Increased insulin sensitivity and hypoglycaemia in mice lacking the p85 alpha subunit of phosphoinositide 3-kinase.

The hallmark of type 2 diabetes, the most common metabolic disorder, is a defect in insulin-stimulated glucose transport in peripheral tissues. Although a role for phosphoinositide-3-kinase (PI3K) activity in insulin-stimulated glucose transport and glucose transporter isoform 4 (Glut4) translocation has been suggested in vitro, its role in vivo and the molecular link between activation of PI3K and translocation has not yet been elucidated. To determine the role of PI3K in glucose homeostasis, we generated mice with a targeted disruption of the gene encoding the p85alpha regulatory subunit of PI3K (Pik3r1; refs 3-5). Pik3r1-/- mice showed increased insulin sensitivity and hypoglycaemia due to increased glucose transport in skeletal muscle and adipocytes. Insulin-stimulated PI3K activity associated with insulin receptor substrates (IRSs) was mediated via full-length p85 alpha in wild-type mice, but via the p50 alpha alternative splicing isoform of the same gene in Pik3r1-/- mice. This isoform switch was associated with an increase in insulin-induced generation of phosphatidylinositol(3,4,5)triphosphate (PtdIns(3,4,5)P3) in Pik3r1-/- adipocytes and facilitation of Glut4 translocation from the low-density microsome (LDM) fraction to the plasma membrane (PM). This mechanism seems to be responsible for the phenotype of Pik3r1-/- mice, namely increased glucose transport and hypoglycaemia. Our work provides the first direct evidence that PI3K and its regulatory subunit have a role in glucose homeostasis in vivo.

CD3+CD16+NK1.1+B220+ large granular lymphocytes arise from both alpha-beta TCR+CD4-CD8- and gamma-delta TCR+CD4-CD8- cells.

Cultivation of CD4-CD8- double negative (DN) mouse thymocytes and splenocytes with recombinant interleukin 2 (IL2) in the absence of other stimulation results in the generation of DN-CD3/TCR+CD16+NK1.1+B220+ large granular lymphocytes (LGL). Purified DN alpha-beta TCR+ thymocytes and splenocytes are CD16+IL2R alpha-IL2R beta+NK1.1+B220-CD5high. These cells are unique in that they express both CD16 and T cell receptor (TCR) which are usually mutually exclusive. In addition, they express the natural killer (NK) marker, NK1.1. Cultivation of these cells with IL2 for several days results in the generation of DN alpha-beta TCR+CD16+NK1.1+B220+CD5- LGL, suggesting that DN alpha-beta TCR+ cells in thymus and spleen are the precursors of the DN LGL reported previously. DN gamma-delta TCR+CD16-NK1.1-B220-CD5high thymocytes and splenocytes also give rise to DN gamma-delta TCR+CD16+NK1.1+B220+CD5- LGL which, as shown previously with DN alpha-beta TCR+ LGL cells, are cytotoxic against NK-sensitive YAC-1 cells. Cytotoxic activity is also induced through either CD16 or the gamma-delta TCR. DN alpha-beta TCR+ and DN gamma-delta TCR+ LGL cells are thus similar in phenotype to TCR- NK cells. DN alpha-beta TCR+ thymocytes express low levels of the gamma subunit of the high affinity immunoglobulin E receptor (Fc epsilon RI gamma) molecule, an essential component of CD16 expression. Fc epsilon RI gamma expression is greatly enhanced after cultivation with IL2, resulting in a higher surface expression of CD16. In contrast to DN alpha-beta TCR+ thymocytes, DN gamma-delta TCR+ thymocytes do not express detectable CD16 or Fc epsilon RI gamma mRNA but expression of both is induced by cultivation with IL2, leading to the expression of CD16 on the surface. Whereas CD16 molecules on both DN alpha-beta TCR+ and DN gamma-delta TCR+ LGL are associated with only Fc epsilon RI gamma homodimers, the TCR on these cells are associated with an Fc epsilon RI gamma homodimer and/or CD3 zeta-Fc epsilon RI gamma heterodimers. These results demonstrate that the Fc epsilon RI gamma subunit is a component of the TCR in a fraction of T lineage cells.

T cell receptor-independent CD2 signal transduction in FcR+ cells.

CD2 subserves both adhesion and signal transduction functions in T cells, thymocytes, and natural killer (NK) cells. In mature T lymphocytes, CD2-mediated signaling function apparently requires surface expression of T cell receptors (TCRs). In contrast, in CD2+ CD3- NK cells and thymocytes, signal transduction through CD2 is TCR independent. To resolve this paradox and characterize TCR-independent triggering mechanisms, we transfected a human CD2 cDNA into a murine mast cell line, C1.MC/57 (Fc epsilon RI+, Fc gamma RII+, Fc gamma RIII+), which is known to produce interleukin 6 (IL-6) as well as release histamine in response to crosslinking of Fc epsilon RI. In the CD2 transfectant, a combination of anti-T11(2) + anti-T11(3) monoclonal antibodies (mAbs) induced a rise in intracellular free calcium [( Ca2+]i), IL-6 production, and histamine release. As expected, no activation was mediated by the same mAbs in C1.MC/57. F(ab)'s fragments of the activatory combination of anti-T11(2) + anti-T11(3) mAbs induced IL-6 in the CD2-transfected mast cells, demonstrating an Fc gamma receptor ectodomain-independent triggering mechanism. In addition, either intact anti-T11(2) or anti-T11(3) IgG alone, which failed to induce [Ca2+]i mobilization in the transfectant, was able to induce IL-6 production. A mAb directed against both Fc gamma RII (previously denoted as Fc gamma RIIb) and Fc gamma RIII (previously denoted as Fc gamma RIIa) inhibits this induction. These results indicate that: (a) Ca2+ mobilization is not essential for IL-6 production; and (b) crosslinking of CD2 and Fc gamma receptors via intact anti-CD2 IgG stimulates IL-6 production. Thus, CD2-mediated IL-6 production occurs by both Fc receptor ectodomain-independent as well as Fc receptor ectodomain-dependent mechanisms in these nonlymphoid cells. Northern blot analysis demonstrates that although the mast cells do not express CD3 zeta or CD3 eta mRNA, they express Fc epsilon RI gamma mRNA. The latter is a known component of Fc gamma RIII as well as Fc epsilon RI, has significant homology to CD3 zeta/eta, and is thought to have a signal transduction function. In these mast cells, CD2 signaling machinery does not require CD3 zeta/eta and may be linked to the Fc epsilon RI gamma subunit. We predict that this subunit or a related structure may confer a TCR-independent signal transduction pathway upon CD2 in CD3- NK cells, thymocytes, and certain B lymphocytes.

Interleukin 12-dependent interferon gamma production by CD8alpha+ lymphoid dendritic cells.

We investigated the role of antigen-presenting cells in early interferon (IFN)-gamma production in normal and recombinase activating gene 2-deficient (Rag-2(-/-)) mice in response to Listeria monocytogenes (LM) infection and interleukin (IL)-12 administration. Levels of serum IFN-gamma in Rag-2(-/-) mice were comparable to those of normal mice upon either LM infection or IL-12 injection. Depletion of natural killer (NK) cells by administration of anti-asialoGM1 antibodies had little effect on IFN-gamma levels in the sera of Rag-2(-/-) mice after LM infection or IL-12 injection. Incubation of splenocytes from NK cell-depleted Rag-2(-/-) mice with LM resulted in the production of IFN-gamma that was completely blocked by addition of anti-IL-12 antibodies. Both dendritic cells (DCs) and monocytes purified from splenocytes were capable of producing IFN-gamma when cultured in the presence of IL-12. Intracellular immunofluorescence analysis confirmed the IFN-gamma production from DCs. It was further shown that IFN-gamma was produced predominantly by CD8alpha+ lymphoid DCs rather than CD8alpha- myeloid DCs. Collectively, our data indicated that DCs are potent in producing IFN-gamma in response to IL-12 produced by bacterial infection and play an important role in innate immunity and subsequent T helper cell type 1 development in vivo.

T cell receptor complexes containing Fc epsilon RI gamma homodimers in lieu of CD3 zeta and CD3 eta components: a novel isoform expressed on large granular lymphocytes.

CD3 zeta and CD3 eta form disulfide-linked homo- or heterodimers important in targeting partially assembled Ti alpha-beta/CD3 gamma delta epsilon T cell receptor (TCR) complexes to the cell surface and transducing stimulatory signals after antigen recognition. Here we identify a new TCR isoform expressed on splenic CD2+, CD3/Ti alpha-beta+, CD4-, CD8-, CD16+, NK1.1+ mouse large granular lymphocytes (LGL), which are devoid of CD3 zeta and CD3 eta proteins. The TCRs of this subset contain homodimers of the gamma subunit of the high affinity receptor for IgE (Fc epsilon RI gamma) in lieu of CD3 zeta and/or CD3 eta proteins. The LGL display natural killer-like activity and are cytotoxic for B cell hybridomas producing anti-CD3 epsilon and anti-CD16 monoclonal antibodies, demonstrating the signaling capacity of both TCR and CD16 in this cell type. These findings provide evidence for an additional level of complexity of TCR signal transduction isoforms in naturally occurring T cell subsets.

A soluble multimeric recombinant CD2 protein identifies CD48 as a low affinity ligand for human CD2: divergence of CD2 ligands during the evolution of humans and mice.

To search for possible ligands of CD2 distinct from CD58 (lymphocyte function-associated antigen 3), we have produced a soluble pentameric CD2-immunoglobulin (Ig) fusion protein (spCD2) linking the 182-amino acid human CD2 extracellular segment with CH2-CH3-CH4 domains of human IgM heavy chain, thus enhancing the micromolar affinity of the CD2 monomer through multimeric interaction. Using quantitative immunofluorescence and standard stringency wash conditions, we observed that the binding of spCD2 to human B lymphoblastoid JY cells and red blood cells is virtually inhibited by anti-CD58 TS2/9 monoclonal antibody, even though these cells express levels of CD48 and CD59 comparable to CD58. Consistent with these results, spCD2 did not show any binding to Chinese hamster ovary (CHO) cells transfected with human CD48 or CD59. However, binding studies on CD48-, CD58-, or CD59-transfected CHO cells with spCD2 under low stringency wash conditions revealed that human CD48 is a low affinity ligand of human CD2 compared with CD58 (Kd approximately 10(-4) vs. approximately 10(-6) M, respectively). The findings are noteworthy given that in the murine system CD48 is the major ligand for CD2. No detectable binding was observed to CD59-transfected CHO cells despite a report suggesting that CD59 may bind to the human CD2 adhesion domain. Importantly, in cell-cell adhesion assays between CD2+ Jurkat T cells and CD48- or CD59-transfected CHO cells, there was no conjugate formation, whereas binding of Jurkat T cells to CD58-transfected CHO cells was readily detected. Collectively, our findings provide evidence for a conservation of the CD2-CD48 interaction in the human species that may be of limited, if any, functional significance. Given the importance of the CD2-CD48 interaction in the murine system and CD2-CD58 interaction in humans, it would appear that there has been a divergence of functional CD2 ligands during the evolution of humans and mice.

Characterization of thymus-derived lymphocytes expressing Ti alpha-beta CD3 gamma delta epsilon zeta-zeta, Ti alpha-beta CD3 gamma delta epsilon eta-eta or Ti alpha-beta CD3 gamma delta epsilon zeta-zeta/zeta-eta antigen receptor isoforms: analysis by gene transfection.

To characterize the function of the CD3 eta subunit of the T cell receptor (TCR), we have used cDNAs encoding CD3 zeta, CD3 eta, or both to reconstitute a variant of a cytochrome c-specific, I-Ek-restricted murine T cell hybridoma, termed MA5.8, which lacks CD3 zeta and CD3 eta proteins. We provide direct evidence that assembly and surface expression of TCRs can be mediated by either of these subunits separately or together. However, the level of TCR expression on zeta transfectants is up to one order of magnitude greater than that on eta transfectants, implying that CD3 eta is weakly associated with the pentameric Ti alpha-beta CD3 gamma delta epsilon complex and/or inefficient at salvaging the incomplete TCR from lysosomal degradation. As a component of the TCR, the CD3 eta subunit preferentially forms a heterodimer with CD3 zeta, but is also able to form a CD3 eta-eta homodimer. Crosslinking of Ti alpha-beta CD3 gamma delta epsilon zeta-zeta, Ti alpha-beta CD3 gamma delta epsilon eta-eta, or Ti alpha-beta CD3 gamma delta epsilon zeta-zeta/zeta-eta TCR isotypes with anti-CD3 epsilon monoclonal antibody or a cytochrome c peptide epitope on I-Ek antigen-presenting cells mediates signal transduction resulting in reversible cell-cycle arrest of transfected clones. Given the potential for diversity of signals generated by these functional TCR isotypes and the expression of the CD3 eta gene product in the thymus, CD3 eta is likely to play a role in selection and/or activation of thymocytes during development.

Xid-like immunodeficiency in mice with disruption of the p85alpha subunit of phosphoinositide 3-kinase.

Mice with a targeted gene disruption of p85alpha, a regulatory subunit of phosphoinositide 3-kinase, had impaired B cell development at the pro-B cell stage, reduced numbers of mature B cells and peritoneal CD5+ Ly-1 B cells, reduced B cell proliferative responses, and no T cell-independent antibody production. These phenotypes are nearly identical to those of Btk-/- or xid (X-linked immunodeficiency) mice. These results provide evidence that p85alpha is functionally linked to the Btk pathway in antigen receptor-mediated signal transduction and is pivotal in B cell development and functions.

Restoration of T cell development in RAG-2-deficient mice by functional TCR transgenes.

Introduction of TCR alpha transgene, TCR beta transgene, or both into RAG-2-/-mice differentially rescues T cell development. RAG-2-/- mice have small numbers of TCR-CD4-CD8-(double negative, DN) thymocytes that express CD3 gamma delta epsilon and zeta proteins intracellularly. Introduction of a TCR beta transgene, but not a TCR alpha transgene, into the RAG-2-/- background restored normal numbers of thymocytes. These cells were CD4+CD8+ (double positive, DP) and expressed small amounts of surface TCR beta chain dimers in association with CD3 gamma delta epsilon but not zeta. RAG-2-/- mice that expressed alpha and beta TCR transgenes developed both DP and single positive thymocytes. Thus, the TCR beta subunit, possibly in association with a novel CD3 complex, participates in the DN to the DP transition.

Autoantibodies to the heat-shock protein hsp90 in systemic lupus erythematosus.

Patients with systemic lupus erythematosus (SLE) develop multiple autoantibodies to self-antigens. Analysis of autoantibody systems in this and related autoimmune disorders can provide information of etiologic and pathogenetic significance. We report here a previously unrecognized autoantibody to the 90,000-D heat-shock protein, hsp90, a molecule thought to have important functions in the cellular response to stress, virus-induced transformation, steroid hormone receptor action, and cellular activation. Autoantibodies to hsp90 were exclusively of the IgG class, and were detected in approximately 50% of unselected patients with SLE and 2/6 patients with idiopathic polymyositis. Anti-hsp90 antibodies were not detected in sera from 10 normal subjects, 10 patients with rheumatoid arthritis, or 7 patients with scleroderma. The identity of this major intracytoplasmic antigen was established by its specific removal from nonionic detergent cell lysates following immunoabsorption with monospecific rabbit anti-hsp90, and by demonstration of increased synthesis following a 10-min 45 degrees C heat shock. These data define the frequent occurrence of a novel autoantibody to a major heat-shock protein in patients with SLE.

Use of autoantigen-knockout mice in developing an active autoimmune disease model for pemphigus.

The development of experimental models of active autoimmune diseases can be difficult due to tolerance of autoantigens, but knockout mice, which fail to acquire tolerance to the defective gene product, provide a useful tool for this purpose. Using knockout mice lacking desmoglein 3 (Dsg3), the target antigen of pemphigus vulgaris (PV), we have generated an active disease model for this autoantibody-mediated disease. Dsg3(-/-) mice, but not Dsg3(+/-) littermates, produced anti-Dsg3 IgG that binds native Dsg3, when immunized with recombinant mouse Dsg3. Splenocytes from the immunized Dsg3(-/-) mice were then adoptively transferred into Rag-2(-/-) immunodeficient mice expressing Dsg3. Anti-Dsg3 IgG was stably produced in the recipient mice for more than 6 months without further boosting. This IgG bound to Dsg3 in vivo and disrupted the cell-cell adhesion of keratinocytes. Consequently, the recipient mice developed erosions in their oral mucous membranes with typical histologic findings of PV. In addition, the recipient mice showed telogen hair loss, as found in Dsg3(-/-) mice. Collectively, the recipient mice developed the phenotype of PV due to the pathogenic anti-Dsg3 IgG. This model will be valuable for developing novel therapeutic strategies. Furthermore, our approach can be applied broadly for the development of various autoimmune disease models.

T-cell receptor isoforms and signal transduction.

Recent cDNA and genomic cloning have identified CD3 eta as an alternatively spliced product of the same gene locus that encodes CD3 zeta. Three distinct T-cell receptor isoforms have now been identified. A current view of the signal transduction function of these isoforms in thymocytes and T cells is discussed.

ERK and p38 MAPK, but not NF-kappaB, are critically involved in reactive oxygen species-mediated induction of IL-6 by angiotensin II in cardiac fibroblasts.

We recently reported that angiotensin II (Ang II) induced IL-6 mRNA expression in cardiac fibroblasts, which played an important role in Ang II-induced cardiac hypertrophy in paracrine fashion. The present study investigated the regulatory mechanism of Ang II-induced IL-6 gene expression, focusing especially on reactive oxygen species (ROS)-mediated signaling in cardiac fibroblasts. Ang II increased intracellular ROS in cardiac fibroblasts, and the increase was completely inhibited by the AT-1 blocker candesartan and the NADH/NADPH oxidase inhibitor diphenyleneiodonium (DPI). We first confirmed that antioxidant N-acetylcysteine, superoxide scavenger Tiron, and DPI suppressed Ang II-induced IL-6 expression. Because we observed that exogenous H(2)O(2) also increased IL-6 mRNA, the signaling pathways downstream of Ang II and exogenous H(2)O(2) were compared. Ang II, as well as exogenous H(2)O(2), activated ERK, p38 MAPK, and JNK, which were significantly inhibited by N-acetylcysteine and DPI. In contrast with exogenous H(2)O(2), however, Ang II did not influence phosphorylation and degradation of IkappaB-alpha/beta or nuclear translocation of p65, nor did it increase NF-kappaB promoter activity. PD98059 and SB203580 inhibited Ang II-induced IL-6 expression. Truncation and mutational analysis of the IL-6 gene promoter showed that CRE was an important cis-element in Ang II-induced IL-6 gene expression. NF-kappaB-binding site was important for the basal expression of IL-6, but was not activated by Ang II. Ang II phosphorylated CREB through the ERK and p38 MAPK pathway in a ROS-sensitive manner. Collectively, these data indicated that Ang II stimulated ROS production via the AT1 receptor and NADH/NADPH oxidase, and that these ROS mediated activation of MAPKs, which culminated in IL-6 gene expression through a CRE-dependent, but not NF-kappaB-dependent, pathway in cardiac fibroblasts.

A metal sulfide photocatalyst composed of ubiquitous elements for solar hydrogen production.

A visible-light-sensitive tin sulfide photocatalyst was designed based on a ubiquitous element strategy and density functional theory (DFT) calculations. Computational analysis suggested that tin monosulfide (SnS) would be more efficient than SnS2 as a photocathode for hydrogen production because of the low ionization potential and weak ionic character of SnS. To test this experimentally, nanoparticles of SnS were loaded onto a mesoporous electrode using a wet chemical method, and the bandgap of the synthesized SnS quantum dots was found to be tunable by adjusting the number of successive ionic layer adsorption and reaction (SILAR) cycles, which controls the magnitude of the quantum confinement effect. Efficient hydrogen production was achieved when the bandgap of SnS was wider than that of the bulk form.

Caulobacter crescentus flagellar filament has a right-handed helical form.

Caulobacter crescentus flagellar filaments were examined for their shape and handedness. Contour length, wavelength and height of the helical filaments were 1.34 +/- 0.14 micron, 1.08 +/- 0.05 micron and 0.27 +/- 0.04 micron, respectively. Together with the value of the filament diameter, 14 +/- 1.5 nm, the parameters of the curvature (alpha) and twist (phi) were calculated as 3.9(%) for alpha and 0.026 (rad) for phi, which are similar to those of the curly I filament of Salmonella typhimurium. Dark-field light microscopic analysis revealed that the C. crescentus wild-type filament possesses a right-handed helical form. Given the result that C. crescentus cells normally swim forward, in the opposite direction to a polar flagellum, it is likely that C. crescentus swims by rotation of a right-handed curly shaped flagellum in a clockwise sense, whereas S. typhimurium and Escherichia coli swim by rotation of left-handed normal type flagella in a counterclockwise sense.

On flagellar formation in Caulobacter crescentus: novel flagellin synthesis in stub-forming non-motile mutants of C. crescentus.

Defective flagellar organella, stubs, and flagellin synthesis by L-[35S]methionine labeling of two non-motile mutants of C. crescentus CB15 were studied. CB15 wild type synthesized two flagellin species, flagellin A of molecular weight 26,000 and flagellin B of molecular weight 28,000. A DNA phage phi0CbK-resistant non-motile mutant, CB15 pdr-816 [Fukuda et al. (1981) J. Bacteriol. 145, 559-572] did not synthesize flagellin B but synthesized large amounts of flagellin A and a novel flagellin of smaller molecular weight 22,000 (22K flagellin), and formed stubs composed of both flagellin A and 22K flagellin. The other mutant, CB15 fla-801 synthesized only 22K flagellin and formed very short stubs composed of 22K flagellin. Motile revertants were isolated from these mutants. Full revertants were isolated from CB15 fla-801 at a frequency of ca. 10(-7). However, only partial revertants swimming slowly were obtained from CB15 pdr-816 at a similar frequency. Such partial revertants synthesized only flagellin A and formed short flagella in spite of the fact that flagellin A was still synthesized at a higher level than in the wild type. Given the previous finding that flagellin B first polymerizes onto the hook followed by addition of flagellin A onto flagellin B filament [Koyasu et al. (1981) J. Mol. Biol. 153, 471-475], flagellin B seems very important in the formation of normal flagella in C. crescentus. A small amount of 22K flagellin was detected even in wild type cell. Neither conversion nor alteration was observed in flagellin A or B or 22K flagellin by short-pulse and pulse-chase experiments. It was thus suggested that 22K flagellin was not a degradation product formed from other flagellins but was of different origin from flagellins A and B. C. crescentus flagellins consist of several subspecies of different isoelectric points. 22K flagellin consisted of at least three subspecies, in two-dimensional gel, with more basic isoelectric points than other flagellins. Heterogeneity in isoelectric point of polymerized and nascent flagellins was similar, indicating that each subspecies of different isoelectric point can polymerize into filament, and that conversion between subspecies is not needed to form flagellar filament.

The penicillin-binding proteins of Caulobacter crescentus.

The proteins that specifically bind penicillin G were studied with Caulobacter crescentus. This organism possessed at least 5 penicillin-binding proteins (PBPs): PBP 1A (132 K), PBP 1Bs (98 K), PBP 2 (77 K), PBP 3 (64 K), and PBP 4 (50 K). As expected from the unique morphology, the C. crescentus PBPs were different from those of other sources in molecular weight and localization: C. crescentus did not possess PBPs of low molecular weight. PBP 4 was found only in the outer membrane, while the other PBPs were present mostly in the inner membrane.

Alteration in penicillin-binding patterns during cell cycle of Caulobacter crescentus.

Alteration in penicillin-binding patterns during asymmetrical cell division in the cell cycle of Caulobacter crescentus was examined. Most penicillin-binding proteins (PBPs) were turned over slowly, and their penicillin-binding patterns did not alter appreciably during the cell cycle. One of the PBPs, PBP S2 in the soluble fraction, however, was turned over quickly and was detected only in swarmer cells, suggesting that PBP S2 is synthesized in swarmer cells and turned over and/or inactivated in the following stalked cell cycle.

Role of antigen-presenting cells in innate immune system.

Activation of antigen-presenting cells (APC) and natural killer (NK) cells initiates the production of various proinflammatory cytokines including interleukin 12 (IL-12), interferon gamma (IFN-gamma) and nitric oxide (NO), which are important in the innate immune response for controlling infection by intracellular pathogens. In this review, we focus on these cytokines produced by APC and summarize the current understanding of how APC functions are regulated by cytokines in innate immunity.

[Regulatory mechanisms of NK cell functions].

Natural Killer (NK) cells discriminate self from non-self in a manner distinct from T cells. NK cells exhibit cytotoxicity against "missing-self" by killing any cells in principle except normal self-cells. Cells expressing low levels of self MHC class I molecules such as tumor cells and foreign cells are killed, whereas normal self cells are neglected by NK cells. Although identities of activation receptors triggering NK activity are still unclear, recent studies have revealed molecules inhibiting cytotoxicity against normal self cells. In this review, we summarize current understanding of molecular basis for missing-self hypothesis and control mechanisms of NK cell activation.