Regulation of antiviral CD8+ T cells by inhibitory natural killer cell receptors

Recent evidence indicates that CD8(+) T cells express natural killer cell receptors that constrain the range and magnitude of their activities. For virus-specific CD8(+) T cells, upregulation of these receptors serves to control infection, while concurrently minimizing bystander pathology. Dysregulated expression of these receptors, however, may foster the establishment of persistent virus infection.     

Unravelling natural killer cell function: triggering and inhibitory human NK receptors

Natural killer (NK) cells represent a highly specialized lymphoid population characterized by a potent cytolytic activity against tumor or virally infected cells. Their function is finely regulated by a series of inhibitory or activating receptors. The inhibitory receptors, specific for major histocompatibility complex (MHC) class I molecules, allow NK cells to discriminate between normal cells and cells that have lost the expression of MHC class I (e.g., tumor cells). The major receptors responsible for NK cell triggering are NKp46, NKp30, NKp44 and NKG2D. The NK-mediated lysis of tumor cells involves several such receptors, while killing of dendritic cells involves only NKp30. The target-cell ligands recognized by some receptors have been identified, but those to which major receptors bind are not yet known. Nevertheless, functional data suggest that they are primarily expressed on cells upon activation, proliferation or tumor transformation. Thus, the ability of NK cells to lyse target cells requires both the lack of surface MHC class I molecules and the expression of appropriate ligands that trigger NK receptors.     

Activating and inhibitory receptors and their role in natural killer cell function

Last decade has seen a significant advancement in our understanding of the Natural Killer (NK) cell biology and function. Several receptors present on NK cells have been identified, which are involved in either their activation or inhibition. Similarly, a large number of interacting ligands have been identified on the target cells that upon interaction transmit either activating or inhibitory signals. This review is oriented towards understanding the role of these receptors on the NK cells, which are considered as the first line of defense.     

Effects of interleukin-18 on natural killer cells: costimulation of activation through Fc receptors for immunoglobulin

The antitumor activity of monoclonal antibodies is mediated by effector cells, such as natural killer (NK) cells, that express Fc receptors for immunoglobulin. Efficacy of monoclonal antibodies, including the CD20 antibody rituximab, could be improved by agents that augment the function of NK cells. Interleukin (IL)-18 is an immunostimulatory cytokine that has antitumor activity in preclinical models. The effects of IL-18 on NK cell function mediated through Fcγ receptors were examined. Human NK cells stimulated with immobilized IgG in vitro secreted IFN-γ as expected; such IFN-γ production was partially inhibited by blocking CD16 with monoclonal antibodies. IL-18 augmented IFN-γ production by NK cells stimulated with immobilized IgG or CD16 antibodies. NK cell IFN-γ production in response to immobilized IgG and/or IL-18 was inhibited by chemical inhibitors of Syk and several other kinases involved in CD16 signaling pathways. IL-18 augmented antibody-dependent cellular cytotoxicity (ADCC) of human NK cells against rituximab-coated Raji cells in vitro. IL-18 and rituximab acted synergistically to promote regression of human lymphoma xenografts in SCID mice. Inasmuch as IL-18 costimulates IFN-γ production and ADCC of NK cells activated through Fc receptors in vitro and augments antitumor activity of rituximab in vivo, it is an attractive cytokine to combine with monoclonal antibodies for treatment of human cancer.     

Lymphokine-activated killer cells in rats: generation of natural killer cells and lymphokine-activated killer cells from bone marrow progenitor cells

The coculture of rat bone marrow cells with recombinant interleukin-2 induced the generation of cells mediating natural killer (NK) activity and subsequent lymphokine-activated killer (LAK) activity depending upon the dose of IL-2 and time of culture. NK activity was detected as early as 4 to 5 days after the addition of IL-2 and could be evoked with as little as 5 to 50 U/ml. The induced NK cells had large granular lymphocyte (LGL) morphology and expressed 0X8 and asialo GM1 surface markers but did not express 0X19 or W3/25 markers. LAK activity was detected only after 5 days of culture, and required above 100 U/ml IL-2. Cells mediating LAK activity also expressed 0X8 and asialo GM1 but not 0X19. The generation of detectable NK and subsequent LAK activity was due to induction of early progenitor cells and not contaminating mature LGL/NK cells within the bone marrow population since of removal of such mature NK cells with L-leucine methyl ester (L-LME) did not affect the subsequent generation of either activity. Moreover, the removal of actively dividing cells as well as mature NK cells from the bone marrow by treatment with 5-fluorouracil (5-FU) in vivo enriched the remaining bone marrow population for both NK and LAK progenitor cells. The phenotype of the L-LME- and 5-FU-resistant NK and LAK progenitor cells within populations of bone marrow was determined by antibody plus complement depletion analysis. Although treatment of normal bone marrow with anti-asialo GM1 + C reduced the induction of NK and LAK activity in 5-day cultures, treatment of 5-FU marrow with anti-asialo GM1 + C did not affect either activity. Treatment with a pan-T cell antibody + C did not affect the development of NK or LAK activity under any conditions. Thus, the 5-FU-resistant NK/LAK progenitors were asialo GM1 negative but became asialo GM1+ after induction by IL-2. Finally, evidence that bone marrow-derived LAK cells were generated directly from the IL-2-induced NK cells was obtained by treating the IL-2-induced LGL/NK cells with L-LME.(ABSTRACT TRUNCATED AT 400 WORDS)     

Recognition of vaccinia virus-infected cells by human natural killer cells depends on natural cytotoxicity receptors

Natural Killer (NK) cells are important in the immune response to a number of viruses; however, the mechanisms used by NK cells to discriminate between healthy and virus-infected cells are only beginning to be understood. Infection with vaccinia virus provokes a marked increase in the susceptibility of target cells to lysis by NK cells, and we show that recognition of the changes in the target cell induced by vaccinia virus infection depends on the natural cytotoxicity receptors NKp30, NKp44, and NKp46. Vaccinia virus infection does not induce expression of ligands for the activating NKG2D receptor, nor does downregulation of major histocompatibility complex class I molecules appear to be of critical importance for altered target cell susceptibility to NK cell lysis. The increased susceptibility to lysis by NK cells triggered upon poxvirus infection depends on a viral gene, or genes, transcribed early in the viral life cycle and present in multiple distinct orthopoxviruses. The more general implications of these data for the processes of innate immune recognition are discussed.     

Identification and characterization of urokinase receptors in natural killer cells and T-cell-derived lymphokine activated killer cells.

Fluorescence-activated cell scanning analysis of human blood cells revealed novel urokinase receptors in large granular lymphocytes and a small subset of T-cells (CD3+). Culturing of T-cells with interleukin-2 to generate CD3+ lymphokine-activated killer cells caused a large increase in urokinase binding, suggesting that the urokinase receptor is an activation antigen. The receptor in lymphocytes was similar to that in monocytes with regard to size, affinity and ligand specificity, but did not mediate degradation of urokinase-inhibitor complexes. It is suggested that lymphocyte-bound pro-urokinase is activated, e.g. by the human T-cell-specific serine proteinase, HuTSP-1, and thereby starts a cascade of plasminogen activation important for extravasation of the cells.     

Expression of chemokine receptors on natural killer cells in HIV-infected individuals

Chemokine receptors CCR5 and CXCR4 are of major importance in the pathogenesis of HIV-1 infection because they are co-receptors for human immunodeficiency virus (HIV) entry. We examined the frequency of CD3⁻CD56⁺CCR5⁺ and CD3⁻CD56⁺CXCR4⁺ in HIV-infected long-term slow progressors (SPs), HIV typical progressors (TPs) with or without highly active antiretroviral therapy (HAART), and HIV-seronegative controls. The results showed that the frequency of CD3⁻CD56⁺CCR5⁺ was up-regulated, and frequency of CD3⁻CD56⁺CXCR4⁺ was down-regulated in HAART-naïve HIV TPs group compared with HIV SPs group and HIV-seronegative controls (P < 0.05). The frequency of CD3⁻CD56⁺CCR5⁺ was down-regulated by HAART therapy (P < 0.05). The frequency of CD3⁻CD56⁺CCR5⁺ was lower in HIV SPs compared with controls (P < 0.05). Lower frequency of CD3⁻CD56⁺CXCR4⁺ and higher frequency of CD3⁻CD56⁺CCR5⁺ positively correlated with the level of HIV viral loads and negatively correlated with CD4 T cell counts (P < 0.05). These results indicated that the expression of chemokine receptors on NK cells correlated with HIV disease progression.     

Characterization of corticosteroid receptors in natural killer cells: comparison with circulating lymphoid and myeloid cells

Lymphocytes mediating natural killer (NK) and antibody-dependent cellular cytotoxicity (ADCC) activities are relatively refractory to the changes in circulatory traffic and intrinsic function induced in other cell types by in vivo and in vitro corticosteroids (CS). To investigate if such drug resistance could be attributed to differences in the CS receptor number of affinity (Kd) of these cells, these characteristics were determined in purified populations of large granular lymphocytes (LGL), monocytes, neutrophils (PMN), and T cells. All cell types displayed a single class of CS receptor of uniform affinity; however, LGL resembled monocytes and PMN in receptor number and Kd while T cells had significantly fewer sites per cell with lower Kd. These studies suggest that the unresponsiveness of NK activity to CS is not secondary to differences in CS receptor capacity or affinity.     

自然杀伤细胞受体的研究进展

自然杀伤细胞（ＮＫ细胞）可表达两类功能相悖的识别受体,即活化受体（ＫＡＲ）和抑制受体（ＫＩＲ）。ＫＩＲ能识别自身细胞上的ＭＨＣⅠ类分子与自身或外来肽形成２的复合物,所产生的抑制信号可阴断ＫＡＲ的活化,以此抑制ＮＫ细胞的细胞毒作用。如果靶细胞失去ＫＩＲ所识别的配体,ＮＫ细胞即可通过ＫＡＲ对靶细胞进行攻击。本文将介绍此类受体的结构及基识别与信号转导机制的研究进展。     

The evolution of natural killer cell receptors

Natural killer (NK) cells are immune cells that play a crucial role against viral infections and tumors. To be tolerant against healthy tissue and simultaneously attack infected cells, the activity of NK cells is tightly regulated by a sophisticated array of germline-encoded activating and inhibiting receptors. The best characterized mechanism of NK cell activation is “missing self” detection, i.e., the recognition of virally infected or transformed cells that reduce their MHC expression to evade cytotoxic T cells. To monitor the expression of MHC-I on target cells, NK cells have monomorphic inhibitory receptors which interact with conserved MHC molecules. However, there are other NK cell receptors (NKRs) encoded by gene families showing a remarkable genetic diversity. Thus, NKR haplotypes contain several genes encoding for receptors with activating and inhibiting signaling, and that vary in gene content and allelic polymorphism. But if missing-self detection can be achieved by a monomorphic NKR system why have these polygenic and polymorphic receptors evolved? Here, we review the expansion of NKR receptor families in different mammal species, and we discuss several hypotheses that possibly underlie the diversification of the NK cell receptor complex, including the evolution of viral decoys, peptide sensitivity, and selective MHC-downregulation.     

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.     

Virus-driven evolution of natural killer cell receptors

Natural killer (NK) cells can mount an immediate response against viral infection, secreting cytokines and killing virus-infected cells. However, viruses have devised strategies to avoid immune detection. Here, we discuss NK cell recognition of viruses and propose that viruses may provide the evolutionary pressure causing the diversification of the NK cell receptors.     

Selective reduction of natural killer cells and T cells expressing inhibitory receptors for MHC class I in the livers of patients with hepatic malignancy

Natural killer (NK) and CD56(+) T cells are thought to play a central role in antitumour immunity. Their cytolytic activities are controlled by a variety of receptors including CD94 and killer immunoglobulin-like receptors (KIR), which bind to major histocompatibility complex (MHC) class I molecules on target cells and mediate cell activation or inhibition. We have examined the numbers, phenotypes and antitumour cytotoxic functions of hepatic NK and CD56(+) T cells isolated from 22 patients with hepatic malignancy and 19 healthy donors. Flow cytometry revealed that NK cell numbers were increased among hepatic mononuclear cells in malignancy compared to histologically normal livers (mean: 38% vs 27%; P=0.03), but CD56(+) T cell numbers were not (28% vs 27%). NK cells and CD56(+) T cells from tumour-bearing livers exhibited lymphokine-activated killing of K562 targets and T cell receptor-mediated lysis of P815 cells. The expression of CD94 and the KIR isotypes CD158a, CD158b and KIR3DL1 by CD56(+) T cells and NK cells was significantly and consistently reduced in tumour-bearing livers compared to healthy livers ( P<0.05 in all cases). Simultaneous ligation of CD158a, CD158b and KIR3DL1 caused an overall partial inhibition of CD56(+) T cell cytotoxic activity, suggesting that the observed reductions in KIR(+) cell numbers in malignancy are likely to lead to enhanced cytotoxicity. Our results suggest that, while hepatic CD56(+) T cells are not expanded in malignancy, downregulation of KIR and CD94 expression may be a mechanism by which the hepatic immune system can be activated to facilitate tumour rejection.