Platelets Drive Inflammation and Target Gray Matter and the Retina in Autoimmune-Mediated Encephalomyelitis

Abstract

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.

Platelet Depletion is Effective in Ameliorating Anxiety-Like Behavior and Reducing the Pro-Inflammatory Environment in the Hippocampus in Murine Experimental Autoimmune Encephalomyelitis

“…To determine whether a direct relationship exists between platelet and MOG 35–55 -CD4 + accumulations, ICS was repeated in the presence of platelet depletion, which was induced from 7 dpi using an antibody against CD42b (or GP1bα). In our hands [25], this approach results in reduction in platelet numbers in all control and experimental groups by above 96%, as well as maintenance of low platelet numbers by repeated anti-CD42b administration every 48 h. Evaluation of MOG 35–55 -CD4 + accumulation at 14 dpi, showed significant reduction in blood, lymphoid organs and CNS tissues (Figure 1Bi–Biv) in the EAE-induced/platelet depleted group, relative to EAE-induced/isotype antibody-treated group. This group did not develop disease symptoms, as demonstrated by absence of clinical scores by experimental end point, whilst their isotype antibody-treated counterparts reached a mean clinical score of 2.25 ± 1.75 (Figure 1Ci).…”

“…A previous study from our laboratory showed a cause and effect relationship between platelet accumulation and subsequent clinical disease development [25]. Here, we confirm this relationship with (1) evidence of a temporal relationship between platelet accumulation in the circulation and that of MOG 35–55 -CD4 + cells; (2) the demonstration of a dependence for MOG 35–55 -CD4 + cell accumulation in the blood, lymphoid organs as well as the CNS, on prior platelet accumulation; and (3) absence of immune cell infiltration and clinical EAE with platelet depletion.…”

“…Additionally, a potential platelet-related neurodegenerative mechanism [24] was suggested by the demonstration that platelets form associations with neurons as well as astrocytes, via sialated gangliosides within lipid rafts, resulting in the release of proinflammatory cytokines from platelets, immune cascades, and EAE development. More recent studies by our own group [25] identified platelet accumulation in the circulation in pre-clinical EAE and showed that the timing of this accumulation clearly preceded the earliest identifiable inflammatory cell extravasation. Platelet accumulation was immediately followed by entry into the parenchyma and association with neurons in the spinal cord and retina [25].…”

“…More recent studies by our own group [25] identified platelet accumulation in the circulation in pre-clinical EAE and showed that the timing of this accumulation clearly preceded the earliest identifiable inflammatory cell extravasation. Platelet accumulation was immediately followed by entry into the parenchyma and association with neurons in the spinal cord and retina [25]. Furthermore, a cause and effect relationship between platelet depletion and absence of clinical disease was identified, thereby establishing a driving rather than exacerbating role for platelets in neuroinflammation.…”

Platelets as Mediators of Neuroinflammation and Thrombosis

“…Unequivocally demonstrating a crucial contribution of platelets to EAE disease pathogenesis, platelet depletion has attenuated EAE in mice, particularly in the effector phase of the disease; thereby, reducing CNS mRNA levels of CCL-2, CCL-5, CCL-19, CXCR-4, and IL-1β as well as the expression of adhesion molecule ICAM-1 ( 14 ) ( Figure 2 ). Consistently, recruitment of leukocytes to the inflamed CNS has been diminished by platelet depletion ( 14 , 160 ). Furthermore, administration of blocking antibodies against GPIIb/IIIa as well as platelet GPIb and its interaction with leukocyte counterreceptor Mac-1 has ameliorated EAE; thus, the involvement of platelets in EAE is regarded to be multi-faceted ( 14 ).…”