Neutrophil emperipolesis as a modulator of platelet function

• Applicant: Julia Katharina Kuehn
• Subject Area: Immunology ; Hematology, Oncology ; Clinical Immunology and Allergology ; Rheumatology
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 540366457

Platelets are small blood cells that are essential for preventing bleeding and critical for immune defense. Circulating platelets form functional subpopulations with clinical implications: platelet subpopulations with a high prothrombotic capacity promote coagulation in cardiovascular events, whereas those with immunomodulatory functions exacerbate pathological inflammation in autoimmunity and sepsis. However, the mechanisms controlling the formation of platelet subpopulations remain largely undefined. Platelets originate from megakaryocytes (MKs) via thrombopoiesis. MK heterogeneity has been proposed to contribute to platelet heterogeneity, but evidence is scarce. An intriguing factor potentially contributing to platelet heterogeneity is a process termed emperipolesis. Initially observed in the 1970s as the histological appearance of neutrophils (Nφs) within MKs, the Nigrovic laboratory has recently identified emperipolesis as a novel type of cell-in-cell interaction in which live Nφs enter MKs within a specialized vacuole termed the emperisome and then exit this vacuole to enter the MK cytoplasm. Cytoplasmic Nφs transiently fuse with an MK organelle termed the demarcation membrane system, transferring lipids and proteins to MKs and giving rise to Nφ:MK hybrid platelets, before Nφs themselves exit the MK intact. The function of hybrid platelets is undefined. Evolutionarily conserved across every mammalian species studied to date, emperipolesis is common, occurring in 3-5% of MKs in healthy mice and humans, a frequency that more than doubles in sepsis, thrombocytopenia, and myeloproliferative neoplasms. These observations suggest that emperipolesis is relevant in normal physiology as well as in inflammatory and hematological diseases. Based on strong preliminary data, we propose here the hypothesis that emperipolesis-derived Nφ:MK hybrid platelets are pro-coagulant and pro-inflammatory compared with non-hybrid platelets, such that emperipolesis represents a novel mechanism for how MK heterogeneity shapes platelet heterogeneity. We outline two independent aims to investigate how emperipolesis modulates platelet phenotype (Aim I) and function (Aim II). Aim I will characterize the phenotype of emperipolesis-derived platelets by defining the content and kinetics of material transfer from Nφs to platelets using lattice light-sheet microscopy and isotope labeling tracers. Aim II will define the function of emperipolesis-derived platelets in hemostasis and their interaction with circulating Nφs. We predict the transfer of a carefully regulated subset of proteins and lipids during emperipolesis, resulting in a distinct phenotype and function for hybrid platelets. We expect to unravel a novel source of platelet heterogeneity, providing a solid foundation for future translational research critical for targeted therapies in platelet-related disorders.

• DFG Programme: WBP Fellowship
• International Connection: USA
• Host: Professor Dr. Peter Nigrovic