Complement activation and regulation in rheumatic disease

Complement is a key component of the innate immune defence and in addition forms a bridge to the adaptive immune responses. As such complement is of vital importance for efficient protection against infections. However, the activity of the complement system can also aberrantly be directed against the tissues of the body itself and contribute to organ damage in a variety of diseases. In several rheumatic diseases complement activation is suggested to play a pronounced role. This review will highlight the role of both complement activation and complement regulation in rheumatic disease. A contribution of complement to the disease process is often suggested based on the presence of complement activation fragments in the target tissues or the presence of complement activation fragments in the circulation. The role that complement plays in different rheumatic diseases is often unknown but is thought to contribute to tissue damage as a consequence of autoantibody mediated immune complex formation and deposition. In addition reduced complement inhibition mediated by endogenous complement regulators can also enhance complement activity and tissue damage. In observational studies, it is difficult to distinguish whether complement activation is a result of enhanced activation or decreased regulation. Until recently, strong conclusions on the relative importance of complement activation to the pathology were largely restricted to animal experiments. Usage of complement targeting therapeutics in humans will hopefully give us the opportunity to study the actual contribution of complement activation towards disease progression and tissue damage in rheumatic disease into more detail.