Staphylococcus aureus α-Hemolysin Activates the NLRP3-Inflammasome in Human and Mouse Monocytic Cells

Abstract

Community Acquired Methicillin Resistant Staphylococcus aureus (CA-MRSA) causes severe necrotizing infections of the skin, soft tissues, and lungs. Staphylococcal α-hemolysin is an essential virulence factor in mouse models of CA-MRSA necrotizing pneumonia. S. aureus α-hemolysin has long been known to induce inflammatory signaling and cell death in host organisms, however the mechanism underlying these signaling events were not well understood. Using highly purified recombinant α-hemolysin, we now demonstrate that α-hemolysin activates the Nucleotide-binding domain and leucine-rich repeat containing gene family, pyrin domain containing 3 protein (NLRP3)-inflammasome, a host inflammatory signaling complex involved in responses to pathogens and endogenous danger signals. Non-cytolytic mutant α-hemolysin molecules fail to elicit NLRP3-inflammasome signaling, demonstrating that the responses are not due to non-specific activation of this innate immune signaling system by bacterially derived proteins. In monocyte-derived cells from humans and mice, inflammasome assembly in response to α-hemolysin results in activation of the cysteine proteinase, caspase-1. We also show that inflammasome activation by α-hemolysin works in conjunction with signaling by other CA-MRSA-derived Pathogen Associated Molecular Patterns (PAMPs) to induce secretion of pro-inflammatory cytokines IL-1β and IL-18. Additionally, α-hemolysin induces cell death in these cells through an NLRP3-dependent program of cellular necrosis, resulting in the release of endogenous pro-inflammatory molecules, like the chromatin-associated protein, High-mobility group box 1 (HMGB1). These studies link the activity of a major S. aureus virulence factor to a specific host signaling pathway. The cellular events linked to inflammasome activity have clear relevance to the disease processes associated with CA-MRSA including tissue necrosis and inflammation.

Caspases as the Key Effectors of Inflammatory Responses Against Bacterial Infection

“…Accumulating evidence indicates that NLRP3 inflammasomes play a significant role in the recognition of and response to bacterial infection. Although there is no direct evidence for the interaction of bacterial ligands with NLRP3, a variety of pathogenic bacteria including Staphylococcus aureus (Craven et al 2009;Holzinger et al 2012;Kebaier et al 2012;Maher et al 2013;McGilligan et al 2013;Muñoz-Planillo et al 2009), Streptococcus pneumoniae (Hoegen et al 2011;McNeela et al 2010;Witzenrath et al 2011) and Listeria monocytogenes (Meixenberger et al 2010) have been Arch. Immunol.…”

The inflammasomes in health and disease: from genetics to molecular mechanisms of autoinflammation and beyond

“…[39][40][41][42][43] Interestingly, NLRP3 also contributes to the ability of dendritic cells and macrophages to detect endogenous danger molecules released by dying cells. NLRP3 can signal the presence of adenosine triphosphate (ATP), crystals of monosodium urate (MSU) or calcium pyrophosphate crystals, 39,44 and other large particulates of non-microbial origin, such as alum, silica and asbestos ( Figure 2).…”

Aggregatibacter actinomycetemcomitans targets NLRP3 and NLRP6 inflammasome expression in human mononuclear leukocytes

“…The most well characterised is the NLRP3 inflammasome that is activated in response to cell stress signals [31], whole bacteria and viruses, or their virulence factors [32][33][34][35], and is likely to play a key role in inflammatory periodontal disease [26]. NLRP2 is less well studied, but it is considered to inhibit the NLRP3-ASC interaction [36,37].…”