THE PLASMA-CONCENTRATION OF SOLUBLE FC-GAMMA RIII IS RELATED TO PRODUCTION OF NEUTROPHILS

Fc gamma RIII (the CD16-antigen), a low-affinity receptor for IgG, is expressed by neutrophils, natural killer lymphocytes and macrophages. A soluble form of Fc gamma RIII has been identified in human plasma. T his soluble form of Fc gamma RIII (sFc gamma RIII) originates from rel ease by neutrophils. In the present study we show by transfusions of p lasma that contains sFc gamma RIII of one allotype (NA1-Fc gamma RIII) in recipients homozygous for the other allotype (NA2-Fc gamma RIII) t hat the clearance of sFc gamma RIII is about 0.7 ml/min. Because the c oncentration of sFc gamma RIII was found to be constant in a small coh ort of donors followed for about 1.5 years, the half-life of NA1-sFc g amma RIII is about 1.8 d, assuming a one-compartment model. The plasma concentration of sFc gamma RIII depended mainly on the production of neutrophils in the bone marrow, and was not influenced by shifts of ne utrophils from one pool to another (storage, marginating or circulatin g pool). Because Fc gamma RlII is only expressed on mature neutrophils , this implies that the concentration of sFc gamma RIII depends on pro duction of mature neutrophils.