Expression of complement components and inhibitors on platelet microparticles

Abstract

Platelet microparticles (PMP) are released from activated platelets and play an important role in hemostasis, thrombosis and inflammation. Since platelets were recently found to demonstrate an intrinsic capacity for activating both classical and alternative pathways of the complement system, the present study extended these observations to PMP. PMP were generated by treating platelets with 10 µM A23187 (37°C, 5 min). PMP were identified by flow cytometry, based on size, Annexin V binding, and expression of P-selectin and GPIIb (CD41). PMP expressed gC1qR/p33, a multifunctional cellular protein that was recently described to activate the classical complement cascade. PMP also expressed the classical pathway and contact system regulator, CI inhibitor (C1-INH), as well as CD55 and CD59. Despite C1-INH expression, PMP supported classical pathway C4 activation in the presence of purified C1 and C4. Moreover, statistically significant deposition of C3b and C5b-9 was detected on PMP exposed to plasma, concurrently with expression of CD55 and CD59. These data provide the first evidence for the ability of PMP to support in situ complement activation. Complement activation contributes to a variety of vascular and inflammatory disease states including atherosclerosis and ischemia/reperfusion injury.

Microparticles (Ectosomes) Shed by Stored Human Platelets Downregulate Macrophages and Modify the Development of Dendritic Cells

“…and further deposition of C4 and C3 fragments as well as C5b-C9 complexes (24,39,40). These MP appear to be equivalent in size and AnV expression to the ectosomes described in this work.…”

“…We confirmed and extended this observation by demonstrating immune adherence of PLT-Ect to erythrocytes in a whole blood system. Evidently, the many molecules capable of blocking complement activation present on PLT-Ect [C1inh (41), FH (27), CD55 (24)] are insufficient to inhibit efficient C3b deposition on PLT-Ect and its binding to CR1 on erythrocytes. The CD35/CR1 receptor for C3b of erythrocytes has already been shown to bind complement-activating bacteria, viruses, and immune complexes, and for immune complexes to deliver them to the fixed macrophages of liver and spleen (42).…”

Coagulation and innate immune responses: can we view them separately?

“…80 It is likely that platelets and procoagulant plateletderived microparticles focus complement and TF to sites of injury, recruiting immune cells to that region, for a rapid and highly localized inflammatory and procoagulant response. 81 TLR2 expression on platelets can be activated by bacteria, which promotes platelet adhesion/aggregation with release of reactive oxygen species and expression of P-selectin. 82 P-selectin has several procoagulant and proinflammatory properties.…”

Megakaryocyte-derived microparticles: direct visualization and distinction from platelet-derived microparticles

“…Megakaryocyte-derived microparticles may be less capable of supporting inflammatory responses and complement activation compared with those derived from activated platelets. 55,56 The ability to distinguish PMPs from megakaryocytederived microparticles could be useful in evaluating the effects of disease states on microparticle levels and for monitoring platelet microparticles for diagnostic and prognostic purposes. Future comparisons of the 2 microparticle populations will assess these possibilities.…”