Granzyme B and tumor cells: An enzymatic death

Apart of a family of closely related serine proteases called granzymes GrB is a key component of cytotoxic granules of CTLs and NK cells^6,7. In humans there are 5 different granzymes (A. B, H, K and M) and while our understanding of their functions is limited GrB is the most extensively studied family member⁸. Prior to release Grb exists as an inactive proenzyme that has an additional peptide sequence on its N-terminus. Only after the removal of 2 amino acids by cathepsin C or cathepsin H does Grb become activated and capable of activating death pathways⁹.

While still debated exactly how, classically Grb is released from CTL’s and NK cells alongside with PEN which creates a pore in the target cells membrane and nucleus allowing GrB to enter¹⁰. Once inside GrB can induce cell death via several signaling pathways. For example, GrB can cleave and activate executioner caspases 3 and 7 and initiator caspases 8 and 1010. In this scenario caspase 7 is the most sensitive to GrB, whereas, caspases 3, 8. 10 are only cleaved partially and require additional enzymes11. GrB is also capable of cleaving BID leading to the oligomerization of BAX/BAX and the release of cytochrome c from the mitochondria triggering apoptosis¹⁰.

Growing evidence suggests that GrB is also expressed non-cytotoxic cells including B cells1 among others^2,3. This has led to possible new roles for GrB including in the early cell-mediated immune response to viral infection and new mechanisms for inducing cell death. For example, B cells can express and secrete GrB after stimulation by the B-cell receptor and interleukin-21 (IL-21)¹¹. Because B cells target in an MHC-independent manner versus CTLs and NK cells this makes them likely to be the first responders in the cases of viral infection which produce IL-21 very early on¹². Another key point is that GrB is capable of inducing target cell death independent of PEN since B cells do not express PEN.

Because GrB is effective in killing tumor cells that are resistant to other cytotoxic drugs there is interest in utilizing GrB as part of targeted therapy¹⁴. An example of this is with the packaging GrB as part of an immunotoxin complex, but this and other treatment designs have been limited due to several factors⁵. The main roadblock is the GrB inhibitor serpin B9 which can be overexpressed by tumor cells allowing tumor cells to evade and minimize the impact of GrB15. Another key limitation is that GrB has a highly positive surface charge making it capable of binding non-specifically to negatively-charged cell surfaces⁸.

Taken together, GrB plays an incredibly important role in the ability for cytotoxic cells like CTLs and NK cells to kill targeted cells. More recent evidence has expanded this to even include traditionally non-cytotoxic cells such as B cells. All of this and more have made GrB an interesting therapeutic option in cancer and other diseases.