Background: Effect of Ninjurin1 deletion in the experimental autoimmune encephalomyelitis (EAE) mice has not been examined. Results: Ninjurin1 knock-out (KO) mice are resistance to EAE due to a defect of leukocyte recruitment into lesion sites. Conclusion: Ninjurin1 is a potent target molecule for treating inflammatory diseases such as multiple sclerosis. Significance: Our study proved contribution of Ninjurin1 in EAE pathogenesis in vivo and supports the importance of its targeting strategies. Ninjurin1 is a homotypic adhesion molecule that contributes to leukocyte trafficking in experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. However, in vivo gene deficiency animal studies have not yet been done. Here, we constructed Ninjurin1 knock-out (KO) mice and investigated the role of Ninjurin1 on leukocyte trafficking under inflammation conditions such as EAE and endotoxin-induced uveitis. Ninjurin1 KO mice attenuated EAE susceptibility by reducing leukocyte recruitment into the injury regions of the spinal cord and showed less adhesion of leukocytes on inflamed retinal vessels in endotoxin-induced uveitis mice. Moreover, the administration of a custom-made antibody (Ab26-37) targeting the Ninjurin1 binding domain ameliorated the EAE symptoms, showing the contribution of its adhesion activity to leukocyte trafficking. In addition, we addressed the transendothelial migration (TEM) activity of bone marrow-derived macrophages and Raw264.7 cells according to the expression level of Ninjurin1. TEM activity was decreased in Ninjurin1 KO bone marrow- derived macrophages and siNinj1 Raw264.7 cells. Consistent with this, GFP-tagged mNinj1-overexpressing Raw264.7 cells increased theirTEMactivity. Taken together, we have clarified the contribution of Ninjurin1 to leukocyte trafficking in vivo and delineated its direct functions to TEM, emphasizing Ninjurin1 as a beneficial therapeutic target against inflammatory diseases such as multiple sclerosis.