Cell surface expression of HLA I molecules as a marker of young platelets

Abstract

Background Accurate identification of the proportion of young platelets is important to distinguish peripheral thrombocytopenia from a deficit in platelet production. Young platelets are defined by their higher RNA content and are often assessed as thiazole orange bright (TObright) by flow cytometry. In clinical practice, their proportion is estimated by automatic blood counter according to their greater RNA content, which identifies a so‐called immature platelet fraction (IPF). However, the detected IPFs are not strictly identical to the young TObright platelet population observed by flow cytometry. Objectives The aim of this study was to assess the reliability of HLA I/major histocompatibility I (MHC I) cell surface expression as a marker of young platelets. Methods The HLA I/MHC I expression was evaluated by flow cytometry after costaining blood with TO and antibodies directed against HLA I/MHC I molecules. Results We found that platelets with a higher expression of plasma membrane‐localized MHC I molecules displayed an increased TO staining and a higher content in ribosomal P‐antigen. Transfusion experiments in mice showed that the number of MHC I molecules expressed on the cell surface of young murine platelets decreased during platelet aging, reaching basal levels within 24 h. Finally, we demonstrated that for patients with thrombocytopenias, the identification of young platelets is better assessed by the flow cytometric determination of the level of HLA I expression than by TO staining or the use of hematological blood counter. Conclusion Overall, our results highlight the relevance of MHC I/HLA I expression as a valuable parameter to identify young platelets.

STAT3 and STAT5B Mutations in T/NK-Cell Chronic Lymphoproliferative Disorders of Large Granular Lymphocytes (LGL): Association with Disease Features

“…The overall (more pronounced) reduction of blood-cell counts observed for several myeloid and lymphoid populations in STAT3mutated vs. WT LGLL cases might be explained by the fact that common surface molecules shared by all the reduced hematopoietic cells could trigger cytotoxicity by clonal LGLs. In this sense, markers such as CD45, CD244, and HLA-I are present or expressed with higher intensity on the surface of neutrophils, eosinophils, dendritic cells, nonclassical monocytes, and NK cells [38][39][40][41][42] which could be potentially recognized by clonal LGLs, and trigger LGL-mediated cytotoxic mechanisms involved in the death/elimination of the target cells. Further studies are needed to confirm which of these or other markers shared by the different leukocyte-cell subsets decreased in T/NK-LGLL patients are potentially targeted by cytotoxic clonal T/NK-LGLs, particularly in STAT3-mutated cases, leading to LGLL-associated cytopenias.…”

Proteomics: A Tool to Study Platelet Function

“…Even though megakaryocytes are found to express both MHC class I and II molecules, the platelets proteome and experimental evidence show only active MHC class I presentation [ 59 , 63 , 64 ]. Interestingly, the declined level of MHC class I molecules on the surface could indicate platelet aging [ 65 ].…”

Structure and function of the ubiquitin‐proteasome system in platelets

“…Activated platelets form aggregates with T cells, which most likely establish immunological synapses between these two cell types . Although platelets mainly express denatured MHC class I heavy chains on their surface, which are adsorbed from plasma and cannot stimulate cytotoxic T lymphocytes (CTL)‐mediated cytotoxicity, especially young platelets express intact MHC class I . In addition, intact MHC molecules including β2‐microglobulin are stored in platelet α‐granules and can be translocated to the cell surface upon platelet activation .…”