Natural killer lymphocytes: biology,development, and function

Natural killer (NK) lymphocytes represent the first line of defense against virally infected cells and tumor cells. The role of NK cells in immune responses has been markedly explored, mainly due to the identification of NK cell receptors and their ligands, but also through the analysis of mechanisms underlying the effects of various cytokines on NK cell development and function. A population of lymphocytes that shares function and receptors with NK cells is represented by natural killer T (NKT) cells. NKT lymphocytes are regulators of both innate and adaptive immune responses, but have also been reported to function as effector antitumor cells. The marked progress in our understanding of the biology, development, and function of NK/NKT cells has provided the basis for their potential application in tumor clinical trials.This work was presented at the first Cancer Immunology and Immunotherapy Summer School, 8–13 September 2003, Ionian Village, Bartholomeio, Peloponnese, Greece.     

Recognition of virus infected cells by NK cells

NK cells show cytotoxicity against virus-infected cells and tumor cells and play an important role in host defense. Although mecheanism of target cell recognition by NK cells have been unclear for a long time, it has recently been elucidated that certain NK cell receptors specifically recognize virus products. Furthermore, expression pattern of NK cell receptors, which consist of activating and inhibitory receptors, determines susceptibility to virus-infection. Here, we review recent progress of mechanism of recognition of virus-infected by NK cells.     

Human NK cells and their receptors

The past decade has witnessed important progress in our understanding of how natural killer (NK) cells function. This is primarily consequent to the identification and functional characterization of MHC-specific inhibitory receptors that allow NK cells to discriminate between normal cells and potentially harmful cells that have lost or express insufficient amounts of MHC class I molecules. More recently, a number of activating receptors or coreceptors have been identified that are involved in the process of natural cytotoxicity but may also play a role in the direct recognition of pathogen-associated structures. Surprisingly, none of the triggering receptors identified in NK cells appears to be involved in the "NK-like activity" of a subset of CD8(+) cytolytic T lymphocytes. In this case, lysis of NK-susceptible tumor target cells is the result of the TCR alpha/beta-mediated recognition of HLA-E. The potent cytolytic activity of NK cells as well as their unique mode of functioning may be exploited in therapy. An important breakthrough is the recent report that "alloreactive" NK cells, generated in haploidentical bone marrow transplantation in patients with acute myeloid leukemias, may efficiently prevent leukemic relapses as well as graft rejection and graft-vs.-host disease. This may lead to a true revolution in bone marrow transplantation, based on the exploitation of appropriate HLA-Cl I mismatches that can put NK cells in action.     

Chemokine networks modulating natural killer cell trafficking to solid tumors

Natural killer (NK) cell-based cell therapy has been emerging as a powerful weapon in the treatment of multiple malignancies. However, the inadequate infiltration of the therapeutic NK cells into solid tumors remains a big challenge to their clinical utility. Chemokine networks, which play essential roles in the migration of lymphocytes, have been recognized as critical in driving the intratumoral infiltration of NK cells via interactions between soluble chemokines and their receptors. Often, such interactions are complex and disease-specific. In the context of NK cells, chemokine receptors of note have included CCR2, CCR5, CCR7, CXCR3, and CX3CR1. The immunobiology of chemokine-receptor interactions has fueled the development of approaches that hope to improve the infiltration of NK cells into the microenvironment of solid tumors. Stimulation of NK cells ex vivo in the presence of various cytokines (such as IL-2, IL-15, and IL-21) and genetic engineering of NK cells have been utilized to alter the chemokine receptor profile and generate NK cells with higher infiltrating capacity. Additionally, the immune-suppressive tumor microenvironment has also been targeted, by introducing, either directly or indirectly, chemokine ligands which NK cells are able to respond to, ultimately creating a more hospitable niche for NK cell trafficking. Such strategies have promoted the infiltration and activity of infused NK cells into multiple solid tumors. In this review, we discuss how chemokine receptors and their ligands coordinate and how they can be manipulated to regulate the trafficking, distribution, and residence of NK cells in solid tumors.     

Advantages and clinical applications of natural killer cells in cancer immunotherapy

The past decade has witnessed a burgeoning of research and further insight into the biology and clinical applications of natural killer (NK) cells. Once thought to be simple innate cells important only as cytotoxic effector cells, our understanding of NK cells has grown to include memory-like responses, the guidance of adaptive responses, tissue repair, and a delicate paradigm for how NK cells become activated now termed “licensing” or “arming.” Although these cells were initially discovered and named for their spontaneous ability to kill tumor cells, manipulating NK cells in therapeutic settings has proved difficult and complex in part due to our emerging understanding of their biology. Therapies involving NK cells may either activate endogenous NK cells or involve transfers of exogenous cells by hematopoietic stem cell transplantation or adoptive cell therapy. Here, we review the basic biology of NK cells, highlighting characteristics which make NK cells particularly useful in cancer therapies. We also explore current treatment strategies that have been used for cancer as well as discuss potential future directions for the field.     

NK cells and cancer

In this review, we overview the main features and functions of NK cells, focusing on their role in cell-mediated immune response to tumor cells. In parallel, we discuss the information available in the field of NK cell receptors and offer a wide general overview of functional aspects of cell targeting and killing, focusing on the recent acknowledgments on the efficacy of NK cells after cytokine and mAb administration in cancer therapy. Since efficacy of NK cell-based immunotherapy has been proven in KIR-mismatch regimens or in TRAIL-dependent apoptosis, the ability to manipulate the balance of activating and inhibitory receptors on NK cells and of their cognate ligands, as well as the sensitivity of tumor cells to apoptosis, opens new perspectives for NK cell-based immunotherapy.     

Immune functions encoded by the natural killer gene complex

There has been marked progress in our understanding of the role of natural killer (NK) cells in immune responses, mainly due to the identification of NK-cell receptors and their ligands. The genes encoding many NK-cell receptors are located in the NK-gene complex (NKC). Here, we review the properties of NKC-encoded receptors, and provide a genomic and conceptual framework for an insight into NK-cell function and biology.     

Current progress in cancer immunotherapy based on natural killer cells

One of the most common diseases in the present era is cancer. The common treatment methods used to control cancer include surgery, chemotherapy, and radiotherapy. Despite progress in the treatment of cancers, there still is no definite therapeutic approach. Among the currently proposed strategies, immunotherapy is a new approach that can provide better outcomes compared with existing therapies. Employing natural killer (NK) cells is one of the means of immunotherapy. As innate lymphocytes, NK cells are capable of rapidly responding to cancer cells without being sensitized or restricted to the cognate antigen in advance, as compared to T cells that are tumor antigen-specific. Latest insights into the biology of NK cells have clarified the underlying molecular mechanisms of NK cell maturation and differentiation, as well as controlling their effector functions through the investigation of the ligands and receptors engaged in recognizing cancer cells by NK cells. Elucidating the fact that NK cells recognize cancer cells could similarly show the mechanism through which cancer cells possibly avoid NK cell-dependent immune surveillance. Additionally, the expectations for novel immunotherapies by targeting NK cells have increased through the latest clinical outcomes of T–cell-targeted cancer immunotherapy. For this emerging method, researchers are still attempting to develop protocols for conferring the best proliferation and expansion medium, activation pathways, utilization dosage, transferring methods, as well as reducing possible side effects in cancer therapy. This study reviews the NK cells, their proliferation and expansion methods, and their recent applications in cancer immunotherapy.     

Target cell expression of MHC antigens is not (always) a turn-off signal to natural killer cells

Recent evidence has demonstrated that the lytic function of natural killer cells might be regulated by potential target cells through the target cells' expression of cell surface components that are able to inhibit the lytic process. Specifically, it has been shown in many target cell systems that the expression of class I MHC proteins by target cells is inversely proportional to their susceptibility to lysis by NK cells. It has been suggested, therefore, that MHC proteins may act as important negative regulatory elements in the ongoing control of NK cell function. Herein, we examined two closely related murine lymphoma cells (ASL1 and ASL1w), both in terms of their susceptibility to lysis by NK cells as well as their expression of both H-2K and H-2D class I MHC proteins. The results of these studies showed that whereas ASL1 and ASL1w cells differed greatly in their susceptibility to NK cell lysis (ASL1 was much more NK resistant than ASL1w), both expressed high levels of H-2K and D proteins. In contrast to what might have been predicted base on reports from other target cell systems, the more NK susceptible ASL1w cells expressed somewhat higher levels of H-2K Ag than did ASL1 cells. These results indicate that expression of H-2 class I proteins by target cells, in and of itself, is not sufficient to inhibit the lytic activity of murine NK cells.     

Molecular and functional characterization of NKG2D, NKp80, and NKG2C triggering NK cell receptors in rhesus and cynomolgus macaques: monitoring of NK cell function during simian HIV infection

An involvement of innate immunity and of NK cells during the priming of adaptive immune responses has been recently suggested in normal and disease conditions such as HIV infection and acute myelogenous leukemia. The analysis of NK cell-triggering receptor expression has been so far restricted to only NKp46 and NKp30 in Macaca fascicularis. In this study, we extended the molecular and functional characterization to the various NK cell-triggering receptors using PBMC and to the in vitro-derived NK cell populations by cytofluorometry and by cytolytic activity assays. In addition, RT-PCR strategy, cDNA cloning/sequencing, and transient transfections were used to identify and characterize NKp80, NKG2D, CD94/NKG2C, and CD94/NKG2A in M. fascicularis and Macaca mulatta as well as in the signal transducing polypeptide DNAX-activating protein DAP-10. Both M. fascicularis and M. mulatta NK cells express NKp80, NKG2D, and NKG2C molecules, which displayed a high degree of sequence homology with their human counterpart. Analysis of NK cells in simian HIV-infected M. fascicularis revealed reduced surface expression of selected NK cell-triggering receptors associated with a decreased NK cell function only in some animals. Overall surface density of NK cell-triggering receptors on peripheral blood cells and their triggering function on NK cell populations derived in vitro was not decreased compared with uninfected animals. Thus, triggering NK cell receptor monitoring on macaque NK cells is possible and could provide a valuable tool for assessing NK cell function during experimental infections and for exploring possible differences in immune correlates of protection in humans compared with cynomolgus and rhesus macaques undergoing different vaccination strategies.     

Regulatory NK cells suppress antigen-specific T cell responses

The immune system has a variety of regulatory/suppressive processes, which are decisive for the development of a healthy or an allergic immune response to allergens. NK1 and NK2 subsets have been demonstrated to display counterregulatory and provocative roles in immune responses, similar to Th1 and Th2 cells. T regulatory cells suppressing both Th1 and Th2 responses have been the focus of intensive research during the last decade. In this study, we aimed to investigate regulatory NK cells in humans, by characterization of NK cell subsets according to their IL-10 secretion property. Freshly purified IL-10-secreting NK cells expressed up to 40-fold increase in IL-10, but not in the FoxP3 and TGF-beta mRNAs. PHA and IL-2 stimulation as well as vitamin D3/dexamethasone and anti-CD2/CD16 mAbs are demonstrated to induce IL-10 expression in NK cells. The effect of IL-10+ NK cells on Ag-specific T cell proliferation has been examined in bee venom major allergen, phospholipase A2- and purified protein derivative of Mycobecterium bovis-induced T cell proliferation. IL-10+ NK cells significantly suppressed both allergen/Ag-induced T cell proliferation and secretion of IL-13 and IFN-gamma, particularly due to secreted IL-10 as demonstrated by blocking of the IL-10 receptor. These results demonstrate that a distinct small fraction of NK cells display regulatory functions in humans.     

NK细胞与HIV/SIV感染相关性的研究进展

NK细胞作为天然免疫系统的重要组成部分,其在HIV/SIV感染后的免疫机制及如何发挥抗病毒作用成为近几年艾滋病研究的热点之一。研究中发现,伴随HIV/SIV的感染,NK细胞亚群比例发生改变同时伴有功能缺陷,这种变化与HIV/SIV慢性感染阶段病毒复制水平有显著相关性。并且由于归巢受体表达的改变引起NK细胞在HIV/SIV感染者体内不同组织间的重新分布。NK细胞表面的受体KIR3DL1和KIR3DS也表现出对HIV感染的抵抗作用。这些发现为我们进一步研究NK细胞的抗HIV/SIV病毒感染的免疫机制提供了新的思路和方向。     

Investigating the morphology, function and genetics of cytotoxic cells in bony fish

Bony fish (teleosts) possess multiple cytotoxic cell lineages that recognize and destroy virally infected and transformed cells. In general, these lineages parallel their functional equivalents in mammals and include neutrophilic granulocytes, macrophages, cytotoxic T lymphocytes (CTL) and natural killer (NK) cells. These four cell types have been morphologically identified in multiple fish species but only limited information is available about their function. In contrast, much work has gone into examining the function of a fifth cytotoxic cell lineage, termed nonspecific cytotoxic cells (NCC), that has been referred to as the bony fish equivalent of NK cells. However, evidence suggesting that NCC do not represent the NK lineage has come through the development of multiple cytotoxic catfish cell lines that are morphologically and functionally similar to human NK cells and are distinct from NCC. In addition to characterizing cytotoxic cells from fish, recent work has identified the novel immune-type receptors (NITR) and cichlid killer leukocyte receptors (cKLR) that are structurally related to mammalian NK receptors and likely play a role in cytotoxic function in fish. This review summarizes the morphological and functional evidence for cytotoxic cells within bony fish and discusses future directions for examining cytotoxicity through genomics and transgenics.     

Cytomegalovirus MHC class I homologues and natural killer cells: an overview

Viruses that establish a persistent infection with their host have evolved numerous strategies to evade the immune system. Consequently, they are useful tools to dissect the complex cellular processes that comprise the immune response. Rapid progress has been made in recent years in defining the role of cellular MHC class I molecules in regulating the response of natural killer (NK) cells. Concomitantly, the roles of the MHC class I homologues encoded by human and mouse cytomegaloviruses in evading or subverting NK cell responses has received considerable interest. This review discusses the results from a number of studies that have pursued the biological function of the viral MHC class I homologues. Based on the evidence from these studies, hypotheses for the possible role of these intriguing molecules are presented.     

N-acetylcysteine reduces susceptibility of transformed and embryonic cells to lytic activity of natural killer cells

Changes in the sensitivity of several transformed and embryonic cells to the lytic activity of natural killers (NK) due to N-acetylcysteine (NAC) has been studied. We found that epidermoid carcinoma A431 cells and murine hepatoma MH22a cells, as well as the transformed mouse fibroblasts 3T3-SV40 treated with 10 mM NAC that we investigated previously normalized their phenotype to the various extent. Like normal cells these cells exposed to NAC are not recognized and destroyed by NK. Murine embryonic fibroblasts (MEFs) similar to transformed cells were destroyed by NK. MEFs treated with 10 mM NAC lost their susceptibility to NK, as did transformed cells. The loss of cell sensitivity to NK cytolytic activity was accompanied by the reorganization of the actin cytoskeleton and generation of well-pronounced stress-fibers.     

母胎界面自然杀伤细胞的研究进展

自然杀伤(natural killer,NK)细胞是母胎界面丰度最高的免疫细胞,在妊娠早期的子宫蜕膜中大量积聚.研究表明母胎界面NK细胞具有独特表型和功能,在妊娠期免疫耐受调节、子宫内膜蜕膜化、滋养细胞侵袭、子宫螺旋动脉重塑、胎盘形成和胎儿生长、发育等多方面都发挥关键作用,但是其在妊娠中的功能及其作用机制还有待深入研究...     

Cutting edge: Thymic NK cells develop independently from T cell precursors

Although NK cells in the mouse are thought to develop in the bone marrow, a small population of NK cells in the thymus has been shown to derive from a GATA3-dependent pathway. Characteristically, thymic NK cells express CD127 and few Ly49 molecules and lack CD11b. Because these NK cells develop in the thymus, the question of their relationship to the T cell lineage has been raised. Using several different mouse models, we find that unlike T cells, thymic NK cells are not the progeny of Rorc-expressing progenitors and do not express Rag2 or rearrange the TCRγ locus. We further demonstrate that thymic NK cells develop independently of the Notch signaling pathway, supporting the idea that thymic NK cells represent bona fide NK cells that can develop independently of all T cell precursors.     

Human B cell activation by autologous NK cells is regulated by CD40-CD40 ligand interaction: role of memory B cells and CD5+ B cells.

NK cells are a subpopulation of lymphocytes characterized primarily by their cytolytic activity. They are recognized as an important component of the immune response against virus infection and tumors. In addition to their cytolytic activity, NK cells also participate either directly or indirectly in the regulation of the ongoing Ab response. More recently, it has been suggested that NK cells have an important role in the outcome of autoimmune diseases. Here, we demonstrate that human NK cells can induce autologous resting B cells to synthesize Ig, including switching to IgG and IgA, reminiscent of a secondary Ab response. B cell activation by the NK cell is contact-dependent and rapid, suggesting an autocrine B cell-regulated process. This NK cell function is T cell-independent, requires an active cytoplasmic membrane, and is blocked by anti-CD40 ligand (anti-CD154) or CD40-mIg fusion protein, indicating a critical role for CD40-CD40 ligand interaction. Depletion studies also demonstrate that CD5+ B cells (autoreactive B-1 cells) and a heterogeneous population of CD27+ memory B cells play a critical role in the Ig response induced by NK cells. The existence of this novel mechanism of B cell activation has important implications in innate immunity, B cell-mediated autoimmunity, and B cell neoplasia.     

Missing self by heterogeneous natural killer cells

Some murine (YAC, P815 and SP20) and human (Molt4, Raji and HR7) tumour cell lines were (i) treated with IFN-γ for inducing enhanced expression of MHC class I antigen, or (ii) given a brief treatment with citrate buffer (pH 3.0), which resulted in denaturation of class I MHC antigens on these tumour cells. IFL-γ or acid treated tumour cells were used as unlabelled competing targets in cold target inhibition assays. The results indicated that the competing ability of acid-treated tumour cells remained unaltered, whereas IFN-γ treated tumour cells competed with significantly less efficiency. These results have been evaluated in light of the current view of NK cell development and the expression of inhibitory receptors for MHC class I molecules (IRMs), on NK cells. A modified view on NK cell heterogeneity based upon IRM expression has been proposed which reconciles several apparently discordant observations about the activity and role of NK cells. Two classes of NK cells have been proposed. Type I NK ceils have target recognition receptors which do not recognize autologous normal cells, lack IRMs, and may participate in first line of defence against transformed cells in vivo. Type II NK cells have target recognition receptors for autologous normal cells and express at least one self-reactive IRM in order to prevent auto-killing. Type II NK cells participate in killing those transformed cells which down-regulate their MHC class I expression in order to escape cytotoxic T-cell surveillance. It is also postulated that mechanism of inverse correlation of target cell MHC class I expression levels and their susceptibility to NK cells, involves interference model of missing self hypothesis for type I NK cells and inhibitory signal model of missing self hypothesis for type II NE cells. Finally, it is proposed that acid treatment of tumour cells enhances their lysis susceptibility by making them additionally susceptible to type II NK cells, rather than enhancing their killing by type I NK cells. This proposition would explain the lack of effect of acid treatment on the competing ability of tumour cells, when target cells are only lysed by type I NE cells.