Recruitment of tyrosine phosphatase HCP by the killer cell inhibitor receptor

Abstract

Cytolysis of target cells by natural killer (NK) cells and by some cytotoxic T cells occurs unless prevented by inhibitory receptors that recognize MHC class I on target cells. Human NK cells express a p58 inhibitory receptor specific for HLA-C. We report association of the tyrosine phosphatase HCP with the p58 receptor in NK cells. HCP association was dependent on tyrosine phosphorylation of p58. Phosphotyrosyl peptides corresponding to the p58 tail bound and activated HCP in vitro. Furthermore, introduction of an inactive mutant HCP into an NK cell line prevented the p58-mediated inhibition of target cell lysis. These data imply that the inhibitory function of p58 is dependent on its tyrosine phosphorylation and on recruitment and activation of HCP.

Figure 1. Tyrosine Residues in Cytoplasmic Tails of the p58 NK Receptor Family

Schematic representation of two types of cytoplasmic tails found in members of the p58 receptor family expressed on NK cells. The hatched and open boxes represent the transmembrane and cytoplasmic domains, respectively. Amino acid residues are shown in single letter code. Spacing between the amino acid residues is shown to scale. The ITAM as found in the CD3 subunits of the TCR is aligned with the first YxxL of the p58 tail. The underlined regions correspond to peptides used for generation of antisera.

Figure 2. Cross-Linking p58 with Antibody in NK Clones Induces Tyrosine Phosphorylation of a 58 kDa Protein

Three NK clones expressing the GL183 determinant (SR70, SR47, and SR64) were pooled. The cells (3 × 10⁶ per condition) were preincubated in the absence of primary antibody (minus), with F(ab′)₂–GL183 (F–GL183), or intact GL183 (GL183). Following addition of F(ab′)₂–goat anti-mouse IgG, cells were incubated for 1 min at 37°C and lysed. GL183 was added to the control samples (minus) after lysis. The GL183 immune complexes (αp58) were collected with mouse anti-goat IgG and the supernatants were subjected to a second immunoprecipitation with anti-phosphotyrosine MAb 4G10 (αptyr). The immunoprecipitated samples were electrophoresed on the same gel and analyzed by Western blot with anti-phosphotyrosine (αptyr). The right panel represents a sequential probing of the membrane in the middle panel using antisera αcyt49. Molecular mass markers are indicated in kilodaltons on the left.

Figure 3. p58 Association with HCP in NK Cells

NK cell populations derived from two donors were treated with MAbs specific for CD56 (Leu19) and p58 (GL183) and cross-linked for 1 min at 37°C with goat anti-mouse IgG. The cells were lysed in Lysis-L buffer and the associated proteins immunoprecipitated (the control precipitation is with MOPC-21) and Western blotted for HCP (a). The samples correspond to 4 × 10⁷ cells/lane from donor A and 3 × 10⁷ cells/lane from donor B. The far-left lane contains total cell lysate from Jurkat cells and indicates the position of HCP. The membrane was reprobed with anti-phosphotyrosine (b). The membrane was then stripped and reprobed to determine the amount of p58 reactive with αcyt6 (c). The same region of the gel is presented in each panel, and the position of a 66 kDa molecular mass marker is indicated by a bar on the right.

Figure 4. Tyrosine Phosphorylation of p58 and Phosphorylation-Dependent Association with HCP Reconstituted in Nonhematopoietic

Cells 3T3E cells (5 × 10⁶) were infected for 6 hr with 5 pfu/cell of recombinant vaccinia viruses in combination with the appropriate dose of the control virus (Vac–c) to normalize the infections to 15 pfu/cell. The recombinant viruses express lyn tyrosine kinase (Vac–lyn), HCP (Vac–HCP), p58-cl6 (Vac–6). Following lysis with Lysis-L buffer, all samples were subjected to immunoprecipitation with GL183. The samples were divided into two lanes of the same gel such that, following transfer to a membrane, they were probed in parallel with anti-phosphotyrosine (αptyr) or anti-HCP (αHCP). The amount of p58 in each sample was determined by reprobing with αcyt6 the membrane that had been first probed with αHCP. The position corresponding to the migration of p58 is indicated on the right, along with those of molecular mass markers.

Figure 5. p58-Derived Phosphotyrosyl Peptides Bind and Activate HCP

(A) Binding of cellular HCP to p58 phosphotyrosyl peptides. Peptides corresponding to each of the two p58 YxxL sequences were synthesized with a tyrosine (Y1 and Y2) or a phosphotyrosine (pY1 and pY2). Bead-conjugated peptides were incubated with a lysate of Jurkat cells. Cellular proteins associated with the peptides were analyzed by SDS–PAGE and Western blotting with antibody against HCP. (B) HCP interacts via its SH2c domain with the p58 peptides. Beads bearing the indicated peptides were incubated with GST fusion proteins containing either one of the two SH2 domains of HCP as indicated in the diagram. Proteins associated with the peptides were analyzed by SDS–PAGE and Western blotting with an antibody against GST. (C) Activation of phosphatase activity by p58 peptides. The PTPase activity of a complete HCP–GST fusion protein was determined in the presence of 0–240 μM of peptide as indicated. The data represent the mean ± SD values of duplicate samples from three independent experiments.

Figure 6. Endogenous HCP in NK Cells

The amount of HCP in 1 × 10⁵ cell equivalents of total cell lysate from the cell line NK-92 is compared with that in two NK clones (SR58 and SR67) by Western blot analysis. Commercially supplied Jurkat lysate is included as a migration marker for HCP but not for quantitative comparison. The intact HCP has an apparent molecular mass of 60 kDa. The 40 kDa band detected with the anti-sera corresponds to a degradation product of HCP.

Figure 7. A Catalytically Inactive Mutant HCP Prevents the p58-Mediated Inhibition of Target Cell Lysis

NK-92 cells were infected with recombinant vaccinia viruses encoding p58 molecules cl6 and cl42 and a catalytically inactive form of HCP, HCP-C453S, as indicated. The mean fluorescence intensities (MFI) given on the left correspond to the surface expression of p58-cl6 or p58-cl42 for each infection as determined with MAbs GL183 and EB6. The same infected NK-92 cells were then tested for their ability to lyse the target cells .221, .221-Cw3, and .221-Cw4 cells by infected NK-92 cells was determined in a 3 hr ⁵¹Cr release assay. The data presented are for an E:T ratio of 4. Similar results were observed at an E:T of 1 and in three independent experiments.

Figure 8. NK Receptors and HCP Binding Proteins Share a Unique Immunoreceptor Tyrosine-based Inhibition Motif (ITIM)

Amino acid sequences (in single letter code) surrounding the YxxL motifs of the indicated proteins are aligned. Dashes indicate identity with the sequence around the membrane-proximal tyrosine of the p58 cytoplasmic tail, p58 (Y1). Sequences were taken from the NK inhibitory receptors p58-cl6 (Wagtmann et al., 1995a), p70 (D’Andrea et al., 1995; Wagtmann et al., 1995b), and Ly-49 (Yokoyama and Seaman, 1993). NKG2-A is a molecule of unknown function expressed in human NK cells (Yabe et al., 1993). The single tyrosine residue in mouse FcγRIIB (D’Ambrosio et al., 1995) and three of the six tyrosines (i.e., Y2, Y5, and Y6) in CD22 (Doody et al., 1995) are known to interact with HCP.