Abstract
Lysosomal cathepsin B (CTSB) has been proposed to play a role in the induction of acute inflammation. We hypothesised that the presence of active CTSB in the cytosol is crucial for NLRP3-inflammasome assembly and, consequently, for mature IL-1β generation after mycobacterial infection in vitro. Elevated levels of CTSB was observed in the lungs of mice and rabbits following infection with Mycobacterium tuberculosis (Mtb) H37Rv as well as in plasma from acute tuberculosis patients. H37Rv-infected murine bone marrow-derived macrophages (BMDMs) displayed both lysosomal leakage, with release of CTSB into the cytosol, as well as increased levels of mature IL-1β. These responses were diminished in BMDM infected with a mutant H37Rv deficient in ESAT-6 expression. Pharmacological inhibition of cathepsin activity with CA074-Me resulted in a substantial reduction of both mature IL-1β production and caspase-1 activation in infected macrophages. Moreover, cathepsin inhibition abolished the interaction between NLRP3 and ASC, measured by immunofluorescence imaging in H37Rv-infected macrophages, demonstrating a critical role of the enzyme in NLRP3-inflammasome activation. These observations suggest that during Mtb infection, lysosomal release of activated CTSB and possibly other cathepsins inhibitable by CA07-Me is critical for the induction of inflammasome-mediated IL-1β processing by regulating NLRP3-inflammasome assembly in the cytosol.
Highlights
The pro-inflammatory cytokine IL-1β is thought to play a major role in host protection against Mycobacterium tuberculosis (Mtb) infection [1,2,3,4,5,6,7,8]
Using macrophages infected in vitro with Mtb, we demonstrate that when secreted into the cytosol by an ESAT-6-dependent mechanism, lysosomal cathepsin activity inhibitable by CA074-Me is crucial for NLRP3-inflammasome activation and for mature IL-1β generation
We first investigated whether Mtb infection increases the expression or activity of cathepsin B (CTSB) in macrophages
Summary
The pro-inflammatory cytokine IL-1β is thought to play a major role in host protection against Mycobacterium tuberculosis (Mtb) infection [1,2,3,4,5,6,7,8]. IL-1β has been shown to facilitate phagolysosomal fusion, which enhances the mycobactericidal activity of host macrophages [9]. IL-1β through its induction of prostaglandin E2 could influence the cell death modality of Mtb-infected macrophages favoring an apoptotic over necrotic fate containing bacterial spread [7, 10,11,12,13]. This could occur in part thru the generation of cyclooxygenase-2, which plays critical role in plasma membrane repair [7, 10]
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