Low cytomegalovirus seroprevalence in early multiple sclerosis: a case for the 'hygiene hypothesis'?

BACKGROUND AND PURPOSE

Cytomegalovirus (CMV) infection has recently been associated with a lower multiple sclerosis (MS) susceptibility, although it remains controversial whether it has a protective role or is merely an epiphenomenon related to westernization and early-life viral infections. We aimed to evaluate whether CMV serostatus may differ in patients with early MS as compared with patients with non-early MS, analyzing the putative association of this virus with MS clinical course and humoral immune responses against other herpesviruses.

METHODS

Multicentric analysis was undertaken of 310 patients with MS (early MS, disease duration ≤5 years, n = 127) and controls (n = 155), evaluating specific humoral responses to CMV, Epstein-Barr virus and human herpesvirus-6, as well as T-cell and natural killer (NK)-cell immunophenotypes.

RESULTS

Cytomegalovirus seroprevalence in early MS was lower than in non-early MS or controls (P < 0.01), being independently associated with disease duration (odds ratio, 1.04; 95% confidence interval, 1.01-1.08, P < 0.05). CMV+ patients with MS displayed increased proportions of differentiated T-cells (CD27-CD28-, CD57+, LILRB1+) and NKG2C+ NK-cells, which were associated with a lower disability in early MS (P < 0.05). CMV+ patients with early MS had an age-related decline in serum anti-EBNA-1 antibodies (P < 0.01), but no CMV-related differences in anti-human herpesvirus-6 humoral responses.

CONCLUSIONS

Low CMV seroprevalence was observed in patients with early MS. Modification of MS risk attributed to CMV might be related to the induction of differentiated T-cell and NK-cell subsets and/or modulation of Epstein-Barr virus-specific immune responses at early stages of the disease.

Circulating NK-cell subsets in renal allograft recipients with anti-HLA donor-specific antibodies

Detection of posttransplant donor-specific anti-HLA antibodies (DSA) constitutes a risk factor for kidney allograft loss. Together with complement activation, NK-cell antibody-dependent cell mediated cytotoxicity (ADCC) has been proposed to contribute to the microvascular damage associated to humoral rejection. In the present observational exploratory study, we have tried to find a relationship of circulating donor-specific and non donor-specific anti-HLA antibodies (DSA and HLA non-DSA) with peripheral blood NK-cell subsets and clinical features in 393 renal allograft recipients. Multivariate analysis indicated that retransplantation and pretransplant sensitization were associated with detection of posttransplant DSA. Recipient female gender, DR mismatch and acute rejection were significantly associated with posttransplant DSA compared to HLA non-DSA. In contrast with patients without detectable anti-HLA antibodies, DSA and HLA non-DSA patients displayed lower proportions of NK-cells, associated with increased CD56(bright) and NKG2A(+) subsets, the latter being more marked in DSA cases. These differences appeared unrelated to retransplantation, previous acute rejection or immunosuppressive therapy. Although preliminary and observational in nature, our results suggest that the assessment of the NK-cell immunophenotype may contribute to define signatures of alloreactive humoral responses in renal allograft recipients.

Assessment of copy-number variation in the NKG2C receptor gene in a single-tube and characterization of a reference cell panel, using standard polymerase chain reaction

Natural killer (NK) and T-lymphocytes monitor human leukocyte antigen (HLA)-E expression through CD94:NKG2 heterodimers. Structural polymorphism is not a hallmark for NK-complex genes on chromosome 12, except for complete NKG2C deletion in some humans. We present a method for fast, simple and accurate assessment of NKG2C copy-number variation - presence or absence in the genome of an NKG2C gene, in homo- or heterozygosis, is detected by a single conventional polymerase chain reaction that yields amplicons of different lengths in each genotype. We have also determined the NKG2C genotypes of a reference cell panel comprising 13 NK- and tumour-cell lines and 39 Epstein-Barr virus transformed cells from the International Histocompatibility Workshop. Our results should facilitate research on the importance of NKG2C and its deletion for immunity.

Natural killer cell phenotype and clinical response to interferon-beta therapy in multiple sclerosis

CD56(bright) NK cells, which may play a role in immunoregulation, are expanded in multiple sclerosis (MS) patients treated with immunomodulatory therapies such as daclizumab and interferon-beta (IFNβ). Yet, whether this NK cell subset is directly involved in the therapeutic effect is unknown. As NK receptor (NKR) expression by subsets of NK cells and CD8+ T lymphocytes is related to MS clinical course, we addressed whether CD56(bright) NK cells and NKR in IFNβ-treated MS patients differ according to the clinical response. IFNβ was associated to lower LILRB1+ and KIR+NK cells, and higher NKG2A+NK cell proportions, an immunophenotypic pattern mainly found in responders. After IFNβ treatment, a CD56(bright) NK cell expansion was significantly related to a positive clinical response. Our results reveal that IFNβ may promote in responders changes in the NK cell immunophenotype, corresponding to the profile found at early maturation stages of this lymphocyte lineage.

Natural killer receptors distribution in multiple sclerosis: Relation to clinical course and interferon-beta therapy

NK cell receptors (NKR) are expressed in subsets of NK and CD8+ T cells, lymphocytes involved in multiple sclerosis (MS) pathogenesis. Clinical implications of NKR expression in MS are unknown. Here, we show that the proportions of CD8+ T cells displaying LILRB1, an inhibitory NKR expressed at late stages of T cell differentiation, were directly related with age and MS duration, and inversely with the immunomodulatory therapy-dependent increase of CD56(bright) NK cells. Similar associations were found for KIR+ and CD56+ CD8+ T cells, whereas no age-related NKR distribution was perceived in controls. Moreover, active MS had lower LILRB1+ NK cells, and IFN-β-treated patients exhibited a phenotypic profile related to shorter disease evolution. Progressive accumulation of terminally differentiated T lymphocytes and experienced NK cells in MS, presumably stimulated in response to a persistent challenge and modulated by IFN-β therapy, may support the analysis of NKR distribution as new biomarkers.

Analysis of expression and function of the inhibitory receptor ILT2 (CD85j/LILRB1/LIR-1) in peripheral blood mononuclear cells from patients with systemic lupus erythematosus (SLE)

The aim of this work was to study the expression and function of the inhibitory receptor ILT2/CD85j in peripheral blood mononuclear cells (PBMC) from patients with systemic lupus erythematosus (SLE). We studied 23 SLE patients as well as 17 patients with rheumatoid arthritis, 10 with fibromyalgia, and 23 healthy individuals. We found a variable level of expression of ILT2 in the PBMC from both SLE patients and controls, with no significant differences among them. However, when the expression of this receptor was assessed in cell subsets, significantly lower levels were detected in CD19+ lymphocytes from SLE patients compared with healthy controls. Functional assays performed in unfractionated PBMC, showed a significant diminished inhibitory activity of ILT2 in CD4+ and CD8+ cell subsets from SLE patients compared to either rheumatoid arthritis or fibromyalgia patients, and healthy individuals. Our results show that the PBMC from some patients with SLE show a defective expression of ILT2, and that most of them exhibit a poor function of this inhibitory receptor.

NK cell receptors involved in the response to human cytomegalovirus infection

Human cytomegalovirus (HCMV) infection is a paradigm of the complexity reached by host-pathogen interactions. To avoid recognition by cytotoxic T lymphocytes (CTL) HCMV inhibits the expression of HLA class I molecules. As a consequence, engagement of inhibitory killer immunoglobulin-like receptors (KIR), CD94/NKG2A, and CD85j (ILT2 or LIR-1) natural killer cell receptors (NKR) specific for HLA class I molecules is impaired, and infected cells become vulnerable to an NK cell response driven by activating receptors. In addition to the well-defined role of the NKG2D lectin-like molecule, the involvement of other triggering receptors (i.e., activating KIR, CD94/NKG2C, NKp46, NKp44, and NKp30) in the response to HCMV is being explored. To escape from NK cell-mediated surveillance, HCMV interferes with the expression of NKG2D ligands in infected cells. In addition, the virus may keep NK inhibitory receptors engaged preserving HLA class I molecules with a limited role in antigen presentation (i.e., HLA-E) or, alternatively, displaying class I surrogates. Despite considerable progress in the field, a number of issues regarding the involvement of NKR in the innate immune response to HCMV remain uncertain.

Natural killer cell receptors for major histocompatibility complex class I and related molecules in cytomegalovirus infection

Downmodulation of major histocompatibility complex (MHC) class I molecules by cytomegalovirus (CMV) impairs the engagement of specific leucocyte-inhibitory receptors, rendering infected cells vulnerable to natural killer (NK) cells. Members of the murine Ly49 and human KIR families, CD85j (ILT2 or leucocyte Ig-like receptor-1), as well as the CD94/NKG2A-inhibitory killer lectin-like receptor (KLR) fulfil this surveillance role. On the other hand, NK-activating receptors specific to ligands expressed on virus-infected cells may overcome the control by inhibitory receptors. In this regard, NKG2D and Ly49H lectin-like molecules trigger NK-cell functions recognizing, respectively class I-related stress-inducible molecules and the m157 murine CMV glycoprotein. Among a variety of immune evasion strategies, CMV promotes the synthesis of class I surrogates and selectively preserves the expression of some class I molecules in infected cells; moreover, CMV interferes with the expression of ligands for NKG2D. We herein review these aspects of the host-pathogen interaction, discussing a number of open issues.

Molecular studies and NK cell function of a new case of TAP2 homozygous human deficiency

In this paper we describe the clinical and molecular features of a new case (GOR) of homozygous human TAP2 deficiency, analysing the phenotype and function of NK cells. The patient presented from infancy with recurrent sinopulmonary infections; a selective IgG2 deficiency, negative antibody response to polysaccharide vaccination and low level of cell surface expression of HLA class I antigens were found. The sequence of TAP2 gene identified a single mutation, a C to T substitution changing the CGA arg codon at amino acid 220 into TGA stop codon in exon 3. By using MoAbs for KIRs, CD94, CD94/NKG2A and ILT2 we observed, in agreement with others, that the latter two receptors were overexpressed on TAP2-deficient NK cells. The inhibitory CD94/NKG2A and triggering CD94/NKG2C NK receptors, specific for HLA-E, appeared to be functional in a limited number of NK clones that could be expanded in vitro. Expression of HLA-E was virtually undetectable in GOR B-LCL and very faint in PBMC, further supporting that interactions of class I leader sequence nonamers with HLA-E in the ER depend on a functional TAP complex.

Human cytomegalovirus and natural killer-mediated surveillance of HLA class I expression: a paradigm of host-pathogen adaptation

Among various strategies to evade the host immune response, some viruses like human cytomegalovirus (HCMV) interfere with surface MHC class I expression and antigen presentation to T lymphocytes. The ability of natural killer (NK) cells to detect MHC class I molecules through inhibitory receptors can be envisaged as an adaptation of the immune system for responding to such pathological alterations. To fulfil that role, rodents use members of the Ly49 C-type lectin superfamily, whereas primates employ killer immunoglobulin-like receptors and the immunoglobulin-like transcript 2/leucocyte immunoglobulin-like receptor-1 receptor. CD94/NKG2 lectin-like heterodimers represent the most conserved receptor system for MHC class I molecules; by interacting with human HLA-E or murine Qa-1b, CD94/NKG2A inhibitory receptors broadly probe the biosynthesis pathway of other class I molecules. Reciprocally, HCMV has developed mechanisms to evade the NK response while modulating HLA class Ia expression. The ability of HCMV to maintain surface levels of HLA-E and to express an HLA class I surrogate (UL18) are herein discussed in the context of the interplay with human NKR systems.

High expression of the ILT2 (LIR-1) inhibitory receptor for major histocompatibility complex class I molecules on clonal expansions of T large granular lymphocytes in asymptomatic patients

BACKGROUND AND OBJECTIVES

The lymphoproliferative disorders of large granular lymphocytes (LGLD) are divided into two groups: T-cell type and NK-cell type. These entities may be either asymptomatic or associated with autoimmune manifestations (especially cytopenias). A number of surface receptors, expressed by NK-cells and some T-lymphocyte subsets repress cytotoxicity and cytokine production upon ligation with HLA class I molecules and are clonally expressed in theses lymphoproliferative disorders. These cytotoxic lymphocytes can lyse erythroid progenitors in vitro, and the physiologic lower levels of HLA class I antigens on the erythroid lineage may contribute to this form of autoimmunity. It is conceivable that the clinical outcome of T-LGLD might be influenced by the expression of MHC class I inhibitory receptors.

DESIGN AND METHODS

We analyzed the surface expression of these molecules, lectin-like heterodimers (CD94/NKG2A) or killer immunoglobulin (Ig)-like receptors (KIR) and another Ig-like inhibitory receptor, termed ILT2 or LIR-1 in CD8+ cells from 12 cases of ab T-LGLD using specific monoclonal antibodies.

RESULTS

None of the LGLD cases had anemia and 11 of 12 patients remain asymptomatic. KIR and CD94/NKG2A expression was detected on CD8+ populations only in some cases of T-LGLD. By contrast, our observations revealed that ILT2 expression was markedly higher in CD8+ cells from LGLD patients than from healthy donors.

INTERPRETATION AND CONCLUSIONS

Expression of the ILT2 inhibitory receptor for HLA class I molecules on LGLD cells might indeed contribute to preventing their autoreactivity. Further studies are required to evaluate the expression/function of the ILT2 receptor in patients who eventually become symptomatic. The development of cytopenias in LGLD patients must involve other self-reactive activating receptors. Analysis of the expression and function of triggering NKR in LGLD needs to be carefully addressed.

Mitogen-activated protein kinase activity is involved in effector functions triggered by the CD94/NKG2-C NK receptor specific for HLA-E

The CD94/NKG2C heterodimer constitutes an activating receptor involved in NK cell-mediated recognition of the class lb molecule HLA-E. It transduces the triggering signal through an ITAM-bearing molecule, DAP12/KARAP, coupled non-covalently to the receptor. Here we show that specific engagement of the receptor complex expressed on the surface of an NK clone induced the phosphorylation of mitogen-activated protein kinase (MAPK). By the use of the MEK inhibitor PD098059 we demonstrate that the MAPK pathway participates in the CD94-dependent TNF-alpha production and cytotoxicity. Moreover, we transferred the activating function by transfection of the heterologous RBL cell line with CD94/NKG2-C/DAP12. In this system, cross-linking of the receptor induced calcium mobilization, serotonin release and phosphorylation of MAPK.

NK cell recognition of non-classical HLA class I molecules

NK cells recognize several HLA class Ib molecules employing both immunoglobulin-like (Ig-like) and C-type lectin receptors. The CD94/NKG2 and NKG2D lectin-like molecules, respectively, interact with HLA-E and MICA; CD94/NKG2A functions as an inhibitory receptor, while CD94/NKG2C and NKG2D trigger NK cell activity. HLA-E predominantly presents nonamers from the leader sequences of other class I molecules; a peptide derived from HLA-G1 constitutes the highest affinity ligand for both CD94/NKG2 receptors. Members of the Ig-like transcript (ILT) or leucocyte Ig-like receptor (LIR) family (ILT2 or LIR-1 and ILT4 or LIR-2), expressed by other leucocyte lineages, interact with a broad spectrum of HLA class Ia molecules and HLA-G1. Among Ig-like KIRs, the KIR2DL4 (p49) receptor has been shown to specifically recognize HLA-G1; this molecule displays an unusual hybrid structure, sharing features with inhibitory and triggering KIRs.

Paired inhibitory and triggering NK cell receptors for HLA class I molecules

Human natural killer (NK) cells specifically interact with major histocompatibility complex (MHC) class I molecules employing different receptor systems, shared with subsets of alphabeta and gammadelta T lymphocytes. Killer cell immunoglobulin-like receptors (KIRs) recognize groups of human leukocyte antigen (HLA) class Ia proteins displaying common structural features at the alpha-1 domain; among them, KIR2DL4 has been proposed to specifically interact with the class Ib molecule HLA-G1. Members of a related family of immunoglobulin (Ig)-like receptors (ILT2 or LIR-1 and ILT4 or LIR-2), expressed by other leukocyte lineages, interact with a broad spectrum of class Ia molecules and HLA-G1. On the other hand, CD94/NKG2-A(-C) and NKG2D lectin-like receptors, respectively, recognize the class Ib molecules HLA-E and MICA. A recurrent finding within the different receptor families is the existence of pairs of homologous molecules that often share the same ligands but display divergent functions. Inhibitory receptors tend to exhibit an affinity for HLA molecules higher than their activating counterparts. Recruitment of SH2 domain-bearing tyrosine phosphatases (SHP) by cytoplasmic phosphorylated immunoreceptor tyrosine-based inhibition motifs (ITIMs) is a crucial event for the inhibitory signalling pathway. By contrast, triggering receptors assemble with homodimers of immune tyrosine-based activation motif (ITAM)-bearing adaptor molecules (i.e., DAP12, CD3 xi) that engage tyrosine kinases (ZAP70 and syk).

A novel family of Ig-like receptors for HLA class I molecules that modulate function of lymphoid and myeloid cells

We review what is presently known about structure, cellular distribution, biochemical characteristics, and function of a new family of human cell-surface receptors referred to as immunoglobulin-like transcripts (ILTs), leukocyte Ig-like receptors (LIRs), or monocyte/macrophage Ig-like receptors (MIRs). These receptors are genetically, structurally, and functionally related to a group of natural killer (NK) cell receptors for HLA class I molecules known as killer cell Ig-like receptors (KIRs). Distinct ILT/LIR/MIR isotypes are differentially expressed on lymphocytes, monocytes, macrophages, dendritic cells, and granulocytes; at least some of them recognize HLA class I molecules. Whereas some isotypes either inhibit or induce cell activation, others may be secreted as soluble receptors. ILT/LIR/MIR receptors may allow all immune cells to monitor class I expression on other cells and to respond in its absence, just as NK cells do. In addition, they may contribute to homeostasis by establishing activation thresholds that can be overcome only by relevant triggering stimuli and not by bystander cells.

Kinetics and peptide dependency of the binding of the inhibitory NK receptor CD94/NKG2-A and the activating receptor CD94/NKG2-C to HLA-E

The lytic function of human natural killer (NK) cells is markedly influenced by recognition of class I major histocompatibility complex (MHC) molecules, a process mediated by several types of activating and inhibitory receptors expressed on the NK cell. One of the most important of these mechanisms of regulation is the recognition of the non-classical class I MHC molecule HLA-E, in complex with nonamer peptides derived from the signal sequences of certain class I MHC molecules, by heterodimers of the C-type lectin-like proteins CD94 and NKG2. Using soluble, recombinant HLA-E molecules assembled with peptides derived from different leader sequences and soluble CD94/NKG2-A and CD94/NKG2-C proteins, the binding of these receptor-ligand pairs has been analysed. We show first that these interactions have very fast association and dissociation rate constants, secondly, that the inhibitory CD94/NKG2-A receptor has a higher binding affinity for HLA-E than the activating CD94/NKG2-C receptor and, finally, that recognition of HLA-E by both CD94/NKG2-A and CD94/NKG2-C is peptide dependent. There appears to be a strong, direct correlation between the binding affinity of the peptide-HLA-E complexes for the CD94/NKG2 receptors and the triggering of a response by the NK cell. These data may help to understand the balance of signals that control cytotoxicity by NK cells.

Natural killer cell activation and inhibition by receptors for MHC class I

Several recent advances have been made in our understanding of the mechanisms which human natural killer cells recognize MHC class I molecules. Three are of special relevance: the identification of a novel molecule (DAP12) with a key role in the activation pathways; the observation that certain immunoglobulin-like receptors for HLA class I molecules are also utilized by other leucocyte lineages; and the definition of MHC class Ib proteins (i.e. HLA-E and Qa-1b) as specific ligands for the phylogenetically conserved CD94-NKG2 lectin-like receptors.

Triggering of effector functions on a CD8+ T cell clone upon the aggregation of an activatory CD94/kp39 heterodimer

Some T lymphocytes express the CD94 Ag, which is known to form heterodimers with members of the NKG2 family. We have studied the expression pattern and function of CD94 heterodimers in different alphabeta or gammadelta T cell clones. Most of the CD94+NKG2A- T cells have a low to intermediate expression of CD94 Ag. The cross-linking of the CD94/NKG2 heterodimer in one of these CD8 alphabeta CD94+NKG2A- T cell clones (K14B06) was able to: 1) increase the intracellular concentration of Ca2+, 2) induce the up-regulation of CD25 Ag expression and the secretion of IFN-gamma, and 3) trigger redirected cytotoxicity in a TCR-independent manner. This activatory property was not shared by any other costimulatory molecule expressed by the K14B06 T cell clone, including CD8, CD28, CD45, CD69, or CD2 Ags. The immunoprecipitation of CD94 heterodimer showed a 39-kDa band with a similar m.w. to the activatory heterodimer found on some NK clones. A novel form of the NKG2 family (NKG2H) was identified in K14B06. NKG2H protein represents an alternative spliced form of the NKG2E gene, displaying a charged residue in the transmembrane portion and a cytoplasmic tail that lacks immunoreceptor tyrosine-based inhibitory motifs. The expression of NKG2H in the cell membrane through its association to CD94 and DAP-12 molecules supports that it could form part of the activatory CD94/Kp39 heterodimer present on K14B06 cells.

How do NK cells sense the expression of HLA-G class Ib molecules?

The nature of NK cell receptors involved in recognition of HLA-G1 has remained an unclear issue; we herein review this topic based on our experience. We found no evidence that well characterized p58 and p70 KIRs may interact with HLA-G1. By contrast, our data support that NK recognition of cells expressing HLA-G1 involves at least two non-overlapping receptor-ligand systems: (1) the direct engagement of the ILT2 (LIR1) receptor by HLA-G1; and (2) the interaction of CD94/NKG2A and CD94/NKG2C receptors with the non-classical class I molecule HLA-E, co-expressed on the surface upon binding to a nonamer (VMAPRTLFL) from the HLA-G leader sequence.

The ILT2(LIR1) and CD94/NKG2A NK cell receptors respectively recognize HLA-G1 and HLA-E molecules co-expressed on target cells

Previous studies on NK recognition of HLA-G1 employed as targets 721.221 transfectants (.221-G1) that unknowingly co-expressed the HLA-E molecule, subsequently found to be a major ligand for the CD94/NKG2 receptors. In the present study we re-evaluated the relative role played by CD94/NKG2 and ILT2(LIR1) molecules in recognition of HLA-G1 by NK clones. We employed as targets .221-G1 cells and a surface HLA-E-negative transfectant, .221-G1(Eneg), generated by site-directed mutagenesis of the HLA-G1 leader sequence. The antagonistic effects of receptor- (ie. CD94/NKG2A, ILT2) and ligand-specific mAb (i.e. HLA-G, HLA-E) were assessed. In addition, binding of an ILT2-Ig fusion protein to the .221-AEH, expressing only HLA-E, and the .221-G1(Eneg) transfectants was analyzed. Our data demonstrate that NK recognition of cells expressing HLA-G1 involves at least two non-overlapping receptor-ligand systems: the CD94/NKG2 interaction with HLA-E, and the engagement of the ILT2(LIR1) receptor by HLA-G1 molecules.

Roles of chemokines and receptor polarization in NK-target cell interactions

We report that the ability of NK cells to produce chemokines is increased in NK-target cell conjugates. The chemokines produced play a critical role in the polarization and recruitment of NK cells as well as in the NK effector-target cell conjugate formation. Chemokines induce the formation of two specialized regions in the NK cell: the advancing front or leading edge, where chemokine receptors CCR2 and CCR5 cluster, which might guide the cells toward the chemotactic source, and the uropod, where adhesion molecules ICAM-1 and -3 are redistributed. NK cell polarity was intrinsically involved in conjugate formation. The redistribution of both adhesion receptors and CCR was preserved during the formation of NK-target cell conjugates. Time-lapse videomicroscopy studies of the formation of effector-target conjugates showed that morphologic poles are also functionally distinct; while the binding to target cells was preferentially mediated through the leading edge, the uropod was found at the rear of migrating NK cells and recruited additional NK cells to the vicinity of K562 target cells. Inhibition of cell polarization and adhesion receptor redistribution blocked the formation of NK-K562 cell conjugates and the cytotoxic activity of NK cells. We discuss the implication of NK-cell polarization in the development of cytotoxic responses.

HLA-E-bound peptides influence recognition by inhibitory and triggering CD94/NKG2 receptors: preferential response to an HLA-G-derived nonamer

The HLA-E class Ib molecule constitutes a major ligand for the lectin-like CD94/NKG2 natural killer (NK) cell receptors. Specific HLA class I leader sequence-derived nonapeptides bind to endogenous HLA-E molecules in the HLA-defective cell line 721.221, inducing HLA-E surface expression, and promote CD94/NKG2A-mediated recognition. We compared the ability of NK clones which expressed either inhibitory or activating CD94/NKG2 receptors to recognize HLA-E molecules on the surface of 721.221 cells loaded with a panel of synthetic nonamers derived from the leader sequences of most HLA class I molecules. Our results support the notion that the primary structure of the HLA-E-bound peptides influences CD94/ NKG2-mediated recognition, beyond their ability to stabilize surface HLA-E. Further, CD94/ NKG2A+ NK clones appeared more sensitive to the interaction with most HLA-E-peptide complexes than did effector cells expressing the activating CD94/NKG2C receptor. However, a significant exception to this pattern was HLA-E loaded with the HLA-G-derived nonamer. This complex triggered cytotoxicity very efficiently over a wide range of peptide concentrations, suggesting that the HLA-E/G-nonamer complex interacts with the CD94/NKG2 triggering receptor with a significantly higher affinity. These results raise the possibility that CD94/NKG2-mediated recognition of HLA-E expressed on extravillous cytotrophoblasts plays an important role in maternal-fetal cellular interactions.

Intrahepatic enhanced expression of beta2-microglobulin conformational epitope in acute liver allograft rejection: evidence of modulation by glucocorticoids

The mechanisms by which glucocorticoids are effective in acute liver rejection therapy are not entirely clear. The aims of this study were to characterize the intrahepatic immunological phenotype in acute liver rejection, as well as the effect of glucocorticoids on cytokine-stimulated hepatocyte cell lines. Biopsy sections from these patients were studied by immunohistochemistry. Cytokine-stimulated hepatocyte cell lines treated with glucocorticoids were evaluated by flow cytometry. The intrahepatic expression of both beta2-microglobulin conformational epitope and intercellular adhesion molecule-1 was higher in acute rejection than in resolving rejection. Interestingly, glucocorticoids were able to modulate in vitro the cytokine-induced expression of these molecules on hepatocyte cell lines. Beneficial effects of the glucocorticoid treatment appear to be associated with a modulation of a beta2-microglobulin conformational epitope and the intercellular adhesion molecule-1 on intrahepatic cellular targets in the acute rejection process.

HLA-E is a major ligand for the natural killer inhibitory receptor CD94/NKG2A

We previously showed that the availability of a nonamer peptide derived from certain HLA class I signal sequences is a necessary requirement for the stabilization of endogenous HLA-E expression on the surface of 721.221 cells. This led us to examine the ability of HLA-E to protect HLA class I transfectants from natural killer (NK) cell-mediated lysis. It was possible to implicate the CD94/NKG2A complex as an inhibitory receptor recognizing this class Ib molecule by using as target a .221 transfectant selectively expressing surface HLA-E. HLA-E had no apparent inhibitory effect mediated through the identified Ig superfamily (Ig-SF) human killer cell inhibitory receptors or ILT2/LIR1. Further studies of CD94/NKG2+ NK cell-mediated recognition of .221 cells transfected with different HLA class I allotypes (i.e., -Cw4, -Cw3, -B7) confirmed that the inhibitory interaction was mediated by CD94/NKG2A recognizing the surface HLA-E molecule, because only antibodies directed against either HLA-E, CD94, or CD94/NKG2A specifically restored lysis. Surface stabilization of HLA-E in cold-treated .221 cells loaded with appropriate peptides was sufficient to confer protection, resulting from recognition of the HLA class Ib molecule by the CD94/NKG2A inhibitory receptor. Consistent with the prediction that the ligand for CD94/NKG2A is expressed ubiquitously, our examination of HLA-E antigen distribution indicated that it is detectable on the surface of a wide variety of cell types.

Identification of a common developmental pathway for thymic natural killer cells and dendritic cells

Current data support the notion that the thymus is seeded by a yet uncommitted progenitor cell able to generate T cells, B cells, natural killer (NK) cells, and dendritic cells (DCs). We assess in this report the developmental relationship of DCs and NK cells derived from a small subset of CD34(+) human postnatal thymocytes that, like the earliest precursors in the fetal thymus, display low CD33 surface expression. Culture of these isolated CD34(+) CD33(lo) thymic progenitors with a mixture of cytokines, including interleukin-7 (IL-7), IL-1alpha, IL-6, granulocyte-macrophage colony-stimulating factor, and stem cell factor, results in predominant generation of DCs. However, the addition of IL-2 to the cytokine mixture leads to the simultaneous development of DCs and NK cells. Both developmental pathways progress through a transient population of CD34(+)CD44(bright) CD5(lo/-)CD33(+) large-sized cells, distinct from small-sized T-lineage precursors, that contain bipotential NK/DC progenitors. These data provide evidence of linked pathways of NK cell and DC development from intrathymic precursors and suggest that NK cells and DCs branch off the T lineage through a common intermediate progenitor.

Specific engagement of the CD94/NKG2-A killer inhibitory receptor by the HLA-E class Ib molecule induces SHP-1 phosphatase recruitment to tyrosine-phosphorylated NKG2-A: evidence for receptor function in heterologous transfectants

It has been recently demonstrated that the CD94/NKG2-A killer inhibitory receptor (KIR) specifically recognizes the HLA-E class Ib molecule. Moreover, the apparent CD94-mediated specific recognition of different HLA class Ia allotypes, transfected into the HLA-defective cell line 721.221, indeed depends on their selective ability to concomitantly stabilize the surface expression of endogenous HLA-E molecules, which confer protection against CD94/NKG2-A+ effector cells. In the present study, we show that a selective engagement of the CD94/NKG2-A inhibitory receptor with a specific monoclonal antibody (mAb) (Z199) was sufficient to induce tyrosine phosphorylation of the NKG2-A subunit and SHP-1 recruitment. These early biochemical events, commonly related to negative signaling pathways, were also detected upon the specific interaction of NK cells with an HLA-E+ 721.221 transfectant (.221-AEH), and were prevented by pre-incubation of .221-AEH with an anti-HLA class I mAb. Furthermore, mAb cross-linking of the CD94/NKG2-A receptor, segregated from other NK-associated molecules by transfection into a rat basophilic leukemia cell line (RBL-2H3), promoted tyrosine phosphorylation of NKG2-A and co-precipitation of SHP-1, together with an inhibition of secretory events triggered via Fc epsilonRI. Remarkably, interaction of CD94/NKG2-A+ RBL cells with the HLA-E+ .221-AEH transfectant specifically induced a detectable association of SHP-1 with NKG2-A, constituting a more formal evidence for the receptor-HLA class I interaction.

Structure of the human CD94 C-type lectin gene

The genomic structure of the human CD94 gene was obtained by analyzing genomic clones obtained from two different libraries. The CD94 gene contains six exons separated by five introns. The carbohydrate-recognition domain (CRD) is encoded by three exons, and the conservation of intron positions within the CRD indicated that CD94 is closely related to group V of C-type lectins. Primer extension and S1 nuclease protection assays showed that initiation of transcription in the CD94 gene is heterogeneous, but restricted to a 60 base pair region around the major initiation site enclosed within a putative initiator element (TTA+1TTCA). The study of the promoter region of CD94 may help to understand the selective expression of this C-type lectin glycoprotein on NK cells and subsets of cytotoxic T cells.

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

Our understanding of the biology of human natural killer (NK) cells has significantly advanced in recent years upon identification of a family of NK cell-expressed genes that encode killer cell inhibitory receptors (KIR). Individual KIR can selectively bind various HLA class I allotypes and consequently transduce inhibitory signals that block NK cell lysis of ligand-bearing target cells. A distinct subset of related and linked genes express truncated versions of KIR that are otherwise highly homologous in amino acid sequence. Interestingly, these receptors appear to transmit stimulatory signals into NK cells and have been termed killer cell activating receptors (KAR). In this report, we demonstrate that recognition of HLA-Cw3 by the p50 KAR, NKAT8, can potentiate the cytotoxic response of appropriate NK cell clones. Specific cross-linking of this KAR with a monoclonal antibody resulted in intracellular calcium mobilization, protein tyrosine phosphorylation, and phosphorylation of the MAP kinases, ERK1 and ERK2. In addition, we identified a KAR-associated disulfide-linked dimer of a 13-kDa protein that was absent in the Jurkat T cell line and is predicted to participate in these activation signaling events. Upon treatment of NK cells with pervanadate, the disulfide-linked p13 and additional proteins of 25, 30, 37 and 50-95 kDa were identified as KAR-associated tyrosine phosphoproteins. Importantly, p13 was inducibly tyrosine phosphorylated upon cross-linking of NKAT8, which strongly suggests that the associated p13 provides KAR with appropriate cytoplasmic structure to couple with tyrosine kinase-mediated signaling effectors.

CD94/NKG2 inhibitory receptor complex modulates both anti-viral and anti-tumoral responses of polyclonal phosphoantigen-reactive V gamma 9V delta 2 T lymphocytes

Viral, bacterial, protozoal, and cancer-associated Ags elicit strong responses in human gammadelta T lymphocytes. The majority of these cells in the peripheral blood express the Vgamma9Vdelta2-encoded TCR and recognize nonpeptidic phosphoantigens without an apparent MHC restriction. We have shown that Vgamma9Vdelta2 T cells express the inhibitory CD94/NKG2 receptor for HLA class I molecules. The anti-CD94 mAb inhibits 1) the Vgamma9Vdelta2 T cell proliferation in response mycobacterial phosphoantigens and 2) the HIV-induced Vgamma9Vdelta2 T cell expansion. Vgamma9Vdelta2 T cells stimulated with nonpeptidic mycobacterial antigens produce IFN-gamma and TNF-alpha. Signaling through the CD94/NKG2 receptor interferes with the synthesis of these cytokines. The CD94/HLA class I interaction is also involved in the cytotoxic activity of Vgamma9Vdelta2 T cells. The Vgamma9Vdelta2 T cell regulation through the CD94 receptor may be important for the potentially dual function in innate immunity, i.e., 1) NK-like and 2) TCR ligand-induced cytolytic activities.

CD94/NKG2 inhibitory receptor complex modulates both anti-viral and anti-tumoral responses of polyclonal phosphoantigen-reactive V gamma 9V delta 2 T lymphocytes

Viral, bacterial, protozoal, and cancer-associated Ags elicit strong responses in human gammadelta T lymphocytes. The majority of these cells in the peripheral blood express the Vgamma9Vdelta2-encoded TCR and recognize nonpeptidic phosphoantigens without an apparent MHC restriction. We have shown that Vgamma9Vdelta2 T cells express the inhibitory CD94/NKG2 receptor for HLA class I molecules. The anti-CD94 mAb inhibits 1) the Vgamma9Vdelta2 T cell proliferation in response mycobacterial phosphoantigens and 2) the HIV-induced Vgamma9Vdelta2 T cell expansion. Vgamma9Vdelta2 T cells stimulated with nonpeptidic mycobacterial antigens produce IFN-gamma and TNF-alpha. Signaling through the CD94/NKG2 receptor interferes with the synthesis of these cytokines. The CD94/HLA class I interaction is also involved in the cytotoxic activity of Vgamma9Vdelta2 T cells. The Vgamma9Vdelta2 T cell regulation through the CD94 receptor may be important for the potentially dual function in innate immunity, i.e., 1) NK-like and 2) TCR ligand-induced cytolytic activities.

A common inhibitory receptor for major histocompatibility complex class I molecules on human lymphoid and myelomonocytic cells

Natural killer (NK) cell-mediated lysis is negatively regulated by killer cell inhibitory receptors specific for major histocompatibility complex (MHC) class I molecules. In this study, we characterize a novel inhibitory MHC class I receptor of the immunoglobulin-superfamily, expressed not only by subsets of NK and T cells, but also by B cells, monocytes, macrophages, and dendritic cells. This receptor, called Ig-like transcript (ILT)2, binds MHC class I molecules and delivers a negative signal that inhibits killing by NK and T cells, as well as Ca2+ mobilization in B cells and myelomonocytic cells triggered through the B cell antigen receptor and human histocompatibility leukocyte antigens (HLA)-DR, respectively. In addition, myelomonocytic cells express receptors homologous to ILT2, which are characterized by extensive polymorphism and might recognize distinct HLA class I molecules. These results suggest that diverse leukocyte lineages have adopted recognition of self-MHC class I molecules as a common strategy to control cellular activation during an immune response.

[Production of tumor necrosis factor alpha and interleukin-6 by alveolar macrophages from patients with rheumatoid arthritis and interstitial pulmonary disease]

The aim of this study was to measure the production of tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) by alveolar macrophages in patients with rheumatoid arthritis and interstitial lung disease (ILD). Rheumatoid arthritis patients diagnosed by ACR criteria (n = 8) with associated ILD documented by pulmonary function tests and high resolution computerized tomography scanning, and 12 healthy volunteers (6 smokers and 6 nonsmokers). We determined the spontaneous and induced production of bacterial lipopolysaccharides (LSP), TNF-alpha and IL-6 by alveolar macrophages obtained by bronchoalveolar lavage. The macrophages were isolated by Ficoll-Hypaque gradient centrifugation and plastic adherence and cultured in serum-containing medium (low endotoxin) in the presence and absence of LPS (100 ng/ml). TNF-alpha and IL-6 levels contents were determined in supernatants by ELISA. In the patient group both spontaneous and induced production of TNF-alpha were significantly higher than in controls (p < 0.01). Macrophages stimulated with LPS in patients with rheumatoid arthritis and ILD also released greater amounts of IL-6 than did those of the healthy controls. IL-6 spontaneous and induced production was significantly lower in smokers than in nonsmokers. TNF-alpha and IL-6 production in patients with rheumatoid arthritis and ILD, studied in bronchoalveolar lavage specimens reveals that alveolar macrophages are hyperreactive in these patients, who are possibly sensitized as a consequence of the inflammatory lung process inherent to the disease. Further study is needed to define the pathogenic role of these mediators.

The CD94/NKG2-A inhibitory receptor complex is involved in natural killer cell-mediated recognition of cells expressing HLA-G1

Human NK cells bear surface receptors that inhibit their cytolytic activity upon specific recognition of MHC class Ia Ags; little is known about the capacity of class Ib molecules to regulate NK cell function. We have studied the roles of different NK inhibitory receptors in recognition of the class Ib HLA-G. To this end, we analyzed the ability of an HLA-defective tumor cell line (721.221) transfected with the membrane form of HLA-G1, which contains the three external domains, to inhibit the cytolytic activity mediated by a panel of NK clones from several donors. A substantial proportion of peripheral blood NK clones appeared to be significantly inhibited by the HLA-G1-transfected cell line (referred to as .221-G1); nevertheless, no relation was observed between the expression and the function of serologically identifiable Ig-SF receptors (p58/p70) and specific recognition of .221-G1 cells. Moreover, p58 killer cell inhibitory receptor-IgG soluble fusion proteins specifically bound to 721.221 transfectants bearing their corresponding HLA-C ligands, but only a weak reactivity with .221-G1 cells was detectable. By contrast, most NK clones blocked by HLA-G1 expressed the CD94/NKG2-A inhibitory receptor, and moreover, CD94-specific mAbs reconstituted their cytolytic activity comparably to anti-HLA class I mAbs. These data support the idea that the CD94/NKG2 receptor complex is involved in the recognition of cells expressing HLA-G1.

The CD94/NKG2-A inhibitory receptor complex is involved in natural killer cell-mediated recognition of cells expressing HLA-G1

Human NK cells bear surface receptors that inhibit their cytolytic activity upon specific recognition of MHC class Ia Ags; little is known about the capacity of class Ib molecules to regulate NK cell function. We have studied the roles of different NK inhibitory receptors in recognition of the class Ib HLA-G. To this end, we analyzed the ability of an HLA-defective tumor cell line (721.221) transfected with the membrane form of HLA-G1, which contains the three external domains, to inhibit the cytolytic activity mediated by a panel of NK clones from several donors. A substantial proportion of peripheral blood NK clones appeared to be significantly inhibited by the HLA-G1-transfected cell line (referred to as .221-G1); nevertheless, no relation was observed between the expression and the function of serologically identifiable Ig-SF receptors (p58/p70) and specific recognition of .221-G1 cells. Moreover, p58 killer cell inhibitory receptor-IgG soluble fusion proteins specifically bound to 721.221 transfectants bearing their corresponding HLA-C ligands, but only a weak reactivity with .221-G1 cells was detectable. By contrast, most NK clones blocked by HLA-G1 expressed the CD94/NKG2-A inhibitory receptor, and moreover, CD94-specific mAbs reconstituted their cytolytic activity comparably to anti-HLA class I mAbs. These data support the idea that the CD94/NKG2 receptor complex is involved in the recognition of cells expressing HLA-G1.

The CD94 and NKG2-A C-type lectins covalently assemble to form a natural killer cell inhibitory receptor for HLA class I molecules

CD94, a type II membrane protein containing a C-type lectin domain, has been shown to be involved in natural killer (NK) cell-mediated recognition of different HLA allotypes. The inhibitory form of the CD94 receptor has recently been identified by the specific monoclonal antibody (mAb) Z199. Herein, we demonstrate that the inhibitory receptor is in fact a complex formed by the covalent association of CD94 with the NKG2-A molecule (Mr approximately 43 kDa), another member of the C-type lectin superfamily, and that Z199 mAb specifically recognize NKG2-A molecules. Although the NKG2-A-encoding cDNA has been known for several years, the corresponding protein and its possible function remained undefined. Moreover, we show that the NKG2-B protein, an alternatively spliced product of the NKG2-A gene, can also assemble with CD94. Remarkably, both NKG2-A and NKG2-B proteins contain cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIM). This may provide the molecular basis of the inhibitory function mediated by the CD94/NKG2-A receptor complexes.

Implications for immunosurveillance of altered HLA class I phenotypes in human tumours

HLA class I downregulation is a frequent event associated with tumour invasion and development. Altered HLA class I tumour phenotypes can have profound effects on T-cell and natural killer (NK)-cell antitumour responses. Here, Federico Garrido and colleagues analyse these altered tumour phenotypes in detail, indicating their potential relevance for implementation of immunotherapeutic protocols and strategies to overcome tumour escape mechanisms.

Structure and function of the CD94 C-type lectin receptor complex involved in recognition of HLA class I molecules

A multigene family of immunoglobulin superfamily (Ig-SF) killer cell inhibitory receptors (KIRs) specifically recognize HLA class I molecules, while the interaction with H-2 products is mediated by members of the murine Ly49 C-type lectin family. A common structural feature of these receptors with inhibitory function is the presence of cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIMs) that couple them to SHP phosphatases. Strong support for the involvement of the CD94 C-type lectin receptor complex in NK cell-mediated recognition of Bw6+ HLA-B, HLA-A and HLA-C alleles has been obtained. The cloned CD94 molecule covalently assembles with at least two different glycoproteins (43 kDa and 39 kDa) to form functional receptors. NK cells inhibited upon HLA recognition express the CD94/p43 dimer, whose specificity for HLA molecules partially overlaps the Ig-SF receptor system. By contrast, NK clones bearing the homologous CD94/p39 receptor are triggered upon its ligation by CD94-specific mAbs. Remarkably, a set of Ig-SF receptors (p50) homologous to p58 KIRs also display an activating function. CD94-associated molecules belong to the NKG2 family of C-type lectins; the NKG2-A gene encodes for the p43 subunit, which contains cytoplasmic ITIMS. Expression of the different CD94 heterodimeric receptors will enable precise analysis of their putative interaction with HLA class I molecules.

The CD94/NKG2 C-type lectin receptor complex: involvement in NK cell-mediated recognition of HLA class I molecules

A multigene family to human Ig-SF receptors and members of the murine Ly49 C-type lectin family are involved in natural killer (NK) cell-mediated recognition of MHC class I molecules. The human CD94 glycoprotein covalently assembles with different C-type lectins of the NKG2 family. By functional criteria, the CD94/NKG2-A (kp43) receptor complex appears also involved in NK cell-mediated recognition of different HLA class I allotypes. Similarly to the other NK inhibitory receptors, NKG2-A contains cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIMS). By contrast, NK clones bearing different receptor complex (CD94/p39) are triggered upon ligation by CD94-specific monoclonal antibodies (MAbs); the p39 subunit is likely encoded by other member(s) of the NKG2 family. Expression of different CD94/NKG2 complexes is warranted to precisely assess their specific interaction with HLA class I molecules, and the molecular basis for their divergent functional properties.

The human natural killer gene complex is located on chromosome 12p12-p13

Natural killer (NK) cells preferentially express several type II glycoproteins of the calcium-dependent lectin superfamily. The genes coding for these molecules are clustered on the distal mouse chromosome 6 and on the rat chromosome 4 in a region designated the NK gene complex. To date, no definite evidence of the presence of a NK gene complex has been found in humans. Here we report the assignment by fluorescence in situ hybridization of the CD94 gene to human chromosome 12p12-p13, in the same region where the CD69 and NKG2A genes had been previously mapped. In addition, using a yeast artificial chromosome contig spanning this region we determined that the human CD94, NKG2A, NKG2C, NKG2E, and NKR-P1A (NKR) genes map to the short arm of chromosome 12. The distal to proximal position of these loci are: NKR- CD69 - CD94/NKG2A/NKG2C/NKG2E. These data demonstrate the existence of a human NK gene complex located within a 5.6 cM interval flanked by the genetic markers D12S397 and D12S89. The physical distance spanned by the NK gene complex in humans ranges between 0.7 and 2.4 megabases.

Biochemical and serologic evidence for the existence of functionally distinct forms of the CD94 NK cell receptor

The CD94 NK cell receptor is assembled as a disulfide-linked dimer and appears to be encoded by a single-copy gene of the C-type lectin superfamily. In reverse Ab-dependent cellular cytotoxicity assays, CD94-specific mAbs may either trigger or inhibit cytotoxicity in distinct subsets of NK clones, termed groups A and B, respectively. The molecular basis for this functional ambivalence of CD94 has been addressed. CD94 molecules immunoprecipitated with the HP-3B1 mAb from the two different subsets of NK clones were comparatively analyzed by SDS-PAGE. Under reducing conditions, the stimulating form of CD94 from group A clones displayed a significantly lower Mr (39 kDa) than the inhibitory form of group B clones (Mr = 43 kDa). Analyses of N-glycanase and V8 protease-digested samples indicated that the two CD94 forms are homologous. A CD94-specific mAb (Z199) that did not recognize cells transfected with a CD94 cDNA (LL288) was characterized. Z199 did not bind to group A clones, whereas its reactivity with group B NK cells was indistinguishable from that of other CD94-specific mAbs. Different from the HP-3B1 mAb, the Z199 mAb displayed only inhibitory effects in reverse Ab-dependent cellular cytotoxicity assays. Immunoprecipitation studies confirmed that Z199 selectively identified the 43-kDa CD94. Our study proves the existence of at least two biochemically and serologically distinct CD94 molecules, whose selective/predominant expression at the clonal level correlates with the pattern of response (i.e., inhibition vs activation) of NK cells to ligation by CD94-specific mAbs.

Biochemical and serologic evidence for the existence of functionally distinct forms of the CD94 NK cell receptor

The CD94 NK cell receptor is assembled as a disulfide-linked dimer and appears to be encoded by a single-copy gene of the C-type lectin superfamily. In reverse Ab-dependent cellular cytotoxicity assays, CD94-specific mAbs may either trigger or inhibit cytotoxicity in distinct subsets of NK clones, termed groups A and B, respectively. The molecular basis for this functional ambivalence of CD94 has been addressed. CD94 molecules immunoprecipitated with the HP-3B1 mAb from the two different subsets of NK clones were comparatively analyzed by SDS-PAGE. Under reducing conditions, the stimulating form of CD94 from group A clones displayed a significantly lower Mr (39 kDa) than the inhibitory form of group B clones (Mr = 43 kDa). Analyses of N-glycanase and V8 protease-digested samples indicated that the two CD94 forms are homologous. A CD94-specific mAb (Z199) that did not recognize cells transfected with a CD94 cDNA (LL288) was characterized. Z199 did not bind to group A clones, whereas its reactivity with group B NK cells was indistinguishable from that of other CD94-specific mAbs. Different from the HP-3B1 mAb, the Z199 mAb displayed only inhibitory effects in reverse Ab-dependent cellular cytotoxicity assays. Immunoprecipitation studies confirmed that Z199 selectively identified the 43-kDa CD94. Our study proves the existence of at least two biochemically and serologically distinct CD94 molecules, whose selective/predominant expression at the clonal level correlates with the pattern of response (i.e., inhibition vs activation) of NK cells to ligation by CD94-specific mAbs.

Clonotypic differences in signaling from CD94 (kp43) on NK cells lead to divergent cellular responses

Ligation of MHC class I-recognizing receptors on NK cells dramatically modulates their secretory and cytotoxic function. This study focuses on characterizing key signaling events regulating these activities after ligation of the C-type lectin superfamily member, CD94. We isolated separate clonal populations of human NK cells in which ligation of CD94 (kp43) either triggered cell-mediated cytotoxicity (group A clones) or potently inhibited NK cell activation (group B clones). We then evaluated the proximal signaling events that regulate these alternative responses. CD94 stimulation of group A clones induced the rapid activation of intracellular protein tyrosine kinases (i.e., lck and ZAP-70), phospholipase C, and phosphatidylinositol 3-kinase. In contrast, CD94 ligation on group B clones had none of the above noted effects and instead inhibited the FcR-induced tyrosine phosphorylations of ZAP-70 and phospholipase C-gamma 2, the formation of phospho-zeta/ZAP-70 complexes, and the release of inositol phosphates. These results define distinct proximal signaling events initiated after CD94 ligation and suggest that clonotypic differences in signaling generate fundamentally different NK cell-mediated responses.

Clonotypic differences in signaling from CD94 (kp43) on NK cells lead to divergent cellular responses

Ligation of MHC class I-recognizing receptors on NK cells dramatically modulates their secretory and cytotoxic function. This study focuses on characterizing key signaling events regulating these activities after ligation of the C-type lectin superfamily member, CD94. We isolated separate clonal populations of human NK cells in which ligation of CD94 (kp43) either triggered cell-mediated cytotoxicity (group A clones) or potently inhibited NK cell activation (group B clones). We then evaluated the proximal signaling events that regulate these alternative responses. CD94 stimulation of group A clones induced the rapid activation of intracellular protein tyrosine kinases (i.e., lck and ZAP-70), phospholipase C, and phosphatidylinositol 3-kinase. In contrast, CD94 ligation on group B clones had none of the above noted effects and instead inhibited the FcR-induced tyrosine phosphorylations of ZAP-70 and phospholipase C-gamma 2, the formation of phospho-zeta/ZAP-70 complexes, and the release of inositol phosphates. These results define distinct proximal signaling events initiated after CD94 ligation and suggest that clonotypic differences in signaling generate fundamentally different NK cell-mediated responses.