Distinct chemokine and cytokine gene expression pattern of murine dendritic cells and macrophages in response to Mycobacterium tuberculosis infection

Changes in the Immune Phenotype and Gene Expression Profile Driven by a Novel Tuberculosis Nanovaccine: Short and Long-Term Post-immunization

Deciphering protection mechanisms against Mycobacterium tuberculosis (Mtb) remains a critical challenge for the development of new vaccines and therapies. We analyze the phenotypic and transcriptomic profile in lung of a novel tuberculosis (TB) nanoparticle-based boosting mucosal vaccine Nano-FP1, which combined to BCG priming conferred enhanced protection in mice challenged with low-dose Mtb. We analyzed the vaccine profile and efficacy at short (2 weeks), medium (7 weeks) and long term (11 weeks) post-vaccination, and compared it to ineffective Nano-FP2 vaccine. We observed several changes in the mouse lung environment by both nanovaccines, which are lost shortly after boosting. Additional boosting at long-term (14 weeks) recovered partially cell populations and transcriptomic profile, but not enough to enhance protection to infection. An increase in both total and resident memory CD4 and CD8 T cells, but no pro-inflammatory cytokine levels, were correlated with better protection. A unique gene expression pattern with differentially expressed genes revealed potential pathways associated to the immune defense against Mtb. Our findings provide an insight into the critical immune responses that need to be considered when assessing the effectiveness of a novel TB vaccine.

Mycobacterium tuberculosis Primary Infection and Dissemination: A Critical Role for Alveolar Epithelial Cells

Globally, tuberculosis (TB) has reemerged as a major cause of morbidity and mortality, despite the use of the Mycobacterium bovis BCG vaccine and intensive attempts to improve upon BCG or develop new vaccines. Two lacunae in our understanding of the Mycobacterium tuberculosis (M. tb)-host pathogenesis have mitigated the vaccine efforts; the bacterial-host interaction that enables successful establishment of primary infection and the correlates of protection against TB. The vast majority of vaccine efforts are based on the premise that cell-mediated immunity (CMI) is the predominating mode of protection against TB. However, studies in animal models and in humans demonstrate that post-infection, a period of several weeks precedes the initiation of CMI during which the few inhaled bacteria replicate dramatically and disseminate systemically. The “Trojan Horse” mechanism, wherein M. tb is phagocytosed and transported across the alveolar barrier by infected alveolar macrophages has been long postulated as the sole, primary M. tb:host interaction. In the current review, we present evidence from our studies of transcriptional profiles of M. tb in sputum as it emerges from infectious patients where the bacteria are in a quiescent state, to its adaptations in alveolar epithelial cells where the bacteria transform to a highly replicative and invasive phenotype, to its maintenance of the invasive phenotype in whole blood to the downregulation of invasiveness upon infection of epithelial cells at an extrapulmonary site. Evidence for this alternative mode of infection and dissemination during primary infection is supported by in vivo, in vitro cell-based, and transcriptional studies from multiple investigators in recent years. The proposed alternative mechanism of primary infection and dissemination across the alveolar barrier parallels our understanding of infection and dissemination of other Gram-positive pathogens across their relevant mucosal barriers in that barrier-specific adhesins, toxins, and enzymes synergize to facilitate systemic establishment of infection prior to the emergence of CMI. Further exploration of this M. tb:non-phagocytic cell interaction can provide alternative approaches to vaccine design to prevent infection with M. tb and not only decrease clinical disease but also decrease the overwhelming reservoir of latent TB infection.

Mycobacterium tuberculosis PPE60 antigen drives Th1/Th17 responses via Toll-like receptor 2-dependent maturation of dendritic cells

Targeting of Mycobacterium tuberculosis (MTB) PE/PPE antigens that induce type 1 helper T cell (Th1) and Th17 responses represents a crucial strategy for the development of tuberculosis (TB) vaccines. However, only a few PE/PPE antigens induce these responses. Here, we sought to determine how the cell wall-associated antigen PPE60 (Rv3478) activates dendritic cell (DC) maturation and T-cell differentiation. We observed that PPE60 induces DC maturation by augmenting the protein expression of cluster of differentiation 80 (CD80) and CD86 and major histocompatibility complex (MHC) class I and MHC class II on the cell surface. PPE60 also stimulated the production of tumor necrosis factor-α (TNFα), interleukin (IL)-1β, IL-6, IL-12p70, and IL-23p19 but not IL-10. This induction was mediated by Toll-like receptor 2 (TLR2) and followed by activation of p38, c-Jun N-terminal kinase (JNK), and NF-κB signaling. PPE60 enhanced MHC-II expression and promoted antigen processing by DCs in a TLR2-dependent manner. Moreover, PPE60-stimulated DCs directed naïve CD4⁺ T cells to produce IFN-γ, IL-2, and IL-17A, expanding the Th1 and Th17 responses, along with activation of T-bet and RAR-related orphan receptor C (RORγt) but not GATA-3. Moreover, PPE60 activated the NLRP3 inflammasome followed by caspase-1-dependent IL-1β and IL-18 synthesis in DCs. Of note, pharmacological inhibition of NLRP3 activation specifically attenuated IFN-γ and IL-17A secretion into the supernatant from CD4⁺ T cells cocultured with PPE60-activated DCs. These findings indicate that PPE60 induces Th1 and Th17 immune responses by activating DCs in a TLR2-dependent manner, suggesting PPE60's potential for use in MTB vaccine development.

IRF5 distinguishes severe asthma in humans and drives Th1 phenotype and airway hyperreactivity in mice

Severe asthma (SA) is a significant problem both clinically and economically, given its poor response to corticosteroids (CS). We recently reported a complex type 1-dominated (IFN-γ-dominated) immune response in more than 50% of severe asthmatics despite high-dose CS treatment. Also, IFN-γ was found to be critical for increased airway hyperreactivity (AHR) in our model of SA. The transcription factor IRF5 expressed in M1 macrophages can induce a Th1/Th17 response in cocultured human T cells. Here we show markedly higher expression of IRF5 in bronchoalveolar lavage (BAL) cells of severe asthmatics as compared with that in cells from milder asthmatics or healthy controls. Using our SA mouse model, we demonstrate that lack of IRF5 in lymph node migratory DCs severely limits their ability to stimulate the generation of IFN-γ- and IL-17-producing CD4+ T cells and IRF5-/- mice subjected to the SA model displayed significantly lower IFN-γ and IL-17 responses, albeit showing a reciprocal increase in Th2 response. However, the absence of IRF5 rendered the mice responsive to CS with suppression of the heightened Th2 response. These data support the notion that IRF5 inhibition in combination with CS may be a viable approach to manage disease in a subset of severe asthmatics.

Subversion of mouse dendritic cell subset function by bacterial pathogens

Dendritic cells (DCs) play an important role as sentinels of the immune system in initiating and controlling the quality of adaptive immune responses. Located at entry points of the host they can sense and alert the body from dangers such as infection by pathogenic bacteria. Considering their strategic localization it is not surprising that DCs have evolved in a series of DC subtypes, which are well adapted to their microenvironment. Nowadays, the advent of the identification of specific DC subtypes has opened the way for the study of pathogen-DCs interactions and the involved mechanisms of these interactions. Due to key aspect of DCs, several bacterial pathogens have taken advantage of these cells and developed mechanisms to subvert DC function and thereby evade the immune system. This review brings recent insights into DC-pathogenic bacteria cross-talk using the mouse model of infection with an emphasis on DC subtypes.

Distinct Strategies Employed by Dendritic Cells and Macrophages in Restricting Mycobacterium tuberculosis Infection: Different Philosophies but Same Desire

Dendritic cells (DCs) and macrophages (Mϕs) are professional antigen-presenting cells (APCs) that can efficiently phagocytose Mycobacterium tuberculosis (Mtb), the etiological agent of tuberculosis (TB). It is quite interesting to mention here that DCs and Mϕs use distinct strategies to combat and eliminate Mtb. Similarly, Mtb employs different mechanisms to counteract the action of DCs and Mϕs. Mϕs are evolved with specialized, innate, defensive machinery to restrict growth of Mtb at the initial phase of infection. However, DCs are more endowed toward initiating adaptive immunity by activating naïve T cells. During encounter with Mtb, DCs and Mϕs deliver discrete functions via triggering through different pattern recognition receptors (PRRs) expressed by these APCs. Mtb-infected DCs and Mϕs show differential expression of genes encoding cytokines, chemokines, costimulatory molecules, and adhesion molecules. Interestingly, Mtb impairs the immune defensive machinery by exploiting various PRRs. Remarkably, selective signaling through PRRs by Mtb abrogates the bactericidal activity of Mϕs, but subverts differentiation of monocytes to DCs. In this article, we highlight the role of PRRs in inducing distinct immune response by DCs and Mϕs against Mtb. Concurrently, we also discuss smart strategies exploited by Mtb to impair the function of host DCs and Mϕs.

Microarray analysis of the in vitro granulomatous response to Mycobacterium tuberculosis H37Ra

BACKGROUND

The hallmark of tuberculosis is the granuloma, an organized cellular accumulation playing a key role in host defense against Mycobacterium tuberculosis. These structures sequester and contain mycobacterial cells preventing active disease, while long term maintenance of granulomas leads to latent disease. Clear understanding on mechanisms involved in granuloma formation and maintenance is lacking.

OBJECTIVE

To monitor granuloma formation and to determine gene expression profiles induced during the granulomatous response to M. tuberculosis (H37Ra).

METHODS

We used a previously characterized in vitro human model. Cellular aggregation was followed daily with microscopy and Wright staining for 5 days. Granulomas were collected at 24 h, RNA extracted and hybridized to Affymetrix human microarrays.

RESULTS

Daily microscopic examination revealed gradual formation of granulomas in response to mycobacterial infection. Granulomatous structures persisted for 96 h, and then began to disappear.

CONCLUSIONS

Microarray analysis identified genes in the innate immune response and antigen presentation pathways activated during the in vitro granulomatous response to live mycobacterial cells, revealing very early changes in gene expression of the human granulomatous response.

Differential dermal expression of CCL17 and CCL18 in tuberculoid and lepromatous leprosy

BACKGROUND

Leprosy is characterized by polar clinical, histologic and immunological presentations. Previous immunologic studies of leprosy polarity were limited by the repertoire of cytokines known at the time.

METHODOLOGY

We used a candidate gene approach to measure mRNA levels in skin biopsies from leprosy lesions. mRNA from 24 chemokines and cytokines, and 6 immune cell type markers were measured from 85 Nepalese leprosy subjects. Selected findings were confirmed with immunohistochemistry.

PRINCIPAL RESULTS

Expression of three soluble mediators (CCL18, CCL17 and IL-10) and one macrophage cell type marker (CD14) was significantly elevated in lepromatous (CCL18, IL-10 and CD14) or tuberculoid (CCL17) lesions. Higher CCL18 protein expression by immunohistochemistry and a trend in increased serum CCL18 in lepromatous lesions was observed. No cytokines were associated with erythema nodosum leprosum or Type I reversal reaction following multiple comparison correction. Hierarchical clustering suggested that CCL18 was correlated with cell markers CD209 and CD14, while neither CCL17 nor CCL18 were highly correlated with classical TH1 and TH2 cytokines.

CONCLUSIONS

Our findings suggest that CCL17 and CCL18 dermal expression is associated with leprosy polarity.

CXCL10 controls inflammatory pain via opioid peptide-containing macrophages in electroacupuncture

Acupuncture is widely used for pain treatment in patients with osteoarthritis or low back pain, but molecular mechanisms remain largely enigmatic. In the early phase of inflammation neutrophilic chemokines direct opioid-containing neutrophils in the inflamed tissue and stimulate opioid peptide release and antinociception. In this study the molecular pathway and neuroimmune connections in complete Freund's adjuvant (CFA)-induced hind paw inflammation and electroacupuncture for peripheral pain control were analyzed. Free moving Wistar rats with hind paw inflammation were treated twice with electroacupuncture at GB30 (Huan Tiao - gall bladder meridian) (day 0 and 1) and analyzed for mechanical and thermal nociceptive thresholds. The cytokine profiles as well as the expression of opioid peptides were quantified in the inflamed paw. Electroacupuncture elicited long-term antinociception blocked by local injection of anti-opioid peptide antibodies (beta-endorphin, met-enkephalin, dynorphin A). The treatment altered the cytokine profile towards an anti-inflammatory pattern but augmented interferon (IFN)-gamma and the chemokine CXCL10 (IP-10: interferon gamma-inducible protein) protein and mRNA expression with concomitant increased numbers of opioid peptide-containing CXCR3⁺ macrophages. In rats with CFA hind paw inflammation without acupuncture repeated injection of CXCL10 triggered opioid-mediated antinociception and increase opioid-containing macrophages. Conversely, neutralization of CXCL10 time-dependently decreased electroacupuncture-induced antinociception and the number of infiltrating opioid peptide-expressing CXCR3⁺ macrophages. In summary, we describe a novel function of the chemokine CXCL10 - as a regulator for an increase of opioid-containing macrophages and antinociceptive mediator in inflammatory pain and as a key chemokine regulated by electroacupuncture.

Biomarkers of latent TB infection

For the last 100 years, the tuberculin skin test (TST) has been the only diagnostic tool available for latent TB infection (LTBI) and no biomarker per se is available to diagnose the presence of LTBI. With the introduction of M. tuberculosis-specific IFN-gamma release assays (IGRAs), a new area of in vitro immunodiagnostic tests for LTBI based on biomarker readout has become a reality. In this review, we discuss existing evidence on the clinical usefulness of IGRAs and the indefinite number of potential new biomarkers that can be used to improve diagnosis of latent TB infection. We also present early data suggesting that the monocyte-derived chemokine inducible protein-10 may be useful as a novel biomarker for the immunodiagnosis of latent TB infection.

Immunotherapy for TB

Mycobacterium tuberculosis was one of the first human pathogens to be identified as the cause of a specific disease – TB. TB was also one of the first specific diseases for which immunotherapy was attempted. In more than a century since, multiple different immunotherapies have been attempted, alongside vaccination and antibiotic treatment, with varying degrees of success. Despite this, TB remains a major worldwide health problem that causes nearly 2 million deaths annually and has infected an estimated 2 billion people. A major reason for this is that M. tuberculosis is an ancient human pathogen that has evolved complex strategies for persistence in the human host. It has thus been long understood that, to effectively control TB, we will need to address the ability of the pathogen to establish a persistent, latent infection in most infected individuals. This review discusses what is presently known about the interaction of M. tuberculosis with the immune system, and how this knowledge has been used to design immunotherapeutic strategies.

Bone marrow-specific deficiency of nuclear receptor Nur77 enhances atherosclerosis

RATIONALE

Nuclear receptor Nur77, also known as NR4A1, TR3, or NGFI-B, is expressed in human atherosclerotic lesions in macrophages, endothelial cells, T cells and smooth muscle cells. Macrophages play a critical role in atherosclerosis and the function of Nur77 in lesion macrophages has not yet been investigated.

OBJECTIVE

This study aims to delineate the function of Nur77 in macrophages and to assess the effect of bone marrow-specific deficiency of Nur77 on atherosclerosis.

METHODS AND RESULTS

We investigated Nur77 in macrophage polarization using bone marrow-derived macrophages (BMM) from wild-type and Nur77-knockout (Nur77(-/-)) mice. Nur77(-/-) BMM exhibit changed expression of M2-specific markers and an inflammatory M1-phenotype with enhanced expression of interleukin-12, IFNγ, and SDF-1α and increased NO synthesis in (non)-stimulated Nur77(-/-) BMM cells. SDF-1α expression in nonstimulated Nur77(-/-) BMM is repressed by Nur77 and the chemoattractive activity of Nur77(-/-) BMM is abolished by SDF-1α inhibiting antibodies. Furthermore, Nur77(-/-) mice show enhanced thioglycollate-elicited migration of macrophages and B cells. The effect of bone marrow-specific deficiency of Nur77 on atherosclerosis was studied in low density lipoprotein receptor-deficient (Ldlr(-/-)) mice. Ldlr(-/-) mice with a Nur77(-/-)-deficient bone marrow transplant developed 2.1-fold larger atherosclerotic lesions than wild-type bone marrow-transplanted mice. These lesions contain more macrophages, T cells, smooth muscle cells and larger necrotic cores. SDF-1α expression is higher in lesions of Nur77(-/-)-transplanted mice, which may explain the observed aggravation of lesion formation.

CONCLUSIONS

In conclusion, in bone marrow-derived cells the nuclear receptor Nur77 has an anti-inflammatory function, represses SDF-1α expression and inhibits atherosclerosis.

Modulation of cell death by M. tuberculosis as a strategy for pathogen survival

It has been clearly demonstrated that in vitro, virulent M. tuberculosis can favor necrosis over apoptosis in infected macrophages, and this has been suggested as a mechanism for evading the host immune response. We recently reported that an effect consistent with this hypothesis could be observed in cells from the blood of TB patients, and in this paper, we review what is known about evasion strategies employed by M. tuberculosis and in particular consider the possible interaction of the apoptosis-inhibiting effects of M. tuberculosis infection with another factor (IL-4) whose expression is thought to play a role in the failure to control M. tuberculosis infection. It has been noted that IL-4 may exacerbate TNF-α-induced pathology, though the mechanism remains unexplained. Since pathology in TB typically involves inflammatory aggregates around infected cells, where TNF-α plays an important role, we predicted that IL-4 would inhibit the ability of cells to remove M. tuberculosis by apoptosis of infected cells, through the extrinsic pathway, which is activated by TNF-α. Infection of human monocytic cells with mycobacteria in vitro, in the presence of IL-4, appears to promote necrosis over apoptosis in infected cells—a finding consistent with its suggested role as a factor in pathology during M. tuberculosis infection.

Diabetic mice display a delayed adaptive immune response to Mycobacterium tuberculosis

Diabetes mellitus (DM) is a major risk factor for tuberculosis (TB) but the defect in protective immunity responsible for this has not been defined. We previously reported that streptozotocin-induced DM impaired TB defense in mice, resulting in higher pulmonary bacterial burden, more extensive inflammation, and higher expression of several proinflammatory cytokines known to play a protective role in TB. In the current study, we tested the hypothesis that DM leads to delayed priming of adaptive immunity in the lung-draining lymph nodes (LNs) following low dose aerosol challenge with virulent Mycobacterium tuberculosis. We show that M. tuberculosis-specific IFN-gamma-producing T cells arise later in the LNs of diabetic mice than controls, with a proportionate delay in recruitment of these cells to the lung and stimulation of IFN-gamma-dependent responses. Dissemination of M. tuberculosis from lung to LNs was also delayed in diabetic mice, although they showed no defect in dendritic cell trafficking from lung to LNs after LPS stimulation. Lung leukocyte aggregates at the initial sites of M. tuberculosis infection developed later in diabetic than in nondiabetic mice, possibly related to reduced levels of leukocyte chemoattractant factors including CCL2 and CCL5 at early time points postinfection. We conclude that TB increased susceptibility in DM results from a delayed innate immune response to the presence of M. tuberculosis-infected alveolar macrophages. This in turn causes late delivery of Ag-bearing APC to the lung draining LNs and delayed priming of the adaptive immune response that is necessary to restrict M. tuberculosis replication.

Mycobacterium tuberculosis infection induces il12rb1 splicing to generate a novel IL-12Rbeta1 isoform that enhances DC migration

RNA splicing is an increasingly recognized regulator of immunity. Here, we demonstrate that after Mycobacterium tuberculosis infection (mRNA) il12rb1 is spliced by dendritic cells (DCs) to form an alternative (mRNA) il12rb1Δtm that encodes the protein IL-12Rβ1ΔTM. Compared with IL-12Rβ1, IL-12Rβ1ΔTM contains an altered C-terminal sequence and lacks a transmembrane domain. Expression of IL-12Rβ1ΔTM occurs in CD11c⁺ cells in the lungs during M. tuberculosis infection. Selective reconstitution of il12rb1^−/− DCs with (mRNA) il12rb1 and/or (mRNA) il12rb1Δtm demonstrates that IL-12Rβ1ΔTM augments IL-12Rβ1-dependent DC migration and activation of M. tuberculosis-specific T cells. It cannot mediate these activities independently of IL12Rβ1. We hypothesize that M. tuberculosis-exposed DCs express IL-12Rβ1ΔTM to enhance IL-12Rβ1-dependent migration and promote M. tuberculosis–specific T cell activation. IL-12Rβ1ΔTM thus represents a novel positive-regulator of IL12Rβ1-dependent DC function and of the immune response to M. tuberculosis.

Spotlight on Mycobacteria and dendritic cells: will novel targets to fight tuberculosis emerge?

Over thousands of years microbes and mammals have co-evolved, resulting in extraordinarily sophisticated molecular mechanisms permitting the organisms to survive together. Mycobacterium tuberculosis is one of the best examples of successful co-evolution, since the bacilli have infected one third of the human population, but in 90% of the cases without causing overt disease. Despite this, increasing incidence of Human Immunodeficiency Virus (HIV) infection and the emergence of drug-resistant strains means that tuberculosis is in fact an extremely serious emerging threat to global health. Decades of work have focused on the interaction of this pathogen with its established cellular host, the macrophage, but still novel therapeautics remain elusive. While the macrophage is clearly important, recent evidence suggests that understanding the role of dendritic cells, which are key regulators of immunity, may be a crucial step in identifying new means of controlling this disease. Novel technologies, in particular genome-wide transcriptome analyses, are advancing our ability to dissect the complex dynamic relationships between dendritic cells and mycobacteria, highlighting new areas for study that have not been previously explored.

Requirement of TLR4 and CD14 in dendritic cell activation by Hemagglutinin B from Porphyromonas gingivalis

Porphyromonas gingivalis is a Gram-negative anaerobic bacterium that is one of the causative agents of chronic adult periodontal disease. Among the potential virulence factors of P. gingivalis are the hemagglutinins. Recombinant Hemagglutinin B (rHagB) from P. gingivalis has been shown to activate the immune system by inducing specific antibodies that protect against experimental periodontal bone loss following P. gingivalis infection. Since different microbial products can stimulate dendritic cells (DC) through Toll-like receptors (TLRs), subsequently leading to T cell activation and antibody production, we wanted to investigate the immunostimulatory effect of rHagB on DC and the role of TLR signaling in this process. Using an endotoxin free rHagB preparation, our results show that stimulation of murine bone marrow-derived DC with rHagB leads to upregulation of the costimulatory molecules CD86 and CD40, activation of p38 and ERK MAP kinases, transcription factors NF-kappaB, CREB and IRF-3 and the production of IL-6, TNF-alpha, IL-12p40 and to a lesser extent IL-10 and IFN-beta. This activation process was absolutely dependent on TLR4 and CD14. While upregulation of CD86 was independent of the adaptor molecule MyD88, CD40 upregulation and optimal cytokine (IL-6, TNF-alpha, IL-12p40, IL-10 and IFN-beta) production required both MyD88 and TRIF molecules. These results are of importance since they are the first to provide insights into the interaction of rHagB with DC and TLRs. The information from this study will aid in the design of effective vaccines strategies against chronic adult periodontal disease.

Interaction of Mycobacterium tuberculosis with the host: consequences for vaccine development

Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), remains a major worldwide health problem that causes more than 2 million deaths annually. In addition, an estimated 2 billion people are latently infected with M. tuberculosis. The bacterium is one of the oldest human pathogens and has evolved complex strategies for survival. Therefore, to be successful in the high endemic regions, any future TB vaccine strategy will have to be tailored in accordance with the resulting complexity of the TB infection and anti-mycobacterial immune response. In this review, we will discuss what is presently known about the interaction of M. tuberculosis with the immune system, and how this knowledge is used in new and more advanced vaccine strategies.

Dendritic cells in the pathogenesis of sarcoidosis

Sarcoidosis is a noncaseating granulomatous disease, likely of autoimmune etiology, that causes inflammation and tissue damage in multiple organs, most commonly the lung, but also skin, and lymph nodes. Reduced dendritic cell (DC) function in sarcoidosis peripheral blood compared with peripheral blood from control subjects suggests that blunted end organ cellular immunity may contribute to sarcoidosis pathogenesis. Successful treatment of sarcoidosis with tumor necrosis factor (TNF) inhibitors, which modulate DC maturation and migration, has also been reported. Together, these observations suggest that DCs may be important mediators of sarcoidosis immunology. This review focuses on the phenotype and function of DCs in the lung, skin, blood, and lymph node of patients with sarcoidosis. We conclude that DCs in end organs are phenotypically and functionally immature (anergic), while DCs in the lymph node are mature and polarize pathogenic Th1 T cells. The success of TNF inhibitors is thus likely secondary to inhibition of DC-mediated Th1 polarization in the lymph node.

Innate invariant NKT cells recognize Mycobacterium tuberculosis-infected macrophages, produce interferon-gamma, and kill intracellular bacteria

Cellular immunity to Mycobacterium tuberculosis (Mtb) requires a coordinated response between the innate and adaptive arms of the immune system, resulting in a type 1 cytokine response, which is associated with control of infection. The contribution of innate lymphocytes to immunity against Mtb remains controversial. We established an in vitro system to study this question. Interferon-γ is produced when splenocytes from uninfected mice are cultured with Mtb-infected macrophages, and, under these conditions, bacterial replication is suppressed. This innate control of bacterial replication is dependent on CD1d-restricted invariant NKT (iNKT) cells, and their activation requires CD1d expression by infected macrophages as well as IL-12 and IL-18. We show that iNKT cells, even in limiting quantities, are sufficient to restrict Mtb replication. To determine whether iNKT cells contribute to host defense against tuberculosis in vivo, we adoptively transferred iNKT cells into mice. Primary splenic iNKT cells obtained from uninfected mice significantly reduce the bacterial burden in the lungs of mice infected with virulent Mtb by the aerosol route. Thus, iNKT cells have a direct bactericidal effect, even in the absence of synthetic ligands such as α-galactosylceramide. Our finding that iNKT cells protect mice against aerosol Mtb infection is the first evidence that CD1d-restricted NKT cells mediate protection against Mtb in vivo.

Host resistance to Mycobacterium tuberculosis (Mtb) requires a coordinated response by the different components of the immune system. We established an in vitro model to study the contribution of innate lymphocytes to immunity against Mtb. When co-cultured with Mtb-infected macrophages, splenocytes from uninfected mice become activated and suppress bacterial replication. By fractionating the different splenocyte cell populations, we discovered that the invariant NKT (iNKT) cell is essential for suppressing intracellular bacterial replication. iNKT cells, which are conserved in rodents and humans, recognize lipids presented by the antigen-presenting molecule CD1d. While we had previously shown that iNKT cell-deficient mice are not more susceptible to tuberculosis, a potential contribution of iNKT cells during the early phase of immunity may have been masked. To address this issue, we showed that highly purified iNKT cells were sufficient to reduce the lung bacterial burden of mice infected with virulent Mtb. This is the first evidence that CD1d-restricted iNKT cells play a physiological role in mediating protection against aerosol Mtb infection in vivo. Thus, by being an early producer of interferon-g and suppressing intracellular bacterial growth, iNKT cells function as an important part of the early immune response against Mtb.