NK cells are key mediators of immune cell-mediated cytotoxicity toward infected and transformed cells, being one of the main executors of cell death in the immune system. NK cells recognize target cells through an array of inhibitory and activating receptors for endogenous or exogenous pathogen-derived ligands, which together with adhesion molecules form a structure known as immunological synapse that regulates NK cell effector functions. The main and best characterized mechanisms involved in NK cell-mediated cytotoxicity are the granule exocytosis pathway (perforin/granzymes) and the expression of death ligands. These pathways are recognized as activators of different cell death programmes on the target cells leading to their destruction. However, most studies analyzing these pathways have used pure recombinant or native proteins instead of intact NK cells and, thus, extrapolation of the results to NK cell-mediated cell death might be difficult. Specially, since the activation of granule exocytosis and/or death ligands during NK cell-mediated elimination of target cells might be influenced by the stimulus received from target cells and other microenvironment components, which might affect the cell death pathways activated on target cells. Here we will review and discuss the available experimental evidence on how NK cells kill target cells, with a special focus on the different cell death modalities that have been found to be activated during NK cell-mediated cytotoxicity; including apoptosis and more inflammatory pathways like necroptosis and pyroptosis. In light of this new evidence, we will develop the new concept of cell death induced by NK cells as a new regulatory mechanism linking innate immune response with the activation of tumour adaptive T cell responses, which might be the initiating stimulus that trigger the cancer-immunity cycle. The use of the different cell death pathways and the modulation of the tumour cell molecular machinery regulating them might affect not only tumour cell elimination by NK cells but, in addition, the generation of T cell responses against the tumour that would contribute to efficient tumour elimination and generate cancer immune memory preventing potential recurrences.
Sepsis is a serious global health problem. In addition to a high incidence, this syndrome has a high mortality and is responsible for huge health expenditure. The pathophysiology of sepsis is very complex and it is not well-understood yet. However, it is widely accepted that the initial phase of sepsis is characterized by a hyperinflammatory response while the late phase is characterized by immunosuppression and immune anergy, increasing the risk of secondary infections. Granzymes (Gzms) are a family of serine proteases classified according to their cleavage specificity. Traditionally, it was assumed that all Gzms acted as cytotoxic proteases. However, recent evidence suggests that GzmB is the one with the greatest cytotoxic capacity, while the cytotoxicity of others such as GzmA and GzmK is not clear. Recent studies have found that GzmA, GzmB, GzmK, and GzmM act as pro-inflammatory mediators. Specially, solid evidences show that GzmA and GzmK function as extracellular proteases that regulate the inflammatory response irrespectively of its ability to induce cell death. Indeed, studies in animal models indicate that GzmA is involved in the cytokine release syndrome characteristic of sepsis. Moreover, the GZM family also could regulate other biological processes involved in sepsis pathophysiology like the coagulation cascade, platelet function, endothelial barrier permeability, and, in addition, could be involved in the immunosuppressive stage of sepsis. In this review, we provide a comprehensive overview on the contribution of these novel functions of Gzms to sepsis and the new therapeutic opportunities emerging from targeting these proteases for the treatment of this serious health problem.
Aims: Peritonitis is one of the most common causes of sepsis, a serious syndrome characterized by a dysregulated systemic inflammatory response. Recent evidence suggests that Granzyme A (GzmA), a serine protease mainly expressed by NK and T cells, could act as a proinflammatory mediator and could play an important role in the pathogenesis of sepsis. This work aims to analyze the role and the therapeutic potential of GzmA in the pathogenesis of peritoneal sepsis. Methods: The level of extracellular GzmA as well as GzmA activity were analyzed in serum from healthy volunteers and patients with confirmed peritonitis and were correlated with the Sequential Organ Failure Assessment (SOFA) score. Peritonitis was induced in C57Bl/6 (WT) and GzmA -/- mice by cecal ligation and puncture (CLP). Mice were treated intraperitoneally with antibiotics alone or in combination serpinb6b, a specific GzmA inhibitor, for 5 days. Mouse survival was monitored during 14 days, levels of some proinflammatory cytokines were measured in serum and bacterial load and diversity was analyzed in blood and spleen at different times. Results: Clinically, elevated GzmA was observed in serum from patients with abdominal sepsis suggesting that GzmA plays an important role in this pathology. In the CLP model GzmA deficient mice, or WT mice treated with an extracellular GzmA inhibitor, showed increased survival, which correlated with a reduction in proinflammatory markers in both serum and peritoneal lavage fluid. GzmA deficiency did not influence bacterial load in blood and spleen and GzmA did not affect bacterial replication in macrophages in vitro, indicating that GzmA has no role in bacterial control. Analysis of GzmA in lymphoid cells following CLP showed that it was mainly expressed by NK cells. Mechanistically, we found that extracellular active GzmA acts as a proinflammatory mediator in macrophages by inducing the TLR4-dependent expression of IL-6 and TNFα. Conclusions: Our findings implicate GzmA as a key regulator of the inflammatory response during abdominal sepsis and provide solid evidences about its therapeutic potential for the treatment of this severe pathology.
Aims: Recent in vitro findings suggest that the serine protease Granzyme K (GzmK) may act as a proinflammatory mediator. However, its role in sepsis is unknown. Here we aim to understand the role of GzmK in a mouse model of bacterial sepsis and compare it to the biological relevance of Granzyme A (GzmA). Methods: Sepsis was induced in WT, GzmA -/- and GzmK -/- mice by an intraperitoneal injection of 2x10 8 CFU from E. coli . Mouse survival was monitored during 5 days. Levels of IL-1α, IL-1β, TNFα and IL-6 in plasma were measured and bacterial load in blood, liver and spleen was analyzed. Finally, profile of cellular expression of GzmA and GzmK was analyzed by FACS. Results: GzmA and GzmK are not involved in the control of bacterial infection. However, GzmA and GzmK deficient mice showed a lower sepsis score in comparison with WT mice, although only GzmA deficient mice exhibited increased survival. GzmA deficient mice also showed reduced expression of some proinflammatory cytokines like IL1-α, IL-β and IL-6. A similar result was found when extracellular GzmA was therapeutically inhibited in WT mice using serpinb6b, which improved survival and reduced IL-6 expression. Mechanistically, active extracellular GzmA induces the production of IL-6 in macrophages by a mechanism dependent on TLR4 and MyD88. Conclusions: These results suggest that although both proteases contribute to the clinical signs of E. coli -induced sepsis, inhibition of GzmA is sufficient to reduce inflammation and improve survival irrespectively of the presence of other inflammatory granzymes, like GzmK.
SUMMARYThe cytotoxic proteases Granzymes A (GzmA) and B (GzmB) are important components of the arsenal used by cytotoxic immune cells to kill virally infected/damaged cells. Until now, there has been limited evidence that GzmA/B proteins contribute to combating intracellular bacterial pathogens in mammals. Here, we find that the route of infection of the intracellular bacterial pathogen Salmonella enterica serovar Typhimurium reveals the distinct roles that Granzymes play in defending against bacterial infection. We used Gzma-/-Gzmb-/- mice to discover that Granzymes are required to protect mice against oral infection with Salmonella. However, Granzymes do not play a role in systemic infection. We investigated the tissue-specific expression of Granzymes and determined that intestinal intraepithelial lymphocytes (IEL) are the only cell types that express Granzymes in healthy non-infected mice. In fact, IEL are sufficient to mediate the protective effects of Granzymes against Salmonella infection. Intriguingly, we found that GzmA and GzmB play opposing roles in Salmonella control, with GzmA being protective against infection whilst GzmB promoted infection. Both GzmA and GzmB proteins functioned independently of the pore-forming molecule Perforin, suggesting extracellular action. Our study reveals that IEL-expressed Granzymes play significant and distinct functions in host defense from oral bacterial infection.
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