Granzyme A Contributes to Inflammatory Arthritis in Mice Through Stimulation of Osteoclastogenesis.

OBJECTIVE: Granzyme A (GzmA) levels are elevated in the plasma and synovium of patients with rheumatoid arthritis (RA), suggesting involvement of this protease in the pathogenesis of the disease. GzmA contributes to sepsis by regulating the production of proinflammatory cytokines. The purpose of this study was to evaluate the contribution of GzmA to the pathogenesis of RA in vivo and to examine the possibility that GzmA acting via tumor necrosis factor (TNF) stimulates osteoclastogenesis. METHODS: Inflammatory arthritis induced by type II collagen was evaluated in wild-type, GzmA-deficient, and perforin-deficient mice. The osteoclastogenic potential of GzmA was examined in vitro using bone marrow cells and colony-forming unit-granulocyte-macrophage (CFU-GM) cells and in vivo using GzmA-deficient mice. RESULTS: Gene deletion of GzmA attenuated collagen-induced arthritis, including serum levels of proinflammatory cytokines, joint damage, and bone erosion in affected mice, suggesting that osteoclast activity is reduced in the absence of GzmA. Accordingly, GzmA-treated bone marrow cells produced multinucleated cells that fulfilled the criteria for mature osteoclasts: tartrate-resistant acid phosphatase (TRAP) activity, ß integrin expression, calcitonin receptor expression, and resorptive activity on dentin slices. GzmA appeared to act without accessory cells, and its activity was not affected by osteoprotegerin, suggesting a minor contribution of RANKL. It also induced the expression and secretion of TNF. Neutralization of TNF or stimulation of CFU-GM cells from TNF-/- mice prevented GzmA-induced osteoclastogenesis. GzmA-deficient mice had reduced osteoclastogenesis in vivo (fewer calcitonin receptor-positive multinucleated cells and fewer transcripts for cathepsin K, matrix metalloproteinase 9, and TRAP in joints) and reduced serum levels of C-terminal telopeptide of type I collagen. CONCLUSION: GzmA contributes to the joint destruction of RA partly by promoting osteoclast differentiation.

Granzymes A and K differentially potentiate LPS-induced cytokine response.

Granzymes are serine proteases that, upon release from cytotoxic cells, induce apoptosis in tumor cells and virally infected cells. In addition, a role of granzymes in inflammation is emerging. Recently, we have demonstrated that extracellular granzyme K (GrK) potentiates lipopolysaccharide (LPS)-induced cytokine response from monocytes. GrK interacts with LPS, disaggregates LPS micelles, and stimulates LPS-CD14 binding and Toll-like receptor signaling. Here we show that human GrA also potentiates cytokine responses in human monocytes initiated by LPS or Gram-negative bacteria. Similar to GrK, this effect is independent of GrA catalytic activity. Unlike GrK, however, GrA does not bind to LPS, has little influence on LPS micelle disaggregation, and does not augment LPS-CD14 complex formation. We conclude that GrA and GrK differentially modulate LPS-Toll-like receptor signaling in monocytes, suggesting functional redundancy among cytotoxic lymphocyte proteases in the anti-bacterial innate immune response.

Mouse cytotoxic T cell-derived granzyme B activates the mitochondrial cell death pathway in a Bim-dependent fashion.

Cytotoxic T cells (Tc) use perforin and granzyme B (gzmB) to kill virus-infected cells and cancer cells. Recent evidence suggests that human gzmB primarily induces apoptosis via the intrinsic mitochondrial pathway by either cleaving Bid or activating Bim leading to the activation of Bak/Bax and subsequent generation of active caspase-3. In contrast, mouse gzmB is thought to predominantly induce apoptosis by directly processing pro-caspase-3. However, in certain mouse cell types gzmB-mediated apoptosis mainly occurs via the mitochondrial pathway. To investigate whether Bim is involved under the latter conditions, we have now employed ex vivo virus-immune mouse Tc that selectively kill by using perforin and gzmB (gzmB(+)Tc) as effector cells and wild type as well as Bim- or Bak/Bax-deficient spontaneously (3T9) or virus-(SV40) transformed mouse embryonic fibroblast cells as targets. We show that gzmB(+)Tc-mediated apoptosis (phosphatidylserine translocation, mitochondrial depolarization, cytochrome c release, and caspase-3 activation) was severely reduced in 3T9 cells lacking either Bim or both Bak and Bax. This outcome was related to the ability of Tc cells to induce the degradation of Mcl-1 and Bcl-XL, the anti-apoptotic counterparts of Bim. In contrast, gzmB(+)Tc-mediated apoptosis was not affected in SV40-transformed mouse embryonic fibroblast cells lacking Bak/Bax. The data provide evidence that Bim participates in mouse gzmB(+)Tc-mediated apoptosis of certain targets by activating the mitochondrial pathway and suggest that the mode of cell death depends on the target cell. Our results suggest that the various molecular events leading to transformation and/or immortalization of cells have an impact on their relative resistance to the multiple gzmB(+)Tc-induced death pathways.

Membrane pore formation at protein-lipid interfaces.

Pore-forming proteins (PFPs) interact with lipid bilayers to compromise membrane integrity. Many PFPs function by inserting a ring of oligomerized subunits into the bilayer to form a protein-lined hydrophilic channel. However, mounting evidence suggests that PFPs can also generate 'proteolipidic' pores by contributing to the fusion of inner and outer bilayer leaflets to form a toroidal structure. We discuss here toroidal pore formation by peptides including melittin, protegrin, and Alzheimer's Aß1-41, as well as by PFPs from several evolutionarily unrelated families: the colicin/Bcl-2 grouping including the pro-apoptotic protein Bax, actinoporins derived from sea anemones, and the membrane attack complex-perforin/cholesterol dependent cytolysin (MACPF/CDC) set of proteins. We also explore how the structure and biological role of toroidal pores might be investigated further.

Elucidating sources and roles of granzymes A and B during bacterial infection and sepsis.

During bacterial sepsis, proinflammatory cytokines contribute to multiorgan failure and death in a process regulated in part by cytolytic cell granzymes. When challenged with a sublethal dose of the identified mouse pathogen Brucella microti, wild-type (WT) and granzyme A (gzmA)(-/-) mice eliminate the organism from liver and spleen in 2 or 3 weeks, whereas the bacteria persist in mice lacking perforin or granzyme B as well as in mice depleted of Tc cells. In comparison, after a fatal challenge, only gzmA(-/-) mice exhibit increased survival, which correlated with reduced proinflammatory cytokines. Depletion of natural killer (NK) cells protects WT mice from sepsis without influencing bacterial clearance and the transfer of WT, but not gzmA(-/-) NK, cells into gzmA(-/-) recipients restores the susceptibility to sepsis. Therefore, infection-related pathology, but not bacterial clearance, appears to require gzmA, suggesting the protease may be a therapeutic target for the prevention of bacterial sepsis without affecting immune control of the pathogen.

Granzyme A produced by γ(9)δ(2) T cells induces human macrophages to inhibit growth of an intracellular pathogen.

Human γ(9)δ(2) T cells potently inhibit pathogenic microbes, including intracellular mycobacteria, but the key inhibitory mechanism(s) involved have not been identified. We report a novel mechanism involving the inhibition of intracellular mycobacteria by soluble granzyme A. γ(9)δ(2) T cells produced soluble factors that could pass through 0.45 µm membranes and inhibit intracellular mycobacteria in human monocytes cultured below transwell inserts. Neutralization of TNF-α in co-cultures of infected monocytes and γ(9)δ(2) T cells prevented inhibition, suggesting that TNF-α was the critical inhibitory factor produced by γ(9)δ(2) T cells. However, only siRNA- mediated knockdown of TNF-α in infected monocytes, but not in γ(9)δ(2) T cells, prevented mycobacterial growth inhibition. Investigations of other soluble factors produced by γ(9)δ(2) T cells identified a highly significant correlation between the levels of granzyme A produced and intracellular mycobacterial growth inhibition. Furthermore, purified granzyme A alone induced inhibition of intracellular mycobacteria, while knockdown of granzyme A in γ(9)δ(2) T cell clones blocked their inhibitory effects. The inhibitory mechanism was independent of autophagy, apoptosis, nitric oxide production, type I interferons, Fas/FasL and perforin. These results demonstrate a novel microbial defense mechanism involving granzyme A-mediated triggering of TNF-α production by monocytes leading to intracellular mycobacterial growth suppression. This pathway may provide a protective mechanism relevant for the development of new vaccines and/or immunotherapies for macrophage-resident chronic microbial infections.

Effects of MACPF/CDC proteins on lipid membranes.

Recent work on the MACPF/CDC superfamily of pore-forming proteins has focused on the structural analysis of monomers and pore-forming oligomeric complexes. We set the family of proteins in context and highlight aspects of their function which the direct and exclusive equation of oligomers with pores fails to explain. Starting with a description of the distribution of MACPF/CDC proteins across the domains of life, we proceed to show how their evolutionary relationships can be understood on the basis of their structural homology and re-evaluate models for pore formation by perforin, in particular. We furthermore highlight data showing the role of incomplete oligomeric rings (arcs) in pore formation and how this can explain small pores generated by oligomers of proteins belonging to the family. We set this in the context of cell biological and biophysical data on the proteins' function and discuss how this helps in the development of an understanding of how they act in processes such as apicomplexan parasites gliding through cells and exiting from cells.

Interleukin-1R signaling is essential for induction of proapoptotic CD8 T cells, viral clearance, and pathology during lymphocytic choriomeningitis virus infection in mice.

The T cell granule exocytosis pathway is essential to control hepatotropic lymphocytic choriomeningitis virus strain WE (LCMV-WE) but also contributes to the observed pathology in mice. Although effective antiviral T cell immunity and development of viral hepatitis are strictly dependent on perforin and granzymes, the molecular basis underlying induction of functionally competent virus-immune T cells, including participation of the innate immune system, is far from being resolved. We demonstrate here that LCMV-immune T cells of interleukin-1 receptor (IL-1R)-deficient mice readily express transcripts for perforin and granzymes but only translate perforin, resulting in the lack of proapoptotic potential in vitro. LCMV is not cleared in IL-1R-deficient mice, and yet the infected mice develop neither splenomegaly nor hepatitis. These results demonstrate that IL-1R signaling is central to the induction of proapoptotic CD8 T cell immunity, including viral clearance and associated tissue injuries in LCMV infection.

Perforin activity at membranes leads to invaginations and vesicle formation.

The cytotoxic cell granule secretory pathway is essential for immune defence. How the pore-forming protein perforin (PFN) facilitates the cytosolic delivery of granule-associated proteases (granzymes) remains enigmatic. Here we show that PFN is able to induce invaginations and formation of complete internal vesicles in giant unilamellar vesicles. Formation of internal vesicles depends on native PFN and calcium and antibody labeling shows the localization of PFN at the invaginations. This vesiculation is recapitulated in large unilamellar vesicles and in this case PFN oligomers can be seen associated with the necks of the invaginations. Capacitance measurements show PFN is able to increase a planar lipid membrane surface area in the absence of pore formation, in agreement with the ability to induce invaginations. Finally, addition of PFN to Jurkat cells causes the formation of internal vesicles prior to pore formation. PFN is capable of triggering an endocytosis-like event in addition to pore formation, suggesting a new paradigm for its role in delivering apoptosis-inducing granzymes into target cells.

Perforin rapidly induces plasma membrane phospholipid flip-flop.

The cytotoxic cell granule secretory pathway is essential for host defense. This pathway is fundamentally a form of intracellular protein delivery where granule proteases (granzymes) from cytotoxic lymphocytes are thought to diffuse through barrel stave pores generated in the plasma membrane of the target cell by the pore forming protein perforin (PFN) and mediate apoptotic as well as additional biological effects. While recent electron microscopy and structural analyses indicate that recombinant PFN oligomerizes to form pores containing 20 monomers (20 nm) when applied to liposomal membranes, these pores are not observed by propidium iodide uptake in target cells. Instead, concentrations of human PFN that encourage granzyme-mediated apoptosis are associated with pore structures that unexpectedly favor phosphatidylserine flip-flop measured by Annexin-V and Lactadherin. Efforts that reduce PFN mediated Ca influx in targets did not reduce Annexin-V reactivity. Antigen specific mouse CD8 cells initiate a similar rapid flip-flop in target cells. A lipid that augments plasma membrane curvature as well as cholesterol depletion in target cells enhance flip-flop. Annexin-V staining highly correlated with apoptosis after Granzyme B (GzmB) treatment. We propose the structures that PFN oligomers form in the membrane bilayer may include arcs previously observed by electron microscopy and that these unusual structures represent an incomplete mixture of plasma membrane lipid and PFN oligomers that may act as a flexible gateway for GzmB to translocate across the bilayer to the cytosolic leaflet of target cells.

Human perforin employs different avenues to damage membranes.

Perforin (PFN) is a pore-forming protein produced by cytotoxic lymphocytes that aids in the clearance of tumor or virus-infected cells by a mechanism that involves the formation of transmembrane pores. The properties of PFN pores and the mechanism of their assembly remain unclear. Here, we studied pore characteristics by functional and structural methods to show that perforin forms pores more heterogeneous than anticipated. Planar lipid bilayer experiments indicate that perforin pores exhibit a broad range of conductances, from 0.15 to 21 nanosiemens. In comparison with large pores that possessed low noise and remained stably open, small pores exhibited high noise and were very unstable. Furthermore, the opening step and the pore size were dependent on the lipid composition of the membrane. The heterogeneity in pore sizes was confirmed with cryo-electron microscopy and showed a range of sizes matching that observed in the conductance measurements. Furthermore, two different membrane-bound PFN conformations were observed, interpreted as pre-pore and pore states of the protein. The results collectively indicate that PFN forms heterogeneous pores through a multistep mechanism and provide a new paradigm for understanding the range of different effects of PFN and related membrane attack complex/perforin domain proteins observed in vivo and in vitro.

Human perforin permeabilizing activity, but not binding to lipid membranes, is affected by pH.

The various steps that perforin (PFN), a critical mediator of innate immune response, undertakes to form a transmembrane pore remains poorly understood. We have used surface plasmon resonance (SPR) to dissect mechanism of pore formation. The membrane association of PFN was calcium dependent irrespective of pH. However, PFN does not permeabilize large or giant unilamellar vesicles (GUV) at pH 5.5 even though the monomers bind to the membranes in the presence of calcium. It was possible to activate adsorbed PFN and to induce membrane permeabilization by simply raising pH to a physiological level (pH 7.4). These results were independently confirmed on GUV and Jurkat cells. The conformational state of PFN at either pH was further assessed with monoclonal antibodies Pf-80 and Pf-344. Pf-344 maps to a linear epitope within region 373-388 of epidermal growth factor (EGF)-like domain while the Pf-80 appears to recognize a conformational epitope. Pf-344 interacts with the EGF-like domain after PFN monomers undergo pore formation, the site recognized by Pf-80 is only accessible at acidic but not neutral pH. Thus, the Pf-80 mAb likely interacts with a region of the monomer that participates in oligomerization prior to insertion of the monomer into the lipid bilayer and thus may have therapeutic utility against PFN-mediated immunopathology.

Granzyme B-induced and caspase 3-dependent cleavage of gelsolin by mouse cytotoxic T cells modifies cytoskeleton dynamics.

Granule-associated perforin and granzymes (gzms) are key effector molecules of cytotoxic T lymphocytes (Tc cells) and natural killer cells and play a critical role in the control of intracellular pathogens and cancer. Based on the notion that many gzms, including A, B, C, K, H, and M exhibit cytotoxic activity in vitro, all gzms are believed to serve a similar function in vivo. However, more recent evidence supports the concept that gzms are not unidimensional but, rather, possess non-cytotoxic potential, including stimulation of pro-inflammatory cytokines and anti-viral activities. The present study shows that isolated mouse gzmB cleaves the actin-severing mouse protein, cytoplasmic gelsolin (c-gelsolin) in vitro. However, when delivered to intact target cells by ex vivo immune Tc cells, gzmB mediates c-gelsolin proteolysis via activation of caspases 3/7. The NH(2)-terminal c-gelsolin fragment generated by either gzmB or caspase 3 in vitro constitutively severs actin filaments without destroying the target cells. The observation that gzmB secreted by Tc cells initiates a caspase cascade that disintegrates the actin cytoskeleton in target cells suggests that this intracellular process may contribute to anti-viral host defense.

Deregulation of mitochondrial membrane potential by mitochondrial insertion of granzyme B and direct Hax-1 cleavage.

The cytoplasm and the nucleus have been identified as activity sites for granzyme B (GrB) following its delivery from cytotoxic lymphocyte granules into target cells. Here we report on the ability of exogenous GrB to insert into and function within a proteinase K-resistant mitochondrial compartment. We identified Hax-1 (HS-1-associated protein X-1), a mitochondrial protein involved in the maintenance of mitochondrial membrane potential, as a GrB substrate within the mitochondrion. GrB cleaves Hax-1 into two major fragments: an N-terminal fragment that localizes to mitochondria and a C-terminal fragment that localizes to the cytosol after being released from GrB-treated mitochondria. The N-terminal Hax-1 fragment major cellular impact is on the regulation of mitochondrial polarization. Overexpression of wild-type Hax-1 or its uncleavable mutant form protects the mitochondria against GrB or valinomycin-mediated depolarization. The N-terminal Hax-1 fragment functions as a dominant negative form of Hax-1, mediating mitochondrial depolarization in a cyclophilin D-dependent manner. Thus, induced expression of the N-terminal Hax-1 fragment results in mitochondrial depolarization and subsequent lysosomal degradation of such altered mitochondria. This study is the first to demonstrate GrB activity within the mitochondrion and to identify Hax-1 cleavage as a novel mechanism for GrB-mediated mitochondrial depolarization.

The biology of cytotoxic cell granule exocytosis pathway: granzymes have evolved to induce cell death and inflammation.

The granule exocytosis pathway of cytotoxic lymphocytes (Tc and NK cells) is critical for control of tumor development and viral infections. Granule-associated perforin and granzymes are key components in Tc cell-mediated function(s). On the basis of studies that showed granzymes A, B, C, K and M, to induce apoptosis in vitro, all granzymes were thought to also induce cell death in vivo. This review summarizes our present understanding of the biological processes elicited by purified granzyme A and granzyme as well as the processes induced by the more physiologically relevant cytotoxic cells secreting these proteases. The combined evidence supports the concept that the granule secretion pathway is not mono-specific but rather poly-functional including induction of pro-inflammatory cytokines, besides their widely appreciated apoptotic properties.

Granule-associated serine proteases: granzymes might not just be killer proteases.

The cytotoxic cell granule secretory pathway is viewed as indispensable for eliminating tumor and virally infected cells through a process in which the pore-forming protein, perforin, delivers the serine protease granzymes into cells targeted for destruction. Residing in cytotoxic cells, granzymes were originally anticipated to act both extracellularly and intracellularly. With the discovery that isolated granzymes induce apoptosis when combined with perforin, the broader functionality of the granzymes became unattractive. The purpose of this article is to describe observations indicating that granzymes possess non-cytotoxic activities that might include such diverse biologic effects as stimulation of pro-inflammatory cytokines, remodeling of extracellular matrices and inactivation of intracellular pathogens.

Human and mouse granzyme A induce a proinflammatory cytokine response.

Granzyme A (GzmA) is considered a major proapoptotic protease. We have discovered that GzmA-induced cell death involves rapid membrane damage that depends on the synergy between micromolar concentrations of GzmA and sublytic perforin (PFN). Ironically, GzmA and GzmB, independent of their catalytic activity, both mediated this swift necrosis. Even without PFN, lower concentrations of human GzmA stimulated monocytic cells to secrete proinflammatory cytokines (interleukin-1beta [IL-1beta], TNFalpha, and IL-6) that were blocked by a caspase-1 inhibitor. Moreover, murine GzmA and GzmA(+) cytotoxic T lymphocytes (CTLs) induce IL-1beta from primary mouse macrophages, and GzmA(-/-) mice resist lipopolysaccharide-induced toxicity. Thus, the granule secretory pathway plays an unexpected role in inflammation, with GzmA acting as an endogenous modulator.

Mcl-1 is a key regulator of apoptosis resistance in Chlamydia trachomatis-infected cells.

Chlamydia are obligate intracellular bacteria that cause variety of human diseases. Host cells infected with Chlamydia are protected against many different apoptotic stimuli. The induction of apoptosis resistance is thought to be an important immune escape mechanism allowing Chlamydia to replicate inside the host cell. Infection with C. trachomatis activates the Raf/MEK/ERK pathway and the PI3K/AKT pathway. Here we show that inhibition of these two pathways by chemical inhibitors sensitized C. trachomatis infected cells to granzyme B-mediated cell death. Infection leads to the Raf/MEK/ERK-mediated up-regulation and PI3K-dependent stabilization of the anti-apoptotic Bcl-2 family member Mcl-1. Consistently, interfering with Mcl-1 up-regulation sensitized infected cells for apoptosis induced via the TNF receptor, DNA damage, granzyme B and stress. Our data suggest that Mcl-1 up-regulation is primarily required to maintain apoptosis resistance in C. trachomatis-infected cells.

NK cell protease granzyme M targets alpha-tubulin and disorganizes the microtubule network.

Serine protease granzyme M (GrM) is highly expressed in the cytolytic granules of NK cells, which eliminate virus-infected cells and tumor cells. The molecular mechanisms by which GrM induces cell death, however, remain poorly understood. In this study we used a proteomic approach to scan the native proteome of human tumor cells for intracellular substrates of GrM. Among other findings, this approach revealed several components of the cytoskeleton. GrM directly and efficiently cleaved the actin-plasma membrane linker ezrin and the microtubule component alpha-tubulin by using purified proteins, tumor cell lysates, and tumor cells undergoing cell death induced by perforin and GrM. These cleavage events occurred independently of caspases or other cysteine proteases. Kinetically, alpha-tubulin was more efficiently cleaved by GrM as compared with ezrin. Direct alpha-tubulin proteolysis by GrM is complex and occurs at multiple cleavage sites, one of them being Leu at position 269. GrM disturbed tubulin polymerization dynamics in vitro and induced microtubule network disorganization in tumor cells in vivo. We conclude that GrM targets major components of the cytoskeleton that likely contribute to NK cell-induced cell death.