Identification of a Transcription Factor That Regulates Host Cell Exit and Virulence of Mycobacterium tuberculosis.

The interaction of Mycobacterium tuberculosis (Mtb) with host cell death signaling pathways is characterized by an initial anti-apoptotic phase followed by a pro-necrotic phase to allow for host cell exit of the bacteria. The bacterial modulators regulating necrosis induction are poorly understood. Here we describe the identification of a transcriptional repressor, Rv3167c responsible for regulating the escape of Mtb from the phagosome. Increased cytosolic localization of MtbΔRv3167c was accompanied by elevated levels of mitochondrial reactive oxygen species and reduced activation of the protein kinase Akt, and these events were critical for the induction of host cell necrosis and macroautophagy. The increase in necrosis led to an increase in bacterial virulence as reflected in higher bacterial burden and reduced survival of mice infected with MtbΔRv3167c. The regulon of Rv3167c thus contains the bacterial mediators involved in escape from the phagosome and host cell necrosis induction, both of which are crucial steps in the intracellular lifecycle and virulence of Mtb.

Activator of G-Protein Signaling 3-Induced Lysosomal Biogenesis Limits Macrophage Intracellular Bacterial Infection.

Many intracellular pathogens cause disease by subverting macrophage innate immune defense mechanisms. Intracellular pathogens actively avoid delivery to or directly target lysosomes, the major intracellular degradative organelle. In this article, we demonstrate that activator of G-protein signaling 3 (AGS3), an LPS-inducible protein in macrophages, affects both lysosomal biogenesis and activity. AGS3 binds the Gi family of G proteins via its G-protein regulatory (GoLoco) motif, stabilizing the Gα subunit in its GDP-bound conformation. Elevated AGS3 levels in macrophages limited the activity of the mammalian target of rapamycin pathway, a sensor of cellular nutritional status. This triggered the nuclear translocation of transcription factor EB, a known activator of lysosomal gene transcription. In contrast, AGS3-deficient macrophages had increased mammalian target of rapamycin activity, reduced transcription factor EB activity, and a lower lysosomal mass. High levels of AGS3 in macrophages enhanced their resistance to infection by Burkholderia cenocepacia J2315, Mycobacterium tuberculosis, and methicillin-resistant Staphylococcus aureus, whereas AGS3-deficient macrophages were more susceptible. We conclude that LPS priming increases AGS3 levels, which enhances lysosomal function and increases the capacity of macrophages to eliminate intracellular pathogens.

Interaction of Mycobacterium tuberculosis with host cell death pathways.

Mycobacterium tuberculosis (Mtb) has coevolved with humans for tens of thousands of years. It is thus highly adapted to its human host and has evolved multiple mechanisms to manipulate host immune responses to its advantage. One central host pathogen interaction modality is host cell death pathways. Host cell apoptosis is associated with a protective response to Mtb infection, whereas a necrotic response favors the pathogen. Consistently, Mtb inhibits host cell apoptosis signaling but promotes induction of programmed necrosis. The molecular mechanisms involved in Mtb-mediated host cell death manipulation, the consequences for host immunity, and the potential for therapeutic and preventive approaches will be discussed.

Cutting edge: Mycobacterium tuberculosis but not nonvirulent mycobacteria inhibits IFN-ß and AIM2 inflammasome-dependent IL-1ß production via its ESX-1 secretion system.

Mycobacterium tuberculosis extracellular DNA gains access to the host cell cytosol via the ESX-1 secretion system. It is puzzling that this extracellular DNA of M. tuberculosis does not induce activation of the AIM2 inflammasome because AIM2 recognizes cytosolic DNA. In this study, we show that nonvirulent mycobacteria such as Mycobacterium smegmatis induce AIM2 inflammasome activation, which is dependent on their strong induction of IFN-ß production. In contrast, M. tuberculosis, but not an ESX-1-deficient mutant, inhibits the AIM2 inflammasome activation induced by either M. smegmatis or transfected dsDNA. The inhibition does not involve changes in host cell AIM2 mRNA or protein levels but led to decreased activation of caspase-1. We furthermore demonstrate that M. tuberculosis inhibits IFN-ß production and signaling, which was partially responsible for the inhibition of AIM2 activation. In conclusion, we report a novel immune evasion mechanism of M. tuberculosis that involves the ESX-1-dependent, direct or indirect, suppression of the host cell AIM2 inflammasome activation during infection.

Mycobacterium tuberculosis infection of dendritic cells leads to partially caspase-1/11-independent IL-1ß and IL-18 secretion but not to pyroptosis.

BACKGROUND: Interleukin-1ß (IL-1ß) is important for host resistance against Mycobacterium tuberculosis (Mtb) infections. The response of the dendritic cell inflammasome during Mtb infections has not been investigated in detail. METHODOLOGY/PRINCIPAL FINDINGS: Here we show that Mtb infection of bone marrow-derived dendritic cells (BMDCs) induces IL-1ß secretion and that this induction is dependent upon the presence of functional ASC and NLRP3 but not NLRC4 or NOD2. The analysis of cell death induction in BMDCs derived from these knock-out mice revealed the important induction of host cell apoptosis but not necrosis, pyroptosis or pyronecrosis. Furthermore, NLRP3 inflammasome activation and apoptosis induction were both reduced in BMDCs infected with the esxA deletion mutant of Mtb demonstrating the importance of a functional ESX-1 secretion system. Surprisingly, caspase-1/11-deficient BMDCs still secreted residual levels of IL-1ßand IL-18 upon Mtb infection which was abolished in cells infected with the esxA Mtb mutant. CONCLUSION: Altogether we demonstrate the partially caspase-1/11-independent, but NLRP3- and ASC-dependent IL-1ß secretion in Mtb-infected BMDCs. These findings point towards a potential role of DCs in the host innate immune response to mycobacterial infections via their capacity to induce IL-1ß and IL-18 secretion.