Signaling through human killer cell activating receptors triggers tyrosine phosphorylation of an associated protein complex

The role of transient receptor potential channels in metastasis

Metastasis is the hallmark of failed tumor treatment and is typically associated with death due to cancer. Transient receptor potential (TRP) channels affect changes in intracellular calcium concentrations and participate at every stage of metastasis. Further, they increase the migratory ability of tumor cells, promote angiogenesis, regulate immune function, and promote the growth of tumor cells through changes in gene expression and function. In this review, we explore the potential mechanisms of action of TRP channels, summarize their role in tumor metastasis, compile inhibitors of TRP channels relevant in tumors, and discuss current challenges in research on TRP channels involved in tumor metastasis.

Engagement of TRAIL triggers degranulation and IFNγ production in human natural killer cells

NK cells utilize a large array of receptors to screen their surroundings for aberrant or virus‐infected cells. Given the vast diversity of receptors expressed on NK cells we seek to identify receptors involved in the recognition of HIV‐1‐infected cells. By combining an unbiased large‐scale screening approach with a functional assay, we identify TRAIL to be associated with NK cell degranulation against HIV‐1‐infected target cells. Further investigating the underlying mechanisms, we demonstrate that TRAIL is able to elicit multiple effector functions in human NK cells independent of receptor‐mediated induction of apoptosis. Direct engagement of TRAIL not only results in degranulation but also IFNγ production. Moreover, TRAIL‐mediated NK cell activation is not limited to its cognate death receptors but also decoy receptor I, adding a new perspective to the perceived regulatory role of decoy receptors in TRAIL‐mediated cytotoxicity. Based on these findings, we propose that TRAIL not only contributes to the anti‐HIV‐1 activity of NK cells but also possesses a multifunctional role beyond receptor‐mediated induction of apoptosis, acting as a regulator for the induction of different effector functions.

The Primary Microglial Leukodystrophies: A Review

Primary microglial leukodystrophy or leukoencephalopathy are disorders in which a genetic defect linked to microglia causes cerebral white matter damage. Pigmented orthochromatic leukodystrophy, adult-onset orthochromatic leukodystrophy associated with pigmented macrophages, hereditary diffuse leukoencephalopathy with (axonal) spheroids, and adult-onset leukoencephalopathy with axonal spheroids and pigmented glia (ALSP) are different terms apparently used to designate the same disease. However, ALSP linked to dominantly inherited mutations in CSF1R (colony stimulating factor receptor 1) cause CSF-1R-related leukoencephalopathy (CRP). Yet, recessive ALSP with ovarian failure linked to AARS2 (alanyl-transfer (t)RNA synthase 2) mutations (LKENP) is a mitochondrial disease and not a primary microglial leukoencephalopathy. Polycystic membranous lipomembranous osteodysplasia with sclerosing leukoencephalopathy (PLOSL; Nasu–Hakola disease: NHD) is a systemic disease affecting bones, cerebral white matter, selected grey nuclei, and adipose tissue The disease is caused by mutations of one of the two genes TYROBP or TREM2, identified as PLOSL1 and PLOSL2, respectively. TYROBP associates with receptors expressed in NK cells, B and T lymphocytes, dendritic cells, monocytes, macrophages, and microglia. TREM2 encodes the protein TREM2 (triggering receptor expressed on myeloid cells 2), which forms a receptor signalling complex with TYROBP in macrophages and dendritic cells. Rather than pure microglial leukoencephalopathy, NHD can be considered a multisystemic “immunological” disease.

Rhesus Macaque Activating Killer Immunoglobulin-Like Receptors Associate With Fc Receptor Gamma (FCER1G) and Not With DAP12 Adaptor Proteins Resulting in Stabilized Expression and Enabling Signal Transduction

Activating killer cell immunoglobulin-like receptors (KIR) in macaques are thought to be derived by genetic recombination of the region encoding the transmembrane and intracellular part of KIR2DL4 and a KIR3D gene. As a result, all macaque activating KIR possess a positively charged arginine residue in the transmembrane region. As human KIR2DL4 associates with the FCER1G (also called Fc receptor-gamma, FcRγ) adaptor, we hypothesized that in contrast to human and great ape the activating KIRs of macaques associate with FcRγ instead of DAP12. By applying co-immunoprecipitation of transfected as well as primary cells, we demonstrate that rhesus macaque KIR3DS05 indeed associates with FcRγ and not with DAP12. This association with FcRγ results in increased and substantially stabilized surface expression of KIR3DS05. In addition, we demonstrate that binding of specific ligands of KIR3DS05, Mamu-A1*001 and A1*011, resulted in signal transduction in the presence of FcRγ in contrast to DAP12.

TRP Channels as Interior Designers: Remodeling the Endolysosomal Compartment in Natural Killer Cells

Cytotoxic lymphocytes, including natural killer (NK) cells and T cells are distinguished by their ability to eliminate target cells through release of secretory lysosomes. Conventional lysosomes and secretory lysosomes are part of the pleomorphic endolysosomal system and characterized by its highly dynamic nature. Several calcium-permeable TRP calcium channels play an essential role in endolysosomal calcium signaling to ensure proper function of these organelles. In NK cells, the expression of self MHC-specific inhibitory receptors dynamically tunes their secretory potential in a non-transcriptional, calcium-dependent manner. New insights suggest that TRPML1-mediated lysosomal calcium fluxes are tightly interconnected to NK cell functionality through modulation of granzyme B and perforin content of the secretory lysosome. Lysosomal TRP channels show a subset-specific expression pattern during NK differentiation, which is paralleled with gradually increased loading of effector molecules in secretory lysosomes. Methodological advances, including organellar patch-clamping, specific pharmacological modulators, and genetically-encoded calcium indicators open up new possibilities to investigate how TRP channels influence communication between intracellular organelles in immune cells. This review discusses our current understanding of lysosome biogenesis in NK cells with an emphasis on the TRP mucolipin family and the implications for NK cell functionality and cancer immunotherapy.

Human Natural Killer Receptors, Co-Receptors, and Their Ligands

In the last 20 years, the study of human natural killer (NK) cells has moved from the first molecular characterizations of very few receptor molecules to the identification of a plethora of receptors displaying surprisingly divergent functions. We have contributed to the description of inhibitory receptors and their signaling pathways, important in fine regulation in many cell types, but unknown until their discovery in the NK cells. Inhibitory function is central to regulating NK-mediated cytolysis, with different molecular structures evolving during speciation to assure its persistence. More recently, it has become possible to characterize the NK triggering receptors mediating natural cytotoxicity, unveiling the existence of a network of cellular interactions between effectors of both natural and adaptive immunity. This unit reviews the contemporary history of molecular studies of receptors and ligands involved in NK cell function, characterizing the ligands of the triggering receptor and the mechanisms for finely regulating their expression in pathogen-infected or tumor cells. © 2018 by John Wiley & Sons, Inc.

Prognostic values of the mRNA expression of natural killer receptor ligands and their association with clinicopathological features in breast cancer patients

BACKGROUND

Natural killer (NK) cells are lymphocytes of the innate immune system that have potent cytotoxic activity against tumor cells. NK cell recognition and activity towards cancer cells are regulated by an integrated interplay between numerous inhibitory and activating receptors acting in concert to eliminate tumor cells expressing cognate ligands. Despite strong evidence supporting the role of NK cells in breast cancer (BC) control, BC still develops and progresses to form large tumors and metastases. A major mechanism of BC escape from NK immunity is the alteration of the expression of NK receptor ligands. The aim of this study was to determine whether NK receptor ligands' mRNA expression might influence prognosis in BC patients and whether these effects differ by molecular subtypes and clinicopathological features.

METHODS

We used the KM plotter platform to analyze the correlation between mRNA expression of 32 NK receptor ligands and relapse-free survival (RFS) and overall survival (OS) in 3951 and 1402 BC patients, respectively. The association with tumor subtypes and clinicopathological features was determined. BC samples were split into high and low expression groups according to the best cutoff value and the two patient cohorts were compared by Kaplan-Meier survival plots. The hazard ratios with 95% confidence intervals and log rank P values were calculated and FDR-adjusted for multiple testing correction. The data was considered to be statistically significant when FDR-adjusted P value < 0.05.

RESULTS

High mRNA expression of around 80% of ligands for NK activating and inhibitory receptors associated with better RFS, which correlated with longer OS for only about half of the NK-activating ligands but for most NK-inhibitory ligands. Also, five NK-activating ligands correlated with worse prognosis. These prognostic values were differentially associated with the BC clinical criteria. In addition, the favorable prognostic influence of NK-activating ligands' upregulation, as a whole, was mainly significantly associated with HER2-positive and basal-like subtypes, lymph node positive phenotype, and high-grade tumors.

CONCLUSIONS

NK receptor ligands appear to play an important role in defining BC patient prognosis. Identification of a group of patients with worse prognosis expressing high levels of NK-activating ligands and low levels of NK-inhibitory ligands makes them ideal potential candidates for NK-based immunotherapy to eliminate residual tumor cells, prevent relapse and improve patient survival.

Inhibitory and Coactivating Receptors Recognising the Same Ligand: Immune Homeostasis Exploited by Pathogens and Tumours

Coactivating and inhibitory receptors that share at least one ligand interact with a wide variety of ligands, indicating their importance in a range of situations. Here, we discuss principles of mainly human paired receptor function and ligand recognition, and possible therapeutic implications of targeting these receptors in cancer, autoimmune diseases, and allergy. We summarise and emphasise the idea that these receptors, which have evolved in part in response to pathogen pressure, fine-tune the immune response, preserve homeostasis, and that pathogens and tumours use the dominance of the inhibitory receptors over the coactivating receptors to avoid immune elimination. Finally, we discuss the options of using paired receptors and their ligand for immune cell education and therapy.

Modulation of Human Leukocyte Antigen-C by Human Cytomegalovirus Stimulates KIR2DS1 Recognition by Natural Killer Cells

The interaction of inhibitory killer cell Ig-like receptors (KIRs) with human leukocyte antigen (HLA) class I molecules has been characterized in detail. By contrast, activating members of the KIR family, although closely related to inhibitory KIRs, appear to interact weakly, if at all, with HLA class I. KIR2DS1 is the best studied activating KIR and it interacts with C2 group HLA-C (C2-HLA-C) in some assays, but not as strongly as KIR2DL1. We used a mouse 2B4 cell reporter system, which carries NFAT-green fluorescent protein with KIR2DS1 and a modified DAP12 adaptor protein. KIR2DS1 reporter cells were not activated upon coculture with 721.221 cells transfected with different HLA-C molecules, or with interferon-γ stimulated primary dermal fibroblasts. However, KIR2DS1 reporter cells and KIR2DS1⁺ primary natural killer (NK) cells were activated by C2-HLA-C homozygous human fetal foreskin fibroblasts (HFFFs) but only after infection with specific clones of a clinical strain of human cytomegalovirus (HCMV). Active viral gene expression was required for activation of both cell types. Primary NKG2A^-KIR2DS1⁺ NK cell subsets degranulated after coculture with HCMV-infected HFFFs. The W6/32 antibody to HLA class I blocked the KIR2DS1 reporter cell interaction with its ligand on HCMV-infected HFFFs but did not block interaction with KIR2DL1. This implies a differential recognition of HLA-C by KIR2DL1 and KIR2DS1. The data suggest that modulation of HLA-C by HCMV is required for a potent KIR2DS1-mediated NK cell activation.

Typing of killer-cell immunoglobulin-like receptors and their cognate human leukocyte antigen class I ligands predicts survival of Chinese Han patients with metastatic non-small-cell lung cancer

Non-small-cell lung cancer (NSCLC) may establish an immunosuppressive tumor microenvironment that is conducive to tumor growth. Natural killer (NK) cells play a pivotal role in immunological surveillance. Activation of NK cells partially depends on the interactions between killer-cell immunoglobulin-like receptors (KIRs) and human leukocyte antigen (HLA) class I ligands. We herein investigated the association of KIRs and HLA ligands with survival in metastatic NSCLC (mNSCLC) patients treated with chemotherapy in a Chinese Han population. Polymerase chain reaction with sequence-specific primers was used to type 15 KIRs at the DNA and mRNA level and 6 HLA ligands in 70 mNSCLC patients. Survival curves were estimated using the Kaplan-Meier method and compared with the log-rank test. Cox proportional hazard regression model was applied for multivariate survival analysis, with the stepwise selection, to determine independent predictors of survival. It was observed that patients with KIR2DS4del gene expression at the mRNA level or HLA-Bw4T80 exhibited poor overall survival (OS). The multivariate analysis revealed that HLA-Bw4T80 and KIR2DS4del expression were independent predictors of OS. This observation indicated that the KIR/HLA ligand is a promising predictor of survival in mNSCLC and may also provide a strategy for treatment stratification and patient management.

The antileukemic potential of natural killer cells

The antileukemic potential of natural killer (NK) cells has over the years raised considerable interest and new immune-based treatment protocols characterized by the infusion of freshly isolated or ex vivo activated and expanded effectors have been designed. Several aspects still need to be addressed, including the optimal timing of NK infusion during the course of the disease, the best preparative regimen, the origin of NK cells and the possible need of ex vivo NK cell manipulation before the infusion. The aims of this review are to discuss the experimental and clinical data available on the role played by NK cells for leukemia patients and to revise the different good manufacturing practice protocols for ex vivo manipulation of these effector cells.

The Myeloid LSECtin Is a DAP12-Coupled Receptor That Is Crucial for Inflammatory Response Induced by Ebola Virus Glycoprotein

Fatal Ebola virus infection is characterized by a systemic inflammatory response similar to septic shock. Ebola glycoprotein (GP) is involved in this process through activating dendritic cells (DCs) and macrophages. However, the mechanism is unclear. Here, we showed that LSECtin (also known as CLEC4G) plays an important role in GP-mediated inflammatory responses in human DCs. Anti-LSECtin mAb engagement induced TNF-α and IL-6 production in DCs, whereas silencing of LSECtin abrogated this effect. Intriguingly, as a pathogen-derived ligand, Ebola GP could trigger TNF-α and IL-6 release by DCs through LSECtin. Mechanistic investigations revealed that LSECtin initiated signaling via association with a 12-kDa DNAX-activating protein (DAP12) and induced Syk activation. Mutation of key tyrosines in the DAP12 immunoreceptor tyrosine-based activation motif abrogated LSECtin-mediated signaling. Furthermore, Syk inhibitors significantly reduced the GP-triggered cytokine production in DCs. Therefore, our results demonstrate that LSECtin is required for the GP-induced inflammatory response, providing new insights into the EBOV-mediated inflammatory response.

Effector Functions of Natural Killer Cell Subsets in the Control of Hematological Malignancies

Treatment of hematological malignant disorders has been improved over the last years, but high relapse rate mainly attributable to the presence of minimal residual disease still persists. Therefore, it is of great interest to explore novel therapeutic strategies to obtain long-term remission. Immune effector cells, and especially natural killer (NK) cells, play a crucial role in the control of hematological malignancies. In this regard, the efficiency of allogeneic stem cell transplantation clearly depends on the immune-mediated graft versus leukemia effect without the risk of inducing graft versus host disease. Alloreactive donor NK cells generated following hematopoietic stem cell transplantation ameliorate the outcome of leukemia patients; in addition, in vivo transfer of in vitro expanded NK cells represents a crucial tool for leukemia treatment. To improve NK cell effector functions against resistant leukemia cells, novel immunotherapeutic strategies are oriented to the identification, isolation, expansion, and administration of particular NK cell subsets endowed with multifunctional anti-tumor potential and tropism toward tumor sites. Moreover, the relationship between the emergence and persistence of distinct NK cell subsets during post-graft reconstitution and the maintenance of a remission state is still rather unclear.

Revving up Natural Killer Cells and Cytokine-Induced Killer Cells Against Hematological Malignancies

Natural killer (NK) cells belong to innate immunity and exhibit cytolytic activity against infectious pathogens and tumor cells. NK-cell function is finely tuned by receptors that transduce inhibitory or activating signals, such as killer immunoglobulin-like receptors, NK Group 2 member D (NKG2D), NKG2A/CD94, NKp46, and others, and recognize both foreign and self-antigens expressed by NK-susceptible targets. Recent insights into NK-cell developmental intermediates have translated into a more accurate definition of culture conditions for the in vitro generation and propagation of human NK cells. In this respect, interleukin (IL)-15 and IL-21 are instrumental in driving NK-cell differentiation and maturation, and hold great promise for the design of optimal NK-cell culture protocols. Cytokine-induced killer (CIK) cells possess phenotypic and functional hallmarks of both T cells and NK cells. Similar to T cells, they express CD3 and are expandable in culture, while not requiring functional priming for in vivo activity, like NK cells. CIK cells may offer some advantages over other cell therapy products, including ease of in vitro propagation and no need for exogenous administration of IL-2 for in vivo priming. NK cells and CIK cells can be expanded using a variety of clinical-grade approaches, before their infusion into patients with cancer. Herein, we discuss GMP-compliant strategies to isolate and expand human NK and CIK cells for immunotherapy purposes, focusing on clinical trials of adoptive transfer to patients with hematological malignancies.

Improving the outcome of leukemia by natural killer cell-based immunotherapeutic strategies

Blurring the boundary between innate and adaptive immune system, natural killer (NK) cells are widely recognized as potent anti-leukemia mediators. Alloreactive donor NK cells have been shown to improve the outcome of allogeneic stem-cell transplantation for leukemia. In addition, in vivo transfer of NK cells may soon reveal an important therapeutic tool for leukemia, if tolerance to NK-mediated anti-leukemia effects is overcome. This will require, at a minimum, the ex vivo generation of a clinically safe NK cell product containing adequate numbers of NK cells with robust anti-leukemia potential. Ideally, ex vivo generated NK cells should also have similar anti-leukemia potential in different patients, and be easy to obtain for convenient clinical scale-up. Moreover, optimal clinical protocols for NK therapy in leukemia and other cancers are still lacking. These and other issues are being currently addressed by multiple research groups. This review will first describe current laboratory NK cell expansion and differentiation techniques by separately addressing different NK cell sources. Subsequently, it will address the mechanisms known to be responsible for NK cell alloreactivity, as well as their clinical impact in the hematopoietic stem cells transplantation setting. Finally, it will briefly provide insight on past NK-based clinical trials.

A physical interaction between the adaptor proteins DOK3 and DAP12 is required to inhibit lipopolysaccharide signaling in macrophages

DNAX-activating protein of 12 kD (DAP12) is an immunoreceptor tyrosine-based activation motif (ITAM)-containing adaptor protein found in myeloid cells and natural killer cells, and it couples to various receptors that mediate either cellular activation or inhibition. DAP12 inhibits Toll-like receptor (TLR) signaling, such as that of TLR4 in response to its ligand lipopolysaccharide (LPS), as well as cytokine responses by coupling to TREM2 (triggering receptor expressed on myeloid cells 2) at the plasma membrane. Understanding the mechanisms that inhibit inflammatory responses in macrophages is important for the development of therapies to treat inflammatory diseases. We show that inhibition of LPS responses by DAP12 is mediated by the adaptor protein DOK3 (downstream of kinase 3). DOK3 physically associated with the ITAM of DAP12 through its phosphotyrosine-binding domain. In response to LPS, DOK3 was phosphorylated in a DAP12- and Src-dependent manner, which led to translocation of phosphorylated DOK3 to the plasma membrane. DOK3-deficient cells exhibited increased production of proinflammatory cytokines and activation of extracellular signal-regulated kinase (ERK). Compared to wild-type mice, DOK3-deficient mice had increased susceptibility to challenge with a sublethal dose of LPS and produced increased serum concentrations of the inflammatory cytokine tumor necrosis factor-α (TNF-α). Together, these data suggest the mechanism by which DAP12 and TREM2 inhibit LPS signaling in macrophages to prevent inflammation.

Theoretical elucidation of the origin for assembly of the DAP12 dimer with only one NKG2C in the lipid membrane

In this work, we have investigated in details the origin of the assembly of the DAP12 dimer with only one NKG2C in the activating immunoreceptor complex from thew two aspects of electronic properties and dynamic structures by performing density functional theory (DFT) calculations and molecular dynamics (MD) simulations. In the DFT calculations, we studied the aggregation ability of the NKG2C(TM) with the DAP12(TM) dimer and the DAP12(TM)-DAP12(TM)-NKG2C(TM) complex by analyzing the electrostatic potentials and frontier molecular orbitals (FMOs), and in the MD simulations we mainly investigated the dynamic structures of the DAP12(TM)-DAP12(TM)-NKG2C(TM) complex and its mutants, as well as the tetramer complex consisting of two DAP12(TM) and two NKG2C(TM) helixes without any restriction. Through the studies of the electrostatic potential, the FMOs, and the dynamic structures, we have provided reasonable explanations to some extent for the experimental observation that only one NKG2C can associate with the DAP12 homodimer. The present theoretical results are expected to give valuable information for further studying the assembly between receptors and signaling subunits.

[Natural killer cells and cancer. Regulation by the killer cell Ig-like receptors (KIR)]

自然杀伤（natural killer, NK）细胞是先天性免疫效应细胞，约占人外周血淋巴细胞总数的10%-15%，主要参与免疫监视，以消除转化细胞和病毒感染细胞。NK细胞最初被界定是由于它们具有自发消除少数主要组织相容性复合物Ⅰ类（major histocompatibility class Ⅰ, MHC-Ⅰ）自身分子表达缺乏细胞的能力，即常说的“丢失自我”识别能力。NK细胞表面表达的MHC-Ⅰ特异性抑制性受体，可使NK细胞对表达MHC-Ⅰ的正常细胞耐受，此为丢失自我识别能力的分子基础。由于缺乏抑制性受体的配体，表面MHC-Ⅰ表达下调的肿瘤细胞和病毒感染细胞易受NK细胞攻击。杀伤细胞免疫球蛋白样受体（KIR; CD158）组成MHC-Ⅰ结合受体家族，对调节人NK细胞和部分T细胞的活化阈值起重要作用。KIR多样性使NK细胞具有多种功能，在此我们将综述多个水平上的KIR多样性，并诠释KIR多样性是如何影响各种疾病（包括癌症）的易感性的。我们将进一步阐述通过针对KIR进行癌症治疗的策略：利用KIR/MHC-Ⅰ配体的错配以强化造血干细胞移植的效果，以及通过阻滞KIR以增强对肿瘤细胞的杀伤力。

Genomic organisation analysis of novel immunoglobulin-like transcripts in Atlantic salmon (Salmo salar) reveals a tightly clustered and multigene family

Background

Several novel immunoglobulin-like transcripts (NILTs) which have previously been identified in the salmonid species rainbow trout (Oncorhynchus mykiss) contain either one or two extracellular Ig domains of the V-type. NILTs also possess either an immunoreceptor tyrosine-based activating motif (ITAM) or immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in the cytoplasmic region resulting in different signalling abilities. Here we report for the first time the genomic organisation and structure of the multigene family of NILTs in Atlantic salmon (Salmo salar) using a BAC sequencing approach.

Results

We have identified six novel Atlantic salmon NILT genes (Ssa-NILT1-6), two pseudogenes (Ssa-NILTp1 and Ssa-NILTp2) and seven genes encoding putative transposable elements in one BAC covering more than 200 kbp. Ssa-NILT1, 2, 4, 5 and 6 contain one Ig domain, all having a CX₃C motif, whereas Ssa-NILT3 contains two Ig domains, having a CX₆C motif in Ig1 and a CX₇C motif in Ig2. Atlantic salmon NILTs possess several ITIMs in the cytoplasmic region and the ITIM-bearing exons are in phase 0. A comparison of identity between the amino acid sequences of the CX₃C Ig domains from NILTs varies from 77% to 96%. Ssa-NILT1, 2, 3 and 4 were all confirmed to be expressed either by their presence in EST databases (Ssa-NILT1) or RT-PCR (Ssa-NILT2, 3, and 4) using cDNA as template. A survey of the repertoire of putative NILT genes in a single individual revealed three novel genes (Ssa-NILT7-9) represented by the Ig domain, which together with Ig domains from Ssa-NILT1-6 could be divided into different groups based on specific motifs.

Conclusions

This report reveals a tightly clustered, multigene NILT family in Atlantic salmon. By screening a highly redundant Atlantic salmon BAC library we have identified and characterised the genomic organisation of six genes encoding NILT receptors. The genes show similar characteristics to NILTs previously identified in rainbow trout, having highly conserved cysteines in the Ig domain and several inhibitory signalling motifs in the cytoplasmic region. In a single individual three unique NILT Ig domain sequences were discovered at the genomic DNA level, which were divided into two different groups based on a four residue motif after the third cysteine. Our results from the BAC screening and analysis on the repertoire of NILT genes in a single individual indicates that many genes of this expanding Ig containing NILT family are still to be discovered in fish.

The impact of KIR2DS4 alleles and the expression of KIR in the development of acute GVHD after unrelated allogeneic hematopoietic SCT

The role of killer Ig-like receptors (KIR) in SCT was analyzed. A total of 75 Chinese patients were transplanted with T-depleted hematopoietic stem cells from unrelated donors. Among the 75 donor-recipient pairs, 60 were HLA 10/10 matched and 15 had some mismatches at HLA-C. Transplants from KIR haplotype B/x group donors showed significantly higher overall survival rates compared with those from KIR haplotype A/A donors (relative risk (RR) 3.1 (95% confidence interval (CI) 1.1-8.6), P=0.007). In the haplotype A/A group, a higher risk of acute GVHD (aGVHD) (RR 9.0 (95% CI 1.2-66.9), P=0.01), especially grade III-IV aGVHD (P=0.006), was observed when the donor was homozygous for the full-length expressed KIR2DS4*00101 allele. Real-time PCR showed that a high expression of inhibitory KIR (2DL1 and 3DL1) in the early stages (<90 days) after transplantation correlated with the development of aGVHD (z=2.558, P=0.011). Our findings indicated a significant association of full-length KIR2DS4 or KIR2DL1/3DL1 expression with the occurrence of aGVHD. In aggregate these results suggested that combining KIR and HLA genotyping could help in the selection of transplant donors and improve the outcome of transplantation. Dynamic detection of KIR2DL1/3DL1 expression would be beneficial for prediction of aGVHD after transplantation.

Natural killer cells and cancer: regulation by the killer cell Ig-like receptors (KIR)

Natural killer (NK) cells are innate immune effector cells that make up approximately 10-15% of the peripheral blood lymphocytes in humans and are primarily involved in immunosurveillance to eliminate transformed and virally-infected cells. They were originally defined by their ability to spontaneously eliminate rare cells lacking expression of class I major histocompatibility complex (MHC-I) self molecules, which is commonly referred to as "missing self" recognition. The molecular basis for missing self recognition emerges from the expression of MHC-I-specific inhibitory receptors on the NK cell surface that tolerize NK cells toward normal MHC-I-expressing cells. By lacking inhibitory receptor ligands, tumor cells or virus-infected cells that have down-modulated surface MHC-I expression become susceptible to attack by NK cells. Killer cell Ig-like receptors (KIR; CD158) constitute a family of MHC-I binding receptors that plays a major role in regulating the activation thresholds of NK cells and some T cells in humans. Here, we review the multiple levels of KIR diversity that contribute to the generation of a highly varied NK cell repertoire and explain how this diversity can influence susceptibility to a variety of diseases, including cancer. We further describe strategies by which KIR can be manipulated therapeutically to treat cancer, through the exploitation of KIR/MHC-I ligand mismatch to potentiate hematopoietic stem cell transplantation and the use of KIR blockade to enhance tumor cell killing.

KIR2DS4 is a product of gene conversion with KIR3DL2 that introduced specificity for HLA-A*11 while diminishing avidity for HLA-C

Human killer cell immunoglobulin-like receptors (KIRs) are distinguished by expansion of activating KIR2DS, whose ligands and functions remain poorly understood. The oldest, most prevalent KIR2DS is KIR2DS4, which is represented by a variable balance between “full-length” and “deleted” forms. We find that full-length 2DS4 is a human histocompatibility leukocyte antigen (HLA) class I receptor that binds specifically to subsets of C1⁺ and C2⁺ HLA-C and to HLA-A*11, whereas deleted 2DS4 is nonfunctional. Activation of 2DS4⁺ NKL cells was achieved with A*1102 as ligand, which differs from A*1101 by unique substitution of lysine 19 for glutamate, but not with A*1101 or HLA-C. Distinguishing KIR2DS4 from other KIR2DS is the proline–valine motif at positions 71–72, which is shared with KIR3DL2 and was introduced by gene conversion before separation of the human and chimpanzee lineages. Site-directed swap mutagenesis shows that these two residues are largely responsible for the unique HLA class I specificity of KIR2DS4. Determination of the crystallographic structure of KIR2DS4 shows two major differences from KIR2DL: displacement of contact loop L2 and altered bonding potential because of the substitutions at positions 71 and 72. Correlation between the worldwide distributions of functional KIR2DS4 and HLA-A*11 points to the physiological importance of their mutual interaction.

In contrast to other stimulatory natural killer cell immunoglobulin-like receptor loci, several KIR2DS5 alleles predominate in African Americans

The five two-domain stimulatory KIR genes carried by 100 random African Americans were characterized by DNA sequencing of genomic DNA covering the majority of coding exons. The frequency of individual loci was similar to that found in European Americans, with the exception of a reduced frequency for KIR2DS1 in African Americans. New alleles were identified at the KIR2DS1 (*008), KIR2DS2 (*006), KIR2DS3 (*00104, *00105, *00106, *004), KIR2DS4 (*00103, *00104, *009, *011, *012, *013), and KIR2DS5 (*006, *007, *00801, *00802, *009) loci. The distribution of alleles at each locus was similar to that found in a European American population except for KIR2DS5. KIR2DS5 exhibits a single allele in European Americans; the same allele is found at a reduced frequency (41% of gene-positive individuals), accompanied by KIR2DS5*006 (18%), KIR2DS5*007 (26%), and six other alleles (25%), in African Americans.

Rainbow trout (Oncorhynchus mykiss) possess multiple novel immunoglobulin-like transcripts containing either an ITAM or ITIMs

The recognition of pathogens by the innate immune system relies on a wide range of inhibitory and activating receptors. Some of these non-rearranging receptors belong to the immunoglobulin superfamily (IgSF) and in teleost fish the novel immune-type receptor (NITR) and the novel immunoglobulin-like transcript (NILT) have been reported. Here we describe the identification and characterisation of three new NILTs from rainbow trout (Oncorhynchus mykiss), with one NILT alternatively spliced into a long isoform containing two Ig domains and a short isoform containing one Ig domain. The cytoplasmic regions contain either immunoreceptor tyrosine-based inhibitory motifs (ITIMs) or an immunoreceptor tyrosine-based activating motif (ITAM) for downstream signalling. Alignment of the various NILT Ig domains revealed a high similarity, especially between Ig domains from NILTs found in this study. Furthermore, a phylogenetic tree showed that NILTs are more closely related to the triggering receptor expressed on myeloid (TREM) cells and NKp44 than to NITRs. The expression of NILTs was studied in six different tissues and two different cell lines, with expression apparent in immunologically important tissues. Expression of NILTs was also shown to be an early event in development, with both eyed eggs and embryos expressing all four genes. The results obtained in this study and future experiments will contribute to our knowledge of the immune system in fish and provide useful information for the control of inflammatory processes in rainbow trout.

Human natural killer receptors, co-receptors, and their ligands

In the last 20 years, the study of human natural killer (NK) cells has moved from the first molecular characterizations of very few receptor molecules to the identification of a plethora of receptors displaying surprisingly divergent functions. Our laboratory has contributed to the description of inhibitory receptors and their signaling pathways, important in fine regulation in many cell types, but unknown until their discovery in the NK cells. Inhibitory function is central to regulating NK-mediated cytolysis, with different molecular structures evolving during speciation to assure its persistence. Only in the last ten years has it become possible to characterize the NK triggering receptors mediating natural cytotoxicity, leading to an appreciation of the existence of a cellular interaction network between effectors of both natural and adaptive immunity. This report reviews the contemporary history of molecular studies of receptors and ligands involved in NK cell function, characterizing the ligands of the triggering receptor and the mechanisms for finely regulating their expression in pathogen-infected or tumor cells.

Dramatically reduced surface expression of NK cell receptor KIR2DS3 is attributed to multiple residues throughout the molecule

Using flow cytometry, fluorescent microscopy and examination of receptor glycosylation status, we demonstrate that an entire killer cell immunoglobulin-like receptor (KIR) locus (KIR2DS3)--assumed earlier to be surface expressed--appears to have little appreciable surface expression in transfected cells. This phenotype was noted for receptors encoded by three allelic variants including the common KIR2DS3*001 allele. Comparing the surface expression of KIR2DS3 with that of the better-studied KIR2DS1 molecule in two different cell lines, mutational analysis identified multiple polymorphic amino-acid residues that significantly alter the proportion of molecules present on the cell surface. A simultaneous substitution of five residues localized to the leader peptide (residues -18 and -7), second domain (residues 123 and 150) and transmembrane region (residue 234) was required to restore KIR2DS3 to the expression level of KIR2DS1. Corresponding simultaneous substitutions of KIR2DS1 to the KIR2DS3 residues resulted in a dramatically decreased surface expression. Molecular modeling was used to predict how these substitutions contribute to this phenotype. Alterations in receptor surface expression are likely to affect the balance of immune cell signaling impacting the characteristics of the response to pathogens or malignancy.

Extracellular domain alterations impact surface expression of stimulatory natural killer cell receptor KIR2DS5

In the human killer cell immunoglobulin-like receptors, KIR2DL2, and KIR2DL3, a triad of amino acids in the D1 domain interact to stabilize protein structure. Substitution of any one of these residues caused significant loss of cell surface expression. Although KIR2DS4 and KIR2DS5, two homologous receptors, differ for this triad, flow cytometry analysis of NK and T cell lines transfected with stimulatory KIR genes KIR2DS4 (allele *001) and KIR2DS5 (allele *002) demonstrated cell surface expression. For KIR2DS5, restoration of the triad sequence increased surface expression. Further studies of the receptor encoded by KIR2DS5*002 showed both mature and immature protein isoforms upon gel electrophoresis coupled with surface biotinylation or deglycosylation. In contrast, the KIR2DS5*001 allelic product was not expressed on the cell surface of either NK or T cells and exhibited only a single immature isoform upon gel electrophoresis. Site-directed mutagenesis demonstrated that absence of the KIR2DS5*001-encoded protein at the cell surface was imparted primarily by two amino acid polymorphisms in the D2 domain. Analysis using molecular dynamics simulations suggested that the substitution of a proline for a serine at residue 111 or the substitution of a serine for a phenylalanine at residue 164 caused destabilization of the domain structure and intracellular retention. A third polymorphism at residue 174 impacted the level of KIR2DS5 surface expression. This is the first description at a stimulatory KIR locus of the impact of specific amino acid variations on receptor maturation and the level of surface expression.

Analysis of the linker for activation of T cells and the linker for activation of B cells in natural killer cells reveals a novel signaling cassette, dual usage in ITAM signaling, and influence on development of the Ly49 repertoire

The linker for activation of T cells (LAT) and the linker for activation of B cells (LAB/NTAL/LAT2) are integral proteins in receptor coupling to downstream events. Both proteins are expressed in natural killer (NK) cells and LAT is phosphorylated during target cell interactions or ligation of the immunoreceptor tyrosine-based activation motif (ITAM)-coupled CD16. Regardless, Lat(-/-) mice exhibit normal natural and antibody-mediated killing. Here we place both LAT and LAB in the DAP12 pathway of NK cells. Moreover, we unveil a LAT-independent pathway that requires expression of Syk. Mice lacking either LAT or LAB have a skewed Ly49 repertoire, and activated NK cells from Lat(-/-) mice have reduced responses to the ITAM-coupled receptor NK1.1. In contrast, resting Lat(-/-) NK cells show intact NK1.1 responses, whereas NK cells without LAB are hyperactive. Elimination of both adaptors severely reduces NK1.1 signaling under both conditions. Together these data show that NK ITAMs preferentially use a signaling cassette regulated by interplay between LAT and LAB. Activation by interleukin-2 causes a shift to greater dependency on LAT due to suppression of Syk signaling. The overlapping use of multiple adaptors permits fine-tuning of NK-cell ITAM responses over the course of an immune response.

Human natural killer receptors and their ligands

Human Natural Killer Receptors and Their Ligands (Roberto Biassoni and Cristina Bottino, Istituto Nazionale per la Ricerca sul Cancro, Genova, Italy; Claudia Cantoni, Universita degli Studi di Genova, Istituto Giannina Gaslini, Genova, Italy; Alessandro Moretta, Universita degli Studi di Genova, Genova, Italy). Natural killer (NK) cells are a lymphocyte subpopulation that are important effectors of innate immune responses against infectious pathogens. They are thought to play an important role in host defense, not only against virally infected cells, but also in killing of tumor cells. Recent progress indicates that NK cells express an array of receptors, some of them clonally distributed, able to modulate the natural cytotoxicity. Three NK-specific activating receptors have been characterized; they belong to a novel receptor family called natural cytotoxicity receptors (NCR) and are represented by NKp46, NKp44, and NKp30. These receptors, upon engagement by their specific ligands, induce a strong activation of NK-mediated cytotoxic activity. This overview discusses the receptors (both activating and inhibitory) expressed by NK cells and their ligands. Finally, the dysfunction of one of these molecules occurring in a genetically inherited immunodeficiency is discussed.

The zebrafish activating immune receptor Nitr9 signals via Dap12

Both inhibitory and activating forms of natural killer (NK) cell receptors are found in mammals. The activating receptors play a direct role in the recognition of virally infected or transformed cells and transduce activating signals into the cell by partnering with an adaptor protein, which contains a cytoplasmic activation motif. Activating NK receptors encoded by the mammalian leukocyte receptor complex (e.g., killer cell immunoglobulin-like receptors) and the natural killer complex (e.g., Ly49s) partner with the adaptor protein DAP12, whereas NK receptors encoded in the CD94/NKG2 complex partner with the adaptor protein DAP10. Novel immune-type receptors (NITRs) found in bony fish share several common features with immunoglobulin-type NK receptors. Nitr9 is a putative activating receptor in zebrafish that induces cytotoxicity within the context of human NK cells. One isoform of Nitr9, Nitr9L, is shown here to preferentially partner with a zebrafish ortholog of Dap12. Cross-linking the Nitr9L-Dap12 complex results in activation of the phosphytidylinositol 3-kinase-->AKT-->extracellular signal-regulated kinase pathway suggesting that the DAP12-based activating pathway is conserved between bony fish and mammals.

KIR2DS1-positive NK cells mediate alloresponse against the C2 HLA-KIR ligand group in vitro

The inhibitory 2DL1 and activating 2DS1 killer Ig-like receptors (KIR) both have shared ligand specificity for codon sequences in the C2 group HLA-Cw Ags. In this study, we have investigated NK cell activation by allogeneic target cells expressing different combinations of the HLA-KIR ligand groups C1, C2, and Bw4. We demonstrate that fresh NK cells as well as IL-2-propagated NK cells from 2DS1-positive donors that are homozygous for the C1 ligand group are activated in vitro by B lymphoblastoid cell lines expressing the C2 group. This response is, in part, due to the absence of C1 group recognition mediated by the inhibitory receptor 2DL2/3. This "missing self" alloresponse to C2, however, is rarely observed in NK cells from donors lacking 2DS1. Even in presence of 2DS1, the NK alloresponse is dramatically reduced in donors that have C2 group as "self." Analysis of selected NK clones that express 2DS1 mRNA and lack mRNA for 2DL1 demonstrates that activation by the C2 ligand and mAb cross-linking of 2DS1 in these clones induces IFN-gamma. Furthermore, this C2 group-induced activation is inhibited by Abs to both HLA class I and the receptor. Collectively, these studies demonstrate that NK cells from 2DS1-positive donors are activated by target cells that express the C2 group as an alloantigen. This leads to increased IFN-gamma-positive fresh NK cells and induces NK allocytotoxicity in IL2-propagated polyclonal NK cells and NK clones. This study also provides support for the concept that incompatibility for the HLA-KIR ligand groups C1, C2, and Bw4 dominates NK alloactivation in vitro.

Cutting Edge: KIR3DS1, a gene implicated in resistance to progression to AIDS, encodes a DAP12-associated receptor expressed on NK cells that triggers NK cell activation

The killer cell Ig-like receptor (KIR) gene, KIR3DS1, has been implicated in slowing disease progression in HIV infection; however, little is known about its expression, function, or ligand specificity. Using retrovirally transduced NKL cells and peripheral blood NK cells from KIR3DS1-positive donors we assessed expression of this gene by flow cytometry and its function by in vitro assays measuring KIR3DS1-induced cell-mediated cytotoxicity and cytokine production. In the present study, we demonstrate that KIR3DS1 is expressed on peripheral blood NK cells and triggers both cytotoxicity and IFN-gamma production. Using cotransfection and coimmunoprecipitation, we found that KIR3DS1 associates with the ITAM-bearing adaptor, DAP12. Soluble KIR3DS1-Ig fusion proteins did not bind to EBV-transformed B lymphoid cell lines transfected with HLA-Bw4 80I or 80T allotypes, suggesting that if KIR3DS1 does recognize HLA-Bw4 ligands, this may be peptide dependent.

Non-T Cell Activation Linker (NTAL) Negatively Regulates TREM-1/DAP12-Induced Inflammatory Cytokine Production in Myeloid Cells

The engagement of triggering receptor expressed on myeloid cells 1 (TREM-1) on macrophages and neutrophils leads to TNF-alpha and IL-8 production and enhances inflammatory responses to microbial products. For signal transduction, TREM-1 couples to the ITAM-containing adapter DNAX activation protein of 12 kDa (DAP12). In general, ITAM-mediated signals lead to cell activation, although DAP12 was recently implicated in inhibitory signaling in mouse macrophages and dendritic cells. To date, signals downstream of the TREM-1 and DAP12 complex in myeloid cells are poorly defined. By analyzing receptor-induced tyrosine phosphorylation patterns, we discovered that the ligation of TREM-1 leads to tyrosine phosphorylation of the non-T cell activation linker (NTAL; also called linker of activation in B cells or LAB) in a myelomonocytic cell line and primary human granulocytes. Using RNA interference to decrease the expression levels of NTAL, we demonstrate that in NTAL knockdown cell lines the phosphorylation of ERK1/2 is enhanced. In addition, low levels of NTAL are correlated with decreased and delayed mobilization of Ca(2+) after TREM-1 triggering. Most importantly, we demonstrate that NTAL acts as a negative regulator of TNF-alpha and IL-8 production after stimulation via TREM-1. Our results show that activation signals delivered via DAP12 can be counterbalanced by the adaptor NTAL, identifying NTAL as gatekeeper of TREM-1/DAP12-induced signaling in myeloid cells.

Signal transduction in natural killer cells

Tolerance of natural killer (NK) cells toward normal cells is mediated through their expression of inhibitory receptors that detect the normal expression of self in the form of class I major histocompatibility complex (MHC-I) molecules on target cells. These MHC-I-binding inhibitory receptors recruit tyrosine phosphatases, which are believed to counteract activating receptor-stimulated tyrosine kinases. The perpetual balance between signals derived from inhibitory and activating receptors controls NK cell responsiveness and provides an interesting paradigm of signaling cross talk. This review summarizes our knowledge of the intracellular mechanisms by which cell surface receptors influence biological responses by NK cells. Special emphasis focuses on the dynamic signaling events at the NK immune synapse and the unique signaling characteristics of specific receptors, such as NKG2D, 2B4, and KIR2DL4.

ITAM-based signaling beyond the adaptive immune response

Classical immunoreceptors like lymphocyte antigen receptors and Fc-receptors (FcR) are central players of the adaptive immune response. These receptors utilize a common signal transduction mechanism, which relies on immunoreceptor tyrosine-based activation motifs (ITAMs) present in the receptor complex. Upon ligand binding to the receptors, tyrosines within the ITAM sequence are phosphorylated by Src-family kinases, leading to an SH2-domain mediated recruitment and activation of the Syk or the related ZAP-70 tyrosine kinase. These kinases then initiate further downstream signaling events. Here we review recent evidence indicating that components of this ITAM-based signaling machinery are also present in a number of non-lymphoid or even non-immune cell types and they participate in diverse biological functions beyond the adaptive immune response, including innate immune mechanisms, platelet activation, bone resorption or tumor development. These results suggest that the ITAM-based signaling paradigm has much wider implications than previously anticipated.

KARAP/DAP12/TYROBP: three names and a multiplicity of biological functions

The signaling adaptor protein KARAP/DAP12/TYROBP (killer cell activating receptor-associated protein / DNAX activating protein of 12 kDa / tyrosine kinase binding protein) belongs to the family of transmembrane polypeptides bearing an intracytoplasmic immunoreceptor tyrosine-based activation motif (ITAM). This adaptor, initially characterized in NK cells, is associated with multiple cell-surface activating receptors expressed in both lymphoid and myeloid lineages. We review here the main features of KARAP/DAP12, describing findings from its identification to recently published data, showing its involvement in a broad array of biological functions. KARAP/DAP12 is a wiring component for NK cell anti-viral function (e.g. mouse cytomegalovirus via its association with mouse Ly49H) and NK cell anti-tumoral function (e.g. via its association with mouse NKG2D or human NKp44). KARAP/DAP12 is also involved in inflammatory reactions via its coupling to myeloid receptors, such as the triggering receptors expressed by myeloid cells (TREM) displayed by neutrophils, monocytes/macrophages and dendritic cells. Finally, bone remodeling and brain function are also dependent upon the integrity of KARAP/DAP12 signals, as shown by the analysis of KARAP/DAP12-deficient mice and KARAP/DAP12-deficient Nasu-Hakola patients.

Receptors and lytic mediators regulating anti-tumor activity by the leukemic killer T cell line TALL-104

The major histocompatibility complex nonrestricted cytotoxic leukemic T cell line T acute lymphoblastic leukemia (TALL)-104 is being pursued as a therapeutic agent for cancer. However, the receptors and effector mechanisms responsible for its broad tumoricidal function remain undefined. Here, we examined the roles played by natural cytotoxicity receptors (NCR), killer cell immunoglobulin-like receptors, cytolytic granule components, and tumor necrosis factor (TNF) family members in tumor recognition and lysis by TALL-104 cells. The perforin-granzyme pathway, TNF-related apoptosis-inducing ligand (TRAIL), and Fas were each involved in the lysis of particular tumor targets by TALL-104. Furthermore, phorbol 12-myristate 13-acetate/ionomycin treatment induced surface expression of Fas-L and TRAIL. In addition, supernatants from CD3-stimulated TALL-104 cultures exhibited antiproliferative activity, which was blocked 50-90% by anti-TNF-alpha monoclonal antibody (mAb). Although negative for the NCR natural killer (NK)p44, this cell line was found to express NKp46. An anti-NKp46 antibody strongly blocked TALL-104-mediated lysis of certain targets and directly induced cytokine production, granule release, and redirected lysis responses. Anti-NKG2D and anti-2B4 also stimulated redirected cytotoxicity by TALL-104. By contrast, anti-NKG2A mAb did not stain the cells or inhibit killing responses. Alternatively, KIR3DL2 was detected on TALL-104, and expression of its reported ligand, human leukocyte antigen (HLA)-A, on target cells provided protection from cytotoxicity. Thus, NKp46, NKG2D, and 2B4 are activating receptors, and KIR3DL2 is an inhibitory receptor on TALL-104. The data demonstrate the ability of TALL-104 cells to recognize a wide variety of tumors with NK cell receptors and kill them with a broad arsenal of cytolytic effector mechanisms, including cytolytic granules and TNF family ligands.

Cutting Edge: KIR2DL4 Transduces Signals into Human NK Cells through Association with the Fc Receptor γ Protein

KIR2DL4 (2DL4, CD158d), a member of the human killer cell Ig-like receptor (KIR) family, triggers potent IFN-gamma responses but weak cytotoxicity in resting NK cells. 2DL4 mRNA has been detected in most NK cell clones from most humans examined, but surface protein expression is detectable only on CD56(high) NK cells from certain donors. The receptor possesses a transmembrane arginine residue, suggesting association with a signaling accessory protein that has remained elusive. We provide biochemical and functional evidence that FcepsilonRI-gamma (gamma) associates with 2DL4 to promote surface expression and provide signal transducing function. Weak cytolytic responses triggered through 2DL4 may result from low stoichiometric association with gamma. Selective association with gamma distinguishes 2DL4 from all other activating forms of the KIR family, which alternatively associate with DNAX-activating protein (DAP)12.

Immunotherapy of cancer by active vaccination: does allogeneic bone marrow transplantation after non-myeloablative conditioning provide a new option?

The critical role of antigen-specific T cells in cancer immunotherapy has been amply demonstrated in many model systems. Though success of clinical trials still remains far behind expectation, the continuous improvement in our understanding of the biology of the immune response will provide the basis of optimized cancer vaccines and allow for new modalities of cancer treatment. This review focuses on the current status of active therapeutic vaccination and future prospects. The latter will mainly be concerned with allogeneic bone marrow cell transplantation after non-myeloablative conditioning, because it is my belief that this approach could provide a major breakthrough in cancer immunotherapy. Concerning active vaccination protocols the following aspects will be addressed: i) the targets of immunotherapeutic approaches; ii) the response elements needed for raising a therapeutically successful immune reaction; iii) ways to achieve an optimal confrontation of the immune system with the tumor and iv) supportive regimen of immunomodulation. Hazards which one is most frequently confronted with in trials to attack tumors with the inherent weapon of immune defense will only be briefly mentioned. Many question remain to be answered in the field of allogeneic bone marrow transplantation after non-myeloablative conditioning to optimize the therapeutic setting for this likely very powerful tool of cancer therapy. Current considerations to improve engraftment and to reduce graft versus host disease while strengthening graft versus tumor reactivity will be briefly reviewed. Finally, I will discuss whether tumor-reactive T cells can be "naturally" maintained during the process of T cell maturation in the allogeneic host. Provided this hypothesis can be substantiated, a T cell vaccine will meet a pool of virgin T cells in the allogeneically reconstituted host, which are tolerant towards the host, but not anergised towards tumor antigens presented by MHC molecules of the host.

The three-dimensional structure of the human NK cell receptor NKp44, a triggering partner in natural cytotoxicity

Natural killer (NK) cells direct cytotoxicity against tumor or virally infected cells. NK cell activation depends on a fine balance between inhibitory and activating receptors. NKp44 is a cytotoxicity activating receptor composed of one Ig-like extracellular domain, a transmembrane segment, and a cytoplasmic domain. The 2.2 A crystal structure shows that the NKp44 Ig domain forms a saddle-shaped dimer, where a charged surface groove protrudes from the core structure in each subunit. NKp44 Ig domain disulfide bridge topology defines a new Ig structural subfamily. The data presented are a first step toward understanding the molecular basis for ligand recognition by natural cytotoxicity receptors, whose key role in the immune system is established, but whose cellular ligands are still elusive.

The killer cell immunoglobulin-like receptor (KIR) genomic region: gene-order, haplotypes and allelic polymorphism

Recent genetic studies have established that the killer cell immunoglobulin-like receptor (KIR) genomic region displays extensive diversity through variation in gene content and allelic polymorphism within individual KIR genes. It is demonstrated by family segregation analysis, genomic sequencing, and gene order determination that genomic diversity by gene content alone gives rise to more than 20 different KIR haplotypes and at least 40-50 KIR genotypes. In the most reductionist format, KIR haplotypes can be accommodated within one of 10 different prototypes, each with multiple permutations. Our haplotype model considers the KIR haplotype as two separate halves: the centromeric half bordered upstream by KIR3DL3 and downstream by 2DL4, and the telomeric half bordered upstream by 2DL4 and downstream by 3DL2. There are rare KIR haplotypes that do not fit into this model. Recombination, gene duplication, and inversion can however, readily explain these haplotypes. Additional allelic polymorphism imposes extensive individual variability. Accordingly, this segment of the human genome displays a level of diversity similar to the one observed for the human major histocompatibility complex. Recent application of immunogenetic analysis of KIR genes in patient populations implicates these genes as important genetic disease susceptibility factors.

KIR: diverse, rapidly evolving receptors of innate and adaptive immunity

KIR genes have evolved in primates to generate a diverse family of receptors with unique structures that enable them to recognize MHC-class I molecules with locus and allele-specificity. Their combinatorial expression creates a repertoire of NK cells that surveys the expression of almost every MHC molecule independently, thus antagonizing the spread of pathogens and tumors that subvert innate and adaptive defense by selectively downregulating certain MHC class I molecules. The genes encoding KIR that recognize classical MHC molecules have diversified rapidly in human and primates; this contrasts with conservation of immunoglobulin- and lectin-like receptors for nonclassical MHC molecules. As a result of the variable KIR-gene content in the genome and the polymorphism of the HLA system, dissimilar numbers and qualities of KIR:HLA pairs function in different humans. This diversity likely contributes variability to the function of NK cells and T-lymphocytes by modulating innate and adaptive immune responses to specific challenges.

Syk regulation of phosphoinositide 3-kinase-dependent NK cell function

Emerging evidence suggests that NK-activatory receptors use KARAP/DAP12, CD3zeta, and FcepsilonRIgamma adaptors that contain immunoreceptor tyrosine-based activatory motifs to mediate NK direct lysis of tumor cells via Syk tyrosine kinase. NK cells may also use DAP10 to drive natural cytotoxicity through phosphoinositide 3-kinase (PI3K). In contrast to our recently identified PI3K pathway controlling NK cytotoxicity, the signaling mechanism by which Syk associates with downstream effectors to drive NK lytic function has not been clearly defined. In NK92 cells, which express DAP12 but little DAP10/NKG2D, we now show that Syk acts upstream of PI3K, subsequently leading to the specific signaling of the PI3K-->Rac1-->PAK1-->mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase-->ERK cascade that we earlier described. Tumor cell ligation stimulated DAP12 tyrosine phosphorylation and its association with Syk in NK92 cells; Syk tyrosine phosphorylation and activation were also observed. Inhibition of Syk function by kinase-deficient Syk or piceatannol blocked target cell-induced PI3K, Rac1, PAK1, mitogen-activated protein/ERK kinase, and ERK activation, perforin movement, as well as NK cytotoxicity, indicating that Syk is upstream of all these signaling events. Confirming that Syk does not act downstream of PI3K, constitutively active PI3K reactivated all the downstream effectors as well as NK cytotoxicity suppressed in Syk-impaired NK cells. Our results are the first report documenting the instrumental role of Syk in control of PI3K-dependent natural cytotoxicity.

A DAP12-mediated pathway regulates expression of CC chemokine receptor 7 and maturation of human dendritic cells

Gene targeting of the adaptor molecule DAP12 in mice caused abnormal distribution and impaired antigen presentation capacity of dendritic cells (DCs). However, the DAP12-associated receptors expressed on DCs and their functions have not been identified yet. Here we show that the triggering receptor expressed on myeloid cells-2 (TREM-2) is a cell surface receptor on human monocyte-derived DCs, which is associated with DAP12. TREM-2/DAP12 promotes upregulation of CC chemokine receptor 7, partial DC maturation, and DC survival through activation of protein tyrosine kinases and extracellular signal-regulated kinase. In contrast to Toll-like receptor-mediated signaling, TREM2/DAP12 stimulation is independent of nuclear factor-kappaB and p38 stress-activated protein kinase. This novel DC activation pathway may regulate DC homeostasis and amplify DC responses to pathogens, explaining the phenotype observed in DAP12-deficient mice.

Activating receptors and coreceptors involved in human natural killer cell-mediated cytolysis

Natural killer cells can discriminate between normal cells and cells that do not express adequate amounts of major histocompatibility complex (MHC) class I molecules. The discovery, both in mouse and in human, of MHC-specific inhibitory receptors clarified the molecular basis of this important NK cell function. However, the triggering receptors responsible for positive NK cell stimulation remained elusive until recently. Some of these receptors have now been identified in humans, thus shedding some light on the molecular mechanisms involved in NK cell activation during the process of natural cytotoxicity. Three novel, NK-specific, triggering surface molecules (NKp46, NKp30, and NKp44) have been identified. They represent the first members of a novel emerging group of receptors collectively termed natural cytotoxicity receptors (NCR). Monoclonal antibodies (mAbs) to NCR block to differing extents the NK-mediated lysis of various tumors. Moreover, lysis of certain tumors can be virtually abrogated by the simultaneous masking of the three NCRs. There is a coordinated surface expression of the three NCRs, their surface density varying in different individuals and also in the NK cells isolated from a given individual. A direct correlation exists between the surface density of NCR and the ability of NK cells to kill various tumors. NKp46 is the only NCR involved in human NK-mediated killing of murine target cells. Accordingly, a homologue of NKp46 has been detected in mouse. Molecular cloning of NCR revealed novel members of the Ig superfamily displaying a low degree of similarity to each other and to known human molecules. NCRs are coupled to different signal transducing adaptor proteins, including CD3 zeta, Fc epsilon RI gamma, and KARAP/DAP12. Another triggering NK receptor is NKG2D. It appears to play either a complementary or a synergistic role with NCRs. Thus, the triggering of NK cells in the process of tumor cell lysis may often depend on the concerted action of NCR and NKG2D. In some instances, however, it may uniquely depend upon the activity of NCR or NKG2D only. Strict NKG2D-dependency can be appreciated using clones that, in spite of their NCR(dull) phenotype, efficiently lyse certain epithelial tumors or leukemic cell lines. Other triggering surface molecules including 2B4 and the novel NKp80 appear to function as coreceptors rather than as true receptors. Indeed, they can induce natural cytotoxicity only when co-engaged with a triggering receptor. While an altered expression or function of NCR or NKG2D is being explored as a possible cause of immunological disorders, 2B4 dysfunction has already been associated with a severe form of immunodeficiency. Indeed, in patients with the X-linked lymphoproliferative disease, the inability to control Epstein-Barr virus infections may be consequent to a major dysfunction of 2B4 that exerts inhibitory instead of activating functions.

Recognition of HLA-Cw4 but not HLA-Cw6 by the NK cell receptor killer cell Ig-like receptor two-domain short tail number 4

NK cells are cytotoxic to virus-infected and tumor cells that have lost surface expression of class I MHC proteins. Target cell expression of class I MHC proteins inhibits NK cytotoxicity through binding to inhibitory NK receptors. In contrast, a similar family of activating NK receptors, characterized by the presence of a charged residue in their transmembrane portion and a truncated cytoplasmic tail, augment lysis by NK cells when ligated by an appropriate class I MHC protein. However, the class I MHC specificity of many of these activating NK receptors is still unknown. Here, we show enhanced lysis of HLA-Cw4 but not HLA-Cw6-expressing cells, by a subset of NK clones. This subset may express killer cell Ig-like receptor two-domain short tail number 4 (KIR2DS4), as suggested by staining with various mAb. It is still possible, however, that these clones may express receptors other than KIR2DS4 that might recognize HLA-Cw4. Binding of KIR2DS4-Ig fusion protein to cells expressing HLA-Cw4 but not to those expressing HLA-Cw6 was also observed. The binding of KIR2DS4-Ig to HLA-Cw4 is weaker than that of killer cell Ig-like receptor two-domain long tail number 1 (KIR2DL1)-Ig fusion protein; however, such weak recognition is capable of inhibiting lysis by an NK transfectant expressing a chimeric molecule of KIR2DS4 fused to the transmembrane and cytoplasmic portion of KIR2DL1. Residue alpha14 is shown to be important in the KIR2DS4 binding to HLA-Cw4. Implications of the role of the activating NK receptors in immunosurveillance are discussed.

Human natural killer cell receptors and signal transduction

Natural killer (NK) cells express numerous receptors, which continually engage with ligands on cell surfaces. Until 1995, only a handful of these receptors were characterized and the molecular basis of NK cell activation was obscure. Recently, considerable advances have been made in characterizing the receptor repertoire on human NK cells. Both activating and inhibitory receptors can transduce positive or negative signals to regulate NK cell cytotoxicity and cytokine release responses. The inhibitory receptors normally predominate in this balance of signals. Certain tumor cells and virally infected cells that lack major histocompatibility complex (MHC) class I molecules, however, can rapidly trigger NK cell activation. The basis of this activation is the loss of negative signals that are normally transmitted by MHC class I-binding inhibitory receptors, and the corresponding domination of activating receptor signals. While ligand specificity for a number of the recently described receptors is still a mystery, their signal transduction properties have begun to be defined. The dynamic crosstalk between these receptors ultimately governs the NK cell activation state. Although the complexities of NK cell signalling are only marginally understood, several overall themes have been defined by characterizing the roles of distinct pathways during NK cell responses.