The pro-inflammatory potential of microparticles in red blood cell units

Microparticles (MPs) are submicron size fragments which are released from leucocytes, platelets (Plts), red blood cells (RBCs) and endothelial cells (Dey-Hazra et al., 2010). Although their role in disease is still not completely known, they have been theorised to be involved in inflammation, atherosclerosis, coagulation and tumour metastasis (Piccin et al., 2007; Italiano, et al., 2010; Grimshaw et al., 2011). Recently, MPs have been postulated to have a cyto-protective/anti-inflammatory effect in the transfused host (Sadallah et al., 2008; Morel et al., 2011). One of the difficulties of assessing MPs has been detection and isolation due to their small size. MPs have multiple names, typically relating to their relative size. The smallest MPs, which are membrane-derived, are called ectosomes with diameters <200 nm. Larger membrane derived MPs, with sizes in the 200–1200 nm range, are referred to as microvessicles. The collective group of MPs typically range anywhere from 100 to 1000 nm (Distler et al., 2006; Jy et al., 2010). Other particles similar to MPs include exosomes and apoptotic bodies, the former are by-products of exocytosis, and the later are released from apoptotic bodies, both typically in the same size range as ectosomes (Mathivanan et al., 2010). Careful analysis using flow cytometry can help identify MPs of varying sizes and the inclusion of sizing beads may help in identifying particles of the right size and determine the lower limit of detection for the flow cytometer (Lacroix et al., 2010). One of the major interests in MPs relates to adverse events in the transfused host, namely transfusion-related reactions where MPs have been theorised to play a key role (Jy et al., 2011; Belizaire et al., 2012; Kriebardis et al., 2012; Almizraq et al., 2013). Because MPs have been shown to contain pro-inflammatory activity, particularly neutrophil (PMN) priming, which has been linked to transfusion-related acute lung injury (TRALI), there has been interest in classifying and understanding their biology (Cardo et al., 2008; Jy et al., 2011). In addition, MPs have been reported to increase during storage of RBC units and are considered part of the ‘storage lesion’ (Kim-Shapiro et al., 2011). Two recent studies have looked at indirect mediators of PMN priming, particularly via P-selectin: P-selectin glycoprotein ligand-1 (PSGL-1) interaction and elevation of C-receptors and immunoglobulins, as well as elevation of CD11b in supernatant from RBC’s (Cardo et al., 2008; Jy et al., 2011). Pre-storage leucoreduction (LR) has been reported to decrease the risk of TRALI (Blumberg et al., 2010), which has been postulated to be due to a reduction in MPs (Sugawara et al., 2010). The objective of this article is to determine if the pro-inflammatory activity that accumulates during RBC storage resides in the MPs or in the microparticle-poor (MPP) supernatant. To this end, the cell-free supernatant of RBC units was separated into microparticle-rich (MPR) and the MPP-supernatant, and we hypothesise that MPs increase during routine RBC storage, contribute to the priming activity that accumulates in the supernatant of RBCs, and that pre-storage LR affects these processes.