Regulation of activated CD4+ T cells by NK cells via the Qa-1-NKG2A inhibitory pathway

NKp46 regulates allergic responses

Natural killer (NK) cells are cytotoxic cells that are able to rapidly kill viruses, tumor cells, parasites, bacteria, and even cells considered "self". The activity of NK cells is controlled by a fine balance of inhibitory and activating signals mediated by a complex set of different receptors. However, the function of NK cells is not restricted only to the killing of target cells, NK cells also possess other properties such as the secretion of proangiogenic factors during pregnancy. Here, we demonstrate another unique NK-cell activity, namely the regulation of T-cell mediated allergic responses, which is dependent on the NK-cell specific receptor NKp46 (Ncr1 in mice). Using mice in which the Ncr1 gene has been replaced with a green fluorescent protein, we demonstrate reduced delayed-type hypersensitivity and airway hypersensitivity. Interestingly, we show that this reduction in airway hypersensitivity is due to differences in the stimulation of T cells resulting in an altered cytokine profile.

Anti-NKG2A autoantibodies in a patient with systemic lupus erythematosus

OBJECTIVES

To characterize a novel anti-NKG2A autoantibody detected in a patient with SLE during a severe flare, and in a cross-sectional study investigate the occurrence of such autoantibodies in patients with SLE and primary SS (pSS).

METHODS

Serum or IgG from patients with SLE, pSS and healthy volunteers were assayed for blocking of anti-NKG2A or HLA-E binding to peripheral blood mononuclear cells or CD94/NKG2A- and CD94/NKG2C-transfected Ba/F3 cells. The anti-NKG2A autoantibodies were evaluated for effect on NK cell degranulation in response to HLA-E-transfected K562 cells. IFN-α was determined by an immunoassay and disease activity by the SLEDAI score.

RESULTS

Anti-NKG2A autoantibodies, which blocked binding of HLA-E tetramers to CD94/NKG2A-transfected cells and impaired NKG2A-mediated inhibition of NK cell activation, were observed in a patient with SLE. The presence of anti-NKG2A autoantibodies was associated with high SLE disease activity (SLEDAI score 14 and 16) and increased serum IFN-α. Of 94 SLE, 60 pSS and 30 healthy donor sera, only the index patient serum contained anti-NKG2A autoantibodies.

CONCLUSION

The presence of autoantibodies targeting NKG2A is a rare event, but when such autoantibodies occur they may promote excessive NK cell function. This can contribute to the pathogenesis by increasing the killing of cells and the release of autoantigens. Our findings highlight the possible importance of NK cells in the SLE disease process.

What causes alopecia areata?

The pathobiology of alopecia areata (AA), one of the most frequent autoimmune diseases and a major unsolved clinical problem, has intrigued dermatologists, hair biologists and immunologists for decades. Simultaneously, both affected patients and the physicians who take care of them are increasingly frustrated that there is still no fully satisfactory treatment. Much of this frustration results from the fact that the pathobiology of AA remains unclear, and no single AA pathogenesis concept can claim to be universally accepted. In fact, some investigators still harbour doubts whether this even is an autoimmune disease, and the relative importance of CD8(+) T cells, CD4(+) T cells and NKGD2(+) NK or NKT cells and the exact role of genetic factors in AA pathogenesis remain bones of contention. Also, is AA one disease, a spectrum of distinct disease entities or only a response pattern of normal hair follicles to immunologically mediated damage? During the past decade, substantial progress has been made in basic AA-related research, in the development of new models for translationally relevant AA research and in the identification of new therapeutic agents and targets for future AA management. This calls for a re-evaluation and public debate of currently prevalent AA pathobiology concepts. The present Controversies feature takes on this challenge, hoping to attract more skin biologists, immunologists and professional autoimmunity experts to this biologically fascinating and clinically important model disease.

Cytokine complex-expanded natural killer cells improve allogeneic lung transplant function via depletion of donor dendritic cells

RATIONALE

Natural killer (NK) cells are innate lymphocytes that target virus-infected and tumor cells. Much less is known about their ability to limit adaptive immune responses.

OBJECTIVES

Thus, we investigated to what extent NK cells can influence mouse lung allograft rejection.

METHODS

For this purpose, we employed an orthotopic lung transplantation model in mice.

MEASUREMENTS AND MAIN RESULTS

We demonstrate here that NK cells infiltrate mouse lung allografts before T cells and thereby diminished allograft inflammation, and that NK-cell deficiency enhanced allograft rejection. In contrast, expansion of recipient NK cells through IL-15/IL-15Rα complex treatment resulted in decreased T-cell infiltration and alloreactive T-cell priming as well as improved function of the allogeneic lung transplant. Only perforin-competent, but not perforin-deficient, NK cells were able to transfer these beneficial effects into transplanted NK cell-deficient IL-15Rα(-/-) mice. These NK cells killed allogeneic dendritic cells (DCs) in vitro and significantly decreased the number of allogeneic DCs in transplanted lungs in vivo. Furthermore, DC-depleted lung allografts presented decreased signs of rejection.

CONCLUSIONS

These results suggest that NK cells favor allograft acceptance by depleting donor-derived DCs, which otherwise would prime alloreactive T-cell responses. Thus, conditioning regimens that augment NK-cell reactivity should be clinically explored to prepare lung allograft recipients.

Natural killer cells regulate diverse T cell responses

Natural killer (NK) cells are important mediators of the immune response against microbial pathogens and tumors. There is growing evidence from mouse and human studies that, NK cells exhibit immunoregulatory functions and can limit T cell immunity. NK cell regulatory activity has been demonstrated in a variety of disease models including chronic viral infection, autoimmunity, and transplantation. Depending on the nature of the immune challenge, NK cells use different strategies to limit T cell function, including via cytokines, interactions with NK receptors NKG2D and NKp46, or by perforin-mediated T cell death. Future work should address whether specific subsets of NK cells inhibit T cell responses, and how NK cells acquire immunosuppressive functions.

Neurodegeneration in multiple sclerosis: novel treatment strategies

In recent years it has become clear that the neuronal compartment already plays an important role early in the pathology of multiple sclerosis (MS). Neuronal injury in the course of chronic neuroinflammation is a key factor in determining long-term disability in patients. Viewing MS as both inflammatory and neurodegenerative has major implications for therapy, with CNS protection and repair needed in addition to controlling inflammation. Here, the authors' review recently elucidated molecular insights into inflammatory neuronal/axonal pathology in MS and discuss the resulting options regarding neuroprotective and regenerative treatment strategies.

Amelioration of arthritis through mobilization of peptide-specific CD8+ regulatory T cells

Current therapies to treat autoimmune disease focus mainly on downstream targets of autoimmune responses, including effector cells and cytokines. A potentially more effective approach would entail targeting autoreactive T cells that initiate the disease cascade and break self tolerance. The murine MHC class Ib molecule Qa-1b (HLA-E in humans) exhibits limited polymorphisms and binds to 2 dominant self peptides: Hsp60(p216) and Qdm. We found that peptide-induced expansion of tetramer-binding CD8(+) Tregs that recognize Qa-1-Hsp60(p216) but not Qa-1-Qdm strongly inhibited collagen-induced arthritis, an animal model of human rheumatoid arthritis. Perforin-dependent elimination of autoreactive follicular Th (T(FH)) and Th17 cells by CD8(+) Tregs inhibited disease development. Infusion of in vitro-expanded CD8(+) Tregs increased the efficacy of methotrexate treatment and halted disease progression after clinical onset, suggesting an alternative approach to this first-line treatment. Moreover, infusion of small numbers of Qa-1-Hsp60(p216)-specific CD8(+) Tregs resulted in robust inhibition of autoimmune arthritis, confirming the inhibitory effects of Hsp60(p216) peptide immunization. These results suggest that strategies designed to expand Qa-1-restricted (HLA-E-restricted), peptide-specific CD8(+) Tregs represent a promising therapeutic approach to autoimmune disorders.

Lymphocytes with cytotoxic activity induce rapid microtubule axonal destabilization independently and before signs of neuronal death

MS (multiple sclerosis) is the most prevalent autoimmune disease of the CNS (central nervous system) historically characterized as an inflammatory and demyelinating disease. More recently, extensive neuronal pathology has lead to its classification as a neurodegenerative disease as well. While the immune system initiates the autoimmune response it remains unclear how it orchestrates neuronal damage. In our previous studies, using in vitro cultured embryonic neurons, we demonstrated that MBP (myelin basic protein)-specific encephalitogenic CD4 T-cells induce early neuronal damage. In an extension of those studies, here we show that polarized CD4 Th1 and Th17 cells as wells as CD8 T-cells and NK (natural killer) cells induce microtubule destabilization within neurites in a contact-independent manner. Owing to the cytotoxic potential of these immune cells, we isolated the luminal components of lytic granules and determined that they were sufficient to drive microtubule destabilization. Since lytic granules contain cytolytic proteins, we determined that the induction of microtubule destabilization occurred prior to signs of apoptosis. Furthermore, we determined that microtubule destabilization was largely restricted to axons, sparing dendrites. This study demonstrated that lymphocytes with cytolytic activity have the capacity to directly drive MAD (microtubule axonal destabilization) in a bystander manner that is independent of neuronal death.

NK cells inhibit T-bet-deficient, autoreactive Th17 cells

The differentiation and maintenance of Th17 cells require a unique cytokine milieu and activation of lineage-specific transcription factors. This process appears to be antagonized by the transcription factor T-bet, which controls the differentiation of Th1 cells. Considering that T-bet-deficient (T-bet(-/-) ) mice are largely devoid of natural killer (NK) cells due to a defect in the terminal maturation of these cells, and because NK cells can influence the differentiation of T helper cells, we investigated whether the absence of NK cells in T-bet-deficient mice contributes to the augmentation of autoreactive Th17 cell responses. We show that the loss of T-bet renders the transcription factors Rorc and STAT3 highly responsive to activation by stimuli provided by NK cells. Furthermore, reconstitution of T-bet(-/-) mice with wild-type NK cells inhibited the development of autoreactive Th17 cells through NK cell-derived production of IFN-γ. These results identify NK cells as critical regulators in the development of autoreactive Th17 cells and Th17-mediated pathology.

NKG2D and DNAM-1 activating receptors and their ligands in NK-T cell interactions: role in the NK cell-mediated negative regulation of T cell responses

The negative regulation of adaptive immunity is relevant to maintain lymphocyte homeostasis. Several studies on natural killer (NK) cells have shown a previously unappreciated immunomodulatory role, as they can negatively regulate T cell-mediated immune responses by direct killing and by secretion of inhibitory cytokines. The molecular mechanisms of T cell suppression by NK cells, however, remained elusive. Only in the last few years has it become evident that, upon activation, human T cells express MICA-B, ULBP1-3, and PVR, ligands of the activating receptors NKG2D and DNAM-1, respectively. Their expression renders T cells targets of NK cell lysis, representing a new mechanism taking part to the negative regulation of T cell responses. Studies on the expression of NKG2D and DNAM-1 ligands on T cells have also contributed in understanding that the activation of ATM (ataxia-telangiectasia, mutated)/ATR (ATM/Rad3-related) kinases and the DNA damage response is a common pathway regulating the expression of activating ligands in different types of cells and under different conditions. The functional consequences of NKG2D and DNAM-1 ligand expression on activated T cells are discussed in the context of physiologic and pathologic processes such as infections, autoimmunity, and graft versus host disease.

Evolving immune circuits are generated by flexible, motile, and sequential immunological synapses

The immune system is made up of a diverse collection of cells, each of which has distinct sets of triggers that elicit unique and overlapping responses. It is correctly described as a 'system' because its overall properties (e.g. 'tolerance', 'allergy') emerge from multiple interactions of its components cells. To mobilize a response where needed, the majority of the cells of the system are obligatorily highly motile and so must communicate with one another over both time and space. Here, we discuss the flexibility of the primary immunological synapse (IS) with respect to motility. We then consider the primary IS as an initiating module that licenses 'immunological circuits': the latter consisting of two or more cell-cell synaptic interactions. We discuss how two or three component immunological circuits interact might with one another in sequence and how the timing, stoichiometry, milieu, and duration of assembly of immunological circuits are likely to be key determinants in the emergent outcome and thus the system-wide immune response. An evolving consideration of immunological circuits, with an emphasis on the cell-cell modules that complement T-antigen-presenting cell interaction, provides a fundamental starting point for systems analysis of the immune response.

Up-regulation of a death receptor renders antiviral T cells susceptible to NK cell-mediated deletion

Hepatic NK cells eliminate HBV-specific T cells dependent on TRAIL and TRAIL-R2 interactions to limit antiviral immunity in chronic infection.

Antiviral T cell responses in hepatotropic viral infections such as hepatitis B virus (HBV) are profoundly diminished and prone to apoptotic deletion. In this study, we investigate whether the large population of activated NK cells in the human liver contributes to this process. We show that in vitro removal of NK cells augments circulating CD8⁺ T cell responses directed against HBV, but not against well-controlled viruses, in patients with chronic hepatitis B (CHB). We find that NK cells can rapidly eliminate HBV-specific T cells in a contact-dependent manner. CD8⁺ T cells in the liver microcirculation are visualized making intimate contact with NK cells, which are the main intrahepatic lymphocytes expressing TNF-related apoptosis-inducing ligand (TRAIL) in CHB. High-level expression of the TRAIL death receptor TRAIL-R2 is found to be a hallmark of T cells exposed to the milieu of the HBV-infected liver in patients with active disease. Up-regulation of TRAIL-R2 renders T cells susceptible to caspase-8–mediated apoptosis, from which they can be partially rescued by blockade of this death receptor pathway. Our findings demonstrate that NK cells can negatively regulate antiviral immunity in chronic HBV infection and illustrate a novel mechanism of T cell tolerance in the human liver.

The depletion of NK cells prevents T cell exhaustion to efficiently control disseminating virus infection

NK cells have well-established functions in immune defense against virus infections and cancer through their cytolytic activity and production of cytokines. In this study, we examined the frequency of NK cells and their influence on T cell responses in mice given variants of lymphocytic choriomeningitis virus that cause acute or persisting infection. We found increased frequencies of circulating NK cells during disseminating infection compared with uninfected or acutely infected mice. Consistent with recent reports, we observed that the depletion of NK cells in mice with disseminated infection increased peak numbers of virus-specific cytokine producing CD8(+) T cells and resulted in the rapid resolution of disseminated infection. Additionally, we show that NK cell depletion sustained T cell responses across time and protected against T cell exhaustion. The positive effects of NK cell depletion on T cell responses only occurred when NK cells were depleted within the first 2 d of infection. We find that the improved CD8(+) T cell response correlated with an enhanced ability of APCs from NK cell-depleted mice to stimulate T cell proliferation, independently of the effects of NK cells on CD4(+) T cells. These results indicate that NK cells play an integral role in limiting the CD8 T cell response and contribute to T cell exhaustion by diminishing APC function during persisting virus infection.

NK cells are required for costimulatory blockade induced tolerance to vascularized allografts

BACKGROUND

The role of natural killer (NK) cells in organ transplantation is poorly understood because studies link these cells to both regulatory and inflammatory functions. NK cells exacerbate inflammation and adaptive immunity under conditions of allograft rejection, but little is known regarding their roles in allograft tolerance. We test the hypothesis that NK cells have regulatory function and promote tolerance induction to murine cardiac allografts.

METHODS

Murine hearts were transplanted as fully vascularized heterotopic grafts from BALB/c donors into C57BL/6 recipients. Allograft tolerance was achieved using donor splenocyte transfusion + anti-CD40L monoclonal antibody (mAb) before transplantation. The requirement for NK cells in tolerance induction was tested by administering anti-NK1.1-depleting mAb or anti-NKG2D-blocking mAb. Intragraft and peripheral immune cell populations were determined by flow cytometry and immunohistochemistry. CD4 T-cell alloantigen-specific responses and donor-specific alloantibody were also determined.

RESULTS

NK cell-depleted recipients acutely reject allografts despite anti-CD40L blockade, but rejecting recipients lacked alloantibody and alloantigen-specific CD4 T-cell responses. NK cell depletion resulted in elevated numbers of graft-infiltrating macrophages. NKG2D blockade in tolerized recipients did not cause acute rejection but increased macrophage graft infiltration and increased the expression of NKG2D ligand Rae-1γ on these cells.

CONCLUSIONS

Our data show that NK cells are required for tolerance induction in recipients given donor splenocyte transfusion + anti-CD40L mAb. Our data suggest NK cells regulate monocyte or macrophage activation and infiltration into allografts by a mechanism partially dependent on NKG2D receptor-ligand interactions between NK cells and monocytes/macrophages.

Role of Natural Killer Cells in Multiple Sclerosis

Although the etiology of multiple sclerosis (MS) is not known, the consensus is that Th1 cells sensitized to myelin proteins in the periphery are recruited into the CNS and damage the myelin sheath. Natural killers (NK) are cells that spontaneously lyse tumor target cells and have immunoregulatory activity secreting multiple cytokines and chemokines, as well as interacting with cells of innate and adaptive immune systems. A great discovery in the field is the cloning of several inhibitory and activating receptors. Another important contribution is the discovery that these cells express many seven-transmembrane-spanning domain receptors which aid them in extravasations into injured tissues. Despite all this progress, the role of NK cells in autoimmune diseases including MS is still not quite clear. In this paper, I will summarize recent findings related to the effects of these cells in both MS and the animal model of experimental autoimmune encephalomyelitis (EAE). Hence, I will discuss the effects of drugs used to treat MS/EAE and then explain their effects on NK cells. These include anti-CD25 or daclizumab, interferon-β (IFN-β), natalizumab, glatiramer acetate (GA), and fingolimod (FTY720). Finally, I will explain the contribution of the recently discovered NK17/NK1 cells in MS disease.

Activated mouse CD4(+)Foxp3(-) T cells facilitate melanoma metastasis via Qa-1-dependent suppression of NK-cell cytotoxicity

The regulatory activities of mouse CD4⁺Foxp3⁺ T cells on various immune cells, including NK cells, have been well documented. Under some conditions, conventional CD4⁺Foxp3⁻ T cells in the periphery are able to acquire inhibitory function on other T cells, but their roles in controlling innate immune cells are poorly defined. As a potential cellular therapy for cancer, ex vivo activated CD4⁺Foxp3⁻ effector T cells are often infused back in vivo to suppress tumor growth and metastasis. Whether such activated T cells could affect NK-cell control of tumorigenesis is unclear. In the present study, we found that mitogen-activated CD4⁺Foxp3⁻ T cells exhibited potent suppressor function on NK-cell proliferation and cytotoxicity in vitro, and notably facilitated B16 melanoma metastasis in vivo. Suppression of NK cells by activated CD4⁺Foxp3⁻ T cells is cell-cell contact dependent and is mediated by Qa-1:NKG2A interaction, as administration of antibodies blocking either Qa-1 or NKG2A could completely reverse this suppression, and significantly inhibited otherwise facilitated melanoma metastasis. Moreover, activated CD4⁺Foxp3⁻ cells from Qa-1 knockout mice completely lost the suppressor activity on NK cells, and failed to facilitate melanoma metastasis when transferred in vivo. Taken together, our findings indicate that innate anti-tumor response is counter regulated by the activation of adaptive immunity, a phenomenon we term as “activation-induced inhibition”. Thus, the regulatory role of activated CD4⁺Foxp3⁻ T cells in NK-cell activity must be taken into consideration in the future design of cancer therapies.

Natural killer cells and their receptors in multiple sclerosis

The immune system has crucial roles in the pathogenesis of multiple sclerosis. While the adaptive immune cell subsets, T and B cells, have been the main focus of immunological research in multiple sclerosis, it is now important to realize that the innate immune system also has a key involvement in regulating autoimmune responses in the central nervous system. Natural killer cells are innate lymphocytes that play vital roles in a diverse range of infections. There is evidence that they influence a number of autoimmune conditions. Recent studies in multiple sclerosis and its murine model, experimental autoimmune encephalomyelitis, are starting to provide some understanding of the role of natural killer cells in regulating inflammation in the central nervous system. Natural killer cells express a diverse range of polymorphic cell surface receptors, which interact with polymorphic ligands; this interaction controls the function and the activation status of the natural killer cell. In this review, we discuss evidence for the role of natural killer cells in multiple sclerosis and experimental autoimmune encephalomyelitis. We consider how a change in the balance of signals received by the natural killer cell influences its involvement in the ensuing immune response, in relation to multiple sclerosis.

Macrophages help NK cells to attack tumor cells by stimulatory NKG2D ligand but protect themselves from NK killing by inhibitory ligand Qa-1

Natural killer (NK) cells and their crosstalk with other immune cells are important for innate immunity against tumor. To explore the role of the interaction between NK cells and macrophages in the regulation of anti-tumor activities of NK cells, we here demonstrate that poly I:C-treated macrophages increased NK cell-mediated cytotoxicity against target tumor cells in NKG2D-dependent manner. In addition, IL-15, IL-18, and IFN-β secreted by poly I:C-treated macrophages are also involved in NKG2D expression and NK cell activation. Interestingly, the increase in expression of NKG2D ligands on macrophages induced a highly NK cell-mediated cytotoxicity against tumor cells, but not against macrophages themselves. Notably, a high expression level of Qa-1, a NKG2A ligand, on macrophages may contribute to such protection of macrophages from NK cell-mediated killing. Furthermore, Qa-1 or NKG2A knockdown and Qa-1 antibody blockade caused the macrophages to be sensitive to NK cytolysis. These results suggested that macrophages may activate NK cells to attack tumor by NKG2D recognition whereas macrophages protect themselves from NK lysis via preferential expression of Qa-1.

Supplementation with galacto-oligosaccharides increases the percentage of NK cells and reduces colitis severity in Smad3-deficient mice

The gut microbiota plays an essential role in intestinal immunity. Prebiotics, including galacto-oligosaccharides (GOS), are fermentable fibers that beneficially affect the host by stimulating the growth of specific microbial populations. We investigated the effect of GOS on colitis development and on immune variables in Smad3-deficient mice treated with the pathogen Helicobacter hepaticus. Mice were supplemented daily with 5000 mg GOS/kg body weight 2 wk prior to infection and 4 wk postinfection, a time period during which colitis severity peaks in this model. Mice (n = 4-8/treatment at each time) were killed preinfection (0 d) and at 3, 7, and 28 d postinfection to evaluate immune variables in the spleen and in mesenteric lymph nodes (MsLN) by flow cytometry. Colon and cecum samples were collected for histopathologic analysis. Fecal pellets (n = 8-9/treatment) were collected prior to infection to measure relative changes in Bifidobacterium ssp. and Lactobacillum ssp. by real-time PCR. GOS significantly reduced colitis severity in response to H. hepaticus (P < 0.0001). This was associated with a significant increase in the percentage of NK cells in the spleen (P < 0.001) and in MsLN (P < 0.001) at 3 d postinfection and a 1.5-fold increase in fecal Bifidobacterium ssp. (P = 0.003). GOS stimulated NK expression of CCR9, a chemokine receptor involved in lymphocyte trafficking to the gut preinfection (0 d) in the blood (P = 0.02), spleen (P = 0.033), and MsLN (P = 0.017). In addition, GOS stimulated colonic IL-15 production 3 d postinfection (P < 0.001). These data suggest that GOS reduces colitis by modulating the function and trafficking of NK cells and may provide a novel therapeutic strategy for individuals with inflammatory bowel disease.

Regulation of self-tolerance by Qa-1-restricted CD8(+) regulatory T cells

Mounting an efficient immune response to pathogens while avoiding damage to host tissues is the central task of the immune system. Emerging evidence has highlighted the contribution of the CD8(+) lineage of regulatory T cells to the maintenance of self-tolerance. Specific recognition of the MHC class Ib molecule Qa-1 complexed to peptides expressed by activated CD4(+) T cells by regulatory CD8(+) T cells triggers an inhibitory interaction that prevents autoimmune responses. Conversely, defective Qa-1-restricted CD8(+) regulatory activity can result in development of systemic autoimmune disease. Here, we review recent research into the cellular and molecular basis of these regulatory T cells, their mechanism of suppressive activity and the potential application of these insights into new treatments for autoimmune disease and cancer.

Cytotoxicity of CD56(bright) NK cells towards autologous activated CD4+ T cells is mediated through NKG2D, LFA-1 and TRAIL and dampened via CD94/NKG2A

In mouse models of chronic inflammatory diseases, Natural Killer (NK) cells can play an immunoregulatory role by eliminating chronically activated leukocytes. Indirect evidence suggests that NK cells may also be immunoregulatory in humans. Two subsets of human NK cells can be phenotypically distinguished as CD16⁺CD56^dim and CD16^dim/−CD56^bright. An expansion in the CD56^bright NK cell subset has been associated with clinical responses to therapy in various autoimmune diseases, suggesting an immunoregulatory role for this subset in vivo. Here we compared the regulation of activated human CD4⁺ T cells by CD56^dim and CD56^bright autologous NK cells in vitro. Both subsets efficiently killed activated, but not resting, CD4⁺ T cells. The activating receptor NKG2D, as well as the integrin LFA-1 and the TRAIL pathway, played important roles in this process. Degranulation by NK cells towards activated CD4⁺ T cells was enhanced by IL-2, IL-15, IL-12+IL-18 and IFN-α. Interestingly, IL-7 and IL-21 stimulated degranulation by CD56^bright NK cells but not by CD56^dim NK cells. NK cell killing of activated CD4⁺ T cells was suppressed by HLA-E on CD4⁺ T cells, as blocking the interaction between HLA-E and the inhibitory CD94/NKG2A NK cell receptor enhanced NK cell degranulation. This study provides new insight into CD56^dim and CD56^bright NK cell-mediated elimination of activated autologous CD4⁺ T cells, which potentially may provide an opportunity for therapeutic treatment of chronic inflammation.

The development of colitogenic CD4(+) T cells is regulated by IL-7 in collaboration with NK cell function in a murine model of colitis

We previously reported that IL-7(-/-)RAG(-/-) mice receiving naive T cells failed to induce colitis. Such abrogation of colitis may be associated with not only incomplete T cell maintenance due to the lack of IL-7, but also with the induction of colitogenic CD4(+) T cell apoptosis at an early stage of colitis development. Moreover, NK cells may be associated with the suppression of pathogenic T cells in vivo, and they may induce apoptosis of CD4(+) T cells. To further investigate these roles of NK cells, RAG(-/-) and IL-7(-/-)RAG(-/-) mice that had received naive T cells were depleted of NK cells using anti-asialo GM1 and anti-NK1.1 Abs. NK cell depletion at an early stage, but not at a later stage during colitogenic effector memory T cell (T(EM)) development, resulted in exacerbated colitis in recipient mice even in the absence of IL-7. Increased CD44(+)CD62L(-) T(EM) and unique CD44(-)CD62L(-) T cell subsets were observed in the T cell-reconstituted RAG(-/-) recipients when NK cells were depleted, although Fas, DR5, and IL-7R expressions in this subset differed from those in the CD44(+)CD62L(-) T(EM) subset. NK cell characteristics were the same in the presence or absence of IL-7 in vitro and in vivo. These results suggest that NK cells suppress colitis severity in T cell-reconstituted RAG(-/-) and IL-7(-/-)RAG(-/-) recipient mice through targeting of colitogenic CD4(+)CD44(+)CD62L(-) T(EM) and, possibly, of the newly observed CD4(+)CD44(-)CD62L(-) subset present at the early stage of T cell development.

Incorporation of innate immune effector mechanisms in the formulation of a vaccine against HIV-1

The realization of a major role for events that occur during acute viremia that dictate the course of disease both in HIV-1 infected humans and susceptible SIV infected non-human primates has prompted an intense interest in studies of the contribution of innate immune effector mechanisms. It is reasoned that findings from such studies may be important and need to be incorporated into the design and formulation of potential candidate vaccines against HIV-1. This review serves to outline the various non-human primate models that can best serve to address this issue, a summary of our knowledge on the various subsets of NK cells (one of the major innate immune cell lineage) that have an impact on the course of disease, the potential pathways that regulate their function and the potential role of the KIRs on SIV-induced disease course. Finally, the major points from this report and the data presented on similar subjects by other investigators is utilized to provide a summary of the potential future directions that we need to take in efforts to move this field forward.

A structural basis for antigen presentation by the MHC class Ib molecule, Qa-1b

The primary function of the monomorphic MHC class Ib molecule Qa-1(b) is to present peptides derived from the leader sequences of other MHC class I molecules for recognition by the CD94-NKG2 receptors expressed by NK and T cells. Whereas the mode of peptide presentation by its ortholog HLA-E, and subsequent recognition by CD94-NKG2A, is known, the molecular basis of Qa-1(b) function is unclear. We have assessed the interaction between Qa-1(b) and CD94-NKG2A and shown that they interact with an affinity of 17 μM. Furthermore, we have determined the structure of Qa-1(b) bound to the leader sequence peptide, Qdm (AMAPRTLLL), to a resolution of 1.9 Å and compared it with that of HLA-E. The crystal structure provided a basis for understanding the restricted peptide repertoire of Qa-1(b). Whereas the Qa-1(b-AMAPRTLLL) complex was similar to that of HLA-E, significant sequence and structural differences were observed between the respective Ag-binding clefts. However, the conformation of the Qdm peptide bound by Qa-1(b) was very similar to that of peptide bound to HLA-E. Although a number of conserved innate receptors can recognize heterologous ligands from other species, the structural differences between Qa-1(b) and HLA-E manifested in CD94-NKG2A ligand recognition being species specific despite similarities in peptide sequence and conformation. Collectively, our data illustrate the structural homology between Qa-1(b) and HLA-E and provide a structural basis for understanding peptide repertoire selection and the specificity of the interaction of Qa-1(b) with CD94-NKG2 receptors.

A LysM and SH3-domain containing region of the Listeria monocytogenes p60 protein stimulates accessory cells to promote activation of host NK cells

Listeria monocytogenes (Lm) infection induces rapid and robust activation of host natural killer (NK) cells. Here we define a region of the abundantly secreted Lm endopeptidase, p60, that potently but indirectly stimulates NK cell activation in vitro and in vivo. Lm expression of p60 resulted in increased IFNγ production by naïve NK cells co-cultured with treated dendritic cells (DCs). Moreover, recombinant p60 protein stimulated activation of naive NK cells when co-cultured with TLR or cytokine primed DCs in the absence of Lm. Intact p60 protein weakly digested bacterial peptidoglycan (PGN), but neither muropeptide recognition by RIP2 nor the catalytic activity of p60 was required for NK cell activation. Rather, the immune stimulating activity mapped to an N-terminal region of p60, termed L1S. Treatment of DCs with a recombinant L1S polypeptide stimulated them to activate naïve NK cells in a cell culture model. Further, L1S treatment activated NK cells in vivo and increased host resistance to infection with Francisella tularensis live vaccine strain (LVS). These studies demonstrate an immune stimulating function for a bacterial LysM domain-containing polypeptide and suggest that recombinant versions of L1S or other p60 derivatives can be used to promote NK cell activation in therapeutic contexts.

Tolerogen-induced interferon-producing killer dendritic cells (IKDCs) protect against EAE

Natural killer (NK) cells and dendritic cells (DCs) have been shown to link the innate and adaptive immune systems. Likewise, a new innate cell subset, interferon-producing killer DCs (IKDCs), shares phenotypic and functional characteristics with both DCs and NK cells. Here, we show IKDCs play an essential role in the resolution of experimental autoimmune encephalomyelitis (EAE) upon treatment with the tolerizing agent, myelin oligodendrocyte glycoprotein (MOG), genetically fused to reovirus protein σ1 (termed MOG-pσ1). Activated IKDCs were recruited subsequent MOG-pσ1 treatment of EAE, and disease resolution was abated upon NK1.1 cell depletion. These IKDCs were able to kill activated CD4(+) T cells and mature dendritic DCs, thus, contributing to EAE remission. In addition, IKDCs were responsible for MOG-pσ1-mediated MOG-specific regulatory T cell recruitment to the CNS. The IKDCs induced by MOG-pσ1 expressed elevated levels of HVEM for interactions with cognate ligand-positive cells: LIGHT(+) NK and T(eff) cells and BTLA(+) B cells. Further characterization revealed these activated IKDCs being MHC class II(high), and upon their adoptive transfer (CD11c(+)NK1.1(+)MHC class II(high)), IKDCs, but not CD11c(+)NK1.1(+)MHC class II(intermediate/low) (unactivated) cells, conferred protection against EAE. These activated IKDCs showed enhanced CD107a, PD-L1, and granzyme B expression and could present OVA, unlike unactivated IKDCs. Thus, these results demonstrate the interventional potency induced HVEM(+) IKDCs to resolve autoimmune disease.

Tregs and infections: on the potential value of modifying their function

CD4(+) T cells, which express a master transcription factor, Foxp3, have been recognized as bona fide Tregs. These cells are essential to maintain immune homeostasis in healthy as well as infected mice and humans. Extensive investigations in the last decade have provided ways to manipulate the Foxp3(+) Treg response therapeutically so the role of such cells in microbe-induced inflammatory reactions can be evaluated. This review focuses on our current understanding of the mechanisms required for the generation and sustenance of Tregs in vivo and the potential value of modulating Tregs to control microbe-induced immunopathological responses.

Mobilization of natural killer cells inhibits development of collagen-induced arthritis

Although natural killer (NK) cells have been implicated in regulating immune responses, their ability to modulate disease development in autoimmune arthritis has not been analyzed. Here we investigate the contribution of NK cells to regulating collagen-induced arthritis, a well-characterized preclinical model of human rheumatoid arthritis. We find that the disease is induced by the combined action of two CD4(+) T helper (T(H)) subsets: follicular T(H) cells and T(H)17 cells. Both CD4(+) T(H) subsets are highly susceptible to lysis by NK cells after activation. Administration of antibody that activates NK cells through blockade of its inhibitory CD94/NKG2A receptor allows enhanced elimination of pathogenic follicular T(H) and T(H)17 cells and arrest of disease progression. These results suggest that antibody-dependent enhancement of NK activity may yield effective, previously undescribed therapeutic approaches to this autoimmune disorder.

TRAIL/DR5 plays a critical role in NK cell-mediated negative regulation of dendritic cell cross-priming of T cells

Dendritic cells (DCs) are critical in initiating immune responses by cross-priming of tumor Ags to T cells. Previous results showed that NK cells inhibited DC-mediated cross-presentation of tumor Ags both in vivo and in vitro. In this study, enhanced Ag presentation was observed in draining lymph nodes in TRAIL(-/-) and DR5(-/-) mice compared with that of wild-type mice. NK cells inhibit DC cross-priming of tumor Ags in vitro, but not direct presentation of endogenous Ags. NK cells lacking TRAIL, but not perforin, were not able to inhibit DC cross-priming of tumor Ags. DCs that lack expression of TRAIL receptor DR5 were less susceptible to NK cell-mediated inhibition of cross-priming, and cross-linking of DR5 receptor led to reduced generation of MHC class I-Ag peptide complexes, followed by attenuated cross-priming of CD8(+) T cells. In addition, key molecules involved in the TRAIL/DR5 pathway during DC/NK cell interactions were determined. In summary, these data indicate a novel alternative pathway for DC/NK cell interactions in antitumor immunity and may reflect homeostasis of both DCs and NK cells for regulation of CD8(+) T cell function in physiological conditions.

Properties and applications of single-chain major histocompatibility complex class I molecules

Stable major histocompatibility complex (MHC) class I molecules at the cell surface consist of three separate, noncovalently associated components: the class I heavy chain, the β(2)-microglobulin light chain, and a presented peptide. These three components are assembled inside cells via complex pathways involving many other proteins that have been studied extensively. Correct formation of disulfide bonds in the endoplasmic reticulum is central to this process of MHC class I assembly. For a single specific peptide to be presented at the cell surface for possible immune recognition, between hundreds and thousands of peptide-containing precursor polypeptides are required, so the overall process is relatively inefficient. To increase the efficiency of antigen presentation by MHC class I molecules, and for possible therapeutic purposes, single-chain molecules have been developed in which the three, normally separate components have been joined together via flexible linker sequences in a single polypeptide chain. Remarkably, these single-chain MHC class I molecules fold up correctly, as judged by functional recognition by cells of the immune system, and more recently by X-ray crystallographic structural data. This review focuses on the interesting properties and potential of this new type of engineered MHC class I molecule.

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.

TLRs, macrophages, and NK cells: our understandings of their functions in uterus and ovary

Inflammation involves multiple changes in many aspects of immune system. Interactions between immune system and female reproductive system strongly impact fertility and reproductive health in general. Many normal events of female reproduction system including ovulation, menstruation, implantation and labor onset are considered as inflammatory process. Emerging evidence reveals that three components of immune system that are critical to initiate and resolve inflammation, Toll-like receptors (TLRs), macrophages, and natural killer (NK) cells, play important roles not only to provide protection against infections by exogenous pathogens but also to regulate essential functions of uterus and ovary. This review will briefly summarize our understanding of the functions of TLRs, macrophages and NK cells in uterus and ovary.

Mechanisms of self-nonself discrimination and possible clinical relevance

This review discusses different mechanisms that result in immunological tolerance, such as intrathymic deletion of immature T cells, intrathymic and extrathymic generation of regulatory T cells, effector mechanisms of regulatory T cells as well as molecular pathways involved in extrathymic generation of regulatory T cells in vivo and in vitro. These molecular mechanisms should enable investigators to develop clinical protocols aiming at the specific prevention of unwanted immune responses, thereby replacing indiscriminate immunosuppression that often has fatal consequences.

Mixed chimerism, lymphocyte recovery, and evidence for early donor-specific unresponsiveness in patients receiving combined kidney and bone marrow transplantation to induce tolerance

BACKGROUND

We have previously reported operational tolerance in patients receiving human leukocyte antigen-mismatched combined kidney and bone marrow transplantation (CKBMT). We now report on transient multilineage hematopoietic chimerism and lymphocyte recovery in five patients receiving a modified CKBMT protocol and evidence for early donor-specific unresponsiveness in one of these patients.

METHODS

Five patients with end-stage renal disease received CKBMT from human leukocyte antigen-mismatched, haploidentical living-related donors after modified nonmyeloablative conditioning. Polychromatic flow cytometry was used to assess multilineage chimerism and lymphocyte recovery posttransplant. Limiting dilution analysis was used to assess helper T-lymphocyte reactivity to donor antigens.

RESULTS

Transient multilineage mixed chimerism was observed in all patients, but chimerism became undetectable by 2 weeks post-CKBMT. A marked decrease in T- and B-lymphocyte counts immediately after transplant was followed by gradual recovery. Initially, recovering T cells were depleted of CD45RA+/CD45RO(-) "naïve-like" cells, which have shown strong recovery in two patients, and CD4:CD8 ratios increased immediately after transplant but then declined markedly. Natural killer cells were enriched in the peripheral blood of all patients after transplant.For subject 2, a pretransplant limiting dilution assay revealed T helper cells recognizing both donor and third-party peripheral blood mononuclear cells. However, the antidonor response was undetectable by day 24, whereas third-party reactivity persisted.

CONCLUSION

These results characterize the transient multilineage mixed hematopoietic chimerism and recovery of lymphocyte subsets in patients receiving a modified CKBMT protocol. The observations are relevant to the mechanisms of donor-specific tolerance in this patient group.

Impaired IFN-γ production and proliferation of NK cells in multiple sclerosis

NK cells are multicompetent lymphocytes of the innate immune system with a central role in host defense and immune regulation. Studies in experimental animal models of multiple sclerosis (MS) provided evidence for both pathologic and protective effects of NK cells. Humans harbor two functionally distinct NK-cell subsets exerting either predominantly cytotoxic (CD56(dim)CD16(+)) or immunoregulatory (CD56(bright)CD16(-)) functions. We analyzed these two subsets and their functions in the peripheral blood of untreated patients with relapsing-remitting MS compared with healthy blood donors. While ex vivo frequencies of CD56(bright)CD16(-) and CD56(dim)CD16(+) NK cells were similar in patients and controls, we found that cytokine-driven in vitro accumulation and IFN-γ production of CD56(bright)CD16(-) NK cells but not of their CD56(dim)CD16(+) counterparts were substantially diminished in MS. Impaired expansion of CD56(bright)CD16(-) NK cells was cell intrinsic because the observed effects could be reproduced with purified NK cells in an independent cohort of patients and controls. In contrast, cytolytic NK-cell activity toward the human erythromyeloblastoid leukemia cell line K562, the allogeneic CD4(+) T cell line CEM and allogeneic primary CD4(+) T-cell blasts was unchanged. Thus, characteristic functions of CD56(bright)CD16(-) NK cells, namely cytokine-induced NK cell expansion and IFN-γ production, are compromised in the NK cell compartment of MS patients.

The rapid induction of HLA-E is essential for the survival of antigen-activated naive CD4 T cells from attack by NK cells

Increasing evidence shows that NK cells regulate adaptive immunity, but the underlying mechanisms are not well understood. In this study, we show that activated human NK cells suppress autologous naive CD4 T cell proliferation in response to allogeneic dendritic cells (DCs) by selectively killing Ag-activated T cells. Naive CD4 T cells, which were initially resistant to NK cell-mediated cytotoxicity, became substantially susceptible to NK cells within a day after priming with DCs. Ag-activated T cells showed various degrees of susceptibility to NK cells. After 1 d of priming with LPS-matured DCs, T cells were less susceptible to NK cells than were T cells primed with TNF-α-matured DCs. Subsequently at day 3, Ag-activated T cells regained resistance to NK cells. The level of HLA-E expression on Ag-activated T cells was closely correlated with resistance to NK cells. HLA-E was highly expressed at day 1 by T cells primed with LPS-matured DCs but not by T cells primed with TNF-α-matured DCs. An Ab blockade revealed a critical role for the HLA-E-NKG2A interaction in the protection of Ag-activated T cells from NK cells. Collectively, this study demonstrates that NK cells impact adaptive immunity through the finely controlled kinetics of HLA-E expression on T cells. Thus, HLA-E may be a new target for immunoregulation.

The other Janus face of Qa-1 and HLA-E: diverse peptide repertoires in times of stress

The non-polymorphic MHC molecule Qa-1 and its human counterpart HLA-E present monomorphic signal peptides to innate receptors and thereby regulate lymphocyte activity. Under stress, this peptide content is replaced with a surprisingly diverse repertoire of novel peptides that are associated with heat-shock proteins, infectious agents or antigen processing defects.

Differential gene expression in acute lymphoblastic leukemia cells surviving allogeneic transplant

The effectiveness of allogeneic graft-versus-leukemia (GVL) activity in control of acute lymphoblastic leukemia is generally regarded as poor. One possible factor is dynamic adaptation of the leukemia cell to the allogeneic environment. This work tested the hypothesis that the pattern of gene expression in acute lymphoblastic leukemia cells in an allogeneic environment would differ from that in a non-allogeneic environment. Expression microarray studies were performed in murine B lineage acute lymphoblastic leukemia cells recovered from mice that had undergone allogeneic MHC-matched but minor histocompatibility antigen mismatched transplants. A limited number of genes were found to be differentially expressed in ALL cells surviving in the allogeneic environment. Functional analysis demonstrated that genes related to immune processes, antigen presentation, ubiquitination and GTPase function were significantly enriched. Several genes with known immune activities potentially relevant to leukemia survival (Ly6a/Sca-1, TRAIL and H2-T23) were examined in independent validation experiments. Increased expression in vivo in allogeneic hosts was observed, and could be mimicked in vitro with soluble supernatants of mixed lymphocyte reactions or interferon-gamma. The changes in gene expression were reversible when the leukemia cells were removed from the allogeneic environment. These findings suggest that acute lymphoblastic leukemia cells respond to cytokines present after allogeneic transplantation and that these changes may reduce the effectiveness of GVL activity.

Natural killer cell education and tolerance

Natural killer (NK) cells play a key role in the immune response to certain infections and malignancies by direct cytolysis of infected or transformed cells and by secretion of potent immune mediators. NK cells express an array of activating receptors that recognize self-molecules. If not restrained by inhibitory receptors recognizing major histocompatibility complex (MHC) class I proteins on the surface of self cells, NK cells are able to kill normal, healthy cells. Not all NK cells express inhibitory receptors for self-MHC class I; thus, other tolerance mechanisms are necessary to prevent NK cell-mediated autoimmunity. Here we review the major mechanisms of NK cell education and tolerance.

Inhibition of follicular T-helper cells by CD8(+) regulatory T cells is essential for self tolerance

The ability to produce vigorous immune responses that spare self tissues and organs depends on elimination of autoreactive T and B cells. However, purging of immature and mature self-reactive T and B cells is incomplete and may require additional censorship by cells programmed to suppress immune responses ¹. Regulatory T cells belonging to the CD4⁺ T cell subset may play a role in preventing untoward inflammatory responses, but T cell subsets programmed to inhibit the development of autoantibody formation and SLE-like disease have not been defined ². Here we delineate a CD8⁺ regulatory T cell lineage that is essential for maintenance of self tolerance and prevention of autoimmune disease. Genetic disruption of the inhibitory interaction between these CD44⁺ ICOSL⁺ CD8⁺ T cells and their target Qa-1⁺ follicular T helper cells results in the development of a lethal SLE-like autoimmune disease. These findings define a sublineage of CD8 T cells programmed to suppress rather than activate immunity that represents an essential regulatory element of the immune response and a guarantor of self tolerance.

HLA-E-restricted regulatory CD8(+) T cells are involved in development and control of human autoimmune type 1 diabetes

A key feature of the immune system is its ability to discriminate self from nonself. Breakdown in any of the mechanisms that maintain unresponsiveness to self (a state known as self-tolerance) contributes to the development of autoimmune conditions. Recent studies in mice show that CD8(+) T cells specific for the unconventional MHC class I molecule Qa-1 bound to peptides derived from the signal sequence of Hsp60 (Hsp60sp) contribute to self/nonself discrimination. However, it is unclear whether they exist in humans and play a role in human autoimmune diseases. Here we have shown that CD8(+) T cells specific for Hsp60sp bound to HLA-E (the human homolog of Qa-1) exist and play an important role in maintaining peripheral self-tolerance by discriminating self from nonself in humans. Furthermore, in the majority of type 1 diabetes (T1D) patients tested, there was a specific defect in CD8(+) T cell recognition of HLA-E/Hsp60sp, which was associated with failure of self/nonself discrimination. However, the defect in the CD8(+) T cells from most of the T1D patients tested could be corrected in vitro by exposure to autologous immature DCs loaded with the Hsp60sp peptide. These data suggest that HLA-E-restricted CD8(+) T cells may play an important role in keeping self-reactive T cells in check. Thus, correction of this defect could be a potentially effective and safe approach in the therapy of T1D.