Natural cytotoxicity receptors that trigger human NK-cell-mediated cytolysis

Identification and molecular characterization of a natural mutant of the p50.2/KIR2DS2 activating NK receptor that fails to mediate NK cell triggering

P50/KIR2DS molecules represent the activating form of the HLA-C-specific inhibitory NK receptors. They are characterized, in the transmembrane portion, by a charged amino acid that is involved in coupling with signal-transducing adaptor polypeptides. In this study we identified a novel p50.2/KIR2DS2 surface molecule, isolated from NK cell clones derived from an otherwise normal donor, that was unable to transduce activating signals. Sequence analysis of the cDNA encoding this molecule revealed six non-conservative codon mutations in the exon coding for the putative transmembrane portion. Notably, one of such mutations involved the charged residue lysine thought to be important for the association with signal-transducing polypeptides. Indeed, co-transfection experiments revealed that this naturally occurring p50.2/KIR2DS2 mutant, termed Mp50.2, displayed a sharply reduced ability to associate with DAP12 polypeptides. These data provide the first in vivo demonstration of the crucial role played by the transmembrane region of p50.2 receptor molecules in the functional association with DAP12 adaptor molecules and in the process of activation of NK-mediated cytotoxicity.

Human natural killer cell activating receptors

Natural killer (NK) cells were poorly characterized until 10 years ago and few molecules expressed on their cell surface were known. Now the situation has changed dramatically, since a plethora of receptors characterized by opposite functions have been functionally and molecularly defined. NK cells express clonally distributed inhibitory receptors specific for different groups of HLA class I alleles, thus protecting normal cells from NK-mediated lysis. On the contrary, various activating receptors are involved in triggering of NK-mediated natural cytotoxicity. Their engagement induces human NK cells to kill target cells that are either HLA class I-negative or -deficient. Here a brief description of the activating receptors and coreceptor and of their ligand(s) is given.

Natural cytotoxic activity of peripheral-blood lymphocytes and cancer incidence: an 11-year follow-up study of a general population

BACKGROUND

One of the most critical questions in immunosurveillance is whether differences between individuals with regards to natural immunological host defence can predict future development of cancer. Although this question has so far remained open, there are clear indications of significant roles of several naturally cytotoxic lymphocytes in preventing the development of cancer. We began a prospective cohort study among a Japanese general population in 1986, using various immunological and biochemical markers.

METHODS

Natural cytotoxic activity of peripheral-blood mononuclear cells was assessed by isotope-release assay in 3625 residents of a Japanese population mostly older than 40 years of age, between 1986 and 1990. Immunological and biochemical markers were also measured, and participants were given a questionnaire on lifestyle. We did an 11-year follow-up survey of the cohort members looking at cancer incidence and death from all causes, and analysed the association between cytotoxic activity of peripheral-blood lymphocytes assessed at baseline and cancer incidence found in the subsequent follow-up.

FINDINGS

154 cancer cases were used in the analysis. When we categorised the cytotoxic activity of peripheral-blood lymphocytes by tertiles, age-adjusted relative risk of cancer incidence (all sites) was 0.72 (95% CI 0.45-1.16) for men with high cytotoxic activity, and 0.62 (0.38-1.03) for men with medium cytotoxic activity, taking the risk of those with low cytotoxic activity as reference. For women with high cytotoxic activity relative risk was 0.52 (0.28-0.95), and for those with medium cytotoxic activity 0.56 (0.31-1.01). For both sexes with high and medium cytotoxic activity risk was 0.63 (0.43-0.92) and 0.59 (0.40-0.87), respectively.

INTERPRETATION

Our results indicate that medium and high cytotoxic activity of peripheral-blood lymphocytes is associated with reduced cancer risk, whereas low activity is associated with increased cancer risk suggesting a role for natural immunological host defence mechanisms against cancer.

Porcine aortic endothelial cells transfected with HLA-G are partially protected from xenogeneic human NK cytotoxicity

In this study we tested whether the expression of HLA-G protects porcine endothelial cells (PEC) from the lysis mediated by human natural killer (NK) cells. Because HLA-E is not present in PEC, this model provides an ideal tool to study the direct role of HLA-G in NK inhibition. Immortalized porcine aortic endothelial cells (PED) were stably transfected with a vector coding for the HLA-G1 protein and surface expression was demonstrated by flow cytometry analysis. Although the adhesion of human NK cells to PED was not compromised by HLA-G, the expression of HLA-G partially protected PED from the lysis mediated by polyclonal NK lines derived from different donors. A decrease of the surface expression of HLA-G on PED corresponded to a loss of the capacity of PED to inhibit NK cytotoxicity, indicating that the surface density of HLA-G molecules must exceed a certain threshold to protect target cells. In summary, these data show that HLA-G, independent from the presence of HLA-E, can only partially and inefficiently protect PED from human NK cell-mediated cytotoxicity. Because ILT-2/LIR-1 expression did not correlate with HLA-G mediated inhibition, we hypothesize that other yet unidentified receptors expressed by peripheral blood NK cells are involved in the recognition of HLA-G.

Triggering receptors involved in natural killer cell-mediated cytotoxicity against choriocarcinoma cell lines

The lack of classical HLA-class I molecules on trophoblast is necessary to prevent allorecognition by maternal CTL, but may induce activation of NK cells. A protective role against NK cells equipped of suitable inhibitory receptors has been proposed for nonclassical HLA-class I molecules including HLA-E and HLA-G. In the present study we show that the NK-mediated killing of two choriocarcinoma cell lines, JAR and JEG3, is induced upon engagement of natural cytotoxicity receptors (NCR) with their specific ligands. In particular, we show that NKp44, a triggering receptor expressed at the NK cell surface only after in vitro culture in the presence of IL-2, plays a central role in triggering NK cytotoxicity against trophoblast cells. Also NKp46 appear to contribute to this function by cooperating with NKp44. On the other hand, other triggering receptors such as NKp30, 2B4, and NKG2D are not involved in killing of choriocarcinoma. Our findings suggest that resistance of trophoblast to NK-mediated cytotoxicity is the result of insufficient activating interactions between the various triggering NK receptors and their target cell ligands. On the other hand, the interaction of nonclassical HLA class I molecules with inhibitory NK receptors appears to play only a marginal role in regulating the susceptibility of choriocarcinoma to NK mediated cytotoxicity.

NK cell inhibitory receptors prevent tyrosine phosphorylation of the activation receptor 2B4 (CD244)

2B4 is an NK cell activation receptor that can provide a co-stimulatory signal to other activation receptors and whose mode of signal transduction is still unknown. We show that cross-linking of 2B4 on NK cells results in its rapid tyrosine phosphorylation, implying that this initial step in 2B4 signaling does not require coligation of other receptors. Ligation of 2B4 in the context of an NK cell-target cell interaction leads to 2B4 tyrosine phosphorylation, target cell lysis, and IFN-gamma release. Coligation of 2B4 with the inhibitory receptors killer cell Ig-like receptor (KIR)2DL1 or CD94/NKG2 completely blocks NK cell activation. The rapid tyrosine phosphorylation of 2B4 observed upon contact of NK cells with sensitive target cells is abrogated when KIR2DL1 or CD94/NKG2 are engaged by their cognate MHC class I ligand on resistant target cells. These results demonstrate that NK inhibitory receptors can interfere with a step as proximal as phosphorylation of an activation receptor.

Combined natural killer cell and dendritic cell functional deficiency in KARAP/DAP12 loss-of-function mutant mice

KARAP/DAP12 is a transmembrane polypeptide with an intracytoplasmic immunoreceptor tyrosine-based activation motif (ITAM). KARAP/DAP12 is associated with several activating cell surface receptors in hematopoietic cells. Here, we report that knockin mice bearing a nonfunctional KARAP/DAP12 ITAM present altered innate immune responses. Although in these mice NK cells are present and their repertoire of inhibitory MHC class I receptors is intact, the NK cell spectrum of natural cytotoxicity toward tumor cell targets is restricted. KARAP/DAP12 loss-of-function mutant mice also exhibit a dramatic accumulation of dendritic cells in muco-cutaneous epithelia, associated with an impaired hapten-specific contact sensitivity. Thus, despite its homology with CD3zeta and FcRgamma, KARAP/DAP12 plays a specific role in innate immunity, emphasizing the nonredundancy of these ITAM-bearing polypeptides in hematopoietic cells.

X-linked lymphoproliferative disease. 2B4 molecules displaying inhibitory rather than activating function are responsible for the inability of natural killer cells to kill Epstein-Barr virus-infected cells

2B4 is a surface molecule involved in activation of the natural killer (NK) cell-mediated cytotoxicity. It binds a protein termed Src homology 2 domain-containing protein (SH2D1A) or signaling lymphocyte activation molecule (SLAM)-associated protein (SAP), which in turn has been proposed to function as a regulator of the 2B4-associated signal transduction pathway. In this study, we analyzed patients with X-linked lymphoproliferative disease (XLP), a severe inherited immunodeficiency characterized by critical mutations in the SH2D1A gene and by the inability to control Epstein-Barr virus (EBV) infections. We show that, in these patients, 2B4 not only fails to transduce triggering signals, but also mediates a sharp inhibition of the NK-mediated cytolysis. Other receptors involved in NK cell triggering, including CD16, NKp46, NKp44, and NKp30, displayed a normal functional capability. However, their activating function was inhibited upon engagement of 2B4 molecules. CD48, the natural ligand of 2B4, is highly expressed on the surface of EBV(+) B cell lines. Remarkably, NK cells from XLP patients could not kill EBV(+) B cell lines. This failure was found to be the consequence of inhibitory signals generated by the interaction between 2B4 and CD48, as the antibody-mediated disruption of the 2B4-CD48 interaction restored lysis of EBV(+) target cells lacking human histocompatibility leukocyte antigen (HLA) class I molecules. In the case of autologous or allogeneic (HLA class I(+)) EBV(+) lymphoblastoid cell lines, restoration of lysis was achieved only by the simultaneous disruption of 2B4-CD48 and NK receptor-HLA class I interactions. Molecular analysis revealed that 2B4 molecules isolated from either XLP or normal NK cells were identical. As expected, in XLP-NK cells, 2B4 did not associate with SH2D1A, whereas similar to 2B4 molecules isolated from normal NK cells, it did associate with Src homology 2 domain-containing phosphatase 1.