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Characterization of Fc gamma receptors on human megakaryocytes
Z Wu1, B Markovic, C N Chesterman
1Centre for Thrombosis and Vascular Research, University of New South Wales, Randwick, Australia.
Abstract:
Megakaryocyte and platelet Fc gamma receptors (FcR) are of importance in the pathophysiology of immune complex-mediated thrombocytopenias such as heparin-induced thrombocytopenia. In this study, Fc gamma R proteins and mRNAs in normal human megakaryocytes were examined. Fc gamma R proteins were studied with immunocytochemical staining, dual colour flow cytometry and immunoprecipitation using monoclonal antibodies against Fc gamma R I, Fc gamma R II and Fc gamma R III. Fc gamma R mRNAs were measured with biotinylated cDNA of oligonucleotide probes using a novel quantitative in situ hybridization technique. Using these techniques, Fc gamma R II protein and mRNA, but not Fc gamma R I and Fc gamma R III proteins and transcripts were detected in megakaryocytes. Further, transcript analysis showed that megakaryocytes contain only the transcript of Fc gamma R IIA gene but no transcripts of Fc gamma R IIB nor Fc gamma R IIC genes; Fc gamma R IIA transcripts with and without the transmembrane (TM) exon are present in approximately equal proportions. In contrast, neutrophils and macrophages also contain Fc gamma R IIA transcript but Fc gamma R IIA transcript with the TM exon predominates suggesting cell lineage-specific Fc gamma R IIA expression. Fc gamma R IIA transcript lacking the TM exon predicts the presence of a potential soluble form of Fc gamma R in platelets and megakaryocytes which may have a physiological role as it can compete with the membrane-bound Fc gamma R IIA for binding of IgG-containing immune complexes and thus protect these cells from excessive binding and injurious effects of immune complexes.
Insights
Megakaryocytes express Fc gamma receptor IIA (FcγRIIA) protein and mRNA, but not FcγRI or FcγRIII. This FcγRIIA expression may protect platelets and megakaryocytes from immune complex-mediated damage.
Area of Science:
- Immunology
- Hematology
- Molecular Biology
Background:
- Fc gamma receptors (FcR) on megakaryocytes and platelets are crucial in immune complex-mediated thrombocytopenias.
- Heparin-induced thrombocytopenia is a significant clinical condition involving FcR.
Purpose of the Study:
- To investigate the presence and characteristics of Fc gamma receptor (FcR) proteins and mRNAs in human megakaryocytes.
- To understand the cell lineage-specific expression of Fc gamma R IIA.
Main Methods:
- Immunocytochemical staining, dual-color flow cytometry, and immunoprecipitation were used to detect Fc gamma R proteins.
- Quantitative in situ hybridization with biotinylated cDNA probes measured Fc gamma R mRNAs.
- Monoclonal antibodies against Fc gamma R I, II, and III were utilized.
Main Results:
- Fc gamma R II protein and mRNA were detected in megakaryocytes, while Fc gamma R I and Fc gamma R III were not.
- Megakaryocytes exclusively express the Fc gamma R IIA gene transcript, with roughly equal proportions of transcripts containing or lacking the transmembrane (TM) exon.
- Neutrophils and macrophages predominantly express Fc gamma R IIA transcripts with the TM exon.
Conclusions:
- Megakaryocytes express Fc gamma R IIA, with transcripts existing in forms that can lead to both membrane-bound and soluble receptors.
- The presence of Fc gamma R IIA transcripts lacking the TM exon suggests a potential soluble Fc gamma R in platelets and megakaryocytes.
- This soluble form may play a physiological role by competing with membrane-bound Fc gamma R IIA for immune complex binding, protecting cells from injury.