The lectin-like NK cell receptor Ly-49A recognizes a carbohydrate-independent epitope on its MHC class I ligand

The mouse NK inhibitory Ly-49A receptor specifically interacts with a peptide-induced conformational determinant on its MHC class I ligand, H-2Dd. In addition, it binds the polysaccharide fucoidan, consistent with its C-type lectin homology and the hypothesis that Ly-49A interacts with carbohydrates on Dd. Herein, however, we demonstrate that Ly-49A recognizes Dd mutants lacking N-glycosylation. Fucoidan competes for binding with anti-Ly-49A antibodies that inhibit Ly-49A-Dd interaction, and blocks apparent Ly-49A binding to unglycosylated Dd. We confirm that Ly-49A recognizes the alpha1 and amino-terminal alpha2 domains of Dd by analysis of recombinant H-2Kd-H-2Dd molecules. These studies indicate that Ly-49A recognizes carbohydrate-independent epitope(s) on Dd and suggest that Ly-49A has two distinct ligands, carbohydrate and MHC class I.

bca: an activation-related B-cell gene

We have identified a novel activation related B-cell gene (bca) through differential hybridization screening of a murine B cell cDNA library. The deduced amino acid sequence predicted a protein of 482 amino acids with strong sequence similarity to the SH2 and SH3 domains present within the non-catalytic regions of several protein tyrosine kinases. Northern analysis of RNA from several murine B-cell lines revealed a transcript of 1.8 kb, which was not detected in T-cell and non-lymphoid cell lines. bca was transcribed at low levels in resting spleen cells from a variety of normal mouse strains and was strongly expressed in kidney RNA. bca expression was markedly increased in RNA prepared from mitogen activated B cells, and in freshly isolated spleen and lymph node cells of MRL/lpr and NZB autoimmune strains. The unique sequence of bca, which bears no obvious similarity to any specific class of proteins containing SH2 and SH3 domains, suggests that this gene encodes a novel protein potentially involved in B-cell signal transduction.

Rigidification of the alpha2 helix of an MHC class I molecule by a valine to proline mutation in position 165 does not prevent peptide-specific antigen presentation

Although classical MHC class I glycoproteins bind peptide Ags for display at the cell surface, some MHC class I-related molecules such as the neonatal Fc receptor (FcRn) execute their function without binding peptide ligands. The three-dimensional structure of the FcRn suggested that a substitution of the conserved valine at position 165 of the alpha2 helix by proline contributed to a kink in the position of this helix relative to the alpha1 helix, and resulted in closing of the potential peptide-binding cleft. To test the contribution of proline 165 to the occlusion of the cleft and the binding of potential antigenic peptides, we introduced this mutation into the classical murine MHC class I molecule, H-2Dd, and characterized the ability of such a mutant to present peptide Ags to either a peptide-specific, H-2Dd-restricted T cell hybridoma (B4.2.3), or an allospecific, peptide-dependent, T cell hybridoma (3DT52.5.8). We show that the V165P mutation, expressed at the cell surface either in H-2Dd or in a single chain membrane version of H-2Dd, fails to eliminate recognition of the peptide/MHC complexes by two different T cells. Evaluation of a panel of synthetic substituted peptides suggests that subtle differences in the fine specificity of presentation can be discerned. Thus, the proline substitution at position 165 of FcRn and some other class I-like molecules is not the sole cause of the lack of peptide presentation.

Rigidification of the alpha2 helix of an MHC class I molecule by a valine to proline mutation in position 165 does not prevent peptide-specific antigen presentation

Although classical MHC class I glycoproteins bind peptide Ags for display at the cell surface, some MHC class I-related molecules such as the neonatal Fc receptor (FcRn) execute their function without binding peptide ligands. The three-dimensional structure of the FcRn suggested that a substitution of the conserved valine at position 165 of the alpha2 helix by proline contributed to a kink in the position of this helix relative to the alpha1 helix, and resulted in closing of the potential peptide-binding cleft. To test the contribution of proline 165 to the occlusion of the cleft and the binding of potential antigenic peptides, we introduced this mutation into the classical murine MHC class I molecule, H-2Dd, and characterized the ability of such a mutant to present peptide Ags to either a peptide-specific, H-2Dd-restricted T cell hybridoma (B4.2.3), or an allospecific, peptide-dependent, T cell hybridoma (3DT52.5.8). We show that the V165P mutation, expressed at the cell surface either in H-2Dd or in a single chain membrane version of H-2Dd, fails to eliminate recognition of the peptide/MHC complexes by two different T cells. Evaluation of a panel of synthetic substituted peptides suggests that subtle differences in the fine specificity of presentation can be discerned. Thus, the proline substitution at position 165 of FcRn and some other class I-like molecules is not the sole cause of the lack of peptide presentation.

Interactions of TCRs with MHC-peptide complexes: a quantitative basis for mechanistic models

The activation of T lymphocytes is initiated by the binding of MHC-peptide complexes on antigen-presenting cells to MHC-restricted, peptide specific TCRs. Significant progress has recently been made in understanding the structure of the TCR and in the direct quantitative examination of the primary binding interactions between MHC-peptide complexes and the TCR. Attempts to develop quantitative models for the differential activation of T cells by MHC-peptide ligands that differ subtly in their structure have largely been based on either the affinity of the MHC-peptide complexes for the TCR in question or on the dissociation kinetics of the MHC-peptide complex from the T cell.

Degenerate MHC restriction reveals the contribution of class I MHC molecules in determining the fine specificity of CTL recognition of an immunodominant determinant of HIV-1 gp160 V3 loop

The novel allogeneic presentation of an immunodominant determinant within the HIV-1 gp160 V3 loop by three different class I MHC molecules to the same CD8+ CTL is used to study the influence of the MHC molecule on the fine specificity of CTL recognition. We previously reported that four distinct class I molecules of H-2d,u,p,q presented the V3 decapeptide P18-I10 (RGPGRAFVTI) to CTL. Surprisingly, we found that H-2d,u,p cells mutually cross-present the P18-I10 peptide to allogeneic CTL clones of each of the other haplotypes, whereas none of these cross-presents to H-2q CTL, nor do H-2q targets present to CTL of the other haplotypes. Here, we explore the critical amino acid residues for the cross-presentation using 10 variant peptides with single amino acid substitutions. The fine specificity examined using these mutant peptides presented by the same MHC class I molecule showed striking similarity among the CTL of each haplotype, expressing either V beta 8.1 or V beta 14. In contrast, the fine specificity is different between the distinct MHC class I molecules even for the lysis by the same CTL, as shown by reciprocal effects of the same substitutions. Thus, peptide fine specificity of a single TCR is influenced by changes in the class I MHC molecules presenting the Ag.

Degenerate MHC restriction reveals the contribution of class I MHC molecules in determining the fine specificity of CTL recognition of an immunodominant determinant of HIV-1 gp160 V3 loop

The novel allogeneic presentation of an immunodominant determinant within the HIV-1 gp160 V3 loop by three different class I MHC molecules to the same CD8+ CTL is used to study the influence of the MHC molecule on the fine specificity of CTL recognition. We previously reported that four distinct class I molecules of H-2d,u,p,q presented the V3 decapeptide P18-I10 (RGPGRAFVTI) to CTL. Surprisingly, we found that H-2d,u,p cells mutually cross-present the P18-I10 peptide to allogeneic CTL clones of each of the other haplotypes, whereas none of these cross-presents to H-2q CTL, nor do H-2q targets present to CTL of the other haplotypes. Here, we explore the critical amino acid residues for the cross-presentation using 10 variant peptides with single amino acid substitutions. The fine specificity examined using these mutant peptides presented by the same MHC class I molecule showed striking similarity among the CTL of each haplotype, expressing either V beta 8.1 or V beta 14. In contrast, the fine specificity is different between the distinct MHC class I molecules even for the lysis by the same CTL, as shown by reciprocal effects of the same substitutions. Thus, peptide fine specificity of a single TCR is influenced by changes in the class I MHC molecules presenting the Ag.

A three-domain T cell receptor is biologically active and specifically stains cell surface MHC/peptide complexes

We have expressed in bacteria a single-chain T cell receptor (scTCR) with specificity for an HIV gp120-derived peptide bound to the murine MHC-I molecule, H-2Dd. This scTCR consists of V alpha covalently linked to the VbetaCbeta domains that was solubilized, refolded, and purified in high yield. Specific binding of the scTCR to MHC/peptide complexes was demonstrated by surface plasmon resonance, with a Kd of 2 to 8 x 10(-6) M. This scTCR specifically inhibited T cell activation, and stained cell surface MHC/peptide complexes as measured by cytofluorimetry. The preservation of binding specificity by such a three-domain scTCR suggests that this structure is sufficient for specific MHC/peptide recognition and that this strategy will be of general use as applied to other TCR.

A three-domain T cell receptor is biologically active and specifically stains cell surface MHC/peptide complexes

We have expressed in bacteria a single-chain T cell receptor (scTCR) with specificity for an HIV gp120-derived peptide bound to the murine MHC-I molecule, H-2Dd. This scTCR consists of V alpha covalently linked to the VbetaCbeta domains that was solubilized, refolded, and purified in high yield. Specific binding of the scTCR to MHC/peptide complexes was demonstrated by surface plasmon resonance, with a Kd of 2 to 8 x 10(-6) M. This scTCR specifically inhibited T cell activation, and stained cell surface MHC/peptide complexes as measured by cytofluorimetry. The preservation of binding specificity by such a three-domain scTCR suggests that this structure is sufficient for specific MHC/peptide recognition and that this strategy will be of general use as applied to other TCR.

Split tolerance to the MHC class I molecule H-2Dd in animals transgenic for its soluble analog

To determine whether the function of MHC molecules in tolerance and education is related to cell surface expression, we have produced two strains of transgenic mice in the C57Bl/6 background that express soluble analogs of the H-2D(d) class I protein. The transgenes were stably integrated and genetically transmitted in a Mendelian fashion. Messenger RNA for the hybrid genes was detected in all tissues analyzed in a class I-like pattern of expression, with the highest levels in lymphoid tissues. All mice bearing the transgenes expressed relatively high levels (0.1 mg/ml) of the encoded protein in their serum as assessed by Western blotting and enzyme-linked immunosorbent assay (ELISA). Gel filtration chromatography showed that the soluble H-2D(d) protein exists as a heterodimer with beta2-microglobulin and as higher order multimers in serum. Lymphoid cells from the transgenic mice showed no cell surface expression of the soluble class I protein in indirect immunofluorescence assays. Splenocytes from two independently derived transgenic lines generated primary cytotoxic and proliferative responses directed against membrane H-2D(d) antigens. Mice of both strains rejected tail skin from donors that differed from the B6 background at the H-2D(d) locus only, but with delayed kinetics compared to nontransgenic littermate controls. Mice expressing the transgenic protein on immunization did not produce antibodies that recognized soluble H-2D(d) in ELISA, whereas B6 mice generated strong antibody responses to challenge with splenocytes bearing cell surface H-2D(d). Thus, transgenic mice expressing soluble H-2D(d) were partially tolerant to stimulation by membrane-bound H-2D(d). As with the activation of T-cells, the induction and maintenance of immunologic tolerance apparently displayed different requirements depending upon the T-cell subpopulation involved.

Reciprocal cytotoxic T lymphocyte cross-reactivity interactions between two major epitopes within HIV-1 gp160

We have observed and analyzed an unexpected cross-reactivity of CD8+ CTL between two nonhomologous peptides of the HIV-1 IIIB gp160 envelope protein, P18 (residues 315-329) and HP53 (834-848, also called TH4.1), in the context of four different class I MHC molecules, Dd, Dp, Dq (or Lq), and H-2u. In strains expressing Dd, the cross-reactivity between peptides was bidirectional, whereas in other strains (H-2u, H-2p, and H-2q), the cross-reactivity was unidirectional; that is, P18-specific CTLs showed no killing against targets pulsed with HP53, although HP53 stimulated CTL showed cross-reactive lysis against P18-pulsed target cells. Cross-reactivity was also shown in immunization in vivo and with target cells endogenously expressing viral protein in vitro using two different recombinant vaccinia viruses expressing only the N-terminal portion of gp160, containing P18 but not HP53. Peptide cross-contamination was excluded. Cold target inhibition and single cell cloning experiments indicated that the same CTL was responding to both peptides. Using substituted and truncated peptides, we explored amino acid residues critical for cross-reactive CTL recognition, identified fine specificity similarities among all cross-reactive CTL lines but not non-cross-reactive lines, and mapped cross-reactivity to a 10-residue core of P18 and to an eight-residue core of HP53. A comparison of these peptide sequences and recent data on residues of P18 interacting with H-2Dd provided us with clues to residues involved in the interaction of the CTL with the MHC-peptide complex.

Reciprocal cytotoxic T lymphocyte cross-reactivity interactions between two major epitopes within HIV-1 gp160

We have observed and analyzed an unexpected cross-reactivity of CD8+ CTL between two nonhomologous peptides of the HIV-1 IIIB gp160 envelope protein, P18 (residues 315-329) and HP53 (834-848, also called TH4.1), in the context of four different class I MHC molecules, Dd, Dp, Dq (or Lq), and H-2u. In strains expressing Dd, the cross-reactivity between peptides was bidirectional, whereas in other strains (H-2u, H-2p, and H-2q), the cross-reactivity was unidirectional; that is, P18-specific CTLs showed no killing against targets pulsed with HP53, although HP53 stimulated CTL showed cross-reactive lysis against P18-pulsed target cells. Cross-reactivity was also shown in immunization in vivo and with target cells endogenously expressing viral protein in vitro using two different recombinant vaccinia viruses expressing only the N-terminal portion of gp160, containing P18 but not HP53. Peptide cross-contamination was excluded. Cold target inhibition and single cell cloning experiments indicated that the same CTL was responding to both peptides. Using substituted and truncated peptides, we explored amino acid residues critical for cross-reactive CTL recognition, identified fine specificity similarities among all cross-reactive CTL lines but not non-cross-reactive lines, and mapped cross-reactivity to a 10-residue core of P18 and to an eight-residue core of HP53. A comparison of these peptide sequences and recent data on residues of P18 interacting with H-2Dd provided us with clues to residues involved in the interaction of the CTL with the MHC-peptide complex.

The natural killer cell receptor Ly-49A recognizes a peptide-induced conformational determinant on its major histocompatibility complex class I ligand

Natural killer (NK) cells are inhibited from killing cellular targets by major histocompatibility complex (MHC) class I molecules. In the mouse, this can be mediated by the Ly-49A NK cell receptor that specifically binds the H-2Dd MHC class I molecule, then inhibits NK cell activity. Previous experiments have indicated that Ly-49A recognizes the alpha 1/alpha 2 domains of MHC class I and that no specific MHC-bound peptide appeared to be involved. We demonstrate here that alanine-substituted peptides, having only the minimal anchor motifs, stabilized H-2Dd expression and provided resistance to H-2Dd-transfected, transporter associated with processing (TAP)-deficient cells from lysis by Ly-49A+ NK cells. Peptide-induced resistance was blocked only by an mAb that binds a conformational determinant on H-2Dd. Moreover, stabilization of "empty" H-2Dd heavy chains by exogenous beta 2-microglobulin did not confer resistance. In contrast to data for MHC class I-restricted T cells that are specific for peptides displayed MHC molecules, these data indicate that NK cells are specific for a peptide-induced conformational determinant, independent of specific peptide. This fundamental distinction between NK cells and T cells further implies that NK cells are sensitive only to global changes in MHC class I conformation or expression, rather than to specific pathogen-encoded peptides. This is consistent with the "missing self" hypothesis, which postulates that NK cells survey tissues for normal expression of MHC class I.

Phosphorylation of extracellular domains of T-lymphocyte surface proteins. Constitutive serine and threonine phosphorylation of the T cell antigen receptor ectodomains

The extracellular accumulation of ATP after activation of T-lymphocytes, as well as the presence of ecto-protein kinases in these cells, led us to propose that T cell surface receptors could be regulated through the reversible phosphorylation of their extracellular domains (ectodomains). Here, in a model system, we used T cell transfectants which express T cell antigen receptor chains lacking intracellular and transmembrane protein domains and 32Pi metabolic labeling of cells to definitively demonstrate phosphorylation of ectodomains of T cell surface proteins. We show that alphabetaTCR ectodomains were phosphorylated intracellularly and constitutively on serine and threonine residues and were then expressed on the T cell surface in phosphorylated form. TCR ectodomains also could be phosphorylated at the cell surface when extracellular [gamma-32P]ATP or [gamma-32P]GTP were used as phosphate donors with the same cells. Consensus phosphorylation sites for serine and threonine protein kinases were found to be strongly evolutionary conserved in both alpha and beta TCR chains constant regions. These results are consistent with the hypothesis, where T cell surface proteins which are phosphorylated intracellularly on their ectodomains, could subsequently be expressed at the cell surface and then be reversibly modified by ectoprotein phosphatase(s) and by ectokinase(s). Such modifications may change T cells cognate interactions by, e.g. affecting TCR-multimolecular complex formation and antigen binding affinity. It is suggested that alphabetaTCR ectodomain phosphorylation could serve as a potential mechanism for regulation of alphabetaTCR-mediated T-lymphocytes response.

A T cell receptor V alpha domain expressed in bacteria: does it dimerize in solution?

To evaluate the potential for dimerization through a particular T cell receptor (TCR) domain, we have cloned the cDNA encoding a TCR V alpha from a hybridoma with specificity for the human immunodeficiency virus (HIV) envelope glycoprotein 120-derived peptide P18-110 (RGPGRAFVTI) bound to the murine major histocompatibility complex (MHC) class I molecule, H-2Dd. This cDNA was then expressed in a bacterial vector, and protein, as inclusion bodies, was solubilized, refolded, and purified to homogeneity. Yield of the refolded material was from 10 to 50 mg per liter of bacterial culture, the protein was soluble at concentrations as high as 25 mg/ml, and it retained a high level of reactivity with an anti-V alpha 2 monoclonal antibody. This domain was monomeric both by size exclusion gel chromatography and by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Circular dichroism spectra indicated that the folded V alpha domain had secondary structure similar to that of single immunoglobulin or TCR domains, consisting largely of beta sheet. Conditions for crystallization were established, and at least two crystal geometries were observed: hexagonal bipyramids that failed to diffract beyond approximately 6 A, and orthorhombic crystals that diffracted to 2.5 A. The dimerization of the V alpha domain was investigated further by solution nuclear magnetic resonance spectroscopy, which indicated that dimeric and monomeric forms of the protein were about equally populated at a concentration of 1 mM. Thus, models of TCR-mediated T cell activation that invoke TCR dimerization must consider that some V alpha domains have little tendency to form homodimers or multimers.

Peptide libraries define the fine specificity of anti-polysaccharide antibodies to Cryptococcus neoformans

Cryptococcus neoformans is an encapsulated fungus that causes a life-threatening meningoencephalitis in patients with AIDS. Monoclonal antibodies to the capsular glucuronoxylomannan can modulate the infection in mice, but the epitopes on this complex polysaccharide recognized by protective and non-protective antibodies have not been defined. We have used 2H1, one of our most protective antibodies, to screen phage display peptide libraries for peptide mimotopes that would allow us to explore the fine specificity of anti-cryptococcal polysaccharide antibodies. Hexa- and decapeptides have been identified with sequence homologies that define four motifs: 1, (E)TPXWM/LM/L; 2, W/YXWM/ LYE; 3, DWXDW; and 4, (Ar)WDGQ(Ar). Peptides representing these motifs compete with each other for a shared binding site that overlaps the polysaccharide binding site. Motifs 1 and 2 confer high affinity binding, and PA1, which displays a motif 1 peptide with the sequence LQYTPSWMLV, binds to 2H1 with a Kd of 295 nM. Analysis of the interaction between the 2H1 binding peptides and 24 structurally related anti-polysaccharide antibodies reveals a complex pattern of reactivity that strongly suggests binding to or close to the complementary determining regions. Furthermore, those antibodies that have been shown to have different specificity, and in some cases different protective potential, do not bind any of the peptides selected by the protective 2H1 antibody. This study shows that peptide mimotopes for a complex microbial polysaccharide can be identified by screening phage peptide libraries and demonstrates the usefulness of such peptides in analyzing closely related interactive sites of proteins in general and of antibodies in particular.

A recombinant single-chain HLA-A2.1 molecule, with a cis active beta-2-microglobulin domain, is biologically active in peptide binding and antigen presentation

We have constructed a recombinant single-chain human HLA-A2.1 molecule (from A*0201) with a covalently attached beta 2m. This molecule (MSC beta A2.1) can be detected on the surface of transfected beta 2m- human cells by conformational antibodies W6/32 and BB7.2 and by anti-human beta 2m mAb BM-63. The covalent beta 2m, now a domain of the MSC beta A2.1 molecule, does not rescue endogenous Class I surface expression. Instead, it works in cis to achieve correct folding of the single-chain molecule. Immunoprecipitation shows that MSC beta A2.1 is a 60-kDa molecule with no dissociable beta 2m. The half-life of the MSC beta A2.1 molecule on transfected cell surfaces was as long as that of two-chain HLA-A2.1 molecules. The MSC beta A2.1 molecule was active in presentation of HTLV-I Tax 11-19 peptide and an endogenous peptide to specific CTL. MSC beta A2.1 molecules and wild-type HLA-A2.1 molecules on live cells can bind the HBV core peptide 18-27 with comparable affinities. These results show that MSC beta A2.1 molecules retain the functional ability to present both pulsed and endogenous antigens to the appropriate T cells, and thus may be useful components of antiviral vaccines.

Studying interactions involving the T-cell antigen receptor by surface plasmon resonance

T-lymphocyte activation is initiated by the interaction of the alpha beta TCR with a complex consisting of a class I or class II MHC-encoded molecule and an antigenic peptide, displayed on the surface of an antigen-presenting cell. Real-time binding measurements using surface plasmon resonance have revealed kinetic and equilibrium parameters for the interactions between purified MHC molecules and peptides, between TCR and MHC-peptide complexes, and between TRC and superantigens. The MHC-peptide interaction is characterized by its high affinity and long half-life, the TCR-MHC/peptide interaction by its low affinity and short half-life, and the TCR-superantigen interaction by its low-to-moderate affinity, which is dependent on the particular superantigen involved. The consistent finding is that both MHC-peptide complexes and superantigens interact with TCR with a low affinity attributable to rapid dissociation. That an MHC-peptide complex that encounters a single TCR only briefly can still deliver the necessary activation signals offers a mechanistic conundrum for which several solutions have been proposed.

A targeted glucocorticoid receptor antisense transgene increases thymocyte apoptosis and alters thymocyte development

The exquisite sensitivity of thymocytes to steroid-induced apoptosis, the steroidogenic potential of thymic epithelial cells, and the ability of steroid synthesis inhibitors to enhance antigen-specific deletion of thymocytes in fetal thymic organ cultures suggest a role for glucocorticoids in thymocyte development. To address this further, transgenic mice that express antisense transcripts to the glucocorticoid receptor (GR) specifically in immature thymocytes were generated. The consequent hyporesponsiveness of thymocytes to glucocorticoids was accompanied by a reduction in thymic size, primarily owing to a decrease in the number of CD4+CD8+ cells. While an enhanced susceptibility to T cell receptor (TCR)-mediated apoptosis appeared to be partially responsible for this reduction, thymocyte loss could also be detected before thymocytes progressed to the CD4+CD8+ TCR alpha beta-expressing stage. These results suggest that glucocorticoids are necessary for survival and maturation of thymocytes, and are consistent with a role for steroids in both the transition from CD4-CD8- to CD4+CD8+ cells and the survival of CD4+CD8+ cells stimulated via the TCR.

Polymorphism at position nine of the MHC class I heavy chain affects the stability of association with beta 2-microglobulin and presentation of a viral peptide

To identify residues that control the interactions between MHC-heavy chains and (beta 2m) sequence comparisons were made between murine class I MHC molecules with high (H-2Dd, H-2Kb) and low (H-2Ld, H-2Db) affinities for beta 2m. A single residue at position 9 was evaluated for its contribution to the stability of the complex. Mutagenesis of the glutamic acid at position 9 of H-2Ld to valine, as is found in H-2Dd and H-2Kb, resulted in both qualitative and quantitative effects on inter-chain interactions, intracellular transport, peptide binding, and peptide presentation. In in vitro translation and assembly studies, the E9V mutation resulted in a more stable association of beta 2m with the heavy chain after immunoprecipitation with the alpha 2 domain-specific Ab 30-5-7 in the presence of an H-2Ld-restricted peptide. E9V variant expressed in transfected L cells had similar surface expression compared with H-2Ld despite exhibiting a slower rate of maturation. However, cells expressing E9V were unable to present peptide Ag to a specific T cell hybridoma. H-2LdE9V in E-3 cells, which are defective in TAP-dependent peptide transport, was expressed at higher levels than H-2Ld and was stabilized more efficiently by the addition of exogenous human beta 2m. Thus, amino acid position 9 not only plays an important role in the interaction of the MHC-1 molecule with the beta 2m, it also qualitatively and quantitatively influences peptide binding and Ag presentation.

Polymorphism at position nine of the MHC class I heavy chain affects the stability of association with beta 2-microglobulin and presentation of a viral peptide

To identify residues that control the interactions between MHC-heavy chains and (beta 2m) sequence comparisons were made between murine class I MHC molecules with high (H-2Dd, H-2Kb) and low (H-2Ld, H-2Db) affinities for beta 2m. A single residue at position 9 was evaluated for its contribution to the stability of the complex. Mutagenesis of the glutamic acid at position 9 of H-2Ld to valine, as is found in H-2Dd and H-2Kb, resulted in both qualitative and quantitative effects on inter-chain interactions, intracellular transport, peptide binding, and peptide presentation. In in vitro translation and assembly studies, the E9V mutation resulted in a more stable association of beta 2m with the heavy chain after immunoprecipitation with the alpha 2 domain-specific Ab 30-5-7 in the presence of an H-2Ld-restricted peptide. E9V variant expressed in transfected L cells had similar surface expression compared with H-2Ld despite exhibiting a slower rate of maturation. However, cells expressing E9V were unable to present peptide Ag to a specific T cell hybridoma. H-2LdE9V in E-3 cells, which are defective in TAP-dependent peptide transport, was expressed at higher levels than H-2Ld and was stabilized more efficiently by the addition of exogenous human beta 2m. Thus, amino acid position 9 not only plays an important role in the interaction of the MHC-1 molecule with the beta 2m, it also qualitatively and quantitatively influences peptide binding and Ag presentation.

Identification of the peptide binding motif for HLA-B44, one of the most common HLA-B alleles in the Caucasian population

Most peptides that bind to a particular MHC class I molecule share amino acid residues that are thought to physically "anchor" the peptide to polymorphic pockets within the class I binding site. Sequence analysis of endogenous peptides bound to HLA-B44 revealed two potential dominant anchor residues: Glu at P2 and Tyr, or occasionally Phe, at P9. In vitro assembly assays employing synthetic peptides and recombinant HLA-B44 produced by Escherichia coli revealed that an acidic amino acid at P2 was necessary for promoting stable peptide binding to HLA-B44. Surprisingly, although Tyr was almost exclusively found at P9 of the endogenous peptide sequences, a wide variety of amino acid residues such as Leu, Ala, Arg, Lys, His, and Phe could be tolerated at this position. Using this information, we identified antigenic peptides from the influenza virus components nonstructural protein 1 and nucleoprotein that are presented by HLA-B44 to antiinfluenza type A cytotoxic T lymphocytes. In addition, cytotoxic T lymphocytes induced by these antigenic peptides were shown to be capable of recognizing endogenously processed peptides from influenza-infected cells, indicating a potential use for these peptides in vaccine development. Finally, molecular models were created to investigate the possible ways in which the anchor residues might function to stabilize the binding of peptides to HLA-B44, and these models indicate that the acidic residue at P2 most likely interacts primarily with Lys 45 of the HLA-B44 heavy chain and makes additional contacts with Ser 67 and Tyr 9.

Lack of strict correlation of functional sensitization with the apparent affinity of MHC/peptide complexes for the TCR

We describe a comprehensive analysis of the effect of avidity of TCR-MHC/peptide interaction on activation of the (p2Ca). In study, monosubstituted variants of p2Ca were used and assessed for binding to purified H-2Ld, binding of H-2Ld/peptide complexes to sTCR, and ability to activate 2C cells to two independent effector functions. Among the > 20 variants analyzed, functional activity of most peptides that bound the MHC well correlated with the strength of interaction of MHC/peptide complexes with sTCR. However, with some variants, a clear discordance between the apparent TCR-MHC/peptide affinity and biologic function was observed, demonstrating that the former cannot always be gauged by the latter. In the case of L4 peptide (phenylalanine at position 4 substituted with leucine), peptide/MHC complexes showed no detectable binding to sTCR, indicating a 10-fold or greater decrease in affinity. Nevertheless, this peptide sensitized target cells for lysis at a level equivalent to the parental peptide. A clearer understanding was revealed by studying the extent to which activation by variant peptides was dependent on CD8. Our data indicate that resistance to anti-CD8 mAb blocking correlates with strong binding affinity between sTCR and MHC/peptide complexes. These data suggest that, for the activation of CTL function, the absolute level of intrinsic affinity of TCR for MHC/peptide ligand is not a single critical determinant, but rather, that activation is governed by the compound influence of several factors, which ensures a minimum threshold of intracellular triggering is reached to elicit the response.

Lack of strict correlation of functional sensitization with the apparent affinity of MHC/peptide complexes for the TCR

We describe a comprehensive analysis of the effect of avidity of TCR-MHC/peptide interaction on activation of the (p2Ca). In study, monosubstituted variants of p2Ca were used and assessed for binding to purified H-2Ld, binding of H-2Ld/peptide complexes to sTCR, and ability to activate 2C cells to two independent effector functions. Among the > 20 variants analyzed, functional activity of most peptides that bound the MHC well correlated with the strength of interaction of MHC/peptide complexes with sTCR. However, with some variants, a clear discordance between the apparent TCR-MHC/peptide affinity and biologic function was observed, demonstrating that the former cannot always be gauged by the latter. In the case of L4 peptide (phenylalanine at position 4 substituted with leucine), peptide/MHC complexes showed no detectable binding to sTCR, indicating a 10-fold or greater decrease in affinity. Nevertheless, this peptide sensitized target cells for lysis at a level equivalent to the parental peptide. A clearer understanding was revealed by studying the extent to which activation by variant peptides was dependent on CD8. Our data indicate that resistance to anti-CD8 mAb blocking correlates with strong binding affinity between sTCR and MHC/peptide complexes. These data suggest that, for the activation of CTL function, the absolute level of intrinsic affinity of TCR for MHC/peptide ligand is not a single critical determinant, but rather, that activation is governed by the compound influence of several factors, which ensures a minimum threshold of intracellular triggering is reached to elicit the response.