. Conversely, colony growth was observed in only a single instance from 49 bone marrows without overt lymphoma and was not observed in cultures of normal lymph nodes (4), spleens (2), bone marrows (10), or peripheral blood (6). In a group of 12 patients whose bone marrows were studied serially before, during, and after chemotherapy, positive cultures correlated well with bone marrow involvement by lymphoma and clinical status. In positive cultures, colonies appeared within 4 days of plating and reached peak size (nearly 1000 cells per colony) in 7-10 days. Plating efficiency ranged from 0.001% to 0.1%. Morphologic, histochemical, and immunologic studies of cells from the colonies identified them as lymphoid. This assay system should permit further identification of the characteristics of lymphomas that grow in vitro and may prove amenable for evaluating effects of anticancer drugs on the survival of human lymphoma colony-forming cells.

Multiple myeloma has served as a model neoplasm in both mouse and man (21). Studies of myeloma immunoglobulin synthesis and metabolism have been applied to quantitate the total-body number of myeloma cells and to follow changes in tumor mass with treatment. Such serial tumor kinetic studies, and those of the tritiated thymidine labeling index of the tumor, have provided insights on the kinetics of growth and regression of myeloma and on approaches to treatment (21). Such studies, however, did not directly assess the clonigenic tumor cells, the cells that provide the basic renewal system of the tumor.