Restoration of antigen presentation to the mutant cell line RMA-S by an MHC-linked transporter

The majority of H2-M3 is retained intracellularly in a peptide-receptive state and traffics to the cell surface in the presence of N-formylated peptides

We used a new monoclonal antibody (mAb 130) to analyze the intracellular trafficking and surface expression of H2-M3, the major histocompatibility complex class Ib molecule that presents N-formylated peptides to cytotoxic T cells. M3 surface expression is undetectable in most cell types due to the paucity of endogenous antigen. M3 is induced on the cell surface by addition of high-affinity N-formylated peptides from mitochondria and listeria. Peptide-induced M3 expression is most efficient on antigen presenting cells. Basal and inducible expression of M3 is transporter associated with antigen processing (TAP)-dependent, distinguishing M3 from the class Ib molecules TL and CD1. Unlike the expression of class Ia molecules and a previously described M3/L(d) chimera, surface expression of M3 cannot be rescued by lowered temperature, suggesting that the alpha3 domain and transmembrane region of M3 may control trafficking. Pulse-chase analysis and use of trafficking inhibitors revealed a pool of empty M3 in the endoplasmic reticulum or early Golgi apparatus. Addition of exogenous peptide allows maturation with kinetics matching those of D(d). The lack of endogenous N-formylated peptide allows discovery of novel pathogen-derived peptides in normal antigen presenting cells. The nonpolymorphic nature of M3 and its ability to present bacterial antigens rapidly and dominantly make it an attractive target for peptide vaccination strategies.

Expression, Linkage, and Polymorphism of MHC-Related Genes in Rainbow Trout, Oncorhynchus mykiss

The architecture of the MHC in teleost fish, which display a lack of linkage between class I and II genes, differs from all other vertebrates. Because rainbow trout have been examined for a variety of immunologically relevant genes, they present a good teleost model for examining both the expression and organization of MHC-related genes. Full-length cDNA and partial gDNA clones for proteasome δ, low molecular mass polypeptide (LMP) 2, TAP1, TAP2A, TAP2B, class Ia, and class IIB were isolated for this study. Aside from the expected polymorphisms associated with class I genes, LMP2 and TAP2 are polygenic. More specifically, we found a unique lineage of LMP2 (LMP2/δ) that shares identity to both LMP2 and δ but is expressed like the standard LMP2. Additionally, two very different TAP2 loci were found, one of which encodes polymorphic alleles. In general, the class I pathway genes are expressed in most tissues, with highest levels in lymphoid tissue. We then analyzed the basic genomic organization of the trout MHC in an isogenic backcross. The main class Ia region does not cosegregate with the class IIB locus, but LMP2, LMP2/δ, TAP1A, and TAP2B are linked to the class Ia locus. Interestingly, TAP2A (second TAP2 locus) is a unique lineage in sequence composition that appears not to be linked to this cluster or to class IIB. These results support and extend the recent findings of nonlinkage between class I and II in a different teleost order (cyprinids), suggesting that this unique arrangement is common to all teleosts.

Interleukin-10: a cytokine used by tumors to escape immunosurveillance

The mechanisms whereby malignant cells can elude the recognition of the immune system, by what is termed 'immunological escape', have attracted major attention of tumor immunologists during the past decade. In this review, the role of the immunosuppressive cytokine interleukin-10 (IL-10) will be discussed as a strategy used by tumors to avoid recognition by cytotoxic T lymphocytes (CTL).

Involvement of transporter associated with antigen processing 2 (TAP2) gene polymorphisms in hepatitis C virus infection

BACKGROUND & AIMS

Transporter associated with antigen processing (TAP) has essential roles in the antigen-presenting systems, translocating antigenic peptides from the cytosol into the endoplasmic reticulum. The aim of this study was to clarify whether TAP polymorphisms are involved in hepatitis C virus (HCV) infection.

METHODS

The 145 HCV-infected Japanese patients examined in this study were categorized into two groups: 36 carriers with persistently normal alanine transaminase (ALT) values and 109 patients with chronic liver disease (CLD). TAP2 gene phenotypes were determined by means of polymerase chain reaction-restriction fragment length polymorphism, and their frequencies were compared between the two groups.

RESULTS

Frequencies of TAP2*0101, *0102, and *0201 were not different between the two groups. However, TAP2*0103 frequency in carriers with normal ALT levels was significantly higher than that in patients with CLD (44% vs. 16%; P = 0.00064, Pc < 0.005). Although the TAP2*0103 allele was tightly linked with class II DRB1*1302-DQB1*0604 haplotype in this study, the TAP2*0103 frequency in the normal ALT group was also significantly higher than that in the CLD group even in DRB1*1302-DQB1*0604-negative patients (31% vs. 10%; P = 0.0076, Pc < 0.05).

CONCLUSIONS

These findings suggest that TAP2*0103 may be closely associated with low serum ALT activity in HCV-infected Japanese patients.

MHC Class I-Restricted Presentation of Maleylated Protein Binding to Scavenger Receptors

Pathways for loading exogenous protein-derived peptides on MHC class I are thought to be present mainly in monocyte-lineage cells and to involve phagocytosis- or macropinocytosis-mediated antigenic leakage into either cytosol or extracellular milieu to give peptide access to MHC class I. We show that maleylation of OVA enhanced its presentation to an OVA-specific MHC class I-restricted T cell line by both macrophages and B cells. This enhanced presentation involved uptake through receptors of scavenger receptor (SR)-like ligand specificity, was TAP-1-independent, and was inhibited by low levels (2 mM) of ammonium chloride. No peptide loading of bystander APCs by maleylated (maleyl) OVA-pulsed macrophages was detected. Demaleylated maleyl-OVA showed enhanced MHC class I-restricted presentation through receptor-mediated uptake and remained highly sensitive to 2 mM ammonium chloride. However, if receptor binding of maleyl-OVA was inhibited by maleylated BSA, the residual presentation was relatively resistant to 2 mM ammonium chloride. Maleyl-OVA directly introduced into the cytosol via osmotic lysis of pinosomes was poorly presented, confirming that receptor-mediated presentation of exogenous maleyl-OVA was unlikely to involve a cytosolic pathway. Demaleylated maleyl-OVA was well presented as a cytosolic Ag, consistent with the dependence of cytosolic processing on protein ubiquitination. Thus, receptor-specific delivery of exogenous protein Ags to APCs can result in enhanced MHC class I-restricted presentation, suggesting that the exogenous pathway of peptide loading for MHC class I may be a constitutive property dependent mainly on the quantity of Ag taken up by APCs.

Mechanisms of antigen presentation

T-lymphocytes recognize short peptide antigens bound stably to polymorphic major histocompatibility complex (MHC)-encoded glycoproteins expressed on the surface of antigen-presenting cells (APC). Two general pathways have evolved to generate peptide-MHC complexes. The MHC class II antigen processing pathway provides a mechanism for sampling proteins present in endosomal compartments. CD4+ regulatory T-cells recognize peptides bound to MHC class II molecules, which are selectively expressed in specialized APC that have efficient mechanisms for uptake of microbial antigens, and express costimulatory molecules required for activating naive T-cells. CD8+ T-cells recognize peptides bound to MHC class I molecules. Class I molecules are widely expressed and bind peptides derived from the normal turnover of cellular proteins, providing a mechanism to display a sampling of cellular components to be monitored for abnormalities by cytotoxic T-cells. Specialized accessory proteins influence the efficiency of antigen presentation and the specificity of immune responses through their roles in generating peptides, targeting antigen and MHC glycoproteins to selected intracellular compartments, and by direct participation in the peptide-loading mechanism. It has recently been discovered that some viruses have evolved ways to inhibit or subvert discrete steps in antigen processing, providing a mechanism to evade immune recognition.

Down-regulation of HLA class I antigen-processing molecules in malignant melanoma: association with disease progression

Expression of the proteasome subunits LMP2 and LMP7, the MHC-encoded transporter subunits TAP1 and TAP2, and HLA Class I antigens was examined by immunoperoxidase staining in 10 nevi and 98 melanoma lesions (60 primary and 38 metastatic), because these molecules play an important role in the presentation of melanoma-associated peptide antigens to cytotoxic T cells. LMP2 was less frequently expressed than LMP7 in primary and metastatic melanoma lesions. TAP1, TAP2, and HLA Class I antigen expression was more frequently (P < 0.05) down-regulated in metastatic than in primary melanoma lesions and in nevi. A synchronous TAP1, TAP2, and HLA Class I antigen down-regulation was observed in 58% of primary and 52% of metastatic lesions. TAP and HLA Class I antigen down-regulation in primary lesions was significantly associated with lesion thickness, stage of disease, reduced time to disease progression, and reduced survival. These results suggest that TAP down-regulation plays a role in the clinical course of malignant melanoma, probably by providing melanoma cells with a mechanism to escape from cytotoxic T lymphocyte recognition during disease progression.

Processing of HIV-1 Envelope Glycoprotein for Class I-Restricted Recognition: Dependence on TAP1/2 and Mechanisms for Cytosolic Localization

Processing of viral proteins for recognition by CTL involves degradation of the proteins in the cytosol of an infected cell followed by transport of the resulting peptides into the endoplasmic reticulum (ER) by the TAP1/2 complex. Uncertainty exists over the site of processing of viral envelope (env) proteins since the extracellular domains of env proteins are not present in the cytosol where the class I Ag-processing pathway begins. Rather, the ectodomains of env proteins are cotranslationally translocated into the ER during biosynthesis. To analyze env protein processing, we used the herpes simplex virus protein ICP47 to block peptide transport by TAP1/2 and examined the effects of TAP blockade on the processing of the HIV-1 env protein. For the majority of env-specific CD8+ CTL, the processing pathway required TAP1/2-mediated transport of cytosolic peptides into the ER. To determine how env peptides are generated in the cytosol, we analyzed the processing of two TAP1/2-dependent epitopes containing N-linked glycosylation sites. In each case, processing involved glycosylation-dependent posttranslational modification of asparagine residues to aspartic acid. These results are consistent with cotranslational translocation of env into the ER, where glycosylation occurs. This is followed by export of a fraction of the newly synthesized protein into the cytosol, where it is deglycosylated, with conversion of the asparagines to aspartic acid residues. Following cytoplasmic proteolysis, env peptides are retransported by TAP1/2 into the ER, where association with class I occurs. Thus, the env protein can enter the class I pathway through multiple distinct processing mechanisms.

MHC Class I Molecules Compete in the Endoplasmic Reticulum for Access to Transporter Associated with Antigen Processing

We have used the functionally distinct TAP alleles of the rat in cellular transfectants as tools to investigate how newly formed rat class I (RT1.A) molecules with distinct peptide requirements gain access to suitable peptides in the endoplasmic reticulum (ER). Normal maturation of RT1.Aa depends on the presence in the ER of peptides with C-terminal arginine, while restrictive TAP-B allelic group transporters fail to transport such peptides. In this situation, RT1.Aa is retained in the ER. We show that this retention is accompanied by accumulation of RT1.Aa in the ER, partly associated with TAP and partly free. In such cells, access to TAP of a second allelic product, RT1.Au, which does not require C-terminal arginine peptides, is competitively inhibited by the build-up of RT1.Aa. Nevertheless, RT1.Au loads and matures normally. Introduction of a permissive TAP-A allele competent to transport C-terminal arginine peptides releases RT1.Aa from the ER and restores RT1.Au interaction with TAP. Both class I alleles associate indiscriminately with permissive and restrictive TAP alleles. The data support the view that interaction with TAP is not a prerequisite for peptide loading by class I molecules, so long as suitable peptides are available in the ER. They further show that TAP association of a class I molecule depends on a competitive balance in the ER defined by the extent to which the peptide requirements of other class I molecules present are satisfied and not only by the intrinsic strength of the interaction with TAP.

Constitutive IL-10 Production Accounts for the High NK Sensitivity, Low MHC Class I Expression, and Poor Transporter Associated with Antigen Processing (TAP)-1/2 Function in the Prototype NK Target YAC-1

Tumor cells that are treated with rIL-10 or transfected with the IL-10 gene show phenotypic changes. These include low but peptide-inducible expression of MHC class I, low sensitivity to specific CTL-mediated lysis, and increased NK sensitivity. In vitro-established mouse tumor lines were screened for IL-10 expression and production, and a large proportion of plasmocytomas or T cell lymphomas were found to produce IL-10. Since one of these lines was the prototype NK target cell YAC-1, we investigated whether the high IL-10 production of this cell line was related to its high NK sensitivity and its defects in MHC class I expression. The decrease in H-2 expression following the in vitro culture of in vivo-passaged YAC-1 cells was accompanied by a gradual increase in IL-10 production, whereas the reverse was found when passing in vitro-grown YAC-1 in vivo as an ascites tumor in syngenic mice. In addition, differences in YAC-1 MHC class I expression correlated with alterations in the functional activity of TAP-1/2 proteins. YAC-1 cells that were transduced with a retroviral IL-10 antisense construct (Y-IL-10 AS) only produced about half of the IL-10 that was produced by YAC-1 transduced with the control construct (Y-IL-10 Mock). Relative to Y-IL-10 Mock cells, the expression of H-2 on Y-IL-10 AS cells was markedly increased, and NK sensitivity was decreased. These data argue for a mechanism wherein IL-10 production is causally related to the low H-2 expression, decreased TAP function, and high NK sensitivity of YAC-1 cells.

TAP1 polymorphism identified in African-American Graves' disease patients

Polymorphism in transporter associated with antigen processing (TAP)1 gene has been observed in African American Graves' disease patients. Single strand conformational polymorphism has been used to identify variation for the locus. First-strand cDNA was generated from cell lines obtained by Epstein-Barr virus immortalization. Four variant alleles for TAP1 have been observed and the products have been sequenced to compare with the location of observed with SSCP position patterns. Variants were detected and compared with substitutions within TAP1 polypeptide which includes changing valine to leucine and three (3) silent substitutions for glycine, glutamic acid and alanine.

The proteasome inhibitor lactacystin prevents the generation of an endoplasmic reticulum leader-derived T cell epitope

The presentation of viral antigens on MHC class I molecules requires their intracellular fragmentation into peptides of appropriate length and anchor residue positions. Evidence has accumulated that the proteasome is the endoprotease in charge of the generation of MHC class I ligands in the cytoplasm. The generation of T cell epitopes derived from the leader peptides of endoplasmic reticulum (ER) targeted proteins, however. has been reported to be independent of the proteasome. Here we show that the H-2Db restricted antigen presentation of the immunodominant T cell epitope derived from the ER leader of the glycoprotein of lymphocytic choriomeningitis virus (LCMV) is completely abolished by administration of the proteasome inhibitor lactacystin. Thus our data support the role of the proteasome in class I restricted antigen processing and extend it to an ER leader derived epitope from a viral glycoprotein.

Functional Analysis by Site-Directed Mutagenesis of the Complex Polymorphism in Rat Transporter Associated with Antigen Processing

The transporter associated with Ag processing, TAP, is an endoplasmic reticulum resident heterodimeric member of the ATP-binding cassette transporter family. TAP transports short peptides from cytosol to the endoplasmic reticulum lumen for loading into recently synthesized class I MHC molecules. In the rat, two alleles of the TAP2 chain differ in their permissiveness to the transport of peptides with small hydrophobic, polar, or charged amino acids at the C terminus, and this correlates with differences between the peptide sets loaded into certain class I molecules in vivo. We have used segmental exchanges and site-directed mutagenesis to identify the residues in rat TAP2 responsible for differential transport between the two alleles of peptides terminating above all in the positively charged residue, arginine. Of the 25 residues by which the two functional TAP2 alleles differ, we have localized differential transport of peptides with a C-terminal arginine to two adjacent clusters of exchanges in the membrane domain involving a total of five amino acids. Each cluster, transferred by site-directed mutagenesis from the permissive to the restrictive sequence, can independently confer on TAP a partial ability to transport peptides with arginine at the C terminus. The results suggest that the permissive TAP2-A allele evolved in at least two steps, each partially permissive for peptides with charged C termini.

Peptide transport in human lymphoblastoid and tumor cells: effect of transporter associated with antigen presentation (TAP) polymorphism

CD8+ T-cells recognize antigenic peptides presented by major histocompatibility complex (MHC) class I molecules. These peptides bind to MHC class I molecules in the endoplasmic reticulum (ER) lumen. Antigenic peptides are translocated from the cytosol to the lumen of ER by transporter associated with antigen presentation (TAP) proteins. In this study, it is shown that TAP1 polymorphism influences the peptide substrate specificity in human B-lymphoblastoid and tumor cell lines. TAP1A and 1C alleles specifically enhance translocation of model peptides containing basic C-terminal amino acid residue. However, TAP1B allele does not show specificity for the peptide C-terminus. Human basophilic leukemia (Ku812), and hepatocellular carcinoma (PLC/PRF/5) cells express TAP1 molecules and exhibit TAP-mediated allele-specific peptide uptake after gamma-interferon (gamma-IFN) treatment. Ku812 cells express TAP1A and preferentially take up antigenic peptides with a basic C-terminus, however, PLC/PRF/5 cells with the TAP1B allele take up low but equivalent levels of peptides regardless of basic, acidic, or hydrophobic C-termini. Moreover, TAP2 polymorphisms have no influence on the peptide translocation in normal or tumor cell lines. In addition, Daudi, a beta2-microglobulin (beta2m) deficient human Burkitt lymphoma, cell line also showed TAP-dependent peptide uptake. Taken together, these results suggest that human TAP1 but not TAP2 polymorphisms influence the antigenic peptide transport and that this transport is independent of beta2m in this system.

MHC class I expression protects target cells from lysis by Ly49-deficient fetal NK cells

Using appropriate conditions natural killer (NK) cells can be cultured from the liver and thymus of day 14 fetal mice. These fetal NK cells are phenotypically and functionally indistinguishable from adult NK cells with the exception that they lack measurable expression of all of the Ly49 molecules that can currently be detected with antibodies. Despite this, they preferentially kill tumor cells and blast cells deficient in the expression of major histocompatibility complex class I molecules, although the degree of discrimination is usually weaker than that shown by adult NK cells and varies depending on the particular combination of effector and target cells used. Polymerase chain reaction analysis revealed that although fetal NK cells are severely deficient in the expression of mRNA for Ly49A, B, C, D, G, H, and I they express high levels of Ly49E mRNA, raising the possibility that Ly49E may have an important and special function in the early development of the NK lineage.

CD8+ T-cell clones in old mice

Most old mice and human beings contain large clones of CD8+ alpha beta TCR+ T cells. In mice, clones bearing V beta 7 appear more frequently in animals infected with mouse hepatitis virus than in uninfected animals. This property is controlled by some non-MHC gene in the animals. The frequency of old mice containing such clones is affected by the origin of the animals. Although the clones are relatively anergic to acute stimuli in vitro, they can divide in vivo since in old animals they divide and turnover with about the same kinetics as other, non-clonally expanded CD8+ T cells. Moreover the clones expand slowly but inexorably after transfer into recipient animals. These data suggest that the CD8+ alpha beta TCR clones arise because they are specific for some exogenous or auto antigen to which the cells are continuously exposed in vivo.

Screening of the TAP1 gene by denaturing gradient gel electrophoresis in insulin-dependent diabetes mellitus: detection and comparison of new polymorphisms between patients and controls

New protective or disease-associated polymorphisms in the TAP1 gene were sought in insulin-dependent diabetes mellitus (IDDM) patients with the use of denaturing gradient gel electrophoresis (DGGE) screening of genomic DNA. The TAP1 gene is located in the human leukocyte antigen (HLA) class II region of the genome and encodes components of a peptide transporter essential for antigen presentation by HLA class I molecules. Fragments of TAP1 corresponding to the 5' promoter, each of the 11 exons (with portions of adjacent intronic regions) and the 3' flanking region were amplified by the polymerase chain reaction and then subjected to DGGE. DNA fragments of TAP1 yielded DGGE bands with patterns whose frequencies differed between IDDM patients and controls. Specific DGGE band patterns with fragments corresponding to the promoter, exons or introns 3, 6, 7, 8, 9 or 10 of TAP1 were detected exclusively in either patients or controls. Sequencing of TAP1 fragments encompassing exon 7 gave rise to a DGGE band pattern exclusively observed in an IDDM patient and sequencing revealed a previously unidentified polymorphisms at codon 518 (GTC-->ATC, Val-->Ile). Another unique polymorphism uncovered by DGGE revealed by sequencing a polymorphism in intron 2 in a diabetic patient. The genotypes of additional HLA class II matched patients and controls were determined with regard to five exonic and one intronic TAP1 polymorphism. A 10 base pair intronic insertion in intron 9 was exclusively identified in controls and missing from patients (P = 0.017). Further large population-based studies may reveal whether these newly identified at risk or protective TAP1 variants confer markers of statistical risk in diverse population groups.

Costimulation by B7 modulates specificity of cytotoxic T lymphocytes: a missing link that explains some bystander T cell activation

It has been proposed that some bystander T cell activation may in fact be due to T cell antigen receptor (TCR) cross-reactivity that is too low to be detected by the effector cytotoxic T lymphocyte (CTL). However, this hypothesis is not supported by direct evidence since no TCR ligand is known to induce T cell proliferation and differentiation without being recognized by the effector CTL. Here we report that transgenic T cells expressing a T cell receptor to influenza virus A/NT/68 nucleoprotein (NP) 366-374:Db complexes clonally expand and become effector CTLs in response to homologous peptides from either A/PR8/34 (H1N1), A/AA/60 (H2N2), or A/NT/68 (H3N2). However, the effector T cells induced by each of the three peptides kill target cells pulsed with NP peptides from the H3N2 and H2N2 viruses, but not from the H1N1 virus. Thus, NP366-374 from influenza virus H1N1 is the first TCR ligand that can induce T cell proliferation and differentiation without being recognized by CTLs. Since induction of T cell proliferation was mediated by antigen-presenting cells that express costimulatory molecules such as B7, we investigated if cytolysis of H1N1 NP peptide-pulsed targets can be restored by expressing B7-1 on the target cells. Our results revealed that this is the case. These data demonstrated that costimulatory molecule B7 modulates antigen specificity of CTLs, and provides a missing link that explains some of the bystander T cell activation.

Gene therapy with B7.1 and GM-CSF vaccines in a murine AML model

PURPOSE

Characterization of B7.1 and GM-CSF vaccines on the induction of anti-tumor immunity in a murine AML model.

MATERIALS AND METHODS

Primary AML cells were retrovirally transduced with the murine costimulatory molecule B7.1, a natural ligand for the T-cell receptors CD28 and CTLA-4, or the cytokine GM-CSF. Mice were vaccinated with irradiated AML cells expressing B7.1 or GM-CSF before or after inoculation of wild type AML cells.

RESULTS

Intravenous injection of irradiated B7.1 or GM-CSF expressing AML cells can provide long lasting systemic immunity against a subsequent challenge of wild type AML cells. Vaccination with irradiated B7.1 or GM-CSF expressing AML cells results in rejection of established leukemia when the vaccination occurs in the early stages of the disease. However, when the vaccines are administered > 2 weeks after leukemic inoculation, only mice which receive the GM-CSF vaccine are cured of leukemia.

CONCLUSIONS

These results suggest that tumor burden and vaccine efficiency are most likely to be the limiting factors in the curative potential of tumor vaccines. Novel approaches such as this experiment could provide improved therapeutic outcomes in patients with AML and other cancers.

A new MHC locus that influences class I peptide presentation

We have investigated the HLA-B27-restricted CTL response to HY minor histocompatibility antigens in rats and mice transgenic for HLA-B27 and human beta2-microglobulin. A polymorphism was found at a locus within the H2 complex, producing two distinct but overlapping sets of B27-presented HY peptides. The locus, named Cim2, mapped between the K and Pb loci, and its product is therefore distinct from TAP, LMP, and tapasin. Identical findings in rats and mice, including identical HY peptide sequences and the failure of a rat Tap2A transgene to alter CTL recognition, suggest that a homologous locus with similar polymorphism exists in the rat. Cim2, or a closely linked locus, was found to exert a broad effect on peptide loading of both HLA-B27 and mouse class I alleles. The data thus establish a strong, previously unrecognized MHC-encoded influence on the class I antigen pathway.

Peptide binding and photo-crosslinking to detergent solubilized and to reconstituted transporter associated with antigen processing (TAP)

The transporter associated with antigen processing (TAP) is essential for peptide loading onto major histocompatibility (MHC) class I molecules by translocating peptides into the endoplasmic reticulum. We have explored the conditions for detergent solubilization of functionally active, heterologously expressed human TAP from microsomal membranes. The efficiency to solubilize TAP was tested for a variety of detergents as well as for different solubilization conditions. The activity of the solubilized TAP complex was analyzed over time, using a non-radioactive crosslinking assay with a photo-activateable peptide, in the presence or absence of external lipid. The detergent CHAPS was found optimally to retain activity and thus allowed us to reconstitute detergent-solubilized, active TAP into proteoliposomes.

Generation of CD8+ T cells specific for transporter associated with antigen processing deficient cells

Cells with impaired transporter associated with antigen processing (TAP) function express low levels of cell surface major histocompatibility complex (MHC) class I molecules, and are generally resistant to lysis by MHC class I restricted cytotoxic T lymphocytes (CTLs). Here we report the generation of MHC class I restricted CD8(+) CTLs that surprisingly require target cell TAP deficiency for efficient recognition. C57BL/6 (B6) mice immunized with syngenic B7-1 (CD80) expressing TAP-deficient cells generated a potent CTL response against both TAP-deficient RMA-S tumor cells and TAP-deficient Con A blasts, whereas the corresponding TAP-expressing target cells were considerably less susceptible or resistant to lysis. The CTL epitopes recognized were expressed also by the human TAP-deficient cell line T2, transfected with appropriate MHC class I molecules. B6 mice immunized with B7-1-transfected TAP-deficient RMA-S cells were protected from outgrowth of a subsequent RMA-S tumor challenge. These findings are discussed in relation to the biochemical nature of MHC class I dependent CTL epitopes associated with impaired TAP function, as well as implications for immunotherapy and autoimmunity.

Major histocompatibility complex class I molecules interact with both subunits of the transporter associated with antigen processing, TAP1 and TAP2

Prior to loading antigenic peptides, assembled major histocompatibility complex (MHC) class I molecules associate with the transporter associated with antigen processing (TAP) in a complex which also includes calreticulin and a recently described component, tapasin. The interaction of MHC class I molecules has been characterized as occurring exclusively with the TAP1 chain of the TAP heterodimer. In contrast, as described here, in the TAP-deficient human cell line T2, MHC class I molecules interact with a transfected rat TAP2 polypeptide in addition to rat TAP1. Furthermore, this interaction with TAP2 also involves calreticulin and tapasin. An association with both TAP polypeptides would presumably further enhance the efficiency of peptide loading of MHC class I molecules by allowing more than one MHC class I allele proximity to the site of peptide supply on each TAP complex.

TAP gene transporter polymorphism in inflammatory bowel diseases

BACKGROUND

Many studies suggest the implication of genetic factors in inflammatory bowel diseases. Despite some associations with HLA genes, the lack of definite data may be due to ethnic variations, clinical heterogeneity, or the involvement of additional susceptibility genes beside or within the major histocompatibility complex (MHC), such as TAP genes. The aim of this study was to analyze in patients with ulcerative colitis (UC) or Crohn's disease (CD) the polymorphism of TAP genes that encode the proteins necessary for the transfer of antigenic peptides through the endoplasmic reticulum membrane.

METHODS

One hundred and one UC and 148 CD patients were compared with 173 unrelated healthy controls. Dimorphisms within the TAP1 and TAP2 alleles were analyzed by sequence-specific oligonucleotide typing.

RESULTS

No difference was found between patient groups and controls. However, when CD patients were classified on the basis of their responsiveness to steroid therapy, a significant decrease of TAP2 AA (*0101/*0101) genotype was found in CD patients who did not respond to steroid therapy (22.9% versus 43.7% in steroid responder group; Pc < 0.05; odds ratio = 2.6; 95% confidence limits (CL) = 1.2-5.9). These data appear independent of the distribution of HLA DRB1*01 or DRB1*03 alleles despite a significant linkage disequilibrium between these alleles and TAP2A.

CONCLUSIONS

This result suggests, despite the absence of arguments favoring a genetic susceptibility to CD, that the TAP2 gene or other genes located on chromosome 6 may be involved in the genetic heterogeneity of CD.

Recognition principle of the TAP transporter disclosed by combinatorial peptide libraries

Transport of peptides across the membrane of the endoplasmic reticulum for assembly with MHC class I molecules is an essential step in antigen presentation to cytotoxic T cells. This task is performed by the major histocompatibility complex-encoded transporter associated with antigen processing (TAP). Using a combinatorial approach we have analyzed the substrate specificity of human TAP at high resolution and in the absence of any given sequence context, revealing the contribution of each peptide residue in stabilizing binding to TAP. Human TAP was found to be highly selective with peptide affinities covering at least three orders of magnitude. Interestingly, the selectivity is not equally distributed over the substrate. Only the N-terminal three positions and the C-terminal residue are critical, whereas effects from other peptide positions are negligible. A major influence from the peptide backbone was uncovered by peptide scans and libraries containing D amino acids. Again, independent of peptide length, critical positions were clustered near the peptide termini. These approaches demonstrate that human TAP is selective, with residues determining the affinity located in distinct regions, and point to the role of the peptide backbone in binding to TAP. This binding mode of TAP has implications in an optimized repertoire selection and in a coevolution with the major histocompatibility complex/T cell receptor complex.

Increased class Ib antigen display on TAP-2 mutant cells by a mitochondrial function inhibitor

Class I histocompatibility antigen display is defective in the RMA-S mutant cell line due to a mutation in the Tap-2 gene, which encodes a peptide transporter. Incubation of RMA-S cells with oligomycin, an inhibitor of mitochondrial ATPase, strongly increased lysis by cytotoxic T lymphocytes (CTL) specific for the class Ib antigen H2-M3, and lysis by Qa-1b-specific CTL was restored. Oligomycin did not affect normal class I display on RMA cells. Treatment of RMA-S cells with other inhibitors of mitochondrial function failed to increase lysis by anti-H2-M3 or Qa-1b CTL. Lysis by allogenic CTL specific for H-2b antigens was either not enhanced or only weakly increased, depending upon the H-2 haplotype of the alloreactive effector cells used.

Altered kinetics of Tap-1 gene expression in macrophages following stimulation with both IFN-gamma and LPS

With recent studies suggesting a key role for professional antigen presenting cells in the induction of major histocompatibility class I cellular immune responses, we initiated studies on the regulation of Tap-1 and Tap-2 gene expression in macrophages. Stimulation of the human macrophage cell line THP-1 with interferon-gamma (IFN-gamma) resulted in maximal induction of both Tap-1 and Tap-2 mRNA within 24 hr. Nuclear run-on analyses showed that the increased expression of Tap-1 and Tap-2 was controlled at the level of transcription. Half-life studies demonstrated that mRNAs for both genes became destabilized after stimulation of THP-1 cells with IFN-gamma for 24 hr, suggesting that a posttranscriptional mechanism down-regulates TAP gene expression following activation. Treatment of cells with both IFN-gamma and lipopolysaccharide (LPS) altered the kinetics and amount of Tap-1 mRNA and protein expression, compared to those with stimulation with IFN-gamma alone. These data suggest that LPS enhances the ability of macrophages stimulated with IFN-gamma to initiate a cellular immune response by altering the kinetics of TAP gene expression.

Human TAP1 polymorphisms detected by denaturing gradient gel electrophoresis

Presentation of endogenous peptides by major histocompatibility complex class I (MHC) molecules is controlled, in part, by the Tap1 and Tap2 genes in the MHC class II region that encode a heterodimeric peptide transporter. Polymorphisms of human Tap1 in normal individuals have now been investigated systematically by denaturing gradient gel electrophoresis (DGGE) analysis of fragments of genomic DNA generated by the polymerase chain reaction. Polymorphisms identified by distinctive DGGE band patterns were confirmed by DNA sequencing. In addition to four previously described polymorphisms in the open reading frame, DGGE detected three new polymorphisms: a G-->T substitution in the promoter region, a 10-bp insert in intron 9, and a G-->T substitution 80-bp downstream of the translation termination codon.

The unique role of interferon-gamma in the regulation of MHC expression on arterial endothelium

We examined the expression of MHC class I and II in the arterial endothelium of interferon-gamma (IFN-gamma, GKO) and IFN-gamma-R (IFN-gamma-R, GRKO) gene knockout mice in comparison with mice with intact IFN-gamma and IFN-gamma-R genes, BALB/c and 129Sv/J wild-type, respectively. The GKO and GRKO were produced by gene targeting. MHC class I and II expression was assessed by mAb binding to frozen tissue (kidney, spleen, heart, liver) sections by immunoperoxidase staining in the basal state and after various stimuli: allogeneic cells, oxazolone skin sensitization, LPS, and rIFN-gamma. As controls, we also examined the expression of two other IFN-gamma inducible genes present in the endothelium, Ly-6 and ICAM-1. We found that basal class I expression was present in the small arteries and arterioles of BALB/c and 129Sv/J wild-type mice but absent from arterial endothelium of GKO and GRKO mice. Class I was induced in the endothelium of BALB/c and 129Sv/J wild-type mice by three in vivo stimuli: allogeneic, LPS, and oxazolone, whereas class II was only induced after allogeneic stimulus. Administration of rIFN-gamma induced class I in the endothelium of GKO and BALB/c wild-type mice. The basal expression of Ly-6 and ICAM-1 was similar in the arteries of GKO and BALB/c wild-type mice, indicating that, the basal expression of these proteins in endothelium is IFN-gamma independent, unlike class I. In summary, basal class I expression in arterial endothelium is not constitutive as previously believed, but is dependent on basal IFN-gamma production. IFN-gamma has an essential role in the induction of class I and II expression in arterial endothelium. The fact that MHC class I is induced in endothelium may be useful therapeutically for reduction of immune recognition in transplantation.

Role of non-anchor residues of Db-restricted peptides in class I binding and TCR triggering

To understand better, the role of non-anchor residues of class I restricted T cell epitopes in class I binding and TCR stimulation, a panel of peptides was synthesized in which each of the non-anchor positions of the Db-restricted influenza peptide, ASNENMETM, was changed to each of the 20 natural amino acids (AAs). The relative affinity of all the peptides for Db was determined and their ability to stimulate anti-ASNENMETM cytotoxic T cell hybridomas was also assessed. The results illustrated that for Db binding, the AAs with the most solvent exposure had the smallest effect on binding. Changes at other positions affected binding to different degrees. Results for the recognition by the T cell hybridomas indicated that a peptide-MHC complex represents a multitude of epitopes, as each hybridoma recognized a different subset of peptides. Most changes in the highly solvent-exposed residues negatively affected recognition by all hybridomas while changes in other positions affected each hybridoma differently, independent of the direction of the side chain of the AA at that position. Furthermore, the use of saturating concentrations of low and high binding peptides showed that, as long as the class I-peptide complex is formed, the T-cell receptor does not differentiate between high and low binding peptides. This indicates that, although the stability of the class I-peptide complex is highly dependent on peptide affinity, the class I MHC conformation induced by low affinity peptides does not necessarily differ significantly from that induced by high affinity peptides. The results of peptide-class I recognition by one ASNENMETM-specific hybridoma was used to construct a peptide that differed from ASNENMETM at four of the nine residues, yet stimulated the hybridoma to a level comparable to ASNENMETM. In addition, peptides bearing the canonical Db-binding motif but unable to bind to the class I molecule with high affinity could be made to bind Db, by changing unfavorable AAs to favourable ones at appropriate positions. The extended motif determined was used to identify more accurately the peptides derived from Coxsakie b3 virus that would bind Db. It was also shown that some of the canonical characteristics of the peptide motif could be obviated and still obtain high affinity binding, provided optimal AAs, were present at secondary anchor positions.

Peptide selection for presentation by HLA class I: a role for the human transporter associated with antigen processing?

Peptide epitopes presented by HLA class I are supplied by an elaborate system of antigen processing which subjects candidate epitopes to a process of selection according to little-understood rules. Transport of peptides from the cytosol into the endoplasmic reticulum by the TAP (transporter associated with antigen processing) complex is likely to play an important role in peptide selection for presentation. The recent development of an assay measuring substrate binding to the TAP complex has led to the identification of the major structural properties of peptides selected by the human transporter. Human TAP favors transport of peptides with structural features common to HLA class I ligands. However, TAP dislikes peptide ligands preferred by some HLA class I alleles, which may therefore frequently rely on alternative sources of peptide ligands.

Structural requirements for synthetic immunogens to induce measles virus specific CTL responses

We have studied the immunogenicity of a synthetic peptide representing a cytotoxic T cell epitope (CTL) from the nucleoprotein of measles virus (MV). For the induction of peptide and MV-specific CTL responses after subcutaneous immunization, covalent linkage of the CTL epitope to a T-helper epitope was required. The presence of two copies of the T-helper epitope at the amino terminus of the CTL epitope (TT-CTL) resulted in the induction of strong CTL responses after administration in saline. In contrast, a chimeric peptide with one copy of the T-helper epitope at the amino terminus of the CTL epitope (T-CTL) was weakly immunogenic when given in saline. Analysis of the structure of the TT-CTL chimeric peptide by CD spectroscopy revealed an alpha-helical conformation, as compared to the random coil conformation favored by the T-CTL chimeric peptide. In addition, the CD spectra of the TT-CTL peptide in the presence of small unilamellar vesicules (SUV) revealed an increased helicity, as compared to the spectra of the T-CTL chimera in the presence of SUV. This suggests that the amphipathic character of the TT-CTL chimeric construct favors its interaction with the cell membrane of antigen presenting cells, therefore, facilitating its cytosolic delivery for class I presentation. These findings highlight the importance of antigen structure for the in vivo induction of CTL responses and may have implications for the design of synthetic peptide vaccines.

Characterisation of the pvdE gene which is required for pyoverdine synthesis in Pseudomonas aeruginosa

Pseudomonas aeruginosa (Pa) strain PAO synthesises a siderophore, pyoverdine (Pvd), when grown under conditions of iron starvation. Pvd consists of a dihydroxyquinoline group attached to an 8-amino-acid-residue peptide. DNA spanning at least 78 kb of the chromosome is required for Pvd synthesis, but to date only three genes involved in this process have been characterised. We report the characterisation of a fourth Pa gene, pvdE, which we show to be required for Pvd synthesis. The deduced amino acid sequence of PvdE indicates that the protein is a member of the ATP-binding-cassette (ABC) family of membrane transporter proteins, and this is the first example of the involvement of an ABC-type protein in siderophore synthesis.

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.

Molecular mechanisms of class I major histocompatibility complex antigen processing and presentation

The presentation of antigenic peptides by class I major histocompatibility complex molecules plays a central role in the cellular immune response, since immune surveillance for detection of viral infections or malignant transformations is achieved by CD8+ T lymphocytes which inspect peptides, derived from intracellular proteins, bind to class I molecules on the surface of most cells. The transporter associated with antigen processing selectively translocates cytoplasmically derived peptides of appropriate sequence and length into the lumen of the endoplasmic reticulum where they associate with newly synthesized class I molecules. The translocated peptides are generated by multicatalytic and multisubunit proteasomes which degrade cytoplasmic proteins in a ATP-ubiquitin-dependent manner. This review discusses our current molecular understanding of class I antigen processing and presentation.

Translocation of long peptides by transporters associated with antigen processing (TAP)

The major histocompatibility complex (MHC)-encoded transporters associated with antigen processing (TAP) translocate peptides from the cytosol into the lumen of the endoplasmic reticulum (ER) where they associate with MHC class I molecules. The length of class I-binding peptides is usually 8-11 amino acids, but examples of significantly longer peptides have been described. The preferred lengths and upper and lower size limits for peptides translocated by TAP have not been determined in detail because in the currently used test systems, peptides are subject to proteolytic degradation. In the present study, three sets of individual peptides or partially randomized peptide libraries ranging between 6 and 40 residues were used that contained a radiolabeled tyrosine and a consensus sequence for ER-specific N-glycosylation at opposite ends, thus ensuring that only nondegraded peptides were monitored in the transport/glycosylation assay. For three different transporters, rat TAP1/2a, rat TAP1/2u and hTAP, the most efficient ATP-dependent transport was observed for peptides with 8-12 amino acids. Hexamers and longer peptides of up to 40 amino acids were also translocated, albeit less efficiently. For two of the three sets of peptides analyzed, rat TAP1/2a showed a less stringent length selection than rat TAP1/2u and human TAP. The superior transport of the decamer of the TNKT.. Y series was not due to faster degradation or less efficient glycosylation of shorter or longer length variants. A binding assay with TAP-containing microsomes revealed a high affinity for the radiolabeled decamer (KD = 580 nM), while other length variants were clearly inferior in their binding affinities. Thus, TAP binds and preferentially translocates peptides with a length suitable for binding to MHC class I molecules, but peptides that are considerably longer may also be substrates. About 10(5) peptide binding sites per cell equivalent of microsomes were determined, providing an estimate for the number of TAP complexes in the ER membrane.

Peptide transporter (TAP-1 and TAP-2)-independent endogenous processing of Epstein-Barr virus (EBV) latent membrane protein 2A: implications for cytotoxic T-lymphocyte control of EBV-associated malignancies

Major histocompatibility [correction of histocampatability] complex (MHC) class I-restricted cytotoxic T lymphocytes (CTLs) recognizing Epstein-Barr virus (EBV) latent antigens play a pivotal role in restricting the proliferation of EBV-infected normal B cells. However, it is now well established that most of the EBV-associated malignancies escape this potent CTL response in vivo. This resistance to immune surveillance is not due to an obvious CTL dysfunction but has been partly attributed to the down-regulation of the peptide transporters, TAP-1 and TAP-2, thus restricting the endogenous loading of MHC class I molecules with peptides derived from viral nuclear antigens. In the present study we have explored the possibility that EBV latent membrane protein 2A (LMP2A), which is often expressed in many of the EBV-associated malignancies, such as nasopharyngeal carcinoma and Hodgkin's disease tumors, can be endogenously processed through an alternative, TAP-1- and TAP-2-independent pathway. The data presented in this study clearly demonstrate not only that LMP2A can be processed by a TAP-independent mechanism but also that tumor cells with down-regulated TAP expression can be efficiently recognized by LMP2A-specific T cells following infection with recombinant vaccinia virus encoding this protein. We propose that since LMP2A is a membrane protein, it is directly translocated into the secretory pathway and the processing enzymes present in the endoplasmic reticulum are capable of generating the relevant peptide epitopes for MHC binding. The present finding of TAP-1- and TAP-2-independent presentation of LMP2A epitopes suggests a novel mechanism for immune targeting of EBV-positive malignancies, such as nasopharyngeal carcinoma and Hodgkin's disease tumors.

A point mutation in the human transporter associated with antigen processing (TAP2) alters the peptide transport specificity

The heterodimeric transporter associated with antigen processing (TAP1/TAP2) translocates peptides from the cytosol into the endoplasmic reticulum where loading of major histocompatibility complex class I molecules takes place. TAP transporters from different species are known to exhibit distinct transport specificities with regard to the C-terminal amino acid (aa) of peptides. Thus, human TAP (hTAP), and rat TAP (rTAP) containing the rTAP2a allele are rather promiscuous, whereas mouse TAP (mTAP), and rTAP containing the rTAP2a allele are restrictive and select against peptides with C-terminal small polar/hydrophobic or positively charged aa. The structural basis for this selectivity is not clear. To assess the relative contribution of the TAP1 and TAP2 subunits to transport specificity, we have constructed and analyzed interspecies TAP hybrids and point mutants of hTAP2 expressed in Sf9 insect cells and in TAP-deficient T2 cells. Transport assays with 20 C-terminal variants of the peptide RYWANATRSX showed that: first, transport specificity with regard to C-terminal aa is mainly influenced by TAP2, but TAP1 can also contribute. Second, the selective transport of peptides with C-terminal positively charged aa is critically controlled by the amino-terminal region (1-361) on the TAP2 chain, while transport of peptides with C-terminal small polar/hydrophobic aa is determined by residues located within as well as outside the region 1-361. Third, a single point mutation in hTAP2 (374A-->D) resulted in a drastic alteration of the transport pattern. These results indicate that both TAP1 and TAP2 contribute to efficient peptide transport and that single point mutations in hTAP2 are able to alter the peptide transport specificity. This opens the possibility that naturally occurring mutations in one of the hTAP subunits may alter epitope selection in vivo.

Loss of MHC I transcription trans-activator in the bovine B-LCL, BL3.1

BL3.1, a variant derived from the BLV infected bovine B cell line, BL3, is distinguished by a loss of expression of MHC class I. All surface MHC I products were down-regulated in BL3.1 compared with BL3 correlating with a diminution in MHC I heavy chain transcription. Class I genes, including regulatory elements, showed no aberrations. The variant, BL3.1, did not differ from the parent cell line in expression of Bovine Leukemia Virus (BLV) or oncogene, c-myc. Transient transfection experiments determined the defect was trans rather than cis, and was due to loss of a trans-activator rather than gain of a trans-suppressor as evidenced by transient heterokaryon fusions. Southwestern blot analysis indicated that two DNA binding proteins associated with the MHC class I promoter were missing in BL3.1 cells. The specific response elements for these proteins in BL3 did not appear to be within the enhancerA region, the major enhancer region of the MHC I promoter.

Identification of new TAP2 alleles in gorilla: evolution of the locus within hominoids

Transporters associated with antigen processing molecules (TAP1 and TAP2) mediate the transfer of cytosolic peptides into the lumen of the endoplasmic reticulum for association with newly synthesized class I molecules of the major histocompatibility complex. Previous molecular and functional analyses of rat and human TAP2 homologues indicated major differences in gene diversification patterns and selectivity of peptides transported. Therefore, in this study, we analyzed the alleles of the gorilla TAP2 locus to determine whether the pattern of diversification resembled that in either of those two species. Sequence analysis of the TAP2 cDNAs from gorilla Epstein-Barr virus-transformed B-cell lines revealed four alleles with a genetic distance of less than 1%. The nucleotide substitutions distinguishing the alleles are confined to the 3' half of the coding region and occur individually or within two small clusters of variability. Diversification of the locus appears to have resulted from point substitutions and recombinational events. Evolutionary-rate estimates for the TAP2 gene in gorilla and human closely approximate those observed for other hominoid genes. The amino acid polymorphisms within the gorilla molecules are distinct from those in the human homologues. The absence of ancestral polymorphisms suggests that gorilla and human TAP2 genes have not evolved in a trans-species fashion but rather have diversified since the divergence of the lineages.