Invariant natural killer T cells: bridging innate and adaptive immunity

Cells of the innate immune system interact with pathogens via conserved pattern-recognition receptors, whereas cells of the adaptive immune system recognize pathogens through diverse, antigen-specific receptors that are generated by somatic DNA rearrangement. Invariant natural killer T (iNKT) cells are a subset of lymphocytes that bridge the innate and adaptive immune systems. Although iNKT cells express T cell receptors that are generated by somatic DNA rearrangement, these receptors are semi-invariant and interact with a limited set of lipid and glycolipid antigens, thus resembling the pattern-recognition receptors of the innate immune system. Functionally, iNKT cells most closely resemble cells of the innate immune system, as they rapidly elicit their effector functions following activation, and fail to develop immunological memory. iNKT cells can become activated in response to a variety of stimuli and participate in the regulation of various immune responses. Activated iNKT cells produce several cytokines with the capacity to jump-start and modulate an adaptive immune response. A variety of glycolipid antigens that can differentially elicit distinct effector functions in iNKT cells have been identified. These reagents have been employed to test the hypothesis that iNKT cells can be harnessed for therapeutic purposes in human diseases. Here, we review the innate-like properties and functions of iNKT cells and discuss their interactions with other cell types of the immune system.     

Organ-specific features of natural killer cells

Natural killer (NK) cells can be swiftly mobilized by danger signals and are among the earliest arrivals at target organs of disease. However, the role of NK cells in mounting inflammatory responses is often complex and sometimes paradoxical. Here, we examine the divergent phenotypic and functional features of NK cells, as deduced largely from experimental mouse models of pathophysiological responses in the liver, mucosal tissues, uterus, pancreas, joints and brain. Moreover, we discuss how organ-specific factors, the local microenvironment and unique cellular interactions may influence the organ-specific properties of NK cells.     

Natural killer cells and innate immunity to protozoan pathogens

Natural killer (NK) cells are lymphoid cells that mediate significant cytotoxic activity and produce high levels of pro-inflammatory cytokines in response to infection. During viral infection, NK cell cytotoxicity and cytokine production is induced principally by monocyte-macrophage- and dendritic cell-derived cytokines but virally encoded ligands for NK cells are also beginning to be described. NK derived interferon-gamma (IFN-gamma) production is also essential for control of several protozoal infections including toxoplasmosis, trypanosomiasis, leishmaniasis and malaria. The activation of NK cells by protozoan pathogens is also believed to be cytokine-mediated although some recent studies suggest that direct recognition of parasites by NK cells also occurs. Both indirect signalling via accessory cell-derived cytokines and direct signalling, presumably through NK receptors, are needed in order for human malaria parasites (Plasmodium falciparum) to optimally stimulate NK activity.     

Human killer cells and natural killer cells: distinct subpopulations of Fc receptor-bearing lymphocytes

Unstimulated human peripheral blood lymphocytes were depleted of K cells, which mediate antibody-dependent cellular cytotoxicity (ADCC) without removing NK cells, which mediate natural killing (NK). K cell depletion was achieved by buoyant centrifugation removal of lymphocytes that bound to glutaraldehyde-treated P815-AB cells at high lymphocyte-to-target ratios. Likewise, NK cells were removed with glutaraldehyde-treated K562 cells without removing K cells. Furthermore, both cytotoxic cell populations were observed directly in one agarose single-cell cytotoxic assay (ASCA) using P815-AB and K562 cells simultaneously as target cells. Moreover, the percentage of total cytotoxic cells was equal to the sum of the percentage of K and NK cells observed in separate ASCA. Collectively, these results indicate that K cells and NK cells are distinct subsets of FcR-bearing lymphocytes. One subset, K cells, has more avid Fc receptors (fcR) than NK cells and are 'activated' via thier FcR to kill antibody-coated target cells. The second subset, NK cells, have less avid FcR and are not 'activated' through their FcR to kill antibody-coated target cells.     

Interferon-producing killer dendritic cells provide a link between innate and adaptive immunity

Natural killer (NK) cells and dendritic cells (DCs) are, respectively, central components of innate and adaptive immune responses. We describe here a third DC lineage, termed interferon-producing killer DCs (IKDCs), distinct from conventional DCs and plasmacytoid DCs and with the molecular expression profile of both NK cells and DCs. They produce substantial amounts of type I interferons (IFN) and interleukin (IL)-12 or IFN-gamma, depending on activation stimuli. Upon stimulation with CpG oligodeoxynucleotides, ligands for Toll-like receptor (TLR)-9, IKDCs kill typical NK target cells using NK-activating receptors. Their cytolytic capacity subsequently diminishes, associated with the loss of NKG2D receptor (also known as Klrk1) and its adaptors, Dap10 and Dap12. As cytotoxicity is lost, DC-like antigen-presenting activity is gained, associated with upregulation of surface major histocompatibility complex class II (MHC II) and costimulatory molecules, which formally distinguish them from classical NK cells. In vivo, splenic IKDCs preferentially show NK function and, upon systemic infection, migrate to lymph nodes, where they primarily show antigen-presenting cell activity. By virtue of their capacity to kill target cells, followed by antigen presentation, IKDCs provide a link between innate and adaptive immunity.     

Regulation of B lymphocytes by natural killer cells. Role of IFN-gamma

Using a co-culture system of fractionated B cells and highly purified NK cells, we have demonstrated direct interactions between B lymphocytes and NK cells. B cells are able to stimulate the production of IFN-gamma by NK cells. This stimulatory ability is restricted to a subpopulation of large, presumably in vivo activated B lymphocytes. The secreted IFN-gamma in turn inhibits polyclonally induced B cell proliferation. Small resting B cells neither stimulate IFN-gamma production nor are they measurably affected by NK cells.     

Natural killer cell activation receptors in innate immunity to infection

Natural killer (NK) cells are best known for their capacity to kill tumors but they are also critical in early innate responses to infection, especially herpesviruses. Recent studies indicate that NK cell receptors involved in tumor target specificity are also involved in responses to viral infections.     

Invariant natural killer T cells trigger adaptive lymphocytes to churn up bile

How innate immune response causes autoimmunity has remained an enigma. In this issue of Cell Host & Microbe, Mattner et al. demonstrate that invariant natural killer T cells activated by the mucosal commensal Novosphingobium aromaticivorans precipitate chronic T cell-mediated autoimmunity against small bile ducts that mirrors human primary biliary cirrhosis. These findings provide a mechanistic understanding of the role of innate immunity toward a microbe in the development of autoimmunity.     

CD3 negative "small agranular lymphocytes" are natural killer cells

We describe here that CD3-, CD16+ and/or CD56+ small lymphocytes, in a highly reproducible fashion, mediate a significant level of K562 killing that is, on a "per cell" basis, comparable to the cytolytic activity of CD3- LGL. The CD3- small lymphocytes appeared to have no granules based on light and electron microscopy and lack of right-angle scatter on the FACS; we thus refer to them as small "agranular" lymphocytes (SAL). The lytic activity against K562 is inhibited by treatment with either L-leucine methyl ester or EGTA, which are reported to effect granule-dependent killing. We suggest that the SAL have lytic molecules in their cytoplasm (which are sensitive to these treatments) but that these molecules are not organized into discrete granules as found in LGL. The CD3- SAL are phenotypically very similar to LGL and both SAL and LGL mediated equal and reproducible antibody-dependent cell-mediated cytotoxicity. These observations force redefinition of the concept of NK cells to include both CD3- LGL and CD3- SAL.     

NKT细胞在抗肿瘤免疫中的作用及其机制

NKT细胞是免疫细胞中一类具有NK细胞特定标志的T细胞亚群,经活化,既可直接作为抗肿瘤效应细胞发挥杀伤作用,又能通过激活其他免疫效应细胞,如NK细胞,间接实现抗肿瘤作用。NKT细胞在抗肿瘤免疫、获得性免疫应答及免疫调节中起着重要作用。     

Characterization of a subset of human B lymphocytes interacting with natural killer cells

Tonsil B cells were analyzed for their capacity to interact directly with NK cells in vitro. A specific, direct interaction between NK cells and B cells could be detected by direct conjugation and by cold target inhibition using the B lymphoblastoid cell line BJA.B as a labeled target. The data further suggest that the B cell interaction with NK cells specifically activates the NK effectors and induces their production of IFN-gamma. The NK-interactive population of tonsil B cells were characterized as low-buoyant density cells (by Percoll gradient fractionation) that stained more brightly with Hoechst 33342, both characteristics of activated B cells. Immunofluorescent staining of NK cell-B cell conjugates allowed determination of the cell-surface antigenic phenotype of conjugate-forming B cells. B cell targets were ICAM-1bri, 4F2+, TfR+, CD32+, BB1+, and CD77-. They tended to be CD38-, but overlapped the CD38+ population. No correlation was seen with CD37, CD44, CD75, CD76, HC2, or Ig kappa. This phenotype is most consistent with a late activation stage of differentiation, just before and overlapping the expression of CD38. These B cells do not appear significantly sensitive to NK-mediated cytolysis, suggesting that NK cell cytokine synthesis and secretion (e.g., IFN-gamma) may be more important in the NK cell regulation of the humoral response.     

Homeostasis of natural killer cells

Natural killer (NK) cells are important players of innate immunity, dedicated to the host defense against viruses and also involved in the immune surveillance of tumors. NK cells are widely distributed in the body and their number may increase locally during infection. They develop mainly in the bone marrow and perhaps in other lymphoid organs. They are constantly renewed, with a half-life of about 17 days at the periphery. In this article, we review the factors that regulate the homeostasis of NK cells including their development, differentiation, export to the periphery, their turnover, their homeostatic or antigen-induced proliferation and their survival before or after activation. In addition, we discuss the homeostasis of recently described so-called "memory" NK cells.     

Trafficking of natural killer cells

Natural killer (NK) cells comprise a set of lymphocytes that is capable of mediating innate immune responses to viral infections, malignancies, and allogeneic bone marrow grafts. This review summarizes what is known about the mechanisms NK cells use to arrive at their sites of action. NK cells express a wide array of adhesion molecules including alphaLbeta2, alphaMbeta2, alphaXbeta2, and alpha4beta1 integrins, ICAM-1, PSGL-1, and L-selectin. Like other immune and inflammatory cells, NK cells use the blood circulation to enter tissues and organs, which requires that they interact with the vessel wall under flow conditions, arrest, and transmigrate. NK cells are able to chemotax to a variety of cytokines and chemokines, including IL-12, IFN-(alpha/beta, CCL2, 3, 4, 5, 7, 8, CXCL8, and CX3CL1. In many cases, NK cells appear to migrate towards these soluble factors without any kind of priming. These cells also appear to distribute in secondary and tertiary lymphoid sites (i.e., spleen, bone marrow, liver, lung, and lymph nodes) both with and without stimulation. In addition to their ability to move throughout the body in an unprimed state, activated NK cells may have increased specificity in homing to sites of inflammation. NK cells not only react to, but also produce IFN-gamma, TNF-alpha, GM-CSF, CCL3, CCL4, and CCL5, enabling them to recruit various immune cells to sites of immune response.     

Self-tolerance of natural killer cells

Natural killer (NK) cells, similar to other lymphocytes, acquire tolerance to self. This means that NK cells have the potential to attack normal self cells but that there are mechanisms to ensure that this does not usually occur. Self-tolerance is acquired by NK cells during their development, but the underlying molecular and cellular mechanisms remain poorly understood. Recent studies have produced important new information about NK-cell self-tolerance. Here, we review the evidence for and against possible mechanisms of NK-cell self-tolerance, with an emphasis on the role of MHC-specific receptors.     

Natural killer cells and malignant haemopathies: a model for the interaction of cancer with innate immunity

Despite recent progress in the therapeutic approach of malignant haemopathies, their prognoses remain frequently poor. Immunotherapy offers an alternative of great interest in this context but defect or abnormal expression of human leukocyte antigens (HLA), frequently observed in cancer cells, limits its efficiency. Natural killer (NK) cells, which are able to kill target cells in a HLA-independent way, represent a novel tool in the treatment of haematological malignancies. Abnormal NK cytolytic function is observed in all the haematological malignancies studied, such as acute leukaemia, myelodysplastic syndromes or chronic myeloid/lymphoid leukaemia. Several mechanisms are involved in the alterations of NK cytotoxicity: decreased expression of activating receptors, increased expression of inhibitory receptors or defective expression of NK ligands on target cells. Further studies are needed to identify how each type of haematological malignancy escapes from the innate immune response. Attempts to increase the expression of activating receptors, to counteract inhibitory receptors expression, or to increase NK cell cytotoxic capacities could overcome tumour escape from innate immunity. These therapies are based on monoclonal antibodies or culture of NK cells in presence of cytokines or dendritic cells. Moreover, many novel drugs used in haematological malignancies [tyrosine kinase inhibitors, IMIDs^®, proteasome inhibitors, demethylating agents, histone deacetylase inhibitors (HDACis), histamine dihydrochloride] display interesting immunomodulatory properties that affect NK cells. These data suggest that combined modalities associating cytotoxic drugs with innate immunity modulators may represent a major breakthrough in tumour eradication.     

NK cells and innate immunity to malaria

Innate immune response against Plasmodium falciparum (Pf), a causative agent of human malaria, is the result of several thousand years of co-evolution between the parasite and his host. An early IFN-gamma production during infection is associated with a better evolution of the disease. Natural killer (NK) cells are among the first cells in peripheral blood to produce IFN-gamma in response to Pf-infected erythrocytes (Pf-E). NK cells are found in blood, in secondary lymphoid organs as well as in peripheral non-lymphoid tissues. They participate in host innate responses that occur upon viral and intracytoplasmic bacterial infections, but also during the course of tumor development and allogeneic transplantation. These lymphocytes are not only important players of innate effector responses, but also participate in the initiation and development of adaptive immune responses. In addition, direct sensing of Pf infection by NK cells induces their production of the proinflammatory chemokine IL-8, suggesting a role for NK cells in the recruitment and the activation of other cells during malaria infection. Several other cell subsets are involved in the innate immune response to Pf. Dendritic cells, macrophages, gamma delta T cells, NKT cells are able to sense the presence of the parasite. Along this line, the presence of IL-12 is necessary to NK cell IFN-gamma production and a functional cooperation takes place between macrophages and NK cells in the context of this parasitic infection. In particular, IL-18 produced by macrophages is a key factor for this NK response. However, the molecular basis of Pf-E recognition by NK cells as well as the functional role of NK cell responses during the course of the disease remain to be adressed.     

Peyer's patch lymphocytes express natural cytotoxicity but not natural killer activity

The distribution of natural cytotoxic (NC) cells in the gut-associated lymphoid tissues (GALT) and in peripheric lymphoid organs was analyzed in comparison to that of natural killer (NK) cells. It was found that cells from the intestinal epithelium, mesenteric lymph nodes and spleen possess significant levels of NC and NK activity, whereas in thymus and popliteal lymph nodes both the natural activities are negligible. As previously shown for splenocytes, the NC activity of GALT cells is detectable in the 16-hour assays and not in the 4-hour assays. Interestingly, Peyer patches lymphocytes (PPL) possess extremely high NC activity but no NK activity. The NC activity of PPL is still high in NK-deficient mouse strains such as A/J and SJL/J. To further support the observation that the effector PPL are truly NC cells, it was shown that, as previously reported for spleen NC activity, overnight incubation at 37 degrees C of the lymphocytes only marginally affected the cytotoxicity of PPL, which could in turn be augmented by interleukin-3 (IL-3) containing supernatants. On the contrary, IL-2 could not increase NC or NK activity by PPL whilst augmenting NK activity of splenocytes. Thus, for the first time a cell population is identified which expresses only NC activity and not NK and which can be positively regulated only by IL-3.     

Mouse granulated metrial gland cells originate by local activation of uterine natural killer lymphocytes

Natural killer (NK) lymphocytes were identified in the mouse uterus by immunostaining their surface membrane marker, LGL-1. The cells were present in large numbers from before mating through Day 14 of pregnancy. Double immunostaining indicated that uterine NK cells began to contain the pore-forming protein, perforin, on Day 6 of pregnancy in mesometrial decidua. Perforin is a probable mediator of cellular cytotoxicity found in lymphokine-activated NK and cytotoxic T lymphocytes. Activation of NK cells to produce perforin continued in mesometrial decidua on Days 8 and 10 of pregnancy and in the peripheral portion of metrial glands (MGs) on Days 12 and 14 of pregnancy, where cells at 3 stages of activation were simultaneously present: small cells with bright surface membrane staining of LGL-1 but no perforin (nonactivated), larger cells with intermediate staining of both markers (partially activated), and large cells with bright staining of perforin but no LGL-1 (fully activated). These observations indicate that activation of uterine NK cells involves loss of membrane LGL-1 as perforin accumulates in the cytoplasm, that the zone of activation shifts from mesometrial decidua to the MG on about Day 11 of pregnancy, and that nonactivated NK cells probably enter activation zones continuously during this period. Resting NK cells may enter activation zones by proliferation and/or migration from other regions of the uterus, rather than from blood, because depletion of circulating NK cells during pregnancy by treatment with NK-1.1 or asialo GM1 antibodies had no effect or only a small effect on the numbers of LGL-1-or perforin-positive cells seen in the uterus later in pregnancy.(ABSTRACT TRUNCATED AT 250 WORDS)