Characterization of Fc_γ Receptors on Human Megakaryocytes

Summary

Megakaryocyte and platelet Fc_γ receptors (FcR) are of importance in the pathophysiology of immune complex-mediated thrombocytopenias such as heparin-induced thrombocytopenia. In this study, Fc_γR proteins and mRNAs in normal human megakaryocytes were examined. Fc_γR proteins were studied with immunocytochemical staining, dual colour flow cytometry and immunoprecipitation using monoclonal antibodies against Fc_γR I, Fc_γR II and Fc_γR III. Fc_γR mRNAs were measured with biotinylated cDNA or oligonucleotide probes using a novel quantitative in situ hybridization technique. Using these techniques, Fc_γR II protein and mRNA, but not Fc_γR I and Fc_γR III proteins and transcripts were detected in megakaryocytes. Further, transcript analysis showed that megakaryocytes contain only the transcript of Fc_γR IIA gene but no transcripts of Fc_γR IIB nor Fc_γR IIC genes; Fc_γR IIA transcripts with and without the transmembrane (TM) exon are present in approximately equal proportions. In contrast, neutrophils and macrophages also contain Fc_γR IIA transcript but Fc_γR IIA transcript with the TM exon predominates suggesting cell lineage-specific Fc_γR IIA expression. Fc_γR IIA transcript lacking the TM exon predicts the presence of a potential soluble form of Fc_γR in platelets and megakaryocytes which may have a physiological role as it can compete with the membrane-bound Fc_γR IIA for binding of IgG-containing immune complexes and thus protect these cells from excessive binding and injurious effects of immune complexes.

• References

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