Abstract Body: After ischemic stroke (IS), the cytotoxic T (Cyto T) cells infusing to the infarct site enhance the neuroinflammation. Previous studies have shown that increasing the proportion of regulatory T (Treg) cells relative to Cyto T cells can alleviate inflammatory conditions. Poly-Glu/Tyr polypeptide (Poly-YE; P-YE) was reported as a one of the key immune modulators to just modulate T cell balance. Therefore, we focused on screening target protein that affect T cell balance by P-YE. To screen target protein, we carried out protein microarray and showed that P-YE strongly bound to neutrophil cytosolic factor (NCF1) which is a one part of NOX complex. In the inflammatory response, activated NCF1 is known to bind to Moesin (MSN) to migrate forward to the membrane and assemble into the NOX complex. Also, recent study showed that unbound MSN to NCF1 induces FOXP3 expression in helper T cells. Based on this data, we hypothesized that 'NCF1+ P-YE' binding leads to the inhibition of NOX complex assembly and the conversion of Cyto T cells to Treg cells in neuroinflammation. P-YE loaded on Nanoparticles (Np P-YE; 3ug/mL) was treated to primary Cyto T (CD8+ T) cell isolated from the spleen of 8-week-old C57BL/6 male mouse. These cells were cultured in Oxygen-Glucose Deprivation-conditioned microglia medium (OGD-conditioned medium; CM) to mimic exposure to neuroinflammation. The results showed that Np P-YE fused to the cell membrane and entered to cytosol of CD8+ T cell. In the CM group treated with P-YE, superoxide was remarkably decreased and CD8+ T cells expressed FOXP3. In Co-IP, we confirmed that the binding of NCF1 and P-YE liberated MSN. Also, Proteome cytokine array showed that P-YE treatment group reduced cytokines compared with non-treatment group. In tMCAO modeling mice injected with P-YE (1mg/kg), the infarct area decreased and the pro-inflammatory phenotype of microglia was reduced. Finally, through behavior tests, the mice injected P-YE after tMCAO surgery showed improvement in stereotyped behavior and weakness due to ischemic damage. Taken together, NCF1 bound to P-YE can’t form NOX complex and MSN dissociated from NCF1 may lead to FOXP3 expression. Therefore, P-YE binds to NCF1 under neuroinflammatory conditions, converting some CD8+ T cells into CD8+FOXP3+ cells, thereby inhibiting ROS production and exerting anti-inflammatory effects.