Objectives Autoantibodies may play a role in the pathogenesis of myositis and are locally produced within muscle tissue. This study aimed to characterize the local expression of immunoglobulin genes across different subgroups of myositis, identify pathways associated with this expression, and evaluate correlations with disease activity.

Methods Bulk RNA sequencing was performed on muscle biopsies from 289 individuals, including patients with various forms of myositis and healthy controls. Expression levels of immunoglobulin gene regions were compared across clinical and autoantibody-defined subgroups. Pathway enrichment analysis and unsupervised clustering were conducted, and correlations between immunoglobulin gene expression and disease activity were assessed.

Results Local immunoglobulin gene expression was highest in inclusion body myositis (IBM) and antisynthetase syndrome (ASyS), followed by dermatomyositis, and lowest in immune-mediated necrotizing myopathy. Among isotypes, IgG, IgA, and IgM predominated, while IgD and IgE expression was minimal. Certain immunoglobulin VJ segments were more frequently used across all patients, with no significant differences in specific region usage between patient groups. Immunoglobulin gene expression strongly correlated with disease activity, particularly in patients with anti-Mi2, anti-MDA5, anti-Jo1 autoantibodies, and IBM. Pathway analysis revealed a robust association between immunoglobulin expression and interferon-gamma (IFN-γ) signaling. Unsupervised clustering based solely on immunoglobulin gene expression clearly separated healthy controls from patients with ASyS and IBM.

Conclusions Immunoglobulin is locally expressed in the muscle of patients with myositis, particularly IBM and ASyS. This expression correlates with disease activity, involves preferential usage of specific isotypes and gene segments, and is closely linked to IFN-γ-associated immune pathways.

The authors have declared no competing interest.

This study was funded, in part, by the Intramural Research Program of the National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health.

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The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

-Ethics committee/IRB of National Institutes of Health gave ethical approval for this work -Ethics committee/IRB of the Johns Hopkins Hospital gave ethical approval for this work -Ethics committee/IRB of the Clinic Hospital gave ethical approval for this work -Ethics committee/IRB of the Vall d'Hebron Hospital gave ethical approval for this work -Ethics committee/IRB of the Charite-Universitatsmedizin Berlin gave ethical approval for this work

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Competing interests: None.

Contributorship: All authors contributed to the development of the manuscript, including interpretation of results, substantive review of drafts, and approval of the final draft for submission. IPF and ALM are the guarantors.

Funding: This study was funded, in part, by the Intramural Research Program of the National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health.

Ethical approval information: All biopsies were from subjects enrolled in institutional review board (IRB)-approved longitudinal cohorts in the National Institutes of Health, the Johns Hopkins, the Clinic Hospital, the Vall d’Hebron Hospital, and the Charité-Universitätsmedizin Berlin.

Patient and public involvement: Patients and/or the public were not involved in the design, conduct, reporting, or dissemination plans of this research.

Data sharing statement: Any anonymized data not published within the article will be shared by request from any qualified investigator.

All data produced in the present study are available upon reasonable request to the authors