The requirements of clonogenic cells of B cell-type chronic lymphocytic leukemia (B CLL) for interleukin 2 (IL 2) were analyzed. Using the cells of five patients, we measured IL 2 receptor expression on the cell surface and the colony-forming abilities of the cells in response to IL 2. In four of the cases, significant percentages of the CLL cells expressed IL 2 membrane receptors (as assessed with the monoclonal antibody anti-Tac), indicative of their potential sensitivity to IL 2. Pure recombinant interleukin 2 (r-IL2) was added to colony cultures that also contained the lectin phytohemagglutinin (PHA) or the phorbol ester 12–0-tetradecanoylphorbol-13-acetate (TPA) to activate the CLL cells. Colony formation completely depended on the presence of r-IL 2 and PHA or TPA in culture, with the exception of one case, in which the addition of IL 2 was not required for colony growth in TPA-supplemented cultures. Twenty-five to fifty units of r-IL 2 per milliliter of culture medium provided optimal stimulation. Under these conditions, a linear relationship was observed between plated cell numbers and colony numbers formed. Morphological and immunologic analysis of colony cells indicated that these were monoclonal CLL cells that had matured toward plasmacellular lymphocytes and plasma cells.

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