Role of natural killer cells in modulating dendritic cell responses to Leishmania amazonensis infection

The importance of the interaction between natural killer (NK) cells and dendritic cells (DCs) in the expansion of antiviral and antitumor immune responses is well-documented; however, limited information on DC-NK cell interaction during parasitic infections is available. Given that some Leishmania parasites are known to prevent or suppress DC activation, we developed a DC-NK cell coculture system to examine the role of NK cells in modulating the functions of Leishmania-infected DCs. We found that the addition of freshly isolated, resting NK cells significantly promoted the activation of DCs that were preinfected with Leishmania amazonensis promastigotes and that these activated DCs, in turn, stimulated NK cell activation mostly via cell contact-dependent mechanisms. Notably, L. amazonensis amastigote infection failed to activate DCs, and this lack of DC activation could be partially reversed by the addition of preactivated NK (ANK) cells but not resting NK cells. Moreover, the adoptive transfer of ANK cells into L. amazonensis-infected mice markedly increased DC and T-cell activation and reduced tissue parasite loads at 1 and 3 weeks postinfection. These results suggest differential roles of DC-NK cell cross talk at different stages of Leishmania infection and provide new insight into the interplay of components of the innate immune system during parasitic infection.     

Interaction between dendritic cells and natural killer cells during pregnancy in mice

A complex regulation of innate and adaptive immune responses at the maternal fetal interface promotes tolerance of trophoblast cells carrying paternally derived antigens. Such regulatory functions involve uterine dendritic cells (uDC) and natural killer (uNK) cells. The existence of a NK and DC "cross talk" has been revealed in various experimental settings; its biological significance ranging from cooperative stimulation to cell lysis. Little is known about the presence or role of NK and DC cross talk at the maternal fetal interface. The present study shows that mouse NK and DC interactions are subject to modulation by trophoblast cells in vitro. This interaction promotes a tolerogenic microenvironment characterized by downregulation of the expression of activation markers on uNK cells and uDC and dominance of Th2 cytokines. NK and DC interactions would also influence uterine cell proliferation and this process would be strongly modulated by trophoblast-derived signals. Indeed; while low proliferation rates were observed upon regular coculture allowing direct contact between uterine cells and trophoblasts, incubation in a transwell culture system markedly increased uterine cell proliferation suggesting that soluble factors are key mediators in the molecular "dialog" between the mother and the conceptus during the establishment of mouse pregnancy. Our data further reveal that the regulatory functions of trophoblast cells associated with tolerance induction are impaired in high abortion murine matings. Interestingly, we observed that secretion of interleukin-12p70 by uDC is dramatically abrogated in the presence of uNK cells. Taken together, our results provide the first evidence that a delicate balance of interactions involving NK cells, DC, and trophoblasts at the mouse maternal fetal interface supports a successful pregnancy outcome.     

Natural killer cells activated by MHC class I(low) targets prime dendritic cells to induce protective CD8 T cell responses

Conserved molecular patterns derived from pathogenic microorganisms prime antigen-presenting dendritic cells (DC) to induce adaptive T cell responses. In contrast, virus-infected or tumor cells that express low levels of major histocompatibility complex (MHC) class I activate natural killer (NK) cells for direct killing. It is unknown whether NK cell recognition of MHC class I(low) targets can also induce adaptive T cell responses. Here, we show that MHC class I(low) targets initiate a cascade of immune responses, starting with the immediate activation of NK cells. The activated NK cells then prime DC to produce IL-12 and to induce highly protective CD8 T cell memory responses. Therefore, sensing of MHC class I(low) targets by NK cells can link innate and adaptive immunity to induce protective T cell responses and may alarm the immune system during early infection with noncytopathic viruses.     

Regulation of NK cell repertoire and function in the liver

NK cells represent a large proportion of the lymphocyte population in the liver and are involved in early innate immunity to pathogen infection. As a result of liver endothelial cell fenestrations, parenchymal cells are not separated by a basal membrane, and thereby pathogen-infected hepatocytes are extensively capable of interacting with innate immune cells including NK cells. In addition, hepatic NK cells interact with surrounding DC and alter their differentiation and function. Recent studies reveal that NK cells exhibit a regulatory function that modulates T cell responses through their interaction with DC and/or direct effect on T cells. Thus, NK cells play a central role, not only in innate immunity, but also in shaping the adaptive immune response. During pathogen infection, there is a remarkable increase of hepatic NK cells, possibly due to the expansion of resident liver NK cells and/or recruitement of NK cells from the blood. The liver microenvironment is believed to modulate hepatic NK cell function through the induction of activating/inhibitory receptor expression and inflammatory cytokine secretion. Particularly, the liver maintains intrahepatic NK cells in a functionally hyporesponsive state compared to splenic NK cells: liver NK cells displayed a dampened IFN-γ response to IL-12/IL-18 stimulation. Notably, the liver contains a significant population of functionally hyporesponsive NK cells that express high levels of the inhibitory receptor NKG2A and lack expression of MHC class I-binding Ly49 receptors. Importantly, adoptively transferred splenic NK cells that migrate to the liver displayed phenotypic and functional changes, supporting a view that the liver environment modifies NK cell receptor expression and functional responsiveness. In this article, we will review studies on the regulation of NK cell repertoire and function in the hepatic environment and the impact of liver NK cell immunoregulatory function on influencing adaptive immunity.     

The impact of eicosanoids on the crosstalk between innate and adaptive immunity: the key roles of dendritic cells

The innate immune response is essentially the first line of defense against an invading pathogen. Through specialized receptors, known as pattern recognition receptors, especially Toll-like receptors, specialized cells of myeloid origin, including macrophages and dendritic cells (DCs) are able to phagocytose microorganisms and induce an innate inflammatory response. Although B and T lymphocytes recognize tissue antigens with high specificity, they are unable to initiate immune responses. The decision to activate an appropriate immune response is made by unique DC, the most professional antigen-presenting cells (APCs) which control the responses of several types of lymphocytes and play central role in the transition between innate and adaptive immunity. Increased secretion of inflammatory endogenous mediators such as cytokines and arachidonic acid-derived lipid mediators, also termed eicosanoids, can activate APC, particularly DC, which in turn induce an adaptive immune response. There is an increasing evidence that eicosanoids play an important role in connecting innate and adaptive immunity by acting on cells of both systems. Prostanoids, a major class of eicosanoids, have a great impact on inflammatory and immune responses. PGE(2) is one of the best known and most well-characterized prostanoids in terms of immunomodulation. Although cytokines are known as key regulators of immunity, eicosanoids, including PGE(2), PGD(2), LTB(4), and LTC(4), may also affect cells of immune system by modulating cytokine release, cell differentiation, survival, migration, antigen presentation, and apoptosis. By acting on various aspects of immune and inflammatory reactions, these lipid mediators emerge as key regulators of the crosstalk between innate and adaptive immunity.     

The dialogue between human natural killer cells and dendritic cells

The interaction of NK cells with dendritic cells (DCs) appears to play an important role in both innate and adaptive immune responses to pathogens. In peripheral inflamed tissues the simultaneous engagement of receptors for danger (e.g. Toll-like receptors), which are expressed by both NK cells and DCs, results in cell activation and the acquisition of functional properties necessary for controlling, and possibly rapidly eliminating, pathogens by innate effector mechanisms. Moreover, NK cells are needed to select the most appropriate DCs that display the functional properties suitable for subsequent T-cell priming. This NK-cell-mediated programming of DC maturation is modulated by cytokines released during the early stages of inflammatory responses (i.e. IL-12, IFN-gamma, IL-4). NK cells and DCs continue their interactions in secondary lymphoid organs where both cell types play a role in the control of T-cell priming.     

A quantitative proteomic approach for detecting protein profiles of activated human myeloid dendritic cells

Dendritic cells (DC) direct the magnitude, polarity and effector function of the adaptive immune response. DC express toll-like receptors (TLR), antigen capturing and processing machinery, and costimulatory molecules, which facilitate innate sensing and T cell activation. Once activated, DC can efficiently migrate to lymphoid tissue and prime T cell responses. Therefore, DC play an integral role as mediators of the immune response to multiple pathogens. Elucidating the molecular mechanisms involved in DC activation is therefore central in gaining an understanding of host response to infection. Unfortunately, technical constraints have limited system-wide 'omic' analysis of human DC subsets collected ex vivo. Here we have applied novel proteomic approaches to human myeloid dendritic cells (mDCs) purified from 100 mL of peripheral blood to characterize specific molecular networks of cell activation at the individual patient level, and have successfully quantified over 700 proteins from individual samples containing as little as 200,000 mDCs. The proteomic and network readouts after ex vivo stimulation of mDCs with TLR3 agonists are measured and verified using flow cytometry.     

NK cells in innate immunity

NK cells have an important role in innate immune responses, particularly in anti-viral immunity. Recent studies have revealed a molecular basis for NK cell recognition of virus-infected cells, implicating the activating KIR and Ly49 receptors and NKG2D in this process. Additionally, mutual cooperation between NK cells and dendritic cells suggests that these innate cells can shape the nature of an adaptive immune response. These findings, as well as advances in understanding NK cell development and homeostasis, indicate that NK cell biology is more sophisticated than previously appreciated.     

Dendritic cells and natural killer cells in the pathogenesis of HIV infection

Dendritic cells (DC) and natural killer (NK) cells, the main cellular components of the innate immune system, participate in the most ancient first line of defense against infections. Both types of cells patrol peripheral tissues, whereas their rapid recruitment and activation at mucosal surfaces [the major entry point for the human immunodeficiency virus (HIV)] is a hallmark of acute inflammatory response. The ability of HIV to survive and replicate in the human host relies upon several molecular mechanisms eluding the immune surveillance of both adaptive immunity and of DC and NK cells beginning with the acute phase of primary HIV infection. DC and NK cells, unlike CD4⁺ T cells, are impaired more functionally rather than being depleted by HIV infection. In this article we will review some of the aspects of DC/NK cells interaction with HIV infection both in vitro and in vivo, and we will also speculate on the potential consequence for HIV pathogenesis and for the capacity of the virus to escape the surveillance of the innate immune system.     

Regulation of adaptive immunity by natural killer cells

Natural killer (NK) cells are well recognized as cytolytic effector cells of the innate immune system. In the past several years, the structure and function of NK cell receptors for the major histocompatibility complex (MHC) class I molecules and other ligands have been the subject of extensive studies. These studies have focused largely on the mechanisms of target cell recognition for lysis. Another aspect of NK cell function that seems to be underappreciated is their role in immune regulation. Since NK cells produce a number of immunologically relevant cytokines, it has been suggested that these cells may modulate the development of the adaptive immune response. But, is it the only mechanism by which NK cells interact with cells involved in the induction of antigen-specific responses? This article reviews some older and more recent studies and attempts to place NK cells in the context of potent immune regulators of T cell responses.     

Learning how to discriminate between friends and enemies, a lesson from Natural Killer cells

Human Natural Killer cells express a large array of surface receptors that are involved in the regulation of the different NK cell functions. Some of these receptors, by sensing the expression of HLA class I on potential target cells, deliver negative signals that inhibit the NK-mediated killing. In the absence of such inhibitory interactions, other receptors are allowed to transduce activating signals that result in NK-mediated attack of tumor- or virus-transformed cells. The function of these main activating receptors is supported and enhanced by the simultaneous engagement of different co-receptors. Finally, the ability of NK cells to interact with normal, non-transformed other cells such as dendritic cells (DC) appears to crucially contribute to the regulation of both innate and adaptive immune responses.     

Interaction between conventional dendritic cells and natural killer cells is integral to the activation of effective antiviral immunity

Dendritic cells (DCs) regulate various aspects of innate immunity, including natural killer (NK) cell function. Here we define the mechanisms involved in DC-NK cell interactions during viral infection. NK cells were efficiently activated by murine cytomegalovirus (MCMV)-infected CD11b(+) DCs. NK cell cytotoxicity required interferon-alpha and interactions between the NKG2D activating receptor and NKG2D ligand, whereas the production of interferon-gamma by NK cells relied mainly on DC-derived interleukin 18. Although Toll-like receptor 9 contributes to antiviral immunity, we found that signaling pathways independent of Toll-like receptor 9 were important in generating immune responses to MCMV, including the production of interferon-alpha and the induction of NK cell cytotoxicity. Notably, adoptive transfer of MCMV-activated CD11b(+) DCs resulted in improved control of MCMV infection, indicating that these cells participate in controlling viral replication in vivo.     

Soluble HLA-G molecules impair natural killer/dendritic cell crosstalk via inhibition of dendritic cells

HLA-G molecules are known to exert immunosuppressive action on DC maturation and on NK cells, and can in consequence inhibit respectively T cell responses and NK cytolysis. In this study, we show that monocyte-derived DC, differentiated in the presence of GM-CSF and IL-4, are sensitive to soluble (s) HLA-G molecules during LPS/IFN-gamma maturation as demonstrated by the decrease of CD80 and HLA-DR expressions and IL-12 secretion. Moreover, DC pretreated with sHLA-G were found to activate NK/DC crosstalk less than non-treated DC. Early activation of NK cells co-cultured with autologous DC was diminished as assessed by CD69 expression. The IFN-gamma production was impaired whereas a slight inhibition of the NK cell cytotoxicity against Daudi cell line was observed. Since sHLA-G is expressed in grafts or sites of tumour proliferation, its indirect action on NK cells via DC could constitute a pathway of early inhibition for both innate and specific immune responses.     

Molecular Events of Bacterial-Induced Maturation ofDendritic Cells

In order to protect the body from infectious microorganisms, mammals have developed powerful lines of defense, consisting in innate and adaptive immune responses. The innate response is phylogenetically more ancient and, for a long time, it has been considered to be broadly directed to microorganisms. However, the discovery of a new class of receptors, involved in recognition of patterns characteristic of groups of microorganisms (the toll-like receptor family) has re-evaluated the role of the innate immune system as a discriminating system. Indeed, there is increasing evidence that the induction of different types of effector adaptive responses are directed by the innate immune system after recognition of particular groups of pathogens. The central role of Dendritic cells (DC) in the induction of adaptive immune responses towards infectious agents has been extensively described, but, recently, a new role of DC as a link between the non-antigen- and the antigen-specific responses has been proposed. DC have, indeed, the capacity to recruit and activate cells of the innate immune system upon inflammation. Thus, understanding the interaction of bacteria with DC, and the early molecular events resulting from this interaction may shed some light on the mechanisms of initiation of the immune response to infectious agents and on aspects of invasiveness, pathogenicity, and the persistence of certain bacteria.     

The 'T-cell-ness' of NK cells: unexpected similarities between NK cells and T cells

NK cells are considered as prototypical innate immune cells. However, recent discoveries have tended to refine the dogmatic concepts of innate and adaptive immunity. In many ways, NK cells are highly related to T cells and represent the closest innate immune cell lineage to adaptive immune cell populations. Here, we review the relationships between NK cells and T cells and discuss the recently described cell-intrinsic-adaptive features of NK cells.     

Natural killer and dendritic cells collaborate in the immune response induced by the vaccine against uterine cervical cancer

Virus‐like particles (VLPs) of human papillomavirus (HPV) are used as a vaccine against HPV‐induced cancer, and recently we have shown that these VLPs are able to activate natural killer (NK) cells. Since NK cells collaborate with dendritic cells (DCs) to induce an immune response against viral infections and tumors, we studied the impact of this crosstalk in the context of HPV vaccination. NK cells in the presence of HPV‐VLPs enhanced DC‐maturation as shown by an upregulation of CD86 and HLA‐DR and an increased production of IL‐12p70, but not of the immunosuppressive cytokine IL‐10. This activation was bidirectional. Indeed, in the presence of HPV‐VLPs, DCs further activated NK cells by inducing the upregulation of cell surface activation markers (CD69 and HLA‐DR). The function of NK cells was also improved as shown by an increase in IFN‐γ secretion and cytotoxic activity against an HPV⁺ cell line. This crosstalk between NK cells and DCs needed CD40 interaction and IL‐12p70 secretion, whereas NKG2D was not implicated. Our results provide insight into how VLPs interact with innate immune cells and how NK cells and DCs play a role in the immune response induced by this vaccine agent.     

Distinct MAPK pathways are involved in IL-23 production in dendritic cells cocultured with NK cells in the absence or presence of angiotensin II

Accumulated evidence suggests that the crosstalk between dendritic cells (DCs) and natural killer (NK) cells enhances each other's capacity, and results in the production of a variety of soluble factors. However, little is known about the effect of DC-NK crosstalk in interleukin-23 (IL-23) production. In the present study we show that DC-NK coculture caused a high expression of IL-23, angiotensin II (Ang II) alone moderately increased IL-23 production in DCs, but decreased IL-23 secretion in the DC-NK coculture system. We found that Ang II does not influence DC maturation in DC-NK crosstalk. We next investigated the mitogen-activated protein kinase (MAPK) pathway in DCs. We found that Ang II increased IL-23 production through the extracellular signal-related kinase (ERK) pathway. All three MAPK members c-Jun N-terminal kinase (JNK), ERK and p38 were involved in IL-23 production in the DC-NK coculture system. In the presence of Ang II, only the JNK pathway seems to play a role in IL-23 production in DCs cocultured with NK. These data suggest that distinct MAPK pathways are involved in IL-23 production in DCs in response to different stimuli. This work demonstrates for the first time that IL-23 is produced in the DC-NK coculture system, and that Ang II is involved in DC-NK crosstalk. This data will act as a resource that allows further exploitation of role of immune response in atherosclerosis.     

Regulation of adaptive immune responses by innate cells expressing NK markers and antigen-transporting macrophages

A continuing theme of work done in our laboratory involves regulation of adaptive immune response by innate cells, in general, and immuneregulation by natural killer (NK) and NKT cells, in particular. Studies include work with the lung and the eye. In addition to immune surveillance of tumor cells, the NK cell is often associated with secreting cytokines that contribute to the creation of microenvironments conducive to Th1 responses and with defense mechanisms that lessen the initial infecting viral load. Reported studies show that the NKT cells support both T helper cell responses (type 1 and 2), as well as their being absolutely central to the development of antigen-specific T-regulatory cells involved in peripheral tolerance. Because of the multifunctional capabilities of the NKT cell, we propose that yet another cell, such as the antigen-presenting cell (APC), may influence the effector pathway of the NKT cell. We postulate that the APC that transports the antigen from the entry environment provides both trafficking and activation signals for innate cells in the secondary lymphoid organs. Evidence is presented that macrophage-derived signals selectively recruit NKT cells and bias their cytokine synthesis. Data imply that, just as occurs in immune inflammation, a collection of innate and adaptive immune cells interact within the secondary lymphoid tissue to generate antigen-specific tolerance in the periphery.     

Innate and adaptive stimulation of murine diverse NKT cells result in distinct cellular responses

Natural killer T (NKT) cells recognize glycolipids presented on CD1d. They share features of adaptive T lymphocytes and innate NK cells, and mediate immunoregulatory functions via rapid production of cytokines. Invariant (iNKT) and diverse (dNKT) NKT cell subsets are defined by their TCR. The immunological role of dNKT cells, that do not express the invariant TCRα‐chain used by iNKT cells, is less well explored than that of iNKT cells. Here, we investigated signals driving Toll‐like receptor (TLR) ligand activation of TCR‐transgenic murine dNKT cells. IFN‐γ production by dNKT cells required dendritic cells (DC), cell‐to‐cell contact and presence of TLR ligands. TLR‐stimulated DC activated dNKT cells to secrete IFN‐γ in a CD1d‐, CD80/86‐ and type I IFN‐independent manner. In contrast, a requirement for IL‐12p40, and a TLR ligand‐selective dependence on IL‐18 or IL‐15 was observed. TLR ligand/DC stimulation provoked early secretion of pro‐inflammatory cytokines by both CD62L⁺ and CD62L^? dNKT cells. However, proliferation was limited. In contrast, TCR/co‐receptor‐mediated activation resulted in proliferation and delayed production of a broader cytokine spectrum preferentially in CD62L^? dNKT cells. Thus, innate (TLR ligand/DC) and adaptive (TCR/co‐receptor) stimulation of dNKT cells resulted in distinct cellular responses that may contribute differently to the formation of immune memory.     

Mast cells in allergy: innate instructors of adaptive responses

The function of mast cells as effector cells in allergy has been extensively studied. However, increasing insight into mast cell physiology has revealed new mast cell functions and has introduced mast cells as key players in the regulation of innate as well as adaptive immunity. For example, mast cells have recently been found to express Toll-like receptors (TLRs), which enable them to participate in the innate immune response against pathogens. Furthermore, mast cells have been reported to interact with B cells, dendritic cells and T cells and thereby modulate the direction of an adaptive immune response. Finally, recent documentation that mast cells express functional MHC class II and costimulatory molecules and release immunologically active exosomes, has raised the possibility that mast cells also engage in (as yet) poorly understood antigen presentation functions. In this review, we explore the hypothesis that mast cells serve as central mediators between innate and adaptive immunity, rather as pure effector cells, during allergic innate responses.