Critical Role of ASC Inflammasomes and Bacterial Type IV Secretion System in Caspase-1 Activation and Host Innate Resistance to Brucella abortus Infection

Abstract

Pathogens are detected by innate immune receptors that, upon activation, orchestrate an appropriate immune response. Recent studies revealed the intracellular signaling cascades involved in the TLR-initiated immune response to Brucella abortus infection. However, no report has elucidated the role of inflammasome receptors in Brucella recognition. Therefore, we decided to investigate the function of NLRC4, NLRP3, and AIM2 in sensing Brucella. In this study, we showed that NLRC4 is not required to induce caspase-1 activation and further secretion of IL-1β by B. abortus in macrophages. In contrast, we determined that AIM2, which senses Brucella DNA, and NLRP3 are partially required for caspase-1 activation and IL-1β secretion. Additionally, mitochondrial reactive oxygen species induced by Brucella were implicated in IL-1β production. Furthermore, AIM2, NLRP3, ASC, and caspase-1 knockout mice were more susceptible to B. abortus infection than were wild-type animals, suggesting that multiple ASC-dependent inflammasomes contribute to host protection against infection. This protective effect is due to the inflammatory response caused by IL-1β and IL-18 rather than pyroptosis, because we observed augmented bacterial burden in IL-1R and IL-18 knockout mice. Finally, we determined that bacterial type IV secretion system VirB and live, but not heat-killed, Brucella are required for full inflammasome activation in macrophages during infection. Taken together, our results indicate that Brucella is sensed by ASC inflammasomes that collectively orchestrate a robust caspase-1 activation and proinflammatory response.

The Effector Protein BPE005 from Brucella abortus Induces Collagen Deposition and Matrix Metalloproteinase 9 Downmodulation via Transforming Growth Factor β1 in Hepatic Stellate Cells

“…Therefore, our results indicate that BPE005 could have a specific role in the inhibition of MMP-9 secretion; however, we could not rule out other different molecular mechanisms that could be implicated such as the alteration of MMP-9 expression or the inhibition of the host secretory pathway. Also, it has been demonstrated that Brucella bacteria could use this secretion system to translocate effector proteins into host cytosol or even nucleic acids to further activate the inflammasome in a way that involved NLRP3 (22). In addition, it has been demonstrated that NLRP3 inflammasome activation in hepatocytes and nonparenchymal liver cells results in the induction of proinflammatory signaling and hepatocyte pyroptotic cell death (46).…”

“…The type IV secretion system (T4SS) VirB has been shown to be involved in the capacity of different Brucella species to establish an intracellular replication niche (24). In addition, this system has been involved in the induction of inflammatory response during B. abortus infection (21,22). Taking into account that the major mediators of liver fibrosis are liver injury and inflammation (29), we decided to test whether VirB is involved in the ability of B. abortus to replicate within LX-2 cells.…”

“…T4SS protein substrates have been shown to modulate various cellular processes in the host cell, including apoptosis, vesicular traffic, and ubiquitination (18,19). As the Brucella T4SS encoded by the virB gene has been shown to be involved in the modulation of the immune response during infection (20)(21)(22), we decided to investigate whether the effect of Brucella infection on the activation of HSCs is dependent on the presence of a functional T4SS and/or its secreted proteins. To this end, LX-2 cells were infected with B. abortus or its isogenic mutants to determine the levels of production of MMPs, collagen deposition, and TGF-␤1 secretion.…”

Temporal Role for MyD88 in a Model of Brucella-Induced Arthritis and Musculoskeletal Inflammation

“…It is becoming increasingly clear that cytosolic signaling mechanisms may also induce inflammation upon Brucella infection. Although the MyD88-independent inflammatory pathway has yet to be determined in this model, one study demonstrated an important role for inflammasomes that require the apoptosis-associated speck-like protein containing a caspase-recruitment domain (ASC) in the host response to Brucella infection (49). Additionally, neutrophils infected with Brucella have been shown to induce TAK1 and SYK kinase-dependent oxidative bursts (50).…”

Brucella abortus Invasion of Osteocytes Modulates Connexin 43 and Integrin Expression and Induces Osteoclastogenesis via Receptor Activator of NF-κB Ligand and Tumor Necrosis Factor Alpha Secretion

“…Since the type four secretion system (T4SS) VirB from B. abortus has been involved in the induction of inflammatory responses upon infection (43,44), experiments were conducted to determine the role of the T4SS in the secretion of TNF-␣, IL-6, and RANKL by osteocytes. To this end, MLO-Y4 osteocytes were infected with B. abortus or the isogenic mutant virB10 strain, and the B. abortus virB10 mutant was unable to induce significant levels of TNF-␣ secretion ( Fig.…”