Granzymes are a family of serine proteases stored within the cytotoxic granules of cytotoxic T lymphocytes (CTLs) whose functions were once believed to be primarily involved in immune-targeted cell death. There are 5 granzymes expressed in humans: granzymes A, B, H, K, and M. Granzymes A and B are the most abundant granzymes and for this reason, have been the most studied. Granzyme B (GzmB) has received much attention over the past three decades and only recently has a new pathological role been demonstrated.
The evolving role of granzymes (aka granule secreted enzymes) and in particular, GzmB, is illustrated here in Figure 1 and Figure 2. Figure 1 shows the previously well-known mechanism of cytotoxicity: Granzymes were thought to be expressed solely in cytoplasmic granules of certain immune cells (CTLs and natural killer cells) along with the pore forming protein, perforin. On expression from the immune cell, perforin is necessary for the granzyme to enter the targeted cell and begin destruction.
An abundance of data has emerged demonstrating a correlation between elevated GzmB in bodily fluids and increased severity of a number of chronic diseases of inflammation and/or aging. GzmB is not naturally inhibited in the extracellular milieu and so levels will accumulate substantially in disease states without any apparent restraint. High levels result in inflammation and/or connective tissue destruction. We have elucidated the mechanism of action in animal studies and in some cases, have validated GzmB as a key contributor in the pathogenesis of certain diseases in which high extracellular levels are prevalent.
GzmB targets and degrades a number of proteoglycans, including decorin, a proteoglycan that organizes collagen into tight, thick bundles and is abundant in most connective tissues. In the absence of decorin, collagen fibrils become disorganized leading to a loss of collagen tensile strength (Figure 4). This inconsistent structuring leads to fragility and loss of tissue integrity. GzmB has been shown to be responsible for this pathology. Inhibition of GzmB allows for proper collagen fibrillogenesis to proceed, leading to improved organization and tensile strength in the tissue (Figure 4).
[Figure 1] Click to Enlarge
[Figure 1] Certain types of cytotoxic lymphocytes possess a cell-permeabilizing protein (perforin) that facilitates Granzyme B (GzmB) entry into cells where it promotes cell death.
[Figure 1] Click to Enlarge
[Figure 2] Click to Enlarge
[Figure 2] Increased Granzyme B accumulates in the extracellular milieu during excessive inflammation and aging due to its secretion by aged, ‘leaky’ immune cells or by cells that otherwise do not express granzymes that are stimulated to express and release this protease. As granzymes accumulate in the extracellular space, they begin to degrade key structural and functional proteins that cells require for orchestrating tissue repair and remodeling.
[Figure 3] Click to Enlarge
[Figure 3] Granzyme B (GzmB) has been observed to be elevated in a number of bodily fluids (Plasma, synovial fluid, cerebrospinal fluid, broncho-alveolar lavage, wound fluid, blister fluid) in the shown indications.
Figure 2 shows the results of the more recent research conducted by Dr. Granville, and others, granzymes are shown to be expressed by other cell types of immune and non-immune origin such as smooth muscle cells, keratinocytes, and chondrocytes, in certain disease states. Further research has demonstrated that granzymes have a broader, extracellular, perforin independent role resulting in the destruction of tissues and vascular integrity that implicates granzymes in a number of inflammatory and age related diseases.
[Figure 4] Click to Enlarge
[Figure 4] Granzyme B cleaves decorin leading to a loss of collagen organization and tensile strength. Decorin has been previously shown to be responsible for proper collagen fibrillogenesis, organization and tensile strength. Granzyme B has been shown to disrupt collagen organization and fibrillogenesis in several models of tissue injury and impaired repair. Inhibition of Granzyme B restores collagen organization and tensile strength.
We are developing a suite of proprietary inhibitors targeting extracellular granzymes to attenuate conditions associated with inflammation and loss of tissue integrity and function.