Granzyme B is elevated in esophageal biopsies from children with eosinophilic esophagitis

OBJECTIVES

Eosinophilic esophagitis (EoE) is an immune-mediated antigen-triggered inflammatory disease of the esophagus. Our aim was to investigate inflammatory responses by an ex vivo biopsy provocation-based method, stimulating biopsies with milk, wheat, and egg extracts.

METHODS

An experimental study was conducted on esophageal biopsies from children who underwent esophagogastroduodenoscopy. Supernatants were collected before and after stimulation of the biopsies with food extracts and analyzed for 45 different inflammatory markers. Biopsies were also stained for histological analyzes.

RESULTS

Study subjects included 13 controls, 9 active EoE, and 4 EoE in remission, median age 12 years. Of the 45 markers analyzed, three had significant differences between controls and patients with active EoE, Granzyme B, (GzmB), IL-1ra, and CXCL8 (p < .05). Levels of GzmB were higher, and levels of IL-1ra were lower in patients with active EoE compared with controls and EoE in remission both at baseline and after food extract stimulation. CXCL8 increased in active EoE compared with controls only after stimulation. The number of histologically detected GzmB-positive cells were significantly higher in patients with active EoE in contrast to control and EoE remission (p < .05).

CONCLUSIONS

The levels of the barrier-damaging protease GzmB were higher in the supernatant both before and after stimulation with food extract ex vivo in patients with active EoE. GzmB was also observed histologically in biopsies from patients with active EoE. The presence of elevated serine protease GzmB in esophageal mucosa of children with active EoE suggests a role in the pathogenesis of this disorder.

Immunogenic camptothesome nanovesicles comprising sphingomyelin-derived camptothecin bilayers for safe and synergistic cancer immunochemotherapy

Despite the enormous therapeutic potential of immune checkpoint blockade (ICB), it benefits only a small subset of patients. Some chemotherapeutics can switch 'immune-cold' tumours to 'immune-hot' to synergize with ICB. However, safe and universal therapeutic platforms implementing such immune effects remain scarce. We demonstrate that sphingomyelin-derived camptothecin nanovesicles (camptothesomes) elicit potent granzyme-B- and perforin-mediated cytotoxic T lymphocyte (CTL) responses, potentiating PD-L1/PD-1 co-blockade to eradicate subcutaneous MC38 adenocarcinoma with developed memory immunity. In addition, camptothesomes improve the pharmacokinetics and lactone stability of camptothecin, avoid systemic toxicities, penetrate deeply into the tumour and outperform the antitumour efficacy of Onivyde. Camptothesome co-load the indoleamine 2,3-dioxygenase inhibitor indoximod into its interior using the lipid-bilayer-crossing capability of the immunogenic cell death inducer doxorubicin, eliminating clinically relevant advanced orthotopic CT26-Luc tumours and late-stage B16-F10-Luc2 melanoma, and achieving complete metastasis remission when combined with ICB and folate targeting. The sphingomyelin-derived nanotherapeutic platform and doxorubicin-enabled transmembrane transporting technology are generalizable to various therapeutics, paving the way for transformation of the cancer immunochemotherapy paradigm.

T-Cell-Derived Nanovesicles for Cancer Immunotherapy

Although T-cell therapy is a remarkable breakthrough in cancer immunotherapy, the therapeutic efficacy is limited for solid tumors. A major cause of the low efficacy is T-cell exhaustion by immunosuppressive mechanisms of solid tumors, which are mainly mediated by programmed death-ligand 1 (PD-L1) and transforming growth factor-beta (TGF-β). Herein, T-cell-derived nanovesicles (TCNVs) produced by the serial extrusion of cytotoxic T cells through membranes with micro-/nanosized pores that inhibit T-cell exhaustion and exhibit antitumoral activity maintained in the immunosuppressive tumor microenvironment (TME) are presented. TCNVs, which have programmed cell death protein 1 and TGF-β receptor on their surface, block PD-L1 on cancer cells and scavenge TGF-β in the immunosuppressive TME, thereby preventing cytotoxic-T-cell exhaustion. In addition, TCNVs directly kill cancer cells via granzyme B delivery. TCNVs successfully suppress tumor growth in syngeneic-solid-tumor-bearing mice. Taken together, TCNV offers an effective cancer immunotherapy strategy to overcome the tumor's immunosuppressive mechanisms.

A Functional Chemiluminescent Probe for in Vivo Imaging of Natural Killer Cell Activity Against Tumours

Natural killer (NK) cells are immune cells that can kill certain types of cancer cells. Adoptive transfer of NK cells represents a promising immunotherapy for malignant tumours; however, there is a lack of methods to validate anti-tumour activity of NK cells in vivo. Herein, we report a new chemiluminescent probe to image in situ the granzyme B-mediated killing activity of NK cells against cancer cells. We have optimised a granzyme B-specific construct using an activatable phenoxydioxetane reporter so that enzymatic cleavage of the probe results in bright chemiluminescence. The probe shows high selectivity for active granzyme B over other proteases and higher signal-to-noise ratios than commercial fluorophores. Finally, we demonstrate that the probe can detect NK cell activity in mouse models, being the first chemiluminescent probe for in vivo imaging of NK cell activity in live tumours.

Studying the biology of cytotoxic T lymphocytes in vivo with a fluorescent granzyme B-mTFP knock-in mouse

Understanding T cell function in vivo is of key importance for basic and translational immunology alike. To study T cells in vivo, we developed a new knock-in mouse line, which expresses a fusion protein of granzyme B, a key component of cytotoxic granules involved in T cell-mediated target cell-killing, and monomeric teal fluorescent protein from the endogenous Gzmb locus. Homozygous knock-ins, which are viable and fertile, have cytotoxic T lymphocytes with endogeneously fluorescent cytotoxic granules but wild-type-like killing capacity. Expression of the fluorescent fusion protein allows quantitative analyses of cytotoxic granule maturation, transport and fusion in vitro with super-resolution imaging techniques, and two-photon microscopy in living knock-ins enables the visualization of tissue rejection through individual target cell-killing events in vivo. Thus, the new mouse line is an ideal tool to study cytotoxic T lymphocyte biology and to optimize personalized immunotherapy in cancer treatment.

Cytosolic Delivery of Functional Proteins In Vitro through Tunable Gigahertz Acoustics

Intracellular delivery is essential to therapeutic applications such as genome engineering and disease diagnosis. Current methods lack simple, noninvasive strategies and are often hindered by long incubation time or high toxicity. Hydrodynamic approaches offer rapid and controllable delivery of small molecules, but thus far have not been demonstrated for delivering functional proteins. In this work, we developed a robust hydrodynamic approach based on gigahertz (GHz) acoustics to achieve rapid and noninvasive cytosolic delivery of biologically active proteins. With this method, GHz-based acoustic devices trigger oscillations through a liquid medium (acoustic streaming), generating shear stress on the cell membrane and inducing transient nanoporation. This mechanical effect enhances membrane permeability and enables cytosolic access to cationic proteins without disturbing their bioactivity. We evaluated the versatility of this approach through the delivery of cationic fluorescent proteins to a range of cell lines, all of which displayed equally efficient delivery speed (≤20 min). Delivery of multiple enzymatically active proteins with functionality related to apoptosis or genetic recombination further demonstrated the relevance of this method.

Purification and characterization of a fish granzymeA involved in cell-mediated immunity

Granzymes are serine proteases involved in the induction of cell death against non-self cells. The enzymes differ in their primary substrate specificity and have one of four hydrolysis activities: tryptase, Asp-ase, Met-ase and chymase. Although granzyme genes have been isolated from several fishes, evidence for their involvement in cytotoxicity has not yet been reported. In the present study, we attempted to purify and characterize a fish granzyme involved in cytotoxicity using ginbuna crucian carp. The cytotoxicity of leukocytes was significantly inhibited by the serine protease inhibitor ''3, 4-dichloroisocoumarin''. In addition, we found that granzymeA-like activity (hydrolysis of Z-GPR-MCA) was inhibited by the same inhibitor and significantly enhanced by allo-antigen stimulation in vivo. Proteins from leukocyte extracts were subjected to two steps of chromatographic purification using benzamidine-Sepharose and SP-Sepharose. The molecular weight of the purified enzyme was estimated to be 26,900 Da by SDS-PAGE analysis. The purified enzyme displayed a Km of 220 μM, a Kcat of 21.7 sec(-1) and a Kcat/Km of 98,796 sec(-1) M(-1) with an optimal pH of 9.5 for the Z-GPR-MCA substrate. The protease was totally inhibited by serine protease inhibitors and showed granzymeA-like substrate specificity. Therefore, we conclude that the purified enzyme belongs to the mammalian granzymeA (EC 3.4.21.78) and appears to be involved in cytotoxicity in fish.

[Rapid biosynthesis and release of 35 kD granzyme B by NK92 cells bypassing secretory lysosomes]

OBJECTIVE

To investigate the possibility of the biosynthesis and release of granzyme B (GZB) by NK92 cells bypassing the way of secretory lysosomes (SLs) and the possible mechanism.

METHODS

As cell models, NK92 cells were activated by the phorbol myristate acetate (PMA) and ionomycin (ION). Within 4 hours following the activation, immuno- fluorescence and electron microscopy were used to detect the content and distribution of 35 000 (Mr) and 32 000 (Mr) GZB in the cytoplasm of NK92 before and after the protein synthesis was inhibited; Western blotting was performed to detect GZB inside and outside the SLs. After blocking the release of 32 000 (Mr) GZB by inhibiting the exocytosis of SLs with EDTA, we tested the content of Mr 35 000 GZB in activated NK92 supernatant. Activated NK92 cells were co-cultured with K562 cells to observe whether the Mr 35 000 GZB could enter the K562 cells. Activated NK92 cell death rate was determined and the enzyme activity of secreted Mr 35 000 GZB was examined.

RESULTS

Four hours after stimulated by PMA/ION, NK92 cells generated large amount of Mr 35 000 GZB in the cytoplasm outside SLs where Mr 32 000 GZB was located. Immunoelectron microscope and immunofluorescence further approved that Mr 35 000 GZB outside SLs was located in vesicles. In addition, Mr 35 000 GZB could be secreted outside NK92 cells. Further investigation found that GZB/Serpinb9 composite and Mr 35 000 GZB could simultaneously emerge in the cytoplasm outside SLs. However, activated NK92 cell death rate did not rise. Mr 32 000 GZB inside SLs had enzyme activity in contrast with the Mr 35 000 GZB in zymogen form outside SLs, which suggested that Mr 35 000 GZB was not originated from the SLs.

CONCLUSION

The activated human NK cell lines could secreted rapidly inactive Mr 35 000 GZB outside SLs, and the GZB could enter the extracellular matrix or target cells bypassing SLs, which provides a part of the extracellular GZB.

Identification of a novel fish granzyme involved in cell-mediated immunity

Granzymes (Gzms) are serine proteases released from cytoplasmic granules within cytotoxic T lymphocytes and natural killer (NK) cells. Gzms induce apoptosis within virus-infected and transformed cells. In fish as well as mammals, Gzms appear to play a major role in inducing target cell death. However, information on the function of fish Gzms is limited, although Gzm-like genes have been reported in several species. We identified and characterized a fish Gzm (termed gcGzm) in ginbuna crucian carp, Carassius auratus langsdorfii. The primary structure of gcGzm resembled mammalian GzmB, and gcGzm clustered with mammalian GzmB by phylogenetic tree analysis. gcGzm was secreted from HEK293T cells transfected with gcgzm cDNA and was predominantly expressed in CD8(+) T cells, as in mammals. Expression of gcgzm mRNA was greatly enhanced by allo-sensitization and infection with the intracellular pathogen Edwardsiella tarda, indicating that gcGzm is involved in cell-mediated immunity. However, its enzymatic activity was different from mammalian Gzms because gcGzm did not cleave the known substrates for mammalian Gzms. Thus we conclude that the newly discovered gcGzm is a novel secretory serine protease involved in cell-mediated immunity in fish, with similar structure to human GzmB but different substrate specificity.

Identification of Serpinb6b as a species-specific mouse granzyme A inhibitor suggests functional divergence between human and mouse granzyme A

The granzyme family serine proteases are key effector molecules expressed by cytotoxic lymphocytes. The physiological role of granzyme (Gzm) A is controversial, with significant debate over its ability to induce death in target cells. Here, we investigate the natural inhibitors of GzmA. We employed substrate phage display and positional proteomics to compare substrate specificities of mouse (m) and human (h) GzmA at the peptide and proteome-wide levels and we used the resulting substrate specificity profiles to search for potential inhibitors from the intracellular serpin family. We identified Serpinb6b as a potent inhibitor of mGzmA. Serpinb6b interacts with mGzmA, but not hGzmA, with an association constant of 1.9 ± 0.8 × 10(5) M(-1) s(-1) and a stoichiometry of inhibition of 1.8. Mouse GzmA is over five times more cytotoxic than hGzmA when delivered into P815 target cells with streptolysin O, whereas transfection of target cells with a Serpinb6b cDNA increases the EC50 value of mGzmA 13-fold, without affecting hGzmA cytotoxicity. Unexpectedly, we also found that Serpinb6b employs an exosite to specifically inhibit dimeric but not monomeric mGzmA. The identification of an intracellular inhibitor specific for mGzmA only indicates that a lineage-specific increase in GzmA cytotoxic potential has driven cognate inhibitor evolution.

Design of human granzyme B variants resistant to serpin B9

Human granzyme B (hGB) is a serine protease involved in immune-mediated apoptosis. Its cytotoxicity makes it potentially applicable in cancer therapy. However, the effectiveness of hGB can be hampered by the cytosolic expression of a natural protein inhibitor, human Serpin B9 (hSB9). Here, we used computational approaches to identify hGB mutations that can affect its binding to hSB9 without significantly decreasing its catalytic efficiency. Alanine-scanning calculations allowed us to identify residues of hGB important for the interaction with hSB9. Some variants were selected, and molecular dynamic simulations on the mutated hGB in complex with hSB9 in aqueous solution were carried out to investigate the effect of these variants on the stability of the complex. The R28K, R201A, and R201K mutants significantly destabilized the interaction of the protein with hSB9. Consistently, all of these variants also retained their activity in the presence of the Serpin B9 inhibitor in subsequent in vitro assays of wild-type and mutated hGB. In particular, the activity of R201K hGB with and without Serpin B9 is very similar to that of the wild-type protein. Hence, R201K hGB emerges as a promising species for antitumoral therapy applications.

Granzyme A activates another way to die

Granzyme A (GzmA) is the most abundant serine protease in killer cell cytotoxic granules. GzmA activates a novel programed cell death pathway that begins in the mitochondrion, where cleavage of NDUFS3 in electron transport complex I disrupts mitochondrial metabolism and generates reactive oxygen species (ROS). ROS drives the endoplasmic reticulum-associated SET complex into the nucleus, where it activates single-stranded DNA damage. GzmA also targets other important nuclear proteins for degradation, including histones, the lamins that maintain the nuclear envelope, and several key DNA damage repair proteins (Ku70, PARP-1). Cells that are resistant to the caspases or GzmB by overexpressing bcl-2 family anti-apoptotic proteins or caspase or GzmB protease inhibitors are sensitive to GzmA. By activating multiple cell death pathways, killer cells provide better protection against a variety of intracellular pathogens and tumors. GzmA also has proinflammatory activity; it activates pro-interleukin-1beta and may also have other proinflammatory effects that remain to be elucidated.

New approaches for monitoring CTL activity in clinical trials

We have developed a modification of the ELISPOT assay that measures Granzyme B (GrB) release from cytotoxic T lymphocytes (CTLs). The GrB ELISPOT assay is a superior alternative to the 51Cr-release assay since it is significantly more sensitive and provides an estimation of cytotoxic effector cell frequency. Additionally, unlike the IFN-gamma ELISPOT assay, the GrB ELISPOT directly measures the release of a cytolytic protein. We report that the GrB ELISPOT can be utilized to measure ex vivo antigen-specific cytotoxicity of peripheral blood mononuclear cells (PBMCs) from cancer patients vaccinated with a peptide-based cancer vaccine. We compare the reactivity of patients' PBMCs in the GrB ELISPOT, with reactivity in the tetramer, IFN-gamma ELISPOT and chromium (51Cr)-release assays. Differences in immune response over all assays tested were found between patients, and four response patterns were observed. Reactivity in the GrB ELISPOT was more closely associated with cytotoxicity in the 51Cr-release assay than the tetramer or IFN-gamma ELISPOT assays. We also optimized the GrB ELISPOT assay to directly measure immune responses against autologous primary tumor cells in vaccinated cancer patients. A perforin ELISPOT assay was also adapted to evaluate peptide-stimulated reactivity of PMBCs from vaccinated melanoma patients. Modifications of the ELISPOT assay described in this chapter allow a more comprehensive evaluation of low-frequency tumor-specific CTLs and their specific effector functions and can provide a valuable insight into immune responses in cancer vaccine trials.

Hip Is a Pro-survival Substrate of Granzyme B

The extended substrate specificity of granzyme B (GrB) was used to identify substrates among the chaperone superfamily. This approach identified Hsp90 and Bag1-L as novel GrB substrates, and an additional GrB cleavage site was identified in the Hsc70/Hsp70-Interacting Protein, Hip. Hsp90, Bag1L, and Hip were validated as GrB substrates in vitro, and mutational analysis confirmed the additional cleavage site in Hip. Because the role of Hip in apoptosis is unknown, its proteolysis by GrB was used as a basis to test whether it has anti-apoptotic activity. Previous work on Hip was limited to in vitro characterization; therefore, it was important to demonstrate Hip cleavage in a physiological context and to show its relevance to natural killer (NK) cell-mediated death. Hip is cleaved at both GrB cleavage sites during NK-mediated cell death in a caspase-independent manner, and its cleavage is due solely to GrB and not other granule components. Furthermore, Hip is not cleaved upon stimulation of the Fas receptor in the Jurkat T-cell line, suggesting that Hip is a substrate unique to GrB. RNA interference-mediated reduction of Hip within the K562 cell line rendered the cells more susceptible to NK cell-mediated lysis, indicating that proteolysis by GrB of Hip contributes to death induction. The small effect of RNA interference-mediated Hip deficiency on cytotoxicity is in agreement with the inherent redundancy of NK cell-mediated cell death. The identification of additional members of the chaperone superfamily as GrB substrates and the validation of Hip as an anti-apoptotic protein contribute to understanding the interplay between stress response and apoptosis.

Identification and characterization of equine granzyme B

In the present study we describe the isolation and characterization of putative equine granzyme B for which we propose the designation 'eqGrzmB'. Sequence analysis revealed characteristic features of a GrzmB protease such as the presence of a signal (leader-) peptide and an activation di-peptide. The isolated eqGrzmB is functionally active when expressed in human or in insect cells. Furthermore, exchange of any of three putative active site amino acids, which are highly conserved along granzyme B enzymes, led to a complete loss of enzymatic activity in the newly identified eqGrzmB. Phylogenetic analysis places eqGrzmB in the chymase-locus within the large family of granzymes in close proximity to putative equine mast cell protease and to granzyme B from mouse, rat, and human. eqGrzmB proteolytic activity has been kinetically characterized and can be specifically inhibited by granzyme B inhibitors. Taken together, we conclude that we have isolated a new member of the granzyme B family, the first granzyme identified in Equidae. The description of equine granzyme B might facilitate the development of immunological assays for the activity of equine lymphocytes.

ALTERED LEVELS AND MOLECULAR FORMS OF GRANZYME K IN PLASMA FROM SEPTIC PATIENTS

Granzyme K (GrK) is a member of a highly conserved group of potent serine proteases specifically found in the secretory granules of cytotoxic T lymphocytes and natural killer cells. Based on the report indicating that inter-alpha inhibitor proteins are the physiological inhibitors of GrK and on previous findings that showed a significant decrease in plasma inter-alpha inhibitor proteins in patients with sepsis, it was our aim to determine whether increased levels of uninhibited GrK would contribute to the development of sepsis. To test this hypothesis, a competitive enzyme-linked immunosorbent assay system was developed; and the levels of GrK were measured in plasma samples obtained from healthy controls and 2 sets of patients with sepsis: patients admitted to the emergency department with a putative diagnosis of sepsis and patients with severe sepsis enrolled in a clinical trial. In addition, the molecular form(s) of GrK present in these samples was analyzed by Western blot. The levels of GrK were significantly increased in emergency department patients compared with healthy controls and significantly decreased in patients with severe sepsis enrolled in a clinical trial compared with healthy controls. GrK was detected as high-molecular-weight protein complexes in healthy controls but as complexes of lower molecular weight in the septic patients. The decrease in complex size correlated with the appearance of a band at 26 kDa similar to the size of free GrK. Our results indicate that plasma levels of GrK could serve as a useful diagnostic marker to stage sepsis, permitting better classification of septic patients and enabling targeting of specific treatments, and may play a functional role in the development of sepsis.

Granzyme K Directly Processes Bid to Release Cytochrome c and Endonuclease G Leading to Mitochondria-dependent Cell Death

Granule-mediated cytolysis is the major pathway for killer lymphocytes to kill pathogens and tumor cells. Little is known about how granzyme K functions in killer lymphocyte-mediated cytolysis. We previously showed that human GzmK triggers rapid cell death independently of caspase activation with single-stranded DNA nicks, similar to GzmA. In this study we found that GzmK can induce rapid reactive oxygen species generation and collapse of mitochondrial inner membrane potential (DeltaPsim). Blockade of reactive oxygen species production by antioxidant N-acetylcysteine or superoxide scavenger Tiron inhibits GzmK-induced cell death. Moreover GzmK targets mitochondria by cleaving Bid to generate its active form tBid, which disrupts the outer mitochondrial membrane leading to the release of cytochrome c and endonuclease G. Thus, we showed herein that GzmK-induced caspase-independent death occurs through Bid-dependent mitochondrial damage that is different from GzmA.

Human and murine granzyme B exhibit divergent substrate preferences

The cytotoxic lymphocyte protease granzyme B (GzmB) can promote apoptosis through direct processing and activation of members of the caspase family. GzmB can also cleave the BH3-only protein, BID, to promote caspase-independent mitochondrial permeabilization. Although human and mouse forms of GzmB exhibit extensive homology, these proteases diverge at residues predicted to influence substrate binding. We show that human and mouse GzmB exhibit radical differences in their ability to cleave BID, as well as several other key substrates, such as ICAD and caspase-8. Moreover, pharmacological inhibition of caspases clonogenically rescued human and mouse target cells from apoptosis initiated by mouse GzmB, but failed to do so in response to human GzmB. These data demonstrate that human and murine GzmB are distinct enzymes with different substrate preferences. Our observations also illustrate how subtle differences in enzyme structure can radically affect substrate selection.

The major human and mouse granzymes are structurally and functionally divergent

Approximately 2% of mammalian genes encode proteases. Comparative genomics reveals that those involved in immunity and reproduction show the most interspecies diversity and evidence of positive selection during evolution. This is particularly true of granzymes, the cytotoxic proteases of natural killer cells and CD8+ T cells. There are 5 granzyme genes in humans and 10 in mice, and it is suggested that granzymes evolve to meet species-specific immune challenge through gene duplication and more subtle alterations to substrate specificity. We show that mouse and human granzyme B have distinct structural and functional characteristics. Specifically, mouse granzyme B is 30 times less cytotoxic than human granzyme B and does not require Bid for killing but regains cytotoxicity on engineering of its active site cleft. We also show that mouse granzyme A is considerably more cytotoxic than human granzyme A. These results demonstrate that even "orthologous" granzymes have species-specific functions, having evolved in distinct environments that pose different challenges.

Novel conformation of histidyl–transfer RNA synthetase in the lung

OBJECTIVE

We previously proposed that novel expression and/or conformation of autoantigens in the target tissue may play a role in generating phenotype-specific immune responses. The strong association of autoantibodies to histidyl-transfer RNA synthetase (HisRS, Jo-1) with interstitial lung disease in patients with myositis led us to study HisRS expression and conformation in the lung.

METHODS

Normal human tissue specimens were probed with a novel anti-HisRS antibody recognizing its granzyme B-cleavable conformation by immunoblotting and immunohistochemistry. The HisRS granzyme B site was mapped using site-directed mutagenesis, and its relationship to the antibody recognition domain was evaluated in tandem immunoprecipitation/granzyme B cleavage studies.

RESULTS

The HisRS alpha-helical coiled-coil N-terminal domain recognized by autoantibodies is bounded by a granzyme B cleavage site. In immunoprecipitation studies with patient sera, HisRS was found to exist in 2 conformations, defined by sensitivity to cleavage by granzyme B and modification by autoantibody binding. Despite similar global expression of HisRS in different tissue, expression of its granzyme B-cleavable form was enriched in the lung and localized to the alveolar epithelium.

CONCLUSION

A proteolytically sensitive conformation of HisRS exists in the lung, the target tissue associated with this autoantibody response. We thus propose that autoimmunity to HisRS is initiated and propagated in the lung.

Mouse and human granzyme B have distinct tetrapeptide specificities and abilities to recruit the bid pathway

Granzyme B is an important mediator of cytotoxic lymphocyte granule-induced death of target cells, accomplishing this through cleavage of Bid and cleavage and activation of caspases as well as direct cleavage of downstream substrates. Significant controversy exists regarding the primary pathways used by granzyme B to induce cell death, perhaps arising from the use of different protease/substrate combinations in different studies. The primary sequence of human, rat, and mouse granzymes B is well conserved, and the substrate specificity and crystal structure of the human and rat proteases are extremely similar. Although little is known about the substrate specificity of mouse granzyme B, recent studies suggest that it may differ significantly from the human protease. In these studies we show that the specificities of human and mouse granzymes B differ significantly. Human and mouse granzyme B cleave species-specific procaspase-3 more efficiently than the unmatched substrates. The distinct specificities of human and mouse granzyme B highlight a previously unappreciated requirement for Asp(192) in the acquisition of catalytic activity upon cleavage of procaspase-3 at Asp(175). Although human granzyme B efficiently cleaves human or mouse Bid, these substrates are highly resistant to cleavage by the mouse protease, strongly indicating that the Bid pathway is not a major primary mediator of the effects of mouse granzyme B. These studies provide important insights into the substrate specificity and function of the granzyme B pathway in different species and highlight that caution is essential when designing and interpreting experiments with different forms of granzyme B.

Serine and cysteine proteases are translocated to similar extents upon formation of covalent complexes with serpins. Fluorescence perturbation and fluorescence resonance energy transfer mapping of the protease binding site in CrmA complexes with granzyme B and caspase-1

CrmA is a "cross-class" serpin family inhibitor of the proapoptotic serine protease, granzyme B, as well as cysteine proteases of the caspase family. To determine whether crmA inhibits these structurally diverse proteases by a common conformational trapping mechanism, we mapped the position of the protease in crmA complexes with granzyme B or caspase-1 by fluorescence perturbation and fluorescence resonance energy transfer (FRET) analyses of site-specific fluorophore-labeled crmAs. A reactive loop P6 NBD label underwent similar large fluorescence enhancements (>200%) either upon reactive loop cleavage by AspN protease or complex formation with granzyme B or caspase-1, consistent with the insertion of the cleaved reactive loop into sheet A in both types of crmA-protease complexes. NBD labels on the noninserting part of the reactive loop docking site for protease (P1' residue) or midway between the two ends of sheet A (helix F residue 101) showed no significant perturbations due to protease complexation. By contrast, labels at positions 68 and 261, lying at the end of sheet A most distal from the reactive loop, showed marked perturbations distinct from those induced by AspN cleavage and thus ascribable to granzyme B or caspase-1 proximity in the complexes. Substantial FRET between protease tryptophans and 5-dimethylaminonaphthalene-1-sulfonyl-labeled crmAs occurred in protease complexes with crmAs labeled at the 68 and 261 positions, but not the P1' position. These results suggest that granzyme B and caspase-1 are inhibited by crmA by a common mechanism involving full reactive loop insertion into sheet A and translocation of the protease to the distal end of the sheet as previously found for inhibition of other serine proteases by serpins.

Hop cleavage and function in granzyme B-induced apoptosis

Granzyme B (GzmB) is a cytotoxic protease found in the granules of natural killer cells and cytotoxic T lymphocytes. GzmB cleaves multiple intracellular protein substrates, leading to caspase activation, DNA fragmentation, cytoskeletal instability, and rapid induction of target cell apoptosis. However, no known individual substrate is required for GzmB to induce apoptosis. GzmB is therefore thought to initiate multiple cell death pathways simultaneously to ensure the death of target cells. We previously identified Hop (Hsp70/Hsp90-organizing protein) as a GzmB substrate in a proteomic survey (Bredemeyer, A. J., Lewis, R. M., Malone, J. P., Davis, A. E., Gross, J., Townsend, R. R., and Ley, T. J. (2004) Proc. Natl. Acad. Sci. U. S. A. 101, 11785-11790). Hop is a co-chaperone for Hsp70 and Hsp90, which have been implicated in the negative regulation of apoptosis. We therefore hypothesized that Hop may have an anti-apoptotic function that is abolished upon cleavage, lowering the threshold for GzmB-induced apoptosis. Here, we show that Hop was cleaved directly by GzmB in vitro and in cells undergoing GzmB-induced apoptosis. Expression of the two cleavage fragments of Hop did not induce cell death. Although cleavage of Hop by GzmB destroyed Hop function in vitro, both cells overexpressing GzmB-resistant Hop and cells with a 90-95% reduction in Hop levels exhibited unaltered susceptibility to GzmB-induced death. We conclude that Hop per se does not set the threshold for susceptibility to GzmB-induced apoptosis. Although it is possible that Hop may be cleaved by GzmB as an "innocent bystander" during the induction of apoptosis, it may also act to facilitate apoptosis in concert with other GzmB substrates.