The presence of phospholipase A2 activity within human B cell Fc gamma receptors was investigated. Lysate produced by detergent treatment of chronic lymphocytic leukemia cells that had 1% of the cells surface radioiodinated was subjected to affinity chromatography by using either rac-1-(9-carboxynonyl)-2-hexadecylglycero-3-phosphorylcholine-Sepharose (PC-Sepharose) or heat-aggregated human IgG-Sepharose 4B conjugate (IgG-Sepharose). The materials eluted from both adsorbants by ethylenediaminetetraacetate- or urea-containing buffer were further purified by gel filtration and isoelectric focusing in the presence of 6 M urea. Both isolated PC- and IgG-binding materials were homogeneous, when judged by gel filtration and isoelectric focusing, and had identical isoelectric points (pI = 6.5), peptide maps, and amino acid compositions. Furthermore, both preparations catalyzed equally the hydrolysis of phosphatidylcholine to release fatty acid from the 2 position. Optimal enzymatic activity depended on the presence of Ca2+, was maximal at pH 9.5, and was augmented by Fc gamma fragments. Both preparations specifically bound to the Fc portion of IgG and inhibited human antibody-coated erythrocyte rosette formation by peripheral mononuclear cells. Our data thus demonstrate the identity of PC- and IgG-binding materials and suggest that a functional activity of the human B cell Fc gamma receptor is the generation of phospholipase A2 activity within the plasma membrane.

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A case of diffuse well differentiated lymphocytic lymphoma with associated leukopenia, is described. The colony‐forming unit (CFU‐C) assay was used to quantitate myeloid precursor cells and to test for the presence of suppressor cells and/or serum factors inhibiting myeloid cell proliferation. Mild suppression of CFU‐C was noted with the patient's bone marrow cells in coculture studies with normal marrow. The patient's serum, when added to the culture of normal bone marrow cells, produced a marked decrease in CFU‐C. Treatment with combination chemotherapy (prednisone, cyclophosphamide, and vincristine) produced a clinical response with correction of the leukopenia. Post‐treatment serum did not produce a significant reduction in CFU‐C from normal bone marrow. Cancer 52:35‐38, 1983.