What is the pathogenesis of polymyositis?

Pathogenesis of Polymyositis

The pathogenesis of polymyositis involves a complex interplay of immune system dysfunction, autoimmunity, and muscle fiber damage. It is primarily considered a T-cell-mediated autoimmune disorder that targets muscle fibers, leading to chronic inflammation and muscle weakness.

Pathogenesis of Polymyositis


1. Immune System Dysregulation

Polymyositis is thought to be initiated by an abnormal immune response, where the body fails to recognize its own muscle tissues and begins attacking them. This response is primarily cell-mediated rather than antibody-mediated.

2. Role of CD8+ T-Cells

  • Cytotoxic CD8+ T lymphocytes are the main effector cells.

  • These T cells recognize antigens presented on muscle fibers via MHC class I molecules, which are abnormally upregulated on the surface of muscle cells in polymyositis.

  • The T cells then directly invade muscle fibers and release perforin and granzymes, leading to muscle cell necrosis (cell death).

3. Muscle Fiber Injury

  • The inflammatory infiltrates are mostly endomysial (within the muscle fiber compartment).

  • The muscle fibers are invaded and destroyed by CD8+ T-cells and macrophages.

  • This results in fiber degeneration, necrosis, and ultimately muscle weakness.

4. Cytokine Involvement

Several pro-inflammatory cytokines contribute to the progression of the disease:

  • Interleukin-1 (IL-1)

  • Tumor Necrosis Factor-alpha (TNF-α)

  • Interferon-gamma (IFN-γ)

These cytokines amplify the inflammatory response and further damage the muscle tissue.

5. Abnormal MHC Class I Expression

Normally, muscle fibers do not express MHC class I molecules, but in polymyositis:

  • MHC class I is abnormally expressed on the sarcolemma (muscle cell membrane).

  • This abnormal expression is believed to trigger autoimmune recognition, allowing cytotoxic T-cells to target the muscle cells.

Summary of Pathogenesis

Step Process
1 Trigger (e.g., viral infection or unknown factor) initiates immune response
2 Abnormal expression of MHC I on muscle fibers
3 CD8+ T-cells recognize and attack muscle fibers
4 Inflammatory cytokines released, worsening damage
5 Muscle fiber necrosis, weakness, and inflammation develop

Conclusion

The pathogenesis of polymyositis highlights its autoimmune nature, dominated by T-cell infiltration and muscle fiber destruction. Understanding this process is crucial for developing targeted immunosuppressive therapies and improving patient outcomes.


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