1138. Microglia and peripheral myelomonocytic cells contribute to divergent outcomes following cerebrovascular injury

Authors: Panagiotis Mastorakos, MD PhD; Joseph Frank; Dorian McGavern (Bethesda, MD)

Introduction:

Vascular injury within the central nervous system (CNS) results in severe pathology and is associated with intracerebral hemorrhage (ICH), hemorrhagic stroke, and hemorrhagic contusion. While immune response to cerebral ischemia and reperfusion has been extensively studied, little is known about the immune response to capillary rupture and intraparenchymal hemorrhage. Here we aim to study the effect of microglia and invading myeloid cells in ICH pathology, vasogenic edema and subsequent angiogenesis.

Methods:

We developed a model of isolated vascular injury using a combination of ultrasound and intravenously injected microbubbles. This approach allowed us to apply injurious mechanical forces from within cerebral vasculature. We studied the dynamics of the injury response using intravital two-photon laser scanning microscopy.

Results:

We observed that brain-resident microglia rapidly extended their processes to surround injured blood vessels, forming a rosette pattern which was inhibited by transcranial inhibitor application, demonstrating that this phenomenon was dependent on purinergic-receptor signaling and ATP-induced ATP release via connexin hemichannels. Inhibition of microglia rosetting or microglia depletion increased cerebrovascular leakage as well as peripheral myelomonocytic cell invasion. Peripheral myelomonocytic cells participated in cerebrovascular injury response by rapidly invading the brain and meninges within one day. Depletion of these cells as well as inhibition of LFA-1 dependent diapedesis decreased cerebral edema and prevented herniation caused by cerebral edema. 

We also observed formation of new vessels with 10 days after injury. This was prevented following depletion of microglia or circulating myeloid cells. Continuous vascular leakage resulting in cerebral edema was present following chornic depletion of myelomonocytic cells. 

Conclusion:

Collectively, these data demonstrate that primary cerebrovascular injury induces a divergent myeloid cell response, with brain-resident microglia limiting the degree of vascular leakage and peripheral myelomonocytic cells enhancing pathology in the acute phase. Also, both groups of myeloid cells were necessary for angiogenesis during the recovery period.