Granzyme B (GrB) is one of the serine protease granzymes most commonly found in the granules of natural killer cells (NK cells) and cytotoxic T cells. It is secreted by these cells along with the pore forming protein perforin to mediate apoptosis in target cells.
Granzyme B has also been found to be produced by a wide range of non-cytotoxic cells ranging from basophils and mast cells to smooth muscle cells. The secondary functions of granzyme B are also numerous. Granzyme B has shown to be involved in inducing inflammation by stimulating cytokine release and is also involved in extracellular matrix remodelling.
Elevated levels of granzyme B are also implicated in a number of autoimmune diseases, several skin diseases, and type 1 diabetes.
Structure
In humans, granzyme B is encoded by GZMB on chromosome 14q11.2, which is 3.2kb long and consists of 5 exons. It is one of the most abundant granzymes of which there are 5 in humans and 10 in mice.
The enzyme is initially in an inactive precursor zymogen form, with an additional amino terminal peptide sequence. Cathepsin H has also been reported to activate granzyme B.
Granzyme B is active at a neutral pH and is therefore inactive in the acidic CTL granules. The enzyme is also rendered inactive when bound by serglycin in the granules to avoid apoptosis triggering inside the cytotoxic T cells themselves.
Granzyme B has also been proposed to enter a target by first exchanging its bound serglycin for negative phospholipids in a target's plasma membrane. Entry then occurs by the less selective process of absorptive pinocytosis.
Granzyme B can also cleave BID leading to BAX/BAK oligomerisation and cytochrome c release from the mitochondria. Granzyme B can cleave ICAD leading to DNA fragmentation and the laddering pattern associated with apoptosis. The caspase independent pathways of cell death are thought to have arisen to overcome viruses that can inhibit caspases and prevent apoptosis.
Extracellular matrix
Granzyme B can degrade many proteins in the extracellular matrix (ECM) including fibronectin, vitronectin and aggrecan. Cleavage can cause cell death by anoikis and release alarmins from the ECM inducing inflammation.
Cleavage of vitronectin occurs at the RGD integrin binding site interrupting cell growth signalling pathways. Cleavage of laminin and fibronectin disrupts dermal-epidermal junction attachment and cross talk while decorin destruction by granzyme B causes collagen disorganisation, skin thinning and aging. Keratinocytes can express granzyme B after exposure to UVA and UVB which is linked to photoaging of the skin. Granzyme M can also cleave PI-9 in the nucleus and cytoplasm to relieve granzyme B of inhibition.
Granzyme B release with perforin from CD8<sup>+</sup> T cells can cause heart and kidney transplant rejection through killing of allogeneic endothelial cells. The destruction of insulin producing β cells in pancreatic islets is mediated by T cells and granzyme B contributing to Type 1 Diabetes. Granzyme B can also mediate the death of cells after spinal cord injury and is found at elevated levels in rheumatoid arthritis.
Chronic obstructive pulmonary disease (COPD) has been attributed to granzyme B secreted from NK and T cells causing the apoptosis of bronchial epithelial cells. Matrix destabilisation and remodelling by granzyme B is also linked to asthma pathogenesis.
Granzyme B can kill melanocytes causing the skin condition vitiligo and granzyme B overexpression is found in contact dermatitis, lichen sclerosus and lichen planus cases.
Cytotoxic cells expressing granzyme B have been identified close to hair follicles linking a possible role in hair loss.
More recently, a key role for extracellular granzyme B has been forwarded for a number of autoimmune (e.g. arthritis, autoimmune blistering, scleroderma, lupus)(Reviewed in ) and/or age-related chronic inflammatory disorders (Photoaging, aneurysm, atherosclerosis, COPD, macular degeneration, etc.)(Reviewed in ). In many of these conditions, proof-of-concept has been demonstrated through the use of experimental models, genetic approaches and/or pharmacologic approaches.