Page 96 - Haematologica May 2020

M. Chimen et al.
vascular inflammation. This process means that circulat- ing monocytes may be recruited to the vessel wall through a pathway outside of the tightly regulated physiological inflammatory system. We believe that such monocyte recruitment may be particularly relevant in the dysregulat- ed inflammatory responses seen in chronic inflammatory disease, which leads to tissue damage and loss of function (such as atherosclerosis and rheumatoid arthritis). In addi- tion, it may be important in inflammation associated with severe trauma, where the drivers of inflammation are sub- stantial and acute tissue damage, and extensive activation of the coagulation and haemostatic pathway. However, we believe that during acute responses initiated by inflam- matory cytokines in a coordinated and controlled manner, and where timely and comprehensive resolution is the norm, platelet-mediated pathways of leukocyte trafficking are likely to be of lesser importance.
Other studies show that whole platelets can bind leuko- cytes, a process dependent upon platelet and/or leukocyte activation and linked to pathological conditions.46 Moreover, if PEV are mixed with isolated monocytes they are able to activate the leukocytes so that they show enhanced levels of recruitment to EC in vitro, although
direct binding between PEV and leukocytes was not demonstrated in that study.47 Here, we show that mono- cytes preferentially accumulate PEV rather than whole platelets through an adhesive pathway reliant upon P-selectin. In the context of leukocyte recruitment to vas- cular EC, P-selectin supports a distinct form of rolling adhesion which is based on the transient nature of the bonds formed with PSGL-1 under conditions of shear.48 Here we propose that the P-selectin-PSGL-1 mediated interactions between PEV and leukocytes are also tran- sient under the shear conditions of our assay and in flow- ing blood in vivo. However, on monocytes and neutrophils, PS in the PEV membrane acts to stabilise heterotypic adhesion upon interaction with membrane receptors on the leukocytes. In the case of T lymphocytes, which also possess abundant PSGL-1, the transient interactions formed with P-selectin under shear are not stabilised by PS, which has not been reported to bind T cells to our knowledge.
In fact, much of the data on heterotypic aggregate for- mation in human blood does not discriminate between platelets and PEV binding to leukocytes, and it is unclear which is being assessed. Studies that do report platelet
ABC
DE
Figure 6. GPIbα derived from platelet-derived extracellular vesicles supports monocytes recruitment on TGF-β1 stimulated endothelial cells. (A-C) Representative pictures of monocytes (A), monocytes bearing CRP_XL (1 mg/mlL) generated-PEV (B) and monocytes bearing PEV with GPIbα blockade (clone 6B4, 20 mg/mL) (C), adhered on TGF-β1 (10 ng/mL) stimulated EC in flow conditions. (D, E) Total adhesion (D) and transmigration (E) of monocytes with or without PEV, GPIbα blockade and filtered PEV through a 10 KDa filters to remove PEV and leave potential soluble factors on TGF-β1 stimulated EC in flow conditions, n=3-5. Data are mean ± standard error of the mean (SEM). *P≤0.05, **P≤ 0.01 by ANOVA and Bonferroni post-test.
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