Graft tolerance in mice appears to be linked to the regulation of the enzyme Granzyme B (GZB), reported Dartmouth Medical School professor Randolph Noelle and colleagues in a recent paper published in The Journal of Immunology. GZB may play a key role in preventing transplant rejection in humans.

Current research has demonstrated that enzymes like GZB are important components of transplantation immunotherapy, along with regulatory T cells, lymphocytes that are responsible for suppressing activation of the immune system.  Noelle, a professor of microbiology and immunology at Dartmouth Medical School, studied GZB and its effects on building long-term tolerance to skin grafts in mice. Noelle discovered, using in vivo data, that the expression of GZB by the FoxP3 gene is a part of the mechanism in which adaptive regulatory T cells maintain homeostasis in the immune system and create tolerance to self-antigens.

GZB is an important part of suppressing immune system activity because it is involved in the process of apoptosis, or programmed cell death, of helper T cells and cytotoxic T cells. GZB is initiated by the FoxP3 gene expressed in adoptive regulatory T cells and is responsible for the cleavage and activation of the caspase-3 enzyme, which engages in the condensation of chromatin and eventually the breakdown of DNA, leading to apoptosis.

Apoptosis of cytotoxic T cells results in immune system suppression because these T cells are programmed to target and destroy pathogens in the body. Transplant intolerance results in shorter periods of graft survival and complications such as severe inflammation and vascular damage.     

In order to prove that the expression of GZB from FoxP3 was essential for longer graft survival, wild-type mice were compared in vivo with GZB deficient or knockout mice that had a mutation in the FoxP3 gene. Test results revealed that the lack of GZB expression led to the loss of long-term tolerance to tail skin grafts from donor mice. Also, when the wild-type mice were given pharmacological inhibitors of GZB, serine protease inhibitor 6, they showed resistance to alloantigen tolerance.

A further study in the mechanism of FoxP3 and GZB indicates that the regulatory T cells enter and accumulate in graft tissue when inducing transplant tolerance. Noelle was able to detect a build up of GZB in tail skin grafts, which were highly infiltrated with FoxP3 positive T cells, through immunofluorescence. This experiment shows that GZB activity is closely linked with long-term survival of transplants in the mice.

Although previous studies revealed similar results in vitro for humans and mice, Noelle’s research produced the first in vivo data to demonstrate the importance of GZB in graft survival. According to Noelle, this recent data implies that future methods for transplantation immunotherapy must involve the introduction of regulatory T cells and GZB functions in order to facilitate long-term transplant tolerance.