Feeder cell requirements for leukemia cell colony formation in cultures supplemented with phytohemagglutinin

The use of phytohemagglutinin-supplemented colony cultures has offered new opportunities recently for studying acute myeloid leukemia (AML) cell growth in vitro. The active stimulator cells for AML colony-forming cells have not been identified, although this could be important for optimal application of the technique and for elucidating differences in growth between normal and leukemic progenitor cells. In this study, feeder layers were prepared from subpopulations of normal peripheral blood leukocytes which were obtained by centrifugation through Ficoll-Isopaque, erythrocyte rosette sedimentation, and adherence separation. Underlayers containing lymphocytes (B, T, or B plus T) or adherent monocytes failed to stimulate AML colony formation. The colony stimulation capacity of total mononuclear cells was decreased significantly following depletion of T-lymphocytes. The highest AML colony numbers were obtained when adherent monocytes and T-lymphocytes in combination were added to phytohemagglutinin-containing cultures. Stimulation of AML colony formation depended on the quantitative interrelationship of monocytes and T-lymphocytes in the cultures. Thus, AML colony-forming cells, unlike normal marrow granulocyte-monocyte colony-forming cells, do not respond to monocyte stimulation alone and require for their proliferation an inducing factor derived from phytohemagglutinin-exposed T-lymphocytes and monocytes.