Granzyme B in skin inflammation and disease.

Granzyme B (GzmB) is a serine protease emerging as an important mediator of skin injury, inflammation and repair. Found at low levels in healthy skin, GzmB is dramatically elevated in chronic disease and inflammatory skin disorders, including diabetic ulcers, hypertrophic scarring, autoimmune skin disorders, cutaneous leishmaniasis and aging skin. Traditionally known for its pro-apoptotic function, the role of GzmB in disease has been redefined due to the discovery of additional activities involving the cleavage of extracellular matrix proteins, epithelial barrier disruption, fibrosis, vascular permeability, anoikis, inflammation and autoimmunity. In addition to the accumulation of GzmB+ cells in diseased tissue, and critical to the mechanistic redefinition, is the realization that GzmB often accumulates in the extracellular milieu, retains its activity in plasma, and is expressed by both immune and non-immune cells that may or may not express perforin, the pore-forming protein required for GzmB internalization into target cells. As GzmB is not normally found in the extracellular milieu, and does not appear to be regulated, GzmB-mediated proteolysis can impact processes such as tissue remodelling, barrier function, autoantigen generation and angiogenesis. The present review will summarize and critically examine the current knowledge regarding GzmB in inflammatory skin disease, providing an overview of both apoptotic and extracellular mechanisms, but with a focus on the extracellular roles of GzmB in skin health and disease.