Immune-mediated necrotizing myopathy (IMNM) is characterized by muscle fiber necrosis and macrophage infiltration. Our previous study identified aberrant expression of secreted phosphoprotein 1 (SPP1) in the muscle tissues of patients with IMNM compared to controls. SPP1 encodes osteopontin (OPN), a multifunctional extracellular matrix protein that regulates macrophages and influences muscle injury and repair. However, its clinical significance and role in IMNM pathogenesis remain unclear. This study aimed to explore the expression profile of OPN in IMNM, its impact on disease progression, and underlying regulatory mechanisms.

Muscle and serum samples from patients with IMNM and controls were analyzed to assess OPN expression, macrophage infiltration, and their correlation with clinical characteristics. In vitro experiments investigated the effects of OPN on bone marrow-derived macrophage (BMDM) polarization and its involvement in the JAK1/STAT1 signaling pathway. Additionally, indirect co-culture experiments were performed to evaluate the influence of OPN on inflammatory responses in myoblasts.

Both gene and protein expression levels of OPN were significantly upregulated in muscle samples from patients with IMNM compared to healthy controls and other idiopathic inflammatory myopathy (IIM) subtypes. Serum OPN levels were also markedly elevated in patients with IMNM and positively correlated with disease activity and inflammatory markers. Histopathological analysis demonstrated that OPN was primarily localized around necrotic muscle fibers, with its expression intensity closely linked to CD68+ macrophage infiltration, particularly in M1 (iNOS+) macrophage-enriched regions. Dual immunofluorescence staining further confirmed the colocalization of OPN with CD68+ and iNOS+ macrophage markers.

In vitro experiments demonstrated that under inflammatory conditions, OPN facilitated M1 macrophage polarization through activation of the JAK1/STAT1 signaling pathway, resulting in increased secretion of pro-inflammatory cytokines, including tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and IL-1β. Indirect co-culture experiments further revealed that OPN derived from M1-polarized BMDMs amplified the inflammatory response in C2C12 myoblasts, whereas inhibition of OPN secretion significantly attenuated this effect.

OPN was aberrantly expressed in patients with IMNM and was associated with disease activity and M1 macrophage infiltration. OPN promoted M1 polarization and exacerbated inflammatory responses by activating the JAK1/STAT1 signaling pathway, contributing to muscle injury. These findings highlight the pivotal role of OPN in IMNM pathogenesis and suggest its potential as a therapeutic target.