The RD1 Locus in the Mycobacterium tuberculosis Genome Contributes to Activation of Caspase-1 via Induction of Potassium Ion Efflux in Infected Macrophages

Abstract

A genomic locus called "region of difference 1" (RD1) in Mycobacterium tuberculosis has been shown to contribute to the generation of host protective immunity as well as to the virulence of the bacterium. To gain insight into the molecular mechanism, we investigated the difference in the cytokine-inducing ability between H37Rv and a mutant strain deficient for RD1 (⌬RD1). We found that RD1 is implicated in the production of caspase-1-dependent cytokines, interleukin-18 (IL-18) and IL-1␤, from infected macrophages. The expression of these cytokines was similarly induced after infection with H37Rv and ⌬RD1. However, the activation of caspase-1 was observed only in H37Rv-infected macrophages. The cytokine production and caspase-1 activation were induced independently of type I interferon receptor signaling events. We also found that the activation of caspase-1 was markedly inhibited with increasing concentrations of extracellular KCl. Furthermore, the production of IL-18 and IL-1␤ and caspase-1 activation were induced independently of a P2X7 purinergic receptor, and the inability of ⌬RD1 in caspase-1 activation was compensated for by nigericin, an agent inducing the potassium ion efflux. Based on these results, we concluded that RD1 participates in caspase-1-dependent cytokine production via induction of the potassium ion efflux in infected macrophages.

Mycobacterial Secretion Systems ESX-1 and ESX-5 Play Distinct Roles in Host Cell Death and Inflammasome Activation

“…A role for ESX-1 is described in NLRP3 inflammasome activation and IL-1b release (13,14). As mycobacterial peptidoglycan fragment muramyl dipeptide can activate NALP3 inflammasomes (52) and ESX-1 enables phagosomal escape, Koo et al (13) have previously suggested that ESX-1 allows entry of muramyl dipeptide or other bacterial products into cytosol to induce inflammasome activation.…”

“…Caspase-1 itself needs activation by inflammasomes, protein complexes that sense cytosolic bacterial products and endogenous danger signals. Both M. tuberculosis and M. marinum reportedly induce IL-1b secretion through NALP3 inflammasomes in an ESX-1-dependent manner (13)(14)(15)(16). Recently, we observed that ESX-5 is also required for IL-1b secretion upon infection with M. marinum (7).…”

“…Inflammasome activation upon M. marinum infection can exacerbate disease (42) indicating a host-detrimental and bacterium-promoting role of the inflammatory pathway during mycobacterial infection (15). Previous studies demonstrated that infection of macrophages with pathogenic mycobacteria results in the induction of IL-1b release, which is dependent on both ESX-1 and ESX-5 (7,(13)(14)(15). Fig.…”

“…Potassium efflux was shown to be important for inflammasome activation by M. tuberculosis (14). Involvement of this inflammasome trigger during M. marinum infection of type 1 macrophage was investigated by adding KCl to the infection medium.…”

Interaction of Mycobacterium tuberculosis with Host Cell Death Pathways

“…The function of the ESX-1 system was linked to escape of Mtb out of the phagosome at later stages of the infection, which coincided with host cell necrosis induction (Simeone et al 2012). The Mtb EsxA protein is important in forming pores in the phagosomal membrane (Wong and Jacobs 2011), which is essential for subsequent NLRP3-inflammasome activation, because esxA Mtb mutants are deficient in activation of this inflammasome (Koo et al 2008;Kurenuma et al 2009;Wong and Jacobs 2011;Abdalla et al 2012;Dorhoi et al 2012). The activation of NLRP3 inflammasome in Mtb-infected cells is dependent on the tyrosine kinase Syk, and this activation is essential for cell lysis, hence showing a necroptotic host cell death pathway (Wong and Jacobs 2011) (Fig.…”