BLOOD PLATELETS AS ACTIVATORS AND REGULATORS OF INFLAMMATORY AND IMMUNE REACTIONS. PART 2. THROMBOCYTES AS PARTICIPANTS OF IMMUNE REACTIONS

Abstract

Thrombocytes keep a leading role in conjugating thrombosis, inflammation and congenital immune responses. The platelets provide stable adhesion and interaction with immune cells. Activated platelets express CD40L (CD154), a membrane glycoprotein of tumor necrosis factor (TNF) family. Hence, the platelets are the main source of sCD40L in blood plasma. Platelet CD154 may interact with CD40 receptor on endothelial cells, causing an inflammatory response, and enhancing production of immunoglobulins by B-lymphocytes. Membrane and soluble CD154 of platelets combined with other signals can induce maturation and activation of dendritic cells (DC). The platelets possess functional receptors, e.g., TLR2, TLR4, TLR7 and TLR9 they also bear Fc-receptors, including FcγRIIA, FcεRI and FcαRIA. FcγRIIA on platelets mediate protection against bacteria. Cross-linking of FcαRI on platelets results in production of prothrombotic and pro-inflammatory mediators such as tissue factor and IL-1β. Activation of platelets via FcεR1 causes release of chemokine RANTES and serotonin, which contribute to the pro-inflammatory response of other immune cells. Platelets possess receptors for activated complement components and its fragments (CR2, CR3, CR4, C1q, C1 inhibitor and factors D and H). Activated platelets trigger the complement system through the release of protein kinases and ATP, and also by phosphorylation of C3 and C3b. α-granules of platelets contain chemokines which represent the most numerous group of antimicrobial proteins of platelets (kinocidins), and there is an antimicrobial protein of the defensin family – hBD-1 in the cytoplasm of platelets. Ligand and receptor of the TNF superfamily (TRAIL and LIGHT), the SDF-1 chemokine (CXCL12), the IL-1βinterleukins, IL-8 and the soluble IL-6 receptor (sRIL-6) are recognized as platelet products belonging to the family of cytokines and their receptors. The HMGB-1 protein classified as an inflammatory cytokine, is expressed by activated platelets and causes formation of the extracellular traps by neutrophils. Platelets produce numerous growth factors, including EGF-α and EGF-β1, EGF-β2, TGF-α and TGF-β1, TGF-β2, PDGF, HGF, FGF-β, IGF, pro- and antiangiogenic factors, e.g., VEGF-F and angiopoietins Ang-1 and Ang-2. Fulfillment of immune functions by the platelets is carried out by their interaction with leukocytes, which are attracted to the site of infection and inflammation and retained during the development of an “immune thrombus” under conditions of high shear stress. Platelets can not only maintain and guide the immune response, but also initiate these events. They are able to present the antigen in the context of MHC class I molecules, and activate naїve CD8+T lymphocytes. Potential consequences of platelet interaction with neutrophils, monocytes, dendritic cells and lymphocytes are discussed in the review article.