The primary cause of alopecia areata (AA) is autoimmune-mediated hair follicle destruction, while breakdown of immune tolerance due to Treg cell homeostasis disruption critically contributes to this process. However, in AA, the factors leading to Treg cell impairment and the effective regulatory pathways remain unanswered.To assess the therapeutic effects of sCD83 on AA and elucidate the crucial role of sCD83-mediated Treg cell activation in remodeling the perifollicular microenvironment.Blood and scalp tissue were collected from AA patients and healthy controls to characterize sCD83 using ELISA, flow cytometry, and immunofluorescence. In graft-induced C3H/HeJ mouse model of AA, the therapeutic effect of sCD83 on early-onset AA was evaluated by H&E staining, immunofluorescence, and flow cytometry. In vitro, human hair follicle organ culture and primary outer root sheath keratinocyte (ORSK) culture were utilized to clarify the impact of sCD83 on the expression and activity of indoleamine 2,3-dioxygenase (IDO) in hair follicle. Co-culture of ORSKs and PBMCs, together with administration of an IDO inhibitor to AA mice, were performed to determine the necessity of IDO for Treg cell activation. Finally, GST-pulldown and co-IP assays were employed to identify potential sCD83 receptors on ORSKs.We showed here that AA patients exhibit sCD83 deficiency, which may be attributed to reduced sCD83 release from DCs, and that serum sCD83 levels are negatively correlated with disease severity. Supplementation with sCD83 in AA mouse reversed disease manifestations and promoted Treg cell proliferation. Mechanistically, IDO activity in ORSKs is essential for sCD83-mediated Treg cell activation, with TRX potentially serving as a sCD83 receptor to regulate IDO expression.Our study establish the functional role of sCD83 in AA, confirms the therapeutic potential of sCD83 supplementation, and provides some mechanistic insights.