A sensitive fluorometric assay for quantitatively measuring specific peptide binding to HLA class I and class II molecules

Development of an Immortalized Porcine Fibroblast Cell Panel With Different Swine Leukocyte Antigen Genotypes

Immortalized cell lines are valuable resources to expand the molecular characterization of major histocompatibility complex genes and their presented antigens. We generated a panel of immortalized cell lines by transfecting human telomerase reverse transcriptase (hTERT) into primary fibroblast cells prepared from ear, fetal, and lung tissues of 10 pigs from five breeds and successfully cultured them for 30-45 passages. The cell growth characteristic of the immortalized fibroblasts was similar to that of primary fibroblast, which was unable to form colonies on soft agar. The genotypes of major swine leukocyte antigen (SLA) genes, including three classical class I (SLA-1, -2, and -3) and three class II genes (DQB1, DRB1, and DQA), were determined using high-resolution typing. A total of 58 alleles, including a novel allele for SLA-2, were identified. Each cell line was unique. A cell line derived from a National Institutes of Health miniature pig was homozygous across the six major SLA genes. The expression levels of SLA classical class I genes varied among the cell lines and were slightly upregulated in the immortalized compared to the primary cells based on semiquantitative reverse transcription polymerase chain reaction. The immortalized porcine fibroblast cell lines with diverse SLA haplotypes that were developed in this study have potential to be applied in studies regarding the molecular characteristics and genetic structure of SLA genes and epitope-major histocompatibility complex interactions in pigs.

Discovery of HLA-E-Presented Epitopes: MHC-E/Peptide Binding and T-Cell Recognition

Understanding the interactions involved during the immunological synapse between peptide, HLA-E molecules, and TCR is crucial to effectively target protective HLA-E-restricted T-cell responses in humans. Here we describe three techniques based on the generation of MHC-E/peptide complexes (MHC-E generically includes HLA-E-like molecules in human and nonhuman species, while HLA-E specifically refers to human molecules), which allow to investigate MHC-E/peptide binding at the molecular level through binding assays and by using peptide loaded HLA-E tetramers, to detect, isolate, and study peptide-specific HLA-E-restricted human T-cells.

Molecular cancer vaccines: Tumor therapy using antigen-specific immunizations

Vaccination against tumors promises selective destruction of malignant cells by the host's immune system. Molecular cancer vaccines rely on recently identified tumor antigens as immunogens. Tumor antigens can be applied in many forms, as genes in recombinant vectors, as proteins or peptides representing T cell epitopes.Analysis of various aspects indicates some advantage for peptide-based vaccines over the other modalities. Further refinements and extensively monitored clinical trials are necessary to advance molecular cancer vaccines from concepts into powerful therapy.

Discovery of low-affinity preproinsulin epitopes and detection of autoreactive CD8 T-cells using combinatorial MHC multimers

Autoreactive cytotoxic CD8 T-cells (CTLs) play a key pathogenic role in the destruction of insulin-producing beta-cells resulting in type 1 diabetes. However, knowledge regarding their targets is limited, restricting the ability to monitor the course of the disease and immune interventions. In a multi-step discovery process to identify novel CTL epitopes in human preproinsulin (PPI), PPI was digested with purified human proteasomes, and resulting COOH-fragments aligned with algorithm-predicted HLA-binding peptides to yield nine potential HLA-A1, -A2, -A3 or -B7-restricted candidates. An UV-exchange method allowed the generation of a repertoire of multimers including low-affinity HLA-binding peptides. These were labeled with quantum dot-fluorochromes and encoded in a combinatorial fashion, allowing parallel and sensitive detection of specific, low-avidity T-cells. Significantly increased frequencies of T-cells against four novel PPI epitopes (PPI(4-13)/B7, PPI(29-38)/A2, PPI(76-84)/A3 and PPI(79-88)/A3) were detected in stored blood of patients with recent onset diabetes but not in controls. Changes in frequencies of circulating CD8 T-cells against these novel epitopes were detected in blood of islet graft recipients at different time points after transplantation, which correlated with clinical outcome. In conclusion, our novel strategy involving a sensitive multiplex detection technology and requiring minimal volumes of stored blood represents a major improvement in the direct ex-vivo characterization and enumeration of immune cells in the pathogenesis of type 1 diabetes.

Competition-based cellular peptide binding assay for HLA class I

This unit describes a competition assay to determine binding of unlabeled test peptides to thirteen of the most prevalent HLA class I molecules. It uses cells expressing the HLA class I molecule of interest on their surface, fluorescently labeled reference peptides, and unlabeled test peptides. Cells of interest are stripped from their natural HLA-bound peptides using acid treatment and subsequently incubated with a mixture of labeled reference peptide and titrating concentrations of test peptide. Subsequently, FACS analysis is performed to determine the amount of bound reference peptide, which is a measure of the ability of test peptide to compete for binding to HLA. The assay provides IC50 values for binding of test peptides to HLA molecules. It can be performed in a normally equipped cellular laboratory, requires no additional equipment besides a flow cytometer (FACS), and is relatively easy to perform. Assay-specific parameters for several HLA alleles are provided.

Conditional MHC class I ligands and peptide exchange technology for the human MHC gene products HLA-A1, -A3, -A11, and -B7

Major histocompatibility complex (MHC) class I multimer technology has become an indispensable immunological assay system to dissect antigen-specific cytotoxic CD8(+) T cell responses by flow cytometry. However, the development of high-throughput assay systems, in which T cell responses against a multitude of epitopes are analyzed, has been precluded by the fact that for each T cell epitope, a separate in vitro MHC refolding reaction is required. We have recently demonstrated that conditional ligands that disintegrate upon exposure to long-wavelength UV light can be designed for the human MHC molecule HLA-A2. To determine whether this peptide-exchange technology can be developed into a generally applicable approach for high throughput MHC based applications we set out to design conditional ligands for the human MHC gene products HLA-A1, -A3, -A11, and -B7. Here, we describe the development and characterization of conditional ligands for this set of human MHC molecules and apply the peptide-exchange technology to identify melanoma-associated peptides that bind to HLA-A3 with high affinity. The conditional ligand technology developed here will allow high-throughput MHC-based analysis of cytotoxic T cell immunity in the vast majority of Western European individuals.

Direct class I HLA antigen discovery to distinguish virus-infected and cancerous cells

Class I human leukocyte antigen molecules are nature's proteome-scanning chips, presenting thousands of endogenously loaded peptides on the surface of virtually every cell in the body. Cytotoxic T cells survey the class I human leukocyte antigen peptide cargo presented, recognize peptides unique to unhealthy cells and destroy diseased cells. A precise understanding of how class I molecules distinguish diseased cells is positioned to drive immune-based diagnostics, therapies and vaccines. When identifying epitopes unique to unhealthy cells, the most experimentally direct approach is to examine the class I-presented peptides of infected/cancerous cells. Here we discuss the strategies adapted for protein production, protein/peptide purification, peptide separation and for maintaining experimental reproducibility during the direct characterization of class I human leukocyte antigen peptides.

Reverse immunology approach for the identification of CD8 T-cell-defined antigens: advantages and hurdles

One of the challenges of tumour immunology remains the identification of strongly immunogenic tumour antigens for vaccination. Reverse immunology, that is, the procedure to predict and identify immunogenic peptides from the sequence of a gene product of interest, has been postulated to be a particularly efficient, high-throughput approach for tumour antigen discovery. Over one decade after this concept was born, we discuss the reverse immunology approach in terms of costs and efficacy: data mining with bioinformatic algorithms, molecular methods to identify tumour-specific transcripts, prediction and determination of proteasomal cleavage sites, peptide-binding prediction to HLA molecules and experimental validation, assessment of the in vitro and in vivo immunogenic potential of selected peptide antigens, isolation of specific cytolytic T lymphocyte clones and final validation in functional assays of tumour cell recognition. We conclude that the overall low sensitivity and yield of every prediction step often requires a compensatory up-scaling of the initial number of candidate sequences to be screened, rendering reverse immunology an unexpectedly complex approach.

Autoreactive CD8 T cells associated with beta cell destruction in type 1 diabetes

Type 1 diabetes is a T cell-mediated autoimmune disease, and insulin is an important target of the autoimmune response associated with beta cell destruction. The mechanism of destruction is still unknown. Here, we provide evidence for CD8 T cell autoreactivity associated with recurrent autoimmunity and loss of beta cell function in type 1 diabetic islet transplant recipients. We first identified an insulin B chain peptide (insB10-18) with extraordinary binding affinity to HLA-A2(*0201) that is expressed by the majority of type 1 diabetes patients. We next demonstrated that this peptide is naturally processed by both constitutive and immuno proteasomes and translocated to the endoplasmic reticulum by the peptide transporter TAP1 to allow binding to HLA-A2 in the endoplasmic reticulum and cell surface presentation. Peripheral blood mononuclear cells from a healthy donor were primed in vitro with this peptide, and CD8 T cells were isolated that specifically recognize target cells expressing the insulin B chain peptide. HLA-A2(insB10-18) tetramer staining revealed a strong association between detection of autoreactive CD8 T cells and recurrent autoimmunity after islet transplantation and graft failure in type 1 diabetic patients. We demonstrate that CD8 T cell autoreactivity is associated with beta cell destruction in type 1 diabetes in humans.

Development and validation of a fluorescence polarization-based competitive peptide-binding assay for HLA-A*0201--a new tool for epitope discovery

Various approaches are currently proposed to successfully develop therapies for the prevention and treatment of infectious diseases and cancer. One of the most promising approaches is the development of vaccines that elicit cytotoxic T lymphocyte (CTL) responses. Consequently, identification and exact definition of molecular parameters involved in peptide-MHC class-I interactions of putative CTL epitopes are of prime importance for the development of immunomodulating compounds. To better facilitate epitope discovery, we developed and validated a novel state-of-the-art biochemical HLA-A0201 assay, which is comprised of technologically advanced cutting edge reagents. The technique is based on competition and uses a FITC-labeled reference peptide and highly purified soluble HLA-A0201 molecules to quantitatively measure the binding capacity of nonlabeled peptide candidates. Detection by fluorescence polarization allows real-time measurement of binding ratios without separation steps. During standardization, the problem of assay parameter variation is discussed, showing the dramatic influence of HLA and reference peptide concentrations as well as the choice of the reference peptide itself on IC(50) determinations. For validation, a panel of 15 well-defined HLA-A0201 ligands from various sources covering a broad range of binding affinities was tested. Binding data were used to compare against pre-existing quantitative assay systems. The results obtained demonstrated significant correlation among assay procedures, suggesting that the application of fluorescence polarization in combination with recombinant sHLA molecules is highly advantageous for the accurate assessment of peptide binding. Furthermore, the assay also features high-throughput screening capacity, providing uniquely efficient means of identifying and evaluating immune target molecules.

Diverse CD8+ T-Cell Responses to Renal Cell Carcinoma Antigens in Patients Treated with an Autologous Granulocyte-Macrophage Colony-Stimulating Factor Gene-Transduced Renal Tumor Cell Vaccine

A phase I clinical trial with granulocyte-macrophage colony-stimulating factor tumor cell vaccines in patients with metastatic renal cell carcinoma (RCC) showed immune cell infiltration at vaccine sites and delayed-type hypersensitivity (DTH) responses to autologous tumor cells indicative of T-cell immunity. To further characterize RCC T-cell responses and identify relevant RCC-associated antigens, we did a detailed analysis of CD8+ T-cell responses in two vaccinated RCC patients who generated the greatest magnitude of DTH response and also displayed a strong clinical response to vaccination (>90% reduction in metastatic tumor volume). Three separate CD8+ T-cell lines (and subsequent derived clones) derived from patient 24 recognized distinct RCC-associated antigens. One recognized a shared HLA-A*0201-restricted antigen expressed by both renal cancer cells and normal kidney cells. This recognition pattern correlated with a positive DTH test to normal kidney cells despite no evidence of impairment of renal function by the patient's remaining kidney after vaccination. A second line recognized a shared HLA-C7-restricted antigen that was IFN-γ inducible. A third line recognized a unique HLA-A*0101-restricted RCC antigen derived from a mutated KIAA1440 gene specific to the tumor. In addition, two independent CTL lines and three clones were also generated from patient 26 and they recognized autologous tumor cells restricted through HLA-A*0205, HLA-A/B/C, and HLA-B/C. These results show that paracrine granulocyte-macrophage colony-stimulating factor tumor vaccines may generate a diverse repertoire of tumor-reactive CD8+ T-cell responses and emphasize the importance of polyvalency in the design of cancer immunotherapies.

Competition-based cellular peptide binding assays for 13 prevalent HLA class I alleles using fluorescein-labeled synthetic peptides

We report the development, validation, and application of competition-based peptide binding assays for 13 prevalent human leukocyte antigen (HLA) class I alleles. The assays are based on peptide binding to HLA molecules on living cells carrying the particular allele. Competition for binding between the test peptide of interest and a fluorescein-labeled HLA class I binding peptide is used as read out. The use of cell membrane-bound HLA class I molecules circumvents the need for laborious biochemical purification of these molecules in soluble form. Previously, we have applied this principle for HLA-A2 and HLA-A3. We now describe the assays for HLA-A1, HLA-A11, HLA-A24, HLA-A68, HLA-B7, HLA-B8, HLA-B14, HLA-B35, HLA-B60, HLA-B61, and HLA-B62. Together with HLA-A2 and HLA-A3, these alleles cover more than 95% of the Caucasian population. Several allele-specific parameters were determined for each assay. Using these assays, we identified novel HLA class I high-affinity binding peptides from HIVpol, p53, PRAME, and minor histocompatibility antigen HA-1. Thus these convenient and accurate peptide-binding assays will be useful for the identification of putative cytotoxic T lymphocyte epitopes presented on a diverse array of HLA class I molecules.

Activation requirements of circulating antigen-specific human CD8(+) memory T cells probed with insect cell-based artificial antigen-presenting cells

We sought to define the molecular setup of an antigen-presenting cell that elicits antigen-specific T cell responses in vitro using insect cells that were infected with recombinant baculoviruses. Expression of single-chain HLA was complemented step-by-step with costimulatory molecules, including CD54 and CD80, by co-infection with the relevant viruses. Role of CD8 was assessed by introducing hybrid class I molecules where the alpha-3 domain of the HLA heavy chain molecule was replaced by its murine K(b) counterpart. Circulating T cells that respond to the EBV-derived HLA-A2-restricted peptide GLGCTLVAML were previously shown to bear hallmarks of memory cells. We found that the HLA+peptide complex alone displayed on the surface of insect cells was sufficient to elicit IFN-gamma secretion from these freshly isolated CD8(+) T cells in ELISpot assays. Binding of CD8 was absolutely required, but coexpression of costimulatory molecules resulted only in minimal increase in the number of spots. Tumor antigen-specific CTL clones also reacted in a strictly antigen-specific manner, but required CD54 for quantitative responses. The amount of IFN-gamma produced by the individual reactive T cells was evaluated as spot size, and was also influenced by the costimulatory molecules: CD54 increased also the response magnitude of cultured CTL lines, while CD80 enhanced cytokine release from freshly isolated CD8(+) T cells. Understanding the stimulatory requirements of functionally competent effector/memory T cells and their exact enumeration will be helpful for increasing the efficacy of vaccines.

The identification of a common pathogen-specific HLA class I A*0201-restricted cytotoxic T cell epitope encoded within the heat shock protein 65

Bacterial antigens recognized by CD8(+) T cells in the context of MHC class I are thought to play a crucial role in protection against pathogenic intracellular bacteria. Here, we demonstrate the induction of HLA-A*0201-restricted CD8(+) T cell responses against six new high-affinity HLA-A*0201-binding CTL epitopes, encoded within an immunodominant and highly conserved antigen of Mycobacteria, the heat shock protein 65 (hsp65). One of these epitopes, Mhsp65(9(369)), is identical in a large number of pathogenic bacteria, and is recognized in a CD8-independent fashion. Mhsp65(9(369)) could be presented by either mycobacterial hsp65-pulsed target cells or BCG-infected macrophages. Interestingly, T cells specific for this epitope did not recognize the corresponding human hsp65 homologue, probably due to structural differences as revealed by modeling studies. Furthermore, in vitro proteasome digestion analyses show that, whereas the mycobacterial hsp65 epitope is efficiently generated, the human hsp65 homologue is not, thus avoiding the induction of autoreactivity. Collectively, these findings describe high-affinity HLA class I-binding epitopes that are naturally processed and are recognized efficiently by MHC class I-restricted CD8(+) T cells, providing a rational basis for the development of subunit vaccine strategies against tuberculosis and other intracellular infectious diseases.

Use of fluorescence polarization to monitor MHC-peptide interactions in solution

We describe here fluorescence polarization-based methods to investigate class I MHC-peptide interactions in solution. Fluorescein-labelled peptides were used to determine MHC/peptide complex association and dissociation constants as well as the equilibrium binding constant (KD). Furthermore, we developed a competition assay for the determination of IC50 values of nonlabelled compounds. Both kinetic and equilibrium parameters are of prime importance for the development of immunomodulating compounds. The assays described here show a good reproducibility and require only picomolar amounts of labelled tracers. A high ratio between the experimental values obtained for bound and free labelled ligand as well as a low standard deviation, permits the detection of class I MHC ligands with low affinity. Fluorescence polarization allows the direct measurement of the ratio between free and bound labelled ligand in solution without any separation step. Thus, in combination with microtiter-plates, the time for analysis is significantly decreased to 10 s per sample. Our assays represent versatile tools for characterizing the binding of single ligands as well as for rapid screening of large numbers of compounds.

Beta-amino acid scan of a class I major histocompatibility complex-restricted alloreactive T-cell epitope

An HLA-B27-restricted self-octapeptide known to react with an alloreactive T-cell receptor has been modified by systematic substitution of a beta-amino acid for the natural alpha-amino acid residue, over the whole length of the parent epitope. All modified peptides were shown to bind to recombinant HLA-B*2705 and induce stable major histocompatibility complex-peptide complexes, but with some variation depending on the position of the beta-amino acid on the peptide sequence. Alteration of the natural peptide sequence at the two N-terminal positions (positions 1 and 2) decreases binding affinity and thermodynamic stability of the refolded complex, but all other positions (from position 3 to the C-terminal residue) were insensitive to the beta-amino acid substitution. All modified peptides were recognized by an alloreactive T-cell clone specific for the parent epitope with decreased efficiency, to an extent dependent of the position that was modified. Furthermore, the introduction of a single beta-amino acid at the first two positions of the modified peptide was shown to be sufficient to protect them against enzymatic cleavage. Thus, beta-amino acids represent new interesting templates for alteration of T-cell epitopes to design either synthetic vaccines of T-cell receptor antagonists.

Mutation of Cys-67 alters the thermodynamic stability of the human leukocyte antigen HLA0-B*2705

The B pocket of the class I major histocompatibility complex-encoded protein HLA-B*2705 has recently been suggested to be responsible for the misfolding of this HLA haplotype and thus to induce susceptibility to autoimmune inflammatory diseases. Four mutants of the B*2705 heavy chain were refolded in the presence of three control peptides. The monitoring of the thermal unfolding of the B*2705-peptide complexes by circular dichroism spectroscopy showed that all heterotrimeric mutants were markedly less stable than the corresponding complexes with the wild-type heavy chain. Among the four heavy chain mutations, the C67S change was investigated for unfolding and peptide binding properties because this position may mediate disulfide pair bridging and alter T-cell recognition of HLA-B*2705. Wild-type heterotrimers completely unfold in a single transition at mild acidic pH whereas increase of the pH to mild basic conditions induce only a partial biphasic unfolding. Cys-67 seems to play a crucial role in controlling the thermodynamic stability of the B*2705-peptide complexes as the C67S mutant unfolds faster and with a single transition, independent of pH. Fluorescence polarization and size exclusion chromatography of unfolding intermediates suggest that the peculiar unfolding of the B*2705 wild-type heavy chain cannot be explained by modified peptide binding properties but more likely by the formation of high molecular weight species.

Quantitative determination of TCR cross-reactivity using peptide libraries and protein databases

A single T cell clone can be activated by many different peptides in the context of a particular HLA molecule. To quantify the number of peptides that can be recognized by a CD4(+) T cell clone, we screened a one-bead-one-peptide synthetic peptide library and a protein database for peptides that stimulate an HLA-DR3-restricted, human glutamic acid decarboxylase (GAD65)-reactive CD4(+) T cell clone. Both the library screening and the database analysis indicated that this T cell clone is able to recognize approximately 10(6) 11-mer peptides at low nanomolar concentration. Furthermore, we determined that the frequency of cross-reactivity increased only 1.5-3 times when the peptide concentration increased 10 times, in the range of 0.01 - 1 microM. These data imply that there is a considerable potential for T cell cross-reactivity and are useful for studies on the role of molecular mimicry in the etiology of T cell-mediated disease.

Identification of a Conserved Universal Th Epitope in HIV-1 Reverse Transcriptase That Is Processed and Presented to HIV-Specific CD4+ T Cells by at Least Four Unrelated HLA-DR Molecules

CD4+ Th cells play an important role in the induction and maintenance of specific T cell immunity. Indications for a protective role of CD4+ T cells against HIV-1 infection were found in subjects who were able to control HIV-1 viremia as well as in highly HIV-1-exposed, yet seronegative, individuals. This study describes the identification of an HIV-1-specific Th epitope that exhibits high affinity binding as well as high immunogenicity in the context of at least four different HLA-DR molecules that together cover 50–60% of the Caucasian, Oriental, and Negroid populations. This HIV-1 reverse transcriptase-derived peptide (RT171–190) is highly conserved among different HIV-1 isolates. Importantly, stimulation of PBL cultures from HIV-1 seronegative donors with this peptide resulted in Th1-type lymphocytes capable of efficient recognition of HIV-1-pulsed APCs. Taken together, these data indicate that peptide RT171–190 constitutes an attractive component of vaccines aiming at induction or enhancement of HIV-1-specific T cell immunity.

The Second Exon-Encoded Factor XII Region Is Involved in the Interaction of Factor XII With Factor XI and Does Not Contribute to the Binding Site for Negatively Charged Surfaces

Contact system activation, in vitro, is triggered by activation of factor XII (FXII) on binding to an activator, such as negatively charged surfaces. A putative surface-binding site of FXII has been located within the amino acid residues 1-28 by identifying the epitope recognized by a monoclonal antibody (MoAb), B7C9, which inhibits kaolin-induced clotting activity. To further elucidate the role of the amino terminal binding site in the regulation of FXII activation, we have characterized a FXII recombinant protein (rFXII-▵19) deleted of the amino acid residues 3-19, which are encoded by the second exon of FXII gene. A plasmid encoding for rFXII-▵19 was constructed and expressed in HepG2 cells by using vaccinia virus. Purified rFXII-▵19 migrated as a single band of Mr 77,000 on sodium dodecyl sulfate (SDS)-polyacrylamide gel, did not bind to MoAb B7C9 immobilized on Protein A-Sepharose, thus confirming that it lacked the epitope for this MoAb, and had no amidolytic activity towards the chromogenic substrate S-2302 in the absence of activator. rFXII-▵19 specific clotting activity was lower (44%) than that of native FXII. The activation rate of rFXII-▵19 by kallikrein in the absence of dextran sulfate was about four times higher than that of full-length FXII and was increased in the presence of dextran sulfate. However, rFXII-▵19 underwent autoactivation in the presence of dextran sulfate. Labeled rFXII-▵19 bound to kaolin, which binding was equally well inhibited by either, rFXII-▵19 or full-length FXII (IC50 = 7.2 ± 2.2 nmol/L for both proteins). Accordingly, a synthetic peptide corresponding to FXII amino acid residues 3-19 did not inhibit the binding of labeled full-length FXII to kaolin. rFXII-▵19 generated a similar amount of FXIIa- and kallikrein-C1–inhibitor complexes in FXII-deficient plasma in the presence of kaolin, as did full-length FXII; but generated less factor XIa-C1–inhibitor complexes (50%) than full-length FXII. This impaired factor XI activation by rFXII-▵19a was also observed in a purified system and was independent of the presence of high molecular weight kininogen. Furthermore, the synthetic peptide 3-19, preincubated with factor XI, inhibited up to 30% activation of factor XI both in the purified system as well as in plasma. These results together indicate that amino acid residues 3-19 of FXII are involved in the activation of factor XI and do not contribute to the binding of FXII to negatively charged surfaces.

The Second Exon-Encoded Factor XII Region Is Involved in the Interaction of Factor XII With Factor XI and Does Not Contribute to the Binding Site for Negatively Charged Surfaces

Contact system activation, in vitro, is triggered by activation of factor XII (FXII) on binding to an activator, such as negatively charged surfaces. A putative surface-binding site of FXII has been located within the amino acid residues 1-28 by identifying the epitope recognized by a monoclonal antibody (MoAb), B7C9, which inhibits kaolin-induced clotting activity. To further elucidate the role of the amino terminal binding site in the regulation of FXII activation, we have characterized a FXII recombinant protein (rFXII-▵19) deleted of the amino acid residues 3-19, which are encoded by the second exon of FXII gene. A plasmid encoding for rFXII-▵19 was constructed and expressed in HepG2 cells by using vaccinia virus. Purified rFXII-▵19 migrated as a single band of Mr 77,000 on sodium dodecyl sulfate (SDS)-polyacrylamide gel, did not bind to MoAb B7C9 immobilized on Protein A-Sepharose, thus confirming that it lacked the epitope for this MoAb, and had no amidolytic activity towards the chromogenic substrate S-2302 in the absence of activator. rFXII-▵19 specific clotting activity was lower (44%) than that of native FXII. The activation rate of rFXII-▵19 by kallikrein in the absence of dextran sulfate was about four times higher than that of full-length FXII and was increased in the presence of dextran sulfate. However, rFXII-▵19 underwent autoactivation in the presence of dextran sulfate. Labeled rFXII-▵19 bound to kaolin, which binding was equally well inhibited by either, rFXII-▵19 or full-length FXII (IC50 = 7.2 ± 2.2 nmol/L for both proteins). Accordingly, a synthetic peptide corresponding to FXII amino acid residues 3-19 did not inhibit the binding of labeled full-length FXII to kaolin. rFXII-▵19 generated a similar amount of FXIIa- and kallikrein-C1–inhibitor complexes in FXII-deficient plasma in the presence of kaolin, as did full-length FXII; but generated less factor XIa-C1–inhibitor complexes (50%) than full-length FXII. This impaired factor XI activation by rFXII-▵19a was also observed in a purified system and was independent of the presence of high molecular weight kininogen. Furthermore, the synthetic peptide 3-19, preincubated with factor XI, inhibited up to 30% activation of factor XI both in the purified system as well as in plasma. These results together indicate that amino acid residues 3-19 of FXII are involved in the activation of factor XI and do not contribute to the binding of FXII to negatively charged surfaces.

A europium fluoroimmunoassay for measuring peptide binding to MHC class I molecules

A fluoroimmunoassay employing europium-streptavidin and time-resolved fluorimetry was used to measure binding of biotin-labeled peptides to H-2Dk molecules. A fluorescein-labeled octameric peptide from the middle T (MT) protein of mouse polyoma virus was identified that specifically binds to purified Dk using a previously-established assay based on size exclusion chromatography. A europium immunoassay was adapted to measure binding of a biotin-derivative of this peptide to purified Dk. The assay was found to be sensitive and highly specific. Binding was optimal at pH 5.0 and 24-27 degrees C, and it was not enhanced in the presence of additional beta2-microglobulin (beta2m). Excellent results were also obtained in experiments with fixed cells that express Dk. This assay is expected to be useful for high volume, routine analysis of peptide binding to MHC class I molecules.

The minor histocompatibility antigen HA-1: a diallelic gene with a single amino acid polymorphism

The minor histocompatibility antigen (mHag) HA-1 is the only known mHag for which mismatching is correlated with the development of severe graft versus host disease (GvHD) after human leukocyte antigen-identical bone marrow transplantation. HA-1 was found to be a nonapeptide derived from an allele of the KIAA0223 gene. The HA-1-negative allelic counterpart encoded by KIAA0223 had one amino acid difference from HA-1. Family analysis with HA-1 allele-specific polymerase chain reaction showed an exact correlation between this allelic polymorphism and the HA-1 phenotype. HA-1 allele typing of donor and recipient should improve donor selection and allow the determination of bone marrow transplantation recipients with high risk for HA-1-induced GvHD development.

A novel, highly efficient peptide-HLA class I binding assay using unfolded heavy chain molecules: identification of HIV-1 derived peptides that bind to HLA-A*0201 and HLA-A*0301

A novel cell-free, highly automated peptide-HLA binding assay has been designed during which a mixture of unfolded recombinant HLA heavy chain molecules, beta 2-microglobulin and a fluorescent labeled standard peptide is allowed to form peptide-HLA complexes. The binding of a peptide of interest is monitored as the ability to inhibit the formation of fluorescent peptide-HLA complexes. The assay was validated using published, known HLA-A* 0201 and HLA-A* 0301 binding peptides. In addition a selected set of HIV-1LAI reverse transcriptase derived 10-mer peptides, that had been selected on the basis of HLA-A* 0201 or HLA-A* 0301 binding motifs, were tested for HLA-A* 0201/A* 0301 binding. In that set we identified 8 peptides which bound with high affinity to HLA-A* 0201 and 5 peptides which bound with high affinity to HLA-A* 0301. The major advantage of the use of denatured heavy chain is the improved economy and efficiency, as unfolded protein material is in principle easily accessible by recombinant technology.