Platelet-lymphocyte cross-talk in atherogenesis : experimental studies on platelet-regulated lymphocyte adhesion and CD4+ T cell activation

Abstract: Atherosclerosis is an inflammatory and thrombotic disease. Platelets and lymphocytes, especially CD4+ T cells, are indispensable players during the initiation and progression of atherosclerotic lesion formation. During the early stage of atherogenesis, lymphocytes are generally thought to be recruited from arterial blood flow. It is, however, not clear how they are recruited. Platelets can regulate various functional aspects of CD4+ T cells, but the mechanisms of platelet-CD4+ T cell interactions have not be well defined. Therefore, aims of the present thesis work are to investigate how platelets influence lymphocyte adhesion under arterial flow conditions, and how platelets regulate CD4+ T cell functions. Platelet-supported lymphocyte adhesion was first investigated using reconstituted human blood and a collagen-coated parallel-plate flow chamber. Adhered platelets supported lymphocyte adhesion under arterial flow conditions (e.g., 500 s-1), which involved multiple adhesion molecules (e.g., P-selectin, CD40L, and GPIIb/IIIa). Platelet-dependent lymphocyte adhesion was selective among larger CD4+ and CD8+ T cells, and small B cells. In vivo model of arterial thrombosis confirmed that arterial thrombi supported lymphocyte recruitment, and that platelet GPIIb/IIIa blockade prevented thrombus formation and subsequently abolished lymphocyte recruitment. We continued to study how platelets enhance lymphocyte adhesion on subendothelial matrix protein (SEMP)-coated surface under arterial flow conditions by using Cone and Plate(let) analyser (CPA). Collagen markedly, fibrinogen and VWF mildly provoked platelet deposition that proportionally enhanced lymphocyte adhesion. The data confirmed that platelets selectively enhance adhesion of large CD4+ and CD8+ T cells and NK cells, and of small B cells. The lymphocyte adhesion positively correlates with their platelet conjugating potential and expression of PSGL-1, Mac-1, and CD40L. Platelet-regulated CD4+ T cell function was investigated in autologous human platelet-CD4+ T cell co-cultures. Platelets attenuated CD4+ T cell proliferation, but enhanced activation of Th1/Th17/Treg cells. The enhancements were exerted by both direct cell-cell contact and platelet-derived soluble mediators PF4, RANTES, and TGF?. The enhancements were shown as increased CD4+ T effector cell phenotypes and corresponding cytokine production. Dynamics of platelet-regulated CD4+ T cell activation were also monitored. Platelets constantly promoted Treg cell activation, but exerted a biphasic regulation on Th1/Th17 activation, namely a transient enhancement followed by a secondary suppression. The distinct regulations were achieved by a selective, TGF?-mediated inhibition of FoxP3– T cell proliferation. Together, the thesis work has elucidated novel mechanisms of platelet-regulated CD4+ T effector cell responses. Platelets selectively enhance lymphocyte adhesion under arterial flow conditions, and regulate distinct dynamics of Th1/Th17/Treg cell activation.