Tumor necrosis factor is critical to control tuberculosis infection.

Tumor necrosis factor (TNF) is critical and non-redundant to control Mycobacterium tuberculosis infection and cannot be replaced by other proinflammatory cytokines. Overproduction of TNF may cause immunopathology, while TNF neutralization reactivates latent and chronic, controlled infection, which is relevant for the use of neutralizing TNF therapies in patients with rheumatoid arthritis.

Reactivation of tuberculosis by tumor necrosis factor neutralization.

Tumor necrosis factor (TNF) is required in the control of infection with Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis. TNF is essential and non-redundant for forming microbiocidal granulomas, and cannot be replaced by other members of the TNF family. We established a model of latent Mtb infection in mice, allowing investigation of the reactivation of latent Mtb as observed in patients receiving TNF-neutralizing therapy used in rheumatoid arthritis and Crohn's disease. Antibody neutralization of TNF is able to reactivate clinically silent Mtb infection. Using mutant mice expressing solely membrane, but not soluble TNF, we demonstrated that membrane TNF is sufficient to control acute Mtb infection. Therefore, we hypothesize that TNF-neutralizing therapy, sparing membrane TNF, may have an advantage as compared to complete neutralization. In conclusion, endogenous TNF is critical for the control of tuberculosis infection. Genetic absence or pharmacological neutralization of TNF results in uncontrolled infection, while selective neutralization might retain the desired anti-inflammatory effect but reduce the infectious risk.

Toll-like receptors and control of mycobacterial infection in mice.

Microbial products including mycobacterial antigens are recognized by distinct Toll-like receptors (TLRs) resulting in activation of cells of the innate immune system. Ablation of most of the TLR signalling in mice deficient for the common adaptor protein MyD88 revealed that TLRs are crucial for the activation of an innate immune response as MyD88-deficient mice are highly sensitive to infection with Mycobacterium tuberculosis. Despite the profound defect of the innate immune response, MyD88 deficiency allows the emergence of an adaptive immunity. These data demonstrate that activation of multiple TLRs contributes to an efficient innate response to mycobacteria, while MyD88-dependent signalling is dispensable to generate adaptive immunity.

Innate immunity to mycobacterial infection in mice: critical role for toll-like receptors.

Toll-like receptors (TLRs) play a critical role in the recognition of several pathogens, including Mycobacterium tuberculosis. Mycobacterial antigens recognize distinct TLRs resulting in rapid activation of cells of the innate immune system. Ablation of most of the TLR signalling as in mice deficient for the common adaptor protein MyD88 reveals that TLR is crucial for the activation of an innate immune response. MyD88-deficient mice are unable to clear virulent mycobacteria and succumb to acute necrotic pneumonia. Despite the profound defect of the innate immune response, MyD88 deficiency allows the emergence of an adaptive immunity. These data demonstrate that activation of multiple TLRs contributes to an efficient innate response to mycobacteria, while MyD88-dependent signalling is dispensable to generate adaptive immunity.

Replication dynamics of Mycobacterium tuberculosis in chronically infected mice.

The dynamics of host-pathogen interactions have important implications for the design of new antimicrobial agents to treat chronic infections such as tuberculosis (TB), which is notoriously refractory to conventional drug therapy. In the mouse model of TB, an acute phase of exponential bacterial growth in the lungs is followed by a chronic phase characterized by relatively stable numbers of bacteria. This equilibrium could be static, with little ongoing replication, or dynamic, with continuous bacterial multiplication balanced by bacterial killing. A static model predicts a close correspondence between "viable counts" (live bacteria) and "total counts" (live plus dead bacteria) in the lungs over time. A dynamic model predicts the divergence of total counts and viable counts over time due to the accumulation of dead bacteria. Here, viable counts are defined as bacterial CFU enumerated by plating lung homogenates; total counts are defined as bacterial chromosome equivalents (CEQ) enumerated by using quantitative real-time PCR. We show that the viable and total bacterial counts in the lungs of chronically infected mice do not diverge over time. Rapid degradation of dead bacteria is unlikely to account for the stability of bacterial CEQ numbers in the lungs over time, because treatment of mice with isoniazid for 8 weeks led to a marked reduction in the number of CFU without reducing the number of CEQ. These observations support the hypothesis that the stable number of bacterial CFU in the lungs during chronic infection represents a static equilibrium between host and pathogen.

Reduced local growth and spread but preserved pathogenicity of a DeltapurC Mycobacterium tuberculosis auxotrophic mutant in gamma interferon receptor-deficient mice after aerosol infection.

We compared the growth levels and pathogenicities of the Mycobacterium tuberculosis MT103 clinical strain with those of the DeltapurC mutant strain MYC1551, which is auxotrophic for purine in wild-type and gamma interferon receptor (IFN-gammaR)-deficient mice. The DeltapurC strain MYC1551 grew initially in both wild-type and IFN-gammaR-deficient mice upon aerosol infection, but it grew much less than strain MT103 did. Despite the comparable bacterial burdens of the mice, IFN-gammaR-deficient mice succumbed to infection with DeltapurC strain MYC1551 from necrotic pneumonia within 6 weeks of those infected with MT103. In conclusion, the DeltapurC mutant MYC1551 displays reduced growth but retains pathogenicity. Therefore, the use of mutant strains of M. tuberculosis as live vaccines may not be recommended.

Toll-like receptor pathways in the immune responses to mycobacteria.

The control of Mycobacterium tuberculosis infection depends on recognition of the pathogen and the activation of both the innate and adaptive immune responses. Toll-like receptors (TLR) were shown to play a critical role in the recognition of several pathogens. Mycobacterial antigens recognise distinct TLR resulting in rapid activation of cells of the innate immune system. Recent evidence from in vitro and in vivo investigations, summarised in this review demonstrates TLR-dependent activation of innate immune response, while the induction of adaptive immunity to mycobacteria may be TLR independent.

Reactivation of latent tuberculosis infection in TNF-deficient mice.

TNF-deficient mice are highly susceptible to Mycobacterium tuberculosis H37Rv infection. Here we asked whether TNF is required for postinfectious immunity in aerosol-infected mice. Chemotherapy for 4 wk commencing 2 wk postinfection reduced CFU to undetectable levels. While wild-type mice had a slight rise in CFU, but controlled infection upon cessation of chemotherapy, TNF-deficient mice developed reactivation of infection with high bacterial loads in lungs, spleen, and liver, which was fatal within 13-18 wk. The increased susceptibility of TNF-deficient mice was accompanied by diminished recruitment and activation of T cells and macrophages into the lung, with defective granuloma formation and reduced inducible NO synthase expression. Reduced chemokine production in the lung might explain suboptimal recruitment and activation of T cells and uncontrolled infection. Therefore, despite a massive reduction of the mycobacterial load by chemotherapy, TNF-deficient mice were unable to compensate and mount a protective immune response. In conclusion, endogenous TNF is critical to maintain latent tuberculosis infection, and in its absence no specific immunity is generated.

Reactivation of latent tuberculosis by an inhibitor of inducible nitric oxide synthase in an aerosol murine model.

Exposure to Mycobacterium tuberculosis results in clinical tuberculosis only in a small percentage of healthy individuals. In most instances the bacilli are controlled by the immune system and survive in a latent state within granuloma. Immunosuppression, however, may result in reactivation of infection, resulting in clinical disease. Using a low-dose aerosol infection (30 colony-forming units) in mice, we describe a short-duration model for studying spontaneous and drug-induced reactivation of anti-tuberculous drug-treated, latent tuberculosis infection. Although a 4-week treatment with rifampicin and isoniazid reduced the number of bacilli to undetectable levels, the infection spontaneously reactivated following therapy. By contrast, an 8-week treatment period induced a state of latent infection, requiring immunosuppression to reactivate infection. Finally, a 12-week treatment period eliminated the bacilli completely and aminoguanidine did not induce reactivation of infection. In view of the fact that therapy in the selected protocol reduces the mycobacterial load to undetectable levels, the data suggest that an 8-week treatment period is necessary and sufficient to mount protective immunity in mice.