Recognition of major histocompatibility class I molecules on target cells by natural killer (NK) cells confers selective protection from NK-mediated lysis. Cross-linking of the p58 NK receptor, involved in the recognition of HLA-C alleles, delivers a negative signal that prevents target cell lysis. Molecular cloning of the p58 NK receptor reported here revealed a new member of the immunoglobulin superfamily. Five distinct p58 receptors, with sequence diversity in the immunoglobulin-related domains, were identified in a single individual. All NK clones tested expressed at least one p58 member. Three different types of transmembrane and cytoplasmic domains exist, even among receptors with closely related extracellular domains. These data revealed a repertoire of NK cells with clonally distributed p58 receptors exhibiting diversity in both extracellular and intracellular domains.

SummaryHuman natural killer (NK) cells express inhibitory receptors that are specific for different groups of HLA-C or HLA-B alleles. The majority of these receptors belong to the immunoglobulin (Ig) superfamily and are characterized by two or three extracellular Ig-hke domains. Here we describe a novel inhibitory NK receptor that is specific for a group of HLA-A alleles. The HLA-A3-specific NK cell clone DP7 has been used for mice immunization. Two mAbs, termed Q66 and Q241, bound to the immunizing clone and stained only a subset of NK cell populations or clones. Among Q66 mAb-reactive clones, we further selected those that did not express any of the previously identified HLA-class I-specific NK receptors. These clones did not lyse HLA-A3 + (or -All +) target cells, but lysis of these targets could be detected in the presence of Q66 or Q241 mAbs. On the other hand, target cells expressing other HLA-A alleles, including -A1, -A2, and -A24, were efficiently lysed. Moreover, none of the HLA-C or HLA-B alleles that were tested exerted a protective effect. Q66 +, but not Q66-NK cell clones, expressed messenger RNA coding for a novel 3 Ig domain protein homologous to the HLA-C (p58) and HLA-B (p70) receptors. The corresponding cDNA (c1.1.1) was used to generate transient and stable transfectants in COS7 and NIH3T3 cell lines, respectively. Both types of transfectants were specifically stained by Q66 and Q241 mAbs. Since the cytoplasmic tail of Q66-reactive molecules was at least 11 amino acid longer than the other known p58/p70 molecules, we could generate an antiserum specific for the COOH-terminus of Q66-reactive molecules, termed PGP-3. PGP-3 immunoprecipitated, only from Q66 + NK cells, molecules displaying a molecular mass of 140 kD, under nonreducing conditions, which resolved, under reducing conditions, in a 70-kD band. Thus, differently from the other p58/p70 receptors, Q66-reactive molecules appear to be expressed as disulphide-linked dimers and were thus termed p140. The comparative analysis of the amino acid sequences of p58, p70, and p140 molecules revealed the existence of two cysteins proximal to the transmembrane region, only in the amino acid sequence of p140 molecules.

SummaryNatural killer cells express clonally distributed receptors specific for major histocompatibility complex class I molecules. The human leukocyte antigen (HLA)-C-specific receptors have been molecularly identified and cloned. They exist not only as inhibitory (p58) but also as activatory (1050) receptors. Here we show that p50 and p58 are highly homologous in their extracellular regions formed by two Ig-like domains. In contrast, major differences exist in their transmembrane and cytoplasmic portions. Whereas p58 displays a 76-84-amino acid cytoplasmic tail containing an unusual antigen receptor activation motif, p50 is characterized by a shorter 39-amino acid tail. In addition, whereas p58 has a nonpolar transmembrane portion, p50 contains the charged amino acid Lys. These data strongly suggest that receptors with identical HLA-C allele specificity can mediate functions of opposite sign owing to their different transmembrane/cytoplasmic portions.T he cytolytic activity of NK cells is regulated by several clonally distributed inhibitory receptors specific for different groups of HLA class I andes. Interaction between these NK receptors and HLA class I molecules leads to inhibition of the NK-mediated target cell lysis (1--4). The receptors specific for two distinct groups of HLA-C alleles have been molecularly identified as p58 molecules, which react with either GL183 (5) or EB6 mAb (6). p58-encoding genes have recently been isolated and cloned (7). They encode for type 1 transmembrane proteins, which belong to the Ig supeffamily. They are characterized by two extracellular Ig-hke domains and display a high level of sequence homology. Recently, additional 50-kD molecular forms of EB6 or GL183 mAb-reactive receptors have been identified. They display the same HLA-C allele specificity of the corresponding p58 receptors but mediate NK cell triggering rather than inhibition (8). p50 receptors were found to be expressed only in some donors and to display a clonal distribution. Importantly, single NK cells did not coexpress p58 and p50 receptors with an identical HLA-C specificity (8). The protein backbone of p58 or p50 molecules has been shown to be 42 and 36 kD, respectively, after deglycosylation. These data, together with identical HLA-C specificity and mAb reactivity, suggested that p58 and p50 could be homologous in their extracellular domains but different in their intracellular portions. Cloning of several p58-homologous genes revealed the existence of molecules displaying cytoplasmic tails of different length. Here we show that, different from p58, the activatory p50 receptors are characterized by a short intracytoplasmic tail associated with a transmembrane portion containing a polar residue. Materials and Methods Isolation of EB6 + or GL183 § NK Clones and Definition of theFunction of their Receptors. EB6 § or GL183 § NK cell clones were isolated as previously described (4). Screening of clones for the expression of triggering (p50) or inhibitory (p58) receptors was performed by cytolytic tests using P...

This study was designed to identify the target molecules of the natural killer (NK) cell-mediated recognition of normal allogeneic target cells. As previously shown, the gene(s) governing the first NK-defined allospecificity (specificity 1) were found to be localized in the major histocompatibility complex region between BF gene and HLA-A. In addition, the analysis of a previously described family revealed that a donor (donor 81) was heterozygous for three distinct NK-defined allospecificities (specificities 1, 2, and 5). HLA variants were derived from the B-Epstein-Barr virus cell line of donor 81 by gamma irradiation followed by negative selection using monoclonal antibodies specific for the appropriate HLA allele. Several variants were derived that lacked one or more class I antigen expressions. These variants were analyzed for the susceptibility to lysis by NK clones recognizing different allospecificities. The loss of HLA-A did not modify the phenotype (i.e., "resistance to lysis"). On the other hand, a variant lacking expression of all class I antigens became susceptible to lysis by all alloreactive clones. Variants characterized by the selective loss of class I antigens coded for by the maternal chromosome became susceptible to lysis by anti-2-specific clones. Conversely, variants selectively lacking class I antigens coded for by paternal chromosome became susceptible to lysis by anti-1 and anti-5 clones (but not by anti-2 clones). Since the Cw3 allele was lost in the variant that acquired susceptibility to lysis by anti-2 clones and, in informative families, it was found to cosegregate with the character "resistance to lysis" by anti-2 clones, we analyzed whether Cw3 could represent the element conferring selective resistance to lysis by anti-2 clones. To this end, murine P815 cells transfected with HLA Cw3 (or with other HLA class I genes) were used as target cells in a cytolytic assay in which effector cells were represented by alloreactive NK clones directed against different specificities. Anti-2-specific clones efficiently lysed untransfected or A2-, A3-, and A24-transfected P815 cells, while they failed to lyse Cw3-transfected cells. NK clones recognizing specificities other than specificity 2 lysed untransfected or Cw3- transfected cells. Thus, the loss of Cw3 resulted in the de novo appearance of susceptibility to lysis, and transfection of the HLA- negative P815 cells with Cw3 resulted in resistance to lysis by anti-2 clones. Therefore, we can infer that Cw3 expression on (both human and murine) target cells confers selective protection from lysis mediated by anti-2 NK clones.

SummaryNatural killer (NK) cells have been shown to express a clonally distributed ability to recognize HLA class I alleles. The previously defined NK clones belonging to "group 1" recognize HLACw*0401 (Cw4) and other HLA-C alleles sharing Asn at position 77 and Lys at position 80. Conversely, the "group 2" NK clones recognize HLA-Cw*0302 (Cw3) and other HLA-C alleles characterized by Ser at position 77 and Asn at position 80. We assessed directly the involvement of these two residues in the capacity of NK cell clones to discriminate between the two groups of HLA-C alleles. To this end, Cw3 and Cw4 alleles were subjected to site-directed mutagenesis. Substitution of the amino acids typical of the Cw3 allele (Ser-77 and Asn-80) with those present in Cw4 (Ash-77 and Lys-80) resulted in a Cw3 mutant that was no longer recognized by group 2 NK cell clones, but that was recognized by group I clones. Analysis of Cw3 or Cw4 molecules containing single amino acid substitutions indicates roles for Lys-80 in recognition mediated by group I clones and for Ser-77 in recognition mediated by group 2 clones. These results demonstrate that NK-mediated specific recognition of HLA-C allotypes is affected by single natural amino acid substitutions at positions 77 and 80 of the heavy chain.