Dendritic cells prime natural killer cells by trans-presenting interleukin 15

Natural killer (NK) cells are important effector cells in the control of infections. The cellular and molecular signals required for NK cell activation in vivo remain poorly defined. By using a mouse model for the inducible ablation of dendritic cells (DCs), we showed that the in vivo priming of NK cell responses to viral and bacterial pathogens required the presence of CD11c(high) DCs. After peripheral Toll-like receptor (TLR) stimulation, NK cells were recruited to local lymph nodes, and their interaction with DCs resulted in the emergence of effector NK cells in the periphery. NK cell priming was dependent on the recognition of type I IFN signals by DCs and the subsequent production and trans-presentation of IL-15 by DCs to resting NK cells. CD11c(high) DC-derived IL-15 was necessary and sufficient for the priming of NK cells. Our data define a unique in vivo role of DCs for the priming of NK cells, revealing a striking and previously unappreciated homology to T lymphocytes of the adaptive immune system.     

NK cells help to induce CD8(+)-T-cell immunity against Toxoplasma gondii in the absence of CD4(+) T cells

CD8(+) T-cell immunity plays an important role in protection against intracellular infections. Earlier studies have shown that CD4(+) T-cell help was needed for launching in vivo CD8(+) T-cell activity against these pathogens and tumors. However, recently CD4(+) T-cell-independent CD8 responses during several microbial infections including those with Toxoplasma gondii have been described, although the mechanism is not understood. We now demonstrate that, in the absence of CD4(+) T cells, T. gondii-infected mice exhibit an extended NK cell response, which is mediated by continued interleukin-12 (IL-12) secretion. This prolonged NK cell response is critical for priming parasite-specific CD8(+) T-cell immunity. Depletion of NK cells inhibited the generation of CD8(+) T-cell immunity in CD4(-/-) mice. Similarly neutralization of IL-12 reduces NK cell numbers in infected animals and leads to the down-regulation of CD8(+) T-cell immunity against T. gondii. Adoptive transfer of NK cells into the IL-12-depleted animals restored their CD8(+) T-cell immune response, and animals exhibited reduced mortality. NK cell gamma interferon was essential for cytotoxic T-lymphocyte priming. Our studies for the first time demonstrate that, in the absence of CD4(+) T cells, NK cells can play an important role in induction of primary CD8(+) T-cell immunity against an intracellular infection. These observations have therapeutic implications for immunocompromised individuals, including those with human immunodeficiency virus infection.     

Innate immunity: cells, receptors, and signaling pathways

Essential differences between the innate and acquired branches of immunity are described. These differences concern the detection system (receptors and pathogen structures) and the cells engaged in both systems as well as the effectory mechanisms. In contrast to those of the acquired system, receptors of the innate system, which developed during evolution, recognize unchanged structures on large groups of pathogens (e.g. lipopolysaccharide in Gram-negative bacteria). Two lineages, natural killer (NK) and dendritic cells (DCs), play important roles in the innate system. Phenotypic and functional differentiation is observed among NKs and DCs, so each of their sublineages plays a different role in the innate system. Every lineage of cells of the innate immune system express different stimulatory and sometimes also inhibitory receptors on their surfaces (e.g. NK cells). Among the stimulatory are Toll-like receptors (TLRs), mannose and scavenger receptors, and the stimulatory receptors of NK cells. All TLRs show similarity in structure and in the kind of molecules involved in intracellular signaling. The immune reactions of the innate system involve cytokine-dependent resistance of cells against infection with pathogen, production of cytokines (tumor necrosis factor, interferons, interleukins, chemokines) and MHC-independent killing. Although these reactions protect the host from invasion by microorganisms, they can also be responsible for significant tissue damage or may stimulate the development of autoimmunity. Therefore innate immunity must be under rigorous control. The possible regulatory mechanisms of innate immunity are discussed.     

Co-stimulation by dendritic cells maintains the peripheral pool of Tregs

Many aspects of an immune response are regulated by dendritic cells (DCs). DCs play key proinflammatory roles by sensing microbial invaders and promoting the activation of innate immune cells such as NK cells. In addition, DCs are required for the initiation and maintenance of adaptive T-cell responses against invading pathogens. Moreover, DCs also fulfill important anti-inflammatory functions: they induce peripheral T-cell tolerance by silencing self-reactive T cells and supporting the homeostasis of regulatory T cells (Tregs). A study in this issue of the European Journal of Immunology reveals that CD80/CD86 expression on DCs contributes to the maintenance of the peripheral pool of Tregs. This Commentary discusses current findings on this topic, focusing on the role of DCs in the homeostatic control of Tregs and myeloid cells, and the potential consequences for T-cell activation.     

Optimal culture conditions for the generation of natural killer cell-induced dendritic cells for cancer immunotherapy

Dendritic cell (DC)-based vaccines continue to be considered an attractive tool for cancer immunotherapy. DCs require an additional signal from the environment or other immune cells to polarize the development of immune responses toward T helper 1 (Th1) or Th2 responses. DCs play a role in natural killer (NK) cell activation, and NK cells are also able to activate and induce the maturation of DCs. We investigated the types of NK cells that can induce the maturation and enhanced function of DCs and the conditions under which these interactions occur. DCs that were activated by resting NK cells in the presence of inflammatory cytokines exhibited increased expression of several costimulatory molecules and an enhanced ability to produce IL-12p70. NK cell-stimulated DCs potently induced Th1 polarization and exhibited the ability to generate tumor antigen-specific cytotoxic T lymphocyte responses. Our data demonstrate that functional DCs can be generated by coculturing immature DCs with freshly isolated resting NK cells in the presence of Toll-like receptor agonists and proinflammatory cytokines and that the resulting DCs effectively present antigens to induce tumor-specific T-cell responses, which suggests that these cells may be useful for cancer immunotherapy.     

Differential induction of interleukin-10 and interleukin-12 in dendritic cells by microbial toll-like receptor activators and skewing of T-cell cytokine profiles

Dendritic cells (DCs) discriminate different microbial pathogens and induce T-cell responses of appropriate effector phenotypes accordingly. Microbial recognition and differentiation are mediated in part by pattern recognition receptors such as Toll-like receptors (TLRs), whereas the development of T-cell effector functions is critically dependent on DC-derived cytokines such as interleukin-12 (IL-12) and IL-10. However, it is not entirely clear to what extent various microbial TLR activators could induce different functional states of DCs that favor different T-cell effector phenotypes. Toward a better understanding of this issue, we examined IL-10 and IL-12 production and T-cell-polarizing potentials of murine bone marrow-derived DCs after stimulation by three microbial TLR activators, namely, lipopolysaccharide (LPS), peptidoglycan (PGN), and zymosan. We found that the three stimuli induced drastically different profiles of IL-10 and IL-12 production in DCs. Further, these stimuli differentially conditioned CD40-dependent IL-10 and IL-12 production by DCs. Finally, LPS-, PGN-, and zymosan-stimulated DCs primed distinct T-cell cytokine profiles. Our results support the notion that microbe-specific information sensed through different TLRs by DCs is linked to differential Th priming through DC-derived cytokines.     

IL-2 mediates adjuvant effect of dendritic cells

Research into the biology of dendritic cells (DCs) has shown that they play a central role in priming early and late immune responses. The recent finding that DCs produce interleukin-2 (IL-2) after microbial challenge suggests that this could be the mechanism by which DCs link innate and adaptive immunity. This hypothesis is supported also by the observation that murine cytomegalovirus, which is able to establish a persistent infection, can interfere with the ability of infected DCs to produce IL-2 and prime T cells.     

Human NK cells directly recognize Mycobacterium bovis via TLR2 and acquire the ability to kill monocyte-derived DC

NK cells are important players of the early innate defense against various pathogens. In this study, we investigated the interaction between human NK cells and Mycobacterium bovis [bacille Calmette-Guérin (BCG)] and we determined whether and how such an interaction might impact on NK cell activation, cytokine production and cytotoxicity. We show that highly purified NK cells, upon short-term co-culture with BCG, expressed activation markers including CD69 and CD25. Moreover, these NK cells released IFN-gamma and tumor necrosis factor-alpha and killed more efficiently different targets including monocyte-derived immature dendritic cell. All these functions were strongly up-regulated in the presence of exogenous IL-12. Although more efficient responses were detected in NK cell populations displaying an NCR(bright) phenotype, no direct evidence of an involvement of triggering NK receptors in BCG recognition could be obtained. On the other hand, anti-toll-like receptor (TLR)2 mAb inhibited NK cell responses to BCG, suggesting that NK cells may express a functional TLR2, which plays a role in their mechanism of direct BCG recognition. Taken together, these data suggest that BCG, by inducing simultaneous activation of NK and antigen-presenting cells via their 'shared' TLR2, can promote efficient bidirectional NK-dendritic cell interactions necessary for subsequent priming of T(h)1 responses.     

Natural killer cells prime the responsiveness of autologous CD4+ T cells to CTLA4-Ig and interleukin-10 mediated inhibition in an allogeneic dendritic cell-mixed lymphocyte reaction

Cytotoxic T-lymphocyte antigen 4 immunoglobulin (CTLA4-Ig) and interleukin (IL)-10 are immunomodulatory molecules which target CD28 costimulation by acting either directly or indirectly on the CD80/86 receptors on dendritic cells (DCs). This study examined the effect of combined treatment with CTLA4-Ig and IL-10 on T-cell responsiveness in a dendritic cell-mixed lymphocyte reaction (DC-MLR). T cells derived from nylon wool enrichment (NWT cells) demonstrated 15% (P = 0.006) and 10% (P = 0.0015) inhibition of proliferation with suboptimal doses of IL-10 (5 ng/ml) and CTLA4-Ig (20 ng/ml), respectively. Combined treatment with both agents resulted in 38% inhibition (P = 0.004) of the MLR response compared with untreated controls. In contrast to NWT cells, which consisted of CD4+, CD8+ and CD56+ (NK) cells, purified CD4+ T cells were less responsive to immunomodulation by CTLA4-Ig and IL-10. Repletion of the CD4+ T cells with NK cells restored IL-10 and CTLA4-Ig mediated immunomodulation, suggesting a role for NK cells in the regulation of DC-T-cell interactions. The specific effect of NK cells on DC activation was demonstrated by CD80 up-regulation on DCs in the absence of T cells. However, in the absence of DCs, NK cells augmented the proliferation of autologous CD4+ T cells stimulated by anti-CD3 monoclonal antibody (mAb), which was blocked by CTLA4-Ig. It is proposed that, in the MLR, immunomodulation by suboptimal CTLA4-Ig and IL-10 is influenced by cellular interactions of NK cells with DCs and T cells involving DC lysis and costimulation. Thus, NK cells prime both DCs and T cells to low doses of CTLA4-Ig and IL-10 during alloimmune responses, providing evidence for the potential interaction between innate and adaptive immunity.     

Targeting dendritic cells in allergen immunotherapy

Allergen immunotherapy is a well-established strategy for treating allergic diseases with the goal of inducing allergen-specific tolerance. Identified mechanisms contributing to the therapeutic effect of immunotherapy include a shift of T helper 2 (Th2)-type immune responses to a modified Th2 immune response, a change of the balance of IgE-producing B cells to the production of IgG subtypes, in addition to increased IL-10 and TGF-beta secretion and activation of the suppressive functions of regulatory T-cells. Dendritic cells (DCs), which as outposts of the immune system are capable of T-cell priming through efficient allergen uptake by IgE receptors expressed on their cell surface. Most of the hypotheses concerning the function of DCs as facilitators of allergen-specific tolerance in allergen immunotherapy remain speculative. Therefore, studies must focus on the functional changes of DCs under immunotherapy to close the gap of knowledge about their exact role. These experimental data should help confirm the hypothesis of DCs as efficient silencers and potential target cells and take advantage of the bivalent character and tolerogenic properties of DCs.     

The role of CD11c(+) hepatic dendritic cells in the induction of innate immune responses

The role of the liver in the initiation and maintenance of tolerance is a critical immune function that involves multiple lineages of immune cells. Included within these populations are liver dendritic cells (DCs). Although there has been significant work on the phenotypic and functional roles of splenic and bone marrow dendritic cells, as well as their subsets, comparable studies in liver have often been difficult. To address this issue we have isolated, from C57BL/6 mice, relatively pure populations of DCs and compared phenotype and function to the data from spleen using flow cytometry, cell sorter assisted purification and culture, morphology by cytospin and May-Giemsa staining, cell cycle progression, antigen uptake, cytokine production and allo-activation potential. natural killer (NK)1.1(-)CD11c(+) liver DC subsets (conventional DCs, T cell receptor (TcR)beta(-)NK1.1(-)CD11c(+)B220(-) and plasmacytoid DCs, TcRbeta(-)NK1.1(-)CD11c(+)B220(+)) efficiently endocytose dextran and produce significant levels of tumour necrosis factor (TNF)-alpha, interleukin (IL)-6 and IL-12 p40 in response to Toll-like receptor (TLR) ligands, with responses higher than splenic DCs. There is also a differential capability of hepatic DCs to respond to innate signals. Indeed, CD11c(+) hepatic DCs have a greater capacity to respond to innate stimulation but are less capable of inducing CpG activated-allogeneic T cells. These data suggest that hepatic dendritic cells function as a critical bridge between innate and adaptive immunity and are capable of inducing stronger innate responses with a lower capacity for allo-stimulation than splenic dendritic cells. These properties of liver dendritic cells contribute to their unique role in the induction of tolerance.     

Early liaisons between cells of the innate immune system in inflamed peripheral tissues

The crosstalk between natural killer (NK) cells and myeloid dendritic cells (DCs) results in NK-cell activation and DC maturation. Activated NK cells acquire the ability to kill DCs that have failed to undergo complete maturation ('DC editing'). Recent studies have revealed that this crosstalk can be promoted by pathogen-derived products that activate different innate immune cell types directly and simultaneously through their Toll-like receptors (TLRs). These cells include NK cells and DCs, as well as plasmacytoid DCs (PDCs) and mast cells. This crosstalk can have a great impact on the quality and strength of the subsequent adaptive immune response. Thus, NK cells have an important role in the defense against pathogens, acting as regulatory cells as well as effector cells.     

Effector and regulatory events during natural killer–dendritic cell interactions

Summary:  The different cell types of the innate immune system can interact with each other and influence the quality and strength of an immune response. The cross talk between natural killer (NK) cells and myeloid dendritic cells (DCs) leads to NK cell activation and DC maturation. Activated NK cells are capable of killing DCs that fail to undergo proper maturation ('DC editing'). Encounters between NK cells and DCs occur in both inflamed peripheral tissues and lymph nodes, where both cell types are recruited by chemokines released in the early phases of inflammatory responses. Different NK cell subsets (CD56^brightCD16⁻ versus CD56⁺CD16⁺) differ in their homing capabilities. In particular, CD56^brightCD16⁻ NK cells largely predominate the lymph nodes. In addition, these two subsets display major functional differences in their cytolytic activity, cytokine production, and ability to undergo proliferation. NK cell functions are also greatly influenced by the presence of polarizing cytokines such as interleukin (IL)-12 and IL-4. The cytokine microenvironment reflects the presence of different cell types that secrete such cytokines in response to microbial products acting on different Toll-like receptors (TLRs). Moreover, NK cells themselves can respond directly to microbial products by means of TLR3 and TLR9. Thus, it appears that the final outcome of a response to microbial infection may greatly vary as a result of the interactions occurring between different pathogen-derived products and different cell types of the innate immunity system. These interactions also determine the quality and strength of the subsequent adaptive responses. Remarkably, NK cells appear to play a key role in this complex network.     

Mast cells promote Th1 and Th17 responses by modulating dendritic cell maturation and function

Mast cells (MCs) play an important role in the regulation of protective adaptive immune responses against pathogens. However, it is still unclear whether MCs promote such host defense responses via direct effects on T cells or rather by modifying the functions of antigen-presenting cells. To identify the underlying mechanisms of the immunoregulatory capacity of MCs, we investigated the impact of MCs on dendritic cell (DC) maturation and function. We found that murine peritoneal MCs underwent direct crosstalk with immature DCs that induced DC maturation as evidenced by enhanced expression of costimulatory molecules. Furthermore, the MC/DC interaction resulted in the release of the T-cell modulating cytokines IFN-γ, IL-2, IL-6 and TGF-β into coculture supernatants and increased the IL-12p70, IFN-γ, IL-6 and TGF-β secretion of LPS-matured DCs. Such MC-"primed" DCs subsequently induced efficient CD4+ T-cell proliferation. Surprisingly, we observed that MC-primed DCs stimulated CD4+ T cells to release high levels of IFN-γ and IL-17, demonstrating that MCs promote Th1 and Th17 responses. Confirming our in vitro findings, we found that the enhanced disease progression of MC-deficient mice in Leishmania major infection is correlated with impaired induction of both Th1 and Th17 cells.     

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

Summary: 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‐β (TGF‐β) 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 α4β7 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.     

Malassezia enhances natural killer cell-induced dendritic cell maturation

Human natural killer (NK) cells can induce cell death in autologous dendritic cells (DCs), though an interaction between these two cell types can also lead to a reciprocal activation. We have recently shown cell contact between NK cells and DCs in vivo, in Malassezia-induced lesional skin of patients with atopic eczema, where the yeast acts as an allergen although it is part of the normal skin microflora. Here we characterize the interaction of human NK cells and monocyte-derived DCs (MDDCs) by using an in vitro system where short-term activated polyclonal NK cells are cocultured with autologous, immature, Malassezia-stimulated or lipopolysaccharide-matured MDDCs. We found that the number of CD83(+) MDDCs increased in the immature and Malassezia-stimulated MDDCs upon coculture with NK cells, while an increased number of CD86(+) cells was detected in the Malassezia-stimulated MDDCs. NK cell-MDDC interaction induced the production of interleukin-8 (IL-8). In conclusion, our results imply that NK cells provide maturation signals and may play a role in inducing IL-8 production in DCs. Furthermore, the increased expression of CD86 on Malassezia-stimulated MDDCs might have a function in subsequent T-cell activation by DCs, and indicate a role for NK cell-DC interaction in modulating the immune responses to microbial stimuli.     

Natural killer cell immune responses

Natural killer (NK) cells play a vital role in innate immune responses to infection; they express activation receptors that recognize virus-infected cells. Highly related to receptors recognizing tumor cells, the activation receptors trigger cytotoxicity and cytokine production. NK cells also express inhibitory receptors for major histocompatibility complex (MHC) class I molecules that block the action of the activation receptors. Although many ligands for NK cell receptors have MHC class I folds, recent studies also indicate ligands resembling the NK cell receptors themselves. A combination of immunologic, genetic, biophysical, and in vivo approaches is being employed to understand fully how these receptors contribute to NK cell activities in innate immunity to pathogens and tumors.     

Innate Vα14+ natural killer T cells mature dendritic cells, leading to strong adaptive immunity

Summary: The observation that the glycolipid α-galactosylceramide (α-GalCer) is a potent stimulator of natural killer T (NKT) cells has provided an important means for investigating NKT cell biology. α-GalCer is presented on CD1d to the invariant NKT receptor, leading to interleukin-12 (IL-12) production by dendritic cells (DCs) and to NK cell activation. We review our research on the tumor-protective properties of α-GalCer, particularly the major role played by DCs. We compared administration of α-GalCer on mature DCs with soluble glycolipid and found that DCs induced more prolonged interferon-γ (IFN-γ) production by NKT cells and better protection against B16 melanoma. Human α-GalCer-loaded DCs also expanded NKT cell numbers in cancer patients. α-GalCer-activated NKT cells were then found to induce DC maturation in vivo . The maturing DCs produced IL-12, upregulated co-stimulatory molecules, and induced adaptive immunity to captured cellular antigens, including prolonged, combined CD4⁺/CD8⁺ T-cell immunity to dying tumor cells. Surprisingly, co-stimulator-poor tumor cells, if directly loaded with α-GalCer ('tumor/Gal') and injected intravenously, also induced strong NKT- and NK-cell responses. The latter killed the tumor/Gal, which were subsequently cross presented by CD1d on DCs to elicit DC maturation and prolonged adaptive T-cell immunity, which lasted 6–12 months. These findings help explain tumor protection via α-GalCer and urge development of the DC-NKT axis to provide innate and adaptive immunity to human cancers.     

Dendritic cells,T cells and their interaction in rheumatoid arthritis

Dendritic cells (DCs) are the key professional antigen-presenting cells which bridge innate and adaptive immune responses, inducing the priming and differentiation of naive to effector CD4⁺ T cells, the cross-priming of CD8⁺ T cells and the promotion of B cell antibody responses. DCs also play a critical role in the maintenance of immune homeostasis and tolerance. DC–T cell interactions underpin the generation of an autoimmune response in rheumatoid arthritis (RA). Here we describe the function of DCs and review evidence for DC and T cell involvement in RA pathogenesis, in particular through the presentation of self-peptide by DCs that triggers differentiation and activation of autoreactive T cells. Finally, we discuss the emerging field of targeting the DC–T cell interaction for antigen-specific immunotherapy of RA.