IFN-gamma enhances bovine macrophage responsiveness to Mycobacterium bovis: Impact on bacterial replication, cytokine release and macrophage apoptosis

We sought to determine the impact of bovine IFN-gamma on the interaction between Mycobacterium bovis and bovine macrophages. Bovine macrophages released small amounts of nitric oxide (NO), TNF-alpha, IL-1beta and IL-12 upon infection with bacille Calmette-Guérin (BCG). Prior pulsing of cells with IFN-gamma significantly enhanced the release of NO and IL-12. Infection of bovine macrophages with virulent M. bovis led to the release of higher levels of pro-inflammatory mediators, compared to levels released upon BCG infection. IFN-gamma treatment of macrophages enhanced the release of pro-inflammatory mediators, but did not modify bacterial replication in M. bovis-infected macrophages. Treatment of macrophages with a combination of IFN-gamma and LPS led to a reduction in bacterial replication. Infected cells treated with IFN-gamma/LPS progressed mostly through an apoptotic pathway, whereas untreated infected cells eventually died by necrosis. Agents that prevented the acquisition of bacteriostatic activity by activated macrophages also prevented the induction of apoptosis in infected macrophages (IL-10 and neutralizing anti-TNF-alpha). We conclude that virulent M. bovis is a major determinant of release of pro-inflammatory cytokines by macrophages. IFN-gamma amplifies the macrophage cytokine release in response to M. bovis. Induction of apoptosis is closely linked to the emergence of macrophage resistance to M. bovis replication, which is dependent on endogenous TNF-alpha release.     

Potent antimycobacterial activity of mouse secretory leukocyte protease inhibitor

Secretory leukocyte protease inhibitor (SLPI) has multiple functions, including inhibition of protease activity, microbial growth, and inflammatory responses. In this study, we demonstrate that mouse SLPI is critically involved in innate host defense against pulmonary mycobacterial infection. During the early phase of respiratory infection with Mycobacterium bovis bacillus Calmette-Guérin, SLPI was produced by bronchial and alveolar epithelial cells, as well as alveolar macrophages, and secreted into the alveolar space. Recombinant mouse SLPI effectively inhibited in vitro growth of bacillus Calmette-Guérin and Mycobacterium tuberculosis through disruption of the mycobacterial cell wall structure. Each of the two whey acidic protein domains in SLPI was sufficient for inhibiting mycobacterial growth. Cationic residues within the whey acidic protein domains of SLPI were essential for disruption of mycobacterial cell walls. Mice lacking SLPI were highly susceptible to pulmonary infection with M. tuberculosis. Thus, mouse SLPI is an essential component of innate host defense against mycobacteria at the respiratory mucosal surface.     

Role of apoptosis-regulating signal kinase 1 in innate immune responses by Mycobacterium bovis bacillus Calmette-Guérin

Mycobacterium bovis bacillus Calmette-Guérin (BCG) induces innate immune responses through Toll-like receptor (TLR) 2 and TLR4. We investigated the role of apoptosis-regulating signal kinase (ASK) 1 in reactive oxygen species (ROS)-mediated innate immune responses induced by BCG mycobacterial infection. In macrophages, M. bovis BCG stimulation resulted in rapid activation of mitogen-activated protein kinases (MAPKs), secretion of inflammatory cytokines, such as tumor necrosis factor (TNF)-alpha and interleukin (IL)-6, and ROS generation in a TLR2- and TLR4-dependent manner. M. bovis BCG-induced ROS production led to robust activation of ASK1 upstream of the c-jun-N-terminal kinase and p38 MAPK, but not extracellular-regulated kinase 1/2. Blocking ASK1 activity markedly attenuated M. bovis BCG-induced TNF-alpha and IL-6 production by macrophages. Both TLR2 and TLR4 were required for optimal activation of ASK1 in response to M. bovis BCG. Furthermore, we present evidence that TNF receptor-associated factor (TRAF) 6 activities were essential for ROS-mediated ASK1 activation by M. bovis BCG. Finally, ASK1 activities were required for effective control of intracellular mycobacterial survival. Thus, the results of this study suggest a novel role of the TLR-ROS-TRAF6-ASK1 axis in the innate immune response to mycobacteria as a signaling intermediate.     

Impaired protection against Mycobacterium bovis bacillus Calmette-Guerin infection in IL-15-deficient mice

To investigate the potential role of endogenous IL-15 in mycobacterial infection, we examined protective immunity in IL-15-deficient (IL-15(-/-)) mice after infection with Mycobacterium bovis bacillus Calmette-Guérin (BCG) or recombinant OVA-expressing BCG (rBCG-OVA). IL-15(-/-) mice exhibited an impaired protection in the lung on day 120 after BCG infection as assessed by bacterial growth. CD4(+) Th1 response capable of producing IFN-gamma was normally detected in spleen and lung of IL-15(-/-) mice on day 120 after infection. Although Ag-specific CD8 responses capable of producing IFN-gamma and exhibiting cytotoxic activity were detected in the lung on day 21 after infection with rBCG-OVA, the responses were severely impaired on days 70 and 120 in IL-15(-/-) mice. The degree of proliferation of Ag-specific CD8(+) T cells in IL-15(-/-) mice was similar to that in wild-type mice during the course of infection with rBCG-OVA, whereas sensitivity to apoptosis of Ag-specific CD8(+) T cells significantly increased in IL-15(-/-) mice. These results suggest that IL-15 plays an important role in the development of long-lasting protective immunity to BCG infection via sustaining CD8 responses in the lung.     

Progression of pulmonary tuberculosis and efficiency of bacillus Calmette-Guérin vaccination are genetically controlled via a common sst1-mediated mechanism of innate immunity

Using a mouse model for genetic analysis of host resistance to virulent Mycobacterium tuberculosis, we have identified a genetic locus sst1 on mouse chromosome 1, which controls progression of pulmonary tuberculosis. In vitro, this locus had an effect on macrophage-mediated control of two intracellular bacterial pathogens, M. tuberculosis and Listeria monocytogenes. In this report, we investigated a specific function of the sst1 locus in antituberculosis immunity in vivo, especially its role in control of pulmonary tuberculosis. We found that the sst1 locus affected neither activation of Th1 cytokine-producing T lymphocytes, nor their migration to the lungs, but rather controlled an inducible NO synthase-independent mechanism of innate immunity. Although the sst1(S) macrophages responded to stimulation with IFN-gamma in vitro, their responsiveness to activation by T cells was impaired. Boosting T cell-mediated immunity by live attenuated vaccine Mycobacterium bovis bacillus Calmette-Guérin or the adoptive transfer of mycobacteria-activated CD4(+) T lymphocytes had positive systemic effect, but failed to improve control of tuberculosis infection specifically in the lungs of the sst1(S) animals. Thus, in the mouse model of tuberculosis, a common genetic mechanism of innate immunity mediated control of tuberculosis progression in the lungs and the efficiency of antituberculosis vaccine. Our data suggest that in immunocompetent humans the development of pulmonary tuberculosis and the failure of the existing vaccine to protect against it, in some cases, may be explained by a similar defect in a conserved inducible NO synthase-independent mechanism of innate immunity, either inherited or acquired.     

Serpin 2a is induced in activated macrophages and conjugates to a ubiquitin homolog

After i.p. infection of mice with the intracellular bacterium Mycobacterium bovis bacillus Calmette-Guérin, macrophages recovered from the peritoneal cavity display classical signs of immune activation. We have identified a member of the serine protease inhibitor (serpin) family which is highly induced in macrophages during bacillus Calmette-Guérin infection. Serpin 2a (spi2a) expression is also induced in macrophages in vivo during infection with Salmonella typhimurium and Listeria monocytogenes, and in vitro by a variety of bacteria and bacterial products. The cytokine IFN-gamma also induces spi2a expression in macrophages, and this induction is synergistic with bacterial products. We also demonstrate here that a ubiquitin homolog, IFN-stimulated gene of 15-kDa (ISG15), is strongly induced during in vitro and in vivo activation of macrophages and that it conjugates to spi2a in activated macrophages. The ISG15-spi2a conjugates were identified by tandem mass spectrometry and contained spi2a conjugated to either one or two molecules of ISG15. Whereas spi2a was induced by either bacterial products or IFN-gamma, ISG15 was induced only by bacterial products. Although many protein targets have been described for ubiquitin conjugation, spi2a is the first ISG15-modified protein to be reported. Macrophage activation is accompanied by the activation of a variety of proteases. It is of interest that a member of the serine protease inhibitor family is concomitantly induced and modified by a ubiquitin-like protein.     

IL-17-mediated regulation of innate and acquired immune response against pulmonary Mycobacterium bovis bacille Calmette-Guerin infection

IL-17 is a cytokine that induces neutrophil-mediated inflammation, but its role in protective immunity against intracellular bacterial infection remains unclear. In the present study, we demonstrate that IL-17 is an important cytokine not only in the early neutrophil-mediated inflammatory response, but also in T cell-mediated IFN-gamma production and granuloma formation in response to pulmonary infection by Mycobacterium bovis bacille Calmette-Guérin (BCG). IL-17 expression in the BCG-infected lung was detected from the first day after infection and the expression depended on IL-23. Our observations indicated that gammadelta T cells are a primary source of IL-17. Lung-infiltrating T cells of IL-17-deficient mice produced less IFN-gamma in comparison to those from wild-type mice 4 wk after BCG infection. Impaired granuloma formation was also observed in the infected lungs of IL-17-deficient mice, which is consistent with the decreased delayed-type hypersensitivity response of the infected mice against mycobacterial Ag. These data suggest that IL-17 is an important cytokine in the induction of optimal Th1 response and protective immunity against mycobacterial infection.     

Characterization of lung gamma delta T cells following intranasal infection with Mycobacterium bovis bacillus Calmette-Guérin

The lungs are considered to have an impaired capacity to contain infection by pathogenic mycobacteria, even in the presence of effective systemic immunity. In an attempt to understand the underlying cellular mechanisms, we characterized the gammadelta T cell population following intranasal infection with Mycobacterium bovis bacillus Calmette-Guérin (BCG). The peak of gammadelta T cell expansion at 7 days postinfection preceded the 30 day peak of alphabeta T cell expansion and bacterial count. The expanded population of gammadelta T cells in the lungs of BCG-infected mice represents an expansion of the resident Vgamma2 T cell subset as well as an influx of Vgamma1 and of four different Vdelta gene-bearing T cell subsets. The gammadelta T cells in the lungs of BCG-infected mice secreted IFN-gamma following in vitro stimulation with ionomycin and PMA and were cytotoxic against BCG-infected peritoneal macrophages as well as against the uninfected J774 macrophage cell line. The cytotoxicity was selectively blocked by anti-gammadelta TCR mAb and strontium ions, suggesting a granule-exocytosis killing pathway. Depletion of gammadelta T cells by injection of specific mAb had no effect on the subsequent developing CD4 T cell response in the lungs of BCG-infected mice, but significantly reduced cytotoxic activity and IFN-gamma production by lung CD8 T cells. Thus, gammadelta T cells in the lungs might help to control mycobacterial infection in the period between innate and classical adaptive immunity and may also play an important regulatory role in the subsequent onset of alphabeta T lymphocytes.     

Virulent Mycobacterium tuberculosis strains evade apoptosis of infected alveolar macrophages

Human alveolar macrophages (AMphi) undergo apoptosis following infection with Mycobacterium tuberculosis in vitro. Apoptosis of cells infected with intracellular pathogens may benefit the host by eliminating a supportive environment for bacterial growth. The present study compared AMphi apoptosis following infection by M. tuberculosis complex strains of differing virulence and by Mycobacterium kansasii. Avirulent or attenuated bacilli (M. tuberculosis H37Ra, Mycobacterium bovis bacillus Calmette-Guérin, and M. kansasii) induced significantly more AMphi apoptosis than virulent strains (M. tuberculosis H37Rv, Erdman, M. tuberculosis clinical isolate BMC 96.1, and M. bovis wild type). Increased apoptosis was not due to greater intracellular bacterial replication because virulent strains grew more rapidly in AMphi than attenuated strains despite causing less apoptosis. These findings suggest the existence of mycobacterial virulence determinants that modulate the apoptotic response of AMphi to intracellular infection and support the hypothesis that macrophage apoptosis contributes to innate host defense in tuberculosis.     

Autocrine deactivation of macrophages in transgenic mice constitutively overexpressing IL-10 under control of the human CD68 promoter

IL-10 plays an essential role in blocking cytokine production by activated macrophages. To analyze the consequences of enforced expression of IL-10 by macrophages on innate and adaptive immune responses, we generated transgenic mice (macIL-10tg mice) expressing an epitope-tagged IL-10 (Flag-IL-10) under control of the human CD68 promoter. Expression of Flag-IL-10 was constitutive and restricted to macrophages, as shown by sorting splenocyte cell populations and intracellular staining for IL-10. Transgenic macrophages displayed suppressed production of TNF-alpha and IL-12 upon stimulation with LPS. When macIL-10tg mice were challenged with LPS, serum levels of proinflammatory cytokines were attenuated compared with controls. Infection with Mycobacterium bovis bacille Calmette-Guérin resulted in approximately 10-fold-higher bacterial loads than in wild-type mice. Normal T and B cell responses were observed in macIL-10tg mice, suggesting that macrophage-specific overexpression of IL-10 predominantly acts in an autocrine/paracrine manner, resulting in chronically deactivated macrophages that manifest an impaired ability to control pathogens.     

Enhanced macrophage activity in granulomatous lesions of immune mice challenged with Mycobacterium tuberculosis

In this study, we evaluated the cellular influx and cytokine environment in the lungs of mice made immune by prior vaccination with Mycobacterium bovis bacillus Calmette-Guérin compared with control mice after infection with Mycobacterium tuberculosis to characterize composition of protective lesions in the lungs. Immune mice controlled the growth of the M. tuberculosis challenge more efficiently than control mice. In immune animals, granulomatous lesions were smaller and had a more lymphocytic core, less foamy cells, less parenchymal inflammation, and slower progression of lung pathology than in lungs of control mice. During the chronic stage of the infection, the bacterial load in the lungs of immune mice remained at a level 10 times lower than control mice, and this was associated with reduced numbers of CD4P(+P) and CD8P(+P) T cells, and the lower expression of protective (IL-12, IFN-gamma), inflammatory (TNF-alpha), immunoregulatory (GM-CSF), and immunosuppressive (IL-10) cytokines. The immune mice had higher numbers of CD11b- CD11c(high) DEC-205(low) alveolar macrophages, but lower numbers of CD11b+ CD11c(high) DEC-205(high) dendritic cells, with the latter expressing significantly lower levels of the antiapoptotic marker TNFR-associated factor-1. Moreover, during the early stage of chronic infection, lung dendritic cells from immune mice expressed higher levels of MHC class II and CD40 molecules than similar cells from control mice. These results indicate that while a chronic disease state is the eventual outcome in both control and immune mice infected with M. tuberculosis by aerosol exposure, immune mice develop a protective granulomatous lesion by increasing macrophage numbers and reduced expression of protective and inflammatory cytokines.     

Bovine dendritic cells are more permissive for Mycobacterium bovis replication than macrophages, but release more IL-12 and induce better immune T-cell proliferation

Bovine dendritic cells (DCs) were obtained by incubating blood monocytes with interleukin-4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GM-CSF). The ability of DCs to phagocytose and allow the replication of virulent Mycobacterium bovis in vitro was studied, and compared with bovine blood monocyte-derived macrophages. In addition, the release of cytokines by M. bovis-infected DCs was assessed. DCs were shown to phagocytose M. bovis efficiently, and allowed a more substantial replication of M. bovis when compared to macrophages, as assessed by the metabolic activity of intracellular bacteria. During the course of M. bovis infection, it was found that macrophages released substantial amounts of pro-inflammatory factors such as tumour necrosis factor-alpha (TNF-alpha), nitric oxide (NO) and interleukin-1 beta (IL-1 beta). M. bovis-infected DCs released much smaller quantities of NO, IL-1 beta and TNF-alpha (5- to 10-fold lower amounts), when compared to macrophages. Treating cells with interferon-gamma (IFN-gamma) before and during the in vitro infection process was shown to increase the release of NO, TNF-alpha and IL-1 beta by M. bovis-infected macrophages, but not by M. bovis-infected DCs. M. bovis-infected macrophages released more interleukin-10 (IL-10) than infected DCs. Treating cells with IFN-gamma/LPS was shown to reduce M. bovis metabolic activity in infected macrophages, but had no such impact on M. bovis metabolic activity in infected DCs. A variety of T-cell-derived cytokines (IFN-gamma, GM-CSF, IL-4) had no impact on the replication of M. bovis in infected DCs. On the other hand, DCs infected with M. bovis sustained a more efficient replication of autologous sensitized T lymphocytes compared to M. bovis-infected macrophages. M. bovis-infected DCs released more substantial amounts of interleukin-12 (IL-12) than similarly infected macrophages. These data suggest a complementary role for DCs and macrophages with regard to bacteriostatic activity and induction of an efficient immune response against M. bovis.     

In vivo activity of released cell wall lipids of Mycobacterium bovis bacillus Calmette-Guérin is due principally to trehalose mycolates

The hallmark of Mycobacterium-induced pathology is granulomatous inflammation at the site of infection. Mycobacterial lipids are potent immunomodulators that contribute to the granulomatous response and are released in appreciable quantities by intracellular bacilli. Previously we investigated the granulomagenic nature of the peripheral cell wall lipids of Mycobacterium bovis bacillus Calmette-Guérin (BCG) by coating the lipids onto 90-microm diameter microspheres that were mixed into Matrigel matrix with syngeneic bone marrow-derived macrophages and injected i.p. into mice. These studies demonstrated that BCG lipids elicit proinflammatory cytokines and recruit leukocytes. In the current study we determined the lipids responsible for this proinflammatory effect. BCG-derived cell wall lipids were fractionated and purified by liquid chromatography and preparative TLC. The isolated fractions including phosphatidylinositol dimannosides, cardiolipin, phosphatidylglycerol, phosphatidylethanolamine, trehalose monomycolate, trehalose dimycolate, and mycoside B. Trehalose dimycolate, when delivered to bone marrow-derived murine macrophages, induced the greatest secretion of IL-1beta, IL-6, and TNF-alpha in vitro. Trehalose dimycolate similarly induced the greatest secretion of these proinflammatory cytokines in ex vivo matrices over the course of 12 days. Trehalose monomycolate and dimycolate also induced profound neutrophil recruitment in vivo. Experiments with TLR2 or TLR4 gene-deficient mice revealed no defects in responses to trehalose mycolates, although MyD88-deficient mice manifested significantly reduced cell recruitment and cytokine production. These results demonstrate that the trehalose mycolates, particularly trehalose dimycolate, are the most bioactive lipids in the BCG extract, inducing a proinflammatory cascade that influences granuloma formation.     

Mycobacteria induce IFN-gamma production in human dendritic cells via triggering of TLR2

IFN-gamma is of central importance for the induction of robust cell-mediated immunity and for the activation of APC. Recent studies using experimental murine systems have now suggested a fundamental role for APC-derived IFN-gamma during infection with intracellular pathogens. It is currently unknown whether human dendritic cells (DC) can respond to bacterial stimulation with production of IFN-gamma. To test this question, we used human monocyte-derived DC stimulated by Mycobacterium bovis bacillus Calmette-Guérin as a model system. We demonstrate production of IFN-gamma mRNA and protein on the single cell level. IFN-gamma in DC cultures was not simply produced by contaminating lymphocytes because production of DC-IFN-gamma could also be demonstrated in highly purified DC cultures containing virtually no T, B, and NK cells. TLR2 was identified as a key receptor involved in triggering production of DC-IFN-gamma. Interestingly, DC-IFN-gamma seems to participate in an autocrine DC activation loop, and production of DC-IFN-gamma could be enhanced by costimulation of DC with IL-12/IL-15/IL-18. In conclusion, we have demonstrated production of IFN-gamma by human DC on the single cell level, identified TLR2 as a pattern recognition receptor involved in this process, and elucidated some of the functional consequences of autocrine IFN-gamma production by human DC.     

Bacillus Calmette-Guérin vaccination of human newborns induces T cells with complex cytokine and phenotypic profiles

The immune response to vaccination with bacillus Calmette-Guérin (BCG), the only tuberculosis vaccine available, has not been fully characterized. We used multiparameter flow cytometry to examine specific T cell cytokine production and phenotypic profiles in blood from 10-wk-old infants routinely vaccinated with BCG at birth. Ex vivo stimulation of whole blood with BCG for 12 h induced expression of predominantly IFN-gamma, IL-2, and TNF-alpha in CD4+ T cells in seven distinct cytokine combinations. IL-4 and IL-10 expression was detected in CD4+ T cells at low frequencies and only in cells that did not coexpress type 1 cytokines. Specific CD8+ T cells were less frequent than CD4+ T cells and produced mainly IFN-gamma and/or IL-2 and less TNF-alpha, IL-4, and IL-10. Importantly, many mycobacteria-specific CD4+ and CD8+ T cells did not produce IFN-gamma. The predominant phenotype of BCG-specific type 1 T cells was that of effector cells, i.e., CD45RA-CCR7-CD27+, which may reflect persistence of Mycobacterium bovis BCG in infants until 10 wk of age. Among five phenotypic patterns of CD4+ T cells, central memory cells were more likely to be IL-2+ and effector cells were more likely to be IFN-gamma+. We concluded that neonatal vaccination with BCG induces T cells with a complex pattern of cytokine expression and phenotypes. Measuring IFN-gamma production alone underestimates the magnitude and complexity of the host cytokine response to BCG vaccination and may not be an optimal readout in studies of BCG and novel tuberculosis vaccination.     

NOD2-deficient mice have impaired resistance to Mycobacterium tuberculosis infection through defective innate and adaptive immunity

NOD2/CARD15 mediates innate immune responses to mycobacterial infection. However, its role in the regulation of adaptive immunity has remained unknown. In this study, we examined host defense, T cell responses, and tissue pathology in two models of pulmonary mycobacterial infection, using wild-type and Nod2-deficient mice. During the early phase of aerosol infection with Mycobacterium tuberculosis, Nod2(-/-) mice had similar bacterial counts but reduced inflammatory response on histopathology at 4 and 8 wk postchallenge compared with wild-type animals. These findings were confirmed upon intratracheal infection of mice with attenuated Mycobacterium bovis bacillus Calmette-Guérin. Analysis of the lungs 4 wk after bacillus Calmette-Guérin infection demonstrated that Nod2(-/-) mice had decreased production of type 1 cytokines and reduced recruitment of CD8(+) and CD4(+) T cells. Ag-specific T cell responses in both the spleens and thoracic lymph nodes were diminished in Nod2(-/-) mice, indicating impaired adaptive antimycobacterial immunity. The immune regulatory role of NOD2 was not restricted to the lung since Nod2 disruption also led to reduced type 1 T cell activation following i.m. bacillus Calmette-Guérin infection. To determine the importance of diminished innate and adaptive immunity, we measured bacterial burden 6 mo after aerosol infection with M. tuberculosis and followed a second infected group for assessment of survival. Nod2(-/-) mice had a higher bacterial burden in the lungs 6 mo after infection and succumbed sooner than did wild-type controls. Taken together, these data indicate that NOD2 mediates resistance to mycobacterial infection via both innate and adaptive immunity.     

In vivo immunomodulation following intradermal injection with DNA encoding IL-18.

IL-18, a recently identified cytokine synthesized by different cell types, including Kupffer cells, activated macrophages, and keratinocytes, induces IFN-gamma production by T cells and NK cells. The cDNA encoding IL-18 with its natural signal peptide was cloned under control of the CMV promoter and injected into the skin of mice. A single intradermal injection of this construction led to efficient in vivo expression of IL-18 in cutaneous dermal cells and induced IFN-gamma mRNA production, indicating that it was produced in a biologically active form. In addition, a massive cellular infiltrate was observed in the skin 2 days after injection. When the mice were subsequently infected with Mycobacterium bovis bacillus Calmette-Guérin (BCG), they produced lower levels of anti-BCG Abs than control animals. However, in contrast to their lowered humoral immune response, the mice produced higher amounts of Ag-specific IFN-gamma after in vitro restimulation, as compared with the controls. Therefore, injection of DNA encoding IL-18 into the skin modulates both Ag-specific humoral and T cell responses upon mycobacterial infection. It increases the Th1 type response, which may be particularly useful for the development of new immunotherapeutic or immunoprotective approaches against infections by intracellular parasites, such as mycobacteria.     

Overexpression of IL-15 in vivo enhances protection against Mycobacterium bovis bacillus Calmette-Guérin infection via augmentation of NK and T cytotoxic 1 responses.

To investigate the immunomodulating effects of IL-15 in vivo on mycobacterial infection, we used IL-15-transgenic (Tg) mice, which were recently constructed with cDNA-encoding secretable isoform of IL-15 precursor protein under the control of a MHC class I promoter. The IL-15-Tg mice exhibited resistance against infection with Mycobacterium bovis bacillus Calmette-Guérin (BCG), as assessed by bacteria growth. IFN-gamma level in serum was significantly higher in IL-15-Tg mice than in non-Tg mice after BCG infection. NK cells were remarkably increased, and Ag-specific T cytotoxic 1 response mediated by CD8+ T cells producing IFN-gamma was significantly augmented in the IL-15-Tg mice following BCG infection. Neutralization of endogenous IFN-gamma by in vivo administration of anti-IFN-gamma mAb deteriorated the clearance of the bacteria. Depletion of of NK cells or CD8+ T cells by in vivo administration of anti-asialo-GM(1) Ab or anti-CD8 mAb hampered the exclusion of bacteria. Thus, overexpression of IL-15 in vivo enhanced protection against BCG infection via augmentation of NK and T cytotoxic 1 responses.