Immunohistochemical Characterization of JAK Family Expression in the Primary Inflammatory Infiltrate of Lichen Planopilaris and Folliculitis Decalvans, With an Exploratory Clinical Assessment of Oral JAK Inhibitors

Background Lichen planopilaris (LPP) and folliculitis decalvans (FD) are two of the most common types of primary scarring alopecia. Several Janus kinase inhibitors (JAKis) have been used effectively in their treatment. We conducted an immunohistochemical (IHC) study to evaluate whether any member of the JAK family was overexpressed in the primary inflammatory infiltrate of FD and LPP. Additionally, we performed an exploratory retrospective study on the efficacy and safety of oral JAKis in LPP. Methods Pathology scalp samples from seven healthy controls and 28 patients with LPP or FD were selected for staining with anti‐JAK1, anti‐JAK2, anti‐JAK3, and anti‐TYK2 antibodies. Cases and controls were compared using the H‐score of the inflammatory cells. On the other hand, an exploratory retrospective clinical study was conducted in which patients with LPP who were treated with JAKis for at least three months were included. Clinical characteristics, severity scores, and the presence of adverse events were collected. Results The IHC study determined that JAK1 and JAK2 exhibited similar expression patterns in cases and controls. However, JAK3 and TYK2 were expressed solely in cases of primary scarring alopecia and were negative in the controls. In the exploratory clinical study, 19 patients with LPP were treated with JAKis. Severity scores improved in follow‐up visits, with adverse events generally being mild. Conclusions JAK3 and TYK2 were overexpressed in LPP and FD samples compared to healthy controls, supporting JAKs as potential pharmacological targets of interest. JAKis could be an effective and safe treatment for LPP patients.