Mechanisms and consequences of Staphylococcus aureus Leukocidin AB-mediated activation of the host NLRP3 inflammasome

The NLRP3 inflammasome is a critical innate immune sensor implicated in the pathogenesis of dozens of infectious and non-infectious diseases. Activation of the NLRP3 inflammasome causes IL-1β and IL-18 secretion and necrotic cell death. Staphylococcus aureus is a common cause of infections in humans. S. aureus produces a family of pore-forming toxins that are cytotoxic to human immune cells. One recently discovered pore-forming toxin, Leukocidin AB, is the focus of studies herein. Leukocidin AB is a human-specific, pore-forming toxin that binds CD11b to initiate pore formation. In order to characterize the mechanism of Leukocidin AB cytotoxicity and determine its significance, we evaluated the effects of Leukocidin AB on primary human monocytes and THP1 monocytic cells. Leukocidin AB was one of the most potent toxins in killing primary human monocytes. In THP1 cells, knockdown of NLRP3 or ASC by shRNA diminished Leukocidin AB-induced cytotoxicity and prevented secretion of IL-1β and IL-18. We also characterized the NLRP3 inflammasome in bacterial survival during phagocytosis. When S. aureus was phagocytosed by THP1 cells, LukAB triggered IL-1β secretion and cell death. shRNA-mediated depletion of NLRP3 or ASC suppressed IL-1β secretion but had no effect on Leukocidin AB-induced cell death. These data suggest that a separate mechanism is responsible for triggering cell death when Leukocidin AB binds CD11b on the phagosome membrane instead of the plasma membrane. We also initiated studies to characterize the role of kinases and phosphorylation in the response to Leukocidin AB. Using multiplex inhibitor bead chromatography and quantitative mass spectrometry, we identified eight kinases that rapidly decrease in activity during Leukocidin AB exposure. We demonstrated a role for Death-Associated Protein Kinase in the response to Leukocidin AB by showing that its inhibition suppressed Leukocidin AB-induced cytokine secretion and cytotoxicity. And finally, we used a novel transfection method for overexpressing mutant proteins in THP1 cells to show that the NLRP3 S198D/S201D mutant could spontaneously activate NLRP3 inflammasome signaling. In total, these studies make significant contributions to the understanding of S. aureus pathogenesis and the regulation of innate immune NLRP3 inflammasome signaling in response to a human specific, S. aureus pore-forming toxin.Doctor of Philosoph

Staphylococcus aureus Leukocidin A/B (LukAB) Kills Human Monocytes via Host NLRP3 and ASC when Extracellular, but Not Intracellular

Staphylococcus aureus infections are a growing health burden worldwide, and paramount to this bacterium’s pathogenesis is the production of virulence factors, including pore-forming leukotoxins. Leukocidin A/B (LukAB) is a recently discovered toxin that kills primary human phagocytes, though the underlying mechanism of cell death is not understood. We demonstrate here that LukAB is a major contributor to the death of human monocytes. Using a variety of in vitro and ex vivo intoxication and infection models, we found that LukAB activates Caspase 1, promotes IL-1β secretion and induces necrosis in human monocytes. Using THP1 cells as a model for human monocytes, we found that the inflammasome components NLRP3 and ASC are required for LukAB-mediated IL-1β secretion and necrotic cell death. S. aureus was shown to kill human monocytes in a LukAB dependent manner under both extracellular and intracellular ex vivo infection models. Although LukAB-mediated killing of THP1 monocytes from extracellular S. aureus requires ASC, NLRP3 and the LukAB-receptor CD11b, LukAB-mediated killing from phagocytosed S. aureus is independent of ASC or NLRP3, but dependent on CD11b. Altogether, this study provides insight into the nature of LukAB-mediated killing of human monocytes. The discovery that S. aureus LukAB provokes differential host responses in a manner dependent on the cellular contact site is critical for the development of anti-infective/anti-inflammatory therapies that target the NLRP3 inflammasome

The Staphylococcus aureus superantigen SElX is a bifunctional toxin that inhibits neutrophil function:SElX Inhibits Neutrophil Function

Bacterial superantigens (SAgs) cause Vβ-dependent T-cell proliferation leading to immune dysregulation associated with the pathogenesis of life-threatening infections such as toxic shock syndrome, and necrotizing pneumonia. Previously, we demonstrated that staphylococcal enterotoxin-like toxin X (SElX) from Staphylococcus aureus is a classical superantigen that exhibits T-cell activation in a Vβ-specific manner, and contributes to the pathogenesis of necrotizing pneumonia. Here, we discovered that SElX can also bind to neutrophils from human and other mammalian species and disrupt IgG-mediated phagocytosis. Site-directed mutagenesis of the conserved sialic acid-binding motif of SElX abolished neutrophil binding and phagocytic killing, and revealed multiple glycosylated neutrophil receptors for SElX binding. Furthermore, the neutrophil binding-deficient mutant of SElX retained its capacity for T-cell activation demonstrating that SElX exhibits mechanistically independent activities on distinct cell populations associated with acquired and innate immunity, respectively. Finally, we demonstrated that the neutrophil-binding activity rather than superantigenicity is responsible for the SElX-dependent virulence observed in a necrotizing pneumonia rabbit model of infection. Taken together, we report the first example of a SAg, that can manipulate both the innate and adaptive arms of the human immune system during S. aureus pathogenesis

Functional amyloid signaling via the inflammasome, necrosome, and signalosome: New therapeutic targets in heart failure

As the most common cause of death and disability globally, heart disease remains an incompletely understood enigma. A growing number of cardiac diseases are being characterized by the presence of misfolded proteins underlying their pathophysiology, including cardiac amyloidosis and dilated cardiomyopathy (DCM). At least nine precursor proteins have been implicated in the development of cardiac amyloidosis, most commonly caused by multiple myeloma (MM) light chain disease and disease-causing mutant or wildtype transthyretin (TTR). Similarly aggregates with PSEN1 and COFILIN-2 have been identified in up to 1/3 of idiopathic DCM cases studied indicating the potential predominance of misfolded proteins in heart failure. In this review, we present recent evidence linking misfolded proteins mechanistically with heart failure and present multiple lines of new therapeutic approaches that target the prevention of misfolded proteins in cardiac TTR amyloid disease. These include multiple small molecule pharmacological chaperones now in clinical trials designed specifically to support TTR folding by rational design, such as tafamidis, and chaperones previously developed for other purposes, such as doxycycline and tauroursodeoxycholic acid. Lastly, we present newly discovered non-pathological functional amyloid structures, such as the inflammasome and necrosome signaling complexes, which can be activated directly by amyloid. These may represent future targets to successfully attenuate amyloid-induced proteotoxicity in heart failure as the inflammasome, for example, is being therapeutically inhibited experimentally in autoimmune disease. Together, these studies demonstrate multiple novel points in which new therapies may be used to primarily prevent misfolded proteins or to inhibit their downstream amyloid-mediated effectors, such as the inflammasome, to prevent proteotoxicity in heart failure

<i>Staphylococcus aureus</i> Leukocidin A/B (LukAB) Kills Human Monocytes via Host NLRP3 and ASC when Extracellular, but Not Intracellular

<div><p><i>Staphylococcus aureus</i> infections are a growing health burden worldwide, and paramount to this bacterium’s pathogenesis is the production of virulence factors, including pore-forming leukotoxins. Leukocidin A/B (LukAB) is a recently discovered toxin that kills primary human phagocytes, though the underlying mechanism of cell death is not understood. We demonstrate here that LukAB is a major contributor to the death of human monocytes. Using a variety of <i>in vitro</i> and <i>ex vivo</i> intoxication and infection models, we found that LukAB activates Caspase 1, promotes IL-1β secretion and induces necrosis in human monocytes. Using THP1 cells as a model for human monocytes, we found that the inflammasome components NLRP3 and ASC are required for LukAB-mediated IL-1β secretion and necrotic cell death. <i>S</i>. <i>aureus</i> was shown to kill human monocytes in a LukAB dependent manner under both extracellular and intracellular <i>ex vivo</i> infection models. Although LukAB-mediated killing of THP1 monocytes from extracellular <i>S</i>. <i>aureus</i> requires ASC, NLRP3 and the LukAB-receptor CD11b, LukAB-mediated killing from phagocytosed <i>S</i>. <i>aureus</i> is independent of ASC or NLRP3, but dependent on CD11b. Altogether, this study provides insight into the nature of LukAB-mediated killing of human monocytes. The discovery that <i>S</i>. <i>aureus</i> LukAB provokes differential host responses in a manner dependent on the cellular contact site is critical for the development of anti-infective/anti-inflammatory therapies that target the NLRP3 inflammasome.</p></div

LukAB targets CD11b on human monocytes to potently induce cell death.

<p>(A) THP1 cells were intoxicated with titrations of the indicated purified toxins for 1 hour and LDH release was analyzed. (B) THP1 cells were transduced with either non-targeting shRNA or shRNA against CD11b and surface CD11b levels were evaluated by flow cytometry. (C) THP1 cells described in panel B were intoxicated with titrations of the indicated toxins for 1 hour and LDH release was analyzed. (D) THP1 cells were intoxicated with the indicated concentration of LukAB for 1 hour and analyzed by flow cytometry for permeability to propidium iodide. (E) Primary CD14+ human monocytes were intoxicated with titrations of the indicated toxins for 1 hour and analyzed by flow cytometry for permeability to propidium iodide. EC50 values are also shown. Error bars represent the mean ± standard error of the mean for at least two independent experiments, each performed in triplicate. Primary cell experiments include three independent donors. Asterisks indicate significance at a <i>p</i>-value of ≤ 0.05 by Tukey’s multiple comparisons post-test for 1-way or 2-way ANOVA, as appropriate.</p