Controversies in granzyme biology

Granule proteases of hematopoietic cells, a family of versatile inflammatory mediators – an update on their cleavage specificity, in vivo substrates, and evolution

Cells from several of the hematopoietic cell lineages including mast cells, basophils, neutrophils, cytotoxic T cells, and natural killer (NK) cells store proteases at very high levels within their cytoplasmic granules. In mast cells, these proteases can account for up to 35% of the total cellular protein, and the absolute majority of these belong to the chymotrypsin-related serine protease family. A number of very diverse functions have been identified for these proteases, including apoptosis induction, blood pressure regulation, inactivation of insect and snake toxins, intestinal parasite expulsion, killing of bacteria and fungi, induction, mobilization, or degradation of cytokines, and the degradation of connective tissue components. A very broad spectrum of primary cleavage specificities has also been observed, including chymase, tryptase, asp-ase, elastase, and met-ase specificities, which highlights the large flexibility in the active site of these proteases. Mast cells primarily express chymases and tryptases with chymotryptic or tryptic primary cleavage specificities, respectively. Neutrophils have several enzymes with chymase, elastase, and tryptase specificities. T cells and NK cells express between 5 and 14 different granzymes, depending on the species, and these enzymes have tryptase, asp-ase, chymase, and met-ase specificities. This review focuses on the appearance of these proteases during vertebrate evolution, their primary and extended cleavage specificities, and their potential in vivo substrates. The in vivo substrates and functions are a particular challenging issue because several of these enzymes have a relatively broad specificity and may therefore cleave a wide range of different substrates.

Granzyme M as a novel effector molecule for human cytolytic fusion proteins: CD64-specific cytotoxicity of Gm-H22(scFv) against leukemic cells

Immunotoxins are promising targeted therapeutic agents comprising an antibody-based ligand that specifically binds to diseased cells, and a pro-apoptotic protein. Toxic components from bacteria or plants can trigger a neutralizing immune response, so that human effector molecules are more suitable. In this context, the protease granzyme B has been successfully tested in cytotoxicity assays against different cancer cells in vitro and in vivo. Our aim here was to introduce granzyme M as an alternative and novel component of human cytolytic fusion proteins. We fused it to the humanized single-chain antibody fragment (scFv) H22 which specifically binds to CD64, an FcγRI receptor overexpressed on activated myeloid cells and leukemic cells. We show that the humanized cytolytic fusion protein Gm-H22(scFv) specifically targets the acute myeloid leukemia cell line HL60 in vitro and is cytotoxic with an IC50 between 1.2 and 6.4 nM. These findings were confirmed ex vivo using leukemic primary cells from patients, which were killed by granzyme M despite the presence of the granzyme B inhibitor serpin B9. In conclusion, granzyme M is a promising new cell-death inducing component for hCFPs because it specifically and efficiently kills target cells when fused to a targeting component.

A novel p38 mitogen-activated protein kinase/Elk-1 transcription factor-dependent molecular mechanism underlying abnormal endothelial cell proliferation in plexogenic pulmonary arterial hypertension

Plexiform lesions (PLs), the hallmark of plexogenic pulmonary arterial hypertension (PAH), contain phenotypically altered, proliferative endothelial cells (ECs). The molecular mechanism that contributes to EC proliferation and formation of PLs is poorly understood. We now show that a decrease in intersectin-1s (ITSN-1s) expression due to granzyme B (GrB) cleavage during inflammation associated with PAH and the high p38/Erk1/2(MAPK) activity ratio caused by the GrB/ITSN cleavage products lead to EC proliferation and selection of a proliferative/plexiform EC phenotype. We used human pulmonary artery ECs of PAH subjects (EC(PAH)), paraffin-embedded and frozen human lung tissue, and animal models of PAH in conjunction with microscopy imaging, biochemical, and molecular biology approaches to demonstrate that GrB cleaves ITSN-1s, a prosurvival protein of lung ECs, and generates two biologically active fragments, an N-terminal fragment (GrB-EH(ITSN)) with EC proliferative potential and a C-terminal product with dominant negative effects on Ras/Erk1/2. The proliferative potential of GrB-EH(ITSN) is mediated via sustained phosphorylation of p38(MAPK) and Elk-1 transcription factor and abolished by chemical inhibition of p38(MAPK). Moreover, lung tissue of PAH animal models and human specimens and EC(PAH) express lower levels of ITSN-1s compared with controls and the GrB-EH(ITSN) cleavage product. Moreover, GrB immunoreactivity is associated with PLs in PAH lungs. The concurrent expression of the two cleavage products results in a high p38/Erk1/2(MAPK) activity ratio, which is critical for EC proliferation. Our findings identify a novel GrB-EH(ITSN)-dependent pathogenic p38(MAPK)/Elk-1 signaling pathway involved in the poorly understood process of PL formation in severe PAH.

Conservation of the extended substrate specificity profiles among homologous granzymes across species

Granzymes are structurally related serine proteases involved in cell death and immunity. To date four out of five human granzymes have assigned orthologs in mice; however for granzyme H, no murine ortholog has been suggested and its role in cytotoxicity remains controversial. Here, we demonstrate that, as is the case for granzyme C, human granzyme H is an inefficient cytotoxin that together with their similar pattern of GrB divergence and functional similarity strongly hint to their orthologous relationship. Besides analyzing the substrate specificity profile of granzyme H by substrate phage display, substrate cleavage susceptibility of human granzyme H and mouse granzyme C was assessed on a proteome-wide level. The extended specificity profiles of granzymes C and H (i.e. beyond cleavage positions P4-P4') match those previously observed for granzyme B. We demonstrate conservation of these extended specificity profiles among various granzymes as granzyme B cleavage susceptibility of an otherwise granzyme H/C specific cleavage site can simply be conferred by altering the P1-residue to aspartate, the preferred P1-residue of granzyme B. Our results thus indicate a conserved, but hitherto underappreciated specificity-determining role of extended protease-substrate contacts in steering cleavage susceptibility.