Mycobacterium tuberculosis TlyA Protein Negatively Regulates T Helper (Th) 1 and Th17 Differentiation and Promotes Tuberculosis Pathogenesis

Mycobacterium tuberculosis, the causative agent of tuberculosis, is an ancient pathogen and a major cause of death worldwide. Although various virulence factors of M. tuberculosis have been identified, its pathogenesis remains incompletely understood. TlyA is a virulence factor in several bacterial infections and is evolutionarily conserved in many Gram-positive bacteria, but its function in M. tuberculosis pathogenesis has not been elucidated. Here, we report that TlyA significantly contributes to the pathogenesis of M. tuberculosis. We show that a TlyA mutant M. tuberculosis strain induces increased IL-12 and reduced IL-1β and IL-10 cytokine responses, which sharply contrasts with the immune responses induced by wild type M. tuberculosis. Furthermore, compared with wild type M. tuberculosis, TlyA-deficient M. tuberculosis bacteria are more susceptible to autophagy in macrophages. Consequently, animals infected with the TlyA mutant M. tuberculosis organisms exhibited increased host-protective immune responses, reduced bacillary load, and increased survival compared with animals infected with wild type M. tuberculosis. Thus, M. tuberculosis employs TlyA as a host evasion factor, thereby contributing to its virulence.     

IL-27 inhibits IFN-γ induced autophagy by concomitant induction of JAK/PI3 K/Akt/mTOR cascade and up-regulation of Mcl-1 in Mycobacterium tuberculosis H37Rv infected macrophages

Interleukin-27 (IL-27), a key immunoregulatory cytokine plays an important role in host response to mycobacterial infection as neutralization of IL-27 augments intracellular killing of mycobacteria. Autophagy has a pivotal role in host immunity and is regulated by various cytokines. Here, we report that IL-27 inhibits IFN-γ and starvation induced autophagy and as a result blocks phagosome maturation and promotes intracellular survival of Mycobacterium tuberculosis H37Rv. Addition of exogenous IL-27 induces the activation of mTOR through JAK/PI3 K pathway and inhibits IFN-γ stimulated autophagy. Furthermore, blockade of JAKs obstructs the inhibitory effect of IL-27 on IFN-γ induced autophagy. Besides this, IL-27 also up-regulates Mcl-1through PI3 K pathway. We further show that in mTOR or Mcl-1 silenced THP-1 cells, IL-27 could no longer inhibit IFN-γ mediated autophagy in M. tuberculosis H37Rv infected cells. Altogether, our study demonstrates that IL-27 by concurrent activation of JAK/PI3 K/Akt/mTOR cascade as well as up-regulation of Mcl-1 inhibits IFN-γ induced autophagy and elimination of intracellular mycobacteria in macrophages.     

Polymorphisms in autophagy genes and susceptibility to tuberculosis

Recent data suggest that autophagy is important for intracellular killing of Mycobacterium tuberculosis, and polymorphisms in the autophagy gene IRGM have been linked with susceptibility to tuberculosis (TB) among African-Americans, and with TB caused by particular M. tuberculosis genotypes in Ghana. We compared 22 polymorphisms of 14 autophagy genes between 1022 Indonesian TB patients and 952 matched controls, and between patients infected with different M. tuberculosis genotypes, as determined by spoligotyping. The same autophagy polymorphisms were studied in correlation with ex-vivo production of TNF, IL-1β, IL-6, IL-8, IFN-γ and IL-17 in healthy volunteers. No association was found between TB and polymorphisms in the genes ATG10, ATG16L2, ATG2B, ATG5, ATG9B, IRGM, LAMP1, LAMP3, P2RX7, WIPI1, MTOR and ATG4C. Associations were found between polymorphisms in LAMP1 (p = 0.02) and MTOR (p = 0.02) and infection with the successful M. tuberculosis Beijing genotype. The polymorphisms examined were not associated with M. tuberculosis induced cytokines, except for a polymorphism in ATG10, which was linked with IL-8 production (p = 0.04). All associations found lost statistical significance after correction for multiple testing. This first examination of a broad set of polymorphisms in autophagy genes fails to show a clear association with TB, with M. tuberculosis Beijing genotype infection or with ex-vivo pro-inflammatory cytokine production.     

Involvement of the autophagy pathway in trafficking of Mycobacterium tuberculosis bacilli through cultured human type II epithelial cells

Interactions between Mycobacterium tuberculosis bacilli and alveolar macrophages have been extensively characterized, while similar analyses in epithelial cells have not been performed. In this study, we microscopically examined endosomal trafficking of M. tuberculosis strain Erdman in A549 cells, a human type II pneumocyte cell line. Immuno‐electron microscopic (IEM) analyses indicate that M. tuberculosis bacilli are internalized to a compartment labelled first with Rab5 and then with Rab7 small GTPase proteins. This suggests that, unlike macrophages, M. tuberculosis bacilli traffic to late endosomes in epithelial cells. However, fusion of lysosomes with the bacteria‐containing compartment appears to be inhibited, as illustrated by IEM studies employing LAMP‐2 and cathepsin‐L antibodies. Examination by transmission electron microscopy and IEM revealed M. tuberculosis‐containing compartments surrounded by double membranes and labelled with antibodies against the autophagy marker Lc3, providing evidence for involvement and intersection of the autophagy and endosomal pathways. Interestingly, inhibition of the autophagy pathway using 3‐methyladenine improved host cell viability and decreased numbers of viable intracellular bacteria recovered after 72 h post infection. Collectively, these datasuggest that trafficking patterns for M. tuberculosis bacilli in alveolar epithelial cells differ from macrophages, and that autophagy is involved this process.     

Bacillary replication and macrophage necrosis are determinants of neutrophil recruitment in tuberculosis

We previously determined that burst size necrosis is the chief mode of mononuclear cell death in the lungs of mice with tuberculosis. The present study explored the link between infection-induced necrosis of mononuclear phagocytes and neutrophil accumulation in the lungs of mice challenged with one of four Mycobacterium tuberculosis strains of increasing virulence (RvΔphoPR mutant, H37Ra, H37Rv and Erdman). At all time points studied, Erdman produced the highest bacterial load and the highest proportion and number of M. tuberculosis-infected neutrophils. These parameters, and the proportion of TUNEL-positive cells, tracked with virulence across all strains tested. Differences in neutrophil infection were not reflected by levels of chemoattractant cytokines in bronchoalveolar lavage fluid, while interferon-γ (reported to suppress neutrophil trafficking to the lung in tuberculosis) was highest in Erdman-infected mice. Treating Erdman-infected mice with ethambutol decreased the proportion of mononuclear phagocytes with high bacterial burden and the ratio of infected neutrophils to infected mononuclear cells in a dose-dependent manner. We propose that faster replicating M. tuberculosis strains cause more necrosis which in turn promotes neutrophil recruitment. Neutrophils infected with M. tuberculosis constitute a biomarker for poorly controlled bacterial replication, infection-induced mononuclear cell death, and increased severity of immune pathology in tuberculosis.     

Autophagy Gene Variant IRGM ?261T Contributes to Protection from Tuberculosis Caused by Mycobacterium tuberculosis but Not by M. africanum Strains

The human immunity-related GTPase M (IRGM) has been shown to be critically involved in regulating autophagy as a means of disposing cytosolic cellular structures and of reducing the growth of intracellular pathogens in vitro. This includes Mycobacterium tuberculosis, which is in agreement with findings indicating that M. tuberculosis translocates from the phagolysosome into the cytosol of infected cells, where it becomes exposed to autophagy. To test whether IRGM plays a role in human infection, we studied IRGM gene variants in 2010 patients with pulmonary tuberculosis (TB) and 2346 unaffected controls. Mycobacterial clades were classified by spoligotyping, IS6110 fingerprinting and genotyping of the pks1/15 deletion. The IRGM genotype −261TT was negatively associated with TB caused by M. tuberculosis (OR 0.66, CI 0.52–0.84, P_nominal 0.0009, P_corrected 0.0045) and not with TB caused by M. africanum or M. bovis (OR 0.95, CI 0.70–1.30. P 0.8). Further stratification for mycobacterial clades revealed that the protective effect applied only to M. tuberculosis strains with a damaged pks1/15 gene which is characteristic for the Euro-American (EUAM) subgroup of M. tuberculosis (OR 0.63, CI 0.49–0.81, P_nominal 0.0004, P_corrected 0.0019). Our results, including those of luciferase reporter gene assays with the IRGM variants −261C and −261T, suggest a role for IRGM and autophagy in protection of humans against natural infection with M. tuberculosis EUAM clades. Moreover, they support in vitro findings indicating that TB lineages capable of producing a distinct mycobacterial phenolic glycolipid that occurs exclusively in strains with an intact pks1/15 gene inhibit innate immune responses in which IRGM contributes to the control of autophagy. Finally, they raise the possibility that the increased frequency of the IRGM −261TT genotype may have contributed to the establishment of M. africanum as a pathogen in the West African population.     

Defective Autophagy and mTORC1 Signaling in Myotubularin Null Mice

Autophagy is a vesicular trafficking pathway that regulates the degradation of aggregated proteins and damaged organelles. Initiation of autophagy requires several multiprotein signaling complexes, such as the ULK1 kinase complex and the Vps34 lipid kinase complex, which generates phosphatidylinositol 3-phosphate [PtdIns(3)P] on the forming autophagosomal membrane. Alterations in autophagy have been reported for various diseases, including myopathies. Here we show that skeletal muscle autophagy is compromised in mice deficient in the X-linked myotubular myopathy (XLMTM)-associated PtdIns(3)P phosphatase myotubularin (MTM1). Mtm1-deficient muscle displays several cellular abnormalities, including a profound increase in ubiquitin aggregates and abnormal mitochondria. Further, we show that Mtm1 deficiency is accompanied by activation of mTORC1 signaling, which persists even following starvation. In vivo pharmacological inhibition of mTOR is sufficient to normalize aberrant autophagy and improve muscle phenotypes in Mtm1 null mice. These results suggest that aberrant mTORC1 signaling and impaired autophagy are consequences of the loss of Mtm1 and may play a primary role in disease pathogenesis.     

Molecular mechanism of interaction of Mycobacterium tuberculosis with host macrophages under high glucose conditions

Mycobacterium tuberculosis has the potential to escape various cellular defense mechanisms for its survival which include various oxidative stress responses, inhibition of phagosome-lysosomes fusion and alterations in cell death mechanisms of host macrophages that are crucial for its infectivity and dissemination. Diabetic patients are more susceptible to developing tuberculosis because of impairement of innate immunity and prevailing higher glucose levels. Our earlier observations have demonstrated alterations in the protein profile of M. tuberculosis exposed to concurrent high glucose and tuberculosis conditions suggesting a crosstalk between host and pathogen under high glucose conditions. Since high glucose environment plays crucial role in the interaction of mycobacterium with host macrophages which provide a niche for the survival of M. tuberculosis, it is important to understand various interactive mechanisms under such conditions. Initial phagocytosis and containment of M. tuberculosis by macrophages, mode of macrophage cell death, respiratory burst responses, Mycobacterium and lysosomal co-localization were studied in M. tuberculosis H37Rv infected cells in the presence of varied concentrations of glucose in order to mimic diabetes like conditions. It was observed that initial attachment, phagocytosis and later containment were less effective under high glucose conditions in comparison to normal glucose. Mycobacterium infected cells showed more necrosis than apoptosis as cell death mechanism during the course of infection under high glucose concentrations. Co-localization and respiratory burst assay also indicated evasion strategies adopted by M. tuberculosis under such conditions. This study by using THP1 macrophage model of tuberculosis and high glucose conditions showed immune evasion strategies adapted during co-pathogenesis of tuberculosis and diabetes.     

Vitamin D Inhibits Human Immunodeficiency Virus Type 1 and Mycobacterium tuberculosis Infection in Macrophages through the Induction of Autophagy

Low vitamin D levels in human immunodeficiency virus type-1 (HIV) infected persons are associated with more rapid disease progression and increased risk for Mycobacterium tuberculosis infection. We have previously shown that 1α,25-dihydroxycholecalciferol (1,25D3), the active form of vitamin D, inhibits HIV replication in human macrophages through the induction of autophagy. In this study, we report that physiological concentrations of 1,25D3 induce the production of the human cathelicidin microbial peptide (CAMP) and autophagic flux in HIV and M. tuberculosis co-infected human macrophages which inhibits mycobacterial growth and the replication of HIV. Using RNA interference for Beclin-1 and the autophagy-related 5 homologue, combined with the chemical inhibitors of autophagic flux, bafilomycin A₁, an inhibitor of autophagosome-lysosome fusion and subsequent acidification, and SID 26681509 an inhibitor of the lysosome hydrolase cathepsin L, we show that the 1,25D3-mediated inhibition of HIV replication and mycobacterial growth during single infection or dual infection is dependent not only upon the induction of autophagy, but also through phagosomal maturation. Moreover, through the use of RNA interference for CAMP, we demonstrate that cathelicidin is essential for the 1,25D3 induced autophagic flux and inhibition of HIV replication and mycobacterial growth. The present findings provide a biological explanation for the benefits and importance of vitamin D sufficiency in HIV and M. tuberculosis-infected persons, and provide new insights into novel approaches to prevent and treat HIV infection and related opportunistic infections.     

Identification of Host-Targeted Small Molecules That Restrict Intracellular Mycobacterium tuberculosis Growth

Mycobacterium tuberculosis remains a significant threat to global health. Macrophages are the host cell for M. tuberculosis infection, and although bacteria are able to replicate intracellularly under certain conditions, it is also clear that macrophages are capable of killing M. tuberculosis if appropriately activated. The outcome of infection is determined at least in part by the host-pathogen interaction within the macrophage; however, we lack a complete understanding of which host pathways are critical for bacterial survival and replication. To add to our understanding of the molecular processes involved in intracellular infection, we performed a chemical screen using a high-content microscopic assay to identify small molecules that restrict mycobacterial growth in macrophages by targeting host functions and pathways. The identified host-targeted inhibitors restrict bacterial growth exclusively in the context of macrophage infection and predominantly fall into five categories: G-protein coupled receptor modulators, ion channel inhibitors, membrane transport proteins, anti-inflammatories, and kinase modulators. We found that fluoxetine, a selective serotonin reuptake inhibitor, enhances secretion of pro-inflammatory cytokine TNF-α and induces autophagy in infected macrophages, and gefitinib, an inhibitor of the Epidermal Growth Factor Receptor (EGFR), also activates autophagy and restricts growth. We demonstrate that during infection signaling through EGFR activates a p38 MAPK signaling pathway that prevents macrophages from effectively responding to infection. Inhibition of this pathway using gefitinib during in vivo infection reduces growth of M. tuberculosis in the lungs of infected mice. Our results support the concept that screening for inhibitors using intracellular models results in the identification of tool compounds for probing pathways during in vivo infection and may also result in the identification of new anti-tuberculosis agents that work by modulating host pathways. Given the existing experience with some of our identified compounds for other therapeutic indications, further clinically-directed study of these compounds is merited.     

Autophagy induction by vitamin D inhibits both Mycobacterium tuberculosis and human immunodeficiency virus type 1

Low vitamin D levels in human immunodeficiency virus type-1 (HIV) infected persons are associated with more rapid disease progression and increased risk for Mycobacterium tuberculosis infection. We report that physiological concentrations of 1α,25-dihydroxycholecalciferol (1,25D3), the active form of vitamin D, inhibits M. tuberculosis and HIV replication in co-infected macrophages through human cathelicidin microbial peptide-dependent autophagy that requires phagosomal maturation. These findings provide a biological explanation for the importance of vitamin D sufficiency in HIV and M. tuberculosis-infected persons, and provide new insights into novel approaches to prevent and treat HIV infection and related opportunistic infections.     

Mycobacterium tuberculosis Rv3034c regulates mTORC1 and PPAR‐γ dependant pexophagy mechanism to control redox levels in macrophages

Mycobacterium tuberculosis survives inside the macrophages by employing several host immune evasion strategies. Here, we reported a novel mechanism in which M. tuberculosis acetyltransferase, encoded by Rv3034c, induces peroxisome homeostasis to regulate host oxidative stress levels to facilitate intracellular mycobacterial infection. Presence of M. tuberculosis Rv3034c induces the expression of peroxisome biogenesis and proliferation factors such as Pex3, Pex5, Pex19, Pex11b, Fis‐1 and DLP‐1; while depletion of Rv3034c decreased the expression of these molecules, thereby selective degradation of peroxisomes via pexophagy. Further studies revealed that M. tuberculosis Rv3034c inhibit induction of pexophagy mechanism by down‐regulating the expression of pexophagy associated proteins (p‐AMPKα, p‐ULK‐1, Atg5, Atg7, Beclin‐1, LC3‐II, TFEB and Keap‐1) and adaptor molecules (NBR1 and p62). Inhibition was found to be dependent on the phosphorylation of mTORC1 and activation of peroxisome proliferator activated receptor‐γ. In order to maintain intracellular homeostasis during oxidative stress, M. tuberculosis Rv3034c was found to induce degradation of dysfunctional and damaged peroxisomes through activation of Pex14 in infected macrophages. In conclusion, this is the first report which demonstrated that M. tuberculosis acetyltransferase regulate peroxisome homeostasis in response to intracellular redox levels to favour mycobacterial infection in macrophage.     

Interactions of attenuated Mycobacterium tuberculosis phoP mutant with human macrophages

Background

Mycobacterium tuberculosis phoP mutant SO2 derived from a clinical isolate was shown to be attenuated in mouse bone marrow-derived macrophages and in vivo mouse infection model and has demonstrated a high potential as attenuated vaccine candidate against tuberculosis.

Methodology/Principal Findings

In this study, we analyze the adhesion and the intracellular growth and trafficking of SO2 in human macrophages. Our results indicate an enhanced adhesion to phagocitic cells and impaired intracellular replication of SO2 in both monocyte-derived macrophages and human cell line THP-1 in comparison with the wild type strain, consistent with murine model. Intracellular trafficking analysis in human THP-1 cells suggest that attenuation of SO2 within macrophages could be due to an impaired ability to block phagosome-lysosome fusion compared with the parental M. tuberculosis strain. No differences were found between SO2 and the wild-type strains in the release and mycobacterial susceptibility to nitric oxide (NO) produced by infected macrophages.

Conclusions/Significance

SO2 has enhanced ability to bind human macrophages and differs in intracellular trafficking as to wild-type M. tuberculosis. The altered lipid profile expression of the phoP mutant SO2 and its inability to secrete ESAT-6 is discussed.     

Autophagy induction by tetrahydrobiopterin deficiency

Tetrahydrobiopterin (BH₄) deficiency is a genetic disorder associated with a variety of metabolic syndromes such as phenylketonuria (PKU). In this article, the signaling pathway by which BH₄ deficiency inactivates mTORC1 leading to the activation of the autophagic pathway was studied utilizing BH₄-deficient Spr^-/- mice generated by the knockout of the gene encoding sepiapterin reductase (SR) catalyzing BH₄ synthesis. We found that mTORC1 signaling was inactivated and autophagic pathway was activated in tissues from Spr^-/- mice. This study demonstrates that tyrosine deficiency causes mTORC1 inactivation and subsequent activation of autophagic pathway in Spr^-/- mice. Therapeutic tyrosine diet completely rescued dwarfism and mTORC1 inhibition but inactivated autophagic pathway in Spr^-/- mice. Tyrosine-dependent inactivation of mTORC1 was further supported by mTORC1 inactivation in Pah^enu2 mouse model lacking phenylalanine hydroxylase (Pah). NIH3T3 cells grown under the condition of tyrosine restriction exhibited autophagy induction. However, mTORC1 activation by RhebQ64L, a positive regulator of mTORC1, inactivated autophagic pathway in NIH3T3 cells under tyrosine-deficient conditions. In addition, this study first documents mTORC1 inactivation and autophagy induction in PKU patients with BH₄ deficiency.Key words: tetrahydrobiopterin, autophagy, mTORC1, tyrosine, phenylalanine, phenylketonuria, Akt, AMPK     

Small Molecule-directed Immunotherapy against Recurrent Infection by Mycobacterium tuberculosis

Tuberculosis remains the biggest infectious threat to humanity with one-third of the population infected and 1.4 million deaths and 8.7 million new cases annually. Current tuberculosis therapy is lengthy and consists of multiple antimicrobials, which causes poor compliance and high treatment dropout, resulting in the development of drug-resistant variants of tuberculosis. Therefore, alternate methods to treat tuberculosis are urgently needed. Mycobacterium tuberculosis evades host immune responses by inducing T helper (Th)2 and regulatory T (Treg) cell responses, which diminish protective Th1 responses. Here, we show that animals (Stat-6^−/−CD4-TGFβRIIDN mice) that are unable to generate both Th2 cells and Tregs are highly resistant to M. tuberculosis infection. Furthermore, simultaneous inhibition of these two subsets of Th cells by therapeutic compounds dramatically reduced bacterial burden in different organs. This treatment was associated with the generation of protective Th1 immune responses. As these therapeutic agents are not directed to the harbored organisms, they should avoid the risk of promoting the development of drug-resistant M. tuberculosis variants.     

Mycobacterium tuberculosis-Induced Polarization of Human Macrophage Orchestrates the Formation and Development of Tuberculous Granulomas In Vitro

The tuberculous granuloma is an elaborately organized structure and one of the main histological hallmarks of tuberculosis. Macrophages, which are important immunologic effector and antigen-presenting cells, are the main cell type found in the tuberculous granuloma and have high plasticity. Macrophage polarization during bacterial infection has been elucidated in numerous recent studies; however, macrophage polarization during tuberculous granuloma formation and development has rarely been reported. It remains to be clarified whether differences in the activation status of macrophages affect granuloma formation. In this study, the variation in macrophage polarization during the formation and development of tuberculous granulomas was investigated in both sections of lung tissues from tuberculosis patients and an in vitro tuberculous granuloma model. The roles of macrophage polarization in this process were also investigated. Mycobacterium tuberculosis (M. tuberculosis) infection was found to induce monocyte-derived macrophage polarization. In the in vitro tuberculous granuloma model, macrophage transformation from M1 to M2 was observed over time following M. tuberculosis infection. M2 macrophages were found to predominate in both necrotic and non-necrotic granulomas from tuberculosis patients, while both M1 and M2 polarized macrophages were found in the non-granulomatous lung tissues. Furthermore, it was found that M1 macrophages promote granuloma formation and macrophage bactericidal activity in vitro, while M2 macrophages inhibit these effects. The findings of this study provide insights into the mechanism by which M. tuberculosis circumvents the host immune system as well as a theoretical foundation for the development of novel tuberculosis therapies based on reprogramming macrophage polarization.     

Relieving autophagy and 4EBP1 from rapamycin resistance

The mammalian target of rapamycin complex 1 (mTORC1) is a multiprotein signaling complex regulated by oncogenes and tumor suppressors. Outputs downstream of mTORC1 include ribosomal protein S6 kinase 1 (S6K1), eukaryotic translation initiation factor 4E (eIF4E), and autophagy, and their modulation leads to changes in cell growth, proliferation, and metabolism. Rapamycin, an allosteric mTORC1 inhibitor, does not antagonize equally these outputs, but the reason for this is unknown. Here, we show that the ability of rapamycin to activate autophagy in different cell lines correlates with mTORC1 stability. Rapamycin exposure destabilizes mTORC1, but in cell lines where autophagy is drug insensitive, higher levels of mTOR-bound raptor are detected than in cells where rapamycin stimulates autophagy. Using small interfering RNA (siRNA), we find that knockdown of raptor relieves autophagy and the eIF4E effector pathway from rapamycin resistance. Importantly, nonefficacious concentrations of an ATP-competitive mTOR inhibitor can be combined with rapamycin to synergistically inhibit mTORC1 and activate autophagy but leave mTORC2 signaling intact. These data suggest that partial inhibition of mTORC1 by rapamycin can be overcome using combination strategies and offer a therapeutic avenue to achieve complete and selective inhibition of mTORC1.     

Shigella Effector OspB Activates mTORC1 in a Manner That Depends on IQGAP1 and Promotes Cell Proliferation

The intracellular bacterial pathogen Shigella infects and spreads through the human intestinal epithelium. Effector proteins delivered by Shigella into cells promote infection by modulating diverse host functions. We demonstrate that the effector protein OspB interacts directly with the scaffolding protein IQGAP1, and that the absence of either OspB or IQGAP1 during infection leads to larger areas of S. flexneri spread through cell monolayers. We show that the effect on the area of bacterial spread is due to OspB triggering increased cell proliferation at the periphery of infected foci, thereby replacing some of the cells that die within infected foci and restricting the area of bacterial spread. We demonstrate that OspB enhancement of cell proliferation results from activation of mTORC1, a master regulator of cell growth, and is blocked by the mTORC1-specific inhibitor rapamycin. OspB activation of mTORC1, and its effects on cell proliferation and bacterial spread, depends on IQGAP1. Our results identify OspB as a regulator of mTORC1 and mTORC1-dependent cell proliferation early during S. flexneri infection and establish a role for IQGAP1 in mTORC1 signaling. They also raise the possibility that IQGAP1 serves as a scaffold for the assembly of an OspB-mTORC1 signaling complex.     

Gr1intCD11b+ Myeloid-Derived Suppressor Cells in Mycobacterium tuberculosis Infection

Background

Tuberculosis is one of the world’s leading killers, stealing 1.4 million lives and causing 8.7 million new and relapsed infections in 2011. The only vaccine against tuberculosis is BCG which demonstrates variable efficacy in adults worldwide. Human infection with Mycobacterium tuberculosis results in the influx of inflammatory cells to the lung in an attempt to wall off bacilli by forming a granuloma. Gr1^intCD11b⁺ cells are called myeloid-derived suppressor cells (MDSC) and play a major role in regulation of inflammation in many pathological conditions. Although MDSC have been described primarily in cancer their function in tuberculosis remains unknown. During M. tuberculosis infection it is crucial to understand the function of cells involved in the regulation of inflammation during granuloma formation. Understanding their relative impact on the bacilli and other cellular phenotypes is necessary for future vaccine and drug design.

Methodology/Principal Findings

We compared the bacterial burden, lung pathology and Gr1^intCD11b⁺ myeloid-derived suppressor cell immune responses in M. tuberculosis infected NOS2-/-, RAG-/-, C3HeB/FeJ and C57/BL6 mice. Gr-1⁺ cells could be found on the edges of necrotic lung lesions in NOS2-/-, RAG-/-, and C3HeB/FeJ, but were absent in wild-type mice. Both populations of Gr1⁺CD11b⁺ cells expressed high levels of arginase-1, and IL-17, additional markers of myeloid derived suppressor cells. We then sorted the Gr1^hi and Gr1^int populations from M. tuberculosis infected NOS-/- mice and placed the sorted both Gr1^int populations at different ratios with naïve or M. tuberculosis infected splenocytes and evaluated their ability to induce activation and proliferation of CD4+T cells. Our results showed that both Gr1^hi and Gr1^int cells were able to induce activation and proliferation of CD4+ T cells. However this response was reduced as the ratio of CD4⁺ T to Gr1⁺ cells increased. Our results illustrate a yet unrecognized interplay between Gr1⁺ cells and CD4⁺ T cells in tuberculosis.