Platelets are clearly central to thrombosis and hemostasis. In addition, more recently, evidence has emerged for non-hemostatic roles of platelets including inflammatory and immune reactions/responses. Platelets express immunologically relevant ligands and receptors, demonstrate adhesive interactions with endothelial cells, monocytes and neutrophils, and toll-like receptor (TLR) mediated responses. These properties make platelets central to innate and adaptive immunity and potential candidate key mediators of autoimmune disorders. Multiple sclerosis (MS) is the most common chronic autoimmune central nervous system (CNS) disease. An association between platelets and MS was first indicated by the increased adhesion of platelets to endothelial cells. This was followed by reports identifying structural and functional changes of platelets, their chronic activation in the peripheral blood of MS patients, platelet presence in MS lesions and the more recent revelation that these structural and functional abnormalities are associated with all MS forms and stages. Investigations based on the murine experimental autoimmune encephalomyelitis (EAE) MS model first revealed a contribution to EAE pathogenesis by exacerbation of CNS inflammation and an early role for platelets in EAE development via platelet-neuron and platelet-astrocyte associations, through sialated gangliosides in lipid rafts. Our own studies refined and extended these findings by identifying the critical timing of platelet accumulation in pre-clinical EAE and establishing an initiating and central rather than merely exacerbating role for platelets in disease development. Furthermore, we demonstrated platelet-neuron associations in EAE, coincident with behavioral changes, but preceding the earliest detectable autoreactive T cell accumulation. In combination, these findings establish a new paradigm by asserting that platelets play a neurodegenerative as well as a neuroinflammatory role in MS and therefore, that these two pathological processes are causally linked. This review will discuss the implications of these findings for our understanding of MS, for future applications for imaging toward early detection of MS, and for novel strategies for platelet-targeted treatment of MS.

The neuropsychiatric symptoms of multiple sclerosis (MS), such as anxiety and depression, can result from disease activity itself as well as psychological reaction to an unfavorable diagnosis. Accordingly, the literature reports evidence of increased anxiety-like behavior in experimental autoimmune encephalomyelitis (EAE), an accepted MS model. Due to the recently described critical role of platelets in inflammation and autoimmune disease, we examined the relationship between platelets, inflammation, and anxiety-like behavior in EAE. In the elevated plus maze, EAE-induced C57BL/6J mice showed decreased time spent in the open arms relative to vehicle-only controls, demonstrating an increase in anxiety-like behavior. This effect occurred in the presence of platelet–neuron association, but absence of lymphocytic infiltration, in the hippocampal parenchyma. Platelet depletion at the pre-clinical disease stage, using antibody-mediated lysis prevented the EAE-induced increase in anxiety-like behavior, while no significant difference in distance moved was recorded. Furthermore, platelet depletion was also associated with reduction of the pro-inflammatory environment to control levels in the hippocampus and prevention of EAE disease symptomology. These studies demonstrate the high efficacy of a platelet-targeting approach in preventing anxiety-like symptoms and clinical manifestations of EAE and have implications for the treatment of neuropsychiatric symptoms in MS.

Despite growing evidence for platelets as active players in infection and immunity, it remains unresolved whether platelets contribute to, or are key elements in the development of neuroinflammation. Using the experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis, we identified platelet accumulation in the circulation by 7-day postinduction (dpi), ahead of clinical onset which occurs at 13–14 dpi. By inducing platelet depletion between 7 and 16 dpi, we demonstrate an association between platelet accumulation in the spinal cord and disease development. Additionally, we provide evidence for platelet infiltration in the white and gray matter parenchyma, but with different outcomes. Thus, while in white matter platelets are clearly associated with lesions, in gray matter large-scale platelet infiltration and expression of the platelet-specific molecule PF4 are detectable prior to T cell entry. In the retina, platelet accumulation also precedes clinical onset and is associated with significant increase in retinal thickness in experimental relative to control animals. Platelet accumulation increases over the disease course in this tissue, but without subsequent T cell infiltration. These findings provide definitive confirmation that platelet accumulation is key to EAE pathophysiology. Furthermore, they suggest an undescribed and, most importantly, therapeutically targetable mechanism of neuronal damage.

Multiple sclerosis (MS) is an autoimmune disease targeting the central nervous system, characterized by an unpredictable disease course and a wide range of symptoms. Emotional and cognitive deficits are now recognized as primary disease manifestations and not simply the consequence of living with a chronic condition, raising questions regarding the efficacy of current therapeutics for these specific symptoms. Mechanisms underlying psychiatric sequelae in MS are believed to be similar to those underlying pathogenesis, that is mediated by cytokines and other inflammatory mediators. To gain insight into the pathogenesis of MS depression, we performed behavioral assays in the murine experimental autoimmune encephalomyelitis (EAE) MS model, in the presence or absence of immunomodulation using the drug FTY720, an analogue of the lipid signaling molecule sphingosine-1-phosphate (S1P). Specifically, mice were challenged with the elevated plus maze (EPM) test, a validated experimental paradigm for rodent-specific anxiety-like behavior. FTY720 treatment failed to ameliorate anxiety-like symptoms, irrespective of dosage. On the other hand, it was effective in reducing inflammatory infiltration, microglial reactivity and levels of pro-inflammatory molecules in the hippocampus, confirming the anti-inflammatory capacity of treatment. To explore the absence of FTY720 effect on behavior, we confirmed expression of S1P receptors (S1PR) S1PR1, S1PR3 and S1PR5 in the hippocampus and mapped the dynamics of these receptors in response to drug treatment alone, or in combination with EAE induction. We identified a complex pattern of responses, differing between (1) receptors, (2) dosage and (3) hippocampal sub-field. FTY720 treatment in the absence of EAE resulted in overall downregulation of S1PR1 and S1PR3, while S1PR5 exhibited a dose-dependent upregulation. EAE induction alone resulted in overall downregulation of all three receptors. On the other hand, combined FTY720 and EAE showed generally no effect on S1PR1 and S1PR3 expression except for the fimbrium region, but strong upregulation of S1PR5 over the range of doses examined. These data illustrate a hitherto undescribed complexity of S1PR response to FTY720 in the hippocampus, independent of drug effect on effector immune cells, but simultaneously emphasize the need to explore novel treatment strategies to specifically address mood disorders in MS.