NA-PHENOTYPE-DEPENDENT DIFFERENCES IN NEUTROPHIL FC-GAMMA-RIIIB EXPRESSION CAUSE DIFFERENCES IN PLASMA-LEVELS OF SOLUBLE FC-GAMMA-RIII

Soluble Fc gamma RIII in plasma is primarily derived from neutrophils and is a measure of the total body neutrophil mass. We have developed a new, sensitive 'sandwich' ELISA to measure soluble Fc gamma RIII in plasma and released Fc gamma RIII in cell supernatants. Both sFc gamma RIIIa, derived from NK cells and sFc gamma RIIIb, derived from neutro phils are detected in the assay. However, plasma analysis of Fc gamma RIIIB gene-deficient donors suggested that sFc gamma RIIIa contributes only marginally to the total amount measured in healthy individuals. Furthermore, we observed that plasma of homozygous NA1-positive donors contained lower amounts of sFc gamma RIII than plasma of homozygous N A2-positive donors. Heterozygous donors were found to have intermediat e levels of sFc gamma RIII in their plasma. Hemizygous Fc gamma RIIIB gene-deficient donors were found to have half the amount of sFc gamma RIII in their plasma compared to donors with two Fc gamma RIIIB allele s. These NA phenotype-dependent differences in plasma sFc gamma RIII c ould not be contributed to either an assay artefact or NA-dependent di fferences in shedding of Fc gamma RIIIb upon neutrophil activation. Ca libration curves constructed with plasma of homozygous donors did not reveal NA-dependent differences in antibody affinity. Measurement of r eleased Fc gamma RIIIb in supernatants of neutrophils stimulated with PMA, and inhibition of this signal with human IgG revealed no NA-depen dent differences. However, NA-dependent differences in neutrophil Fc g amma RIIIb expression were present, comparable to the differences foun d in plasma levels of sFc gamma RIII. Differences in the amounts of re leased Fc gamma RIII in supernatants of NA-typed apoptotic neutrophils were similar to initial differences in Fc gamma RIIIb expression, aga in being lower in NA1-positive than in heterozygous and NA2-positive d onors. In conclusion, NA-dependent differences in plasma levels of sol uble Fc gamma RIII seem to be caused by differences in expression of t he receptor on the neutrophil membrane.