A Comprehensive Guide to Idiopathic Inflammatory Myopathies (IIM) | Review of Classification, Immune Mechanisms, and Therapeutic Frontiers

1. Humoral Immunity-Dominant Subtypes (DM/ASyS): Type I Interferon-Complement-Cytokine Network

• Core regulatory axis of DM: Involves IFIH1 gene variants → MDA5 recognition of self-antigens → activation of the Type I interferon-JAK-STAT1 pathway → upregulated expression of interferon-stimulated genes → B cell activation to produce specific autoantibodies (e.g., anti-MDA5/TIF1-γ) → activation of the classical complement pathway → perifascicular myofiber damage. Studies have confirmed that specific immune cells in the tissues of DM patients are the main source of interferons, and serum IFN-α levels are positively correlated with ILD progression.
• Molecular crosstalk mechanism of ASyS: The binding of antisynthetase antibodies to cell surface antigens can activate pathways such as NF-κB, releasing cytokines including IL-6 and CXCL8 to recruit immune cell infiltration and form a pro-inflammatory positive feedback loop. Recent research has revealed that anti-Jo-1 antibodies may induce ferroptosis in lung epithelial cells by upregulating ACSL4, providing a novel mechanistic perspective for ASyS-associated ILD.
   

Figure 2. Pathogenesis and clinical manifestations of anti-synthetase syndrome

2. Cellular Immunity-Dominant Subtype (PM): Clonal Activation and Cytotoxic Mechanism of CD8+ T Cells

The core pathogenesis of PM lies in antigen-specific CD8+ T cell-mediated targeted killing of myofibers: abnormal overexpression of MHC-I molecules on the myofiber surface; restricted TCR clonal expansion of intramuscular CD8+ T cells; high expression of cytotoxic mediators such as perforin and granzyme B by effector T cells; and concurrent dysfunction of Treg cells leading to insufficient immune suppression and amplified tissue damage. Current research focuses on identifying specific self-antigens and deciphering the structure of the TCR-antigen-MHC complex.

3. Subtype with Intrinsic Myocellular Abnormalities (IBM): Dysregulation of the Autophagy-Protein Homeostasis-Mitochondrial Function Network

IBM research centers on "cell-autonomous damage": excessive activation of the mTOR pathway inhibits autophagy, resulting in the accumulation of abnormal proteins such as β-amyloid and TDP-43; mitochondrial DNA mutations and reactive oxygen species accumulation exacerbate protein misfolding; mild CD8+ T cell infiltration further aggravates autophagic impairment through IFN-γ secretion, forming synergistic damage. Currently, there is a lack of ideal animal models, and monotherapy with mTOR inhibitors yields limited efficacy, necessitating combination strategies targeting other pathways.

4. Necroptosis-Dominant Subtype (IMNM): Specific Activation of the RIPK3-MLKL Signaling Axis

The core of IMNM involves an "antibody-complement-necroptosis" cascade: anti-SRP/HMGCR antibodies activate the alternative complement pathway, forming membrane attack complexes that damage the myolemma, which in turn activates the RIPK3-MLKL pathway via DAMPs. A key characteristic is significantly elevated MLKL phosphorylation levels with no substantial change in caspase-3 activity. Small-molecule inhibitors targeting RIPK3-MLKL are currently a major research focus.

Figure 3: IMNM muscle damage and repair

5. Molecular Basis of Pathway Crosstalk

Core pathways of different subtypes interact through key nodes: for example, the Type I interferon pathway in DM can enhance NF-κB activity; IFN-γ induced by T cell activation in PM can feedback-upregulate MHC-I expression; complement components in IMNM can promote RIPK3 phosphorylation. Such network crosstalk provides the molecular rationale for disease heterogeneity and combination therapeutic strategies.