We have reported that TLR2 is crucial for host resistance against chronic Mycobacterium tuberculosis infection; however, which cell types are key players in this response remain unknown. This led us to decipher the relative contribution of TLR2 on nonhematopoietic and hematopoietic cells in resistance against chronic M. tuberculosis infection in mice infected with M. tuberculosis Erdman. Consistent with our previous report, at 8 wk of infection, TLR2 knockout (TLR2KO)→TLR2KO bone marrow chimeric mice exhibited increased bacterial burden, disorganized accumulation of lymphocytes and mononuclear cells, and extensive pulmonary immunopathology compared with wild-type (WT)→WT chimeric mice. Bacterial burden and pulmonary immunopathology of chimeric mice lacking TLR2 in the hematopoietic compartment (TLR2KO→WT) was comparable to TLR2KO mice. In contrast, chimeric mice deficient in TLR2 in the nonhematopoietic compartment (WT→TLR2KO) exhibited a marked attenuation in granulomatous inflammation compared with WT mice. Although the latter mice did not exhibit improved pulmonary bacterial control, significant reductions in bacterial burden in the draining lymph nodes, spleen, and liver were observed. These findings establish that the TLR2-mediated hematopoietic response promotes stable control of pulmonary bacterial burden and granuloma integrity, whereas TLR2 signaling on nonhematopoietic cells may partly facilitate granulomatous inflammation and bacterial dissemination.

Induction of cathelicidin-mediated antimicrobial pathway against intracellular M. tuberculosis by 1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ), the active form of vitamin D, has been documented in vitro . However, in in vivo studies related to inflammatory disorders, 1,25(OH) 2 D 3 has been demonstrated to induce an anti-inflammatory response. We therefore examined whether in the murine model of tuberculosis, the anti-inflammatory effects of 1,25(OH) 2 D 3 would affect the outcome of M. tuberculosis infection. We show here that administration of 1,25(OH) 2 D 3 to M. tuberculosis infected mice led to a change in lung granuloma architecture, characterized by a marked decrease in B cell lymphocytic aggregates. Consistent with the altered granulomas, 1,25(OH) 2 D 3 -treated mice also exhibited significantly higher bacterial burden in the lungs compared to the control group. These findings highlight the need to further investigate the effect of vitamin D on host immunity to M. tuberculosis in the context of the granulomatous response.

ABSTRACT Previously we had reported that Nippostrongylus brasiliensis, a helminth with a lung migratory phase, affected host resistance against Mycobacterium tuberculosis infection through the induction of alternatively activated (M2) macrophages. Several helminth species do not have an obligatory lung migratory phase but establish chronic infections in the host that include potent immune downregulatory effects, in part mediated through induction of a FoxP3+ T regulatory cell (Treg) response. Treg cells exhibit duality in their functions in host defense against M. tuberculosis infection since their depletion leads to enhanced priming of T cells in the lymph nodes and attendant improved control of M. tuberculosis infection, while their presence in the lung granuloma protects against excessive inflammation. Heligmosomoides polygyrus is a strictly murine enteric nematode that induces a strong FoxP3 Treg response in the host. Therefore, in this study we investigated whether host immunity to M. tuberculosis infection would be modulated in mice with chronic H. polygyrus infection. We report that neither primary nor memory immunity conferred by Mycobacterium bovis BCG vaccination was affected in mice with chronic enteric helminth infection, despite a systemic increase in FoxP3+ T regulatory cells. The findings indicate that anti-M. tuberculosis immunity is not similarly affected by all helminth species and highlight the need to consider this inequality in human coinfection studies.

TLR2 polymorphisms have been associated with increased susceptibility to tuberculosis (TB). We have demonstrated that TLR2 is critical for host resistance against Mtb infection and limits immunopathology in the lung. Recent publications illustrating a role for epithelial cells in host immunity against Mtb led us to investigate TLR2’s role in hematopoietic (H) cells and nonhematopoietic (NH) cells in host defense against Mtb. We generated radiation bone marrow chimeric mice, which lack TLR2 either in the H, NH, or both compartments, and infected these mice with Erdman Mtb. Our results show that TLR2 signaling on the H component is crucial for control of chronic murine Mtb infection. Loss of TLR2 in the H component resulted in increased bacterial burden, decreased accumulation of regulatory T cells, and disruption of granuloma architecture. In contrast, loss of TLR2 on the NH compartment resulted in decreased inflammation and dissemination of Mtb to extrapulmonary sites. These data illustrate a novel role for TLR2 and a paradox within TLR2 signaling: TLR2 signaling on H cells is protective to the host both by controlling infection and regulating inflammation while on NH cells it promotes inflammation induced immunopathology. This balanced response is essential to maintaining host control of TB and could lead to the development of cell-specific TLR2 inhibitors.

Helminths are known to exert immunomodulatory effects on the host, an effect largely mediated by their induction of generalized Th2 responses. In a recent study, using the murine nematode Nippostrongylus brasiliensis (Nb), we have shown that a prior infection with Nb makes the host more susceptible to tuberculosis (TB). In this study we sought to investigate whether coinfection would exacerbate chronic infection, a phase where Mycobacterium tuberculosis (Mtb) is contained within granulomas. Mice chronically infected with Mtb were coinfected with Nb. We report herein that despite a helminth induced Th2 component, coinfected animals had a similar bacterial burden in comparison with Mtb alone infected animals. This observation could be due to the fact that the Mtb is already established in the granuloma and any effect of the helminth induced Th2 response is unable to shift this balance. In order to further understand the kinetics of helminths influencing TB, we infected mice with Mtb and these were infected 5days later with Nb. Interestingly we observed a significant increase in lung bacterial burden in coinfected animals. These results suggest that the timing of a helminth infection in an Mtb infected individual could have completely different outcomes. Studies are underway to unravel the mechanisms behind these observations. This finding is significant as a helminth infection can occur even during adulthood in endemic areas of the world, regions which are also endemic with TB

Nitric oxide (NO) defends against intracellular pathogens, but its synthesis must be regulated due to cell and tissue toxicity. During infection, macrophages import extracellular arginine to synthesize NO, generating the byproduct citrulline. Accumulated intracellular citrulline is thought to fuel arginine synthesis catalyzed by argininosuccinate synthase (Ass1) and argininosuccinate lyase (Asl), which would lead to abundant NO production. Instead, we find that citrulline is exported from macrophages during early stages of NO production with <2% retained for recycling via the Ass1-Asl pathway. Later, extracellular arginine is depleted, and Ass1 expression allows macrophages to synthesize arginine from imported citrulline to sustain NO output. Ass1-deficient macrophages fail to salvage citrulline in arginine-scarce conditions, leading to their inability to control mycobacteria infection. Thus, extracellular arginine fuels rapid NO production in activated macrophages, and citrulline recycling via Ass1 and Asl is a fail-safe system that sustains optimum NO production.

Purpose of reviewTuberculosis (TB) continues to be a significant problem and a major cause of morbidity and mortality in the developing world despite decades of intensive efforts to combat the disease. The poverty in these endemic areas is associated with an increased incidence of tropical helminthic infections. The purpose of this review is to bring to the fore, the urgent need to unravel the potential consequences of helminth coinfection to tuberculosis disease pathogenesis and transmission. Recent findingsThere is now strong experimental evidence that helminth-induced T helper (Th)2 and T regulatory (Treg) responses impinge on host resistance against Mycobacterium tuberculosis (Mtb) infection. Several studies show that Th1 response is reduced in helminth coinfected hosts. Emerging studies also indicate that helminth-induced alternatively activated macrophages contribute to enhanced susceptibility to TB. Despite studies showing an association between helminthes and diminished Th1 immunity, maternal antihelminthic treatment had no effect on an infant's response to bacille Calmette-Guerin vaccination. SummaryWhether helminthes affect tuberculosis (TB) disease is still an open question and clinical trials are warranted to determine at the population level whether helminthes enhance TB incidence and transmission and diminish the protective immune response to vaccines. Consequently, mass deworming of infected individuals could contribute toward overall improvement of global public health.

Purpose of review Tuberculosis (TB) continues to be a significant problem and a major cause of morbidity and mortality in the developing world despite decades of intensive efforts to combat the disease. The poverty in these endemic areas is associated with an increased incidence of tropical helminthic infections. The purpose of this review is to bring to the fore, the urgent need to unravel the potential consequences of helminth coinfection to tuberculosis disease pathogenesis and transmission. Recent findings There is now strong experimental evidence that helminth-induced T helper (Th)2 and T regulatory (Treg) responses impinge on host resistance against Mycobacterium tuberculosis (Mtb) infection. Several studies show that Th1 response is reduced in helminth coinfected hosts. Emerging studies also indicate that helminth-induced alternatively activated macrophages contribute to enhanced susceptibility to TB. Despite studies showing an association between helminthes and diminished Th1 immunity, maternal antihelminthic treatment had no effect on an infant's response to bacille Calmette-Guerin vaccination. Summary Whether helminthes affect tuberculosis (TB) disease is still an open question and clinical trials are warranted to determine at the population level whether helminthes enhance TB incidence and transmission and diminish the protective immune response to vaccines. Consequently, mass deworming of infected individuals could contribute toward overall improvement of global public health.

Tuberculosis and helminthic infections coexist in many parts of the world, yet the impact of helminth-elicited Th2 responses on the ability of the host to control Mycobacterium tuberculosis (Mtb) infection has not been fully explored. We show that mice infected with the intestinal helminth Nippostrongylus brasiliensis (Nb) exhibit a transitory impairment of resistance to airborne Mtb infection. Furthermore, a second dose of Nb infection substantially increases the bacterial burden in the lungs of co-infected mice. Interestingly, the Th2 response in the co-infected animals did not impair the onset and development of the protective Mtb-specific Th1 cellular immune responses. However, the helminth-induced Th2 environment resulted in the accumulation of alternatively activated macrophages (AAMs) in the lung. Co-infected mice lacking interleukin (IL) 4Rα exhibited improved ability to control Mtb infection, which was accompanied by significantly reduced accumulation of AAMs. Moreover, IL-4Rα−/− mice adoptively transferred with wild-type macrophages had a significantly higher Mtb load in their lungs compared with those that received IL-4Rα−/− macrophages, suggesting a direct contribution for the IL-4R pathway to the heightened susceptibility of co-infected animals. The Th2 response can thus enhance the intracellular persistence of Mtb, in part by mediating the alternative activation of macrophages via the IL-4Rα signaling pathway.

The lengthy course of treatment with currently used antimycobacterial drugs and the resulting emergence of drug-resistant strains have intensified the need for alternative therapies against Mycobacterium tuberculosis (Mtb), the etiologic agent of tuberculosis. We show that Mtb and Mycobacterium marinum use ABL and related tyrosine kinases for entry and intracellular survival in macrophages. In mice, the ABL family tyrosine kinase inhibitor, imatinib (Gleevec), when administered prophylactically or therapeutically, reduced both the number of granulomatous lesions and bacterial load in infected organs and was also effective against a rifampicin-resistant strain. Further, when coadministered with current first-line drugs, rifampicin or rifabutin, imatinib acted synergistically. These data implicate host tyrosine kinases in entry and intracellular survival of mycobacteria and suggest that imatinib may have therapeutic efficacy against Mtb. Because imatinib targets host, it is less likely to engender resistance compared to conventional antibiotics and may decrease the development of resistance against coadministered drugs.

Costimulation plays a crucial role in priming of naïve T cell to an effector cell type. In a recent study, we have shown that lack of B7.1 and B7.2 (DKO) causes the host to generate a dampened Th1 effector response, disabling it to contain the bacterial burden during chronic phase of infection. Since the development of T cell memory is connected to the effector T cell response, we sought to investigate how a defective primary effector response will affect the induction of the memory. Memory mice were generated by treating infected wildtype (WT) and DKO mice with antibiotics and resting them after the treatment. These memory mice along with age matched naïve controls were then challenged with low dose infection with M. tuberculosis. A significant decrease in bacterial burden was seen in both WT and DKO memory mice at 4 and 6 weeks post infection when compared to naive WT and DKO mice respectively. This decreased bacterial burden was accompanied with a faster Th1 effector response in both WT and DKO memory lungs as indicated by increased number of IFNγ producing cells in the lungs. These results indicate that despite its inability to generate a defective primary response the DKO host is able to mount a memory response against M.tuberculosis infection. Further studies are in progress to study the mechanism of memory induction in the absence of a successful primary response in the DKO mice. This study was supported by NIH Grant AI 49778, Potts Memorial Foundation Award and UMDNJ Foundation Award.