<scp>JAK</scp> Inhibition Restores Epidermal Homeostasis and Modulates Immune Responses in Dermatomyositis Skin: A Spatial Transcriptomic Pilot Study

Dermatomyositis (DM) is an autoimmune disease characterized by diverse skin lesions, including Gottron's papules, heliotrope rash, V-neck, and shawl signs, with or without muscle involvement. JAK inhibitors, targeting the JAK–STAT pathway, including type I interferon (IFN-I)-mediated components, have shown therapeutic promise, although most studies have focused on blood or muscle tissue [1, 2]. Given the skin's central role in disease manifestation, we examined local immune mechanisms in DM skin after JAK inhibitor treatment using spatial transcriptomics. We retrospectively analyzed archived paraffin-embedded skin biopsy specimens from the shawl sign area of the upper back of a 76-year-old man with hypomyopathic DM, collected before and after 4 weeks of daily off-label baricitinib (4 mg) treatment. Spatial transcriptomic profiling was performed using the GeoMx DSP platform (NanoString Technologies, Seattle, WA, USA) [3]. Three regions of interest were selected from each time point and segmented into macrophages, CD4+ T cells, and keratinocytes (Figure 1a). Among 18 677 genes, 1822 (9.76%) normalized genes exceeded the quantitation threshold, of which differentially expressed genes (DEGs, p < 0.05, log2 fold-change > 1) were selected for further analysis: 171 in macrophages, 56 in CD4+ T cells, and 96 in keratinocytes. Interferon-stimulated genes (ISGs) were consistently downregulated across the three cell types in post-treatment samples (Figure 1b). In macrophages, IFI27, IFI16, IFI44L, and RSAD2 expression decreased, while CD4+ T cells exhibited reduced IFI16, ISG15, IFI30, and IFIT3 levels. Similarly, keratinocytes showed decreased IFI6, IFI27, IFI44L, and OAS3 expression. Furthermore, we observed alterations in gene expression specifically linked to regional skin inflammation (Figure 1c). Macrophages showed reduced expression of chemokines (CCL8 and CCL13) and antigen-presentation genes (CD74, HLA-DPA1, HLA-DQB1, and HLA-DRB1). CD4+ T cells displayed downregulation of T-cell activation genes (CD53, PTPRC, and TRAC), whereas genes linked to regulatory T-cell function (PAK2) were upregulated. Keratinocytes in the post-treatment samples showed decreased expression of genes related to epidermal morphogenesis, including those involved in differentiation and adhesion (DMKN, CD44, ITGA3, ITGB4, PKP1, and PKP3). Immunohistochemistry confirmed reduced IFI6 and ITGA3 levels in keratinocytes after treatment (Figure 1d). These findings suggest baricitinib modulates both immune cell responses and keratinocyte homeostasis in DM skin lesions. By inhibiting JAK1/2, baricitinib suppressed IFN-I signaling via IFNAR-driven JAK–STAT activation, thereby reducing downstream ISG expression [2]. In addition, baricitinib decreased the gene expression involved in T-cell activation and recruitment while increasing those related to immune suppression. Reduced ITGA3, critical for basal layer integrity and implicated in inflammatory epidermal remodeling, showed restored expression following treatment, suggesting partial normalization of epidermal architecture [4]. IFI6 overexpression, which promotes apoptosis resistance and aberrant keratinocyte proliferation, was decreased after JAK inhibition, indicating a shift back toward a homeostatic differentiation program [5]. Although further validation is required, assessing interferon signaling and epidermal remodeling gene expression, including IFI6 and ITGA3, in pre- and post-treatment skin lesions may help identify DM treatment response. In conclusion, our research provides spatial transcriptomic evidence that JAK inhibition mitigates IFN-I-driven inflammation and keratinocyte dysregulation in DM skin. Although this study is based on a single patient sample, the spatial transcriptomic analysis provides valuable preliminary data to guide future investigations. This work was supported by a research grant (VHSMC 25008) from VHS Medical Center, Republic of Korea. This retrospective study was approved by the Institutional Review Board of the Veterans Health Service Medical Center (IRB no. 2025-03-016) and was conducted in accordance with the Declaration of Helsinki. Archived skin biopsy specimens, originally obtained during clinical evaluation with informed consent, were used from a participant enrolled in an IRB-approved observational cohort study (IRB no. 2023-12-028). The participant had provided broad consent for future research use of clinical data and biospecimens. No additional procedures were performed solely for research purposes. The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request.