OC 49.3 - Modulating platelet-microglial complexes to limit neuroinflammation post-ischemic stroke

Background: Platelets have traditionally been regarded as small cell fragments that participate only in hemostasis, but their origin, membrane receptors and intracellular contents provide irrefutable support for the view that platelets have a central role in in inflammation. Despite this, the role of platelets in neuroinflammation after stroke remains controversial. Indeed, whether brain-infiltrating platelets contribute to the polarization of microglia, the brain-resident macrophage, to dictate patient prognosis post-stroke is currently unknown.

Aims: To examine the immunomodulatory function of platelets to polarize microglial phenotype and function following an in vitro or in vivo experimental model of stroke.

Methods: in vitro, the polarization of primary and immortalized microglia in stroke-like conditions (oxygen- and glucose-starved) was assessed by microscopy and flow cytometry in the presence, or absence of washed platelets. in vivo, we performed the photothrombotic pre-clinical model of ischemic stroke on Cx3cr1gfp/+ mice, to identify microglia by intravital two-photon microscopy. Animal behavioral response to motor and sensory tests, alongside cortical infarct size demonstrated clinical outcome.

Results: Following hypoxia and glucose starvation, BV2 microglia-like cells, or primary microglia, demonstrated strong activation (CD86+), and pro-inflammatory polarization (iNOS+), and this response was exacerbated by the presence of washed platelets. Neurovascular platelet extravasation has been described during multiple sclerosis and hypertension. Following photothrombotic stroke, we observed microglial activation surrounding the lesion in the brain of post-stroke mice. By using state-of-the-art in vivo imaging, we demonstrate that cerebral ischemia was strongly associated with significant loss of blood-brain-barrier integrity and presence of platelet-positive microglia. This finding provide evidence that platelets extravasate, and their interaction with perivascular microglia may modulate post-stroke neuroinflammation.

Conclusion(s): We have demonstrated platelet extravasation during ischemic stroke in vivo and a novel strategy, modulating microglial-platelet complexes, which may reduce neuroinflammation and improve clinical outcome.