Mechanisms of NK cell activation: CD4+ T cells enter the scene

Natural killer (NK) cells are innate lymphocytes involved in immunosurveillance through their cytotoxic activity and their capacity to secrete inflammatory cytokines. NK cell activation is necessary to initiate effector functions and results from a complex series of molecular and cellular events. We review here the signals that trigger NK cells and discuss recent findings showing that, besides antigen-presenting cells, T cells can play a central role in the initiation of NK cell activation in lymph nodes.     

Natural killer cell activation by dendritic cells: balancing inhibitory and activating signals

Natural killer (NK) cells have originally been identified by their spontaneous cytolytic potential against tumor cells, which, however, might result from pre-activation due to prior pathogen exposure. Resting NK cells, on the contrary, require activation by bystander antigen-presenting cells to reach their full functional competence. In this review, we will summarize studies on how dendritic cells (DCs), the most potent type of antigen-presenting cell, communicate with human NK cells to activate them in secondary lymphoid organs and to integrate signals from activated NK cells at sites of inflammation for their own maturation. Furthermore, we will review aspects of the immunological synapse, which mediates this cross-talk. These studies provide the mechanistic understanding of how mature DCs can activate NK cells and survive to go on for the activation of adaptive immunity. This feature of DCs, to activate different waves of immune responses, could be harnessed for immunotherapies, including vaccinations.     

Granzyme B is recovered by natural killer cells via clathrin-dependent endocytosis

When they recognize a target cell, natural killer (NK) cells mount an attack to kill the target by exerting their cytotoxicity via the exocytosis of cytotoxic granules. Although the details of this process (which includes the movement of cytotoxic granules in the immune synapse and their fusion with the plasma membrane, releasing granzymes and perforin into the synaptic cleft) are relatively better understood, the post-exocytosis regulation of the process is still largely unknown. Here we show that a clathrin-dependent endocytosis stimulated by target cell occurs in NK92 cell line, which is closely correlated with granzyme B recovery. Inhibition of the endocytosis significantly attenuates the cytotoxicity of NK92 cells. The NK cell recovery of its released effector molecules, in turn, suggests that endocytosis may well play a key role in the post exocytosis regulation of immune cells.     

Surface receptors and functional interactions of human natural killer cells: from bench to the clinic

The past 10years have witnessed dramatic progress in our understanding of how natural killer (NK) cells function and their role in innate immunity. Thanks to an array of inhibitory receptors specific for different HLA class I molecules, human NK cells can sense the decrease or loss of even single alleles at the cell surface. This represents a typical condition of a potential danger, i.e. the presence of tumor or virally infected cells. NK cell triggering and lysis of these cells is mediated by several activating receptors and coreceptors that have recently been identified and cloned. While normal cells are usually resistant to NK-mediated attack, a remarkable exception is represented by dendritic cells (DCs). In their immature form they are susceptible to NK-mediated lysis because of the expression of low levels of surface HLA class I molecules. The process of DC maturation (mDCs) is characterized by the surface expression of high levels of HLA class I molecules. Accordingly, mDCs become resistant to NK cells. A recent major breakthrough highlighted the role played by donor NK cells in allogenic bone marrow transplantation to cure acute myeloid leukemias. Alloreactive NK cells derived from donor hematopoietic precursors not only prevented leukemic relapses, but also prevented graft rejection and graft-versus-host disease.Received 12 March 2003; received after revision 18 April 2003; accepted 30 April 2003     

The role of M cells in mucosal immunity

Mucosa-associated lymphoid tissue in the respiratory and digestive tracts are covered by a specialized epithelium, the follicle-associated epithelium, which includes M cells, which are specialized for the uptake and transcytosis of macromolecules and microorganisms. Following transcytosis, antigens are released to cells of the immune system in lymphoid aggregates beneath the epithelium where antigen processing and presentation and stimulation of specific B and T lymphocytes are achieved. Circulation of the lymphoid cells enables their homing to their original, and other, mucosal sites where they exert the effector function. Such a response may be dominated by secretory immunoglobulin A release and may include cytotoxic T lymphocyte action. Binding of particles to the apical M cell membrane may be nonspecific or due to specific interaction between molecules such as integrins and lectins. Exploiting the specific binding to M cells is an aim for mucosal vaccination, for example to increase the efficiency of uptake of an oral vaccine by its conjugation to an M-cell-specific molecule. Alternatively, an M-cell-specific live vector, such as attenuated Salmonella bacteria, may be used to deliver epitopes of other organisms. Mucosal vaccination efficiency may also be enhanced by a temporary increase in the number of M cells. Therefore, investigation of the properties and ontogeny of M cells must be pursued to allow the development of better mucosal vaccines for the future.     

Long-term production and culture of clones of human T-lymphocytes

Culture of blood T lymphocytes collected from normal individuals and cancer patients were carried out in presence of T cell growth factor (TCGF); these cultures presented cytotoxic activity directed against different targets (lectin activated cells, autologous cancer cells, antibody coated cells and K 562). In order to study separately the different effector subpopulations, isolation of single cultured cells were performed with the help of a micropipette under microscope and monoclonal cultures were carried out in presence of TCGF. In the preliminary cytotoxic assays performed in the clones: (1) a marked activity directed against lectin targets was observed in many clones and (2) an important N K activity was exhibited by the clone 45 B9 (65% of the tested cells lysed human lymphoma K 562 cells).     

Cell-mediated cytotoxicity by natural killer and killer cells, lipid peroxidation and glutathione

In the course of spontaneous cell-mediated cytotoxicity (SCMC) and antibody-dependent cell-mediated cytotoxicity (ADCC) with human peripheral lymphocytes as effector cells, no lipid peroxidation occurred as measured by the production of ethane and thiobarbituric acid-reactive material. Furthermore, impairment of major cellular defense systems of target cells (K562 cells for SCMC, Chang liver cells for ADCC), by decreasing their glutathione content, had no effect on either lipid peroxidation or the cytotoxic response. These findings indicate that peroxidative damage is not a mechanism of NK and K cell-mediated cytotoxicity.     

Virus-mediated inhibition of natural cytotoxicity receptor recognition

Natural killer (NK) cells are a part of the innate immune system that functions mainly to kill transformed and infected cells. Their activity is controlled by signals derived from a panel of activating and inhibitory receptors. The natural cytotoxicity receptors (NCRs): NKp30, NKp44, and NKp46 (NCR1 in mice) are prominent among the activating NK cell receptors and they are, notably, the only NK-activating receptors that are able to recognize pathogen-derived ligands. In addition, the NCRs also recognize cellular ligands, the identity of which remains largely unknown. In this review, we summarize the current knowledge regarding viruses that are recognized by the NCRs, focusing on the diverse immune-evasion mechanisms employed by viruses to escape this detection. We also discuss the unique role the NCRs have in regulating NK cell activity with particular emphasis on the in vivo function of NKp46/NCR1.     

O death where is thy sting? Immunologic tolerance to apoptotic self

In higher organisms, innate scavenging cells maintain physiologic homeostasis by removal of the billions of apoptotic cells generated on a daily basis. Apoptotic cell removal requires efficient recognition and uptake by professional and non-professional phagocytic cells, which are governed by an array of soluble and apoptotic cell-integral signals resulting in immunologically silent clearance. While apoptosis is associated with profound suppression of adaptive and innate inflammatory immunity, we have only begun to scratch the surface in understanding how immunologic tolerance to apoptotic self manifest at either the molecular or cellular level. In the last 10 years, data has emerged implicating professional phagocytes, most notably stromal macrophages and CD8α⁺CD103⁺ dendritic cells, as critical in initiation of the regulatory cascade that will ultimately lead to long-term whole-animal immune tolerance. Importantly, recent work by our lab and others has shown that alterations in apoptotic cell perception by the innate immune system either by removal of critical phagocytic sentinels in secondary lymphoid organs or blockage of immunosuppressive pathways leads to pronounced inflammation with a breakdown of tolerance towards self. This challenges the paradigm that apoptotic cells are inherently immunosuppressive, suggesting that apoptotic cell tolerance is a “context-dependent” event.     

Natural killer-like activity in human cultured lymphoid cells propagated in the presence of interleukin-2: acquired resistance to prostaglandin E2- or dexamethasone-mediated suppression

The cytotoxic activity of human peripheral blood lymphocytes against the natural killer-sensitive target K562 was suppressed both by prostaglandin E2 and dexamethasone. On the other hand, cultured lymphoid cells propagated in the presence of interleukin-2 showed strong cytotoxic reactivity against K562 targets, and were resistant to prostaglandin E2- or dexamethasone-mediated suppression.     

Small lymphocyte production and lymphoid cell proliferation in mouse bone marrow

Summary In mice given³H-thymidine systemically during temporary circulatory occlusion of one hind limb, comparison of the labeling of rapidly-renewing small lymphocytes in the tibial marrows demonstrated that these cells were locally produced. Labeling by³H-thymidine infusion revealed that many marrow large lymphoid cells, presumptive small lymphocyte progenitors, had a marked proliferative activity and rapid turnover which varied according to cell size, was maximal in young mice and declined with increasing age.This work was supported by a grant from the Medical Research Council of Canada. The technical assistance of Mrs E. D. Watson is acknowledged.     

B7-H6/NKp30 interaction: a mechanism of alerting NK cells against tumors

Natural killer (NK) cells are lymphocytes of the innate immune system that sense target cells through a panel of activating and inhibitory receptors. Together with NKG2D, the natural cytotoxicity receptors (NCRs) are major activating receptors involved in tumor cell detection. Although numerous NKG2D ligands have been identified, characterization of the molecules interacting with the NCRs is still incomplete. The identification of B7-H6 as a counter structure of the NCR NKp30 shed light on the molecular basis of NK cell immunosurveillance. We review here the current knowledge on NKp30 and B7-H6, and we discuss their potential role in anti-tumor immunity.     

Small lymphocyte production and lymphoid cell proliferation in mouse bone marrow

In mice given 3H-thymidine systemically during temporary circulatory occlusion of one hind limb, comparison of the labeling of rapidly-renewing small lymphocytes in the tibial marrows demonstrated that these cells were locally produced. Labeling by 3H-thymidine infusion revealed that many marrow large lymphoid cells, presumptive small lymphocyte progenitors, had a marked proliferative activity and rapid turnover which varied according to cell size, was maximal in young mice and declined with increasing age.     

Central role of dendritic cells in the regulation and deregulation of immune responses

Dendritic cells (DCs) play a critical role in orchestrating the innate and adaptive components of the immune system so that appropriate, coordinated responses are mounted against infectious agents. Tissue-resident DCs interact with microbes through germline-encoded pattern-recognition receptors (PRRs), which recognize molecular patterns expressed by various microorganisms. Antigens use PRR activation to instruct DCs for the appropriate priming of natural killer (NK) cells, followed by specific T-cell responses. Due to the central role of DCs in regulating the activation and progression of immune responses, minor imbalances in the feedback control of Toll-like receptor (TLR)-activated cells have been associated with autoimmunity in genetically prone individuals. We review here recent findings on the role of DCs in the priming of innate and adaptive immune responses and the possible involvement of DCs in inducing and maintaining autoimmune reactions.     

Insights into NK cell biology from human genetics and disease associations

Rare human primary immunodeficiency disorders with extreme susceptibility to infections in infancy have provided important insights into immune function. Increasingly, however, primary immunodeficiencies are also recognized as a cause of other more common, often discrete, infectious susceptibilities. In a wider context, loss-of-function mutations in immune genes may also cause disorders of immune regulation and predispose to cancer. Here, we review the associations between human diseases and mutations in genetic elements affecting natural killer (NK) cell development and function. Although many such genetic aberrations significantly reduce NK cell numbers or severely impair NK cell responses, inferences regarding the role of NK cells in disease are confounded by the fact that most mutations also affect the development or function of other cell types. Still, data suggest an important role for NK cells in diseases ranging from classical immunodeficiency syndromes with susceptibility to viruses and other intracellular pathogens to cancer, autoimmunity, and hypersensitivity reactions.     

Natural killer-like activity in human cultured lymphoid cells propagated in the presence of interleukin-2: acquired resistance to prostaglandin E2- or dexamethasone-mediated suppression

Summary The cytotoxic activity of human peripheral blood lymphocytes against the natural killer-sensitive target K562 was suppressed both by prostaglandin E₂ and dexamethasone. On the other hand, cultured lymphoid cells propagated in the presence of interleukin-2 showed strong cytotoxic reactivity against K562 targets, and were resistant to prostaglandin E₂- or dexamethasone-mediated suppression.