PROJECT SUMMARY/ABSTRACT Stroke is one of the leading causes of death and disability worldwide and is the third leading cause of death and disability in the United States. Clinical and preclinical studies suggest the importance of inflammation in acute and chronic neuronal tissue damage following ischemic stroke; however, the mechanisms and cells involved in neuroinflammation are not fully understood. There is currently no available treatment for targeting the acute immune response that develops in the brain during cerebral ischemia, and no new treatment has been introduced to stroke therapy since the discovery of tissue plasminogen activator therapy in 1996. We found that interleukin 21 (IL21) is a major contributor to acute brain injury after ischemic stroke. IL21 expressing cells were detected in perivascular regions and the infarcted parenchyma of ischemic human brain tissues. This raises the possibility that IL21?targeting therapies adjunct to current treatments might be beneficial in stroke management. The long-term goal of this work is to understand how to manipulate the acute inflammation following ischemic injury in order to inhibit stroke-induced tissue damage. The objective of this proposal is to define the mechanisms by which IL21 is involved in acute stroke-induced injury. The specific hypothesis is that the recently discovered circulating IL21-producing follicular helper T cells (Tfh) infiltrate into ischemic tissues in the brain, where IL21 contributes to tissue damage by inducing neuronal death and dysfunction of the neurovascular unit (NVU). Three aims will study how IL21 contributes to ischemic injury in the brain. In Aim 1, we will identify cellular source(s) of IL21 in the ischemic brain and determine the regulatory pathway(s) that are important for Tfh cell recruitment to stroke lesions. We propose that selective Tfh cell recruitment blockers could decrease tissue damage in stroke. In Aim 2, we will evaluate spatiotemporal expression of IL21R in the ischemic brain and test potential regulator(s) of IL21R expression on neurons. We propose that inhibiting IL21R expression on neurons could decrease tissue damage in stroke. Aim 3 will focus on the role of IL21R on neurons. We will study the role of neuronal IL21R expression in neuronal damage and subsequent dysfunction of the NVU. Successful completion of this work will lead to a better understanding of the mechanism by which IL21 promotes damage in ischemic injury and identify new therapeutic targets in stroke.