Caspase-4 Mediates Restriction of Burkholderia pseudomallei in Human Alveolar Epithelial Cells

Differentiation in pyroptosis induction by Burkholderia pseudomallei and Burkholderia thailandensis in primary human monocytes, a possible cause of sepsis in acute melioidosis patients

Melioidosis caused by Burkholderia pseudomallei is an infectious disease with a high mortality rate. In acute melioidosis, sepsis is a major cause of death among patients. Once the bacterium enters the bloodstream, immune system dysregulation ensues, leading to cytokine storms. In contrast to B. pseudomallei, a closely related but non-virulent strain B. thailandensis has rarely been reported to cause cytokine storms or death in patients. However, the mechanisms in which the virulent B. pseudomallei causes sepsis are not fully elucidated. It is well-documented that monocytes play an essential role in cytokine production in the bloodstream. The present study, therefore, determined whether there is a difference in the innate immune response to B. pseudomallei and B. thailandensis during infection of primary human monocytes and THP-1 monocytic cells by investigating pyroptosis, an inflammatory death pathway known to play a pivotal role in sepsis. Our results showed that although both bacterial species exhibited a similar ability to invade human monocytes, only B. pseudomallei can significantly increase the release of cytosolic enzyme lactate dehydrogenase (LDH) as well as the increases in caspase-1 and gasdermin D activations in both cell types. The results were consistent with the significant increase in IL-1β and IL-18 production, key cytokines involved in pyroptosis. Interestingly, there was no significant difference in other cytokine secretion, such as IL-1RA, IL-10, IL-12p70, IL-15, IL-8, and IL-23 in cells infected by both bacterial species. Furthermore, we also demonstrated that ROS production played a crucial role in controlling pyroptosis activation during B. pseudomallei infection in primary human monocytes. These findings suggested that pyroptosis induced by B. pseudomallei in the human monocytes may contribute to the pathogenesis of sepsis in acute melioidosis patients.

Strategies of bacterial detection by inflammasomes

Mammalian innate immunity is regulated by pattern-recognition receptors (PRRs) and guard proteins, which use distinct strategies to detect infections. PRRs detect bacterial molecules directly, whereas guards detect host cell manipulations by microbial virulence factors. Despite sensing infection through different mechanisms, both classes of innate immune sensors can activate the inflammasome, an immune complex that can mediate cell death and inflammation. Inflammasome-mediated immune responses are crucial for host defense against many bacterial pathogens and prevent invasion by non-pathogenic organisms. In this review, we discuss the mechanisms by which inflammasomes are stimulated by PRRs and guards during bacterial infection, and the strategies used by virulent bacteria to evade inflammasome-mediated immunity.

Extracorporeal Membrane Oxygenation-Dependent Fulminant Melioidosis From Caspase 4 Mutation Reversed by Interferon Gamma Therapy

We describe bedside-to-bench immunological and genetic elucidation of defective pyroptosis attributable to novel caspase 4 defect mediating pathogen-triggered inflammatory programmed cell death, in the setting of severe pneumonia and abscess-forming melioidosis in an overtly healthy host failing to clear Burkholderia pseudomallei infection, and how targeted adjunctive biological therapy led to a successful outcome.

Increased platelet activation and platelet-inflammasome engagement during chikungunya infection

Chikungunya fever is a viral disease transmitted by mosquitoes of the genus Aedes. The infection is usually symptomatic and most common symptoms are fever accompanied by joint pain and swelling. In most cases symptoms subside within a week. However, severe prolonged and disabling joint pain, that may persist for several months, even years, are reported. Although the pathogenesis of Chikungunya infection is not fully understood, the evolution to severe disease seems to be associated with the activation of immune mechanisms and the action of inflammatory mediators. Platelets are recognized as inflammatory cells with fundamental activities in the immune response, maintenance of vascular stability and pathogenicity of several inflammatory and infectious diseases. Although the involvement of platelets in the pathogenesis of viral diseases has gained attention in recent years, their activation in Chikungunya has not been explored. The aim of this study was to analyze platelet activation and the possible role of platelets in the amplification of the inflammatory response during Chikungunya infection. We prospectively included 132 patients attended at the Quinta D’Or hospital and 25 healthy volunteers during the 2016 epidemic in Rio de Janeiro, Brazil. We observed increased expression of CD62P on the surface of platelets, as well as increased plasma levels of CD62P and platelet-derived inflammatory mediators indicating that the Chikungunya infection leads to platelet activation. In addition, platelets from chikungunya patients exhibit increased expression of NLRP3, caspase 4, and cleaved IL-1β, suggestive of platelet-inflammasome engagement during chikungunya infection. In vitro experiments confirmed that the Chikungunya virus directly activates platelets. Moreover, we observed that platelet activation and soluble p-selectin at the onset of symptoms were associated with development of chronic forms of the disease. Collectively, our data suggest platelet involvement in the immune processes and inflammatory amplification triggered by the infection.

Mechanisms and Consequences of Noncanonical Inflammasome-Mediated Pyroptosis

The noncanonical inflammasome, comprising inflammatory caspases 4, 5, or 11, monitors the cytosol for bacterial lipopolysaccharide (LPS). Intracellular LPS-elicited autoproteolysis of these inflammatory caspases leads to the cleavage of the pore-forming protein gasdermin D (GSDMD). GSDMD pore formation induces a lytic form of cell death known as pyroptosis and the release of inflammatory cytokines and DAMPs, thereby promoting inflammation. The noncanonical inflammasome-dependent innate sensing of cytosolic LPS plays important roles in bacterial infections and sepsis pathogenesis. Exciting studies in the recent past have significantly furthered our understanding of the biochemical and structural basis of the caspase-4/11 activation of GSDMD, caspase-4/11's substrate specificity, and the biological consequences of noncanonical inflammasome activation of GSDMD. This review will discuss these recent advances and highlight the remaining gaps in our understanding of the noncanonical inflammasome and pyroptosis.

IGF2BP2 knockdown inhibits LPS-induced pyroptosis in BEAS-2B cells by targeting caspase 4, a crucial molecule of the non-canonical pyroptosis pathway

Insulin-like growth factor 2 (IGF2) mRNA-binding protein 2 (IGF2BP2) is a secreted protein that can bind to IGF2 and has been reported to promote inflammation. The data from the ENCORI database have predicted that IGF2BP2 can bind caspase 4, which mediates pyroptosis and promotes airway inflammation and lipopolysaccharide (LPS)-induced lung injury. The present study investigated whether IGF2BP2 can regulate LPS-induced lung cell inflammation by targeting caspase 4. Therefore, the non-tumorigenic lung epithelial cell line Beas-2B was transfected with short hairpin RNA (shRNA)-IGF2BP2 and stimulated with LPS. A number of parameters, including cell viability, production of interleukin (IL)-1β and IL-18, activation of gasdermin D (GSDMD) and the expression levels of IGF2BP2, caspase 4 and cleaved-caspase 1, were subsequently assessed using CCK-8, ELISA kits, western blotting and immunofluorescence staining, respectively. RNA pull-down assay was used to probe the possible interaction between IGF2BP2 and caspase 4 RNA. LPS treatment was found to inhibit cell viability, trigger IL-1β and IL-18 production and increase IGF2BP2 expression in a concentration-dependent manner. Compared with cells transfected with shRNA-negative control, cells that were transfected with shRNA-IGF2BP2 exhibited enhanced cell viability, reduced IL-1β and IL-18 concentrations, decreased GSDMD activation in addition to reduced expression levels of caspase 4 and cleaved-caspase 1 following stimulation with 1 µg/ml LPS. Concomitantly, the effects of IGF2BP2 silencing on caspase 4 expression were higher compared with those noted on caspase 1. In addition, binding of IGF2BP2 to caspase 4 RNA was also observed. To conclude, data from the present study suggest that IGF2BP2 knockdown inhibited LPS-induced Beas-2B cell inflammation by targeting caspase 4, thereby inhibiting the non-canonical pyroptosis pathway.