Mechanisms of disease: a 'DAMP' view of inflammatory arthritis.

Innate immunity achieves our primary host defense by recognizing invading microorganisms through pathogen-associated molecular patterns (PAMPs) and by reacting to tissue damage signals called damage-associated molecular patterns (DAMPs). DAMP molecules, including high mobility group box 1 protein (HMGB-1), heat-shock proteins (HSPs), uric acid, altered matrix proteins, and S100 proteins, represent important danger signals that mediate inflammatory responses through the receptor for advanced glycation end-products (RAGE, also known as AGER) and Toll-like receptors, after release from activated or necrotic cells. The terms 'alarmins' and 'endokines' have also been proposed for DAMP molecules. A prototypic DAMP molecule, the nuclear protein HMGB-1, is either passively released by necrotic cells or actively secreted with delay by activated cells. S100A8, S100A9, and S100A12 are calcium-binding proteins expressed in the cytoplasm of phagocytes. They are rapidly secreted by activated monocytes or neutrophils, which are abundant in inflamed synovial tissue. HSPs are involved in the crosstalk between innate and adaptive immune systems, and primarily mediate immune regulatory functions. Multiple positive feedback loops between DAMPs and PAMPs and their overlapping receptors temporally and spatially drive these processes and may represent the molecular basis for the observation that infections, as well as nonspecific stress factors, can trigger flares in rheumatic diseases.