A novel mechanism of antitumor response involving the expansion of CD3+/CD56+ large granular lymphocytes triggered by a tumor-expressed activating ligand

We describe a patient with acute myeloid leukemia (AML) who developed polyclonal large granular lymphocyte (LGL) proliferation. The reciprocal evolution of AML and LGLs suggested that these LGLs had an anti-tumor activity. The patient's LGLs killed autologous leukemia cells in a different way to classical T lymphocyte-mediated cytotoxicity since it did not rely on the recognition of target antigens presented by major histocompatibility complex (MHC) class I molecules by the CD3/TcRalphabeta complex. This killing was also different from natural killer (NK)-mediated cytotoxicity, which depends on the absence of MHC class I molecule recognition by NK inhibitory receptors. The LGLs were polyclonal, had a CD3+/CD8+/CD56+ phenotype, and did not express the natural killer cell receptors (NKRs) for MHC class I molecules. The LGLs did not express the NK-specific activating natural cytotoxicity receptors but expressed the 2B4 non-MHC restricted triggering receptor, whose ligand CD48 was expressed by leukemic cells and normal bone marrow cells. The 2B4 receptor participated in the ability of LGLs to lyse patient's leukemia. This represents a novel function for 2B4 in man, since this molecule, at variance with the murine system, was considered not to have direct effects on CD8+ T cell-mediated cytotoxicity. This case report allowed us to describe a novel T lymphocyte-mediated anti-tumor mechanism which relied on (1) the abnormal expansion of the rare 2B4-positive CD3+/CD8+/CD56+ T lymphocyte subset, (2) an as yet undescribed cytotoxicity mechanism in man which depended on 2B4 molecule. The relevance of this observation in human cancer immunotherapy has to be further investigated.

Natural killer lymphocytes: "null cells" no more

Natural Killer (NK) lymphocytes were initially described as potent effector cells that, unlike T lymphocytes, were able to kill targets in the absence of a priori stimulation and without specific recognition mechanisms. Over the past ten years however, it has been clearly demonstrated that NK cell function is regulated by a number of surface receptors that bind specific ligands expressed by target cells. Some of these receptors display inhibitory functions and recognize MHC class I molecules expressed by normal autologous cells that, as a consequence, are spared from indiscriminate NK-mediated killing. Other receptors are involved in NK cell activation against allogeneic cells or cells that, upon viral infection or tumor transformation, down-regulate MHC Class I expression. Altogether these data provide important advances toward the understanding of the complexity of the molecular mechanisms that regulate NK-mediated functions.

Human natural killer cell receptors and co-receptors

In the absence of sufficient signaling by their HLA class I-specific inhibitory receptors, human natural killer (NK) cells become activated and display potent cytotoxicity against cells that are either HLA class I negative or deficient. This indicates that the NK receptors responsible for the induction of cytotoxicity recognize ligands on target cells different from HLA class I molecules. On this basis, the process of NK-cell triggering can be considered as a mainly non-MHC-restricted mechanism. The recent identification of a group of NK-specific triggering surface molecules has allowed a first series of pioneering studies on the functional/molecular characteristics of such receptors. The first three members of a receptor family that has been termed natural cytotoxicity receptors (NCR) are represented by NKp46, NKp44 and NKp30. These receptors are strictly confined to NK cells, and their engagement induces a strong activation of NK-mediated cytolysis. A direct correlation exists between the surface density of NCR and the ability of NK cells to kill various target cells. Importantly, mAb-mediated blocking of these receptors has been shown to suppress cytotoxicity against most NK-susceptible target cells. However, the process of NK-cell triggering during target cell lysis may also depend on the concerted action of NCR and other triggering receptors, such as NKG2D, or surface molecules, including 2B4 and NKp80, that appear to function as co-receptors rather than as true receptors. Notably, a dysfunction of 2B4 has been associated with a severe form of immunodeficiency termed X-linked lymphoproliferative disease. Future studies will clarify whether also the altered expression and/or function of other NK-triggering molecules may represent a possible cause of immunological disorders.

CD4(+) cutaneous T-cell lymphoma cells express the p140-killer cell immunoglobulin-like receptor

Tumor cells of patients with cutaneous T-cell lymphoma (CTCL) have the cell surface phenotype of mature T-helper lymphocytes, and it may be impossible to differentiate them from nonmalignant lymphocytes in skin and blood. Until now, no specific cell membrane marker of CTCL has been reported. In the current study, it is reported for the first time that CTCL cells express the major histocompatibility complex class I binding p140-killer cell immunoglobulin-like receptor, which has been described on a minor subset of natural killer lymphocytes and on a marginal circulating CD8(+) T lymphocyte subset. Interestingly, the molecular characterization of this KIR expressed by CTCL allowed us to isolate a novel allelic form of p140-KIR3DL, resulting in 4 amino acid substitutions, 3 in the extracellular immunoglobulin-like domain of the protein and one in the cytoplasmic region. This finding is likely to be important both for the pathophysiology and for the clinical treatment of patients with CTCL.

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

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

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

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

Involvement of natural cytotoxicity receptors in human natural killer cell-mediated lysis of neuroblastoma and glioblastoma cell lines

The surface receptors involved in natural killer (NK) cell triggering during the process of target cell lysis have been at least in part identified. These are members of a novel family of receptors that has been termed natural cytotoxicity receptors (NCR). The first three members of this emerging group of receptors are the NKp46, NKp44 and NKp30 molecules that all belong to the immunoglobulin superfamily. Blocking of these receptors inhibits NK-mediated cytotoxicity against a wide variety of tumor target cells. In the present study, we show that these NCR are also involved in NK-mediated killing of tumor cells of neural origin. Glioblastoma and neuroblastoma target cells were efficiently killed by all NK clones analyzed since little protection from NK lysis was mediated by HLA class I molecules. Blocking of one or another NCR inhibited cytotoxicity; however, optimal inhibition was only observed when the three receptors were blocked simultaneously. A sharp difference in cytotoxicity against neural tumors was demonstrated between NCR(bright) and NCR(dull) NK clones, further supporting the notion that NCR play a critical role in the induction of cytotoxicity against tumor target cells of different histotype. Finally, our data also indicate that CD16 does not function as a triggering receptor involved in lysis of neural tumors since no difference in cytotoxicity could be substantiated between CD16(+) and CD16(-) NK clones and no correlation could be detected between the NCR(bright)/NCR(dull) phenotype and CD16 expression.

2B4 functions as a co-receptor in human NK cell activation

Natural cytotoxicity receptors (NKp46, NKp44 and NKp300) play a predominant role in human NK cell triggering during natural cytotoxicity. Human 2B4 also induced NK cell activation in redirected killing assays using anti-2B4 monoclonal antibodies (mAb) and murine targets. Since this effect was confined to a fraction of NK cells, this suggested a functional heterogeneity of 2B4 molecules. Here we show that activation via 2B4 in redirected killing against murine targets is strictly dependent upon the engagement of NKp46 by murine ligand (s) on target cells. Thus, NK cell clones expressing high surface density of NKp46 (NKp46bright) were triggered by anti-2B4 mAb, whereas NKp46dull clones were not although they expressed a comparable surface density of 2B4. mAb-mediated modulation of NKp46 molecules in NKp46bright clones had no effect on the expression of 2B4 while it rendered cells unresponsive to anti-2B4 mAb. Finally, anti-2B4 mAb could induce NK cell triggering in NKp46dull clones provided that suboptimal doses of anti-NKp44 or anti-CD16 mAb were added to the redirected killing assay. These results indicate that differences in responses do not reflect a functional heterogeneity of 2B4 but rather depend on the co-engagement of triggering receptors.

Molecular and functional characterization of IRp60, a member of the immunoglobulin superfamily that functions as an inhibitory receptor in human NK cells

In this study we describe the functional and molecular characterization of IRp60 (inhibitory receptor protein 60), an inhibitory receptor expressed on all human NK cells. The IRp60 molecule has been identified by the generation of three novel monoclonal antibodies (mAb). Cross-linking of IRp60 by specific mAb strongly inhibits the spontaneous cytotoxicity of NK cells as well as the NK-mediated cytolytic activity induced via different non-HLA-specific or HLA-specific activating receptors. IRp60 is a 60-kDa glycoprotein that, upon sodium pervanadate treatment, becomes tyrosine phosphorylated and associates with the SH2-containing phosphatases SHP-1 and SHP-2. The IRp60 gene is located on human chromosome 17 and encodes a molecule belonging to the immunoglobulin (Ig) superfamily characterized by a single V-type Ig-like domain in the extracellular portion. The cytoplasmic tail contains three classical immunoreceptor tyrosine-based inhibitory motifs. Southern blot analysis revealed cross-hybridization with monkey and mouse genomic DNA, thus suggesting that IRp60 may be conserved among different species. Moreover, based on the use of different anti-IRp60 mAb, we could identify two IRp60 allelic variants. Since IRp60 is also expressed by other cell types, including T cell subsets, monocytes and granulocytes, it may play a more general role in the negative regulation of different leukocyte populations.

Identification and molecular cloning of p75/AIRM1, a novel member of the sialoadhesin family that functions as an inhibitory receptor in human natural killer cells

In this study, by the generation of a specific monoclonal antibody, we identified p75/AIRM1 (for adhesion inhibitory receptor molecule 1), a novel inhibitory receptor that is mostly confined to human natural killer cells. p75/AIRM1 is a 75-kD glycoprotein that, upon sodium pervanadate treatment, becomes tyrosine phosphorylated and associates to src homology 2 domain-bearing protein tyrosine phosphatase (SHP)-1. The p75/AIRM1 gene is located on human chromosome 19 and encodes a novel member of the sialoadhesin family characterized by three immunoglobulin-like extracellular domains (one NH(2)-terminal V-type and two C2-type) and a classical immunoreceptor tyrosine-based inhibitory motif (ITIM) in the cytoplasmic portion. The highest amino acid sequence similarity has been found with the myeloid-specific CD33 molecule and the placental CD33L1 protein. Similar to other sialoadhesin molecules, p75/AIRM1 appears to mediate sialic acid-dependent ligand recognition.

NKp46 is the major triggering receptor involved in the natural cytotoxicity of fresh or cultured human NK cells. Correlation between surface density of NKp46 and natural cytotoxicity against autologous, allogeneic or xenogeneic target cells

NKp46 is a novel triggering receptor expressed by all human NK cells that is involved in natural cytotoxicity. In this study we show that the surface density of NKp46 may vary in different NK cells and that a precise correlation exists between the NKp46 phenotype of NK clones and their natural cytotoxicity against HLA-class I-unprotected allogeneic or xenogeneic cells. Thus, NKp46bright clones efficiently lysed human and murine tumor cells while NKp46dull clones were poorly cytolytic against both types of target cells. We also show that the NKp46 phenotype of NK clones correlates with their ability to lyse HLA-class I-unprotected autologous cells. Finally, NKp46 was found to be deeply involved in the natural cytotoxicity mediated by freshly derived NK cells. This was indicated both by the inhibition of cytolysis after monoclonal antibody-mediated masking of NKp46 and by the correlation existing between the natural cytotoxicity of fresh NK cells derived from different donors and their NKp46 phenotype. In conclusion, these studies strongly support the concept that NKp46 plays a central role in the physiological triggering of NK cells and, as a consequence (in concert with killer inhibitory receptors), in the NK-mediated clearance of abnormal cells expressing inadequate amounts of HLA-class I molecules.

Molecular cloning of NKp46: a novel member of the immunoglobulin superfamily involved in triggering of natural cytotoxicity

NKp46 has been shown to represent a novel, natural killer (NK) cell-specific surface molecule, involved in human NK cell activation. In this study, we further analyzed the role of NKp46 in natural cytotoxicity against different tumor target cells. We provide direct evidence that NKp46 represents a major activating receptor involved in the recognition and lysis of both human and murine tumor cells. Although NKp46 may cooperate with other activating receptors (including the recently identified NKp44 molecule) in the induction of NK-mediated lysis of human tumor cells, it may represent the only human NK receptor involved in recognition of murine target cells. Molecular cloning of the cDNA encoding the NKp46 molecule revealed a novel member of the immunoglobulin (Ig) superfamily, characterized by two C2-type Ig-like domains in the extracellular portion. The transmembrane region contains the positively charged amino acid Arg, which is possibly involved in stabilizing the association with CD3zeta chain. The cytoplasmic portion, spanning 30 amino acids, does not contain immunoreceptor tyrosine-based activating motifs. Analysis of a panel of human/hamster somatic cell hybrids revealed segregation of the NKp46 gene on human chromosome 19. Assessment of the NKp46 mRNA expression in different tissues and cell types unambiguously confirmed the strict NK cell specificity of the NKp46 molecule. Remarkably, in line with the ability of NKp46 to recognize ligand(s) on murine target cells, the cDNA encoding NKp46 was found to be homologous to a cDNA expressed in murine spleen. In conclusion, this study reports the first characterization of the molecular structure of a NK-specific receptor involved in the mechanism of NK cell activation during natural cytotoxicity.

NKp44, a novel triggering surface molecule specifically expressed by activated natural killer cells, is involved in non-major histocompatibility complex-restricted tumor cell lysis

After culture in interleukin (IL)-2, natural killer (NK) cells acquire an increased capability of mediating non-major histocompatibility complex (MHC)-restricted tumor cell lysis. This may reflect, at least in part, the de novo expression by NK cells of triggering receptors involved in cytolysis. In this study we identified a novel 44-kD surface molecule (NKp44) that is absent in freshly isolated peripheral blood lymphocytes but is progressively expressed by all NK cells in vitro after culture in IL-2. Different from other markers of cell activation such as CD69 or VLA.2, NKp44 is absent in activated T lymphocytes or T cell clones. Since NKp44 was not detected in any of the other cell lineages analyzed, it appears as the first marker specific for activated human NK cells. Monoclonal antibody (mAb)-mediated cross-linking of NKp44 in cloned NK cells resulted in strong activation of target cell lysis in a redirected killing assay. This data indicated that NKp44 can mediate triggering of NK cell cytotoxicity. mAb-mediated masking of NKp44 resulted in partial inhibition of cytolytic activity against certain (FcgammaR-negative) NK-susceptible target cells. This inhibition was greatly increased by the simultaneous masking of p46, another recently identified NK-specific triggering surface molecule. These data strongly suggest that NKp44 functions as a triggering receptor selectively expressed by activated NK cells that, together with p46, may be involved in the process of non-MHC-restricted lysis. Finally, we show that p46 and NKp44 are coupled to the intracytoplasmic transduction machinery via the association with CD3zeta or KARAP/DAP12, respectively; these associated molecules are tyrosine phosphorylated upon NK cell stimulation.

The activating form of CD94 receptor complex: CD94 covalently associates with the Kp39 protein that represents the product of the NKG2-C gene

Inhibitory receptor complexes formed by CD94 and NKG2-A (Kp43) molecules have been implicated in HLA class I recognition by human natural killer (NK) cells. Additional forms of CD94 receptors have recently been described in NK cells characterized by the lack of NKG2-A expression. These CD94 receptors were shown to display activating functions. Immunoprecipitation with anti-CD94 monoclonal antibodies (mAb) led to the identification, in these cells, of a 39-kDa (Kp39) molecule that was originally believed to represent an activating isoform of the CD94 molecules. In the present study we show that the Kp39 molecule is covalently associated with CD94 and displays a protein backbone (26 kDa) similar to that of NKG2-A (Kp43) glycoproteins. Peptide mapping analysis indicates that Kp39 and NKG2-A glycoproteins belong to the same molecular family. A novel NKG2-specific mAb (termed P25) has been generated that specifically reacts with both NKG2-A and NKG2-C molecules, but fails to recognize NKG2-E molecules. Analysis of polyclonal and clonal NK cells shows that P25 mAb reacts with all NKG2-A+ cells and with a fraction of CD94+ cells lacking the expression of NKG2-A. These data indicate that NKG2-C molecules are indeed expressed only in a subset of cells lacking the expression of NKG2-A. The CD94-associated Kp39 molecule can be detected only in NKG2-A- P25+ cells, i.e. cells expressing NKG2-C molecules. Indeed, reverse transcription-polymerase chain reaction analysis performed on a large panel of NK clones indicates that NKG2-A- P25+ NK clones express the NKG2-C transcript. Notably, the cytolytic activity of these clones can be triggered by the P25 mAb in redirected killing analysis. Finally, biochemical analysis of COS7 cells cotransfected with CD94 and NKG2-C demonstrates the identity between Kp39 and NKG2-C molecules. Altogether, our data demonstrate that NKG2-C molecules associate with CD94 to form an activating receptor complex in a subset of human NK cells.

Function and specificity of human natural killer cell receptors

In humans, natural killer lymphocytes express HLA class I-specific inhibitory receptors belonging to at least two different molecular families. The first is represented by members of the Ig superfamily that are involved in the recognition of different groups of HLA class I alleles, and the second is represented by a molecular complex formed by CD94 and NKG2A that displays a broad specificity for various class I molecules including the 'non-classical' HLA-G molecules. In addition to the inhibitory receptors, a series of activating receptors has been identified. Some display the same specificities as the corresponding inhibiting receptors and can be viewed as HLA class I-specific activating receptors. Another group of activating receptors appear to be involved in the cytolytic activity against HLA-'negative' target cells. These receptors are clearly non-MHC specific and, under physiological conditions, their function is suppressed by the HLA class I-specific inhibitory receptors.

Role of amino acid position 70 in the binding affinity of p50.1 and p58.1 receptors for HLA-Cw4 molecules

In an attempt to identify the amino acid position(s) of the HLA-C-specific p58.1/p50.1 natural killer cell receptors that determine the binding affinity for their ligand, we used soluble fusion proteins formed by the ectodomain of either receptor and the Fc portion of human IgG1. We show that the soluble p50.1 (activating) receptor binds weakly to 221-Cw4 transfectants. In contrast, the soluble p58.1 (inhibitory) receptor binds with high affinity. A single amino acid mutation at position 70, obtained by site-directed mutagenesis, was found to affect the binding affinity of both the p50.1 and the p58.1 receptors. Thus, substitution in p50.1 of lysine 70 by threonine (typical of the inhibitory p58.1 molecule) resulted in a dramatic increase in binding affinity, comparable to that of the p58.1 molecule. On the other hand, substitution of threonine 70 by lysine in p58.1 almost abolished binding to 221-Cw4 cells. Our present data indicate that a single amino acid difference greatly influences the p58.1/p50.1 affinity for their HLA-C ligand and suggests a possible role of position 70 as a contact site in the natural killer cell receptor/major histocompatibility complex class I interaction.

p46, a novel natural killer cell-specific surface molecule that mediates cell activation

Limited information is available on the surface molecules that are involved in natural killer (NK) cell triggering. In this study, we selected the BAB281 monoclonal antibody (mAb) on the basis of its ability to trigger NK-mediated target cell lysis. BAB281 identified a novel NK cell-specific surface molecule of 46 kD (p46) that is expressed by all resting or activated NK cells. Importantly, unlike the NK cell antigens identified so far, the expression of p46 was strictly confined to NK cells. Upon mAb-mediated cross-linking, p46 molecules induced strong cell triggering leading to [Ca2+]i increases, lymphokine production, and cytolytic activity both in resting NK cells and NK cell clones. The p46-mediated induction of Ca2+ increases or triggering of cytolytic activity was downregulated by the simultaneous engagement of inhibitory receptors including p58, p70, and CD94/NKG2A. Both the unique cellular distribution and functional capability of p46 molecules suggest a possible role in the mechanisms of non-major histocompatibility complex-restricted cytolysis mediated by human NK cells.

HLA-G recognition by human natural killer cells. Involvement of CD94 both as inhibitory and as activating receptor complex

The lack of classical human histocompatibility leukocyte antigen (HLA) molecules in human placenta prevents the recognition and lysis by maternal T lymphocytes but poses the problem of susceptibility to natural killer (NK) cell-mediated lysis. The nonclassical HLA class I molecule HLA-G may mediate protection from NK cells. NK cells are known to express a number of HLA class I-specific inhibitory receptors. These include members of the immunoglobulin (Ig) superfamily (p58, p70, p140), characterized by a defined allele specificity, and CD94/NKG2A with a broad specificity for different HLA class I molecules. We analyzed a series of NK cell clones derived from normal peripheral blood expressing different NK receptors (NKR). Clones were analyzed for their cytolytic activity against the HLA class I-negative 221 cell line either untransfected or transfected with HLA-G (221/G) or other informative alleles, as control. All clones expressing CD94/NKG2A [as identified by the Z199 monoclonal antibody (mAb)] displayed a markedly reduced cytolytic activity against 221/G. Moreover, mAb directed to the CD94/NKG2A complex completely restored target cell lysis. Among NKG2A-negative NK clones, different functional patterns could be detected. Clones expressing inhibitory receptors belonging to the Ig superfamily lysed 221/G target cells with equal or higher efficiency than untransfected 221 cells. These data indicated that p58, p70 and p140 do not function as HLA-G-specific inhibitory NKR, and that HLA-G-specific activating NKR also exist. Further analysis indicated that in these clones (characterized by the CD94+/NKG2A- phenotype) mAb specific for CD94, but not for the other NKR, reversed the activating effect. Infrequent clones were also isolated that, in spite of the lack of CD94/NKG2A, displayed HLA-G specificity, thus suggesting the existence of a different, still unknown NKR.

CD94 functions as a natural killer cell inhibitory receptor for different HLA class I alleles: identification of the inhibitory form of CD94 by the use of novel monoclonal antibodies

CD94 molecules have been suggested to function as inhibitory natural killer cell (NK) receptors involved in the recognition of HLA-B alleles sharing the Bw6 supertypic specificity. In this study, we show that CD94 molecules may play a more general role: they are also involved in the recognition of other HLA class I molecules, including HLA-C and at least some HLA-A alleles. The inhibitory effect mediated by CD94 molecules on NK cytolytic activity is lower in magnitude than that of bona fide inhibitory receptors such as p58 or p70. Distinct from the other human NK receptors involved in HLA class I recognition, CD94 is expressed on virtually all NK cells. In addition, it has been shown to be functionally heterogeneous since, in different clones, CD94 mediated either cell triggering or inhibition. Although NK cells expressing inhibitory CD94 molecules are usually characterized by a CD94bright phenotype, there is no precise correlation between fluorescence intensity and inhibitory or activating function. Here, we describe two novel monoclonal antibodies (mAb) which selectively recognize inhibitory CD94 molecules and bind to a subset (variable in size among different donors) of CD94bright cells. The use of these mAb allows the direct assessment of NK cells expressing inhibitory CD94 receptors both at the population and at the clonal level.

A novel surface molecule homologous to the p58/p50 family of receptors is selectively expressed on a subset of human natural killer cells and induces both triggering of cell functions and proliferation

Human natural killer (NK) cells express inhibitory (p58) or activatory (p50) receptors for HLA-C alleles. Here, we describe a novel member of the p58/p50 family that is expressed by a subset of NK cells in about one third of donors. This molecule, termed p50.3, mediates NK cell triggering as revealed by the induction of intracellular free calcium mobilization, cytokine release and cytotoxicity. In addition, anti-p50.3 monoclonal antibody (mAb) induced a selective, strong proliferation of p50.3+ NK cells in peripheral blood lymphocytes. Although p50.3 molecules do not appear to display an obvious HLA class I specificity, they are usually coexpressed with known inhibitory receptors for HLA class I alleles, mAb-mediated cross-linking of these receptors leads to inhibition of the anti-p50.3 mAb-induced NK cell activation and proliferation. Surface p50.3 molecules are glycoproteins of approximately 55-58 kDa which, upon deglycosylation, display a relative molecular mass of 36 kDa, similar to that of deglycosylated (activatory) p50 receptors. Analysis of the two-dimensional peptide maps of the 50.3 molecules revealed a high homology with the other HLA-C-specific p58/p50 receptors. The use of a set of oligodeoxynucleotide primers, previously shown to amplify the activatory (p50) forms of HLA-C-specific receptors, consistently amplified in p50.3+ clones a cDNA sequence termed KKA3. This sequence belongs to the p58/p50 multigene family, that encodes for a transmembrane protein specifically stained by anti-p50.3 mAb in cell transfectants. Similar to p50 molecules, the KKA3-encoded molecules are characterized by two extracellular immunoglobulin-like domains, by the presence of a lysine in the transmembrane region and a short (39 amino acids) cytoplasmic tail which does not contain immune receptor tyrosine-based activation motifs (ITAM)-like sequences.

Physical and functional independency of p70 and p58 natural killer (NK) cell receptors for HLA class I: their role in the definition of different groups of alloreactive NK cell clones

Natural killer (NK) cells express clonally distributed receptors for different groups of HLA class I alleles. The Z27 monoclonal antibody described in this study recognizes a p70 receptor specific for HLA-B alleles belonging to the Bw4 supertypic specificity. Single amino acid substitutions in the peptide-binding groove of HLA-B2705 molecules influenced the recognition by some, but not all, p7O/Z27+ clones. This suggests the existence of a limited polymorphism within the p7O family of receptors. The pattern of reactivity of monoclonal antibody Z27 revealed that Bw4-specific receptors may be expressed alone or in combination with different (GL183 and/or EB6) p58 molecules. Analysis of NK clones coexpressing p58 and p7O receptors allowed us to demonstrate that the two molecules represent physically and functionally independent receptors. The expression of p7O molecules either alone or in combination with EB6 molecules provided the molecular basis for understanding the cytolytic pattern of two previously defined groups of "alloreactive" NK cell clones ("group 3" and "group 5").

Existence of both inhibitory (p58) and activatory (p50) receptors for HLA-C molecules in human natural killer cells

The natural killer (NK) cell-specific p58 molecules EB6 and GL183 have been shown to represent the putative surface receptors for two distinct groups of human histocompatibility leukocyte antigen (HLA) C alleles. Interaction between p58 receptors and class I molecules expressed on target cells results in inhibition of the NK-mediated cytolytic activity and thus in target cell protection. In the present study, we show that EB6 molecules may also act as receptors mediating NK cell triggering. Activatory EB6 molecules were found to be confined only to certain donors. Moreover, in these donors, only a fraction of EB6+ NK clones expressed the activatory form of EB6 molecules, while the remaining clones expressed the conventional inhibitory form. Biochemical analysis of the activatory EB6 molecules revealed a molecular mass of approximately 50 kD (p50), thus differing from the 58-kD inhibitory form. This difference was not due to differential glycosylation of the same protein, as revealed by deglycosylation experiments of isolated EB6 molecules. Treatment of purified p58 or p50/EB6 molecules with proteolytic enzymes, including V8-protease, chymotrypsin, and papain, showed only minor differences in the resulting peptides. Treatment with pepsin followed by two-dimensional peptide mapping demonstrated that, although the majority of peptides migrated in identical positions, differences between the two forms could be detected for at least one major peptide. Anti-EB6 monoclonal antibody (mAb)-mediated cross-linking of p50 molecules was required to trigger the cytolytic activity and the intracellular calcium ([Ca+2]i) increases in appropriate NK clones. Likewise, mAb-mediated cross linking of the p58 EB6 molecules was needed to inhibit the cytolytic activity; however, in this case, no [Ca+2]i increases could be detected. In NK clones expressing the inhibitory p58 EB6 receptors, soluble anti-EB6 mAb prevented recognition of protective Cw4 molecules and reconstituted target cell lysis. In contrast, in clones expressing the activatory p50/EB6 receptor, EB6 masking frequently resulted in partial inhibition of the cytolytic activity against Cw4+ target cells. Therefore, it appears that NK clones expressing the p50/EB6 receptors are induced to lyse Cw4+ target cells upon specific interaction with Cw4 molecules. This concept was further substantiated by experiments in which target cells were represented by the HLA-negative LCL721.221 cell line transfected with the Cw4 allele. Phenotypic and functional analysis of a large number of NK clones showed that clones expressing the activatory p50/EB6 molecules consistently coexpressed inhibitory receptors for other HLA class I alleles.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional ambivalence of the Kp43 (CD94) NK cell-associated surface antigen

We originally characterized the Kp43 (CD94) surface Ag, whose expression is restricted to human NK cells and a minor T lymphocyte subset. As shown in the present study, anti-Kp43 mAbs but not their F(ab')2 fragments markedly inhibited the cytolytic activity of polyclonal-activated NK cells in a redirected lysis assay against the murine P815 cell line. Furthermore, anti-Kp43 mAbs also down-regulated the CD16-dependent redirected killing and PHA-induced lectin-dependent cellular cytotoxicity. However, the intensity of the inhibitory effect was variable and no significant down-regulation of cytotoxicity was substantiated in NK cell populations from some individuals. A similar variability in the responsiveness to anti-Kp43 mAb was noticed when fresh CD3- lymphocyte populations were tested: in some donors we observed the induction of redirected lysis, whereas in other samples the Kp43-specific mAb inhibited cytotoxicity. The analysis of single cell-derived microcultures provided a clue to interpret the variable responsiveness of polyclonal cell populations; remarkably, the cytolytic activity of some NK clones was inhibited, whereas that of others was either stimulated or unaffected. The pattern of responsiveness in the cytolytic assay correlated with TNF production upon stimulation with solid phase-bound anti-Kp43 mAbs. The different types of clones could be derived from the same individual, although their relative proportions varied from donor to donor. Moreover, Kp43 appeared to be coupled differently to signal transduction pathways, because (Ca2+)i mobilization in the presence of the Kp43-specific mAbs was substantiated only in clones that were activated in the redirected lysis assay.

Functional ambivalence of the Kp43 (CD94) NK cell-associated surface antigen

We originally characterized the Kp43 (CD94) surface Ag, whose expression is restricted to human NK cells and a minor T lymphocyte subset. As shown in the present study, anti-Kp43 mAbs but not their F(ab')2 fragments markedly inhibited the cytolytic activity of polyclonal-activated NK cells in a redirected lysis assay against the murine P815 cell line. Furthermore, anti-Kp43 mAbs also down-regulated the CD16-dependent redirected killing and PHA-induced lectin-dependent cellular cytotoxicity. However, the intensity of the inhibitory effect was variable and no significant down-regulation of cytotoxicity was substantiated in NK cell populations from some individuals. A similar variability in the responsiveness to anti-Kp43 mAb was noticed when fresh CD3- lymphocyte populations were tested: in some donors we observed the induction of redirected lysis, whereas in other samples the Kp43-specific mAb inhibited cytotoxicity. The analysis of single cell-derived microcultures provided a clue to interpret the variable responsiveness of polyclonal cell populations; remarkably, the cytolytic activity of some NK clones was inhibited, whereas that of others was either stimulated or unaffected. The pattern of responsiveness in the cytolytic assay correlated with TNF production upon stimulation with solid phase-bound anti-Kp43 mAbs. The different types of clones could be derived from the same individual, although their relative proportions varied from donor to donor. Moreover, Kp43 appeared to be coupled differently to signal transduction pathways, because (Ca2+)i mobilization in the presence of the Kp43-specific mAbs was substantiated only in clones that were activated in the redirected lysis assay.

Coexpression of two functionally independent p58 inhibitory receptors in human natural killer cell clones results in the inability to kill all normal allogeneic target cells

In the present study, we define a group of natural killer (NK) clones (group 0) that fails to lyse all of the normal allogeneic target cells analyzed. Their specificity for HLA class I molecules was suggested by their ability to lyse class I-negative target cells and by the fact that they could lyse resistant target cells in the presence of selected anti-class I monoclonal antibodies. The use of appropriate target cells represented by either HLA-homozygous cell lines or cell transfectants revealed that these clones recognized all the HLA-C alleles. By the use of monoclonal antibodies directed to either GL183 or EB6 molecules, we showed that the EB6 molecules were responsible for the recognition of Cw4 and related alleles, while the GL183 molecules recognized Cw3 (and related C alleles). These data suggest that the GL183 and the EB6 molecules can function, in individual NK clones, as independent receptors for two different groups of HLA-C alleles, (which include all known alleles for locus C), thus resulting in their inability to lyse all normal HLA-C+ target cells. Indirect immunofluorescence and fluorescence-activated cell sorting analysis revealed that the presently defined GL183+EB6+ group 0 NK clones brightly express EB6 molecules (EB6bright) while the GL183+EB6+ group 2 clones (unable to recognize Cw4) express an EB6dull phenotype. These data also imply that the density of EB6 receptors may be critical for the generation of an optimal negative signal upon interaction with appropriate HLA-C alleles.

General role of HLA class I molecules in the protection of target cells from lysis by natural killer cells: evidence that the free heavy chains of class I molecules are not sufficient to mediate the protective effect

Some HLA-C alleles have been shown to exert a specific protective effect preventing target cells from lysis by groups of natural killer (NK) clones displaying a defined specificity. In this study, we analyzed whether class I-mediated protection is a more general phenomenon involving all NK cells. First, we utilized two anti-class I mAbs (6A4 of IgG1 isotype and A6-136 of IgM isotype), which had been shown to induce lysis of protected target cells by group 1 and group 2 NK clones. Addition of A6-136 or 6A4 used as F(ab')2 mAb resulted in lysis of protected target cells by all NK clones analyzed. Target cells were represented by a panel of HLA homozygous Epstein-Barr virus-transformed B cell lines (B-EBV) while NK clones were representative of clones displaying different GL183/EB6 surface phenotypes and/or different abilities to lyse allogeneic cells. Unselected NK clones derived from seven different individuals were tested against autologous target cells represented by phytohemagglutinin-induced blasts or B-EBV transformed cell lines. In both instances, addition of a mixture of 6A4 F(ab')2 and A6-136 mAbs resulted in lysis of autologous target cells, thus suggesting that class I molecules prevent lysis of normal cells by self NK cells. We further investigated whether the class I-mediated protection requires the complexed form of class I molecules (composed of alpha chain, beta 2-microglobulin and the antigen peptide) or rather the free alpha chain. Acidic treatment of the C1R (Cw4+) target cells or 81.22 (Cw3+, Cw4+) at pH 2.2 resulted in loss of reactivity with 6A4, A6-136 and W6-32 mAb (known to react with the assembled form of class I molecules) and in the de novo reactivity with L31 mAb (specific for the HLA-C free chain). While the untreated Cw+ C1R cells were resistant to lysis by the Cw4-specific group 1 NK clones, the pH 2.2-treated cells became highly susceptible to lysis by the same clones. These data indicate that, at least for the NK clones analyzed, the protection of target cells requires class I molecules in the complexed form.

p40, a novel surface molecule involved in the regulation of the non-major histocompatibility complex-restricted cytolytic activity in humans

Four monoclonal antibodies (mAb) termed NKTA255, NKTA72, 1F1 and 1B1 were selected on the basis of their ability to inhibit the cytolytic activity of natural killer (NK) cell clones against P815 target cells. These mAb selectively reacted with normal or tumor cells of hematopoietic origin and displayed a cellular distribution similar to that of CD45 or CD11a/CD18 antigens. Immunoprecipitation experiments showed that they reacted with molecules with an apparent molecular mass of 40 kDa under both reducing and nonreducing conditions ("p40" molecules), thus differing from CD45 or CD11a/CD18 antigens as well as from the "inhibitory" receptors for HLA class I molecules (i.e. p58, CD94 and NKB1 molecules). Double-immunofluorescence analysis of peripheral blood mononuclear cells allowed the identification of three distinct populations on the basis of the fluorescence intensity of cells stained with anti-p40 mAb. p40bright cells were homogeneously HLA-DR-positive, p40medium cells were HLA-DR-negative but co-expressed CD56 antigens, while p40dull cells were all CD3+. Anti-p40 mAb strongly inhibited the lysis of K562 target cells, mediated by fresh NK cells, as well as the lysis of P815 target cells by NK or T cell clones. In addition, in redirected killing assays, anti-p40 mAb strongly reduced the anti-CD16 mAb-induced cytolytic activity of NK cell clones. On the contrary, they did not inhibit either the anti-CD3 or anti-T cell receptor mAb-mediated cytolytic activity of T cell clones or the lysis of allogeneic phytohemagglutinin blasts mediated by specific cytolytic T cell clones. The p40-induced inhibition of the NK cytotoxicity required optimal cross-linking, as anti-p40 mAb could inhibit the lysis of Fc gamma receptor (Fc gamma R)-positive but not of Fc gamma R-negative target cells. In addition, (Fab')2 fragments of anti-p40 mAb failed to inhibit the lysis of Fc gamma R-positive target cells. In conclusion, p40 molecules represent a new type of inhibitory surface molecule that appears to play a general regulatory role in the NK-mediated cytolysis.

The human natural killer cell receptor for major histocompatibility complex class I molecules. Surface modulation of p58 molecules and their linkage to CD3 zeta chain, Fc epsilon RI gamma chain and the p56lck kinase

The natural killer cell (NK)-specific p58 surface molecules, recognized by the GL183 and EB6 monoclonal antibodies (mAb), have been shown to represent the putative NK receptor for HLA-C molecules. The interaction between p58 receptors and HLA-C results in inhibition of the NK-mediated target cell lysis. In this study, GL183-EB6+ clones (Cw4-specific), after mAb-induced surface modulation of EB6 molecules, acquired the ability to lyse the Cw4+ C1R cells. In NK clones co-expressing both GL183 and EB6 molecules and unable to kill Cw3-protected target cells, the mAb-induced modulation of EB6 molecules resulted both in selective co-modulation of GL183 molecules and in the lysis of Cw3-transfected P815 murine cells. In line with the co-modulation experiments we also show that the GL183 and EB6 molecules can be co-immunoprecipitated from GL183+/EB6+ clones after cell lysis in the presence of digitonin. The p58 receptor also revealed an association with molecules belonging to the zeta family (i.e. CD3 zeta and Fc epsilon RI gamma chains). Two-dimensional diagonal gel analysis of the p58 complex immunoprecipitated from polyclonally activated p58+ NK cells indicated a preferential association with CD3 zeta chains either in the form of covalently linked zeta-zeta homodimers or in the form of zeta-gamma heterodimers, while gamma-gamma homodimers were detectable in low amounts. However, p58+ clones displaying a unique association with gamma-gamma homodimers could also be isolated. Probing the immunoprecipitated p58 complex with anti-p56lck antibody also revealed an association with this member of the src family. In addition, mAb-mediated signaling of NK clones via p58 molecules induced increments of p58/p56lck association. However, under the same experimental conditions that induced optimal in vivo tyrosine phosphorylation of the CD16-associated CD3 zeta chains, no tyrosine phosphorylation was detected in the p58-associated CD3 zeta chains. In these in vivo experiments neither anti-CD16 nor anti-p58 mAb could induce tyrosine phosphorylation of the gamma chains. Finally, the anti-p58-mediated inhibition of the NK cell triggering via CD16 molecules was not accompanied by a down-regulation of the tyrosine phosphorylation of the CD16-associated CD3 zeta chains.

Human natural killer cell receptors for HLA-class I molecules. Evidence that the Kp43 (CD94) molecule functions as receptor for HLA-B alleles

GL183 or EB6 (p58) molecules have been shown to function as receptors for different HLA-C alleles and to deliver an inhibitory signal to natural killer (NK) cells, thus preventing lysis of target cells. In this study, we analyzed a subset of NK cells characterized by a p58-negative surface phenotype. We show that p58-negative clones, although specific for class I molecules do not recognize HLA-C alleles. In addition, by the use of appropriate target cells transfected with different HLA-class I alleles we identified HLA-B7 as the protective element recognized by a fraction of p58-negative clones. In an attempt to identify the receptor molecules expressed by HLA-B7-specific clones, monoclonal antibodies (mAbs) were selected after mice immunization with such clones. Two of these mAbs, termed XA-88 and XA-185, and their F(ab')2 fragments, were found to reconstitute lysis of B7+ target cells by B7-specific NK clones. Both mAbs were shown to be directed against the recently clustered Kp43 molecule (CD94). Thus, mAb-mediated masking of Kp43 molecules interferes with recognition of HLA-B7 and results in target cell lysis. Moreover, in a redirected killing assay, the cross-linking of Kp43 molecules mediated by the XA185 mAb strongly inhibited the cytolytic activity of HLA-B7-specific NK clones, thus mimicking the functional effect of B7 molecules. Taken together, these data strongly suggest that Kp43 molecules function as receptors for HLA-B7 and that this receptor/ligand interaction results in inhibition of the NK-mediated cytolytic activity. Indirect immunofluorescence and FACS analysis of a large number of random NK clones showed that Kp43 molecules (a) were brightly expressed on a subset of p58-negative clones, corresponding to those specific for HLA-B7; (b) displayed a medium/low fluorescence in the p58-negative clones that are not B7-specific as well as in most p58+ NK clones; and (c) were brightly expressed as in the p58+ clone ET34 (GL183-/EB6+, Cw4-specific). Functional analysis revealed that Kp43 functioned as an inhibitory receptor only in NK clones displaying bright fluorescence. These studies also indicate that some NK clones (e.g., the ET34) can coexpress two distinct receptors (p58 and Kp43) for different class I alleles (Cw4 and B7). Finally, we show that Kp43 molecules function as receptors only for some HLA-B alleles and that still undefined receptor(s) must exist for other HLA-B alleles including B27.

CD45-mediated regulation of LFA1 function in human natural killer cells. Anti-CD45 monoclonal antibodies inhibit the calcium mobilization induced via LFA1 molecules

The TA218 and T205 monoclonal antibodies (mAb) were selected on the basis of their ability to inhibit the non-major histocompatibility complex-restricted lysis of the murine mastocytoma P815 cell line mediated by CD3-CD16+ natural killer (NK) cells. Both mAb were found to react with CD45 molecules, as demonstrated by immunoprecipitation after surface iodination and western blot analysis. A panel of tumor target cells susceptible to lysis by polyclonal or clonal CD3-CD16+ NK cells was used to study the mAb-mediated inhibitory effect. The inhibition of cytolysis mediated by TA218 and T205 mAb was found to consistently parallel the inhibition mediated (with the same tumor target cells) by the anti-LFA1 alpha mAb TS.1.22 or by the anti-LFA1 beta mAb TS.1.18. However, different from the anti-LFA1 mAb, T205 or TA218 mAb did not inhibit the binding of activated CD3-CD16+ effector NK cells to the same tumor target cells. This finding supported the concept that the anti-CD45 mAb-mediated inhibition could occur at a post-binding stage. In polyclonal or clonal CD3-CD16+ NK cells T205 or TA218 mAb were found to reduce by 50-70% the intracellular Ca++ ([Ca++]i) mobilization induced by anti-LFA1 alpha or anti-LFA1 beta mAb. On the other hand, TA218 and T205 mAb did not inhibit the Ca++ mobilization induced by anti-CD16 mAb or phytohemagglutinin, thus suggesting that, in NK cells, CD45 molecules may exert a selective inhibitory effect on the signal transduction mediated by LFA1 molecules. In line with this hypothesis, the cytolytic activity of human NK clones was triggered in the presence of the hybridoma cells secreting either anti-CD16 or anti-LFA1 alpha mAb (as "triggering targets"). This effect of anti-LFA1 alpha, but not of anti-CD16 hybridoma was susceptible to inhibition by the anti-CD45 mAb T205 or TA218. Further, experiments on cloned NK cells indicated that T205 or TA218 mAb induced a strong decrease in the constitutive phosphorylation of the LFA1 alpha chain (but not of HLA class I antigens). Taken together, these studies suggest that in human NK lymphocytes, CD45 molecule may regulate both the activation state and the function of the LFA1 molecule.

Extrathymic differentiation of T lymphocytes and natural killer cells from human embryonic liver precursors

Liver cells were isolated on Ficoll/Hypaque gradients from embryos or fetuses at 6-10 weeks of gestation; 2-20% of the cells expressed CD45 or HLA class I surface antigens and 2-6% expressed CD7. Other T- or natural-killer (NK)-cell-lineage-specific markers were undetectable. Liver-cell suspensions cultured in the presence of phytohemagglutinin and recombinant interleukin 2 gave rise to large proportions of CD3+ lymphocytes expressing either alpha/beta or gamma/delta T-cell receptors. This occurred not only in bulk cultures but also when cells were cloned under limiting dilution conditions. Importantly, these figures were obtained also in embryos at 6-8 weeks of gestation, which is before colonization of the thymic rudiment by T-cell precursors. When the same liver-cell suspensions were cultured in the presence of irradiated H9 cells and recombinant interleukin 2 (either in bulk cultures or under cloning conditions), large proportions of cells (or clones) expressed surface CD16 and CD56 antigens and displayed a strong cytolytic activity against both NK-sensitive (K562) and NK-resistant (M14) target cells. In addition, liver-derived T or NK cells expressed functional receptor molecules since they could be activated via either CD3/T-cell receptor or CD16 surface antigens, respectively. Further fractionation of liver cells on the basis of CD45 antigen expression indicated that only CD45+ cells could give rise to T or NK cells in culture. Thus, CD45 can be used as a marker for identification of an early liver-cell population containing T- and NK-cell precursors. That T or NK cells were derived from male embryos and not from the mother was shown by PCR amplification of X and Y chromosomal sequences. Our present data may offer an in vitro model for extrathymic embryonic T-cell maturation that can be used to examine fundamental aspects of human T-cell development and function.

High performance liquid chromatographic strategy for the analysis of exopolysaccharides extracted from pathogenic bacteria

A high performance liquid chromatographic (HPLC) method has been developed to permit the rapid comparison of acidic polysaccharides of diverse compositions and the sensitive determination of their constituents. It is based on two combined analyses of the polysaccharide hydrolysates--a separation of the released compounds by ion-moderated partition chromatography with UV detection at two wavelengths and a separation of the sugar dansylhydrazine derivatives by reversed phase chromatography. The former permits identification and quantitation of uronic and carboxylic acids, the latter permits more sensitive and specific determination of the neutral aldoses. Some bacterial exopolysaccharides have been used to demonstrate the validity of this HPLC procedure for the chemical characterization of uronic acid-containing polysaccharides. This method appears to be useful for studying capsular polysaccharides, which are involved in the evasion of phagocytosis by pathogenic bacteria.