Rheumatoid Arthritis

Rheumatoid Arthritis-Brian Catchpole RVC 2008

Pathogenesis:

• Type IV hypersensitivity: CD4 Th-1 cell mediated.
• Macrophages phagocytose self antigens and can present peptides on their MHC Class II molecules. The autoreactive TH-1 cells release IFN-gamma which activates macrophages to release prostaglandins, MMP enzymes and TNF-alpha (pro-inflammatory mediator), see diagram. These cytokines cause inflammation and tissue destruction.

• Occurs in the dog, mainly smaller breeds
• Uncommon
• Progressive erosive polyarthritis
• Mostly involves elbows, stifles, carpal and tarsal joints
• Grossly:
  • Marked villus hypertrophy of synovial membrane
  • Cartilage erosion
  • Pannus and periarticular osteophyte formation
  • In severe cases ankylosis
• Histologically:
  • Hyperplasia of lining cells
  • Proliferative synovitis
  • Synovial membrane has fibrin deposits
  • Lymphoid and plasma cell infiltration
  • Surrounding haemorrhagic areas
  • Macrophages containing haemosiderin
  • Connective tissue may contain foci of necrosis
  • Areas of erosion of peripheral articular cartilage and underlying subchondral bone
• Pathogenesis:
  • May involve deposition of immune complexes within joints
  • Substances degrading cartilage are released by synovial cells and macrophages involved in pannus formation