Osteoarthritis represents the most frequently diagnosed condition of the musculoskeletal system and accounts for a high amount of direct and indirect socioeconomic costs worldwide. While for rheumatoid arthritis much progress has been made in the past decades both in understanding its pathogenesis and in creating novel therapies, the pathophysiology of osteoarthritis still holds several secrets to be unraveled in the near future in order to also allow for the development of effective novel pharmacotherapeutical options. Though first categorized as a joint disorder being primarily non-inflammatory in nature for a long period of time with research focused on biomechanic aspects and imbalanced wear and tear, recent evidence including immunological processes helped to refine disease interpretation. Thus, showing true inflammatory characteristics that clinically emerge as synovitis, osteoarthritis is nowadays recognized to include signs of inflammation that at least histologically may sometimes be indistinguishable from rheumatoid synovial infiltration. Although this was known already more than 25 years ago, efforts made in solving pathophysiologic key issues did not succeed sufficiently. This review is thought to summarize elementary pathogenic aspects including genetic predisposition and epigenetic regulation and highlights important central innate but also putative adaptive immunological mechanisms today generally accepted to drive inflammation and tissue destruction in osteoarthritis.

Rheumatoid arthritis (RA) is an autoimmune disease characterized by synovitis, synovial hyperplasia and progressive degeneration of affected joints. These processes are mediated by cells of the immune system as well as by synovial fibroblasts (RASF) originating from the lining layer of the synovium. In this scenario RASFs display an activated phenotype: they show an altered expression of adhesion molecules which allows attachment to articular cartilage and by synthesis of proteases they mediate progressive cartilage and bone destruction. Furthermore, they produce various cytokines and chemokines, which are essential for promoting the inflammatory response. In recent years it has become evident that RASFs not only passively respond to the proinflammatory milieu in the joints of RA patients but also actively contribute by the overproduction of several cytokines and chemokines. These proinflammatory cytokines trigger the transformation of RASFs into an aggressive and invasive phenotype. Additionally, the primarily altered genuine RASFs are actively involved in the recruitment and activation of immune cells. Taken together, they are key players in the development of the well-known chronic, destructive inflammatory response in joints affected by RA.

No abstract