The pseudokinase MLKL activates PAD4-dependent NET formation in necroptotic neutrophils

Thrombus NET content is associated with clinical outcome in stroke and myocardial infarction

OBJECTIVE

To investigate whether immune cell composition and content of neutrophil extracellular traps (NETs) in relation to clinical outcome are different between acute ischemic stroke (AIS) and acute myocardial infarction (AMI), we performed histologic analysis and correlated results with clinical and procedural parameters.

METHODS

We retrieved thrombi from patients with AIS (n = 71) and AMI (n = 72) during endovascular arterial recanalization and analyzed their immune cell composition and NET content by immunohistology. We then associated thrombus composition with procedural parameters and outcome in AIS and with cardiac function in patients with AMI. Furthermore, we compared AIS thrombi with AMI thrombi and differentiated Trial of Org 10172 in Acute Stroke Treatment classifications to address potential differences in thrombus pathogenesis.

RESULTS

Amounts of leukocytes (p = 0.133) and neutrophils (p = 0.56) were similar between AIS and AMI thrombi. Monocytes (p = 0.0052), eosinophils (p < 0.0001), B cells (p < 0.0001), and T cells (p < 0.0001) were more abundant in stroke compared with AMI thrombi. NETs were present in 100% of patients with AIS and 20.8% of patients with AMI. Their abundance in thrombi was associated with poor outcome scores in patients with AIS and with reduced ejection fraction in patients with AMI.

CONCLUSION

In our detailed histologic analysis of arterial thrombi, thrombus composition and especially abundance of leukocyte subsets differed between patients with AIS and AMI. The presence and amount of NETs were associated with patients' outcome after AIS and AMI, supporting a critical impact of NETs on thrombus stability in both conditions.

Competitive Cell Death Interactions in Pulmonary Infection: Host Modulation Versus Pathogen Manipulation

In the context of pulmonary infection, both hosts and pathogens have evolved a multitude of mechanisms to regulate the process of host cell death. The host aims to rapidly induce an inflammatory response at the site of infection, promote pathogen clearance, quickly resolve inflammation, and return to tissue homeostasis. The appropriate modulation of cell death in respiratory epithelial cells and pulmonary immune cells is central in the execution of all these processes. Cell death can be either inflammatory or anti-inflammatory depending on regulated cell death (RCD) modality triggered and the infection context. In addition, diverse bacterial pathogens have evolved many means to manipulate host cell death to increase bacterial survival and spread. The multitude of ways that hosts and bacteria engage in a molecular tug of war to modulate cell death dynamics during infection emphasizes its relevance in host responses and pathogen virulence at the host pathogen interface. This narrative review outlines several current lines of research characterizing bacterial pathogen manipulation of host cell death pathways in the lung. We postulate that understanding these interactions and the dynamics of intracellular and extracellular bacteria RCD manipulation, may lead to novel therapeutic approaches for the treatment of intractable respiratory infections.

Necrostatin-1 Ameliorates Neutrophilic Inflammation in Asthma by Suppressing MLKL Phosphorylation to Inhibiting NETs Release

Neutrophilic inflammation occurs during asthma exacerbation, and especially, in patients with steroid-refractory asthma, but the underlying mechanisms are poorly understood. Recently, a significant accumulation of neutrophil extracellular traps (NETs) in the airways of neutrophilic asthma has been documented, suggesting that NETs may play an important role in the pathogenesis. In this study, we firstly demonstrated that NETs could induce human airway epithelial cell damage in vitro. In a mouse asthmatic model of neutrophil-dominated airway inflammation, we found that NETs were markedly increased in bronchoalveolar lavage (BAL), and the formation of NETs exacerbated the airway inflammation. Additionally, a small-molecule drug necrostatin-1 (Nec-1) shown to inhibit NETs formation was found to alleviate the neutrophil-dominated airway inflammation. Nec-1 reduced total protein concentration, myeloperoxidase activity, and the levels of inflammatory cytokines in BAL. Finally, further experiments proved that the inhibition of Nec-1 on NETs formation might be related to its ability to inhibiting mixed lineage kinase domain-like (MLKL) phosphorylation and perforation. Together, these results document that NETs are closely associated with the pathogenesis of neutrophilic asthma and inhibition of the formation of NETs by Nec-1 may be a new therapeutic strategy to ameliorate neutrophil-dominated airway inflammation.

Ptpn6 inhibits caspase-8- and Ripk3/Mlkl-dependent inflammation

Ptpn6 is a cytoplasmic phosphatase that functions to prevent autoimmune and interleukin-1 (IL-1) receptor-dependent, caspase-1-independent inflammatory disease. Conditional deletion of Ptpn6 in neutrophils (Ptpn6^∆PMN) is sufficient to initiate IL-1 receptor-dependent cutaneous inflammatory disease, but the source of IL-1 and the mechanisms behind IL-1 release remain unclear. Here, we investigate the mechanisms controlling IL-1α/β release from neutrophils by inhibiting caspase-8-dependent apoptosis and Ripk1-Ripk3-Mlkl-regulated necroptosis. Loss of Ripk1 accelerated disease onset, whereas combined deletion of caspase-8 and either Ripk3 or Mlkl strongly protected Ptpn6^∆PMN mice. Ptpn6^∆PMN neutrophils displayed increased p38 mitogen-activated protein kinase-dependent Ripk1-independent IL-1 and tumor necrosis factor production, and were prone to cell death. Together, these data emphasize dual functions for Ptpn6 in the negative regulation of p38 mitogen-activated protein kinase activation to control tumor necrosis factor and IL-1α/β expression, and in maintaining Ripk1 function to prevent caspase-8- and Ripk3-Mlkl-dependent cell death and concomitant IL-1α/β release.

Triggering NETosis via protease-activated receptor (PAR)-2 signaling as a mechanism of hijacking neutrophils function for pathogen benefits

Neutrophil-derived networks of DNA-composed extracellular fibers covered with antimicrobial molecules, referred to as neutrophil extracellular traps (NETs), are recognized as a physiological microbicidal mechanism of innate immunity. The formation of NETs is also classified as a model of a cell death called NETosis. Despite intensive research on the NETs formation in response to pathogens, the role of specific bacteria-derived virulence factors in this process, although postulated, is still poorly understood. The aim of our study was to determine the role of gingipains, cysteine proteases responsible for the virulence of P. gingivalis, on the NETosis process induced by this major periodontopathogen. We showed that NETosis triggered by P. gingivalis is gingipain dependent since in the stark contrast to the wild-type strain (W83) the gingipain-null mutant strain only slightly induced the NETs formation. Furthermore, the direct effect of proteases on NETosis was documented using purified gingipains. Notably, the induction of NETosis was dependent on the catalytic activity of gingipains, since proteolytically inactive forms of enzymes showed reduced ability to trigger the NETs formation. Mechanistically, gingipain-induced NETosis was dependent on proteolytic activation of protease-activated receptor-2 (PAR-2). Intriguingly, both P. gingivalis and purified Arg-specific gingipains (Rgp) induced NETs that not only lacked bactericidal activity but instead stimulated the growth of bacteria species otherwise susceptible to killing in NETs. This protection was executed by proteolysis of bactericidal components of NETs. Taken together, gingipains play a dual role in NETosis: they are the potent direct inducers of NETs formation but in the same time, their activity prevents P. gingivalis entrapment and subsequent killing. This may explain a paradox that despite the massive accumulation of neutrophils and NETs formation in periodontal pockets periodontal pathogens and associated pathobionts thrive in this environment.

Periodontitis, or gum disease, is characterized by chronic inflammation and erosion of the tooth-supporting tissues. The condition is fuelled by bacterial accumulation on the tooth surface below the gum line that resists the host innate immune response, including massive accumulation of neutrophils. Despite possessing a formidable array of bactericidal machineries, including neutrophil extracellular traps (NETs) formation whereby neutrophils release DNA-composed fibers decorated with bactericidal proteins and peptides to efficiently trap and kill bacteria. Nevertheless, neutrophils in periodontitis are unable to clear the infection due to the presence of key periodontal pathogens, including Porphyromonas gingivalis. This bacterium secretes a variety of virulence factors, including proteases (gingipains) that allow the organism to manipulate the host immune response to benefit the entire dysbiotic microbial community. Here, we describe a unique strategy whereby P. gingivalis trigger NET formation through gingipain-dependent cleavage of Protease Activated Receptor (PAR)-2 on the neutrophil surface. Importantly, NETs formed in this way are deficient in antibacterial activity but instead, supports bacterial growth due to degradation of bactericidal components by gingipains. This finding may explain a paradox that dysbiotic bacteria flourished in periodontal pockets in spite of massive accumulation of neutrophils and abundant NETs formation.