Cartilage as a target of autoimmunity: a thin layer.

The articular cartilage is an important component of human organism that has elasticity, low-friction surface, and ability to withstand great physical forces. The structure consists of collagens and proteoglycans, whereas non-collagenous proteins are needed for the organization and modulation of the molecular networks. The structural elements of the cartilage are typical to that tissue and could, in part, account for the localization of the inflammatory response to the joint. For this reason cartilage is of particular interest in autoimmunity as it may represent a source of antigens. It is well known that sensitization with collagens can produce autoimmune rheumatic diseases in experimental models. So far, the cartilage proteins that have been clearly characterized to be arthritogenic in experimental models involve types II and XI collagen, cartilage oligomeric matrix protein, and aggrecan. It is likely that these proteins are also recognized at different stages in the development of rheumatoid arthritis and in other autoimmune diseases. The mechanisms determining the trigger of a cartilage-specific immune response, its development and outcome are poorly understood. Most likely, the distribution and concentration of a specific cartilage protein may play a role by eliciting an autoimmune response. Indeed, the inflammatory processes lead to tissue damage mediated by the intervention of several factors such as autoantibodies, cytokines as well as cells of the innate an adaptive immunity. For this reason, even previously-considered degenerative diseases, such as osteoarthritis, should now be re-evaluated as at least partly inflammatory-driven. Thus, the objective of this review is to describe the clinical conditions sustained by the immune-mediated reactions to cartilage, which represents the target organ in a number of autoimmune diseases.