Role of Inflammation and the Immune System in the Progression of Osteoarthritis.

Understanding the molecular drivers and feedback loops of osteoarthritis (OA) may provide future therapeutic strategies to modulate the disease progression. The current paradigm of OA is evolving from a purely mechanical disease caused by cartilage wear toward a complex biological response connecting biomechanics, inflammation, and the immune system. The view of OA as a chronic wound highlights the role inflammation plays and also the body's attempts to repair an ongoing injury. Inflammatory signals, including cytokines such as interleukin-1 and tissue necrosis factor α, surface-expressed pattern recognition receptors such as toll-like receptors 2 and 4, complement factors such as C5, as well as pathogen-associated molecular patterns and damage-associated molecular patterns drive the enzymatic cascade that degrades cartilage matrix in OA. Considering the joint as an entire organ, interactions between the cells that reside in the synovium including macrophages and other immune cells, appear to drive enzymatic activity in cartilage, which, in turn, feeds signals back to the synovium that continues stimulating degradation in a feed-forward loop. This review will explore the potential roles of immune cells such as macrophages and T cells in the synovium in both stimulating and modulating the inflammatory response in OA. © 2019 Orthopedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:253-257, 2020.