A Yersinia Effector with Enhanced Inhibitory Activity on the NF-κB Pathway Activates the NLRP3/ASC/Caspase-1 Inflammasome in Macrophages

Abstract

A type III secretion system (T3SS) in pathogenic Yersinia species functions to translocate Yop effectors, which modulate cytokine production and regulate cell death in macrophages. Distinct pathways of T3SS-dependent cell death and caspase-1 activation occur in Yersinia-infected macrophages. One pathway of cell death and caspase-1 activation in macrophages requires the effector YopJ. YopJ is an acetyltransferase that inactivates MAPK kinases and IKKβ to cause TLR4-dependent apoptosis in naïve macrophages. A YopJ isoform in Y. pestis KIM (YopJKIM) has two amino acid substitutions, F177L and K206E, not present in YopJ proteins of Y. pseudotuberculosis and Y. pestis CO92. As compared to other YopJ isoforms, YopJKIM causes increased apoptosis, caspase-1 activation, and secretion of IL-1β in Yersinia-infected macrophages. The molecular basis for increased apoptosis and activation of caspase-1 by YopJKIM in Yersinia-infected macrophages was studied. Site directed mutagenesis showed that the F177L and K206E substitutions in YopJKIM were important for enhanced apoptosis, caspase-1 activation, and IL-1β secretion. As compared to YopJCO92, YopJKIM displayed an enhanced capacity to inhibit phosphorylation of IκB-α in macrophages and to bind IKKβ in vitro. YopJKIM also showed a moderately increased ability to inhibit phosphorylation of MAPKs. Increased caspase-1 cleavage and IL-1β secretion occurred in IKKβ-deficient macrophages infected with Y. pestis expressing YopJCO92, confirming that the NF-κB pathway can negatively regulate inflammasome activation. K+ efflux, NLRP3 and ASC were important for secretion of IL-1β in response to Y. pestis KIM infection as shown using macrophages lacking inflammasome components or by the addition of exogenous KCl. These data show that caspase-1 is activated in naïve macrophages in response to infection with a pathogen that inhibits IKKβ and MAPK kinases and induces TLR4-dependent apoptosis. This pro-inflammatory form of apoptosis may represent an early innate immune response to highly virulent pathogens such as Y. pestis KIM that have evolved an enhanced ability to inhibit host signaling pathways.

Caspase-8 and RIP kinases regulate bacteria-induced innate immune responses and cell death

“…The effect on caspase-1 cleavage may be mediated by the inflammasome adaptor ASC (Fig. 5E), because ASC can associate with caspase-8 after Francisella or Salmonella infection (38,48), although the role of ASC may differ depending upon conditions and source of YopJ (16,23,25).…”

“…Inflammasome activation, culminating in the activation and processing of caspase-1, leads to the production of IL-18 and IL-1β, key inflammatory cytokines and antibacterial defenses, but can also be associated with caspase-1-dependent pyroptotic cell death (22). YopJ also participates in inflammasome activation (16,23), leading to a host immune response. Thus, this single bacterial effector may induce both protective and harmful effects for the host.…”

Mycobacterium tuberculosis Mce3E Suppresses Host Innate Immune Responses by Targeting ERK1/2 Signaling

“…Such mediators are essential for recruitment and activation of immune cells to respond to bacterial infection (8). In contrast, bacterial pathogens frequently target the MAPK pathway to suppress the production of cytokines to counteract host defense (29,32)…”

Caspases as the Key Effectors of Inflammatory Responses Against Bacterial Infection

“…In accordance with this, NLRP3 inflammasome activation is induced by changes in the concentration and flux of K ? caused by bacterial cytotoxins that form pores on the target cell membrane (McNeela et al 2010;Zheng et al 2011). For example, pore-forming toxins such as a-hemolysin or pneumolysin from S. aureus oligomerizes and recruits the adaptor molecule apoptosis-associated speck-like protein containing a CARD (ASC) by interacting with its pyrin domain (PYD).…”