SummaryThe expression of protective immunity to Mycobacterium tuberculosis in mice is mediated by T lymphocytes that secrete cytokines . These molecules then mediate a variety of roles, including the activation ofparasitized host macrophages, and the recruitment of other mononuclear phagocytes to the site of the infection in order to initiate granuloma formation . Among these cytokines, interferon y (IFN-y) is believed to play a key role is these events. In confirmation ofthis hypothesis, we show in this study that mice in which the IFN-y gene has been disrupted were unable to contain or control a normally sublethal dose of M. tuberculosis, delivered either intravenously or aerogenically. In such mice, a progressive and widespread tissue destruction and necrosis, associated with very high numbers ofacid-fast bacilli, was observed. In contrast, despite the lack of protective immunity, some DTH-like reactivity could still be elicited. These data, therefore, indicate that although IFN-y may not be needed for DTH expression, it plays a pivotal and essential role in protective cellular immunity to tuberculosis infection .C urrent murine models of experimental Mycobacterium tuberculosis infection indicate that the emergence of acquired immunity to this organism is mediated by populations of both class I and II MHC-restricted T lymphocytes, which secrete cytokines that result in the activation of parasitized macrophages and promotion of the granulomatous response (1-4) . The cytokine IFN-y, which is an effective inducer of antimicrobial mechanisms in several systems (5, 6) has been shown to inhibit the growth of mycobacteria in vitro (7-9), but its role in vivo has yet to be precisely defined. In this regard, we report here that mice in which the IFN-y gene has been disrupted (IFN-y gene knockout mice [GKO mice]), develop a fatal, disseminated form of disease when inoculated with a normally sublethal inoculum of M. tuberculosis . These results indicate, therefore, that IFN-'Y plays a pivotal role in the expression ofprotective immunity to this infection in the mouse. Moreover, this new immunodeficient mouse model may prove highly useful in the evaluation of therapeutic intervention strategies in the severely immunocompromised host.Mice. GKO mice were generated as described previously (10). Briefly, a normal IFN-y allele in mouse embryonic stem cells was replaced with a defective gene using a targeted vector which introduced a termination codon after the first 30 amino acids of the mature IFN-y protein . The altered stem cells were injected into C57BL/6J blastocysts and transmitted via the germline. Heterozygous offspring ofthe chimeras were intercrossed to generate mice homozygous for the altered (GKO) and wild type (WT) allele . The GKO mice were previously characterized as normal in terms of spleen and thymus cell number and expression of CD3, B220, CD4, and CD8 surface markers, and were shown to be incapable of IFN-y secretion (10).Experimental Infections. The Erdman strain ofM. tuberculosis was grown in Proskauer B...

Toll-like receptor (TLR) signaling in macrophages is required for antipathogen responses, including the biosynthesis of nitric oxide from arginine, and is essential for immunity to Mycobacterium tuberculosis, Toxoplasma gondii and other intracellular pathogens. Here we report a ‘loophole’ in the TLR pathway that is advantageous to these pathogens. Intracellular pathogens induced expression of the arginine hydrolytic enzyme arginase 1 (Arg1) in mouse macrophages through the TLR pathway. In contrast to diseases dominated by T helper type 2 (TH2) responses, TLR-mediated Arg1 induction was independent of the TH2-associated STAT6 pathway. Specific elimination of Arg1 in macrophages favored host survival in T. gondii infection and decreased lung bacterial load in tuberculosis infection.

Immunity to Mycobacterium tuberculosis infection is associated with the emergence of protective CD4 T cells that secrete cytokines, resulting in activation of macrophages and the recruitment of monocytes to initiate granuloma formation. The cytokine-mediating macrophage activation is interferon-γ (IFN-γ), which is largely dependent on interleukin-12 (IL-12) for its induction. To address the role of IL-12 in immunity to tuberculosis, IL-12 p40−/− mice were infected with M. tuberculosis and their capacity to control bacterial growth and other characteristics of their immune response were determined. The IL-12 p40−/− mice were unable to control bacterial growth and this appeared to be linked to the absence of both innate and acquired sources of IFN-γ. T cell activation as measured by delayed type hypersensitivity and lymphocyte accumulation at the site of infection were both markedly reduced in the IL-12 p40−/− mice. Therefore, IL-12 is essential to the generation of a protective immune response to M. tuberculosis, with its main functions being the induction of the expression of IFN-γ and the activation of antigen-specific lymphocytes capable of creating a protective granuloma.

This study extends earlier reports regarding the in vitro and in vivo efficacies of the nitroimidazopyran PA-824 against Mycobacterium tuberculosis. PA-824 was tested in vitro against a broad panel of multidrugresistant clinical isolates and was found to be highly active against all isolates (MIC < 1 g/ml). The activity of PA-824 against M. tuberculosis was also assessed grown under conditions of oxygen depletion. PA-824 showed significant activity at 2, 10, and 50 g/ml, similar to that of metronidazole, in a dose-dependent manner. In a short-course mouse infection model, the efficacy of PA-824 at 50, 100, and 300 mg/kg of body weight formulated in methylcellulose or cyclodextrin/lecithin after nine oral treatments was compared with those of isoniazid, rifampin, and moxifloxacin. PA-824 at 100 mg/kg in cyclodextrin/lecithin was as active as moxifloxacin at 100 mg/kg and isoniazid at 25 mg/kg and was slightly more active than rifampin at 20 mg/kg. Long-term treatment with PA-824 at 100 mg/kg in cyclodextrin/lecithin reduced the bacterial load below 500 CFU in the lungs and spleen. No significant differences in activity between PA-824 and the other single drug treatments tested (isoniazid at 25 mg/kg, rifampin at 10 mg/kg, gatifloxacin at 100 mg/kg, and moxifloxacin at 100 mg/kg) could be observed. In summary, its good activity in in vivo models, as well as its activity against multidrug-resistant M. tuberculosis and against M. tuberculosis isolates in a potentially latent state, makes PA-824 an attractive drug candidate for the therapy of tuberculosis. These data indicate that there is significant potential for effective oral delivery of PA-824 for the treatment of tuberculosis.Therapy for tuberculosis (TB) is arduous due to its long duration and the need to use multidrug regimens. The current standard regimen of isoniazid (INH), rifampin (RIF), and pyrazinamide (PZA) requires 6 to 8 months of daily treatment. In part due to noncompliance with treatment, therapy is now further complicated by the emergence of drug-resistant strains, with the global prevalence of drug resistance being from 1 to 3% (27). A further, equally important issue with tuberculosis therapy is the treatment of patients in which the infection may be in a latent state. Supposedly, 1:3 people throughout the world harbor latent bacilli, which have the potential to reactivate and cause active disease (21, 23). Current anti-TB drugs are mainly effective against replicating and metabolically active bacteria, and therefore, there is an urgent need for novel drugs that are also effective against persisting or latent bacterial infections, as well as those that can overcome the increasing problem of drug resistance.A series of bicyclic nitroimidazofurans, originally investigated as radiosensitizers for use in cancer chemotherapy (1), were found to possess activity against cultured replicating Mycobacterium tuberculosis and had significant in vivo activity in a murine infection model (3,17,25). A subsequent series of 3-substituted nitroimidazopyrans (NAP...