Dendritic cells and the complexity of microbial infection

Interaction between saliva's adenosine and tick parasitism: effects on feeding and reproduction

Background

It has recently been demonstrated that saliva from Rhipicephalus sanguineus ticks contains adenosine (ADO) and prostaglandin E2 (PGE2), two non-protein molecules that have significant immunomodulatory properties. These molecules can inhibit cytokine production by dendritic cells (DCs), while also reducing the expression of CD40 in these cells. However, more studies are needed for a better understanding of their participation in the feeding of ticks in vivo. This work, therefore, evaluated the importance of ADO during tick infestations. Mice were infested with adult ticks (3 couples/mouse), and their skin was collected at the tick-infested site (3rd and 7th day), and mRNA for receptors of ADO was quantified by real-time PCR.

Results

Tick infestation increased by four and two times the expression of the A2b and A3v1 receptors on day 3, respectively, while expression of other ADO receptors was unaltered. In addition, we treated mice (n = 10/group) daily with 8-(p-Sulfophenyl)theophylline, 8-pSPT, 20 mg/kg, i.p.), a non-selective antagonist of ADO receptors, and evaluated the performance of ticks during infestations. Female ticks fed on 8-pSPT-treated mice presented a reduction in their engorgement, weight and hatching rates of egg masses, and survival times of larvae compared to the same parameters presented by ticks in the control group. To investigate if these 8-pSPT-treated mice presented altered immune responses, we performed three tick infestations and collected their lymph node cells to determine the percentages and activation state of DCs and cytokine production by lymphocytes by flow cytometry (Cytometric Bead Array technique, CBA). Our data showed that 8-pSPT-treated mice presented an increase in the percentage of DCs as well as of their stimulatory and co-stimulatory molecules (CD40, CD80 and MHCII). Regarding production of T cell cytokines, we observed a significant increase in the levels of IL-2 and a significant decrease in IL-10, IL-17, TNF-α and IFN-γ cytokines.

Conclusions

These results suggest that ADO produced by ticks helps them feed and reproduce and that this effect may be due to modulation of host DCs and T cells.

Interaction of a live attenuated Salmonella Gallinarum vaccine candidate with chicken bone marrow-derived dendritic cells

Salmonella enterica serovar Gallinarum (SG) is a Gram-negative intracellular host-adapted pathogen that causes fowl typhoid. Attenuated strains of SG are proven and widely used vaccine candidates because of advantages like induction of strong humoral and cell-mediated immune responses. In the present study, we investigated the interaction of chicken bone marrow-derived dendritic cells (chBM-DCs) with an attenuated SG (JOL1355) strain that secretes a heat-labile enterotoxin B subunit protein previously shown to successfully vaccinate chickens. ChBM-DCs were isolated and cultured in the presence of recombinant chicken GM-CSF and IL-4 cytokines. The chBM-DCs were infected with JOL1355 at an multiplicity of infection of 10. JOL1355 was able to invade dendritic cells (DCs); however, the survival of JOL1355 in DCs decreased over time. At 24 h post infection, IL-6, IL-10 and IFN-γ transcript levels were significantly increased in JOL1355-infected DCs compared to non-stimulated DCs. Flow cytometry analysis showed an increased proportion of cells producing CD40, CD80, and MHC class II in the JOL1355-infected cultures compared to the non-stimulated control. In addition, JOL1355-stimulated chBM-DCs could induce significant expression of IL-2 in co-culture with autologous CD4+ T cells. Based on these results, we conclude that chBM-DCs are capable of internalizing the live attenuated SG vaccine candidate and the infected chBM-DCs show signs of maturation as evidenced by the upregulated expression of costimulatory molecules and cytokines.

Salmonella as an Inducer of Autoimmunity

A clear etiological link has been established between infection with several gram-negative enteric pathogens, including Salmonella spp., and the incidence of reactive arthritis (ReA), an autoimmune disease that largely affects the joints. ReA is sometimes referred to as Reiter's syndrome, particularly when accompanied by uveitis and urethritis. This review reviews the evidence etiologically linking Salmonella infection with autoimmune disease and addresses the roles that bacterial and host elements play in controlling disease outcome. ReA is an autoimmune disease that largely consists of painful joint inflammation but also can include inflammation of the eye, gastrointestinal tract, and skin. ReA is a member of a broad spectrum of chronic inflammatory disorders termed the seronegative spondyloarthropathies (SNSpAs) that includes ankylosing spondylitis (AS), psoriatic arthritis, and enteropathic arthritis. Salmonella species, as well as other enteric pathogens associated with postgastroenteritis ReA, are facultative intracellular gram-negative bacteria. Many studies have analyzed the association of the HLA class I molecule, HLA-B27, with SNSpAs. Whereas B27 has been shown to be present in 90 to 95% of cases of AS, the association of the B27 haplotype with other SNSpAs is more tenuous. The clear association between ReA and infection with Salmonella or other gram-negative enteric pathogens has led to the suggestion that the adaptive immune response to infection has an autoimmune component. In addition to various Salmonella species, other gram-negative enteric pathogens have been linked to the development of ReA. Given their close relationship to Salmonella, this review considers the involvement of Shigella species in ReA.

Ebola virus infection induces irregular dendritic cell gene expression

Filoviruses subvert the human immune system in part by infecting and replicating in dendritic cells (DCs). Using gene arrays, a phenotypic profile of filovirus infection in human monocyte-derived DCs was assessed. Monocytes from human donors were cultured in GM-CSF and IL-4 and were infected with Ebola virus Kikwit variant for up to 48 h. Extracted DC RNA was analyzed on SuperArray's Dendritic and Antigen Presenting Cell Oligo GEArray and compared to uninfected controls. Infected DCs exhibited increased expression of cytokine, chemokine, antiviral, and anti-apoptotic genes not seen in uninfected controls. Significant increases of intracellular antiviral and MHC I and II genes were also noted in EBOV-infected DCs. However, infected DCs failed to show any significant difference in co-stimulatory T-cell gene expression from uninfected DCs. Moreover, several chemokine genes were activated, but there was sparse expression of chemokine receptors that enabled activated DCs to home to lymph nodes. Overall, statistically significant expression of several intracellular antiviral genes was noted, which may limit viral load but fails to stop replication. EBOV gene expression profiling is of vital importance in understanding pathogenesis and devising novel therapeutic treatments such as small-molecule inhibitors.

MAP of F1 and V antigens from Yersinia pestis astride innate and adaptive immune response

Yersinia pestis, a causative agent of plague, has a plethora of armors to fight against major components of innate immunity and survive within host cells. Dendritic cells and macrophages are important antigen presenting cells for effective immune response. This report is focused on the changes in DC activation and TLR2 and TLR4 expression on macrophages induced by MAP of F1 and V antigens of Y. pestis. F1 and V MAPs bear potential synthetic T and B cell epitopes from F1 and V protein respectively. We evaluated these parameters in DC's isolated from spleen and lamina propria and macrophages isolated from peritoneal lavage of mice after intranasal immunization. F1 MAP and V MAP significantly increased the expression of CD80 and CD86 on CD11c(+) dendritic cells isolated from spleen and lamina propria as well as intracellular IL-12 levels. Similarly, in macrophages derived from peritoneal cavity, the above formulation enhanced TLR2 and TLR4 expression. Again after in vitro stimulation with F1 and V MAP these macrophages produced significantly high IL12 and TNFα. The study clearly indicates involvement of DC and macrophages for efficient antigen presentation to immune cells. From this study we conclude that F1MAP and VMAP ameliorate innate immune mechanism. These two synthetic constructs exert their effect via TLR2 and TLR4, leading to the production of proinflammatory cytokines by macrophages and are able to increase DC activation, that could be helpful in generation of adaptive immunity as well as is important strong immune response.

The MAPK-Activated Kinase MK2 Attenuates Dendritic Cell-Mediated Th1 Differentiation and Autoimmune Encephalomyelitis

Dendritic cell (DC)-mediated inflammation induced via TLRs is promoted by MAPK-activated protein kinase (MK)-2, a substrate of p38 MAPK. In this study we show an opposing role of MK2, by which it consolidates immune regulatory functions in DCs through modulation of p38, ERK1/2-MAPK, and STAT3 signaling. During primary TLR/p38 signaling, MK2 mediates the inhibition of p38 activation and positively cross-regulates ERK1/2 activity, leading to a reduction of IL-12 and IL-1α/β secretion. Consequently, MK2 impairs secondary autocrine IL-1α signaling in DCs, which further decreases the IL-1α/p38 but increases the anti-inflammatory IL-10/STAT3 signaling route. Therefore, the blockade of MK2 activity enables human and murine DCs to strengthen proinflammatory effector mechanisms by promoting IL-1α-mediated Th1 effector functions in vitro. Furthermore, MK2-deficient DCs trigger Th1 differentiation and Ag-specific cytotoxicity in vivo. Finally, wild-type mice immunized with LPS in the presence of an MK2 inhibitor strongly accumulate Th1 cells in their lymph nodes. These observations correlate with a severe clinical course in DC-specific MK2 knockout mice compared with wild-type littermates upon induction of experimental autoimmune encephalitis. Our data suggest that MK2 exerts a profound anti-inflammatory effect that prevents DCs from prolonging excessive Th1 effector T cell functions and autoimmunity.

Dietary supplementation with white button mushrooms augments the protective immune response to Salmonella vaccine in mice

We previously showed that dietary white button mushrooms (WBMs) enhanced natural killer cell activity and that in vitro WBM supplementation promotes maturation and function of dendritic cells (DCs). The current study investigated whether WBM consumption would enhance pathogen-specific immune response using a Salmonella vaccination and infection animal model. C57BL/6 mice were fed diets containing 0%, 2%, or 5% WBM for 4 wk before oral vaccination with live attenuated Salmonella typhimurium SL1479. Four weeks after immunization, mice were orally infected with virulent Salmonella typhimurium SL1344. Immunization increased animal survival and, among immunized mice, the 2% WBM group had a higher survival rate than the other groups. Next, we fed mice 2% WBMs to determine the immunological mechanism underlying the WBM-potentiated protective effect. We found that WBM supplementation increased Salmonella-specific blood immunoglobulin (Ig) G and fecal IgA concentrations. WBM-fed mice also had a higher IgG2a and unchanged IgG1 production, leading to an elevated IgG2a:IgG1 ratio and indicating an enhanced T helper 1 response. Consistent with these results, WBM-fed mice had higher interferon-γ, tumor necrosis factor (TNF)-α, and interleukin (IL)-17A production and unchanged IL-4 production in their splenocytes after polyclonal (anti-CD3/CD28) or antigen-specific stimulation. Furthermore, WBM-fed mice had more DCs in the spleen, and these DCs expressed higher levels of activation markers CD40 and major histocompatibility complex-II. These mice also produced more IL-12 and TNF-α postimmunization. Together, these results suggest that WBMs may improve Salmonella vaccine efficacy through an enhanced adaptive immune response.

Heat shock protein profiles on the protein and gene expression levels in olive flounder kidney infected with Streptococcus parauberis

Heat shock proteins (HSPs) have been observed in cells exposed to a variety of stresses, including infectious pathogens. This study used a label-free, quantitative proteomic approach and transcriptional gene expression analysis to investigate infection-related HSP proteins and their encoding genes in whole kidneys from olive flounder (Paralichthys olivaceus). During Streptococcus parauberis infection in the flounder, the genes encoding Hsp10, Hsp40A4, Hsp40B6, Hsp40B11, Hsp60, Hsp70, glucose regulated protein 78 (Grp78), Hsp90α, Hsp90β and Grp94 were induced, and the protein levels of Hsp60, Hsp70, Hsp90α, Hsp90β and Grp94 were differentially regulated over time. Subsequent results also revealed that Hsp60, Hsp70, Hsp90α, Hsp90β and Grp94 appear to be the dominant and critical HSPs in olive flounder during bacterial infection. This is the first estimation of the differential involvement of HSPs in the immune response of olive flounder exposed to bacterial infection.

Mechanisms used by virulent Salmonella to impair dendritic cell function and evade adaptive immunity

Innate and adaptive immunity are inter-related by dendritic cells (DCs), which directly recognize bacteria through the binding of pathogen-associated molecular patterns (PAMPs) to specialized receptors on their surface. After capturing and degrading bacteria, DCs present their antigens as small peptides bound to MHC molecules and prime naive bacteria-specific T cells. In response to PAMP recognition DCs undergo maturation, which is a phenotypic change that increases their immunogenicity and promotes the activation of naive T cells. As a result, a specific immune response that targets bacteria-derived antigens is initiated. Therefore, the characterization of DC-bacteria interactions is important to understand the mechanisms used by virulent bacteria to avoid adaptive immunity. Furthermore, any impairment of DC function might contribute to bacterial survival and dissemination inside the host. An example of a bacterial pathogen capable of interfering with DC function is Salmonella enterica serovar Typhimurium (S. Typhimurium). Virulent strains of this bacterium are able to differentially modulate the entrance to DCs, avoid lysosomal degradation and prevent antigen presentation on MHC molecules. These features of virulent S. Typhimurium are controlled by virulence proteins, which are encoded by pathogenicity islands. Modulation of DC functions by these gene products is supported by several studies showing that pathogenesis might depend on this attribute of virulent S. Typhimurium. Here we discuss some of the recent data reported by the literature showing that several virulence proteins from Salmonella are required to modulate DC function and the activation of host adaptive immunity.

Immunomodulatory activities of surface-layer proteins obtained from epidemic and hypervirulent Clostridium difficile strains

Surface-layer proteins (SLPs) have been detected in all Clostridium difficile strains and play a role in adhesion, although an involvement in the inflammatory process may also be supposed, as they cover the bacterial surface and are immunodominant antigens. The aim of this study was to evaluate the immunomodulatory properties of SLPs obtained from hypervirulent and epidemic (H/E) or non-H/E C. difficile strains, to try to determine whether they contribute to hypervirulence. SLPs were purified from H/E PCR ribotype 027 and 001 and non-H/E PCR ribotype 012 C. difficile strains, and the ability to modulate these properties was studied in human ex vivo models of monocytes and monocyte-derived dendritic cells (MDDCs). The results indicated that SLPs were able to induce immunomodulatory cytokines [interleukin (IL)-1β, IL-6 and IL-10] in monocytes. SLPs induced maturation of MDDCs, which acquired enhanced antigen-presenting activity, a crucial function of the mature stage. SLP-primed MDDCs expressed high levels of IL-10, an important regulatory cytokine. No significant differences were found in the activation induced in monocytes and MDDCs by SLP preparations from H/E and non-H/E strains. Overall, these findings show an important role for SLPs in modulation of the immune response to C. difficile. However, SLPs from H/E strains did not show a specific immunomodulatory pattern compared with SLPs from non-H/E strains, suggesting that SLPs are not involved in the increased severity of infection peculiar to H/E strains.

Salmonella pathogenicity island 1 differentially modulates bacterial entry to dendritic and non-phagocytic cells

Salmonella enterica serovar Typhimurium can enter non-phagocytic cells, such as intestinal epithelial cells, by virtue of a Type Three Secretion System (TTSS) encoded in the Salmonella Pathogenicity Island 1 (SPI-1), which translocates bacterial effector molecules into the host cell. Salmonella can also be taken up by dendritic cells (DCs). Although the role of SPI-1 in non-phagocytic cell invasion is well established, its contribution to invasion of phagocytic cells has not been evaluated. Here, we have tested the invasive capacity of a S. Typhimurium strain lacking a key component of its TTSS-1 (DeltaInvC) leading to defective translocation of SPI-1-encoded effectors. Whereas this mutant Salmonella strain was impaired for invasion of non-phagocytic cells, it was taken up by DCs at a significantly higher rate than wild-type Salmonella. Similar to wild-type Salmonella, the DeltaInvC mutant strain retained the capacity to avoid antigen presentation to T cells. However, mice infected with the DeltaInvC mutant strain showed higher survival rate and reduced organ colonization. Our data suggest that, besides promoting phagocytosis by non-phagocytic cells, SPI-1 modulates the number of bacteria that enters DCs. The SPI-1 could be considered not only as an inducer of epithelial cell invasion but as a controller of DC entry.

Bordetella pertussis commits human dendritic cells to promote a Th1/Th17 response through the activity of adenylate cyclase toxin and MAPK-pathways

The complex pathology of B. pertussis infection is due to multiple virulence factors having disparate effects on different cell types. We focused our investigation on the ability of B. pertussis to modulate host immunity, in particular on the role played by adenylate cyclase toxin (CyaA), an important virulence factor of B. pertussis. As a tool, we used human monocyte derived dendritic cells (MDDC), an ex vivo model useful for the evaluation of the regulatory potential of DC on T cell immune responses. The work compared MDDC functions after encounter with wild-type B. pertussis (BpWT) or a mutant lacking CyaA (BpCyaA-), or the BpCyaA- strain supplemented with either the fully functional CyaA or a derivative, CyaA*, lacking adenylate cyclase activity. As a first step, MDDC maturation, cytokine production, and modulation of T helper cell polarization were evaluated. As a second step, engagement of Toll-like receptors (TLR) 2 and TLR4 by B. pertussis and the signaling events connected to this were analyzed. These approaches allowed us to demonstrate that CyaA expressed by B. pertussis strongly interferes with DC functions, by reducing the expression of phenotypic markers and immunomodulatory cytokines, and blocking IL-12p70 production. B. pertussis-treated MDDC promoted a mixed Th1/Th17 polarization, and the activity of CyaA altered the Th1/Th17 balance, enhancing Th17 and limiting Th1 expansion. We also demonstrated that Th1 effectors are induced by B. pertussis-MDDC in the absence of IL-12p70 through an ERK1/2 dependent mechanism, and that p38 MAPK is essential for MDDC-driven Th17 expansion. The data suggest that CyaA mediates an escape strategy for the bacterium, since it reduces Th1 immunity and increases Th17 responses thought to be responsible, when the response is exacerbated, for enhanced lung inflammation and injury.

Modulation of pathogen-induced CCL20 secretion from HT-29 human intestinal epithelial cells by commensal bacteria

Background

Human intestinal epithelial cells (IECs) secrete the chemokine CCL20 in response to infection by various enteropathogenic bacteria or exposure to bacterial flagellin. CCL20 recruits immature dendritic cells and lymphocytes to target sites. Here we investigated IEC responses to various pathogenic and commensal bacteria as well as the modulatory effects of commensal bacteria on pathogen-induced CCL20 secretion. HT-29 human IECs were incubated with commensal bacteria (Bifidobacterium infantis or Lactobacillus salivarius), or with Salmonella typhimurium, its flagellin, Clostridium difficile, Mycobacterium paratuberculosis, or Mycobacterium smegmatis for varying times. In some studies, HT-29 cells were pre-treated with a commensal strain for 2 hr prior to infection or flagellin stimulation. CCL20 and interleukin (IL)-8 secretion and nuclear factor (NF)-κB activation were measured using enzyme-linked immunosorbent assays.

Results

Compared to untreated cells, S. typhimurium, C. difficile, M. paratuberculosis, and flagellin activated NF-κB and stimulated significant secretion of CCL20 and IL-8 by HT-29 cells. Conversely, B. infantis, L. salivarius or M. smegmatis did not activate NF-κB or augment CCL20 or IL-8 production. Treatment with B. infantis, but not L. salivarius, dose-dependently inhibited the baseline secretion of CCL20. In cells pre-treated with B. infantis, C. difficile-, S. typhimurium-, and flagellin-induced CCL20 were significantly attenuated. B. infantis did not limit M. Paratuberculosis-induced CCL20 secretion.

Conclusion

This study is the first to demonstrate that a commensal strain can attenuate CCL20 secretion in HT-29 IECs. Collectively, the data indicate that M. paratuberculosis may mediate mucosal damage and that B. infantis can exert immunomodulatory effects on IECs that mediate host responses to flagellin and flagellated enteric pathogens.

[IL-2/IL-2R pathway in dendritic cell modulates both their cytokine synthesis profiles and their capacity to activate allogeneic CD4+ T lymphocytes]

The results provide new insights into the role of IL-2/IL-2R pathway in DC. We report that stimulation of human monocyte-derived DC with LPS strongly upregulated CD25 (α chain of the IL-2R) expression. In addition, by using a humanized monoclonal antibody against CD25, we demonstrated that the IL-2 signalling in DC upregulated both IL-12 and γIFN production but decreased IL-10 synthesis. Anti-CD25 treatment reduced the ability of LPS-DC to induce allogeneic CD4(+) T cell proliferation as compared to LPS-matured DC. In addition, LPS-matured DC treated with IL-2 had a higher allostimulatory capacity compared to LPS-DC. We also found that LPS-matured DC produced IL-2. Thus, IL-2 seems to contribute actively to DC activation through an autocrine pathway. Moreover, IL-2 pathway in DC is involved in T helper priming. These findings might be useful for protocols in cellular therapy and a valuable tool to understand graft rejection versus the acquisition of peripheral tolerance.

Involvement of dendritic cells in the pathogenesis of inflammatory bowel disease

In conclusion, during inflammation, DCs are likely activated by inflammatory signals and induced to migrate to T cell zones of organized lymphoid tissues where the cells induce T cell responses. In addition to their established role in T cell priming and the induction of tolerance, DCs may act to enhance (or possibly suppress) T cell responses at sites of mucosal inflammation. Determining the importance of DCs in this regard, as well as establishing a potential role for DCs in continuous activation of naive or central memory cells in lymph nodes draining inflammatory sites, will elucidate the role of DCs as a potential therapeutic target for chronic inflammatory diseases, like IBD. Resident intestinal macrophages are noninflammatory and do not efficiently present antigens to intestinal T cells, yet are avidly phagocytic and able to kill internalized organisms. During intestinal inflammation, monocytes are recruited from the blood, become inflammatory macrophages in the inflamed tissue, and are major contributors to tissue destruction and perpetuation of inflammation via their production of chemokines and pro-inflammatory cytokines. Macrophages may also contribute directly to DC activation and maturation, which would drive DCs to present antigens from the bacterial flora to T cells locally within tissue or to more efficiently traffic to T cell zones of lymphoid tissue. Thus, DCs and macrophages have evolved functional niches that promote cooperation in the prevention of untoward intestinal inflammation in the steady state and in the eradication of invasive microorganisms during infection. The balance between suppressing inflammation and promoting host defense is altered in humans with IBD allowing a persistent inflammatory response to commensal bacteria. Based on studies from animal models, the pathogenesis of IBD likely involves either the lack of appropriate regulation from T cells, or an over-production of effector T cells. The end result of these potential mechanisms is the abnormal induction and/or survival of effector T cells and the production of factors such as cytokines by inflammatory macrophages and neutrophils that result in tissue destruction. The destructive process likely involves normally tolerizing DCs, which in the microenvironment of the inflamed mucosa activate T cell responses to normal flora in both draining lymphoid tissues and at sites of inflammation, with macrophages and neutrophils contributing the bulk of inflammatory and destructive cytokines.

Down-modulation of TCR expression by Salmonella enterica serovar Typhimurium

T cell-mediated adaptive immunity is required to help clear infection with the facultative intracellular bacterial pathogen Salmonella enterica serovar Typhimurium (S. Typhimurium), yet development of T cell-mediated adaptive immunity to S. Typhimurium has been described as slow and inefficient. A key step in inducing T cell-mediated adaptive immunity is T cell priming; the activation, proliferation, and differentiation of naive T cells following initial encounter with Ag. We previously demonstrated that S. Typhimurium had a direct inhibitory effect on naive T cells from mouse, blocking their proliferation. In this study, we show that S. Typhimurium down-modulates expression of the TCR beta-chain, a molecule that is essential for Ag recognition and T cell function. Specifically, we demonstrate that reduced amounts of surface and intracellular TCR-beta protein and decreased levels of tcrbeta transcript are expressed by T cells cultured in the presence of S. Typhimurium. We further show that the down-modulation of TCR-beta expression requires contact between S. Typhimurium and the T cells and that once contact occurs, a factor capable of reducing TCR-beta expression is secreted. These results provide new insight into the mechanism by which S. Typhimurium may inhibit T cell priming and avoid clearance by the adaptive immune system.

Monocyte-derived dendritic cells from Crohn patients show differential NOD2/CARD15-dependent immune responses to bacteria

BACKGROUND

Three common mutations in the NOD2/CARD15 gene are strongly associated with Crohn's disease (CD). NOD2 is an intracellular receptor of muramyl dipeptide (MDP), a component of peptidoglycan present in the cell wall of gram-positive (G+) and gram-negative (G-) bacteria.

METHODS

We generated monocyte-derived dendritic cells (MoDCs) from CD patients mutated or not for CARD15 (n = 53) or from healthy donors (n = 12) and analyzed their activation in response to live Salmonella typhimurium as a model of pathogenic G- bacteria.

RESULTS

MoDCs carrying the L1007fs mutation, although phenotypically activated by bacteria, produced a significantly reduced amount of tested cytokines. MoDCs carrying R702W or compound G908R/R702W NOD2 mutations displayed an increased basal level of IL-8 release. After a bacterial encounter, these cells were phenotypically activated and produced levels of cytokines similar to healthy controls. Interestingly, although L1007fs/WT mutations conferred reduced production of cytokines, including IL-12, these cells were perfectly capable of inducing T-cell polarization toward the Th1 phenotype.

CONCLUSIONS

NOD2 mutations affect the basal characteristics of MoDCs and their response to G- bacteria differently. MoDCs could be involved in CD onset because they have defects in releasing inflammatory cytokines and in polarizing T-cell responses.

Wild-type Leishmania donovani promastigotes block maturation, increase integrin expression and inhibit detachment of human monocyte-derived dendritic cells--the influence of phosphoglycans

The protective immune response against the parasite, including the role of dendritic cells (DC) in the course of infection, plays a fundamental role. This study shows that wild-type (WT) Leishmania promastigotes and specifically the phosphoglycans family of virulence-associated antigens inhibit human monocyte-derived dendritic cells (MoDC) maturation and detachment to distinct surfaces. Immature phagocytosis of Leishmania donovani promastigotes by immature MoDC results in the increased expression of CD11b and CD51, and inhibition of cell detachment to distinct surfaces, which was dependent on the presence of phosphoglycans. These findings demonstrate that phosphoglycans of WT L. donovani might also inhibit human DC migration to lymphoid organs.

The capacity of Salmonella to survive inside dendritic cells and prevent antigen presentation to T cells is host specific

Infection with Salmonella enterica serovar Typhimurium (S. Typhimurium) causes a severe and lethal systemic disease in mice, characterized by poor activation of the adaptive immune response against Salmonella-derived antigens. Recently, we and others have reported that this feature relies on the ability of S. Typhimurium to survive within murine dendritic cells (DCs) and avoid the presentation of bacteria-derived antigens to T cells. In contrast, here we show that infection of murine DCs with either S. Typhi or S. Enteritidis, two serovars adapted to different hosts, leads to an efficient T-cell activation both in vitro and in vivo. Accordingly, S. Typhi and S. Enteritidis failed to replicate within murine DCs and were quickly degraded, allowing T-cell activation. In contrast, human DCs were found to be permissive for survival and proliferation of S. Typhi, but not for S. Typhimurium or S. Enteritidis. Our data suggest that Salmonella host restriction is characterized by the ability of these bacteria to survive within DCs and avoid activation of the adaptive immune response in their specific hosts.

Anti CD25 treatment of human dendritic cells modulates both their cytokine synthesis profiles and their capacity to activate allogeneic CD4 T cells: a potential tolerogenic effect

Recently, attention has been focused on the role for dendritic cells (DCs) in the promotion of peripheral tolerance. It is currently believed that the maturation/activation state of DCs might be a control point for the induction of graft tolerance through modifications of the activation state of T cells. We have observed IL-2, and IL-2 receptor gene expression by reverse transcriptase-polymerase chain reaction (RT-PCR) in human DC lysates following bacterial stimulation. It has been demonstrated in the present study IL-2R alpha (CD25) expression on human DCs upon LPS activation. DCs differentiated from monocytes were exposed to anti CD25 during maturation, anti CD25 treatment affected the abilities of human DCs to induce CD4+ T cell proliferation in response to alloantigens, maintained endocytic capacity, Anti CD25 treated DCs produce low levels of IL-12 and IFN and high level of IL-10. All these characteristics suggest that DCs may be used in cellular therapy either to induce allograft tolerance (anti CD25 treated DCs) or to restore immunity against tumors (IL-2 treated DCs).

Haemophilus ducreyi partially activates human myeloid dendritic cells

Dendritic cells (DC) orchestrate innate and adaptive immune responses to bacteria. How Haemophilus ducreyi, which causes genital ulcers and regional lymphadenitis, interacts with DC is unknown. H. ducreyi evades uptake by polymorphonuclear leukocyte and macrophage-like cell lines by secreting LspA1 and LspA2. Many H. ducreyi strains express cytolethal distending toxin (CDT), and recombinant CDT causes apoptosis of DC in vitro. Here, we examined interactions between DC and H. ducreyi 35000HP, which produces LspA1, LspA2, and CDT. In human volunteers infected with 35000HP, the ratio of myeloid DC to plasmacytoid DC was 2.8:1 in lesions, compared to a ratio of 1:1 in peripheral blood. Using myeloid DC derived from monocytes as surrogates for lesional DC, we found that DC infected with 35000HP remained as viable as uninfected DC for up to 48 h. Gentamicin protection and confocal microscopy assays demonstrated that DC ingested and killed 35000HP, but killing was incomplete at 48 h. The expression of LspA1 and LspA2 did not inhibit the uptake of H. ducreyi, despite inactivating Src kinases. Infection of DC with live 35000HP caused less cell surface marker activation than infection with heat-killed 35000HP and lipopolysaccharide (LPS) and inhibited maturation by LPS. However, infection of DC with live bacteria caused the secretion of significantly higher levels of interleukin-6 and tumor necrosis factor alpha than infection with heat-killed bacteria and LPS. The survival of H. ducreyi in DC may provide a mechanism by which the organism traffics to lymph nodes. Partial activation of DC may abrogate the establishment of a full Th1 response and an environment that promotes phagocytosis.

T cell immunity evasion by virulent Salmonella enterica

Salmonella enterica are Gram-negative bacteria that cause systemic disease in their specific hosts. One of the recently appreciated features of Salmonella pathogenicity is the capacity of the bacteria to impair host adaptive immunity by interfering with DC function and T cell activation. It is likely that this feature of virulent Salmonella is needed to promote systemic dissemination in the host. Recent studies have suggested explanations for some of the molecular mechanisms developed by virulent Salmonella to impair DC and T cell function. Several of these mechanisms require the expression of virulence genes encoded within Salmonella pathogenicity islands. Targeted deletion of these genes diminishes Salmonella pathogenicity and leads to efficient activation of T cells by Salmonella-infected DCs. In this review, recent data that support the subversion of DC function by Salmonella as a means to evade host adaptive immunity and cause systemic infection are discussed. These new findings suggest a new pathogenesis model with DCs as key targets for Salmonella virulence factors.

Nanoparticles as potential oral delivery systems of proteins and vaccines: a mechanistic approach

Peptides and proteins remain poorly bioavailable upon oral administration. One of the most promising strategies to improve their oral delivery relies on their association with colloidal carriers, e.g. polymeric nanoparticles, stable in gastrointestinal tract, protective for encapsulated substances and able to modulate physicochemical characteristics, drug release and biological behavior. The mechanisms of transport of these nanoparticles across intestinal mucosa are reviewed. In particular, the influence of size and surface properties on their non-specific uptake or their targeted uptake by enterocytes and/or M cells is discussed. Enhancement of their uptake by appropriate cells, i.e. M cells by (i) modeling surface properties to optimize access to and transport by M cells (ii) identifying surface markers specific to human M cell allowing targeting to M cells and nanoparticles transcytosis is illustrated. Encouraging results upon in vivo testing are reported but low bioavailability and lack of control on absorbed dose slow down products development. Vaccines are certainly the most promising applications for orally delivered nanoparticles.

Discordance in the effects of Yersinia pestis on the dendritic cell functions manifested by induction of maturation and paralysis of migration

The encounter between invading microorganisms and dendritic cells (DC) triggers a series of events which include uptake and degradation of the microorganism, induction of a maturation process, and enhancement of DC migration to the draining lymph nodes. Various pathogens have developed strategies to counteract these events as a measure to evade the host defense. In the present study we found that interaction of the Yersinia pestis EV76 strain with DC has no effect on cell viability and is characterized by compliance with effective maturation, which is manifested by surface display of major histocompatibility complex class II, of costimulatory markers, and of the chemokine receptor CCR7. This is in contrast to maturation inhibition and cell death induction exerted by the related species Yersinia enterocolitica WA O:8. Y. pestis interactions with DC were found, however, to impair functions related to cytoskeleton rearrangement. DC pulsed with Y. pestis failed to adhere to solid surfaces and to migrate toward the chemokine CCL19 in an in vitro transmembrane assay. Both effects were dependent on the presence of the pCD1 virulence plasmid and on a bacterial growth shift to 37 degrees C prior to infection. Moreover, while instillation of a pCD1-cured Y. pestis strain into mouse airways triggered effective transport of alveolar DC to the mediastinal lymph node, instillation of Y. pestis harboring the plasmid failed to do so. Taken together, these results suggest that virulence plasmid-dependent impairment of DC migration is the major mechanism utilized by Y. pestis to subvert DC function.

Bordetella pertussis inhibition of interleukin-12 (IL-12) p70 in human monocyte-derived dendritic cells blocks IL-12 p35 through adenylate cyclase toxin-dependent cyclic AMP induction

Bordetella pertussis, the causative agent of whooping cough, possesses an array of virulence factors, including adenylate cyclase toxin (ACT), relevant in the establishment of infection. Here we better define the impact of cyclic AMP (cAMP) intoxication due to the action of ACT on dendritic cell (DC)-driven immune response, by infecting monocyte-derived DC (MDDC) with an ACT-deficient B. pertussis mutant (ACT- 18HS19) or its parental strain (WT18323). Both strains induced MDDC maturation and antigen-presenting cell functions; however, only ACT- 18HS19 infected MDDC-induced production of interleukin-12 (IL-12) p70. Gene expression analysis of the IL-12 cytokine family subunits revealed that both strains induced high levels of p40 (protein chain communal to IL-12 p70 and IL-23) as well as p19, a subunit of IL-23. Conversely only ACT- 18HS19 infection induced consistent transcription of IL-12 p35, a subunit of IL-12 p70. Addition of the cAMP analogous D-butyril-cAMP (D-cAMP) abolished IL-12 p70 production and IL-12 p35 expression in ACT- 18HS19-infected MDDC. ACT- 18HS19 infection induced the expression of the transcription factors interferon regulatory factor 1 (IRF-1) and IRF-8 and of beta interferon, involved in IL-12 p35 regulation, and the expression of these genes was inhibited by D-cAMP addition and in WT18323-infected MDDC. The concomitant expression of IL-12 p70 and IL-23 allowed ACT- 18HS19 to trigger a more pronounced T helper 1 polarization compared to WT18323. The present study suggests that ACT-dependent cAMP induction leads to the inhibition of pathways ultimately leading to IL-12 p35 production, thus representing a mechanism for B. pertussis to escape the host immune response.

Lactic acid bacteria inducing a weak interleukin-12 and tumor necrosis factor alpha response in human dendritic cells inhibit strongly stimulating lactic acid bacteria but act synergistically with gram-negative bacteria

The development and maintenance of immune homeostasis indispensably depend on signals from the gut flora. Lactic acid bacteria (LAB), which are gram-positive (G+) organisms, are plausible significant players and have received much attention. Gram-negative (G-) commensals, such as members of the family Enterobacteriaceae, may, however, be immunomodulators that are as important as G+ organisms but tend to be overlooked. Dendritic cells (DCs) are crucial immune regulators, and therefore, the present study aimed at investigating differences among human gut flora-derived LAB and G- bacteria in their patterns of DC polarization. Human monocyte-derived DCs were exposed to UV-killed bacteria, and cytokine secretion and surface marker expression were analyzed. Profound differences in the DC polarization patterns were found among the strains. While strains of LAB varied greatly in their capacity to induce interleukin-12 (IL-12) and tumor necrosis factor alpha (TNF-alpha), G- strains were consistently weak IL-12 and TNF-alpha inducers. All strains induced significant amounts of IL-10, but G- bacteria were far more potent IL-10 inducers than LAB. Interestingly, we found that when weakly IL-12- and TNF-alpha-inducing LAB and strong IL-12- and TNF-alpha-inducing LAB were mixed, the weakly IL-12- and TNF-alpha-inducing LAB efficiently inhibited otherwise strong IL-12- and TNF-alpha-inducing LAB, yet when weakly IL-12- and TNF-alpha-inducing LAB were mixed with G- bacteria, they synergistically induced IL-12 and TNF-alpha. Furthermore, strong IL-12- and TNF-alpha-inducing LAB efficiently up-regulated surface markers (CD40, CD83, CD86, and HLA-DR), which were inhibited by weakly IL-12- and TNF-alpha-inducing LAB. All G- bacteria potently up-regulated surface markers; however, these markers were not inhibited by weakly IL-12- and TNF-alpha-inducing LAB. These much divergent DC stimulation patterns among intestinal bacteria, which encompass both antagonistic and synergistic relationships, support the growing evidence that the composition of the gut flora affects immune regulation and that compositional imbalances may be involved in disease etiology.

Suppression of adaptive immunity to heterologous antigens during Plasmodium infection through hemozoin-induced failure of dendritic cell function

BACKGROUND

Dendritic cells (DCs) are central to the initiation and regulation of the adaptive immune response during infection. Modulation of DC function may therefore allow evasion of the immune system by pathogens. Significant depression of the host's systemic immune response to both concurrent infections and heterologous vaccines has been observed during malaria infection, but the mechanisms underlying this immune hyporesponsiveness are controversial.

RESULTS

Here, we demonstrate that the blood stages of malaria infection induce a failure of DC function in vitro and in vivo, causing suboptimal activation of T cells involved in heterologous immune responses. This effect on T-cell activation can be transferred to uninfected recipients by DCs isolated from infected mice. Significantly, T cells activated by these DCs subsequently lack effector function, as demonstrated by a failure to migrate to lymphoid-organ follicles, resulting in an absence of B-cell responses to heterologous antigens. Fractionation studies show that hemozoin, rather than infected erythrocyte (red blood cell) membranes, reproduces the effect of intact infected red blood cells on DCs. Furthermore, hemozoin-containing DCs could be identified in T-cell areas of the spleen in vivo.

CONCLUSION

Plasmodium infection inhibits the induction of adaptive immunity to heterologous antigens by modulating DC function, providing a potential explanation for epidemiological studies linking endemic malaria with secondary infections and reduced vaccine efficacy.

Salmonella inhibit T cell proliferation by a direct, contact-dependent immunosuppressive effect

Dendritic cells (DC) are of central importance in the initiation of T cell-mediated adaptive immunity because these professional phagocytes internalize, process, and present microbial antigens to T lymphocytes. T lymphocytes have a pivotal role in controlling and clearing infection with intracellular pathogens through cytokine production. T lymphocytes also can mediate direct lysis of infected cells or activate B and T cells. In this article, we report that DC, when cocultured with Salmonella, fail to efficiently stimulate T cells for proliferation. We show that the failure of T lymphocytes to respond to Salmonella-infected DC is not simply due to Salmonella-induced programmed DC death or interference with up-regulation of costimulatory molecules CD80 and CD86. We cocultured bacteria with purified T lymphocytes, and we demonstrate here that Salmonella have a direct, contact-dependent inhibitory effect on the T cells, even in the absence of DC. This direct, Salmonella-induced inhibitory effect reduces the ability of T cells to proliferate and produce cytokines in response to stimulation and appears to require live bacteria. Cumulatively, these results are evidence that Salmonella may interfere with the development of acquired immunity, providing insights into the complex nature of this host-pathogen interaction.

Maturation of function in dendritic cells for tolerance and immunity

The capacity of antigen presenting dendritic cells (DC) to function in both tolerance and immunity is now well documented. The function and characteristics of different DC subsets are reviewed here and their capacity to activate T cells under different conditions of maturation and activation is discussed. The immunogenic potential of exosomes produced by DC is also considered in light of evidence that the capacity of exosomes to activate T cells for tolerance or immunity appears to mirror that of the parent DC. A model is proposed whereby exosomes produced by immature DC can function to maintain peripheral tolerance, while exosomes produced by more mature DC can stimulate effector T cells.

Ceramide catabolism critically controls survival of human dendritic cells

The regulation of dendritic cell (DC) survival is crucial for the modulation of adaptive immunity. Ceramide is a lipid mediator of the stress response, which accumulates intracellularly during DC differentiation. We found that ceramide levels are tightly regulated in human DCs and that the pharmacological inhibition of enzymes responsible for ceramide catabolism, such as ceramidases and sphingosine kinases, sensitizes DCs to ceramide-induced cell death. It is important that inhibition of sphingosine kinases, during lipopolysaccharide stimulation, causes extensive ceramide accumulation and death of DCs. These data indicate that ceramide catabolism regulates survival of human DCs and reveal novel potential targets for the pharmacological manipulation of the immune response.

Involvement of intestinal dendritic cells in oral tolerance, immunity to pathogens, and inflammatory bowel disease

Dendritic cells (DCs) are composed of a family of cells, now recognized to be essential for innate and acquired immunity. DCs at mucosal surfaces have a particular capacity to induce the differentiation of regulatory T cells producing interleukin-10 (IL-10) and transforming growth factor-beta (TGF-beta) in the steady state (non-infected, non-immunized), yet they retain the capacity to induce effector T cells in response to invasive pathogens. This decision between the induction of active immunity and tolerance will depend on the subpopulation of DC involved and the surface receptors engaged during DC activation and T-cell priming. The local microenvironment will likely play an important role both in defining the DC phenotype and in providing direct signals to responding T cells. Furthermore, DCs in organized mucosal lymphoid tissues preferentially induce the expression of CCR9 and alpha4beta7 on T cells, which results in T-cell homing to the intestinal lamina propria. Finally, DCs may play an important role in the maintenance of abnormal intestinal inflammation either by driving pathogenic T-cell responses in mesenteric lymph nodes or by acting to expand or maintain pathogenic T cells locally at sites of inflammation. In this review, a brief discussion of general issues of DC biology that are pertinent to mucosal immunity is followed by a more in-depth discussion of the phenotype and function of DC populations in the intestine.

Humoral and cell-mediated adaptive immune responses are required for protection against Burkholderia pseudomallei challenge and bacterial clearance postinfection

Burkholderia pseudomallei, the causative agent of melioidosis, is a gram-negative bacillus endemic to areas of southeast Asia and northern Australia. Presently, there is no licensed vaccine for B. pseudomallei and the organism is refractive to antibiotic therapy. The bacterium is known to survive and multiply inside both phagocytic and nonphagocytic host cells and may be able to spread directly from cell to cell. Current vaccine delivery systems are unlikely to induce the correct immune effectors to stimulate a protective response to the organism. In this study, we have developed a procedure to utilize dendritic cells as a vaccine delivery vector to induce cell-mediated immune responses to B. pseudomallei. Dendritic cells were produced by culturing murine bone marrow progenitor cells in medium containing granulocyte-macrophage colony-stimulating factor and tumor necrosis factor alpha. Purified dendritic cells were pulsed with heat-killed whole-cell B. pseudomallei and used to immunize syngeneic mice. Strong cellular immune responses were elicited by this immunization method, although antibody responses were low. Booster immunizations of either a second dose of dendritic cells or heat-killed B. pseudomallei were administered to increase the immune response. Immunized animals were challenged with fully virulent B. pseudomallei, and protection was demonstrated in those with strong humoral and cell-mediated immunity. These results indicate the importance of both cell-mediated and humoral immune mechanisms in protection against intracellular pathogens.

Cell-based cancer gene therapy: breaking tolerance or inducing autoimmunity?

This review examines the mechanisms involved in anti-tumor immunity and how peptides present in many tumor types (tumor-associated antigens) are recognized by T cells from tumor-bearing cancer patients. Tumor-associated antigens are derived from proteins that are also expressed in normal cells. It is predicted that immune responses to such peptides will be compromised by self-tolerance or that stimulation of effective immune responses will be accompanied by autoimmunity. We also consider that the immunity induced against two autoantigens, which are highly conserved in vertebrates, involve qualitatively different mechanisms, such as the production of antibodies and cell-mediated immune responses. However, both pathways lead to tumor immunity and identical phenotypic manifestations of autoimmunity. Appropriate selection of the optimal tumor antigen is critical for the induction of an anti-tumor immune response. Thus, we stress that the methods for antigen presentation using dendritic cells play a critical role in the development of tumor vaccines, to break immune tolerance and induce a strong immune response against them. The viability and feasibility of expansion of canine dendritic cells from bone marrow and peripheral blood ex vivo for the treatment of spontaneous cancers in dogs is also discussed.

Monocyte-derived dendritic cells activated by bacteria or by bacteria-stimulated epithelial cells are functionally different

Dendritic cells (DCs) are able to open the tight junctions between adjacent epithelial cells (ECs) and to take up both invasive and noninvasive bacteria directly from the intestinal lumen. In this study, we describe a tight cross talk between ECs and human monocyte-derived DCs (MoDCs) in bacterial handling across epithelial monolayers. We show that the release of proinflammatory mediators by ECs in response to bacteria is dependent on bacterial invasiveness and on the presence of flagella. This correlates with the capacity of EC-derived factors to modulate MoDC function. MoDCs incubated with supernatants of bacteria-treated ECs are "noninflammatory" as they release interleukin-10 (IL-10) but not IL-12 and can drive only T helper (Th)-2 type T cells. Moreover, noninflammatory MoDCs release chemokines aimed at recruiting Th2 and T-regulatory cells. In contrast, when MoDCs are incubated with ECs and bacteria in a transwell coculture system, and can contact directly the bacteria across stimulated EC monolayers, they are more inflammatory as they release IL-12 and IL-10 and induce both Th1 and Th2 responses. These results suggest that ECs are not simply a barrier to bacteria entering via the oral route, but they actively influence the activating properties of DCs.

Intestinal immune homeostasis is regulated by the crosstalk between epithelial cells and dendritic cells

The control of damaging inflammation by the mucosal immune system in response to commensal and harmful ingested bacteria is unknown. Here we show epithelial cells conditioned mucosal dendritic cells through the constitutive release of thymic stromal lymphopoietin and other mediators, resulting in the induction of 'noninflammatory' dendritic cells. Epithelial cell-conditioned dendritic cells released interleukins 10 and 6 but not interleukin 12, and they promoted the polarization of T cells toward a 'classical' noninflammatory T helper type 2 response, even after exposure to a T helper type 1-inducing pathogen. This control of immune responses seemed to be lost in patients with Crohn disease. Thus, the intimate interplay between intestinal epithelial cells and dendritic cells may help to maintain gut immune homeostasis.

Uptake and presentation of orally administered antigens

The mucosae of the gastrointestinal tract are continuously exposed to a myriad of antigens and microorganisms that the immune system has to discriminate between dangerous and harmless. Entry of pathogenic microorganisms occurs mainly via M cells that are concentrated in the follicle-associated epithelium overlying the Peyer's Patches (PPs). M cells are very selective and do not allow entry of all microorganisms. We have recently described an additional mechanism by which dendritic cells (DCs) can monitor the contents of the intestinal lumen. DCs send dendrites outside the epithelium, like periscopes. It is not clear whether this mechanism is constitutively active or is induced in response to signals from epithelial cells that have been in contact with pathogens or high numbers of non pathogenic bacteria in the lumen. Therefore, deciphering the signals that are released by epithelial cells after the encounter with mucosal antigens is of paramount importance to understand the ability of the DCs to respond to the different antigens and to mount immune or tolerogenic responses.

Prostaglandin E2 promotes the survival of bone marrow-derived dendritic cells

Since dendritic cells (DC) participate in both innate and adaptive immunity, their survival and expansion is tightly controlled. Little is known about the mechanisms of DC apoptosis. PGE(2), an arachidonic acid metabolite, plays an essential role in DC migration. We propose a novel function for PGE(2) as a DC survival factor. Our studies demonstrate that PGE(2) protects DC in vitro against apoptosis induced by withdrawal of growth factors or ceramide. DC matured in conditions that inhibit endogenous PGE(2) release are highly susceptible to apoptosis and exogenous PGE(2) re-establishes the more resistant phenotype. The antiapoptotic effect is mediated through EP-2/EP-4 receptors and involves the PI3K --> Akt pathway. PGE(2) leads to increased phosphorylation of Akt, protection against mitochondrial membrane compromise, and decreased caspase 3 activity. Macroarray data indicate that PGE(2) leads to the down-regulation of a number of proapoptotic molecules, i.e., BAD, several caspases, and granzyme B. In vivo, higher numbers of immature and Ag-loaded CFSE-labeled DC are present in the draining lymph nodes of mice inoculated with PGE(2) receptor agonists, compared with animals treated with ibuprofen or controls injected with PBS. This suggests that PGE(2) acts as an endogenous antiapoptotic factor for DC and raises the possibility of using PGE(2) agonists to increase the survival of Ag-loaded DC following in vivo administration.

New trends in antigen uptake in the gut mucosa

The mucosa represents a large surface of the human body that is in contact with the external environment. Mucosal tissues are colonized by an extremely dense and diverse micro flora of commensal bacteria, which compete with the growth of pathogenic strains. The mucosal sites continuously sample foreign material via specific cells, such as M cells and dendritic cells. Many new ways of antigen uptake have been described recently, especially by M cells and dendritic cells in the intestine. Depending on various factors, antigen presentation in the mucosa can lead to tolerance or initiation of an immune response. Mucosal vaccine strategies will certainly require eliciting specific antigen uptake because this initial step has a crucial role in controlling the outcome of immune responses.

Regulation of Antigen Presentation and Cross-Presentation in the Dendritic Cell Network: Facts, Hypothesis, and Immunological Implications

Dendritic cells (DCs) are central to the maintenance of immunological tolerance and the initiation and control of immunity. The antigen-presenting properties of DCs enable them to present a sample of self and foreign proteins, contained within an organism at any given time, to the T-cell repertoire. DCs achieve this communication with T cells by displaying antigenic peptides bound to MHC I and MHC II molecules. Here we review the studies carried out over the past 15 years to characterize these antigen presentation mechanisms, emphasizing their significance in relation to DC function in vivo. The life cycles of different DC populations found in vivo are described. Furthermore, we provide a critical assessment of the studies that examine the mechanisms controlling DC MHC class II antigen presentation, which have often reached contradictory conclusions. Finally, we review findings pertaining to the biological mechanisms that enable DCs to present exogenous antigens on their MHC class I molecules, a process known as cross-presentation. Throughout, we highlight what we consider to be major knowledge gaps in the field and speculate on possible directions for future research.

Intestinal Epithelial Cells Control Dendritic Cell Function

Dendritic cells (DCs) comprise a family of cells specializing in antigen capture and presentation to T cells. We have recently shown that DC play an active role in bacterial uptake across mucosal surfaces. Indeed, DC are able to open tight junctions and to sample antigens directly across epithelia, both in vitro and in vivo. Because DC express tight junction proteins, the integrity of the epithelial barrier is preserved. In this study we have analyzed the possible involvement of epithelial cells in controlling DC function. We developed an in vitro model in our laboratory consisting of a three-player system of dendritic cells, epithelial cell monolayers, and bacteria. The crosstalk between epithelial cells and dendritic cells was analyzed, and epithelial cells were tested for their capacity to release cytokines and chemokines that induce the migration and activation of DC. We show that the capacity of epithelial cells to produce cytokines and activate DC is dependent on the invasiveness of the bacteria tested. In particular, invasive bacteria stimulate epithelial cells to release proinflammatory cytokines and to induce the maturation state of DC. By contrast, noninvasive bacteria are unable to stimulate epithelial cells, but can activate DC directly when DC translocate to the apical side. In conclusion, epithelial cells are not simply a barrier to bacteria entering via the oral route, but actively influence the activating properties of bystander DC.

Dendritic cell trafficking and antigen presentation in the human immune response to Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is an extraordinarily successful human pathogen, one of the major causes of death by infectious disease worldwide. A key issue for the study of tuberculosis is to understand why individuals infected with Mtb experience different clinical outcomes. To better understand the dynamics of Mtb infection and immunity, we coupled nonhuman primate experiments with a mathematical model we previously developed that qualitatively and quantitatively captures important processes of cellular priming and activation. These processes occur between the lung and the nearest draining lymph node where the key cells mediating this process are the dendritic cells (DC). The nonhuman primate experiments consist of bacteria and cell numbers from tissues of 17 adult cynomolgus macaques (Macaca fascicularis) that were infected with Mtb strain Erdman ( approximately 25 CFU/animal via bronchoscope). The main result of this work is that delays in either DC migration to the draining lymph node or T cell trafficking to the site of infection can alter the outcome of Mtb infection, defining progression to primary disease or latent infection and reactivated tuberculosis. Our results also support the idea that the development of a new generation of treatment against Mtb should optimally elicit a fast DC turnover at the site of infection, as well as strong activation of DCs for maximal Ag presentation and production of key cytokines. This will induce the most protective T cell response.

A Toll-like receptor 2 ligand stimulates Th2 responses in vivo, via induction of extracellular signal-regulated kinase mitogen-activated protein kinase and c-Fos in dendritic cells

The adaptive immune system can generate distinct classes of responses, but the mechanisms that determine this are poorly understood. In this study, we demonstrate that different Toll-like receptor (TLR) ligands induce distinct dendritic cell (DC) activation and immune responses in vivo. Thus, Escherichia coli LPS (TLR-4 stimulus), activates DCs to produce abundant IL-12(p70), but little IL-10, and stimulates Th1 and Tc1 responses. In contrast, Pam-3-cys (TLR-2 stimulus) elicits less IL-12(p70), but abundant IL-10, and favors Th2 and T cytotoxic 2 (Tc2) responses. These distinct responses likely occur via differences in extracellular signal-regulated kinase signaling in DCs. Thus, Pam-3-cys induces enhanced extracellular signal-regulated kinase signaling, compared with LPS, resulting in suppressed IL-12(p70) and enhanced IL-10 production, as well as enhanced induction of the transcription factor, c-Fos. Interestingly, DCs from c-fos(-/-) mice produce more IL-12(p70), but less IL-10, compared with control DCs. Therefore, different TLR ligands induce distinct cytokines and signaling in DCs, and differentially bias Th responses in vivo.

The Regulatory Role of Dendritic Cells in the Immune Response

Dendritic cells (DCs) are key regulators of immune reactions. They control early innate responses, regulate long-lasting adaptive immunity and contribute to the maintenance of self-tolerance. DCs continuously monitor the environment through a multifaceted innate antigen receptor repertoire and, in response to perturbations, start a complex genetic reprogramming that leads to a complete activation of innate and, then, adaptive immune responses. This review discusses how DCs become efficient activators of NK and, subsequently, T cells following a microbial encounter.

The human immune response to Mycobacterium tuberculosis in lung and lymph node

A key issue for the study of tuberculosis is to understand why individuals infected with Mycobacterium tuberculosis (Mtb) experience different clinical outcomes. To better understand the dynamics of Mtb infection and immunity, we have previously developed a temporal mathematical model that qualitatively and quantitatively characterizes the cellular and cytokine control network during infection. In this work we extend that model to a two compartmental model to capture the important processes of cellular activation and priming that occur between the lung and the nearest draining lymph node. We are able to reproduce typical disease progression scenarios including primary infection, latency or clearance. Then we use the model to predict key processes determining these different disease trajectories (i.e. identify bifurcation parameters), suggesting directions for further basic science study and potential new treatment strategies.

Salmonella rapidly kill dendritic cells via a caspase-1-dependent mechanism

Dendritic cells provide a critical link between innate and acquired immunity. In this study, we demonstrate that the bacterial pathogen Salmonella enterica serovar Typhimurium can efficiently kill these professional phagocytes via a mechanism that is dependent on sipB and the Salmonella pathogenicity island 1-encoded type III protein secretion system. Rapid phosphatidylserine redistribution, caspase activation, and loss of plasma membrane integrity were characteristic of dendritic cells infected with wild-type Salmonella, but not sipB mutant bacteria. Caspase-1 was particularly important in this process because Salmonella-induced dendritic cell death was dramatically reduced in the presence of a caspase-1-specific inhibitor. Furthermore, dendritic cells obtained from caspase-1-deficient mice, but not heterozygous littermate control mice, were resistant to Salmonella-induced cytotoxicity. We hypothesize that Salmonella have evolved the ability to selectively kill professional APCs to combat, exploit, or evade immune defense mechanisms.