Circulating microparticles. Markers or participants in carotid artery remodelling?

Microparticles initially referred to as ‘cell dust’ were identified more than 40 years ago. Microparticles from different cell types can be identified and consist of a number of subtypes depending on how they are generated. Microparticles called exosomes originate from multivesicular bodies and are released during exocytosis of endocytic bodies, when they fuse with the cell membrane; ectosomes originate from the cell membrane surface. Blood contains a large array of microparticles-derived from different cell types. Platelet-derived microparticles are the most abundant representing 70–90% of all circulating microparticles [1] but microparticles-derived from red cells, granulocytes, monocytes, lymphocytes, mast cells, endothelial cells and vascular smooth muscle cells can also be detected. Increased levels of circulating microparticles have been associated with various physiological and pathological conditions including altered vascular function, vascular remodelling, cardiovascular disease, autoimmunity and cancer. They are detected, quantified and their cellular origin defined using either solid phase capture assays or flow cytometry [2]. These methods rely on antibody detection of specific cellular markers such as CD11a for leucocytes and CD3 for T lymphocytes and frequently annexin V binding of phosphatidylserine. Those originating from apoptotic cells can be identified by their DNA and RNA content [3]. Depending on their composition, microparticles presence in blood can indicate specific cell activation or damage. In addition, microparticles may exert pleiotropic effects on vascular cells. Those from platelets or endothelial cells have procoagulant and proinflammatory activities [4,5] as well as being associated with acute coronary syndromes [6]. Leucocyte-derived microparticles can be markers of cardiovascular diseases and also contribute to pathogenesis. For example, ectosomes released by neutrophils bind to endothelial cells stimulating cytokine release and induction of tissue factor [7]. Microparticles released by T lymphocytes impair endothelial function [8] whereas those derived from mast cells deliver peripheral cytokine signals to remote lymph nodes, facilitating communication between peripheral sites of inflammation and remote secondary lymphoid tissues [9]. Microparticles are being intensely investigated in various cardiovascular disorders including hypertension, atherosclerosis and myocardial infarction. In this issue of the journal [10], rather than focusing on microparticles associated with disease, the authors examine whether microparticles might be associated with early vessel remodelling, before development of significant atherosclerosis. The authors examine associations between microparticles-derived from platelets, endothelium and leucocytes and carotid artery remodelling reflected by alterations in vessel circumference and intima–media thickness (IMT) in a cohort of untreated asymptomatic individuals in whom atherosclerosis could not be detected. A thin intima contains relatively few smooth muscle cells and is considered resistant to atherosclerosis, either not progressing, progressing very slowly or only progressing in humans with very high plasma levels of atherogenic proteins [11], whereas thick intimas rich in smooth muscle cells are considered progression prone. Using the markers Gp1b, CD105 and CD11a to distinguish microparticles-derived from platelets, endothelial cells and leucocytes, respectively, the authors demonstrate a positive association of endothelial-derived microparticles with common carotid IMT to lumen ratio for the whole population. Although this association was not experimentally examined further in relation to questions as to whether the microparticles were derived from activated or functionally impaired endothelium, impaired endothelial cell function reflected by reductions in endothelial nitric oxide synthase expression is known to increase carotid IMT to lumen ratios [12]; nitric oxide is a potent inhibitor of both smooth muscle cell migration and proliferation [13]. More detailed statistical analyses using multivariate models provided somewhat unexpected, interesting and potentially important associations between microparticles-derived from platelets, endothelial cells and leucocytes and vessel remodelling. Subdivision of IMT values into tertiles uncovered the relationship that individuals with IMT values in the highest tertile have common carotid arteries of smaller internal diameter when plasma levels of microparticles are high. This was significant for microparticles-derived from platelets, endothelial cells and leucocytes. Similar associations were found between the microparticles from platelets and leucocytes and common carotid external diameter in individuals with IMT in the highest tertile. In other words carotid artery diameter correlated negatively with microparticles-derived from platelets, endothelial cells and leucocytes whereas external carotid artery diameter correlated negatively with platelet and leucocyte-derived microparticles. Previous studies have reported an increase in common carotid artery diameter associated with increased IMT and suggested that with increasing IMT there is compensatory outward remodelling to maintain shear stress at the interface between blood and the endothelium constant [14]. It should be pointed out that vessel dilatation rather than true remodelling cannot be excluded in this study as the vessels could not be maximally dilated prior to the ultrasound measurements. Also, a time-dependent longitudinal study rather than the cross-sectional study design used by the authors would be necessary to definitively demonstrate remodelling. However, it is not unreasonable to assume vessel remodelling occurs as IMT increases, even in normal vessels, as it is known to occur in atherosclerotic arteries with large IMTs. Remodelling has been associated with variables such as blood pressure, hypercholesterolemia and age but these have been taken into account in their multivariate analyses. Consequently, the authors propose that circulating microparticles are the major factor associated carotid artery remodelling with high circulating levels of microparticles preventing compensatory remodelling in vessels with high IMT values. This interesting study raises two important questions with respect to microparticles. Are the microparticles merely reflecting the functional status of platelets, endothelial cells and leucocytes in individuals with large carotid artery intimas and/or do they also contribute to remodelling of arteries with large IMT? A more detailed investigation as to the types of platelet-derived particles and their composition would have provided greater insight into these questions but the size of the clinical study apparently prevented such investigations. Consequently one can only speculate as to how these microparticles might be associated with remodelling of vessels with high IMT values. Shear stress increases with luminal narrowing. Shear stress on platelets adhering to endothelium contributes to microparticle formation by platelets [15], generating up to four size classes [16]. The smallest size microparticles do not only inhibit thrombus formation but also contain growth factors such as transforming growth factor-β1 and chemokine (C-C motif) ligand 5, which can affect vessel remodelling [16]. Microparticles from endothelial cells can be triggered by both apoptotic and activating signals. Impaired nitric oxide production has also been associated with microparticle production by endothelial cells [17], and as mentioned earlier this may influence intima growth and vessel remodelling. Thus the increase in platelet and endothelial-derived microparticles may not only represent platelet and endothelial cell activation but their microparticles could also participate in vessel remodelling. Leucocyte microparticles are generally generated by activated leucocytes and it is highly probable that their elevation in plasma of individuals with IMT in the highest tertile represents increased leucocyte activation. Leucocyte-derived microparticles can also participate in vessel remodelling by interacting with the endothelium. They are known to bind and be internalized by endothelial cells and have previously been implicated in vascular remodelling [18]. This association of leucocyte-derived microparticles with common carotid artery remodelling suggests that in humans leucocytes play a significant role in the very early stages development of atherosclerotic intima, once the nonatherosclerotic intima has reached a critical size. Small numbers of monocytes are known to accumulate in diffuse intimal thickenings before there is any evidence of developing atherosclerosis. It is interesting to note that circulating leucocyte-derived microparticles predict subclinical atherosclerosis burden in asymptomatic patients [19]. Although endothelial-derived microparticles have not been similarly studied, their elevation in plasma has been associated with clinically significant atherosclerosis [20]. A growing body of evidence indicates that microparticles may not only reflect the functional status of various cell types, that is, whether they are activated or undergoing apoptosis, but that they may also contribute to development of various cardiovascular disorders. The study by Chironi et al. [10] suggest that platelets, endothelial cells and leucocytes may be activated in patients with large IMTs and attenuate compensatory outward carotid artery remodelling. Further examination of these microparticles, in particular their composition will help define how they are generated and may also provide novel insights as to how they attenuate outward vessel remodelling before development of clinically detectable atherosclerotic lesions.