Functional interactions between dendritic cells and NK cells during viral infection

Dendritic cells support the in vivo development and maintenance of NK cells via IL-15 trans-presentation

IL-15 is a key component that regulates the development and homeostasis of NK cells and is delivered through a mechanism termed trans-presentation. During development, multiple events must proceed to generate a functional mature population of NK cells that are vital for tumor and viral immunity. Nevertheless, how IL-15 regulates these various events and more importantly what cells provide IL-15 to NK cells to drive these events is unclear. It is known dendritic cells (DC) can activate NK cells via IL-15 trans-presentation; however, the ability of DC to use IL-15 trans-presentation to promote the development and homeostatic maintenance of NK cell has not been established. In this current study, we show that IL-15 trans-presentation solely by CD11c(+) cells assists the in vivo development and maintenance of NK cells. More specifically, DC-mediated IL-15 trans-presentation drove the differentiation of NK cells, which included the up-regulation of the activating and inhibitory Ly49 receptors. Although these cells did not harbor a mature CD11b(high) phenotype, they were capable of degranulating and producing IFN-gamma upon stimulation similar to wild-type NK cells. In addition, DC facilitated the survival of mature NK cells via IL-15 trans-presentation in the periphery. Thus, an additional role for NK-DC interactions has been identified whereby DC support the developmental and homeostatic niche of NK cells.

Hidden talents of natural killers: NK cells in innate and adaptive immunity

Natural killer (NK) cells are innate immune lymphocytes capable of killing target cells and producing immunoregulatory cytokines. Herein, we discuss recent studies that indicate that NK cells span the conventional boundaries between innate and adaptive immunity. For example, it was recently discovered that NK cells have the capacity for memory-like responses, a property that was previously thought to be limited to adaptive immunity. NK cells have also been identified in multiple tissues, and a subset of cells that specialize in the production of the T(H)17 cytokine IL-22, NK-22s, was recently described in mucosal-associated lymphoid tissue. Finally, we review work that shows that NK cells develop at sites that were traditionally thought to be occupied only by adaptive immune cells, including the thymus and lymph nodes.

Advances in Immunotherapy of Multiple Myeloma: From the Discovery of Tumor-Associated Antigens to Clinical Trials

Tumors aberrantly express tumor-associated antigens that can be specifically recognized by T-cells, thereby providing a scientific rationale for the design and clinical testing of immunotherapeutic strategies targeting these antigens. Multiple myeloma is a fatal hematologic malignancy. Here, we review techniques to discover new tumor-associated antigens in multiple myeloma and the latest immunotherapeutic strategies employed in this disease.

Cutting edge: IL-15-independent NK cell response to mouse cytomegalovirus infection

NK cells respond rapidly during viral infection. The development, function, and survival of NK cells are thought to be dependent on IL-15. In mice lacking IL-15, NK cells are found in severely decreased numbers. Surprisingly, following infection of IL-15- and IL-15Ralpha-deficient mice with mouse CMV, we measured a robust proliferation of Ly49H-bearing NK cells in lymphoid and nonlymphoid organs capable of cytokine secretion and cytolytic function. Remarkably, even in Rag2(-/-) x Il2rg(-/-) mice, a widely used model of NK cell deficiency, we detected a significant number of NK cells 1 wk after mouse CMV infection. In these mice we measured a >300-fold expansion of NK cells, which was dependent on recognition of the m157 viral glycoprotein ligand and IL-12. Together, these findings demonstrate a previously unrecognized independence of NK cells on IL-15 or other common gamma signaling cytokines during their response against viral infection.

Crosstalk between components of the innate immune system: promoting anti-microbial defenses and avoiding immunopathologies

Because it reaches full functional efficacy rapidly upon encounter with a pathogen, the innate immune system is considered as the first line of defense against infections. The sensing of microbes or of transformed or infected cells, through innate immune recognition receptors (referred to as activating I2R2), initiates pro-inflammatory responses and innate immune effector functions. Other I2R2 with inhibitory properties bind self-ligands constitutively expressed in host. However, this dichotomy in the recognition of foreign or induced self versus constitutive self by I2R2 is not always respected in certain non-infectious conditions reminiscent of immunopathologies. In this review, we discuss that immune mechanisms have evolved to avoid inappropriate inflammatory disorders in individuals. Molecular crossregulation exists between components of I2R2 signaling pathways, and intricate interactions between cells from both innate and adaptive immune systems set the bases of controlled immune responses. We also pinpoint that, like T or B cells, some cells of the innate immune system must go through education processes to prevent autoreactivity. In addition, we illustrate how gene expression profiling of immune cell types is a useful tool to find functional homologies between cell subsets of different species and to speculate about unidentified functions of these cells in the responses to pathogen infections.

Dendritic cells in cytomegalovirus infection: viral evasion and host countermeasures

Human cytomegalovirus (HCMV) is a beta-herpesvirus that infects the majority of the population during early childhood and thereafter establishes life-long latency. Primary infection as well as spontaneous reactivation usually remains asymptomatic in healthy hosts but can, in the context of systemic immunosuppression, result in substantial morbidity and mortality. HCMV counteracts the host immune response by interfering with the recognition of infected cells. A growing body of literature has also suggested that the virus evades the immune system by paralyzing the initiators of antiviral immune responses--the dendritic cells (DCs). In the current review, we discuss the effects of CMV (HCMV and murine CMV) on various DC subsets and the ensuing innate and adaptive immune responses. The impact of HCMV on DCs has mainly been investigated using monocyte-derived DCs, which are rendered functionally impaired by infection. In mouse models, DCs are targets of viral evasion as well, but the complex cross-talk between DCs and natural killer cells has, however, demonstrated an instrumental role for DCs in the control and clearance of viral infection. Fewer studies address the role of peripheral blood DC subsets, plasmacytoid DCs and CD11c+ myeloid DCs in the response against HCMV. These DCs, rather than being paralyzed by HCMV, are largely resistant to infection, mount a vigorous first-line defense and induce T-cell responses to the virus. This possibly provides a partial explanation for an intriguing conundrum: the highly efficient control of viral infection and reactivation in immunocompetent hosts in spite of multi-layered viral evasion mechanisms.

NK-cell-mediated killing of target cells triggers robust antigen-specific T-cell-mediated and humoral responses

Previous work showed that administration of antigen-expressing apoptotic cells in vivo results in antigen-specific CD8+ T-cell responses independent of Toll-like receptor signaling. We report here that natural killer (NK) cells can serve a function directly upstream of this pathway and initiate robust adaptive immune responses via killing of antigen-expressing target cells. This pathway is highly sensitive, in that administration of as few as 10(4) target cells induced detectable antigen-specific CD8+ T-cell responses. Importantly, NK cell-mediated cytotoxicity of target cells could also induce robust antigen-specific CD4+ T-cell responses, which were critical for subsequent CD8+ T-cell priming and IgG responses. Unlike adaptive immune responses induced by gamma-irradiated cells, the NK-cell pathway required myeloid differentiating factor 88 (MyD88) and Toll/interleukin-1 receptor domain-containing adapter-inducing interferon-beta (Trif) signaling. NK cells have previously been shown to detect and kill pathogen-infected host cells, as well as neoplastic cells and tissue allografts. The present data provide further evidence that they also discharge a strong tie with their relatives in the adaptive immune system. We think that the recognition and killing of target cells by NK cells represents an important pathway for the generation of robust CD8+ T and humoral responses that may be exploited for vaccine development.

CD28/B7-mediated co-stimulation is critical for early control of murine cytomegalovirus infection

Control of acute murine cytomegalovirus (MCMV) infection is dependent upon both innate and adaptive immune responses, relying primarily upon natural killer (NK) and T-cell responses for control. Although CD28/B7 plays a clear role in T-cell responses in many antigen systems including some viral infections, the importance of co-stimulation during MCMV infection is unconfirmed. In addition, recent data suggest that CD28/B7 co-stimulation might also be important to Ly49H+ NK-cell expansion. We therefore hypothesized that CD28/B7 co-stimulation is critical to viral control after MCMV infection, and further that CD28/B7 co-stimulation plays a role in MCMV-specific T- and NK-cell responses. To test these hypotheses, we utilized C57BL/6 mice lacking the co-stimulatory molecules B7-1 and B7-2 or CD28. After primary infection with MCMV, viral titers are significantly elevated in mice lacking CD28 or B7 compared with wild-type mice. Impaired viral control is associated with significant defects in peripheral T-cell responses to MCMV, which appear to be dependent upon CD28/B7 co-stimulation. Abnormal hepatic T-cell responses in CD28(-/-) mice are preceded by impaired MCMV-specific Ly49H+ NK-cell responses. Cytokine evaluations confirm that CD28/B7 co-stimulation is not required for non-specific antiviral responses. We conclude that CD28-mediated co-stimulation is critical for early viral control during acute MCMV infection.

HLA class I subtype-dependent expansion of KIR3DS1+ and KIR3DL1+ NK cells during acute human immunodeficiency virus type 1 infection

NK cells are critical in the early containment of viral infections. Epidemiological and functional studies have shown an important role of NK cells expressing specific killer immunoglobulin-like receptors (KIRs) in the control of human immunodeficiency virus type 1 (HIV-1) infection, but little is known about the mechanisms that determine the expansion of these antiviral NK cell populations during acute HIV-1 infection. Here we demonstrate that NK cells expressing the activating receptor KIR3DS1(+) and, to a lesser extent, the inhibitory receptor KIR3DL1(+) specifically expand in acute HIV-1 infection in the presence of HLA-B Bw480I, the putative HLA class I ligand for KIR3DL1/3DS1. These data demonstrate for the first time the HLA class I subtype-dependent expansion of specific KIR(+) NK cells during an acute viral infection in humans.

NKT cell immune responses to viral infection

BACKGROUND

Natural killer T (NKT) cells are a heterogeneous population of innate T cells that have attracted interest because of their potential to regulate immune responses to a variety of pathogens. The most widely studied NKT cell subset is the invariant (i)NKT cells that recognize glycolipids in the context of the CD1d molecule. The multifaceted methods of activation iNKT cells possess and their ability to produce regulatory cytokines has made them a primary target for studies.

OBJECTIVE/METHODS

To give insights into the roles of iNKT cells during infectious diseases, particularly viral infections. We also highlight mechanisms leading to iNKT cell activation in response to pathogens.

CONCLUSIONS

iNKT cell's versatility allows them to detect and respond to several viruses. Therapeutic approaches to specifically target iNKT cells will require additional research. Notably, the roles of non-invariant NKT cells in response to pathogens warrant further investigation.

Dendritic cell interactions with NK cells from different tissues

INTRODUCTION

In recent years, it has been realized that innate lymphocytes do not act in isolation but potentiate their efficiency by interacting with each other, resulting even in the regulation of adaptive immune response. One such cross-talk exists between dendritic cells (DCs) and natural killer (NK) cells. Here, we summarize recent studies on which subsets of these two innate immune components participate in this interaction, how it influences immune responses, and to which extent similar stimuli are integrated by DCs and NK cells during innate immunity.

CONCLUSION

We suggest that this cross-talk should be harnessed by activating both of these innate leucocyte populations with new adjuvant formulations for immunotherapies.

Cancer vaccines for established cancer: how to make them better?

If one envisions dendritic cells (DCs) as nature's adjuvant, then it is easy to predict that they would be advantageous for cancer immunotherapy. Advances in culture processes that generate large numbers of purified and functionally mature DCs raised the possibility that DCs might be promising clinical agents to generate effective immune responses against cancer. The use of mature DCs as cellular vaccines was proposed to be superior to conventional strategies aimed at treating cancer, yet a phase III clinical trial in patients with melanoma demonstrated no increased benefit of DCs over standard therapy. Despite this and other apparent failures, we propose that DC-based therapy should not be discarded but rather reassessed. The heterogeneity of DCs and their interaction with other innate cells and regulatory and effector pathways must be clearly understood before the full therapeutic benefit of DCs are recognized. Several aspects of DC vaccination require optimization including the following: effective delivery of vaccines to DCs in lymphoid tissues; incorporation of components that induce appropriate DC activation; and facilitation of innate and adaptive interactions and reduction of regulatory T-cell networks or suppressive microenvironments that hinder the function of immune effectors. Application of this knowledge is resulting in encouraging new data in pre-clinical settings, where multiple arms of the immune system are targeted for cancer therapy.

Cytokine-induced memory-like natural killer cells

The mammalian immune response to infection is mediated by 2 broad arms, the innate and adaptive immune systems. Innate immune cells are a first-line defense against pathogens and are thought to respond consistently to infection, regardless of previous exposure, i.e., they do not exhibit memory of prior activation. By contrast, adaptive immune cells display immunologic memory that has 2 basic characteristics, antigen specificity and an amplified response upon subsequent exposure. Whereas adaptive immune cells have rearranged receptor genes to recognize the universe of antigens, natural killer (NK) cells are innate immune lymphocytes with a limited repertoire of germ-line encoded receptors for target recognition. NK cells also produce cytokines such as IFN-gamma (IFN-gamma) to protect the host during the innate response to infection. Herein, we show that cytokine-activated NK cells transferred into naïve hosts can be specifically detected 7-22 days later when they are phenotypically similar to naïve cells and are not constitutively producing IFN-gamma. However, they produce significantly more IFN-gamma when restimulated. This memory-like property is intrinsic to the NK cell. By contrast, memory-like NK cells do not express granzyme B protein and kill targets similarly to naïve NK cells. Thus, these experiments identify an ability of innate immune cells to retain an intrinsic memory of prior activation, a function until now attributed only to antigen-specific adaptive immune cells.

Dendritic cells are required for optimal activation of natural killer functions following primary infection with herpes simplex virus type 1

Natural killer (NK) cells play an important role in the optimal clearance of herpes simplex virus type 1 (HSV-1) infection in mice. Activated NK cells function via cytokine secretion or direct cytolysis of target cells; dendritic cells (DCs) are thought to make critical contributions in the activation of both of these functions. Yet, the magnitude and physiological relevance of DC-mediated NK cell activation in vivo is not completely understood. To examine the contribution of DC help in regulating NK cell functions after infection with HSV-1, we utilized a transgenic mouse model that allows the transient ablation of DCs. Using this approach, it was found that the gamma interferon (IFN-gamma) expression potential of NK cells is quantitatively and qualitatively impaired in the absence of DCs. With regard to priming of NK cytolytic functions, the ablation of DCs did not significantly affect cytotoxic protein expression by NK cells. An in vivo cytolytic assay did, however, reveal impairments in the magnitude of NK cell cytotoxicity. Overall, this study provides direct evidence that functional DCs are required for optimal IFN-gamma expression and cytolytic function by NK cells following infection with HSV-1.

Killers and beyond: NK-cell-mediated control of immune responses

Effective immunity requires coordinated activation of innate and adaptive immune responses. NK cells are principal mediators of innate immunity, able to respond to challenge quickly and generally without prior activation. The most acknowledged functions of NK cells are their cytotoxic potential and their ability to release large amounts of cytokines, especially IFN-gamma. Recently, it has become clear that NK cells are more than assassins. Indeed, NK cells play critical roles in shaping adaptive immunity.

Cutting edge: the mechanism of invariant NKT cell responses to viral danger signals

Invariant NK T (iNKT) cells influence the response to viral infections, although the mechanisms are poorly defined. In this study we show that these innate-like lymphocytes secrete IFN-gamma upon culture with CpG oligodeoxynucleotide-stimulated dendritic cells (DCs) from mouse bone marrow. This requires TLR9 signaling and IL-12 secretion by the activated DCs, but it does not require CD1d expression. iNKT cells also produce IFN-gamma in response to mouse CMV infection. Their mechanism of mouse CMV detection is quite similar to that of CpG, requiring both TLR9 signaling and IL-12 secretion, while the need for CD1d expression is relatively minor. Consequently, iNKT cells have the ability to respond to a variety of microbes, including viruses, in an Ag-independent manner, suggesting they may play a broad role in antipathogen defenses despite their limited TCR repertoire.

Do adaptive immune cells suppress or activate innate immunity?

Current dogma holds that the innate immune system primes the adaptive immune system in response to infection, which in turn amplifies innate responses in a positive loop to effectively control pathogens. Therefore, it is accepted in most cases that T-cell deficient hosts die of acute infection because of the impaired ability of the innate immune system to control pathogens. Recent studies, however, reveal that adaptive immune cells actively dampen initial innate responses. In contrast to current understanding, there is now evidence that an insufficient number of T cells results in loss of control of innate immune responses. This raises new questions regarding the, as of yet underappreciated, role of the adaptive immune system in early infection and inflammation.

Dendritic cells in viral infections

Antigen presenting cells (APCs) are recognized as key initiators of adaptive immunity, particularly to pathogens, by eliciting a rapid and potent immune attack on infected cells. Amongst APCs, dendritic cells (DCs) are specially equipped to initiate and regulate immune responses in a manner that depends on signals they receive from microbes and their cellular environment. To achieve this, they are equipped with highly efficient mechanisms that allow them to detect pathogens, to capture, process and present antigens, and to activate and guide the differentiation of T cells into effector and memory cells. DCs can no longer be considered as a homogeneous cell type performing a single function, but are heterogeneous both in phenotype, function and dependence on inflammatory stimuli for their formation and responsiveness. Recent studies of DC subtypes have highlighted the contrasting roles of different professional APCs in activating divergent arms of the immune response towards pathogens. In this review, we discuss the progress that has been made in dissecting the attributes of different DC subsets that migrate into, or reside permanently, within lymphoid tissues and their putative roles in the induction of the anti-viral immune response.

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.

Noncytotoxic functions of NK cells: direct pathogen restriction and assistance to adaptive immunity

Natural killer cells were named after their ability to mediate spontaneous cytotoxicity during innate immune responses. However, it has become clear in recent years that they play an equally important role in restricting infections and assisting the development of adaptive immune responses via their ability to produce cytokines. In humans, a dedicated NK cell subset primarily fulfills these later functions. In this review we discuss the noncytotoxic effector functions of NK cells and how they could be harnessed for immunotherapy and vaccine development.

Cutting edge: Priming of NK cells by IL-18

Recent evidence suggests that NK cells require priming to display full effector activity. In this study, we demonstrate that IL-18 contributed to this phenomenon. IL-18 signaling-deficient NK cells were found to be unable to secrete IFN-gamma in response to ex vivo stimulation with IL-12. This was not due to a costimulatory role of IL-18, because blocking IL-18 signaling during the ex vivo stimulation with IL-12 did not alter IFN-gamma production by wild-type NK cells. Rather, we demonstrate that IL-18 primes NK cells in vivo to produce IFN-gamma upon subsequent stimulation with IL-12. Importantly, IL-12-induced IFN-gamma transcription by NK cells was comparable in IL-18 signaling-deficient and -sufficient NK cells. This suggests that priming by IL-18 leads to an improved translation of IFN-gamma mRNA. These results reveal a novel type of cooperation between IL-12 and IL-18 that requires the sequential action of these cytokines.

Adenovirus MART-1-engineered autologous dendritic cell vaccine for metastatic melanoma

We performed a phase 1/2 trial testing the safety, toxicity, and immune response of a vaccine consisting of autologous dendritic cells (DCs) transduced with a replication-defective adenovirus (AdV) encoding the full-length melanoma antigen MART-1/Melan-A (MART-1). This vaccine was designed to activate MART-1-specific CD+8 and CD4+ T cells. Metastatic melanoma patients received 3 injections of 10(6) or 10(7) DCs, delivered intradermally. Cell surface phenotype and cytokine production of the DCs used for the vaccines were tested, and indicated intermediate maturity. CD8+ T-cell responses to MART-1 27-35 were assessed by both major histocompatibility complex class I tetramer and interferon (IFN)-gamma enzyme-linked immunosorbent spot (ELISPOT) before, during, and after each vaccine and CD4+ T-cell responses to MART-1 51-73 were followed by IFN-gamma ELISPOT. We also measured antigen response breadth. Determinant spreading from the immunizing antigen MART-1 to other melanoma antigens [gp100, tyrosinase, human melanoma antigen-A3 (MAGE-A3)] was assessed by IFN-gamma ELISPOT. Twenty-three patients were enrolled and 14 patients received all 3 scheduled DC vaccines. Significant CD8+ and/or CD4+ MART-1-specific T-cell responses were observed in 6/11 and 2/4 patients evaluated, respectively, indicating that the E1-deleted adenovirus encoding the cDNA for MART-1/Melan-A (AdVMART1)/DC vaccine activated both helper and killer T cells in vivo. Responses in CD8+ and CD4+ T cells to additional antigens were noted in 2 patients. The AdVMART1-transduced DC vaccine was safe and immunogenic in patients with metastatic melanoma.

Tolerance of NK cells encountering their viral ligand during development

During development, T and B cells encountering their cognate ligands via antigen-specific receptors are deleted or rendered anergic. Like T and B cells, natural killer (NK) cells express certain receptors, such as Ly49H, associated with immunoreceptor tyrosine-based activation motif-bearing adaptor proteins that transmit activating signals through Syk family kinases. Ly49H binds with high affinity to a mouse cytomegalovirus (MCMV)-encoded glycoprotein, m157, but does not recognize self-antigens. For comparison with the behavior of immature T and B cells exposed to foreign antigens, we addressed the fate of Ly49H(+) NK cells that encountered their viral ligand during development by retroviral transduction of bone marrow stem cells with m157. In chimeric mice expressing m157, we observed a reduction in Ly49H(+) NK cells in multiple tissues and less Ly49H on the cell surface. NK cells exposed to m157 during development appeared less mature, produced less interferon gamma when stimulated through Ly49H, and were unable to kill m157-bearing target cells. After MCMV infection, these NK cells were severely impaired in their ability to proliferate. Thus, if immature NK cells encounter ligands for their activating receptors, regulatory mechanisms exist to keep these cells in an unresponsive state.

Tonsilar NK cells restrict B cell transformation by the Epstein-Barr virus via IFN-gamma

Cells of the innate immune system act in synergy to provide a first line of defense against pathogens. Here we describe that dendritic cells (DCs), matured with viral products or mimics thereof, including Epstein-Barr virus (EBV), activated natural killer (NK) cells more efficiently than other mature DC preparations. CD56^brightCD16⁻ NK cells, which are enriched in human secondary lymphoid tissues, responded primarily to this DC activation. DCs elicited 50-fold stronger interferon-γ (IFN-γ) secretion from tonsilar NK cells than from peripheral blood NK cells, reaching levels that inhibited B cell transformation by EBV. In fact, 100- to 1,000-fold less tonsilar than peripheral blood NK cells were required to achieve the same protection in vitro, indicating that innate immune control of EBV by NK cells is most efficient at this primary site of EBV infection. The high IFN-γ concentrations, produced by tonsilar NK cells, delayed latent EBV antigen expression, resulting in decreased B cell proliferation during the first week after EBV infection in vitro. These results suggest that NK cell activation by DCs can limit primary EBV infection in tonsils until adaptive immunity establishes immune control of this persistent and oncogenic human pathogen.

Epstein-Barr virus (EBV) establishes a persistent infection in nearly all human adults. Due to its tumor causing potential EBV infection has to be continuously controlled by the immune system in virus carriers. We demonstrate here that in the first week after infection, when other EBV-specific immune responses are still being recruited, human natural killer (NK) cells are able to prevent transformation of the main host cell type by EBV, the human B cell. Especially NK cells of tonsils, the primary site of EBV infection, inhibit B cell transformation by EBV after they have been activated by dendritic cells (DCs). For this protective function, EBV can directly stimulate DCs to efficiently activate NK cells. Interestingly, NK cells primarily prevent B cell transformation by EBV via secretion of the anti-viral cytokine IFN-γ, and NK cells from tonsils and lymph nodes produce 5-fold more of this cytokine than their peripheral blood counterparts. These data suggest that specialized NK cells in tonsils, the mucosal entry site of EBV, can be efficiently stimulated by EBV-activated DCs, and then limit EBV-induced B cell transformation until EBV-specific immune control by other components of the immune system is established.

NK cells contribute to intracellular bacterial infection-mediated inhibition of allergic responses

To experimentally examine the hygiene hypothesis, here we studied the effect of chlamydial infection on the development of allergic responses induced by OVA and the involvement of NK cells in this process using a mouse model of airway inflammation. We found that prior Chlamydia muridarum infection can inhibit airway eosinophilic inflammation and mucus production induced by allergen sensitization and challenge. The inhibition was correlated with an alteration of allergen-driven cytokine-producing patterns of T cells. We demonstrated that NK cells were activated following chlamydial infection, showing both cell expansion and cytokine secretion. The in vivo depletion of NK cells using anti-NK Ab before OVA sensitization and challenge partially abolished the inhibitory effect of chlamydial infection, which was associated with a partial restoration of Th2 cytokine production. In contrast, the adoptive transfer of NK cells that were isolated from infected mice showed a significant inhibitory effect on allergic responses, similar to that observed in natural infection. The data suggest that the innate immune cells such as NK cells may play an important role in infection-mediated inhibition of allergic responses.

Dendritic cell-mediated NK cell activation is controlled by Jagged2-Notch interaction

Natural killer (NK) cells regulate various immune responses by exerting cytotoxic activity or secreting cytokines. The interaction of NK cells with dendritic cells (DC) contributes to NK cell-mediated antitumor or antimicrobial responses. However, the cellular and molecular mechanisms for controlling this interaction are largely unknown. Here, we show an involvement of Jagged2-Notch interaction in augmenting NK cell cytotoxicity mediated by DC. Enforced expression of Jagged2 on A20 cells (Jag2-A20 cells) suppressed their growth in vivo, which was abrogated by depleting NK cells. Moreover, Jag2-A20 cells exerted a suppression on the growth of nonmanipulated A20 cells in SCID mice in an NK-dependent manner. Consistently, coinoculation of A20 cells with DC overexpressing Jagged2 (Jag2-DC) suppressed the growth of A20 cells in mice. Stimulation of NK cells with Jagged2 directly enhanced their cytotoxicity, IFN-gamma production, and proliferation. Ligation of Notch2 on NK cells enhanced their cytotoxic activity, and Jag2-DC or CpG-treated DC-mediated NK cell cytotoxicity was suppressed by a gamma-secretase inhibitor. These results indicate that the Jagged2-Notch axis plays a crucial role in DC-mediated NK cell cytotoxicity. Furthermore, manipulation of this interaction may provide an approach to induce potent tumor immunity or to inhibit certain autoimmune diseases caused by NK cell activation.

NK cell survival mediated through the regulatory synapse with human DCs requires IL-15Ralpha

DCs activate NK cells during innate immune responses to viral infections. However, the composition and kinetics of the immunological synapse mediating this interaction are largely unknown. Here, we report the rapid formation of an immunological synapse between human resting NK cells and mature DCs. Although inhibitory NK cell receptors were polarized to this synapse, where they are known to protect mature DCs from NK cell lysis, the NK cell also received activation signals that induced mobilization of intracellular calcium and CD69 upregulation. The high-affinity component of the receptor for IL-15, IL-15Ralpha, accumulated at the synapse center on NK cells, and blocking of IL-15Ralpha increased NK cell apoptosis and diminished NK cell survival during their interaction with DCs. Furthermore, IL-15Ralpha-deficient NK cells, obtained from donors with a history of infectious mononucleosis, showed diminished survival in culture with DCs. Synapse formation was required for IL-15Ralpha-mediated NK cell survival, because synapse disruption by adhesion molecule blocking decreased DC-induced NK cell survival. These results identify what we believe to be a novel regulatory NK cell synapse with hallmarks of spatially separated inhibitory and activating interactions at its center. We suggest that this synapse formation enables optimal NK cell activation by DCs during innate immune responses.

Evolutionary struggles between NK cells and viruses

Natural killer (NK) cells are well recognized for their ability to provide a first line of defence against viral pathogens and they are increasingly being implicated in immune responses against certain bacterial and parasitic infections. Reciprocally, viruses have devised numerous strategies to evade the activation of NK cells and have influenced the evolution of NK-cell receptors and their ligands. NK cells contribute to host defence by their ability to rapidly secrete cytokines and chemokines, as well as to directly kill infected host cells. In addition to their participation in the immediate innate immune response against infection, interactions between NK cells and dendritic cells shape the nature of the subsequent adaptive immune response to pathogens.

Natural killer and dendritic cell liaison: recent insights and open questions

The functional links between natural killer (NK) and dendritic cells (DCs) have been widely investigated in the last years and different studies have demonstrated that reciprocal activations ensue upon NK/DC interactions. More recently, the anatomical sites were these interactions take place have been identified together with the related cell subsets involved. Remarkably, as predicted by pioneering studies, there is now "in vivo" evidence that this cellular cross-talk occurring during the innate phase of the immune response can deeply affects the magnitude and the quality of the subsequent adaptive response. Thus, NK cells are not merely cytotoxic lymphocytes competent in containing viral and tumor spreading but can now rather be considered as crucial fine-tuning effector cells. Despite the large mass of information rapidly obtained in this field, several fundamental questions still remain to be addressed. Among them, two central issues require additional consideration: (a) what mediates the activation of NK cell cytotoxicity induced by DCs and (b) what factors are responsible for NK-dependent maturation of DCs. Unexpectedly, for both of these questions insufficient or inconsistent results are so far available; factors either dependent or independent from cell contacts between DCs and NK cells have been convincingly described, and it is likely that several mechanisms, rather than a single one, are responsible for each of these novel innate functions. Understanding the molecular bases of this complex liaison will pave the way to new and more effective immune adjuvants. The most recent advances about NK/DC interplay are here reviewed and possible answers to still open questions in this field are considered.

Analysis of individual natural killer cell responses

Typical assays for natural killer (NK) cell function assess the responses of entire NK cell populations. It is now possible to determine the responses of individual NK cells. Herein, two representative assays are described along with examples of how they have helped clarify current understanding of NK cell biology.

Natural killer (NK) cells from killers to regulators: distinct features between peripheral blood and decidual NK cells

Natural killer (NK) cells are a key component of innate immunity, particularly crucial during the early phase of immune responses against certain viruses, parasites, and microbial pathogens. The role of NK cell during pregnancy has been vividly discussed over the past years and it is now becoming increasingly clear that NK cells control pregnancy maintenance at several levels. In normal pregnancy, it appears that they provide benefit by properly secreting cytokines, chemokines and angiogenic factors rather than functioning as cytotoxic effector cells. However, as they are endowed with all the cytolytic weapons, they promptly become capable of attacking fetal and maternal tissues during infection and inflammation.

Role of natural killer cells in the pathogenesis and progression of multiple sclerosis

Natural killer (NK) cells are a subset of lymphocytes which have long been alleged to play an immunoregulatory role in the prevention of autoimmune diseases. Here, we briefly review NK cell features and the major findings from studies on NK cells in human and animals susceptible to multiple sclerosis (MS). Although most studies in human seem to suggest an association between disease and deficiencies in NK cells, it is also clear that NK cells can be both protective and pathogenic in MS models. These contrasting observations could result from differences in experimental procedures as well as from differences in NK cell subset targeted. Whatever the case, the functional features of these cells and their potential role in regulation of autoimmunity suggest that NK cell-based therapies might be an interesting approach for the treatment of multiple sclerosis.

Genetic analysis of resistance to viral infection

As machines that reprogramme eukaryotic cells to suit their own purposes, viruses present a difficult problem for multicellular hosts, and indeed, have become one of the central pre-occupations of the immune system. Unable to permanently outpace individual viruses in an evolutionary footrace, higher eukaryotes have evolved broadly active mechanisms with which to sense viruses and suppress their proliferation. These mechanisms have recently been elucidated by a combination of forward and reverse genetic methods. Some of these mechanisms are clearly ancient, whereas others are relatively new. All are remarkably adept at discriminating self from non-self, and allow the host to cope with what might seem an impossible predicament.

Qa-1(b)-dependent modulation of dendritic cell and NK cell cross-talk in vivo

Dendritic cells (DC) trigger activation and IFN-gamma release by NK cells in lymphoid tissues, a process important for the polarization of Th1 responses. Little is known about the molecular signals that regulate DC-induced NK cell IFN-gamma synthesis. In this study, we analyzed whether the interaction between Qa-1(b) expressed on DC and its CD94/NKG2A receptor on NK cells affects this process. Activation of DC using CpG-oligodeoxynucleotides in Qa-1(b)-deficient mice, or transfer of CpG-oligodeoxynucleotide-activated Qa-1(b)-deficient DC into wild-type mice, resulted in dramatically increased IFN-gamma production by NK cells, as compared with that induced by Qa-1(b)-expressing DC. Masking the CD94/NKG2A inhibitory receptor on NK cells in wild-type mice similarly enhanced the IFN-gamma response of these cells to Qa-1(b)-expressing DC. Furthermore, NK cells from CD94/NKG2A-deficient mice displayed higher IFN-gamma production upon DC stimulation. These results demonstrate that Qa-1(b) is critically involved in regulating IFN-gamma synthesis by NK cells in vivo through its interaction with CD94/NKG2A inhibitory receptors. This receptor-ligand interaction may be essential to prevent unabated cytokine production by NK cells during an inflammatory response.

Type I interferon production during herpes simplex virus infection is controlled by cell-type-specific viral recognition through Toll-like receptor 9, the mitochondrial antiviral signaling protein pathway, and novel recognition systems

Recognition of viruses by germ line-encoded pattern recognition receptors of the innate immune system is essential for rapid production of type I interferon (IFN) and early antiviral defense. We investigated the mechanisms of viral recognition governing production of type I IFN during herpes simplex virus (HSV) infection. We show that early production of IFN in vivo is mediated through Toll-like receptor 9 (TLR9) and plasmacytoid dendritic cells, whereas the subsequent alpha/beta IFN (IFN-alpha/beta) response is derived from several cell types and induced independently of TLR9. In conventional DCs, the IFN response occurred independently of viral replication but was dependent on viral entry. Moreover, using a HSV-1 UL15 mutant, which fails to package viral DNA into the virion, we found that entry-dependent IFN induction also required the presence of viral genomic DNA. In macrophages and fibroblasts, where the virus was able to replicate, HSV-induced IFN-alpha/beta production was dependent on both viral entry and replication, and ablated in cells unable to signal through the mitochondrial antiviral signaling protein pathway. Thus, during an HSV infection in vivo, multiple mechanisms of pathogen recognition are active, which operate in cell-type- and time-dependent manners to trigger expression of type I IFN and coordinate the antiviral response.

Adaptive immune cells temper initial innate responses

Toll-like receptors (TLRs) recognize conserved microbial structures called pathogen-associated molecular patterns. Signaling from TLRs leads to upregulation of co-stimulatory molecules for better priming of T cells and secretion of inflammatory cytokines by innate immune cells^1,2,3,4. Lymphocyte-deficient hosts often die of acute infection, presumably owing to their lack of an adaptive immune response to effectively clear pathogens. However, we show here that an unleashed innate immune response due to the absence of residential T cells can also be a direct cause of death. Viral infection or administration of poly(I:C), a ligand for TLR3, led to cytokine storm in T-cell- or lymphocyte-deficient mice in a fashion dependent on NK cells and tumor necrosis factor. We have further shown, through the depletion of CD4⁺ and CD8⁺ cells in wild-type mice and the transfer of T lymphocytes into Rag-1–deficient mice, respectively, that T cells are both necessary and sufficient to temper the early innate response. In addition to the effects of natural regulatory T cells, close contact of resting CD4⁺CD25⁻Foxp3⁻ or CD8⁺ T cells with innate cells could also suppress the cytokine surge by various innate cells in an antigen-independent fashion. Therefore, adaptive immune cells have an unexpected role in tempering initial innate responses.

Natural killer cells promote early CD8 T cell responses against cytomegalovirus

Understanding the mechanisms that help promote protective immune responses to pathogens is a major challenge in biomedical research and an important goal for the design of innovative therapeutic or vaccination strategies. While natural killer (NK) cells can directly contribute to the control of viral replication, whether, and how, they may help orchestrate global antiviral defense is largely unknown. To address this question, we took advantage of the well-defined molecular interactions involved in the recognition of mouse cytomegalovirus (MCMV) by NK cells. By using congenic or mutant mice and wild-type versus genetically engineered viruses, we examined the consequences on antiviral CD8 T cell responses of specific defects in the ability of the NK cells to control MCMV. This system allowed us to demonstrate, to our knowledge for the first time, that NK cells accelerate CD8 T cell responses against a viral infection in vivo. Moreover, we identify the underlying mechanism as the ability of NK cells to limit IFN-α/β production to levels not immunosuppressive to the host. This is achieved through the early control of cytomegalovirus, which dramatically reduces the activation of plasmacytoid dendritic cells (pDCs) for cytokine production, preserves the conventional dendritic cell (cDC) compartment, and accelerates antiviral CD8 T cell responses. Conversely, exogenous IFN-α administration in resistant animals ablates cDCs and delays CD8 T cell activation in the face of NK cell control of viral replication. Collectively, our data demonstrate that the ability of NK cells to respond very early to cytomegalovirus infection critically contributes to balance the intensity of other innate immune responses, which dampens early immunopathology and promotes optimal initiation of antiviral CD8 T cell responses. Thus, the extent to which NK cell responses benefit the host goes beyond their direct antiviral effects and extends to the prevention of innate cytokine shock and to the promotion of adaptive immunity.

To fight viral infections, vertebrates have developed a battery of innate and adaptive immune responses aimed at inhibiting viral replication or at killing infected cells. These responses include the early production of innate antiviral cytokines, especially interferons α and β (IFN-α/β), and the activation of cytotoxic lymphocytes such as the innate natural killer (NK) cells and the adaptive CD8 T cells. While critical for antiviral defense, cytokine or CD8 T cell responses can be detrimental or even fatal to the host when deregulated. Therefore, we need to better understand how the different arms of antiviral immunity are regulated. In particular, NK cells are proposed to play a protective role in a variety of viral infection in humans, but the underlying mechanisms remain poorly understood. Here, in a mouse model of cytomegalovirus infection, we demonstrate that NK cells prevent an excessive production of IFN-α/β and promote more efficient antiviral CD8 T cell responses. We thus show that NK cells can help promote health over disease during viral infections by regulating both innate and adaptive immune responses. It will be important to examine in humans whether NK cells control innate cytokine production to prevent immunopathology and to promote adaptive immunity against herpesviruses, HIV-1, influenza, or SARS.

NKp30-dependent cytolysis of filovirus-infected human dendritic cells

Understanding how protective innate immune responses are generated is crucial to defeating highly lethal emerging pathogens. Accumulating evidence suggests that potent innate immune responses are tightly linked to control of Ebola and Marburg filoviral infections. Here, we report that unlike authentic or inactivated Ebola and Marburg, filovirus-derived virus-like particles directly activated human natural killer (NK) cells in vitro, evidenced by pro-inflammatory cytokine production and enhanced cytolysis of permissive target cells. Further, we observed perforin- and CD95L-mediated cytolysis of filovirus-infected human dendritic cells (DCs), primary targets of filovirus infection, by autologous NK cells. Gene expression knock-down studies directly linked NK cell lysis of infected DCs to upregulation of the natural cytotoxicity receptor, NKp30. These results are the first to propose a role for NK cells in the clearance of infected DCs and the potential involvement of NKp30-mediated cytolysis in control of viral infection in vivo. Further elucidation of the biology of NK cell activation, specifically natural cytotoxicity receptors like NKp30 and NKp46, promises to aid our understanding of microbial pathology.

NK Cells Enhance Dendritic Cell Response against Parasite Antigens via NKG2D Pathway

Recent studies have shown that NK-dendritic cell (DC) interaction plays an important role in the induction of immune response against tumors and certain viruses. Although the effect of this interaction is bidirectional, the mechanism or molecules involved in this cross-talk have not been identified. In this study, we report that coculture with NK cells causes several fold increase in IL-12 production by Toxoplasma gondii lysate Ag-pulsed DC. This interaction also leads to stronger priming of Ag-specific CD8+ T cell response by these cells. In vitro blockade of NKG2D, a molecule present on human and murine NK cells, neutralizes the NK cell-induced up-regulation of DC response. Moreover, treatment of infected animals with Ab to NKG2D receptor compromises the development of Ag-specific CD8+ T cell immunity and reduces their ability to clear parasites. These studies emphasize the critical role played by NKG2D in the NK-DC interaction, which apparently is important for the generation of robust CD8+ T cell immunity against intracellular pathogens. To the best of our knowledge, this is the first work that describes in vivo importance of NKG2D during natural infection.

Intravenous administration of MIP-1α with intra-tumor injection of P. acnes shows potent anti-tumor effect

Dendritic cells (DCs) play a key role as potent biological adjuvants in anti-tumor host responses. However, DC-based vaccination does not always eradicate tumors effectively. Clinical evidence suggests that a strategy to recruit a substantial number of DCs into the tumor mass might provoke proficient anti-tumor immune responses. Here we describe that myeloid DCs (mDCs) efficiently accumulate in tumor sites after intravenous injection of recombinant macrophage inflammatory protein (MIP)-1alpha when pretreated locally with adjuvants like Propionibacterium acnes. Combined treatment of tumor-bearing mice with MIP-1alpha and P. acnes also recruited a large number of natural killer cells (NK cells) to both tumor sites and regional lymph nodes (LNs) and induced a strong T helper 1 immunity at an early time. This early response later led to accumulation of CD8(+) T cells, retraction of tumors and survival of animals treated with P. acnes/MIP-1alpha. In vivo depletion of NK cells or CD8(+) T cells impaired anti-tumor effects, suggesting that activation of NK cells and CD8(+) T cells contributes to anti-tumor immunity in this model. Therefore, this study provides a novel therapeutic strategy for cancer treatment using MIP-1alpha and certain adjuvants.

Acquisition of murine NK cell cytotoxicity requires the translation of a pre-existing pool of granzyme B and perforin mRNAs

Although activated murine NK cells can use the granule exocytosis pathway to kill target cells immediately upon recognition, resting murine NK cells are minimally cytotoxic for unknown reasons. Here, we showed that resting NK cells contained abundant granzyme A, but little granzyme B or perforin; in contrast, the mRNAs for all three genes were abundant. Cytokine-induced in vitro activation of NK cells resulted in potent cytotoxicity associated with a dramatic increase in granzyme B and perforin, but only minimal changes in mRNA abundance for these genes. The same pattern of regulation was found in vivo with murine cytomegalovirus infection as a physiologic model of NK cell activation. These data suggest that resting murine NK cells are minimally cytotoxic because of a block in perforin and granzyme B mRNA translation that is released by NK cell activation.

Viral Interference with Antigen Presentation Does Not Alter Acute or Chronic CD8 T Cell Immunodominance in Murine Cytomegalovirus Infection

Both human CMV and murine CMV (MCMV) elicit large CD8 T cell responses, despite the potent effects of viral genes that interfere with the MHC class I (MHC I) pathway of Ag presentation. To investigate the impact of immune evasion on CD8 T cell priming, we infected mice with wild-type (wt) MCMV or a mutant lacking its MHC I immune evasion genes, Deltam4+m6+m152 MCMV. In acute infection, the two viruses elicited a CD8 T cell response to 26 peptide epitopes that was virtually identical in total size, kinetics, and immunodominance hierarchy. This occurred despite results demonstrating that primary DCs are susceptible to the effects of MCMV's MHC I immune evasion genes. Eight months later, responses to both wt and mutant MCMV displayed the same CD8 T cell "memory inflation" and altered immunodominance that characterize the transition to chronic MCMV infection in C57BL/6 mice. Taken together, these findings suggest either that cross-priming dominates over direct CD8 T cell priming in both acute and chronic MCMV infection, or else that the MHC I immune evasion genes of MCMV are unable to alter direct CD8 T cell priming in vivo. At 2 years postinfection, differences in CD8 T cell immunodominance emerged between individual mice, but on average there were only slight differences between wt and mutant virus infections. Overall, the data indicate that the presence or absence of MHC I immune evasion genes has remarkably little impact on the size or specificity of the MCMV-specific CD8 T cell response over an entire lifetime of infection.

Cellular targets of interleukin-18 in rheumatoid arthritis

Recent data are presented which indicate a critical role for interleukin (IL)-18 in rheumatoid arthritis (RA). The T cells and macrophages invading the synovium or in the synovial fluid are the chief cellular targets of IL-18 in RA. Neutrophils, dendritic cells and endothelial cells may also be cellular mediators of IL-18. The direct effect of IL-18 on fibroblast-like synoviocytes or chondrocytes may not be essential or important. In RA, IL-18, which is mainly produced by macrophages, activates T cells and macrophages to produce proinflammatory cytokines, chemokines, adhesion molecules and RANKL which, in turn, perpetuate chronic inflammation and induce bone and cartilage destruction.

Self-tolerance, dendritic cell (DC)-mediated activation and tissue distribution of natural killer (NK) cells

Natural killer (NK) cells are lymphocytes of the innate immune system that exert a potent function against infected and tumor cells. Although NK cells were originally defined by their capacity to lyse target cells and produce interferon (IFN)-gamma without prior activation, more recent studies found that NK cells display also a potent regulatory function. Following engagement of surface receptors by other cells or signalling by soluble molecules, NK cells release cytokines able to influence the outcome of an immune response. Since their discovery in the 1970s, the biology of NK cells has been deeply investigated; nevertheless some aspects of their maturation process, activation mechanisms, and tissue distribution remain still obscure. These review will focus on three major issues regarding NK cell regulation. In particular we aim to discuss: (i) how NK cells become tolerant to self-tissues during their maturation; (ii) how NK cells become activated, with a particular attention to dendritic cell (DC)-mediated mechanisms of NK cell priming; (iii) where NK cells play their functions and how NK cell tissue distribution can favour their capacity to skew T cell responses.

NK Cell Maturation and Peripheral Homeostasis Is Associated with KLRG1 Up-Regulation

NK cells are important for the clearance of tumors, parasites, and virus-infected cells. Thus, factors that control NK cell numbers and function are critical for the innate immune response. A subset of NK cells express the inhibitory killer cell lectin-like receptor G1 (KLRG1). In this study, we identify that KLRG1 expression is acquired during periods of NK cell division such as development and homeostatic proliferation. KLRG1(+) NK cells are mature in phenotype, and we show for the first time that these cells have a slower in vivo turnover rate, reduced proliferative response to IL-15, and poorer homeostatic expansion potential compared with mature NK cells lacking KLRG1. Transfer into lymphopenic recipients indicate that KLRG1(-) NK cells are precursors of KLRG1(+) NK cells and KLRG1 expression accumulates following cell division. Furthermore, KLRG1(+) NK cells represent a significantly greater proportion of NK cells in mice with enhanced NK cell numbers such as Cd45(-/-) mice. These data indicate that NK cells acquire KLRG1 on their surface during development, and this expression correlates with functional distinctions from other peripheral NK cells in vivo.

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 cell activation in visceral leishmaniasis requires TLR9, myeloid DCs, and IL-12, but is independent of plasmacytoid DCs

Natural killer (NK) cells are sentinel components of the innate response to pathogens, but the cell types, pathogen recognition receptors, and cytokines required for their activation in vivo are poorly defined. Here, we investigated the role of plasmacytoid dendritic cells (pDCs), myeloid DCs (mDCs), Toll-like receptors (TLRs), and of NK cell stimulatory cytokines for the induction of an NK cell response to the protozoan parasite Leishmania infantum. In vitro, pDCs did not endocytose Leishmania promastigotes but nevertheless released interferon (IFN)-alpha/beta and interleukin (IL)-12 in a TLR9-dependent manner. mDCs rapidly internalized Leishmania and, in the presence of TLR9, produced IL-12, but not IFN-alpha/beta. Depletion of pDCs did not impair the activation of NK cells in L. infantum-infected mice. In contrast, L. infantum-induced NK cell cytotoxicity and IFN-gamma production were abolished in mDC-depleted mice. The same phenotype was observed in TLR9(-/-) mice, which lacked IL-12 expression by mDCs, and in IL-12(-/-) mice, whereas IFN-alpha/beta receptor(-/-) mice showed only a minor reduction of NK cell IFN-gamma expression. This study provides the first direct evidence that mDCs are essential for eliciting NK cell cytotoxicity and IFN-gamma release in vivo and demonstrates that TLR9, mDCs, and IL-12 are functionally linked to the activation of NK cells in visceral leishmaniasis.

DAP12 signaling directly augments proproliferative cytokine stimulation of NK cells during viral infections

NK cells vigorously proliferate during viral infections. During the course of murine CMV infection, this response becomes dominated by the preferential proliferation of NK cells that express the activation receptor Ly49H. The factors driving such selective NK cell proliferation have not been characterized. In this study, we demonstrate that preferential NK cell proliferation is dependent on DAP12-mediated signaling following the binding of Ly49H to its virally encoded ligand, m157. Ly49H signaling through DAP12 appears to directly augment NK cell sensitivity to low concentrations of proproliferative cytokines such as IL-15. The impact of Ly49H-mediated signaling on NK cell proliferation is masked in the presence of high concentrations of proproliferative cytokines that nonselectively drive all NK cells to proliferate.