IntroductionAlopecia areata (AA) is a major medical problem and is among the most prevalent autoimmune disease in the US, with a lifetime risk of 1.7% (1). AA affects both sexes across all ethnicities and represents the second most common form of human hair loss, second only to androgenetic alopecia (2). AA usually presents with patchy hair loss. One-third of these patients will experience spontaneous remissions within the first year. However, many patients' disease will progress to alopecia totalis (AT, total scalp hair loss) or alopecia universalis (AU, loss of all body hair). Persistent moderate-to-severe AA causes significant disfigurement and psychological distress in affected individuals (3). In clinical practice, there are no evidence-based treatments for AA (4), yet various treatments are offered, most commonly topical and intralesional steroids, which have limited efficacy.Our recent mechanistic studies demonstrated a dominant role for type I cellular immunity in AA pathogenesis, mediated by IFN-γ-producing NKG2D-bearing CD8 + cytotoxic T lymphocytes (CTLs) (5). The central role of type I cellular immunity is also reflected in the transcriptional landscape of AA lesional skin in humans and mice, which is dominated by IFN response genes and a CTL signature. These findings provided BACKGROUND. Alopecia areata (AA) is a common autoimmune disease with a lifetime risk of 1.7%; there are no FDA-approved treatments for AA. We previously identified a dominant IFN-γ transcriptional signature in cytotoxic T lymphocytes (CTLs) in human and mouse AA skin and showed that treatment with JAK inhibitors induced durable hair regrowth in mice by targeting this pathway. Here, we investigated the use of the oral JAK1/2 inhibitor ruxolitinib in the treatment of patients with moderate-to-severe AA.