In this issue of Blood, Jiang and colleagues1 report that platelet NOD-like receptor family pyrin domain containing 6 (NLRP6) is critical for the prevention of microthrombosis in sepsis. These findings advance our understanding of the pathogenesis of sepsis and pave the way for the development of drugs that can target the cell-specific molecule to mitigate the severity of sepsis.
Sepsis is a dysregulated inflammatory response in multiple organs as a result of systemic bacterial infection that can eventually lead to organ failure.2 Among both the immunocompetent and immunocompromised population, sepsis is responsible for a tremendous burden of disease.2 In the United States alone, sepsis cases exceed 750 000 admissions and account for 270 000 deaths and >$26 billion in health care expenditures annually.2 It is also in the top 10 causes of death in the United States.2 Despite advances in treatment and management of sepsis, specific therapies for the dysregulated immune response are not available. Clinical trials using the US Food and Drug Administration–approved Xigris (drotrecogin alfa; recombinant form of human activated protein C) and eritoran (a toll-like receptor 4 [TLR4]–myeloid differentiation factor 2 antagonist) have not been successful. Therefore, a critical need exists to broaden our knowledge to develop new therapeutic agents against sepsis.
Thromboinflammation is the interplay between thrombosis and inflammation and can be either acute or chronic in nature in response to infection or injury. It is characterized by the induction of inflammation, thrombosis, and vascular leak with eventual multiorgan damage and death. Thromboinflammation is associated with a broad range of inflammatory and cardiovascular/pulmonary diseases, such as severe sepsis, COVID-19, acute lung injury/acute respiratory distress syndrome, myocardial infarction–induced ischemia-reperfusion injury, acute kidney ischemia, cardiac arrest, and organ transplant. Despite numerous years of intensive development of anti-inflammatory and antithrombotic therapies, there is still no convincing effective therapy for the treatment of thromboinflammation. Although the reasons underlying the ineffectiveness of anti-inflammatory and antithrombotic therapies are unclear, a better understanding of the pathogenesis is critical.
Platelets are critical for bridging inflammation and thrombosis. They are the smallest cells in the blood (diameter, 2-3 μm). Platelets are most commonly formed in the bone marrow, due to the breaking apart of megakaryocytes in the blood vasculature. As they are both differentiated from a common myeloid progenitor cell, they share many features with myeloid lineage cells, including the expression of pattern recognition receptors (PRRs), such as TLRs and nucleotide-binding oligomerization domain-like receptors (NLRs), phagocytosis, and the release of cytokines and chemokines on activation. Despite its importance, the role of PRRs in this cell type in the context of thromboinflammation in sepsis is not well understood.
Some NLRs form inflammasomes, which are multiprotein complexes located in the cytosol that are a critical component of the innate immune system that augment host defense against pathogens or other components. An inflammasome consists of a sensor, an adapter, and an effector. The sensor is composed of either a pyrin domain (PYD) or a caspase activation and recruitment domain (CARD) at its N-terminal end; the adapter, usually apoptosis-associated speck-like protein containing a CARD, is composed of PYD and CARD domains, and the effector has a CARD at its end. On the basis of the effector, inflammasomes can be either canonical, which activates caspase-1, or noncanonical, which activates caspase-11. Seminal gene-deletion studies demonstrated that the NLRP6 inflammasome controls host defense during microbial infections through both proinflammatory and anti-inflammatory effects.3 NLRP6 deletion augmented host defense against systemic infections with Salmonella typhimurium,4,Listeria monocytogenes,5 and Escherichia coli4 and intrapulmonary infection with Staphylococcus aureus.6 On the other hand, NLRP6 deletion decreased host protection in intrapulmonary infection with Klebsiella pneumoniae,7 oral infection with Clostridium rodentium,8 and systemic infection with norovirus.9 Therefore, NLRP6-dependent host protection is likely context dependent. In the context of bacterial infections, in which myeloid cells are important, NLRP6 appears to cause destructive inflammation in most cases. In contrast, in intestinal inflammation, in which epithelial cells are essential, the NLRP6-dependent response is protective. From these studies, host protection is dependent on NLRP6-dependent regulation of NF-κB activation. Also, NLRP6 deletion enhanced host protection in sepsis.10 Previous studies have primarily used whole body gene-deficient mice and focused on myeloid cells (neutrophils and macrophages). Therefore, the contribution of cell types other than macrophages and neutrophils, such as platelets in host protection, remained elusive.
In their study, Jiang et al demonstrate how platelet-specific NLRP6 activation can modulate microthrombosis and survival in sepsis and identify NF-κB as a master regulator for platelet activation and granule secretion. The study showed that NLRP6 is highly expressed not only in mouse but also in human platelets. To explore the unique role of NLRP6 in platelets, the authors generated a platelet-specific deletion of NLRP6 using the Cre-lox system. Using platelet-specific NLRP6-gene-deficent mice challenged by cecal ligation and puncture, the most commonly used animal model to investigate human sepsis, the authors establish that platelet NLRP6 activation is essential for decreasing microvascular thrombosis in the lung and liver, and enhancing host protection in sepsis. Mechanistically, the authors illustrate that platelet NLRP6 activation inhibits platelet activation in sepsis. Moreover, the authors demonstrate that platelet NLRP6 activation reduces granule release. The authors reveal that NF-κB signaling is decreased on platelet NLRP6 activation. The authors further establish that NLRP6 enables the interaction of tripartite motif 21 (TRIM21) with TAK1-binding protein (TAB1), causing TAB1 degradation through K48-linked ubiquitination. To validate whether blocking NF-κB signaling can restore the effect of NLRP6 deficiency in the formation of microthrombosis and survival, the authors used an inhibitor and found that inhibition of NF-κB signaling reduced microvascular thrombosis and enhanced survival. Finally, the authors show that plasma from septic mice augments the interaction of NLRP6 with TRIM21 and TAB1 and induces TAB1 degradation in platelets.
Overall, the study by Jiang et al improves our understanding of the mechanism of platelet NLRP6 activation that leads to inhibition of microthrombosis and prolonged survival in sepsis (see figure). These findings also offer additional targets for therapeutic interventions that may benefit to reduce disease burden. This study uncovers a new function of a platelet NLRP6 in addition to its conventional roles, which include pathogen recognition and caspase-1 activation. Nevertheless, several outstanding questions remain related to the additional downstream signaling cascades by which NLRP6-TAB1 axis in platelets contributes to host protection. It is crucial to note that this study is novel, timely, and likely to have broad implications in thrombosis and inflammation, because thromboinflammation is associated with a broad spectrum of inflammatory disorders, ranging from mild to severe in humans.
A schematic representation of how NLRP6 inhibits sepsis-induced microthrombosis formation. During systemic inflammation, pathogen-associated molecular patterns activate NLRP6, which leads to the polyubiquitination and subsequent degradation of the TAB1 protein. This degradation has 2 major downstream effects: (a) it inhibits the phosphorylation of key signaling proteins, including p65, IκBα, IKKβ, and TAK1, thereby impairing platelet function; and (b) it also inhibits the phosphorylation of SNAP23, preventing granule secretion. Together, the suppression of platelet activation and granule secretion disrupts microthrombosis formation during sepsis.
A schematic representation of how NLRP6 inhibits sepsis-induced microthrombosis formation. During systemic inflammation, pathogen-associated molecular patterns activate NLRP6, which leads to the polyubiquitination and subsequent degradation of the TAB1 protein. This degradation has 2 major downstream effects: (a) it inhibits the phosphorylation of key signaling proteins, including p65, IκBα, IKKβ, and TAK1, thereby impairing platelet function; and (b) it also inhibits the phosphorylation of SNAP23, preventing granule secretion. Together, the suppression of platelet activation and granule secretion disrupts microthrombosis formation during sepsis.
Conflict-of-interest disclosure: The author declares no competing financial interests.
This feature is available to Subscribers Only
Sign In or Create an Account Close Modal