Studies of Fc.gamma. receptors of human B lymphocytes: phospholipase A2 activity of Fc.gamma. receptors

Abstract

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 EDTA 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. 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. The identity of PC- and IgG-binding materials is demonstrated, and a functional activity of the human B cell Fc.gamma. receptor is apparently the generation of phospholipase A2 activity within the plasma membrane.