Carl G. Feng

Conventional T-bet+Foxp3− Th1 cells are the major source of host-protective regulatory IL-10 during intracellular protozoan infection

Although interferon γ (IFN-γ) secretion is essential for control of most intracellular pathogens, host survival often also depends on the expression of interleukin 10 (IL-10), a cytokine known to counteract IFN-γ effector functions. We analyzed the source of regulatory IL-10 in mice infected with the protozoan parasite Toxoplasma gondii. Unexpectedly, IFN-γ–secreting T-bet+Foxp3− T helper type 1 (Th1) cells were found to be the major producers of IL-10 in these animals. Further analysis revealed that the same IL-10+IFN-γγ population displayed potent effector function against the parasite while, paradoxically, also inducing profound suppression of IL-12 production by antigen-presenting cells. Although at any given time point only a fraction of the cells appeared to simultaneously produce IL-10 and IFN-γ, IL-10 production could be stimulated in IL-10−IFN-γ+ cells by further activation in vitro. In addition, experiments with T. gondii–specific IL-10+IFN-γ+ CD4 clones revealed that although IFN-γ expression is imprinted and triggered with similar kinetics regardless of the state of Th1 cell activation, IL-10 secretion is induced more rapidly from recently activated than from resting cells. These findings indicate that IL-10 production by CD4+ T lymphocytes need not involve a distinct regulatory Th cell subset but can be generated in Th1 cells as part of the effector response to intracellular pathogens.

TLR9 regulates Th1 responses and cooperates with TLR2 in mediating optimal resistance to Mycobacterium tuberculosis

To investigate the role of Toll-like receptor (TLR)9 in the immune response to mycobacteria as well as its cooperation with TLR2, a receptor known to be triggered by several major mycobacterial ligands, we analyzed the resistance of TLR9−/− as well as TLR2/9 double knockout mice to aerosol infection with Mycobacterium tuberculosis. Infected TLR9−/− but not TLR2−/− mice displayed defective mycobacteria-induced interleukin (IL)-12p40 and interferon (IFN)-γ responses in vivo, but in common with TLR2−/− animals, the TLR9−/− mice exhibited only minor reductions in acute resistance to low dose pathogen challenge. When compared with either of the single TLR-deficient animals, TLR2/9−/− mice displayed markedly enhanced susceptibility to infection in association with combined defects in proinflammatory cytokine production in vitro, IFN-γ recall responses ex vivo, and altered pulmonary pathology. Cooperation between TLR9 and TLR2 was also evident at the level of the in vitro response to live M. tuberculosis, where dendritic cells and macrophages from TLR2/9−/− mice exhibited a greater defect in IL-12 response than the equivalent cell populations from single TLR9-deficient animals. These findings reveal a previously unappreciated role for TLR9 in the host response to M. tuberculosis and illustrate TLR collaboration in host resistance to a major human pathogen.

IL-23 plays a key role in Helicobacter hepaticus–induced T cell–dependent colitis

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder of the gastrointestinal tract that is caused in part by a dysregulated immune response to the intestinal flora. The common interleukin (IL)-12/IL-23p40 subunit is thought to be critical for the pathogenesis of IBD. We have analyzed the role of IL-12 versus IL-23 in two models of Helicobacter hepaticus–triggered T cell–dependent colitis, one involving anti–IL-10R monoclonal antibody treatment of infected T cell–sufficient hosts, and the other involving CD4+ T cell transfer into infected Rag−/− recipients. Our data demonstrate that IL-23 and not IL-12 is essential for the development of maximal intestinal disease. Although IL-23 has been implicated in the differentiation of IL-17–producing CD4+ T cells that alone are sufficient to induce autoimmune tissue reactivity, our results instead support a model in which IL-23 drives both interferon γ and IL-17 responses that together synergize to trigger severe intestinal inflammation.

Macrophage and T Cell Dynamics during the Development and Disintegration of Mycobacterial Granulomas

Granulomas play a key role in host protection against mycobacterial pathogens, with their breakdown contributing to exacerbated disease. To better understand the initiation and maintenance of these structures, we employed both high-resolution multiplex static imaging and intravital multiphoton microscopy of Mycobacterium bovis BCG-induced liver granulomas. We found that Kupffer cells directly capture blood-borne bacteria and subsequently nucleate formation of a nascent granuloma by recruiting both uninfected liver-resident macrophages and blood-derived monocytes. Within the mature granuloma, these myeloid cell populations formed a relatively immobile cellular matrix that interacted with a highly dynamic effector T cell population. The efficient recruitment of these T cells was highly dependent on TNF-alpha-derived signals, which also maintained the granuloma structure through preferential effects on uninfected macrophage populations. By characterizing the migration of both innate and adaptive immune cells throughout the process of granuloma development, these studies provide a new perspective on the cellular events involved in mycobacterial containment and escape.

P47 GTPASES: REGULATORS OF IMMUNITY TO INTRACELLULAR PATHOGENS

Activation of the innate immune system by interferon-γ(IFN-γ) is crucial for host resistance to infection. IFN-γ induces the expression of a wide range of mediators that undermine the ability of pathogens to survive in host cells, including a newly discovered family of 47-kDa GTPases. Elimination of different p47 GTPases in mice by gene targeting severely cripples IFN-γ-regulated defence against Toxoplasma gondii, Listeria monocytogenes, Mycobacterium spp. and other pathogens. In this article, we review our understanding of the role of p47 GTPases in resistance to intracellular infection and discuss the present evidence concerning their mode of action.

The major component in schistosome eggs responsible for conditioning dendritic cells for Th2 polarization is a T2 ribonuclease (omega-1)

Schistosoma mansoni eggs contain factors that trigger potent Th2 responses in vivo and condition mouse dendritic cells (DCs) to promote Th2 lymphocyte differentiation. Using an in vitro bystander polarization assay as the readout, we purified and identified the major Th2-inducing component from soluble egg extract (SEA) as the secreted T2 ribonuclease, omega-1. The Th2-promoting activity of omega-1 was found to be sensitive to ribonuclease inhibition and did not require MyD88/TRIF signaling in DCs. In common with unfractioned SEA, the purified native protein suppresses lipopolysaccharide-induced DC activation, but unlike SEA, it fails to trigger interleukin 4 production from basophils. Importantly, omega-1–exposed DCs displayed pronounced cytoskeletal changes and exhibited decreased antigen-dependent conjugate formation with CD4+ T cells. Based on this evidence, we hypothesize that S. mansoni omega-1 acts by limiting the interaction of DCs with CD4+ T lymphocytes, thereby lowering the strength of the activation signal delivered.

Innate and adaptive interferons suppress IL-1α and IL-1β production by distinct pulmonary myeloid subsets during Mycobacterium tuberculosis infection.

Interleukin-1 (IL-1) receptor signaling is necessary for control of Mycobacterium tuberculosis (Mtb) infection, yet the role of its two ligands, IL-1α and IL-1β, and their regulation in vivo are poorly understood. Here, we showed that both IL-1α and IL-1β are critically required for host resistance and identified two multifunctional inflammatory monocyte-macrophage and DC populations that coexpressed both IL-1 species at the single-cell level in lungs of Mtb-infected mice. Moreover, we demonstrated that interferons (IFNs) played important roles in regulating IL-1 production by these cells in vivo. Type I interferons inhibited IL-1 production by both subsets whereas CD4(+) T cell-derived IFN-γ selectively suppressed monocyte-macrophages. These data provide a cellular basis for both the anti-inflammatory effects of IFNs and probacterial functions of type I IFNs during Mtb infection and reveal differential regulation of IL-1 production by distinct cell populations as an additional layer of complexity in the activity of IL-1 in vivo.

Mice lacking myeloid differentiation factor 88 display profound defects in host resistance and immune responses to Mycobacterium avium infection not exhibited by Toll-like receptor 2 (TLR2)- and TLR4-deficient animals.

To assess the role of Toll-like receptor (TLR) signaling in host resistance to Mycobacterium avium infection, mice deficient in the TLR adaptor molecule myeloid differentiation factor 88 (MyD88), as well as TLR2−/− and TLR4−/− animals, were infected with a virulent strain of M. avium, and bacterial burdens and immune responses were compared with those in wild-type (WT) animals. MyD88−/− mice failed to control acute and chronic M. avium growth and succumbed 9–14 wk postinfection. Infected TLR2−/− mice also showed increased susceptibility, but displayed longer survival and lower bacterial burdens than MyD88−/− animals, while TLR4−/− mice were indistinguishable from their WT counterparts. Histopathological examination of MyD88−/− mice revealed massive destruction of lung tissue not present in WT, TLR2−/−, or TLR4−/− mice. In addition, MyD88−/− and TLR2−/−, but not TLR4−/−, mice displayed marked reductions in hepatic neutrophil infiltration during the first 2 h of infection. Although both MyD88−/− and TLR2−/− macrophages showed profound defects in IL-6, TNF, and IL-12p40 responses to M. avium stimulation in vitro, in vivo TNF and IL-12p40 mRNA induction was impaired only in infected MyD88−/− mice. Similarly, MyD88−/− mice displayed a profound defect in IFN-γ response that was not evident in TLR2−/− or TLR4−/− mice or in animals deficient in IL-18. These findings indicate that resistance to mycobacterial infection is regulated by multiple MyD88-dependent signals in addition to those previously attributed to TLR2 or TLR4, and that these undefined elements play a major role in determining bacterial induced proinflammatory as well as IFN-γ responses.

MyD88-deficient mice display a profound loss in resistance to Mycobacterium tuberculosis associated with partially impaired Th1 cytokine and nitric oxide synthase 2 expression.

ABSTRACT Mycobacterium tuberculosis possesses agonists for several Toll-like receptors (TLRs), yet mice with single TLR deletions are resistant to acute tuberculosis. MyD88−/− mice were used to examine whether TLRs play any role in protection against aerogenic M. tuberculosis H37Rv infection. MyD88−/− mice failed to control mycobacterial replication and rapidly succumbed. Moreover, expressions of interleukin 12, tumor necrosis factor alpha, gamma interferon, and nitric oxide synthase 2 were markedly decreased in the knockout animals. These results argue that resistance to M. tuberculosis must depend on MyD88-dependent signals mediated by an as-yet-undetermined TLR or a combination of TLRs.

Intranasal Poly-IC treatment exacerbates tuberculosis in mice through the pulmonary recruitment of a pathogen-permissive monocyte/macrophage population

Type I IFN has been demonstrated to have major regulatory effects on the outcome of bacterial infections. To assess the effects of exogenously induced type I IFN on the outcome of Mycobacterium tuberculosis infection, we treated pathogen-exposed mice intranasally with polyinosinic-polycytidylic acid condensed with poly-l-lysine and carboxymethylcellulose (Poly-ICLC), an agent designed to stimulate prolonged, high-level production of type I IFN. Drug-treated, M. tuberculosis-infected WT mice, but not mice lacking IFN-alphabeta receptor 1 (IFNalphabetaR; also known as IFNAR1), displayed marked elevations in lung bacillary loads, accompanied by widespread pulmonary necrosis without detectable impairment of Th1 effector function. Importantly, lungs from Poly-ICLC-treated M. tuberculosis-infected mice exhibited a striking increase in CD11b+F4/80+Gr1int cells that displayed decreased MHC II expression and enhanced bacterial levels relative to the same subset of cells purified from infected, untreated controls. Moreover, both the Poly-ICLC-triggered pulmonary recruitment of the CD11b+F4/80+Gr1int population and the accompanying exacerbation of infection correlated with type I IFN-induced upregulation of the chemokine-encoding gene Ccl2 and were dependent on host expression of the chemokine receptor CCR2. The above findings suggest that Poly-ICLC treatment can detrimentally affect the outcome of M. tuberculosis infection, by promoting the accumulation of a permissive myeloid population in the lung. In addition, these data suggest that agents that stimulate type I IFN should be used with caution in patients exposed to this pathogen.