Characterisation of Platelet Releasate Proteome in Relapsing‐Remitting Multiple Sclerosis Reveals Dysregulation of Inflammatory Signalling and Extracellular Vesicle Dynamics

ABSTRACT Purpose Multiple Sclerosis is an inflammatory neurodegenerative disease characterised by blood‐brain barrier dysfunction and leukocyte infiltration into the CNS. Platelets are best known for their contributions to haemostasis, however, upon activation, platelets release an abundance of soluble and vesicular‐associated proteins, termed the platelet releasate (PR). This milieu contains numerous inflammatory and vasoactive proteins, that can attract leukocytes and alter endothelial permeability. Experimental design We aimed to characterise the PR of Relapsing‐Remitting multiple sclerosis (RRMS) patients, previously characterized regarding thrombin generation dynamics compared to healthy controls. We carried out LFQ proteomic profiling of the PR from 15 RRMS and 19 aged‐matched healthy controls. Results We identified 9 proteins increased and 16 proteins decreased in the PR of RRMS patients. Platelet/endothelial cell‐adhesion molecule‐1 (PECAM‐1) was uniquely found in healthy control PR and circulating levels of PECAM‐1 were significantly lower in RRMS patient samples. GO analysis revealed a strong link between altered proteins and extracellular vesicles (EVs). Small EV levels were significantly reduced in RRMS PR compared to healthy PR and showed a negative correlation with PECAM‐1 levels in RRMS plasma. Conclusions and clinical relevance Our findings suggest that platelet reactivity may be linked to disease activity, even in periods of disease remission.