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NA‐phenotype‐dependent differences in neutrophil FcγRIIIb expression cause differences in plasma levels of soluble FcγRIII

Soluble FcγRIII 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 FcγRIII in plasma and released FcγRIII in cell supernatants. Both sFcγRIIIa, derived from NK cells and sFcγRIIIb, derived from neutrophils are detected in the assay. However, plasma analysis of FcγRIIIB gene‐deficient donors suggested that sFcγRIIIa 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 sFcγRIII than plasma of homozygous NA2‐positive donors. Heterozygous donors were found to have intermediate levels of sFcγRIII in their plasma. Hemizygous FcγRIIIB gene‐deficient donors were found to have half the amount of sFcγRIII in their plasma compared to donors with two FcγRIIIB alleles. These NA phenotype‐dependent differences in plasma sFcγRIII could not be contributed to either an assay artefact or NA‐dependent differences in shedding of FcγRIIIb upon neutrophil activation. Calibration curves constructed with plasma of homozygous donors did not reveal NA‐dependent differences in antibody affinity. Measurement of released FcγRIIIb in supernatants of neutrophils stimulated with PMA, and inhibition of this signal with human IgG revealed no NA‐dependent differences. However, NA‐dependent differences in neutrophil FcγRIIIb expression were present, comparable to the differences found in plasma levels of sFcγRIII. Differences in the amounts of released FcγRIII in supernatants of NA‐typed apoptotic neutrophils were similar to initial differences in FcγRIIIb expression, again being lower in NA1‐positive than in heterozygous and NA2‐positive donors. In conclusion, NA‐dependent differences in plasma levels of soluble FcγRIII seem to be caused by differences in expression of the receptor on the neutrophil membrane.