MS Society-funded study explores promising neuroprotective protein in mice with MS-like disease
Cytotoxic T cells are a special kind of white blood cell that act as important warriors in the immune system’s host defense response against infection: they recognize and destroy target cells (such as those infected with a pathogenic virus) by programming them to die. In multiple sclerosis, these cytotoxic T cells cross over into the central nervous system (CNS) in uncontrolled numbers and start to target the body’s own nerve cells, causing the breakdown of nerve fibres (axons) and nerve cells (neurons).
One of the principal weapons used by cytotoxic T cells to destroy other target cells is an enzyme called granzyme B. A great deal of evidence has linked granzyme B to the neurodegeneration seen in MS; granzyme B is found in high concentrations in MS lesions, and decreasing levels of granzyme B inside T cells can lead to reduced destruction of neurons in mice with an MS-like disease. In a recent study funded by the MS Society, researchers at the University of Alberta, including Drs. Chris Bleackley (who discovered granzyme B), Bradley Kerr, Fabrizio Guiliani and colleagues tested the effects of a granzyme B inhibitor called serpina3n on neurodegeneration and disease severity in animals with an MS-like disease. The study was published in the Journal of Inflammation.
The authors performed several different experiments to study the effects of serpina3n on neurodegeneration triggered by granzyme B. Firstly, they incubated activated T cells with serpina3n for an hour to allow them to interact, after which they mixed neuronal cells in cell culture with either the serpina3n-treated T cells or a dummy solution. They then measured two outcomes: how many neuronal cells survived, and; whether certain structural proteins in the neurons were damaged.
Secondly, they investigated the effects of serpina3n in mice with an MS-like disease. Mice were split into groups: one group was injected with serpina3n 7 days after the disease was triggered, while the other group was treated both 7 and 20 days after disease induction. A group of control mice received a dummy solution instead of serpina3n. Clinical signs of disability were assessed for up to 36 days.
Thirdly, the same mice were then evaluated after 36 days to measure the number of injured axons in the spinal cord, the amount of myelin damage, and the infiltration of immune cells into the CNS.
The study authors found that when activated T cells were treated with serpina3n and then mixed with cultured neurons, the number of surviving neurons was twice as high compared to culture dishes where T cells were not exposed to serpina3n. As well, important structural proteins in neurons were not damaged when T cells were treated with serpina3n.
Mice with an MS-like disease who were administered serpina3n experienced significantly fewer symptoms of disability compared to mice given a dummy solution, and this improvement persisted for up to 20 days. Mice who were treated with serpina3n again 20 days later began to once again experience reduced disease severity.
Serpina3n also reduced the number of damaged axons in the spinal cord by about 50% and appeared to curb levels of demyelination, although it did not appear to affect the infiltration of T cells into the CNS.
One of the major challenges facing researchers is the development of disease-modifying therapies for MS that prevent immune cells from attacking the body’s own nervous tissues while at the same time refraining from suppressing the immune system’s ability to detect and fight infection. The inhibition of granzyme B using serpina3n has the potential to be a promising alternative, since it blocks the ability of inflammatory T cells to kill neurons, but at the same time does not prevent the entry of some T cells into the CNS or negatively impact immunosurveillance – the immune system’s ability to detect infection.
In addition to its ability to protect axons from destruction and reduce disease severity, blocking granzyme B had the added benefit of reducing demyelination: this two-pronged approach makes a granzyme B inhibitor like serpina3n a promising therapeutic candidate for targeting several of the disease-causing mechanisms of MS in a single hit. Progressive MS in particular could potentially benefit from these findings, since axonal injury is one of the leading culprits underlying the accumulation of disability seen in this form of the disease. Further translational work will be needed to ensure that these neuroprotective strategies can be adapted into a safe and effective medication for people living with MS.
Haile Y et al. Granzyme B-inhibitor serpina3n induces neuroprotection in vitro and in vivo. J Neuroinflammation. 2015; 12: 157.