Publications

Hepcidin is an antimicrobial peptide involved in regulating iron homeostasis. It is induced by iron overload and decreased by hypoxia and anemia. Hepcidin regulates iron metabolism by inhibiting iron absorption by the duodenum and by inhibiting macrophage iron recycling. Hepcidin is induced in hepatocytes during the acute-phase response by IL-6. Previously, we have shown that hepcidin is not induced in macrophages by IL-6 but is induced by the synergistic interaction of IFN-gamma and Mycobacterium tuberculosis infection. In the present study, we examined the pathways involved in inducing macrophage hepcidin expression. We show that TLRs TLR2 and TLR4 and the transcription factor STAT1 are required for induction of hepcidin mRNA. Hepcidin promoter activity is also synergistically induced in RAW264.7 macrophages by IFN-gamma and M. tuberculosis. NF-kappaB and C/CEBP binding sites are required for promoter activity. Binding of NF-kappaB (p50/p65) to the NF-kappaB site and STAT1 and C/EBPbeta to the C/CEBP site was confirmed by EMSA. Knockdown of STAT1 and C/EBPbeta expression in RAW264.7 cells with siRNA plasmids inhibited hepcidin promoter activity induced by IFN-gamma and M. tuberculosis. Together, these studies demonstrate that macrophage hepcidin expression is induced by the activation of STAT1 and NF-kappaB and the induction of C/EBPbeta expression.

BACKGROUND: Autophagy has been shown recently to play an important role in the intracellular survival of several pathogenic bacteria. In this study, we investigated the effect of a novel small-molecule autophagy-inducing agent, AR-12, on the survival of Francisella tularensis, the causative bacterium of tularemia in humans and a potential bioterrorism agent, in macrophages.

METHODS AND RESULTS: Our results show that AR-12 induces autophagy in THP-1 macrophages, as indicated by increased autophagosome formation, and potently inhibits the intracellular survival of F. tularensis (type A strain, Schu S4) and F. novicida in macrophages in association with increased bacterial co-localization with autophagosomes. The effect of AR-12 on intracellular F. novicida was fully reversed in the presence of the autophagy inhibitor, 3-methyl adenine or the lysosome inhibitor, chloroquine. Intracellular F. novicida were not susceptible to the inhibitory activity of AR-12 added at 12 h post-infection in THP-1 macrophages, and this lack of susceptibility was independent of the intracellular location of bacteria.

CONCLUSION: Together, AR-12 represents a proof-of-principle that intracellular F. tularensis can be eradicated by small-molecule agents that target innate immunity.

Eradication of intracellular pathogenic bacteria with host-directed chemical agents has been an anticipated innovation in the treatment of antibiotic-resistant bacteria. We previously synthesized and characterized a novel small-molecule agent, AR-12, that induces autophagy and inhibits the Akt kinase in cancer cells. As both autophagy and the Akt kinase have been shown recently to play roles in the intracellular survival of several intracellular bacteria, including Salmonella enterica serovar Typhimurium, we investigated the effect of AR-12 on the intracellular survival of Salmonella serovar Typhimurium in macrophages. Our results show that AR-12 induces autophagy in macrophages, as indicated by increased autophagosome formation, and potently inhibits the survival of serovar Typhimurium in macrophages in association with increased colocalization of intracellular bacteria with autophagosomes. Intracellular bacterial growth was partially rescued in the presence of AR-12 by the short hairpin RNA-mediated knockdown of Beclin-1 or Atg7 in macrophages. Moreover, AR-12 inhibits Akt kinase activity in infected macrophages, which we show to be important for its antibacterial effect as the enforced expression of constitutively activated Akt1 in these cells reverses the AR-12-induced inhibition of intracellular serovar Typhimurium survival. Finally, oral administration of AR-12 at 2.5 mg/kg/day to serovar Typhimurium-infected mice reduced hepatic and splenic bacterial burdens and significantly prolonged survival. These findings show that AR-12 represents a proof of principle that the survival of intracellular bacteria can be suppressed by small-molecule agents that target both innate immunity and host cell factors modulated by bacteria.

Alveolar macrophages are thought to play a central role in the pathogenesis of inhalational anthrax. Receptors present on macrophages that mediate phagocytosis of Bacillus anthracis spores have yet to be completely defined. To begin to determine if soluble factors that are present in the lung such as immunoglobulin and complement are involved, we characterized the binding of human IgG and C3 to the surface of B. anthracis spores at different concentrations of nonimmune human serum. Furthermore we investigated the uptake of B. anthracis spores by human monocyte-derived macrophages in the presence of nonimmune human serum. Here we show that C3b is bound to B. anthracis spores and is activated through the classical pathway by IgG bound to the spore surface. Furthermore, we show that C3 serves as an opsonin for B. anthracis spores resulting in enhanced phagocytosis by human macrophages. These studies provide evidence that nonimmune serum contains IgG which binds to B. anthracis spores but is not sufficient to initiate phagocytosis. However, surface-bound IgG does initiate the classical pathway of complement activation, which is active in the lung, resulting in deposition of the opsonin C3b on the spore surface.

In the lung, alternatively activated macrophages (AAM) form the first line of defense against microbial infection. Due to the highly regulated nature of AAM, the lung can be considered as an immunosuppressive organ for respiratory pathogens. However, as infection progresses in the lung, another population of macrophages, known as classically activated macrophages (CAM) enters; these cells are typically activated by IFN-gamma. CAM are far more effective than AAM in clearing the microbial load, producing proinflammatory cytokines and antimicrobial defense mechanisms necessary to mount an adequate immune response. Here, we are concerned with determining the first time when the population of CAM becomes more dominant than the population of AAM. This proposed "switching time" is explored in the context of Mycobacterium tuberculosis (MTb) infection. We have developed a mathematical model that describes the interactions among cells, bacteria, and cytokines involved in the activation of both AAM and CAM. The model, based on a system of differential equations, represents a useful tool to analyze strategies for reducing the switching time, and to generate hypotheses for experimental testing.

The Immune Epitope Database (IEDB), an online resource available at http://immuneepitope.org/, contains data on T cell and B cells epitopes of multiple pathogens, including M. tuberculosis. A workshop held in June, 2007 reviewed the existing database, discussed the utility of reference sets of epitopes, and identified knowledge gaps pertaining to epitopes and immune responses in tuberculosis.

The transmembrane C-type lectin, dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN), has three conserved cytoplasmic tail motifs: the tyrosine (Y)-based, dileucine (LL), and triacidic cluster (EEE), which are believed to regulate ligand binding, uptake, and trafficking. We mutated each of these motifs by alanine substitution and tested their roles in phagocytosis and receptor-mediated endocytosis of the highly mannosylated ligands, Mycobacterium tuberculosis mannose-capped lipoarabinomannan (ManLAM) and HIV-1 surface glycoprotein gp120, respectively, in transfected human myeloid K-562 cells. Compared with wild-type and other mutants, the EEE mutant of DC-SIGN showed a reduced cell-surface expression, near abolishment in the phagocytosis of ManLAM-coated beads (90.5+/-0.4%), and a marked reduction in the endocytosis of soluble gp120 (79.3+/-0.7%). Although, the Y mutant of DC-SIGN did not exhibit any effect on phagocytosis and intracellular trafficking to the phagolysosome, the LL mutant caused the majority of the receptor and/or ligands to remain bound to the cell surface, indicating a role for the LL motif as an internalization signal. The majority of the EEE mutant protein was found to be retained by the intracellular trans-Golgi network and not by the late endosomal/lysosomal compartment of transfected K-562 cells. Collectively, our data indicate a dual role for the EEE motif as a sorting signal in the secretory pathway and a lysosomal targeting signal in the endocytic pathway.

Intracellular pathogens, including Mycobacterium tuberculosis, obtain iron from the host for their survival. Ferroportin 1 (FPN1; SLC40A1) is the sole iron exporter from mammalian cells and is expressed in the duodenum and macrophages. In the present study, we show that FPN1 mRNA levels in the mouse macrophage cell line RAW264.7 are synergistically induced by treatment with live or gamma-irradiated M. tuberculosis and IFN-gamma. FPN1 mRNA levels were also induced by Mycobacterium avium and IFN-gamma in RAW264.7 cells and the mouse alveolar macrophage cell line AMJ2-C8. Treatment of mouse resident peritoneal macrophages with M. tuberculosis and IFN-gamma resulted in a sixfold increase in FPN1 mRNA expression. In contrast, M. tuberculosis and IFN-gamma inhibited FPN1 mRNA expression in bone marrow-derived macrophages and lung macrophages, which have high basal levels of FPN1 mRNA expression. Using confocal microscopy, FPN1 protein localized rapidly to M. tuberculosis phagosomes after infection in RAW264.7 macrophages. In RAW264.7 cells expressing wild-type natural resistance-associated macrophage protein 1 (Nramp1(Gly169)), FPN1 and Nramp1 partially colocalized in late endosomes/lysosomes prior to infection. After 2 h of infection, Nramp1 and FPN1 were present in M. tuberculosis phagosomes. Our studies provide evidence for transcriptional regulation of FPN1 by pathogenic mycobacteria and IFN-gamma, which is dependent on the macrophage type. The trafficking of FPN1 to the M. tuberculosis phagosome suggests that it is involved in regulating iron availability to the mycobacteria in this locale.

Phenotypically distinct clinical isolates of Mycobacterium tuberculosis are capable of altering the balance that exists between the pathogen and human host and ultimately the outcome of infection. This study has identified two M. tuberculosis strains (i.e. HN885 and HN1554) among a bank of clinical isolates with a striking defect in phagocytosis by primary human macrophages when compared with strain Erdman, a commonly used laboratory strain for studies of pathogenesis. Mass spectrometry in conjunction with NMR studies unequivocally confirmed that both HN885 and HN1554 contain truncated and more branched forms of mannose-capped lipoarabinomannan (ManLAM) with a marked reduction of their linear arabinan (corresponding mainly to the inner Araf-alpha(1–>5)-Araf unit) and mannan (with fewer 6-Manp residues and more substitutions in the linear Manp-alpha(1–>6)-Manp unit) domains. The truncation in the ManLAM molecules produced by strains HN885 and HN1554 led to a significant reduction in their surface availability. In addition, there was a marked reduction of higher order phosphatidyl-myo-inositol mannosides and the presence of dimycocerosates, triglycerides, and phenolic glycolipid in their cell envelope. Less exposed ManLAM and reduced higher order phosphatidyl-myo-inositol mannosides in strains HN885 and HN1554 resulted in their low association with the macrophage mannose receptor. Despite reduced phagocytosis, ingested bacilli replicated at a fast rate following serum opsonization. Our results provide evidence that the clinical spectrum of tuberculosis may be dictated not only by the host but also by the amounts and ratios of surface exposed mycobacterial adherence factors defined by strain genotype.