Ontogeny and expansion of human natural killer cells: clinical implications

Effector and cytolytic function of natural killer cells in anticancer immunity

Adaptive immune cells play an important role in mounting antigen-specific antitumor immunity. The contribution of innate immune cells such as monocytes, macrophages, natural killer (NK) cells, dendritic cells, and gamma-delta T cells is well studied in cancer immunology. NK cells are innate lymphoid cells that show effector and regulatory function in a contact-dependent and contact-independent manner. The cytotoxic function of NK cells plays an important role in killing the infected and transformed host cells and controlling infection and tumor growth. However, several studies have also ascribed the role of NK cells in inducing pathophysiology in autoimmune diseases, promoting immune tolerance in the uterus, and antitumor function in the tumor microenvironment. We discuss the fundamentals of NK cell biology, its distribution in different organs, cellular and molecular interactions, and its cytotoxic and noncytotoxic functions in cancer biology. We also highlight the use of NK cell-based adoptive cellular therapy in cancer.

Key Activating and Inhibitory Ligands Involved in the Mobilization of Natural Killer Cells for Cancer Immunotherapies

Natural killer (NK) cells are the most potent arm of the innate immune system and play an important role in immunity, alloimmunity, autoimmunity, and cancer. NK cells recognize “altered-self” cells due to oncogenic transformation or stress due to viral infection and target to kill them. The effector functions of NK cells depend on the interaction of the activating and inhibitory receptors on their surface with their cognate ligand expressed on the target cells. These activating and inhibitory receptors interact with major histocompatibility complex I (MHC I) expressed on the target cells and make decisions to mount an immune response. NK cell immune response includes cytolytic activity and secretion of cytokines to help with the ongoing immune response. The advancement of our knowledge on the expression of inhibitory and activating molecules led us to exploit these molecules in the treatment of cancer. This review discusses the importance of activating and inhibitory receptors on NK cells and their clinical importance in cancer immunotherapy.

Targeting Cancer Stem Cells with Natural Killer Cell Immunotherapy

INTRODUCTION

Standard cytoreductive cancer therapy, such as chemotherapy and radiotherapy, are frequently resisted by a small portion of cancer cells with 'stem-cell' like properties including quiescence and repopulation. Immunotherapy represents a breakthrough modality for improving oncologic outcomes in cancer patients. Since the success of immunotherapy is not contingent on target cell proliferation, it may also be uniquely suited to address the problem of resistance and repopulation exerted by cancer stem cells (CSCs). Areas covered: Natural killer (NK) cells have long been known for their ability to reject allogeneic hematopoietic stem cells, and there are increasing data demonstrating that NK cells can selectively identify and lyse CSCs. The authors review the current knowledge of CSCs and NK cells and highlight recent studies that support the concept that NK cells are capable of targeting CSC in solid tumors, especially in the context of combination therapy simultaneously targeting non-CSCs and CSCs. Expert opinion: Unlike cytotoxic cancer treatments, NK cells can target and eliminate quiescent/non-proliferating cells such as CSCs, and these enigmatic cells are an important source of relapse and metastasis. NK targeting of CSCs represents a novel and potentially high impact method to capitalize on the intrinsic therapeutic potential of NK cells.

Could interleukin-15 potentiate histone deacetylase inhibitor effects in haematological malignancy?

Despite significant progress in cancer therapy, prognosis in acute leukaemia remains dismal, and the development of new therapies is urgently warranted: in acute myeloid leukaemia, the current cure rate is of 30-40% in younger and much less in older patients. Chromatin remodeling through histone acetylation is one of the major mechanisms of transcriptional control of genes, and is involved in 'gene silencing' of antioncogenes in various tumour cells. Chromatin remodeling is also involved in transcriptional control of other genes, such as NKG2D ligand genes. Histone deacetylases and acetyltransferases are involved in the epigenetic regulation of gene expression, and increased/decreased activity of histone deacetylases has been reported in several cancer types. Histone deacetylase inhibitors were reportedly active in many cancers including hematological malignancies, and have been shown in numerous experiments to reduce cancer cell growth and enhance cell differentiation, growth arrest and apoptosis. In acute myeloid leukaemia, histone deacetylase inhibitors alone had limited efficacy, but their combination with other anticancer agents yielded promising results. Interleukin (IL)-15 is regarded with great hope in the immunotherapy of cancer, and IL-15-activated cytokine-induced killer cells showed potent antileukemic activity both in vitro and in vivo. IL-15 increases expression of NKG2D and its ligands and can increase natural killer cell mediated cytotoxicity against tumour cells. The administration of IL-15 was recently shown to be safe in preclinical models, and there are ongoing clinical trials of IL-15 in patients with cancer and HIV infection. We hypothesise that IL-15 will synergise with histone deacetylase inhibitors in increasing the levels of activatory NKG2D receptors on natural killer and CD8(+) T cells and of their ligands, the MHC class I related molecule A and B, on tumor cells, and will enhance innate immune antitumour responses in acute myeloid leukaemia and other haematological malignancies. Up-regulation of NKG2D-NKG2D-ligand antitumour immune response by combining histone deacetylase inhibitors with IL-15 has the potential to improve the efficacy of acute myeloid leukaemia treatment.

Natural killer cell reprogramming with chimeric immune receptors

Natural killer (NK) cells are emerging as a new tool for cell therapy of cancer. However, some cancer subtypes are relatively resistant to NK cell cytotoxicity. Expression of anti-CD19 chimeric signaling receptors can enhance NK-cell reactivity against CD19+ leukemia and lymphoma cells. Here we describe a method to enforce expression of such receptors in human NK cells relying on electroporation of mRNA and compare it to retroviral transduction of cDNA. These methods are applicable to the reprogramming of NK cells with chimeric receptors specific for other antigens expressed on cancer cells as well as with molecules that can modulate NK cell function.

Use of allogeneic NK cells for cancer immunotherapy

Controversy exists as to the role that the immune system plays in cancer therapy. While the immune system has been proposed to scavenge the body to prevent microscopic transformation from forming cancer, it has been difficult to mount its potential of shrinking established tumors. NK cells are components of the innate immune system. They can recognize targets without prior sensitization, making them ideal candidates to manipulate for therapeutic use against cancer. Initially, autologous NK cells were directed against tumors but it was realized that NK cells that recognize self cells are inhibited. More encouraging advances have been made with allogeneic NK cell therapy in clinical trials to overcome this limitation. In this article, we present developments in NK cell adoptive immunotherapy for hematologic and solid tumor malignancies.

MTS dye based colorimetric CTLL-2 cell proliferation assay for product release and stability monitoring of interleukin-15: assay qualification, standardization and statistical analysis

A colorimetric cell proliferation assay using soluble tetrazolium salt [(CellTiter 96(R) Aqueous One Solution) cell proliferation reagent, containing the (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt) and an electron coupling reagent phenazine ethosulfate], was optimized and qualified for quantitative determination of IL-15 dependent CTLL-2 cell proliferation activity. An in-house recombinant Human (rHu)IL-15 reference lot was standardized (IU/mg) against an international reference standard. Specificity of the assay for IL-15 was documented by illustrating the ability of neutralizing anti-IL-15 antibodies to block the product specific CTLL-2 cell proliferation and the lack of blocking effect with anti-IL-2 antibodies. Under the defined assay conditions, the linear dose-response concentration range was between 0.04 and 0.17ng/ml of the rHuIL-15 produced in-house and 0.5-3.0IU/ml for the international standard. Statistical analysis of the data was performed with the use of scripts written in the R Statistical Language and Environment utilizing a four-parameter logistic regression fit analysis procedure. The overall variation in the ED(50) values for the in-house reference standard from 55 independent estimates performed over the period of 1year was 12.3% of the average. Excellent intra-plate and within-day/inter-plate consistency was observed for all four parameter estimates in the model. Different preparations of rHuIL-15 showed excellent intra-plate consistency in the parameter estimates corresponding to the lower and upper asymptotes as well as to the 'slope' factor at the mid-point. The ED(50) values showed statistically significant differences for different lots and for control versus stressed samples. Three R-scripts improve data analysis capabilities allowing one to describe assay variations, to draw inferences between data sets from formal statistical tests, and to set up improved assay acceptance criteria based on comparability and consistency in the four parameters of the model. The assay is precise, accurate and robust and can be fully validated. Applications of the assay were established including process development support, release of the rHuIL-15 product for pre-clinical and clinical studies, and for monitoring storage stability.

Expansion and activation of natural killer cells for cancer immunotherapy

Natural killer (NK) cells can kill a wide range of cancer cells and are a promising tool for cell therapy of cancer. NK cells cytotoxicity is regulated by a balance between stimulatory and inhibitory signals. Interleukin-2 is known to increase NK cell cytotoxicity. Although many cytokines have been studied in efforts to induce durable NK cell expansions, most reports indicate a rather modest effect and the requirement for additional stimuli. We found that contact with the K562 myeloid leukemia cell line, genetically modified to express a membrane-bound form of interleukin-15 and the ligand for the costimulatory molecule 4-1BB, induced vigorous expansion of NK cells from peripheral blood. Based on these findings, we developed a method for large-scale clinical-grade expansion of NK cells. This method is currently used to expand allogeneic NK cells for infusion in patients with leukemia and solid tumors. We here summarize methods for expansion and activation of NK cells from human peripheral blood mononuclear cells as well as clinical-scale methods to produce NK cells for immunotherapy under Current Good Manufacturing Practices (cGMP) conditions.

Retroviral gene transfer into primary human natural killer cells

Modulation of intracellular signaling pathways or receptor expression in natural killer (NK) cells by genetic manipulation is an attractive possibility in studies of NK cell specificity and function. Moreover, feasible applications of these genetic manipulations in the context of gene and NK cell therapy regimens may be considered. However, efficient gene modification of primary NK cells has been largely hampered by the absence of an efficient gene-transfer protocol.A retrovirus-based easy-to-use transduction protocol that can insert the gene of interest permanently into primary NK cells would be an important tool to advance our studies in NK cell biology and NK cell-mediated therapies. We have recently described a protocol for efficient expansion of NK cells under good manufacturing practice (GMP) conditions from the healthy donors and from patients with hematological malignancies. As the active division of cells is a prerequisite for efficient retroviral insertion, the high rate of expansion in this protocol provides more efficient transduction by retroviral vectors. We hereby present this simple and efficient retroviral vector-based gene-transfer protocol for such ex vivo cultured primary human NK cells.

New visions for basic research and primary prevention of pediatric allergy: an iPAC summary and future trends

Hydrolyzed formula feeding, delayed introduction of solid food, indoor allergen avoidance, smoke and pollutants avoidance have been applied for several decades as primary preventive measures for allergic diseases. Unfortunately, some of these strategies have had no or modest success. Therefore, resources need to be focused on better understanding of the early allergic events and on interventional studies to investigate new strategies of primary and secondary prevention. Accordingly, this review summarizes the state-of-the-art of genetic, immunological and clinical aspects of primary prevention of allergic diseases. Studies investigating gene-by-gene and gene-by-environment interactions suggest that prevention of allergic diseases must be tailored to the individual genetic susceptibilities ('gene profiling') and environmental exposures. The expanding knowledge on new T cell populations (Th17, TSLP (thymic stromal derived lymphopoietin)-dependent 'inflammatory Th2 cells') is also inspiring new concepts on the origins of allergic diseases. The old concept of 'blocking immunoglobulin G antibodies' has been re-appraised and it is likely to generate novel preventive and therapeutic strategies. The major task for future clinical research is to clearly define the timing of optimal exposure to potential allergens. In addition, the role of microbial products such as certain bacteria, or their components, and of helminths or their larvae at different times in early life, alone or with potential allergens, definitely need to be further investigated. The benefit of efficient allergy prevention, based on focusing resources on novel and promising research lines, will be of prime importance to both affluent countries and other parts of the world where allergy is only currently emerging.

Human natural killer cell development and biology

Natural killer cells are important innate immune effector cells with potentially broad applications in the treatment of human malignancy due to their ability to lyse neoplastic cells without the need for tumor-specific antigen recognition. Human NK cells can be divided into two functional subsets based on their surface expression of CD56; CD56(bright) immunoregulatory cells and CD56(dim) cytotoxic cells. In addition to functional differences, these NK cell subsets can be modulated differently by interleukin (IL)-2, which has permitted the development of lower dose, better tolerated IL-2 regimens for the in vivo expansion and activation of NK cells. The importance of early hematopoietic growth factors, such as c-kit ligand and flt-3 ligand, and their synergy with IL-15 in the development of human NK cells in the bone marrow has permitted the investigation of novel cytokine combinations for optimizing in vivo expansion of NK cell in the clinic. The importance of lymph nodes as a site for NK cell development has recently been elucidated. Furthermore, progress in the field of how NK cell recognize target cells via activating and inhibitory receptors, and how the balance of signals from these receptors can modulate NK cell activity has revolutionized our understanding of the selective killing of tumor cells by NK cells while sparing normal cells. In this review, we summarize current understanding of NK cell biology, and highlight how such knowledge may be translated to optimize the efficacy of using autologous or allogeneic NK cell for the immunotherapy of cancer.

Efficient gene transfer into primary human natural killer cells by retroviral transduction

OBJECTIVE

To optimize retroviral gene transfer into primary human natural killer (NK) cells.

MATERIALS AND METHODS

NK cells from healthy donors were expanded ex vivo for a period of 21 days. Retroviral transductions were carried out by replacing culture media with retrovirus-containing supernatant during 2-hour incubations on days 3, 4, 5, 6, 10, 15, or 20. In some experiments, NK cells were transduced on 2 consecutive days (days 5 and 6). Green fluorescent protein served as a marker for detection of transduced cells.

RESULTS

NK cells showed a median of 27.2% transduction efficiency after a single transduction round (transduction on day 5) and a median of 47.1% transduction efficiency after two rounds of transduction (transduction on days 5 and 6), 24 hours after exposure to retrovirus-containing supernatants. On day 21 after initial culture, 51.9% of NK cells were transduced after a single transduction round (transduction on day 5) and 75.4% after two rounds of transduction (transduction on days 5 and 6). Gene transfer did not change the function or phenotype of NK cells as determined by phenotypical analysis, nor did the proliferative ability or cytotoxic function change.

CONCLUSION

The results show that NK cells can successfully be transduced with retroviral vectors, without any detectable changes in phenotype or function. This may open up new possibilities in the studies of NK cell biology and the development of NK cells for immunotherapy regimens.

Genetic modification of primary natural killer cells overcomes inhibitory signals and induces specific killing of leukemic cells

Natural killer (NK) cells hold promise for improving the therapeutic potential of allogeneic hematopoietic transplantation, but their effectiveness is limited by inhibitory HLA types. We sought to overcome this intrinsic resistance by transducing CD56+CD3- NK cells with chimeric receptors directed against CD19, a molecule widely expressed by malignant B cells. An abundance of NK cells for transduction was secured by culturing peripheral blood mononuclear cells with K562 cells expressing the NK-stimulatory molecules 4-1BB ligand and interleukin 15, which yielded a median greater than 1000-fold expansion of CD56+CD3- cells at 3 weeks of culture, without T-lymphocyte expansion. Expression of anti-CD19 receptors linked to CD3zeta overcame NK resistance and markedly enhanced NK-cell-mediated killing of leukemic cells. This result was significantly improved by adding the 4-1BB costimulatory molecule to the chimeric anti-CD19-CD3zeta receptor; the cytotoxicity produced by NK cells expressing this construct uniformly exceeded that of NK cells whose signaling receptors lacked 4-1BB, even when natural cytotoxicity was apparent. Addition of 4-1BB was also associated with increased cell activation and production of interferon gamma and granulocyte-macrophage colony-stimulating factor. Our findings indicate that enforced expression of signaling receptors by NK cells might circumvent inhibitory signals, providing a novel means to enhance the effectiveness of allogeneic stem cell transplantation.

Characterization of cord blood natural killer cells: implications for transplantation and neonatal infections

The role of natural killer (NK) cells in hematopoietic stem cell transplantation and in the control of neonatal infections is not yet clear. Donor-versus-recipient NK cell alloreactivity was found to improve outcome in some settings of hematopoietic stem cell transplantation. We hypothesized that the role of NK cells in cord blood (CB) transplantation and neonatal infections may depend on CB NK cell maturation stage. We therefore analyzed the expression of NK cell differentiation/phenotypic markers in human CB, as well as functional properties of purified CB NK cells. CD8 and CD57 expression was lower in CB than in adult NK cells. However, the expression of other differentiation markers was similar, as was cell surface density of CD56, the percentage of late NK cell precursors, interferon-gamma production, and the proliferative response of purified NK cells to IL-2. Spontaneous cytotoxic activity of purified CB NK cells against NK-sensitive targets was low but reached adult levels after treatment with IL-15. Expression of perforin and granzyme B was higher in CB NK cells (90 versus 58% and 86 versus 69%, respectively). intercellular adhesion molecule-1 and CD161 expression was lower in CB. Surprising, fewer CB NK cells expressed L-selectin, a marker of immature NK cells. Taken together, our results suggest that CB NK cells are phenotypically and functionally mature.

Efficient infection of human natural killer cells with an EBV/retroviral hybrid vector

Molecular characterization of human natural killer (NK) cells will require targeted gene delivery to inhibit and activate specific signaling pathways, yet to our knowledge, an effective means to deliver such products for long-term gene expression without disrupting normal cellular processes has not been described. In this study, we have developed a retroviral strategy to effectively express gene products in the NK cell, whereby its effector functions of cytotoxicity and cytokine production remain intact. Using an EBV/retroviral hybrid vector, we demonstrate infection of human peripheral blood NK cells with simultaneous expression of a marker for infection--the enhanced green fluorescent protein (EGFP)--along with various genes of interest. This technique results in successful infection of the CD56dim NK population that predominates among human peripheral blood NK and is the effector of antibody-dependent cellular cytotoxicity and natural killing. In addition, we demonstrate infection of the CD56bright NK subset as well as the NK-92 cell line. In summary, we have devised an efficient and reproducible methodology for the targeted delivery of gene products to human NK cells that should now provide opportunities to dissect the molecular processes critical to normal NK cell physiology.

Interleukin (IL)-15 and IL-2 Reciprocally Regulate Expression of the Chemokine Receptor CX3CR1 through Selective NFAT1- and NFAT2-dependent Mechanisms

We have recently reported that interleukin (IL)-15 and IL-2, which signal through IL-2Rbetagamma, oppositely regulate expression of the proinflammatory chemokine receptor CX3CR1. Here we delineate molecular mechanisms responsible for this paradox. By using a luciferase reporter plasmid, we identified a 433-bp region spanning the major transcriptional start point of human CX3CR1 that, when expressed in human peripheral blood mononuclear cells (PBMCs), possessed strong constitutive promoter activity. IL-2 and IL-15 treatment increased and abolished this activity, respectively, mimicking their effects on endogenous CX3CR1. IL-2 and IL-15 have been reported to also have opposite effects on the immunoregulatory transcription factor NFAT (nuclear factor of activated T cells), and the 433-bp region contains a kappaB-like NFAT site. The effects of IL-15 and IL-2 on both CX3CR1 reporter activity and endogenous CX3CR1 transcription in PBMCs were abolished by the NFAT inhibitors cyclosporin A and VIVIT. Moreover, mutation of the kappaB-like NFAT sequence markedly attenuated IL-2 and IL-15 modulation of CX3CR1 promoter-reporter activity in PBMCs. Furthermore, chromatin immunoprecipitation revealed that IL-15 promoted specific recruitment of NFAT1 but not NFAT2 to the CX3CR1 promoter, whereas IL-2 had the converse effect. This appears to be relevant in vivo because mouse CX3CR1 mRNA was expressed in both PBMCs and splenocytes from NFAT1-/- mice injected with recombinant IL-15 but was undetectable in cells from IL-15-injected NFAT1+/+ BALB/c mice; as predicted, IL-2 up-regulated cx3cr1 in both mouse strains to a similar extent. Thus, by pharmacologic, genetic, and biochemical criteria in vitro and in vivo, our results suggest that IL-15 and IL-2 oppositely regulate CX3CR1 gene expression by differentially recruiting NFAT1 and NFAT2 to a kappaB-like NFAT site within the CX3CR1 promoter. We propose that expression of CX3CR1 and possibly other immunoregulatory genes may be determined in part by the balance of NFAT1 and NFAT2 activity in leukocytes.

Differential effects of interleukin-7 and interleukin-15 on NK cell anti-human immunodeficiency virus activity

The ability of interleukin-7 (IL-7) and IL-15 to expand and/or augment effector cell functions may be of therapeutic benefit to human immunodeficiency virus (HIV)-infected patients. The functional effects of these cytokines on innate HIV-specific immunity and their impact on cells harboring HIV are unknown. We demonstrate that both IL-7 and IL-15 augment natural killer (NK) function by using cells (CD3(-) CD16(+) CD56(+)) from both HIV-positive and -negative donors. Whereas IL-7 enhances NK function through upregulation of Fas ligand, the effect of IL-15 is mediated through upregulation of tumor necrosis factor-related apoptosis-inducing ligand. The difference in these effector mechanisms is reflected by the ability of IL-15-treated but not IL-7-treated NK cells to reduce the burden of replication-competent HIV in autologous peripheral blood mononuclear cells (PBMC) (infectious units per million for control NK cells, 6.79; for IL-7-treated NK cells, 236.17; for IL-15-treated cells, 1.01; P = 0.01 versus control). In addition, the treatment of PBMC with IL-15-treated but not IL-7-treated NK cells causes undetectable HIV p24 (five of five cases), HIV RNA (five of five cases), or HIV DNA (three of five cases). These results support the concept of adjuvant immunotherapy of HIV infection with either IL-7 or IL-15 but suggest that the NK-mediated antiviral effect of IL-15 may be superior.

Biology and clinical impact of human natural killer cells

Natural killer (NK) cells, through elaboration of cytokines and cytolytic activity, are critical to host defense against invading organisms and malignant transformation. Two subsets of human NK cells are identified according to surface CD56 expression. CD56dim cells compose the majority of NK cells and function as effectors of natural cytotoxicity and antibody-dependent cellular cytotoxicity, whereas CD56bright cells have immunomodulatory function through secretion of cytokines. For a long time, NK cells have held promise for cancer immunotherapy because, unlike T-lymphocytes, NK cells can lyse tumor cells without tumor-specific antigen recognition. To date, NK cell therapy, largely focused on in vivo expansion and activation with cytokines, has met with only modest success. However, recent understanding of the importance of NK receptors (NKR) for recognition and lysis of tumor cells while normal cells are spared suggests novel therapeutic strategies. The balance of inhibitory and activating signals through surface receptors that recognize major histocompatibility complex class I and class I-like molecules on target cells determines whether NK cells activate killing. Identification of NKR ligands and their level of expression on normal and neoplastic cells has important implications for the rational design of immunotherapy strategies for cancer. We review recent development in the biology and clinical relevance of NK cells in cancer immunotherapy.

Genetic models in applied physiology: selected contribution: effects of spaceflight on immunity in the C57BL/6 mouse. II. Activation, cytokines, erythrocytes, and platelets

This portion of the study quantified the effects of a 12-day space shuttle mission (Space Transport System-108/UF-1) on body and lymphoid organ masses, activation marker expression, cytokine secretion, and erythrocyte and thrombocyte characteristics in C57BL/6 mice. Animals in flight (Flt group) had 10-12% lower body mass compared with ground controls housed either in animal enclosure modules or under standard vivarium conditions (P < 0.001) and the smallest thymus and spleen masses. Percentages of CD25(+) lymphocytes, CD3(+)/CD25(+) T cells, and NK1.1(+)/CD25(+) natural killer cells from Flt mice were higher compared with both controls (P < 0.05). In contrast, CD71 expression was depressed in the Flt and animal enclosure module control mice compared with vivarium control animals (P < 0.001). Secretion of interferon-gamma, IL-2, and IL-4, but not tumor necrosis factor-alpha and IL-5, by splenocytes from Flt mice was decreased relative to either one or both ground controls (P < 0.05). Flt mice also had high red blood cell and thrombocyte counts compared with both sets of controls; low red blood cell volume and distribution width, percentage of reticulocytes, and platelet volume were also noted (P < 0.05) and were consistent with dehydration. These data indicate that relatively short exposure to the spaceflight environment can induce profound changes that may become significant during long-term space missions.

Natural killer cell activation in mice and men: different triggers for similar weapons?

The signaling pathways that regulate B and T lymphocytes are remarkably conserved between humans and mice. However, recent evidence suggests that the pathways regulating natural killer (NK) cell activation may actually differ between these two species. We discuss the controversies in the field and propose that this divergence could be deceptive: despite some clear differences between human and mouse NK cell receptors, the many ways of activating NK cells and their functions may well be conserved.

The biology of human natural killer-cell subsets

Human natural killer (NK) cells comprise approximately 15% of all circulating lymphocytes. Owing to their early production of cytokines and chemokines, and ability to lyse target cells without prior sensitization, NK cells are crucial components of the innate immune system. Human NK cells can be divided into two subsets based on their cell-surface density of CD56--CD56(bright) and CD56(dim)--each with distinct phenotypic properties. Now, there is ample evidence to suggest that these NK-cell subsets have unique functional attributes and, therefore, distinct roles in the human immune response. The CD56(dim) NK-cell subset is more naturally cytotoxic and expresses higher levels of Ig-like NK receptors and FCgamma receptor III (CD16) than the CD56(bright) NK-cell subset. By contrast, the CD56(bright) subset has the capacity to produce abundant cytokines following activation of monocytes, but has low natural cytotoxicity and is CD16(dim) or CD16(-). In addition, we will discuss other cell-surface receptors expressed differentially by human NK-cell subsets and the distinct functional properties of these subsets.