Dendritic cells are host cells for mycobacteria in vivo that trigger innate and acquired immunity

Der p 2 recombinant bacille Calmette-Guérin targets dendritic cells to inhibit allergic airway inflammation in a mouse model of asthma

BACKGROUND

Previous studies showed that a recombinant bacille Calmette-Guérin (rBCG) which expressed the Der p 2 of house dust mites (Der p 2 rBCG) could suppress asthmatic airway inflammation. There are two possible mechanisms: (1) Der p 2 rBCG elicits immune deviation from Th2 to Th1, and (2) Der p 2 rBCG induces antigen-specific regulatory T cells. However, the role of dendritic cell (DC) Der p 2 rBCG in this protective effect and in reprogramming T-cell commitment still needs to be studied.

OBJECTIVES

The aim of this study was to determine whether DCs play a central role in the Der p 2 rBCG-mediated inhibition of allergic airway inflammation.

METHODS

DCs were collected from Der p 2 rBCG-immunized mice (Der p 2 rBCG-DCs) and adoptively transferred to Der p 2-sensitized mice. The effects of DCs on airway inflammation and immune regulation were analyzed.

RESULTS

Adoptive transfer of DCs from Der p 2 rBCG-immunized mice suppressed asthmatic responses, including airway inflammation, mucin secretion and airway responsiveness. Der p 2 rBCG-DCs could effectively inhibit excessive Th2 immune responses and induced a subtype of CD4+CD25+Foxp3+ anti-specific regulatory T cells in this asthma model. Furthermore, Der p 2 rBCG immunization recruited more plasmacytoid DCs in abdominal draining lymph nodes.

CONCLUSIONS

These findings suggest that DCs played a key role in Der p 2 rBCG-induced immunoregulation. Compared with BCG, Der p 2 rBCG displayed a more potent inhibitory effect on asthma responses, which may be related to the increase in plasmacytoid DC recruitment. These results improve our understanding of the cellular basis of Der p 2 BCG-mediated inhibition of asthma.

Identification of DNA binding motifs of the Mycobacterium tuberculosis PhoP/PhoR two-component signal transduction system

BACKGROUND

The Mycobacterium tuberculosis PhoP/PhoR two-component signal transduction system controls the expression of about 2% of the genome and plays a major role in pathogenicity. However, its regulon has not been well characterized.

METHODOLOGY/PRINCIPAL FINDINGS

The binding site of PhoP transcription regulator was identified in the upstream regions of msl3, pks2, lipF and fadD21 genes, by using gene fusions, electrophoretic mobility shift assays and DNase I footprinting experiments. A consensus sequence for PhoP binding was deduced. It consists of two direct repeats, DR1/DR2, associated with a third repeat, DR3, important in some cases for PhoP binding to DR1/DR2 but located at a variable distance from these direct repeats. DR1/DR2 and DR3 consensus sequences were used to screen the whole-genome sequence for other putative binding sites potentially corresponding to genes directly regulated by PhoP. The identified 87 genes, encoding transcription regulators, and proteins involved in secondary metabolites biosynthesis, transport and catabolism are proposed to belong to the PhoP regulon.

CONCLUSIONS/SIGNIFICANCE

A consensus sequence derived from the analysis of PhoP binding to four gene promoter regions is proposed. We show for the first time the involvement of a third direct repeat motif in this binding reaction. The consensus sequence was instrumented to study the global regulation mediated by PhoP in M. tuberculosis. This analysis leads to the identification of several genes that are potentially regulated by this key player.

Divergent effects of mycobacterial cell wall glycolipids on maturation and function of human monocyte-derived dendritic cells

BACKGROUND

Mycobacterium tuberculosis (Mtb) is able to evade the immune defenses and may persist for years, decades and even lifelong in the infected host. Mtb cell wall components may contribute to such persistence by modulating several pivotal types of immune cells. Dendritic cells (DCs) are the most potent antigen-presenting cells and hence play a crucial role in the initial immune response to infections by connecting the innate with the adaptive immune system.

PRINCIPAL FINDINGS

We investigated the effects of two of the major mycobacterial cell wall-associated types of glycolipids, mannose-capped lipoarabinomannan (ManLAM) and phosphatidylinositol mannosides (PIMs) purified from the Mtb strains H37Rv and Mycobacterium bovis, on the maturation and cytokine profiles of immature human monocyte-derived DCs. ManLAM from Mtb H37Rv stimulated the release of pro-inflammatory cytokines TNF, IL-12, and IL-6 and expression of co-stimulatory (CD80, CD86) and antigen-presenting molecules (MHC class II). ManLAM from M. bovis also induced TNF, IL-12 and IL-6 but at significantly lower levels. Importantly, while ManLAM was found to augment LPS-induced DC maturation and pro-inflammatory cytokine production, addition of PIMs from both Mtb H37Rv and M. bovis strongly reduced this stimulatory effect.

CONCLUSIONS

These results indicate that the mycobacterial cell wall contains macromolecules of glycolipid nature which are able to induce strong and divergent effects on human DCs; i.e while ManLAM is immune-stimulatory, PIMs act as powerful inhibitors of DC cytokine responses. Thus PIMs may be important Mtb-associated virulence factors contributing to the pathogenesis of tuberculosis disease. These findings may also aid in the understanding of some earlier conflicting reports on the immunomodulatory effects exerted by different ManLAM preparations.

The transcription factor NFATp plays a key role in susceptibility to TB in mice

In T cells, the transcription factor nuclear factor of activated T cells p (NFATp) is a key regulator of the cytokine genes tumor necrosis factor (TNF) and interferon-γ (IFN-γ). Here, we show that NFATp-deficient (NFATp(-/-)) mice have a dramatic and highly significant increase in mortality after Mycobacterium tuberculosis (MTb) infection as compared to mortality of control animals after MTb infection. Animals deficient in NFATp have significantly impaired levels of TNF and IFN-γ transcription and protein expression in naïve or total CD4(+) T cells, but display wild-type levels of TNF mRNA or protein from MTb-stimulated dendritic cells (DC). The rapid mortality and disease severity observed in MTb-infected NFATp(-/-) mice is associated with dysregulated production of TNF and IFN-γ in the lungs, as well as with increased levels of TNF, in their serum. Furthermore, global blocking of TNF production by injection of a TNF neutralizaing agent at 6 weeks, but not 12 weeks, post-MTb-infection further decreased the survival rate of both wild-type and NFATp(-/-) mice, indicating an early role for TNF derived from cells from the monocyte lineage in containment of infection. These results thus demonstrate that NFATp plays a critical role in immune containment of TB disease in vivo, through the NFATp-dependent expression of TNF and IFN-γ in T cells.

Mycobacterium tuberculosis infection of dendritic cells leads to partially caspase-1/11-independent IL-1β and IL-18 secretion but not to pyroptosis

Background

Interleukin-1β (IL-1β) is important for host resistance against Mycobacterium tuberculosis (Mtb) infections. The response of the dendritic cell inflammasome during Mtb infections has not been investigated in detail.

Methodology/Principal Findings

Here we show that Mtb infection of bone marrow-derived dendritic cells (BMDCs) induces IL-1β secretion and that this induction is dependent upon the presence of functional ASC and NLRP3 but not NLRC4 or NOD2. The analysis of cell death induction in BMDCs derived from these knock-out mice revealed the important induction of host cell apoptosis but not necrosis, pyroptosis or pyronecrosis. Furthermore, NLRP3 inflammasome activation and apoptosis induction were both reduced in BMDCs infected with the esxA deletion mutant of Mtb demonstrating the importance of a functional ESX-1 secretion system. Surprisingly, caspase-1/11-deficient BMDCs still secreted residual levels of IL-1βand IL-18 upon Mtb infection which was abolished in cells infected with the esxA Mtb mutant.

Conclusion

Altogether we demonstrate the partially caspase-1/11-independent, but NLRP3- and ASC- dependent IL-1β secretion in Mtb-infected BMDCs. These findings point towards a potential role of DCs in the host innate immune response to mycobacterial infections via their capacity to induce IL-1β and IL-18 secretion.

Attenuated Listeria monocytogenes, a Mycobacterium tuberculosis ESAT-6 antigen expression and delivery vector for inducing an immune response

We selectively expressed protective Mycobacterium tuberculosis antigen ESAT-6 in recombinant strains Lm(esat-6) and LmΔactA/plcB(esat-6) to evaluate the capacity of Listeria monocytogenes to deliver antigens from M. tuberculosis, and we studied the pathogenicity and immunogenicity of these strains compared with virulent parental strain yzuLm4 and attenuated strain LmΔactA/plcB. The two recombinant strains retained listeriolysin O hemolytic activity, escaped into the cytosol niche and established replication in the macrophage-like RAW264.7 cell line; however, these strains showed decreased virulence in C57BL/6 mice. Histopathology revealed no obvious pathological changes following administration of the recombinant strains to mice, indicating that they were significantly safer than parental strains. Moreover, intravenous vaccination of mice with the recombinant strains elicited specific Th1-type cellular immunity, splenocyte proliferation and effective CTL activity in vivo. Thus, attenuated L. monocytogenes strains can be used as effective vectors for delivering M. tuberculosis ESAT-6 and inducing a cellular immune response, suggesting that such vectors may be effective as novel vaccines for preventing tuberculosis.

Mycobacterium tuberculosis manipulates pulmonary APCs subverting early immune responses

Alveolar macrophages (AM) and dendritic cells (DCs) are the main antigen presenting cells (APCs) in the respiratory tract. Whereas macrophages have been extensively studied in tuberculosis, in situ interactions of DC with Mycobacterium tuberculosis (Mtb) are poorly explored. We aimed to characterize lung APCs during pulmonary tuberculosis in Balb/C mice infected with Mtb H37Rv. Mtb-infection via the airways induced a delayed and continuous accumulation of DCs and AM in the lungs. While lung DCs increased after day 3 post-infection, macrophages increased after 2-3 weeks. Although both populations accumulated in lungs during the infection, DCs decreased in the late stages. Infection induced differential expression of co-stimulatory molecules in these lung APCs, decreasing to basal levels in both APCs in the late stages. A remarkable segregation was found regarding bacillary burden. Many macrophages contained numerous bacilli, but DC contained scarce mycobacteria or none. Mtb-infection also induced delayed accumulation of DC in draining lymph nodes. This delayed recruitment was not associated with a lack of IL-12p40, which was detected from day 3 post-infection. Although AM and lung DCs behave differently during pulmonary tuberculosis, Mtb apparently manipulates both lung APCs subverting early protective responses resulting in disease progression.

Tuberculosis and HIV co-infection

Tuberculosis (TB) and HIV co-infections place an immense burden on health care systems and pose particular diagnostic and therapeutic challenges. Infection with HIV is the most powerful known risk factor predisposing for Mycobacterium tuberculosis infection and progression to active disease, which increases the risk of latent TB reactivation 20-fold. TB is also the most common cause of AIDS-related death. Thus, M. tuberculosis and HIV act in synergy, accelerating the decline of immunological functions and leading to subsequent death if untreated. The mechanisms behind the breakdown of the immune defense of the co-infected individual are not well known. The aim of this review is to highlight immunological events that may accelerate the development of one of the two diseases in the presence of the co-infecting organism. We also review possible animal models for studies of the interaction of the two pathogens, and describe gaps in knowledge and needs for future studies to develop preventive measures against the two diseases.

Crohn's disease and Mycobacterium avium subsp. paratuberculosis: the need for a study is long overdue

The initial suggestion that Mycobacterium avium subsp. paratuberculosis (Map) might be involved in the pathogenesis of Crohn's disease (CD) was based on the apparent similarity of lesions in the intestine of patients with CD with those present in cattle infected with Map, the etiological agent of Johne's disease (JD). Recent investigations have now revealed the presence of Map or Map DNA in blood or lesions from adults and children with CD. Of special interest, Map has also been found in patients with other diseases as well as healthy subjects. The latter observations indicate all humans are susceptible to infection with Map and that, like with other mycobacterial pathogens such as Mycobacterium tuberculosis, infection does not invariably lead to development of clinical disease but rather development of a persistent latent stage of infection where an immune response controls but does not eliminate the pathogen. Limited information has been obtained on the immune response to Map in healthy subjects and patients with CD. Understanding how Map may be involved in the pathogenesis of CD will require a better understanding of the immune response to Map in one of its common hosts as well as healthy humans and patients with CD.

Deciphering the genetic architecture of variation in the immune response to Mycobacterium tuberculosis infection

Tuberculosis (TB) is a major public health problem. One-third of the world's population is estimated to be infected with Mycobacterium tuberculosis (MTB), the etiological agent causing TB, and active disease kills nearly 2 million individuals worldwide every year. Several lines of evidence indicate that interindividual variation in susceptibility to TB has a heritable component, yet we still know little about the underlying genetic architecture. To address this, we performed a genome-wide mapping study of loci that are associated with functional variation in immune response to MTB. Specifically, we characterized transcript and protein expression levels and mapped expression quantitative trait loci (eQTL) in primary dendritic cells (DCs) from 65 individuals, before and after infection with MTB. We found 198 response eQTL, namely loci that were associated with variation in gene expression levels in either untreated or MTB-infected DCs, but not both. These response eQTL are associated with natural regulatory variation that likely affects (directly or indirectly) host interaction with MTB. Indeed, when we integrated our data with results from a genome-wide association study (GWAS) for pulmonary TB, we found that the response eQTL were more likely to be genetically associated with the disease. We thus identified a number of candidate loci, including the MAPK phosphatase DUSP14 in particular, that are promising susceptibility genes to pulmonary TB.

Concomitant administration of Mycobacterium bovis BCG with the meningococcal C conjugate vaccine to neonatal mice enhances antibody response and protective efficacy

Mycobacterium bovis BCG is administered to human neonates in many countries worldwide. The objective of the study was to assess if BCG could act as an adjuvant for polysaccharide-protein conjugate vaccines in newborns and thereby induce protective immunity against encapsulated bacteria in early infancy when susceptibility is high. We assessed whether BCG could enhance immune responses to a meningococcal C (MenC) conjugate vaccine, MenC-CRM(197), in mice primed as neonates, broaden the antibody response from a dominant IgG1 toward a mixed IgG1 and IgG2a/IgG2b response, and increase protective efficacy, as measured by serum bactericidal activity (SBA). Two-week-old mice were primed subcutaneously (s.c.) with MenC-CRM(197). BCG was administered concomitantly, a day or a week before MenC-CRM(197). An adjuvant effect of BCG was observed only when it was given concomitantly with MenC-CRM(197), with increased IgG response (P = 0.002) and SBA (8-fold) after a second immunization with MenC-CRM(197) without BCG, indicating increased T-cell help. In neonatal mice (1 week old) primed s.c. with MenC-CRM(197) together with BCG, MenC-polysaccharide (PS)-specific IgG was enhanced compared to MenC-CRM(197) alone (P = 0.0015). Sixteen days after the second immunization with MenC-CRM(197), increased IgG (P < 0.05), IgG1 (P < 0.05), IgG2a (P = 0.06), and IgG2b (P < 0.05) were observed, and only mice primed with MenC-CRM(197) plus BCG showed affinity maturation and detectable SBA (SBA > 128). Thus, vaccination with a meningococcal conjugate vaccine (and possibly with other conjugates) may benefit from concomitant administration of BCG in the neonatal period to accelerate and enhance production of protective antibodies, compared to the current infant administration of conjugate which follows BCG vaccination at birth.

Neonatal lung immune responses show a shift of cytokines and transcription factors toward Th2 and a deficit in conventional and plasmacytoid dendritic cells

The high incidence of lung-damaging life-threatening respiratory infections in infants may be related to the immaturity of their immune systems. To determine whether lung immune features differ in early life compared with those in adulthood, whole lung as well as lung T lymphocyte and DC responses were investigated in BALB/c neonates versus adults. Higher expression of GATA-3 and rapid and sustained production of type 2 cytokines by lung explants after in vitro exposure to anti-CD3 was the hallmark of the neonatal period, suggestive of a Th2 bias. Neonatal lung GATA-3-producing cells were identified as CD3(+), CD4 and CD8 double-negative T lymphocytes, a subset found at a higher frequency in neonatal than adult lung. The neonatal lungs contained fewer conventional DCs, with a lower ratio of CD103(+) to CD11b(+) DCs, and a much lower number of plasmacytoid DCs in comparison with adult lungs. Yet, when stimulated in vivo by BCG, neonatal lung DCs matured and primed adult naïve CD4(+) T cells toward Th1 as efficiently as adult BCG-primed lung DCs. Conversely, both adult and neonatal BCG-primed lung DCs induced a Th2 cytokine response from neonatal naïve lymph node T cells, suggestive of an intrinsic feature of neonatal T lymphocytes.

Mycobacterium bovis BCG and purified protein derivative-induced reduction in the CD80 expression and the antigen up-take function of dendritic cells from patients with cervical cancer

OBJECTIVE

Clinical diagnostic studies have reported that co-existence of malignancy and tuberculosis in regions with high incidence of tuberculosis may be possible. However, simultaneous occurrence of cervical cancer (CaCx) and tuberculosis has been reported for it rarity. Our analysis of prevalence of mycobacterial infection among the study group of patients with CaCx had revealed co-infection of mycobacterium species including Mycobacterium tuberculosis. Since M. tuberculosis potentially affects the dendritic cells-mediated protective immune response, the main aim of this study was to analyse the effect of the mycobacterial infection on the immunological functions of dendritic cells (DC) in patients with CaCx.

STUDY DESIGN

Initially, the prevalence of mycobacterial infection was analysed in the blood samples of patients with CaCx using ELISA and PCR techniques. Later to determine the role of mycobacterial infection on immunosuppression of DC in the progression of CaCx, dendritic cell-mediated immune suppression induced by Mycobacterium bovis BCG (M. bovis BCG) and purified protein derivative (PPD) was studied in patients with CaCx. Expression of co-stimulatory molecules CD80 and CD86, and the antigen processing function of the macrophage-derived immature dendritic cells (imDC) were analysed.

RESULTS

When compared to untreated imDC, the PPD and M. bovis BCG-treated imDC displayed 15% and 46.7% reduced expression of CD80, respectively. However, the M. bovis BCG and PPD did not have any significant reduction in the CD86 expression. Moreover, the dextran up-take was reduced by 24.4% and 35.8% with PPD and M. bovis BCG treatment, respectively. In addition, the M. bovis BCG-treated imDC derived from patients with CaCx showed a marginal reduction of lymphocyte proliferation and the PPD-treated imDC did not show a significant stimulation of lymphocyte proliferation.

CONCLUSION

These results indicate that the PPD and M. bovis BCG inhibit the maturation and antigen up-take property of macrophage-derived imDC in patients with cervical cancer, suggesting that the M. tuberculosis infection may favour the progression of cervical cancer through immunosuppressed imDC.

Pathogen-derived oligosaccharides improve innate immune response to intracellular parasite infection

Pathogen glycolipids, including Leishmania spp. lipophosphoglycan (LPG) and Mycobacterium tuberculosis mannosylated lipoarabinomannan (ManLAM), modulate essential interactions with host phagocytic cells. Polysaccharide and lipid components promote immunomodulation. Owing to the stereochemistry required to synthesize oligosaccharides, the roles for oligosaccharides in the pathogenesis of infectious diseases have remained largely unknown. Recent advances in carbohydrate chemistry allowed us to synthesize pathogen surface oligosaccharides to discern their immune response-altering activities. Trimannose cap carbohydrates from ManLAM and LPG altered the production of proinflammatory cytokines via a toll-like receptor (TLR2)-mediated mechanism in vitro and in vivo. In vivo treatment with trimannose led to increased Th1-polarizing, IL-12p40-producing cells from the draining lymph nodes of treated Leishmania major-infected mice compared with cells from untreated infected mice. Trimannose treatment increased the production of other Th1 proinflammatory cytokines (ie, interferon-γ, IL-6, and tumor necrosis factor-α) critical for a productive immune response to either pathogen. This significant difference in cytokine production between trimannose cap sugar-treated and control groups was not observed in draining lymph node cells from TLR2(-/-) mice. Type of inflammation and rate of bead entry into macrophages and dendritic cells were different for trimannose-coated beads compared with control oligosaccharide-coated beads, indicating selective lectin receptor/oligosaccharide interactions mediating cell entry and cytokine production. These novel findings may prompt the development of targeted oligosaccharide adjuvants against chronic infections.

Lipids, apoptosis, and cross-presentation: links in the chain of host defense against Mycobacterium tuberculosis

Eicosanoids regulate whether human and murine macrophages infected with Mycobacterium tuberculosis die by apoptosis or necrosis. The death modality is important since apoptosis is associated with diminished pathogen viability and should be viewed as a form of innate immunity. Apoptotic vesicles derived from infected macrophages are also an important source of bacterial antigens that can be acquired by dendritic cells to prime antigen-specific T cells. This review integrates in vitro and in vivo data on how apoptosis of infected macrophages is linked to development of T cell immunity against M. tuberculosis.

Immunopathology of postprimary tuberculosis: increased T-regulatory cells and DEC-205-positive foamy macrophages in cavitary lesions

Postprimary tuberculosis occurs in immunocompetent people infected with Mycobacterium tuberculosis. It is restricted to the lung and accounts for 80% of cases and nearly 100% of transmission. Little is known about the immunopathology of postprimary tuberculosis due to limited availability of specimens. Tissues from 30 autopsy cases of pulmonary tuberculosis were located. Sections of characteristic lesions of caseating granulomas, lipid pneumonia, and cavitary stages of postprimary disease were selected for immunohistochemical studies of macrophages, lymphocytes, endothelial cells, and mycobacterial antigens. A higher percentage of cells in lipid pneumonia (36.1%) and cavitary lesions (27.8%) were positive for the dendritic cell marker DEC-205, compared to granulomas (9.0%, P < .05). Cavities contained significantly more T-regulatory cells (14.8%) than found in lipid pneumonia (5.2%) or granulomas (4.8%). Distribution of the immune cell types may contribute to the inability of the immune system to eradicate tuberculosis.

Increased intracellular growth of Mycobacterium avium in HIV-1 exposed monocyte-derived dendritic cells

Dendritic cells (DC) are the most potent antigen-presenting cells, and form a link between the innate and adaptive immune system. They sample the periphery of the body for antigens and present them to T cells to elicit a proper immune response. It has been shown that dendritic cells phagocytose mycobacteria, but there have been conflicting reports as to whether the bacteria are capable of intracellular replication in DCs. Mycobacterium avium is a facultative intracellular bacterium, part of the Mycobacterium avium complex (MAC) of mycobacteria and are commonly seen as opportunistic pathogens in patients infected by Human immunodeficiency virus type 1 (HIV-1). To clarify the issue of whether DCs are capable of controlling the intracellular growth of M. avium and whether this control is lost upon HIV-1 exposure, we investigated the intracellular replication of M. avium in monocyte-derived dendritic cells and compared it to bacterial growth in dendritic cultures exposed to HIV-1 for 24 h. Our results show that exposure of DCs to HIV-1 promotes or facilitates the intracellular growth of M. avium.

Generation of self-renewing immature dendritic cells from mouse spleen that can take up mycobacteria and present antigens to T cells

Dendritic cells (DC) play a key role in driving the adaptive immune response to Mycobacterium tuberculosis (MTB), the causative pathogen of tuberculosis (TB). However, studying these important yet very sparse immune cells in the context of MTB pathogenesis is severely restricted by the lack of suitable cell lines and the complexity of culturing of DC progenitors, usually obtained from the bone marrow. However, significant advances have been made towards generating long-term DC cultures from various lymphoid tissues. Here, we report the evidence for generating a long-term, self-renewing DC culture from the Balb/c mouse spleen. We demonstrate that these cells, termed IDC-3, have a myeloid DC origin, i.e. they are CD11c(+) CD11b(++) CD8-α(-) F4/80(+/-) and that they also display a phenotype MHC-II(+) CD16/32(++) CD80(+/-) CD86(+) , indicating that they are immature DC. Following incubation with Mycobacterium bovis BCG (Bacillus Calmette Guerin), the IDC-3 efficiently took up bacteria and acquired the morphology of mature DC. Importantly though, when IDC-3 were pre-stimulated with a mycobacterial antigen in vitro, they were able to induce proliferation of T lymphocytes from mice immunized with the same antigen. The T-cell stimulatory potential of IDC-3 was further enhanced when the cells were co-stimulated with an anti-CD40 mAb. We therefore suggest that the IDC-3 culture system could be a useful tool for studying the interaction of DC with mycobacteria.

Dendritic cells in chronic mycobacterial granulomas restrict local anti-bacterial T cell response in a murine model

BACKGROUND

Mycobacterium-induced granulomas are the interface between bacteria and host immune response. During acute infection dendritic cells (DCs) are critical for mycobacterial dissemination and activation of protective T cells. However, their role during chronic infection in the granuloma is poorly understood.

METHODOLOGY/PRINCIPAL FINDINGS

We report that an inflammatory subset of murine DCs are present in granulomas induced by Mycobacteria bovis strain Bacillus Calmette-guerin (BCG), and both their location in granulomas and costimulatory molecule expression changes throughout infection. By flow cytometric analysis, we found that CD11c(+) cells in chronic granulomas had lower expression of MHCII and co-stimulatory molecules CD40, CD80 and CD86, and higher expression of inhibitory molecules PD-L1 and PD-L2 compared to CD11c(+) cells from acute granulomas. As a consequence of their phenotype, CD11c(+) cells from chronic lesions were unable to support the reactivation of newly-recruited, antigen 85B-specific CD4(+)IFNgamma(+) T cells or induce an IFNgamma response from naïve T cells in vivo and ex vivo. The mechanism of this inhibition involves the PD-1:PD-L signaling pathway, as ex vivo blockade of PD-L1 and PD-L2 restored the ability of isolated CD11c(+) cells from chronic lesions to stimulate a protective IFNgamma T cell response.

CONCLUSIONS/SIGNIFICANCE

Our data suggest that DCs in chronic lesions may facilitate latent infection by down-regulating protective T cell responses, ultimately acting as a shield that promotes mycobacterium survival. This DC shield may explain why mycobacteria are adapted for long-term survival in granulomatous lesions.

Mycobacterium tuberculosis promotes HIV trans-infection and suppresses major histocompatibility complex class II antigen processing by dendritic cells

Mycobacterium tuberculosis is a leading killer of HIV-infected individuals worldwide, particularly in sub-Saharan Africa, where it is responsible for up to 50% of HIV-related deaths. Infection by HIV predisposes individuals to M. tuberculosis infection, and coinfection accelerates the progression of both diseases. In contrast to most other opportunistic infections associated with HIV, an increased risk of M. tuberculosis infection occurs during early-stage HIV disease, long before CD4 T cell counts fall below critical levels. We hypothesized that M. tuberculosis infection contributes to HIV pathogenesis by interfering with dendritic cell (DC)-mediated immune control. DCs carry pathogens like M. tuberculosis and HIV from sites of infection into lymphoid tissues, where they process and present antigenic peptides to CD4 T cells. Paradoxically, DCs can also deliver infectious HIV to T cells without first becoming infected, a process known as trans-infection. Lipopolysaccharide (LPS)-activated DCs sequester HIV in pocketlike membrane invaginations that remain open to the cell surface, and individual virions are delivered from the pocket into T cells at the site of contact during trans-infection. Here we report that M. tuberculosis exposure increases HIV trans-infection and induces viral sequestration within surface-accessible compartments identical to those seen in LPS-stimulated DCs. At the same time, M. tuberculosis dramatically decreases the degradative processing and major histocompatibility complex class II (MHC-II) presentation of HIV antigens to CD4 T cells. Our data suggest that M. tuberculosis infection promotes a shift in the dynamic balance between antigen processing and intact virion presentation, favoring DC-mediated amplification of HIV infections.

Regulation of antigen presentation by Mycobacterium tuberculosis: a role for Toll-like receptors

Mycobacterium tuberculosis survives in antigen-presenting cells (APCs) such as macrophages and dendritic cells. APCs present antigens in association with major histocompatibility complex (MHC) class II molecules to stimulate CD4(+) T cells, and this process is essential to contain M. tuberculosis infection. Immune evasion allows M. tuberculosis to establish persistent or latent infection in macrophages and results in Toll-like receptor 2 (TLR2)-dependent inhibition of MHC class II transactivator expression, MHC class II molecule expression and antigen presentation. This reduction of antigen presentation might reflect a general mechanism of negative-feedback regulation that prevents excessive T cell-mediated inflammation and that M. tuberculosis has subverted to create a niche for survival in infected macrophages and evasion of recognition by CD4(+) T cells.

Autophagy promotes BCG-induced maturation of human dendritic cells

Dendritic cells (DCs) are important for the initiation of the adaptive immune response against Mycobacterium tuberculosis. Autophagy is an innate and adaptive defense mechanism and important for the control of M. tuberculosis. However, the role of autophagy in the adaptive immune response against M. tuberculosis remains to be determined. In the present study, we studied the effects of autophagy on the maturation of DCs infected with Bacillus Calmette- Guerin (BCG). The phenotype and function of the DCs were assessed by measuring the expression of CD86 and HLA-DR and the secretion of IL-10 and IL-6. Autophagy was evaluated by the change in LC3II, a molecular marker for autophagy. Following stimulation of autophagy, DCs that were matured in the presence of BCG showed enhanced expression of CD86 and HLA-DR and increased IL-6 production. The expression of LC3II was increased after the stimulation of autophagy. These results demonstrated that autophagy might result in the increased maturation of BCG-infected DCs, suggesting that autophagy could contribute to an enhanced adaptive immune response against 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.

CD8+ DC, but Not CD8(-)DC, isolated from BCG-infected mice reduces pathological reactions induced by mycobacterial challenge infection

BACKGROUND

Tuberculosis is a mycobacterial infection causing worldwide public health problems but the available vaccine is far from ideal. Type-1 T cell immunity has been shown to be critical for host defence against tuberculosis infection, but the role of dendritic cell (DC) subsets in pathogenesis of mycobacterial infection remains unclear.

METHODOLOGY/PRINCIPAL FINDINGS

We examined the effectiveness of dendritic cell (DC) subsets in BCG-infected mice in generating immune responses beneficial for pathogen clearance and reduction of pathological reactions in the tissues following challenge infection. Our data showed that only the adoptive transfer of the subset of CD8alpha+ DC isolated from infected mice (iCD8+ DC) generated significant protection, demonstrated by less mycobacterial growth and pathological changes in the lung and liver tissues in iCD8+ DC recipients than sham-treated control mice. The adoptive transfer of the CD8alpha(-)DC from the infected mice (iCD8(-) DC) not only failed to reduce bacterial growth, but enhanced inflammation characterized by diffuse heavy cellular infiltration. Notably, iCD8(-) DC produced significantly higher levels of IL-10 than iCD8+ DC and promoted more Th2 cytokine responses in in vitro DC-T cell co-culture and in vivo adoptive transfer experiments.

CONCLUSIONS/SIGNIFICANCE

The data indicate that in vivo BCG-primed CD8+ DC is the dominant DC subset in inducing protective immunity especially for reducing pathological reactions in infected tissues. The finding has implications for the rational improvement of the prophylactic and therapeutic approaches for controlling tuberculosis infection and related diseases.

The role of dendritic cells in mycobacterium-induced granulomas

The presence of dendritic cells (DCs) in mycobacterium-containing granulomas, as well as in other granuloma-inducing diseases, is beginning to be appreciated. This review will summarize what is known about DCs with regards to the granuloma and discuss the potential roles DCs may be playing during mycobacterial infection. Potential functions may include mycobacterial dissemination from lesions or sampling of granuloma-containing mycobacterial antigens and migration to the draining lymph nodes to maintain continuous T cell priming. Additionally, the review will discuss the potential outcomes of DC-T cell cross-talk within the granuloma and whether it results in boosting the effector functions of newly arrived T cells or anergizing systemic T cells locally. Understanding the DCs complex and changing role during this critical stage may help explain how latency is achieved and maintained. Such knowledge might also lead to improved vaccination strategies.

Potential role for ESAT6 in dissemination of M. tuberculosis via human lung epithelial cells

ESAT6 has recently been demonstrated to cause haemolysis and macrophage lysis. Our studies demonstrate that ESAT6 causes cytolysis of type 1 and type 2 pneumocytes. Both types of pneumocytes express membrane laminin, and ESAT6 exhibits dose-dependent binding to both cell types and to purified human laminin. While minimal ESAT6 was detected on the surface of Mycobacterium tuberculosis grown in vitro, exogenously provided ESAT6 specifically associated with the bacterial cell surface, and the bacterium-associated ESAT6 retained its cytolytic ability. esat6 transcripts were upregulated approximately 4- to approximately 13-fold in bacteria replicating in type 1 cells, and approximately 3- to approximately 5 fold in type 2 cells. In vivo, laminin is primarily concentrated at the basolateral surface of pneumocytes where they rest on the basement membrane, which is composed primarily of laminin and collagen. The upregulation of esat6 transcripts in bacteria replicating in pneumocytes, the specific association of ESAT6 with the bacterial surface, the binding of ESAT6 to laminin and the lysis of pneumocytes by free and bacterium-associated ESAT6 together suggest a scenario wherein Mycobacterium tuberculosis replicating in pneumocytes may utilize surface ESAT6 to anchor onto the basolateral laminin-expressing surface of the pneumocytes, and damage the cells and the basement membrane to directly disseminate through the alveolar wall.

Tuberculosis-associated immune restoration syndrome in HIV-1-infected patients involves tuberculin-specific CD4 Th1 cells and KIR-negative gammadelta T cells

Tuberculosis (TB)-associated immune restoration syndrome (IRS) is a frequent event (10 to 30%) in HIV-1-infected patients receiving antiretroviral treatment and is associated with an increased number of IFN-gamma-producing tuberculin-specific cells. To further understand the immune mechanisms of TB-IRS and to identify predictive factors, we prospectively analyzed the Th1 and TCRgammadelta T cells known to be involved in mycobacterial defenses and dendritic cells at baseline and after antiretroviral and TB treatment in 24 HIV-1(+) patients, 11 with and 13 without IRS. At baseline, these two groups differed by significantly lower proportions of TCRgammadelta and Vdelta2(+) T cells displaying the inhibitory receptors CD94/NKG2 and CD158ah,b in IRS patients. The two groups did not differ in the baseline characteristics of CD8 or CD4 T cells or TLR-2 expression on monocytes or myeloid/plasmacytoid dendritic cells. During IRS, the increase in tuberculin-specific IFN-gamma-producing cells involved only highly activated effector memory multifunctional (IFN-gamma(+)TNF-alpha(+)IL-2(-)) CD4 T cells, whereas activated HLA-DR(+) CD4(+) T cells also increased during IRS. In contrast, dendritic cells decreased significantly during IRS and there were no changes in TLR-2 expression. Finally, the Vdelta2(+) T cells, mostly killer Ig-related receptor (KIR) (CD94/NKG2(-) and CD158(-)), significantly peaked during IRS but not in non-IRS patients. In conclusion, IRS is associated with an increase in the number of activated tuberculin-specific effector memory CD4 T cells and of KIR(-)Vdelta2(+) TCRgammadelta(+) T cells. Higher proportions of Vdelta2(+)TCRgammadelta(+) T cells lacking KIR expression are present as baseline and distinguish patients who will develop IRS from those who will not.

Role of phosphatidylinositol mannosides in the interaction between mycobacteria and DC-SIGN

The C-type lectin dendritic cell (DC)-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) is the major receptor on DCs for mycobacteria of the Mycobacterium tuberculosis complex. Recently, we have shown that although the mannose caps of the mycobacterial surface glycolipid lipoarabinomannan (ManLAM) are essential for the binding to DC-SIGN, genetic removal of these caps did not diminish the interaction of whole mycobacteria with DC-SIGN and DCs. Here we investigated the role of the structurally related glycolipids phosphatidylinositol mannosides (PIMs) as possible ligands for DC-SIGN. In a binding assay with both synthetic and natural PIMs, DC-SIGN exhibited a high affinity for hexamannosylated PIM(6), which contains terminal alpha(1-->2)-linked mannosyl residues identical to the mannose cap on ManLAM, but not for di- and tetramannosylated PIM(2) and PIM(4), respectively. To determine the role of PIM(6) in the binding of whole mycobacteria to DC-SIGN, a mutant strain of M. bovis bacillus Calmette-Guérin deficient in the production of PIM(6) (Delta pimE) was created, as well as a double knockout deficient in the production of both PIM(6) and the mannose caps on LAM (Delta pimE Delta capA). Compared to the wild-type strain, both mutant strains bound similarly well to DC-SIGN and DCs. Furthermore, the wild-type and mutant strains induced comparable levels of interleukin-10 and interleukin-12p40 when used to stimulate DCs. Hence, we conclude that, like ManLAM, PIM(6) represents a bona fide DC-SIGN ligand but that other, as-yet-unknown, ligands dominate in the interaction between mycobacteria and DCs.

Toll-like receptor 2 and DC-SIGNR1 differentially regulate suppressors of cytokine signaling 1 in dendritic cells during Mycobacterium tuberculosis infection

A hallmark of protective immunity during Mycobacterium tuberculosis (M. tb) infection is the regulated secretion of pro-inflammatory and regulatory cytokines. Suppressors of Cytokine Signaling (SOCS) are key regulators of cytokine secretion and function. In this study we investigated regulation of Toll-like receptor 2 (TLR2) and dendritic cell-specific ICAM-3 grabbing non-integrin receptor 1 (DC-SIGNR1)-mediated SOCS1 expression in DCs during M. tb infection. We show that, compared with TLR2, stimulating DC-SIGNR1 on DCs induces higher SOCS1 expression and lower interleukin-12 production. Co-stimulating DC-SIGNR1 and TLR2 differentially regulates SOCS1 expression depending on the relative concentration of their ligands. Stimulating DC-SIGNR1 with M. tb infection increases SOCS1 expression, while stimulating TLR2 with M. tb infection reduces SOCS1 expression. Knockdown of SOCS1 in DCs by siRNA enhances interleukin-12 transcription and protein expression upon DC-SIGNR1 stimulation. Raf-1 and Syk differentially regulate TLR2- and DC-SIGNR1-mediated SOCS1 expression. In addition, DC-SIGNR1 shows greater association with SOCS1 when compared with TLR2. Interestingly, compared with healthy asymptomatic individuals, peripheral blood mononuclear cells of patients with active tuberculosis disease showed higher expression of SOCS1, which was reduced following chemotherapy. Similarly, stimulating DC-SIGNR1 on DCs from M. tb-infected TLR2(-/-) mice enhanced SOCS1 expression that was reduced following chemotherapy. Further, knockdown of SOCS1 in mouse DCs or human peripheral blood mononuclear cells resulted in increased killing of virulent M. tb. These results indicate that TLR2 and DC-SIGNR1 differentially regulate SOCS1 expression during M. tb infection. This in turn regulates M. tb survival by governing key cytokine expression.

In situ IL-12/23p40 production during mycobacterial infection is sustained by CD11bhigh dendritic cells localized in tissue sites distinct from those harboring bacilli

Although IL-12/23p40 is known to play a major role in host resistance to Mycobacterium spp, the cellular source, tissue localization, and regulation of p40 production during mycobacterial infection in vivo has been unclear. In this study, we used IL-12/23p40eYFP (yet40) reporter mice to track expression of the cytokine following Mycobacterium bovis bacillus Calmette-Guérin (BCG) infection. We found that in spleens of these mice, p40 production is initiated by a transient burst from CD11b(low)CD11c(+) dendritic cells (DC) which are later replaced at the onset of granuloma formation by CD11b(high)CD11c(+) DC as the major source of the cytokine. The latter subset was also found to be the key producer of DC-derived p40 in nonlymphoid tissue and in both spleen and liver optimal production of the cytokine was regulated by endogenous TNF-alpha. Although BCG and p40-expressing DC were both observed in splenic white pulp, p40(+) DC rarely colocalized with bacilli. Indeed, in vitro flow cytometry and confocal microscopy indicated that the presence of intracellular bacteria is not required for p40 production by DC and Transwell experiments confirmed that soluble mycobacterial components are sufficient for inducing cytokine expression by these cells. Moreover, when stimulated with LPS, DC directly infected with BCG showed impaired IL-12p40 production in vitro. Together, our findings establish CD11b(high) DC as a major source of IL-12/23p40 during mycobacterial infection in situ and implicate both soluble mycobacterial products and TNF-alpha in stimulating sustained production of p40 by these cells.

Intracerebral Mycobacterium bovis bacilli Calmette-Guerin infection-induced immune responses in the CNS

To study whether cerebral mycobacterial infection induces granuloma and protective immunity similar to systemic infection, we intracerebrally infected mice with Mycobacterium bovis bacilli Calmette-Guerin. Granuloma and IFN-gamma(+)CD4(+) T cell responses are induced in the central nervous system (CNS) similar to periphery, but the presence of IFN-gammaIL-17 double-positive CD4(+) T cells is unique to the CNS. The major CNS source of TNF-alpha is microglia, with modest production by CD4(+) T cells and macrophage. Protective immunity is accompanied by accumulation of Foxp3(+)CD4(+) T cells and PD-L2(+) dendritic cells, suggesting that both inflammatory and anti-inflammatory responses develop in the CNS following mycobacterial infection.

Cell-mediated immune responses in tuberculosis

Tuberculosis is primarily a disease of the lung, and dissemination of the disease depends on productive infection of this critical organ. Upon aerosol infection with Mycobacterium tuberculosis (Mtb), the acquired cellular immune response is slow to be induced and to be expressed within the lung. This slowness allows infection to become well established; thus, the acquired response is expressed in an inflammatory site that has been initiated and modulated by the bacterium. Mtb has a variety of surface molecules that interact with the innate response, and this interaction along with the autoregulation of the immune response by several mechanisms results in less-than-optimal control of bacterial growth. To improve current vaccine strategies, we must understand the factors that mediate induction, expression, and regulation of the immune response in the lung. We must also determine how to induce both known and novel immunoprotective responses without inducing immunopathologic consequences.

Impaired phenotype and function of monocyte derived dendritic cells in pulmonary tuberculosis

Pulmonary tuberculosis (PTB) is often associated with impaired immunological functions. Blood monocytes, which can differentiate into dendritic cells upon cytokine stimulation, play a central role in adequate immune reactivity. Here, we investigated the morphologic, phenotypic and functional characteristics of in vitro-generated monocyte derived dendritic cells (MoDC) from PTB patients in comparison with healthy subjects. Phenotypic analysis revealed a defective differentiation of MoDC in PTB patients as assessed by a strong down regulation of CD1a, MHC II, CD80 and CD83 expression and impaired allostimulatory function under the influence of IL-4 and GM-CSF. In contrast, the expression of CD86 was not affected and remained same as in healthy subjects. Furthermore, the maturation status of lipopolysaccharide (LPS) stimulated MoDC was not optimal in PTB. However, the MoDC of PTB patients produced significantly higher levels of TNF-alpha and IL-6 but lower levels of IL-12 compared to healthy subjects. These findings suggest that there is a fundamental defect in the differentiation and maturation of dendritic cells during PTB that may compromise the antigen presentation and subsequent immune functions.

Foal monocyte-derived dendritic cells become activated upon Rhodococcus equi infection

Susceptibility of foals to Rhodococcus equi pneumonia is exclusive to the first few months of life. The objective of this study was to investigate the immediate immunologic response of foal and adult horse antigen-presenting cells (APCs) upon infection with R. equi. We measured the activation of the antigen-presenting major histocompatibility complex (MHC) class II molecule, costimulatory molecules CD40 and CD86, the cytokine interleukin-12 (IL-12), and the transcriptional factor interferon regulatory factor 1 (IRF-1) in monocyte-derived macrophages (mMOs) and dendritic cells (mDCs) of adult horses and foals of different ages (from birth to 3 months of age) infected with virulent R. equi or its avirulent, plasmid-cured derivative. Infection with virulent or avirulent R. equi induced (P <or= 0.01) the expression of IL-12p35 and IL-12p40 mRNAs in foal mMOs and mDCs at different ages. This response was likely mediated by the higher (P=0.008) expression of IRF-1 in foal mDCs at birth than in adult horse mDCs. R. equi infection promoted comparable expression of costimulatory molecules CD86 and CD40 in foal and adult horse cells. The cytokine and costimulatory response by foal mDCs was not accompanied by robust MHC class II molecule expression. These data suggest that foal APCs detect the presence of R. equi and respond with the expression of the Th1-inducing cytokine IL-12. Nevertheless, there seems to be a limitation to MHC class II molecule expression which we hypothesize may compromise the efficient priming of naïve effector cells in early life.

Evasion of peptide, but not lipid antigen presentation, through pathogen-induced dendritic cell maturation

Dendritic cells (DC) present lipid and peptide antigens to T cells on CD1 and MHC Class II (MHCII), respectively. The relative contribution of these systems during the initiation of adaptive immunity after microbial infection is not characterized. MHCII molecules normally acquire antigen and rapidly traffic from phagolysosomes to the plasma membrane as part of DC maturation, whereas CD1 molecules instead continually recycle between these sites before, during, and after DC maturation. We find that in Mycobacterium tuberculosis (Mtb)-infected DCs, CD1 presents antigens quickly. Surprisingly, rapid DC maturation results in early failure and delay in MHCII presentation. Whereas both CD1b and MHCII localize to bacterial phagosomes early after phagocytosis, MHCII traffics from the phagosome to the plasma membrane with a rapid kinetic that can precede antigen availability and loading. Thus, rather than facilitating antigen presentation, a lack of coordination in timing may allow organisms to use DC maturation as a mechanism of immune evasion. In contrast, CD1 antigen presentation occurs in the face of Mtb infection and rapid DC maturation because a pool of CD1 molecules remains available on the phagolysosome membrane that is able to acquire lipid antigens and deliver them to the plasma membrane.

Naive helper T cells from BCG-vaccinated volunteers produce IFN-gamma and IL-5 to mycobacterial antigen-pulsed dendritic cells

Mycobacterium bovis bacillus Calmette-Guérin (BCG) is a live vaccine that has been used in routine vaccination against tuberculosis for nearly 80 years. However, its efficacy is controversial. The failure of BCG vaccination may be at least partially explained by the induction of poor or inappropriate host responses. Dendritic cells (DCs) are likely to play a key role in the induction of immune response to mycobacteria by polarizing the reactivity of T lymphocytes toward a Th1 profile, contributing to the generation of protective cellular immunity against mycobacteria. In this study we aimed to investigate the production of Th1 and Th2 cytokines by naive CD4+ T cells to mycobacterial antigen-pulsed DCs in the group of young, healthy BCG vaccinated volunteers. The response of naive helper T cells was compared with the response of total blood lymphocytes. Our present results clearly showed that circulating naive CD45RA+CD4+ lymphocytes from BCG-vaccinated subjects can become effector helper cells producing IFN-gamma and IL-5 under the stimulation by autologous dendritic cells presenting mycobacterial protein antigen-PPD or infected with live M. bovis BCG bacilli.

Characterization of effector functions of human peptide-specific CD4+ T-cell clones for an intracellular pathogen

CD4+ T cells are believed to play a dominant role in human defenses against Mycobacterium tuberculosis through production of interferon (IFN)-gamma, cytolytic T-cell (CTL) activity, and inhibition of intracellular mycobacterial growth. Most functional studies of CD4+ cells have used bulk T-cells that recognize crude mycobacterial antigens, and the functional capacity of individual human T cells is not well defined. We studied the functional capacity of human CD4+ T-cell clones that recognize a specific mycobacterial peptide. Clone B9 produced high concentrations of IFN-gamma and exhibited potent CTL activity, whereas clone D3 produced IFN-gamma but showed poor CTL activity. The CTL activity of clone B9 was inhibited by SrCl(2) and concanamycin A but not by anti-Fas antibodies. Clone B9 also reduced the mycobacterial burden in dendritic cells by more than 90%, and this antimycobacterial activity was inhibited by SrCl(2) and concanamycin A. We conclude that: (1) individual human peptide-specific CD4+ T-cell clones have differential capacity to produce Th1 cytokines and to lyse M tuberculosis-infected target cells; and (2) both granulysin and perforin contribute to the capacity of human CD4+ T-cells to lyse infected targets and to inhibit intracellular mycobacterial growth.

Decreased pathology and prolonged survival of human DC-SIGN transgenic mice during mycobacterial infection

Dendritic cell (DC)-specific intercellular adhesion molecule-3 grabbing nonintegrin (DC-SIGN: CD209) is a C-type lectin that binds ICAM-2,3 and various pathogens such as HIV, helicobacter, and mycobacteria. It has been suggested that Mycobacterium tuberculosis, the causative agent of pulmonary tuberculosis, interacts with DC-SIGN to evade the immune system. To directly analyze the role of human DC-SIGN during mycobacterial infection, we generated conventional transgenic (tg) mice (termed "hSIGN") using CD209 cDNA under the control of the murine CD11c promoter. Upon mycobacterial infection, DCs from hSIGN mice produced significantly less IL-12p40 and no significant differences were be observed in the secretion levels of IL-10 relative to control DCs. After high dose aerosol infection with the strain M. tuberculosis H37Rv, hSIGN mice showed massive accumulation of DC-SIGN(+) cells in infected lungs, reduced tissue damage and prolonged survival. Based on our in vivo data, we propose that instead of favoring the immune evasion of mycobacteria, human DC-SIGN may have evolved as a pathogen receptor promoting protection by limiting tuberculosis-induced pathology.

Mycobacterium bovis BCG-infected neutrophils and dendritic cells cooperate to induce specific T cell responses in humans and mice

Neutrophils are increasingly thought to modulate dendritic cell (DC) functions. We investigated the role of the neutrophil-DC partnership in the response to Mycobacterium bovis BCG-the vaccine used against tuberculosis. We compared neutrophil-DC crosstalk in humans and mice, searching for functional differences. In both species, neutrophils captured fluorescent BCG-enhanced green fluorescent protein (EGFP) and were more phagocytic than DC. Non-apoptotic BCG-infected neutrophils clustered with immature DC, establishing intimate contacts with dendrites, at which fluorescent intact bacilli were observed. Physical interactions between neutrophils and DC were required for DC activation. Human BCG-infected DC produced interleukin (IL)-10, an inhibitory cytokine, whereas DC exposed to BCG-infected neutrophils produced low to undetectable amounts of the cytokine. Mouse BCG-infected neutrophils induced sustained IL-2 production by DC. Human DC exposed to BCG-infected neutrophils stimulated recall T cell reactivity from vaccinated donors. Mouse DC infected with recombinant ovalbumin (OVA)-producing BCG (rBCG(ova)) elicited proliferation of TCR-OVA-transgenic CD4 and CD8 T cells. Moreover, exposing DC to rBCG(ova)-infected neutrophils enhanced OVA presentation. Thus, in mice and humans, neutrophils help DC to cross-present BCG antigens to T cells. Our results suggest that this "ménage à trois" involving neutrophils, DC and T cells plays a major role in the immune response to BCG.

Mycobacterium tuberculosis antigens specifically modulate CCR2 and MCP-1/CCL2 on lymphoid cells from human pulmonary hilar lymph nodes

Macrophages and dendritic cells are involved in the immune response to Mycobacterium tuberculosis (Mtb). Such a response, although extensively studied using animal models and cells from human blood, has not been characterized in cells from pulmonary hilar lymph nodes (PHLN). We characterized populations of myeloid APC from PHLN and determined their expression of CCR2, CCR5, CCR7, CD40, CD54, CD80, and CD86 as well as the cytokine/chemokine microenvironment before and after purified protein derivative (PPD) and mannosilated lipoarabinomannan (ManLAM) stimulation. Results show that there are at least three APC populations in PHLN, defined as CD14highHLA-DRlow/-, CD14dimHLA-DRdim, and CD14-HLA-DRhigh/dendritic cells (DC), with the largest number represented by CD14dimHLA-DRdim cells (where dim indicates intermediate levels). CD14-HLA-DRhigh/DC expressed higher levels of costimulatory molecules and lower levels of CCR2 and CCR5, but all cell populations showed similar CCR7 levels. PPD and ManLAM specifically down-regulated CCR2 expression but not that of CCR5 and CCR7, and such down-regulation was observed on all APC populations. Mtb Ag did not affect the expression of costimulatory molecules. PPD but not ManLAM specifically induced MCP-1/CCL2 production, which was likely associated with the induction of IFN-gamma because this cytokine was highly induced by PPD. We characterized, for the first time, different APC from human PHLN and show that Mtb Ag exert fine and specific regulation of molecules closely associated with the immune response to Mtb infection. Because knowledge of this response in secondary lymphoid tissues is still poorly understood in humans, such studies are necessary and important for a better understanding of lymphoid cell microenvironment and migrating capacities and their role in the immunopathogenesis of tuberculosis.

A DNA vaccine against tuberculosis based on the 65 kDa heat-shock protein differentially activates human macrophages and dendritic cells

Background

A number of reports have demonstrated that rodents immunized with DNA vaccines can produce antibodies and cellular immune responses presenting a long-lasting protective immunity. These findings have attracted considerable interest in the field of DNA vaccination. We have previously described the prophylactic and therapeutic effects of a DNA vaccine encoding the Mycobacterium leprae 65 kDa heat shock protein (DNA-HSP65) in a murine model of tuberculosis. As DNA vaccines are often less effective in humans, we aimed to find out how the DNA-HSP65 stimulates human immune responses.

Methods

To address this question, we analysed the activation of both human macrophages and dendritic cells (DCs) cultured with DNA-HSP65. Then, these cells stimulated with the DNA vaccine were evaluated regarding the expression of surface markers, cytokine production and microbicidal activity.

Results

It was observed that DCs and macrophages presented different ability to uptake DNA vaccine. Under DNA stimulation, macrophages, characterized as CD11b⁺/CD86⁺/HLA-DR⁺, produced high levels of TNF-alpha, IL-6 (pro-inflammatory cytokines), and IL-10 (anti-inflammatory cytokine). Besides, they also presented a microbicidal activity higher than that observed in DCs after infection with M. tuberculosis. On the other hand, DCs, characterized as CD11c⁺/CD86⁺/CD123^-/BDCA-4⁺/IFN-alpha^-, produced high levels of IL-12 and low levels of TNF-alpha, IL-6 and IL-10. Finally, the DNA-HSP65 vaccine was able to induce proliferation of peripheral blood lymphocytes.

Conclusion

Our data suggest that the immune response is differently activated by the DNA-HSP65 vaccine in humans. These findings provide important clues to the design of new strategies for using DNA vaccines in human immunotherapy.

Probing host pathogen cross-talk by transcriptional profiling of both Mycobacterium tuberculosis and infected human dendritic cells and macrophages

Background

Transcriptional profiling using microarrays provides a unique opportunity to decipher host pathogen cross-talk on the global level. Here, for the first time, we have been able to investigate gene expression changes in both Mycobacterium tuberculosis, a major human pathogen, and its human host cells, macrophages and dendritic cells.

Methodology/Principal Findings

In addition to common responses, we could identify eukaryotic and microbial transcriptional signatures that are specific to the cell type involved in the infection process. In particular M. tuberculosis shows a marked stress response when inside dendritic cells, which is in accordance with the low permissivity of these specialized phagocytes to the tubercle bacillus and to other pathogens. In contrast, the mycobacterial transcriptome inside macrophages reflects that of replicating bacteria. On the host cell side, differential responses to infection in macrophages and dendritic cells were identified in genes involved in oxidative stress, intracellular vesicle trafficking and phagosome acidification.

Conclusions/Significance

This study provides the proof of principle that probing the host and the microbe transcriptomes simultaneously is a valuable means to accessing unique information on host pathogen interactions. Our results also underline the extraordinary plasticity of host cell and pathogen responses to infection, and provide a solid framework to further understand the complex mechanisms involved in immunity to M. tuberculosis and in mycobacterial adaptation to different intracellular environments.

Effects of DNA- and Mycobacterium bovis BCG-based delivery of the Flt3 ligand on protective immunity to Mycobacterium tuberculosis

The control of intracellular pathogens such as Mycobacterium tuberculosis is dependent on the activation and maintenance of pathogen-reactive T cells. Dendritic cells (DCs) are the major antigen-presenting cells initiating antimycobacterial T-cell responses in vivo. To investigate if immunization strategies that aim to optimize DC function can improve protective immunity against virulent mycobacterial infection, we exploited the ability of the hematopoietic growth factor Fms-like tyrosine kinase 3 ligand (Flt3L) to expand the number of DCs in vivo. A DNA fusion of the genes encoding murine Flt3L and M. tuberculosis antigen 85B stimulated enhanced gamma interferon (IFN-gamma) release by T cells and provided better protection against virulent M. tuberculosis than DNA encoding the single components. Vaccination of mice with a recombinant Mycobacterium bovis BCG strain secreting Flt3L (BCG:Flt3L) led to early expansion of DCs compared to immunization with BCG alone, and this effect was associated with increased stimulation of BCG-reactive IFN-gamma-secreting T cells. BCG and BCG:Flt3L provided similar protective efficacies against low-dose aerosol M. tuberculosis; however, immunization of immunodeficient mice revealed that BCG:Flt3L was markedly less virulent than conventional BCG. These results demonstrate the potential of in vivo targeting of DCs to improve antimycobacterial vaccine efficacy.

Non-pathogenic Mycobacterium smegmatis induces the differentiation of human monocytes directly into fully mature dendritic cells

Mycobacterium smegmatis infects human monocytes that can be precursors of dendritic cells. We tested whether the interaction of M. smegmatis with monocytes modulated their differentiation into dendritic cells. We found that M. smegmatis-infected monocytes differentiated into CD1a-CCR7+ dendritic cells in the presence of GM-CSF and IL-4 and acquired a mature phenotype since they expressed CD83 molecules in the absence of maturation stimuli. Dendritic cells derived from M. smegmatis-infected monocytes stimulated with bacterial products, produced IL-10 and still retained the capacity to produce IL-12. Consequently, they polarized naïve T lymphocytes towards a mixed Th1/Th2 immune response inducing both IFN-gamma and IL-4 production. These findings suggest that the exposure to environmental mycobacteria could modulate the differentiation of dendritic cells making them able to migrate into secondary lymphoid organs and modulate the adaptive immune response. This could explain one of the mechanisms by which environmental mycobacteria can influence the immune response to pathogenic species.