A New Perspective on Mechanisms of Neurodegeneration in Experimental Autoimmune Encephalomyelitis and Multiple Sclerosis: the Early and Critical Role of Platelets in Neuro/Axonal Loss.

Multiple sclerosis (MS) is a central nervous system (CNS) autoimmune disorder, with limited treatment options. This disease is characterized by differential pathophysiology between grey matter (GM) and white matter (WM). The predominant WM hallmark is the perivascular plaque, associated with blood brain barrier (BBB) loss of function, lymphocytic infiltration, microglial reactivity, demyelination and axonal injury and is adequately addressed with immunomodulatory drugs. By contrast, mechanisms underlying GM damage remain obscure, with consequences for neuroprotective strategies. Cortical GM pathology is already significant in early MS and characterized by reduced BBB disruption and lymphocytic infiltration relative to WM, but a highly inflammatory environment, microglial reactivity, demyelination and neuro/axonal loss. There is no satisfactory explanation for the occurrence of neurodegeneration without large-scale inflammatory cell influx in cortical GM. A candidate mechanism suggests that it results from soluble factors originating from meningeal inflammatory cell aggregates, which diffuse into the underlying cortical tissue and trigger microglial activation. However, the recent literature highlights the central role of platelets in inflammation, together with the relationship between coagulation factors, particularly fibrinogen, and tissue damage in MS. Using the experimental autoimmune encephalomyelitis (EAE) model, we identified platelets as drivers of neuroinflammation and platelet-neuron associations from the pre-symptomatic stage. We propose that fibrinogen leakage across the BBB is a signal for platelet infiltration and that platelets represent a major and early participant in neurodegeneration. This concept is compatible with the new appreciation of platelets as immune cells and of neuronal damage driven by inflammatory cells sequestered in the meninges.