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Abstract
Platelets are small anucleate blood cells with a life span of 7 to 10 days. They are main regulators of hemostasis. Balanced platelet activity is crucial to prevent bleeding or occlusive thrombus formation. Growing evidence supports that platelets also participate in immune reactions, and interaction between platelets and leukocytes contributes to both thrombosis and inflammation. The ubiquitin‐proteasome system (UPS) plays a key role in maintaining cellular protein homeostasis by its ability to degrade non‐functional self‐, foreign, or short‐lived regulatory proteins. Platelets express standard and immunoproteasomes. Inhibition of the proteasome impairs platelet production and platelet function. Platelets also express major histocompatibility complex (MHC) class I molecules. Peptide fragments released by proteasomes can bind to MHC class I, which makes it also likely that platelets can activate epitope specific cytotoxic T lymphocytes (CTLs). In this review, we focus on current knowledge on the significance of the proteasome for the functions of platelets as critical regulators of hemostasis as well as modulators of the immune response.
Abstract: The main purpose of new stent technologies is to overcome unfavorable material-related
incompatibilities by producing bio- and hemo-compatible polymers with anti-inflammatory and antithrombogenic properties. In this context, wettability is an important surface property, which has a
major impact on the biological response of blood cells. However, the influence of local hemodynamic
changes also influences blood cell activation. Therefore, we investigated biodegradable polymers
with different wettability to identify possible aspects for a better prediction of blood compatibility.
We applied shear rates of 100 s−1 and 1500 s−1 and assessed platelet and monocyte activation as
well as the formation of CD62P+ monocyte-bound platelets via flow cytometry. Aggregation of
circulating platelets induced by collagen was assessed by light transmission aggregometry. Via
live cell imaging, leukocytes were tracked on biomaterial surfaces to assess their average velocity.
Monocyte adhesion on biomaterials was determined by fluorescence microscopy. In response to
low shear rates of 100 s−1
, activation of circulating platelets and monocytes as well as the formation
of CD62P+ monocyte-bound platelets corresponded to the wettability of the underlying material
with the most favorable conditions on more hydrophilic surfaces. Under high shear rates, however,
blood compatibility cannot only be predicted by the concept of wettability. We assume that the
mechanisms of blood cell-polymer interactions do not allow for a rule-of-thumb prediction of the
blood compatibility of a material, which makes extensive in vitro testing mandatory.