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: Platelets are components of the blood that are highly reactive, and they quickly respond
to multiple physiological and pathophysiological processes. In the last decade, it became clear that
platelets are the key components of circulation, linking hemostasis, innate, and acquired immunity.
Protein composition, localization, and activity are crucial for platelet function and regulation. The
current state of mass spectrometry-based proteomics has tremendous potential to identify and quantify thousands of proteins from a minimal amount of material, unravel multiple post-translational
modifications, and monitor platelet activity during drug treatments. This review focuses on the role
of proteomics in understanding the molecular basics of the classical and newly emerging functions
of platelets. including the recently described role of platelets in immunology and the development
of COVID-19.The state-of-the-art proteomic technologies and their application in studying platelet
biogenesis, signaling, and storage are described, and the potential of newly appeared trapped ion
mobility spectrometry (TIMS) is highlighted. Additionally, implementing proteomic methods in
platelet transfusion medicine, and as a diagnostic and prognostic tool, is discussed.
Abstract
Background
Heparin induced thrombocytopenia (HIT) is likely a misdirected bacterial host defense mechanism. Platelet factor 4 (PF4) binds to polyanions on bacterial surfaces exposing neo‐epitopes to which HIT antibodies bind. Platelets are activated by the resulting immune complexes via FcγRIIA, release bactericidal substances, and kill Gram‐negative Escherichia coli.
Objectives
To assess the role of PF4, anti‐PF4/H antibodies and FcγRIIa in killing of Gram‐positive bacteria by platelets.
Methods
Binding of PF4 to protein‐A deficient Staphylococcus aureus (SA113Δspa) and non‐encapsulated Streptococcus pneumoniae (D39Δcps) and its conformational change were assessed by flow cytometry using monoclonal (KKO,5B9) and patient derived anti‐PF4/H antibodies. Killing of bacteria was quantified by counting colony forming units (cfu) after incubation with platelets or platelet releasate. Using flow cytometry, platelet activation (CD62P‐expression, PAC‐1 binding) and phosphatidylserine (PS)‐exposure were analyzed.
Results
Monoclonal and patient‐derived anti‐PF4/H antibodies bound in the presence of PF4 to both S. aureus and S. pneumoniae (1.6‐fold increased fluorescence signal for human anti‐PF4/H antibodies to 24.0‐fold increase for KKO). Staphylococcus aureus (5.5 × 104cfu/mL) was efficiently killed by platelets (2.7 × 104cfu/mL) or their releasate (2.9 × 104cfu/mL). Killing was not further enhanced by PF4 or anti‐PF4/H antibodies. Blocking FcγRIIa had no impact on killing of S. aureus by platelets. In contrast, S. pneumoniae was not killed by platelets or releasate. Instead, after incubation with pneumococci platelets were unresponsive to TRAP‐6 stimulation and exposed high levels of PS.
Conclusions
Anti‐PF4/H antibodies seem to have only a minor role for direct killing of Gram‐positive bacteria by platelets. Staphylococcus aureus is killed by platelets or platelet releasate. In contrast, S. pneumoniae affects platelet viability.