The sequence alteration associated with a mutational hotspot in p53 protects cells from lysis by cytotoxic T lymphocytes specific for a flanking peptide epitope

Pleiotropic effects of post-translational modifications on the fate of viral glycopeptides as cytotoxic T cell epitopes

The fate of viral glycopeptides as cytotoxic T lymphocyte (CTL) epitopes is unclear. We have dissected the mechanisms of antigen presentation and CTL recognition of the peptide GP392-400 (WLVTNGSYL) from the lymphocytic choriomeningitis virus (LCMV) and compared them with those of the previously reported GP92-101 antigen (CSANNSHHYI). Both GP392-400 and GP92-101 bear a glycosylation motif, are naturally N-glycosylated in the mature viral glycoproteins, bind to major histocompatibility complex H-2D(b) molecules, and are immunogenic. However, post-translational modifications differentially affected GP92-101 and GP392-400. Upon N-glycosylation or de-N-glycosylation, a marked decrease in major histocompatibility complex binding was observed for GP392-400 but not for GP92-101. Further, under its N-glycosylated or de-N-glycosylated form, GP392-400 then lost its initial ability to generate a CTL response in mice, whereas GP92-101 was still immunogenic under the same conditions. The genetically encoded form of GP392-400, which on the basis of its immunogenicity could still be presented with H-2D(b) during the course of LCMV infection, does not in fact appear at the surface of LCMV-infected cells. Our results show that post-translational modifications of viral glycopeptides can have pleiotropic effects on their presentation to and recognition by CTL that contribute to either creation of neo-epitopes or destruction of potential epitopes.

Generation of cytotoxic T lymphocytes by MHC class I ligands fused to heat shock cognate protein 70

Immunization with gp96 and heat shock cognate protein 70 (hsc70) purified with in vivo bound naturally occurring peptides or bound to synthetic peptides by in vitro reconstitution has been shown to induce peptide-specific cytotoxic T lymphocytes (CTL). In addition, mycobacterial heat shock protein 70 covalently fused to ovalbumin (OVA)-derived fragments has been shown to generate MHC class I-restricted CTL responses. Here, we genetically fused five different CTL epitopes, including peptides derived from Plasmodium yoelii circumsporozoite protein, tumor antigens, HY antigen and OVA, to either the N- or C-terminus of murine hsc70 and expressed the resulting proteins in Escherichia coli. Vaccination with all five fusion proteins induced peptide-specific CTL, indicating that no cognate flanking regions of CTL epitopes are necessary for the immune response. The point of injection was crucial for CTL induction. CD4(+) T cells were not required for the priming of CD8(+) T cells and vaccination with bone marrow-derived dendritic cells pulsed with hsc70 fusion proteins also elicited CTL responses. Furthermore, by using deletion mutants of hsc70, we identified amino acid residues 280-385 of hsc70 as the region most critical for inducing the CTL response.

Circumventing tolerance to a human MDM2-derived tumor antigen by TCR gene transfer

We identified a tumor-associated cytotoxic T lymphocyte (CTL) epitope derived from the widely expressed human MDM2 oncoprotein and were able to bypass self-tolerance to this tumor antigen in HLA-A*0201 (A2.1) transgenic mice and by generating A2.1-negative, allo-A2.1-restricted human T lymphocytes. A broad range of malignant, as opposed to nontransformed cells, were killed by high-avidity transgenic mouse and allogeneic human CTLs specific for the A2.1-presented MDM2 epitope. Whereas the self-A2.1-restricted human T cell repertoire gave rise only to low-avidity CTLs unable to recognize the natural MDM2 peptide, human A2.1+ T lymphocytes were turned into efficient MDM2-specific CTLs upon expression of wild-type and partially humanized high-affinity T cell antigen receptor (TCR) genes derived from the transgenic mice. These results demonstrate that TCR gene transfer can be used to circumvent self-tolerance of autologous T lymphocytes to universal tumor antigens and thus provide the basis for a TCR gene transfer-based broad-spectrum immunotherapy of malignant disease.

Identification of p53 peptides recognized by CD8(+) T lymphocytes from patients with bladder cancer

In many types of cancer, p53 frequently accumulates in tumor cells and anti-p53 antibodies can be detected. However, only four CD8(+) T-cell epitopes from p53 have been identified in humans so far. To further analyze the development of a T-cell response against p53, peptides having binding motifs specific for HLA-A1, -A2, -A3, -A24, -B7, -B35, -B44, and -B51 molecules have been defined. The HLA-binding capacity of those peptides was tested, and the stability of formed complexes was defined. Thirteen peptides that bound to HLA-A24 and -B44 molecules are presented. The positive peptides were then used to detect the anti-p53 response of CD8(+) T lymphocytes from patients with bladder cancer. Six peptides, presented by HLA-A2, -B51, or -A24, were able to stimulate T cells from two patients (among 16) with tumor cells that strongly accumulated p53. On the contrary, p53 peptides systematically failed to stimulate T cells from healthy donors or patients with low or undetectable levels of p53 in their tumor cells. These results have led to the identification of four new potential T CD8(+) epitopes from p53: 194-203 associating with HLA-B51 and 204-212, 211-218, and 235-243 associating with HLA-A24.

Frameshift peptide-derived T-cell epitopes: a source of novel tumor-specific antigens

Microsatellite instability (MSI) caused by defective DNA mismatch repair (MMR) is a hallmark of hereditary nonpolyposis colorectal cancers (HNPCC) but also occurs in about 15% of sporadic tumors. If instability affects microsatellites in coding regions, translational frameshifts lead to truncated proteins often marked by unique frameshift peptide sequences at their C-terminus. Since MSI tumors show enhanced lymphocytic infiltration and our previous analysis identified numerous coding mono- and dinucleotide repeat-bearing candidate genes as targets of genetic instability, we examined the role of frameshift peptides in triggering cellular immune responses. Using peptide pulsed autologous CD40-activated B cells, we have generated cytotoxic T lymphocytes (CTL) that specifically recognize HLA-A2.1-restricted peptides derived from frameshift sequences. Among 16 frameshift peptides predicted from mutations in 8 different genes, 3 peptides conferred specific lysis of target cells exogenously loaded with cognate peptide. One peptide derived from a (-1) frameshift mutation in the TGFbetaIIR gene gave rise to a CTL bulk culture capable of lysing the MSI colorectal cancer cell line HCT116 carrying this frameshift mutation. Given the huge number of human coding microsatellites and assuming only a fraction being mutated and encoding immunologically relevant peptides in MSI tumors, frameshift protein sequences represent a novel subclass of tumor-specific antigens. It is tempting to speculate that a frameshift peptide-directed vaccination approach not only could offer new treatment modalities for existing MSI tumors but also might benefit asymptomatic at-risk individuals in HNPCC families by a prophylactic vaccination strategy.

Defined flanking spacers and enhanced proteolysis is essential for eradication of established tumors by an epitope string DNA vaccine

Loss of immunogenic epitopes by tumors has urged the development of vaccines against multiple epitopes. Recombinant DNA technologies have opened the possibility to develop multiepitope vaccines in a relatively rapid and efficient way. We have constructed four naked DNA-based multiepitope vaccines, containing CTL, Th cell, and B cell epitopes of the human papillomavirus type 16. Here we show that gene gun-mediated vaccination with an epitope-based DNA vaccine protects 100% of the vaccinated mice against a lethal tumor challenge. The addition of spacers between the epitopes was crucial for the epitope-induced tumor protection, as the same DNA construct without spacers was significantly less effective and only protected 50% of the mice. When tested for therapeutic potential, only the epitope construct with defined spacers significantly reduced the size of established tumors, but failed to induce tumor regression. Only after targeting the vaccine-encoded protein to the protein degradation pathway by linking it to ubiquitin, the vaccine-induced T cell-mediated eradication of 100% of 7-day established tumors in mice. The finding that defined flanking sequences around epitopes and protein targeting dramatically increased the efficacy of epitope string DNA vaccines against established tumors will be of importance for the further development of multiepitope DNA vaccines toward clinical application.

Generation of cytotoxic T cell responses to an HLA-A24 restricted epitope peptide derived from wild-type p53

Mutations in the p53 gene are the most common genetic alterations found in human tumours, and these mutations result in high levels of p53 protein in the tumour cells. Since the expression levels of wild-type p53 in nonmalignant tissue are usually much lower in contrast, the p53 protein is an attractive target for cancer immunotherapy. We tested p53 encoded HLA-A24 binding peptides for their capacity to elicit anti-tumour cytotoxic T lymphocytes (CTL) in vitro. These peptides were in murine p53-derived cytotoxic peptides, which were being presented to CTL by H-2K(d)and H-2K(b)molecules, because the HLA-A24 peptide binding motifs were similar to the H-2K(d)and H-2K(b). For CTL induction, we used CD8(+)T lymphocytes from the peripheral blood mononuclear cells (PBMC) of healthy donors and the peptides from pulsed dendritic cells as antigen-presenting cells. We identified the peptide, p53-161 (AIYKQSQHM), which was capable of eliciting CTL lines that lysed tumour cells expressing HLA-A24 and p53. The effectors lysed C1RA24 cells (p53(+), HLA-A*2402 transfectant), but not their parental cell lines C1R (p53(+), HLA-A,B null cell). These results strongly indicate that the CTL exerts cytotoxic activity in HLA-A24's restricted manner. The identification of this novel p53 epitope for CTL offers the possibility to design and develop specific immunotherapeutic approaches for treating tumours with p53 mutation in HLA-A24-positive patients.

Antigen processing by the proteasome

The proteasome is an essential part of our immune surveillance mechanisms: by generating peptides from intracellular antigens it provides peptides that are then 'presented' to T cells. But proteasomes--the waste-disposal units of the cell--typically do not generate peptides for antigen presentation with high efficiency. How, then, does the proteasome adapt to serve the immune system well?

Tumour immunotherapy: new tools, new treatment modalities and new T-cell antigens

Tumour immunology has seen many exciting developments in the last few years. In addition to tumour antigens that are defined by antitumour T- and B-cell responses in patients, the human telomerase reverse transcriptase has been identified by 'reverse immunology' as the first truly universal tumour antigen. Molecular remission has been associated with a cancer vaccine that targets the clonal idiotype of B-cell malignancies, and sophisticated cellular vaccines (including fusions of tumour cells and antigen-presenting cells) have demonstrated promising results. Moreover, our capabilities of measuring immunity have been significantly enhanced by novel technology, such as major histocompatibility complex (MHC)-peptide tetramers and ELISPOT analysis. We are now capable of tracking antigen-specific T cells at a single cell level. This review will analyse recent developments and highlight some important issues that need to be addressed in the future.

Generation of T cells specific for the wild-type sequence p53(264-272) peptide in cancer patients: implications for immunoselection of epitope loss variants

Alterations in the p53 gene occur frequently and can lead to accumulation of p53 protein in squamous cell carcinomas of the head and neck (SCCHN). Since accumulation of p53 is associated with enhanced presentation of wild-type sequence (wt) p53 peptides to immune cells, the development of pan vaccines against SCCHN has focused on wt p53 epitopes. We used the HLA-A2.1-restricted wt p53(264-272) epitope to generate CTL from circulating precursor T cells of HLA-A2.1(+) healthy donors and patients with SCCHN. Autologous peptide-pulsed dendritic cells were used for in vitro sensitization. CTL specific for the wt p53(264-272) peptide were generated from PBMC obtained from two of seven normal donors and three of seven patients with SCCHN. These CTL were HLA class I restricted and responded to T2 cells pulsed with p53(264-272) peptide as well as HLA-A2-matched SCCHN cell lines naturally presenting the epitope. Paradoxically, none of the tumors in the three patients who generated CTL could adequately present the epitope; two had a wt p53 genotype and no p53 protein accumulation, while the third tumor expressed a point mutation (R to H) in codon 273 that prevents presentation of the p53(264-272) epitope. In contrast, patients who did not generate CTL had tumors that accumulated altered p53 and potentially could present the p53(264-272) epitope. These findings suggest that in vivo, CTL specific for the wt p53(264-272) peptide might play a role in the elimination of tumor cells expressing this epitope and in immunoselection of epitope-loss tumor cells. Immunoselection of tumors that become resistant to anti-p53 immune responses has important implications for future p53-based vaccination strategies.

Different proteasome subtypes in a single tissue exhibit different enzymatic properties

It is concluded from many experiments that mammalian tissues and cells must contain a heterogeneous population of 20 S proteasome complexes. We describe the purification and separation by chromatographic procedures of constitutive 20 S proteasomes, 20 S immuno-proteasomes and intermediate-type 20 S proteasomes from a given tissue. Our data demonstrate that each of these three groups comprises more than one subtype and that the relative ratios of the subtypes differ between different rat tissues. Thus, six subtypes could be identified in rat muscle tissue. Subtypes I and II are constitutive proteasomes, while subtypes V and VI comprise immuno-proteasomes. Subtypes III and IV belong to a group of intermediate-type proteasomes. The subtypes differ with regard to their enzymatic characteristics. Subtypes I-III exhibit high chymotrypsin-like activity and high peptidylglutamylpeptide hydrolysing activity, while these activities are depressed in subtypes IV-VI. In contrast, trypsin-like activity of subtypes IV-VI is enhanced in comparison to subtypes I-III. Importantly, the subtypes also differ in their preferential cleavage site usage when tested by digestion of a synthetic 25mer polypeptide substrate. Therefore, the characteristics of proteasomes purified from tissues or cells represent the average of the different subtype activities which in turn may have different functions in vivo.

A kinetic model of vertebrate 20S proteasome accounting for the generation of major proteolytic fragments from oligomeric peptide substrates

There is now convincing evidence that the proteasome contributes to the generation of most of the peptides presented by major histocompatibility complex class I molecules. Here we present a model-based kinetic analysis of fragment patterns generated by the 20S proteasome from 20 to 40 residues long oligomeric substrates. The model consists of ordinary first-order differential equations describing the time evolution of the average probabilities with which fragments can be generated from a given initial substrate. First-order rate laws are used to describe the cleavage of peptide bonds and the release of peptides from the interior of the proteasome to the external space. Numerical estimates for the 27 unknown model parameters are determined across a set of five different proteins with known cleavage patterns. Testing the validity of the model by a jack knife procedure, about 80% of the observed fragments can be correctly identified, whereas the abundance of false-positive classifications is below 10%. From our theoretical approach, it is inferred that double-cleavage fragments of length 7-13 are predominantly cut out in "C-N-order" in that the C-terminus is generated first. This is due to striking differences in the further processing of the two fragments generated by the first cleavage. The upstream fragment exhibits a pronounced tendency to escape from second cleavage as indicated by a large release rate and a monotone exponential decline of peptide bond accessibility with increasing distance from the first scissile bond. In contrast, the release rate of the downstream fragment is about four orders of magnitude lower and the accessibility of peptide bonds shows a sharp peak in a distance of about nine residues from the first scissile bond. This finding strongly supports the idea that generation of fragments with well-defined lengths is favored in that temporary immobilization of the downstream fragment after the first cleavage renders it susceptible for a second cleavage.

p53: a potential target antigen for immunotherapy of cancer

Approximately 50% of all human malignancies exhibit mutation and aberrant expression of p53, making this protein an interesting candidate target for immunotherapy of cancer. Mutations in p53 are highly diverse. Therefore, targeting of determinants within the wild-type p53 sequence appears most practical. Despite the fact that p53 is ubiquitously expressed, adoptive immunotherapy of tumor-bearing mice with p53-specific cytotoxic T lymphocytes (CTL) results in eradication of p53-overexpressing tumors in the absence of immunopathological damage to normal tissues. These CTL also eliminate tumors that do not show greatly enhanced expression of p53, indicating that the sensitivity of these tumors for p53-specific CTL is determined by the efficiency by which p53-derived peptides are processed into class I MHC, rather than by the steady state levels of p53. Of note, although p53-specific CTL can readily be isolated from p53-/- mice, tolerance for this self antigen may prevent induction of similarly effective CTL in p53+/+ subjects. The T helper (Th) branch of the p53-specific immune response does not seem to be profoundly affected by tolerance. In addition, more and more evidence is obtained for the pivotal role of tumor-specific Th cells in the induction and effector phases of the antitumor response, also against tumors that lack class II MHC expression. The efficacy of Th cells, specific for a recently identified class II MHC-restricted p53 peptide, against p53-overexpressing tumors is currently being investigated. In addition, natural and induced Th responses are analyzed both in a murine tumor model and in a phase I clinical trial involving p53-specific vaccination of colon cancer patients.

CD4+ T cell responses to self- and mutated p53 determinants during tumorigenesis in mice

We analyzed CD4+ T helper responses to wild-type (wt) and mutated (mut) p53 protein in normal and tumor-bearing mice. In normal mice, we observed that although some self-p53 determinants induced negative selection of p53-reactive CD4+ T cells, other p53 determinants (cryptic) were immunogenic. Next, BALB/c mice were inoculated with J774 syngeneic tumor cell line expressing mut p53. BALB/c tumor-bearing mice mounted potent CD4+ T cell responses to two formerly cryptic peptides on self-p53. This response was characterized by massive production of IL-5, a Th2-type lymphokine. Interestingly, we found that T cell response was induced by different p53 peptides depending upon the stage of cancer. Mut p53 gene was shown to contain a single mutation resulting in the substitution of a tyrosine by a histidine at position 231 of the protein. Two peptides corresponding to wt and mutated sequences of this region were synthesized. Both peptides bound to the MHC class II-presenting molecule (Ed) with similar affinities. However, only mut p53.225-239 induced T cell responses in normal BALB/c mice, a result strongly suggesting that high-affinity wt p53.225-239 autoreactive T cells had been eliminated in these mice. Surprisingly, CD4+ T cell responses to both mut and wt p53.225-239 peptides were recorded in J774 tumor-bearing mice, a phenomenon attributed to the recruitment of low-avidity p53.225-239 self-reactive T cells.

A processed pseudogene codes for a new antigen recognized by a CD8(+) T cell clone on melanoma

The M88.7 T cell clone recognizes an antigen presented by HLA B*1302 on the melanoma cell line M88. A cDNA encoding this antigen (NA88-A) was isolated using a library transfection approach. Analysis of the genomic gene's sequence identified it is a processed pseudogene, derived from a retrotranscript of mRNA coding for homeoprotein HPX42B. The NA88-A gene exhibits several premature stop codons, deletions, and insertions relative to the HPX42B gene. In NA88-A RNA, a short open reading frame codes for the peptide MTQGQHFLQKV from which antigenic peptides are derived; a stop codon follows the peptide's COOH-terminal Val codon. Part of the HPX42B mRNA's 3′ untranslated region codes for a peptide of similar sequence (MTQGQHFSQKV). If produced, this peptide can be recognized by M88.7 T cells. However, in HPX42B mRNA, the peptide's COOH-terminal Val codon is followed by a Trp codon. As a result, expression of HPX42B mRNA does not lead to antigen production. A model is proposed for events that participated in creation of a gene coding for a melanoma antigen from a pseudogene.

MHC class I antigen processing of an adenovirus CTL epitope is linked to the levels of immunoproteasomes in infected cells

Proteasomes are the major source for the generation of peptides bound by MHC class I molecules. To study the functional relevance of the IFN-gamma-inducible proteasome subunits low molecular mass protein 2 (LMP2), LMP7, and mouse embryonal cell (MEC) ligand 1 in Ag processing and concomitantly that of immunoproteasomes, we established the tetracycline-regulated mouse cell line MEC217, allowing the titrable formation of immunoproteasomes. Infection of MEC217 cells with Adenovirus type 5 (Ad5) and analysis of Ag presentation with Ad5-specific CTL showed that cells containing immunoproteasomes processed the viral early 1B protein (E1B)-derived epitope E1B192-200 with increased efficiency, thus allowing a faster detection of viral entry in induced cells. Importantly, optimal CTL activation was already achieved at submaximal immunosubunit expression. In contrast, digestion of E1B-polypeptide with purified proteasomes in vitro yielded E1B192-200 at quantities that were proportional to the relative contents of immunosubunits. Our data provide evidence that the IFN-gamma-inducible proteasome subunits, when present at relatively low levels as at initial stages of infection, already increase the efficiency of antigenic peptide generation and thereby enhance MHC class I Ag processing in infected cells.

Abrogation of CTL epitope processing by single amino acid substitution flanking the C-terminal proteasome cleavage site

CTL directed against the Moloney murine leukemia virus (MuLV) epitope SSWDFITV recognize Moloney MuLV-induced tumor cells, but do not recognize cells transformed by the closely related Friend MuLV. The potential Friend MuLV epitope has strong sequence homology with Moloney MuLV and only differs in one amino acid within the CTL epitope and one amino acid just outside the epitope. We now show that failure to recognize Friend MuLV-transformed tumor cells is based on a defect in proteasome-mediated processing of the Friend epitope which is due to a single amino acid substitution (N-->D) immediately flanking the C-terminal anchor residue of the epitope. Proteasome-mediated digestion analysis of a synthetic 26-mer peptide derived from the Friend sequence shows that cleavage takes place predominantly C-terminal of D, instead of V as is the case for the Moloney MuLV sequence. Therefore, the C terminus of the epitope is not properly generated. Epitope-containing peptide fragments extended with an additional C-terminal D are not efficiently translocated by TAP and do not show significant binding affinity to MHC class I-Kb molecules. Thus, a potential CTL epitope present in the Friend virus sequence is not properly processed and presented because of a natural flanking aspartic acid that obliterates the correct C-terminal cleavage site. This constitutes a novel way to subvert proteasome-mediated generation of proper antigenic peptide fragments.

The proteasome activator 11 S REG (PA28) and class I antigen presentation

There are two immune responses in vertebrates: humoral immunity is mediated by circulating antibodies, whereas cytotoxic T lymphocytes (CTL) confer cellular immunity. CTL lyse infected cells upon recognition of cell-surface MHC Class I molecules complexed with foreign peptides. The displayed peptides are produced in the cytosol by degradation of host proteins or proteins from intracellular pathogens that might be present. Proteasomes are cylindrical multisubunit proteases that generate many of the peptides eventually transferred to the cell surface for immune surveillance. In mammalian proteasomes, six active sites face a central chamber. As this chamber is sealed off from the enzyme's surface, there must be mechanisms to promote entry of substrates. Two protein complexes have been found to bind the ends of the proteasome and activate it. One of the activators is the 19 S regulatory complex of the 26 S proteasome; the other activator is '11 S REG' [Dubiel, Pratt, Ferrell and Rechsteiner (1992) J. Biol. Chem. 267, 22369-22377] or 'PA28' [Ma, Slaughter and DeMartino (1992) J. Biol. Chem. 267, 10515-10523]. During the past 7 years, our understanding of the structure of REG molecules has increased significantly, but much less is known about their biological functions. There are three REG subunits, namely alpha, beta and gamma. Recombinant REGalpha forms a ring-shaped heptamer of known crystal structure. 11 S REG is a heteroheptamer of alpha and beta subunits. REGgamma is also presumably a heptameric ring, and it is found in the nuclei of the nematode work Caenorhabditis elegans and higher organisms, where it may couple proteasomes to other nuclear components. REGalpha and REGbeta, which are abundant in vertebrate immune tissues, are located mostly in the cytoplasm. Synthesis of REG alpha and beta subunits is induced by interferon-gamma, and this has led to the prevalent hypothesis that REG alpha/beta hetero-oligomers play an important role in Class I antigen presentation. In the present review we focus on the structural properties of REG molecules and on the evidence that REGalpha/beta functions in the Class I immune response.

Efficient generation of a hepatitis B virus cytotoxic T lymphocyte epitope requires the structural features of immunoproteasomes

Interferon (IFN)-gamma-induced cells express the proteasome subunits low molecular weight protein (LMP)2, LMP7, and MECL-1 (multicatalytic endopeptidase complex-like 1), leading to the formation of immunoproteasomes. Although these subunits are thought to optimize MHC class I antigen processing, the extent of their role and the mechanistic aspects involved remain unclear. Herein, we study the proteolytic generation of an human histocompatibility leukocyte antigen (HLA)-Aw68-restricted hepatitis B virus core antigen (HBcAg) cytotoxic T lymphocyte (CTL) epitope that is recognized by peripheral blood lymphocytes from patients with acute self-limited but not chronic hepatitis B virus (HBV). Immunological data suggest that IFN-gamma-induced rather than uninduced HeLa cells process and present the HBV CTL epitope upon infection with HBcAg-expressing vaccinia viruses. Analyses of 20S proteasome digests of synthetic polypeptides covering the antigenic HBcAg peptide demonstrate that only immunoproteasomes efficiently perform the cleavages needed for the liberation of this HBV CTL epitope. Although the concerted presence of the three immunosubunits appears essential, we find that both catalytically active LMP7 and inactive LMP7 T1A support CTL epitope generation. We conclude that LMP7 influences the structural features of 20S proteasomes, thereby enhancing the activity of the LMP2 and MECL-1 catalytic sites, which provide cleavage specificity. Thus, LMP7 incorporation is of greater functional importance for the generation of an HBV CTL epitope than cleavage specificity.

Cytotoxic T-lymphocyte clones, established by stimulation with the HLA-A2 binding p5365-73 wild type peptide loaded on dendritic cells In vitro, specifically recognize and lyse HLA-A2 tumour cells overexpressing the p53 protein

Mutations in the tumour suppressor gene p53 are among the most frequent genetic alterations in human malignancies, often associated with an accumulation of the p53 protein in the cytoplasm. We have generated a number of cytotoxic T lymphocyte (CTL) clones that specifically recognize the HLA-A*0201 p53 wild type peptide RMPEAAPPV [65-73], designated R9V, by the in vitro stimulation of CD8 enriched peripheral blood lymphocytes from a healthy HLA-A*0201 donor using peptide loaded autologous dendritic cells. A total of 22 CTL clones were generated from the same bulk culture and demonstrated to carry identical T-cell receptors. The CTL clone, 2D9, was shown to specifically lyse the HLA-A*0201+ squamous carcinoma cell line SCC9 and the breast cancer cell line MDA-MB-468. Our data demonstrate that human peripheral blood lymphocytes from normal healthy individuals comprise T cells capable of recognizing p53 derived wild type (self) peptides. Furthermore, the capacity of R9V specific T cell clones to exert HLA restricted cytotoxicity, argues that the R9V peptide is naturally presented on certain cancer cells. This supports the view that p53 derived wild type peptides might serve as candidate target antigens for the immunotherapeutic treatment of cancer.

Sequence signals for generation of antigenic peptides by the proteasome: implications for proteasomal cleavage mechanism

Proteasomal cleavage of proteins is the first step in the processing of most antigenic peptides that are presented to cytotoxic T cells. Still, its specificity and mechanism are not fully understood. To identify preferred sequence signals that are used for generation of antigenic peptides by the proteasome, we performed a rigorous analysis of the residues at the termini and flanking regions of naturally processed peptides eluted from MHC class I molecules. Our results show that both the C terminus (position P1 of the cleavage site) and its immediate flanking position (P1') possess significant signals. The N termini of the peptides show these signals only weakly, consistent with previous findings that antigenic peptides may be cleaved by the proteasome with N-terminal extensions. Nevertheless, we succeed to demonstrate indirectly that the N-terminal cleavage sites contain the same preferred signals at position P1'. This reinforces previous findings regarding the role of the P1' position of a cleavage site in determining the cleavage specificity, in addition to the well-known contribution of position P1. Our results apply to the generation of antigenic peptides and bare direct implications for the mechanism of proteasomal cleavage. We propose a model for proteasomal cleavage mechanism by which both ends of cleaved fragments are determined by the same cleavage signals, involving preferred residues at both P1 and P1' positions of a cleavage site. The compatibility of this model with experimental data on protein degradation products and generation of antigenic peptides is demonstrated.

Cytotoxic T lymphocyte epitopes of HIV-1 Nef: Generation of multiple definitive major histocompatibility complex class I ligands by proteasomes

Although a pivotal role of proteasomes in the proteolytic generation of epitopes for major histocompatibility complex (MHC) class I presentation is undisputed, their precise function is currently the subject of an active debate: do proteasomes generate many epitopes in definitive form, or do they merely generate the COOH termini, whereas the definitive NH(2) termini are cleaved by aminopeptidases? We determined five naturally processed MHC class I ligands derived from HIV-1 Nef. Unexpectedly, the five ligands correspond to only three cytotoxic T lymphocyte (CTL) epitopes, two of which occur in two COOH-terminal length variants. Parallel analyses of proteasomal digests of a Nef fragment encompassing the epitopes revealed that all five ligands are direct products of proteasomes. Moreover, in four of the five ligands, the NH(2) termini correspond to major proteasome cleavage sites, and putative NH(2)-terminally extended precursor fragments were detected for only one of the five ligands. All ligands are transported by the transporter associated with antigen processing (TAP). The combined results from these five ligands provide strong evidence that many definitive MHC class I ligands are precisely cleaved at both ends by proteasomes. Additional evidence supporting this conclusion is discussed, along with contrasting results of others who propose a strong role for NH(2)-terminal trimming with direct proteasomal epitope generation being a rare event.

The proteasome activator 11 S REG (PA28) and class I antigen presentation

There are two immune responses in vertebrates: humoral immunity is mediated by circulating antibodies, whereas cytotoxic T lymphocytes (CTL) confer cellular immunity. CTL lyse infected cells upon recognition of cell-surface MHC Class I molecules complexed with foreign peptides. The displayed peptides are produced in the cytosol by degradation of host proteins or proteins from intracellular pathogens that might be present. Proteasomes are cylindrical multisubunit proteases that generate many of the peptides eventually transferred to the cell surface for immune surveillance. In mammalian proteasomes, six active sites face a central chamber. As this chamber is sealed off from the enzyme's surface, there must be mechanisms to promote entry of substrates. Two protein complexes have been found to bind the ends of the proteasome and activate it. One of the activators is the 19 S regulatory complex of the 26 S proteasome; the other activator is '11 S REG' [Dubiel, Pratt, Ferrell and Rechsteiner (1992) J. Biol. Chem. 267, 22369-22377] or 'PA28' [Ma, Slaughter and DeMartino (1992) J. Biol. Chem. 267, 10515-10523]. During the past 7 years, our understanding of the structure of REG molecules has increased significantly, but much less is known about their biological functions. There are three REG subunits, namely alpha, beta and gamma. Recombinant REGalpha forms a ring-shaped heptamer of known crystal structure. 11 S REG is a heteroheptamer of alpha and beta subunits. REGgamma is also presumably a heptameric ring, and it is found in the nuclei of the nematode work Caenorhabditis elegans and higher organisms, where it may couple proteasomes to other nuclear components. REGalpha and REGbeta, which are abundant in vertebrate immune tissues, are located mostly in the cytoplasm. Synthesis of REG alpha and beta subunits is induced by interferon-gamma, and this has led to the prevalent hypothesis that REG alpha/beta hetero-oligomers play an important role in Class I antigen presentation. In the present review we focus on the structural properties of REG molecules and on the evidence that REGalpha/beta functions in the Class I immune response.

The specificity of proteasomes: impact on MHC class I processing and presentation of antigens

We have studied polypeptide processing by purified proteasomes, with regard to proteolytic specificity and cytotoxic T-lymphocyte (CTL) epitope generation. Owing to defined preferences with respect to cleavage sites and fragment length, proteasomes degrade polypeptide substrates into cohorts of overlapping oligopeptides. Many of the proteolytic fragments exhibit structural features in common with major histocompatibility complex (MHC) class I ligands including fragment size and frequencies of amino acids at fragment boundaries. Proteasomes frequently generate definitive MHC class I ligands and/or slightly longer peptides, while substantially larger peptides are rare. Individual CTL epitopes are produced in widely varying amounts, often consistent with immunohierarchies among CTL epitopes. We further found that polypeptide processing is remarkably conserved among proteasomes of eukaryotic origin and that invertebrate proteasomes can efficiently produce known high-copy MHC class I ligands, suggesting evolutionary adaptation of the transporter associated with antigen processing and MHC class I to ancient constraints imposed by proteasomal protein degradation.

Effect of epitope flanking residues on the presentation of N-terminal cytotoxic T lymphocyte epitopes

We here demonstrate that placing two distinct influenza virus nucleoprotein epitopes at the N terminus of a cytosolic protein selectively blocks their presentation to specific cytotoxic T lymphocytes. The block is a cytosolic phenomenon, which can be overcome by distancing the epitope from the protein N terminus by two or more amino acids. Shortening the protein's C terminus fails to relieve the antigen presentation block. These results demonstrate that events at the N terminus of the target protein, rather than at its C terminus, are responsible for the lack of presentation of N-terminal epitopes. We also show that lack of presentation of N terminal epitopes is associated with a modification of the target protein which affects its electrophoretic mobility and isoelectric focusing point. This modification can be prevented by mutating the epitope's N-terminal flanking sequence, which results in an efficient presentation of the N-terminal epitope. Lack of presentation of the N-terminal epitopes results in a reduced ability of influenza-primed mice to clear acute infection with vaccinia virus encoding an N-terminal nucleoprotein epitope. Our results demonstrate that presentation of epitopes localized at the N terminus of cytosolic proteins can be modulated by events occurring at early stages of antigen processing.

Degradation of cell proteins and the generation of MHC class I-presented peptides

Major histocompatibility complex (MHC) class I molecules display on the cell surface 8- to 10-residue peptides derived from the spectrum of proteins expressed in the cells. By screening for non-self MHC-bound peptides, the immune system identifies and then can eliminate cells that are producing viral or mutant proteins. These antigenic peptides are generated as side products in the continual turnover of intracellular proteins, which occurs primarily by the ubiquitin-proteasome pathway. Most of the oligopeptides generated by the proteasome are further degraded by distinct endopeptidases and aminopeptidases into amino acids, which are used for new protein synthesis or energy production. However, a fraction of these peptides escape complete destruction and after transport into the endoplasmic reticulum are bound by MHC class I molecules and delivered to the cell surface. Herein we review recent discoveries about the proteolytic systems that degrade cell proteins, how the ubiquitin-proteasome pathway generates the peptides presented on MHC-class I molecules, and how this process is stimulated by immune modifiers to enhance antigen presentation.

A theoretical approach towards the identification of cleavage-determining amino acid motifs of the 20 S proteasome

Hitherto the mechanisms controlling the selective cleavage of peptide bonds by the 20 S proteasome have been poorly understood. The observation that peptide bond cleavage may eventually occur at the carboxyl site of either amino acid residue rules out a simple control of cleavage preferences by the P1 residue alone. Here, we follow the rationale that the presence of specific cleavage-determining amino acids motifs (CDAAMs) around the scissile peptide bond are required for the attainment of substrate conformations susceptible to cleavage. We present an exploratory search for these putative motifs based on empirical regression functions relating the cleavage probability for a given peptide bond to some selected side-chain properties of the flanking amino acid residues. Identification of the sequence locations of cleavage-determining residues relative to the scissile bond and of their optimal side-chain properties was carried out by fitting the cleavage probability to (binary) experimental observations on peptide bond cleavage gathered among a set of seven different peptide substrates with known patterns of proteolytic degradation products. In this analysis, all peptide bonds containing the same residue in the P1 position were assumed to be cleaved by the same active sites of the proteasome, and thus to be under control of the same CDAAMs. We arrived at a final set of ten different CDAAMs, accounting for the cleavage of one to five different groups of peptide bonds with an overall predictive correctness of 93 %. The CDAAM is composed of two to four "anchor" positions preferentially located between P5 and P5' around the scissile bond. This implies a length constraint for the usage of cleavage sites, which could considerably suppress the excision of shorter fragments and thus partially explain for the observed preponderance of medium-size cleavage products.

The role of the proteasome system and the proteasome activator PA28 complex in the cellular immune response

The generation of antigenic peptides bound and presented to the immune system by MHC class I molecules predominantly depends on the function of the proteasome system. Stimulation of cells with interferon gamma induces the incorporation of three active site bearing beta-subunits into the 20S proteasome and the formation of the PA28 proteasome modulator complex. PA28 alters the cleavage properties of the proteasome and enhances MHC class I antigen presentation. Thus, by cytokine induced change of the proteasome system cells may alter the proteolytic properties of the 20S proteasome and may render an organism more flexible in its peptide generation capacity.