Inflammatory Mechanisms of Brain - Lymphatic Signaling in Stroke

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Inflammatory Mechanisms of Brain-Lymphatic Signaling in Stroke After stroke, the peripheral immune system becomes activated, and these systemic inflammatory responses are known to amplify brain injury and worsen outcomes. But a major gap in knowledge remains. It is unclear how the stroke-damaged brain sends signals to the periphery. Recently, it has been suggested that some type of specialized lymphatic drainage system may exist in the CNS. Back in 1995, we had used CT imaging to show that tracers injected into rabbit brain can directly drain into the cervical lymph nodes in vivo (Hunter et al, Neuropath Appl Neurobiol 1995). We now propose that this brain-to-cervical-lymph node connection may provide a potential pathway for inflammatory crosstalk between brain and systemic responses after stroke. Based on our pilot data, we propose the hypothesis that after stroke, the injured neurovascular unit releases signals that drain into the cervical lymph node and activate macrophages thus worsening neuroinflammation and stroke outcomes: (i) after focal ischemia, brain astrocytes/pericytes/endothelial cells secrete VEGF-C into CSF; (ii) VEGF-C travels into cervical lymph nodes, enhances pro-inflammatory signals in lymphatic endothelium, and induces M1-like macrophage polarization and recruitment; (iii) M1-like macrophages then contribute to further neuroinflammation and brain injury in both gray and white matter. We will test this hypothesis in three integrated aims, using a combination of molecular tools, cell culture, and in vivo models. In Aim 1, we assess and compare mechanisms for how astrocytes, pericytes and brain endothelial cells release VEGF-C after oxygen-glucose deprivation. In Aim 2, we investigate mechanisms that underlie the ability of VEGF-C to induce inflammation in lymphatic endothelium and activate macrophages. In Aim 3, we will use mouse models of focal cerebral ischemia to confirm these brain-to-lymphatic signals in vivo, and examine therapeutic approaches that may interrupt this pathway to improve stroke outcomes. To assess causality in our pathways, we will conduct gain and loss-of-function experiments using a combination of cell culture, in vivo mouse models, pharmacologic inhibitors, molecular techniques including siRNA and transgenics, long-term neurological outcomes, and imaging. This project should define a novel mechanism wherein the damaged neurovascular unit communicates with peripheral lymphatics after stroke. Our findings may provide a new conceptual framework for seeking potential stroke targets and biomarkers in the lymphatic system. Finally, this project may also help open up new collaborative crosstalk between stroke biology and the well established field of lymphatic vascular biology.