NK cells lyse cellular targets such as certain tumor cells or virus-infected cells. NK-mediated lysis of normal cells is prevented by inhibitory receptors on NK cells that recognize MHC class I molecules on target cells (1, 2). MHC class I molecules consist of a trimolecular complex of a polymorphic heavy chain, β2-microglobulin (β2m), and an intracellularly derived short peptide. MHC class I recognition is mediated by a number of structurally diverse inhibitory receptors present on NK cells in mice and humans. The effect of peptide binding on the structural nature of the class I molecule recognized by the different receptors on NK cells is not well defined. Does peptide fulfill only a generic role in providing proper conformation and adequate cell surface expression of MHC molecules or do receptors on NK cells bind to specific conformational determinants on MHC class I induced only by certain peptides?
Mouse NK cells express inhibitory type II transmembrane glycoproteins called Ly-49 that bind H-2 ligands (1). Studies on the role of peptide in the recognition of H-2Dd by Ly-49A+ NK cells revealed that H-2Dd expressed on transporter associated with antigen presentation (TAP)-deficient cells confers protection from NK-mediated lysis only after loading with peptide but regardless of the sequence of the bound peptide (3, 4). Thus, there is no apparent peptide specificity for the mouse inhibitory receptor Ly-49A and the contribution of the peptide bound to H-2Dd is to ensure proper folding and reasonable cell surface expression.
Human NK cells express at least two different types of inhibitory receptors that are involved in the specific recognition of HLA-A, -B, and -C allotypes on target cells. One type belongs to the C-type lectin family and consists of the type II glycoprotein CD94 covalently associated with structurally heterogeneous members of the NKG2 receptor family (5, 6). The HLA ligands for CD94–NKG2 are not clearly established but appear to include polymorphic HLA-A, -B, and -C molecules (5, 7). There is no information on the role of peptide in the recognition of HLA ligands by the CD94–NKG2 complex. The second type of NK receptors is the p58/p70 family of receptors, called killer cell inhibitory receptors (KIR) that are type I transmembrane glycoproteins of the immunoglobulin superfamily (8–10). Whereas the p70 receptors are specific for certain HLA-A and -B alleles (11–15), members of the p58 group discriminate among defined HLA-C allotypes (allotypes with N77K80, including Cw2, Cw4, Cw5, Cw6 versus allotypes with S77N80, such as Cw1, Cw3, Cw7, Cw8) (13, 16, 17).
In contrast with Ly-49 on mouse NK cells, there is some degree of peptide selectivity in the recognition of HLAB*2705 by NK clones expressing p70 (18, 19). This peptide discrimination can be mediated by a single p70 KIR, cl11 (20). Single–amino acid substitutions along a protective peptide identified residues at positions P7 and P8 as critical for NK recognition of HLA-B*2705 (19). However, there is no data on whether these substitutions affected direct binding of the p70 KIR to HLA-B*2705. Recognition of empty HLA-C molecules expressed by TAP-deficient cells indicated that bound peptide was not required for the inhibition of CTL or NK-mediated lysis (21, 22) by HLA-C. However, the identity of the inhibitory receptor(s) (e.g., KIR, CD94–NKG2) was not established in these studies.
In this study, we investigated the role of peptide in the recognition of HLA-C by KIR cl42, a receptor previously shown to be specific for HLA-Cw4 by binding studies and by functional reconstitution experiments (13). We used a direct in vitro binding assay to show that the recognition of HLA-Cw4 by KIR cl42 is dependent on the presence of peptide in the groove. Moreover, there is peptide selectivity, because certain peptides that stabilized HLA-Cw4 surface expression on the TAP-deficient RMA-S cells did not render these molecules competent to interact with KIR cl42.
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Materials and Methods
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NK Clones and Target Cells.
CD3–CD56+ NK cells were obtained from PBMC, cloned by limiting dilution, and maintained as previously described (20, 23). The TAP-deficient cell line RMA-S transfected with the human β2m gene (referred to as RMA-S below) was obtained from P. Cresswell (Yale University, New Haven, CT) (24). This line was transfected with the cDNA constructs RSV.5gpt-Cw4 and RSV.5gpt-Cw8. A SalI–HpaI fragment containing either the HLA-Cw*0401 cDNA or the HLA-Cw*0802 cDNA (obtained from R. Biassoni, Istituto Nazionale per la Ricerca sul Cancro, Genova, Italy) was cloned into the SalI–HpaI sites of vector RSV.5gpt (25). Cells were electroporated at 250 V, 960 µF with 20 µg of these constructs. 48 h later, cells were selected with 10 µg/ml mycophenolic acid and 10 µg/ml xanthine and plated in V-bottomed 96-well plates at 103 cells/well. Cells were screened by flow cytometry using the mAb F4/326 and subcloned. Clones with the highest class I expression were used for the experiments described here.
Peptides, Antibodies, and Immunostaining.
Peptides were synthesized and purified as described (26). Purity was confirmed by analytical reverse phase HPLC and by mass spectrometry. QYDDAVYKL is a consensus peptide that binds Cw*0401 with an IC50 value of 18 nM (27). It contains residues present in the endogenous peptides bound to Cw*0401, as determined by pool sequencing (28). The anchor motif for peptides bound to Cw*0401 was proposed to be Y,P at position 2, V,I,L at position 6 and L,F,M at position 9 (27, 28). The following mAbs were used: F4/326 (IgG2a), a gift from S.Y. Yang (Memorial Sloan-Kettering Cancer Center, New York); HP3E4 (IgM), a gift from M. Lopez-Botet (Hospital de la Princesa, Madrid, Spain); DX22 (IgG1), a gift from L. Lanier (DNAX, Palo Alto, CA). 1 x 105 cells were incubated for 30 min on ice with primary mAb, washed in PBS containing 2% FCS, and stained with FITC-conjugated goat anti–mouse IgG+IgM (Jackson ImmunoResearch Laboratories, Inc., West Grove, PA). Cells were washed twice and analyzed on a FACScan® (Becton Dickinson, Mountain View, CA). To test the binding of cl42–Ig (13) to RMA-S transfectants in the absence or presence of peptides, cells were incubated with 100 µg/ml of KIR cl42–Ig (or control cl43–Ig) for 30 min on ice, followed by staining with a fluoresceinated goat anti– human Fc-specific reagent as previously described (13). Cells were then washed and analyzed by flow cytometry.
Peptide Binding Assay for Stabilization of HLA-C Surface Expression.
RMA-S transfectants were plated at 2.5 x 105 cells/well in a 48-well plate in a final volume of 0.5 ml. Cells were incubated for 24 h at 25°C. Peptide (100 µM final concentration) was added at the onset of culture and again 8 h later. Cells were harvested, washed in PBS containing 2% FCS, and divided into three aliquots. One aliquot was used to measure the surface levels of class I molecules on peptide-loaded RMA-S transfectants as described above. The second aliquot was used to test for binding of KIR cl42–Ig to peptide-loaded RMA-S as described above. The third aliquot was used as targets in the cytotoxicity assay described below.
Cytotoxicity Assay.
HP3E4+, DX22+ NK clones were tested for killing at several effector to target ratios in duplicate wells. NK clones were resuspended at the desired concentration in a medium consisting of Iscove's modified essential medium with 10% FCS, 2 mM glutamine, and 50 U/ml rIL-2 (Hoffman-La Roche) and plated in V-bottomed 96-well plates in a final volume of 100 or 50 µl in the presence of mAb. Target cells were incubated with peptide as described above and labeled overnight during the incubation at 25°C with sodium 51Cr (50 µCi/well; Amersham, Arlington Heights, IL). Targets were washed and resuspended at a final concentration of 2.5 x 104 cells/ml and 100 µl were added to the NK clones. The assay was incubated at 37°C for 3 h and 51Cr release was measured as previously described (23). mAbs were added at the onset of the assay and were present throughout the assay.
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Results and Discussion
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Cell Surface Expression of HLA-Cw4 and HLA-Cw8 in RMA-S Cells.
TAP-deficient RMA-S cells previously transfected with human β2m (24) (referred to in this report as RMA-S) were used for transfections with HLA-Cw*0401 or HLA-Cw*0802 cDNA. HLA-Cw*0401 belongs to the group of HLA-C allotypes with amino acids N77K80, and HLA-Cw*0802 belongs to the group with amino acids S77N80. Transfectants were incubated at 25°C to faciliate surface expression of empty HLA-C molecules, screened by flow cytometry using the HLA-C-reactive mAb F4/326, and subcloned. One clone each, expressing high cell surface levels of either HLA-Cw4 or HLA-Cw8 at 25°C, was chosen for further study (Fig. 1). RMA-S-Cw4 transfectants were incubated with the peptide QYDDAVYKL, a high affinity consensus peptide that bound Cw*0401 (27). Peptide loading resulted in a dramatic increase in cell surface expression of HLA-Cw4 as depicted by the greater than fivefold increase in fluorescence intensity. This peptide did not affect the cell surface expression of HLA-Cw8 (see Fig. 2).
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