T cell clones specific for p21 ras-derived peptides: characterization of their fine specificity and HLA restriction

Colorectal cancer vaccines: Tumor-associated antigens vs neoantigens

Therapeutic options for the treatment of colorectal cancer (CRC) are diverse but still not always satisfying. Recent success of immune checkpoint inhibition treatment for the subgroup of CRC patients suffering from hyper-mutated tumors suggests a permanent role of immune therapy in the clinical management of CRC. Substantial improvement in treatment outcome could be achieved by development of efficient patient-individual CRC vaccination strategies. This mini-review summarizes the current knowledge on the two general classes of targets: tumor-associated antigens (TAAs) and tumor-specific antigens. TAAs like carcinoembryonic antigen and melanoma associated antigen are present in and shared by a subgroup of patients and a variety of clinical studies examined the efficacy of different TAA-derived peptide vaccines. Combinations of several TAAs as the next step and the development of personalized TAA-based peptide vaccines are discussed. Improvements of peptide-based vaccines achievable by adjuvants and immune-stimulatory chemotherapeutics are highlighted. Finally, we sum up clinical studies using tumor-specific antigens - in CRC almost exclusively neoantigens - which revealed promising results; particularly no severe adverse events were reported so far. Critical progress for clinical outcomes can be expected by individualizing neoantigen-based peptide vaccines and combining them with immune-stimulatory chemotherapeutics and immune checkpoint inhibitors. In light of these data and latest developments, truly personalized neoantigen-based peptide vaccines can be expected to fulfill modern precision medicine’s requirements and will manifest as treatment pillar for routine clinical management of CRC.

Pilot study of mutant ras peptide-based vaccine as an adjuvant treatment in pancreatic and colorectal cancers

INTRODUCTION

There is mounting evidence describing the immunosuppressive role of bulky metastatic disease, thus countering the therapeutic effects of tumor vaccine. Therefore, adjuvant immunotherapy may have a better impact on clinical outcome. In this phase II clinical trial, we aimed to test the feasibility of using a specific mutant ras peptide vaccine as an adjuvant immunotherapy in pancreatic and colorectal cancer patients.

MATERIALS AND METHODS

Twelve patients with no evidence of disease (NED), five pancreatic and seven colorectal cancer patients were vaccinated subcutaneously with 13-mer mutant ras peptide, corresponding to their tumor's ras mutation. Vaccinations were given every 4 weeks, up to a total of six vaccines.

RESULTS

No serious acute or delayed systemic side effects were seen. We detected specific immune responses to the relevant mutant ras peptide by measuring IFN-gamma mRNA expression by quantitative real-time PCR. Five out of eleven patients showed a positive immune response. Furthermore, the five pancreatic cancer patients have shown a mean disease-free survival (DFS) of 35.2+ months and a mean overall survival (OS) of 44.4+ months. The seven colorectal cancer patients have shown a mean disease-free survival (DFS) of 27.2+ months and a mean overall survival (OS) of 41.5+ months.

CONCLUSION

In this study, we found that it is feasible to use mutant ras vaccine in the adjuvant setting. This vaccine is safe, can induce specific immune responses, and it appears to have a positive outcome in overall survival. Therefore, we believe that such an approach warrants further investigation in combination with other therapies.

Molecular technology and pancreatic cancer

BACKGROUND

Pancreatic cancer is the fifth leading cause of cancer death in the Western world. Despite improvement in operative mortality rates, little impact has been made on overall 5-year survival. This review discusses the molecular changes peculiar to pancreatic cancer and how the use of molecular technology might affect detection, screening, diagnosis and treatment of the disease.

METHODS

A literature review was performed using the National Library of Medicine's Pubmed database; this was combined with ongoing work within the Queen Elizabeth Hospital, Birmingham.

RESULTS

Over the past 20 years great strides have been made in our understanding of the molecular basis of disease. Advances in molecular biology are now reshaping how diseases are screened for, diagnosed, investigated and treated. In recent years collaboration between clinicians and basic scientists has revealed a unique pattern of genetic and molecular events in pancreatic cancer. This review discusses how these advances may impact on patients with this disease.

CONCLUSION

The past decade has seen some improvement in outlook for patients with pancreatic cancer, but the 'molecular age' promises to deliver even better results.

c-erbB-2 and episialin challenge host immune response by HLA class I expression in human non-small-cell lung cancer

The role of major histocompatibility complex expression in cancer prognosis and pathogenesis is contradictory. The aim of the current study was to compare the expression of HLA class I molecules and of oncoproteins that may be sources of peptides presented by HLA class I antigens in non-small-cell lung cancer. For this purpose, the expression of HLA class I antigen and TAP-1 molecule (a transporter in the antigen-processing 1 transport protein) were studied with epidermal growth factor, receptor; c-erbB-2; episialin; wild-type and mutant p53; bcl-2 oncoprotein expression; and angiogenic factor expression (vascular endothelial growth factor and thymidine phosphorylase). The degree of lymphocytic stromal infiltration and of platelet-endothelial cell adhesion molecule-expressing lymphocytes was also studied. A strong association of c-erbB-2 and MUC1 (episialin) expression with HLA class I expression was observed (p = 0.005 and 0.009, respectively). Intense CD31-positive lymphocytic infiltration was also more frequent in HLA class I-positive cases (p = 0.05). Although there was no association of HLA class I expression with survival, loss of the HLA class I expression in MUC1 or c-erbB-2 overexpressing cases conferred a poorer clinical outcome (p = 0.04). Both c-erbB-2 and MUC1 are well-known targets of T-cell-mediated cytotoxicity and cell-cell or cell-matrix adhesion-regulating proteins. The authors provide evidence that the sequence of cell adhesion-disrupting oncoprotein expression, HLA class I induction, and enhanced epitope presentation followed by lymphocytic response is an important pathogenetic three-step sequence of events that define, in part, the clinical outcome in non-small-cell lung cancer.

Cellular but not humoral immune responses generated by vaccination with dendritic cells protect mice against leukaemia

Dendritic cells (DC) are extremely efficient at generating both prophylactic and therapeutic anti-tumour immunity. We aimed to analyse the respective roles of humoral and cellular immune responses generated in mice vaccinated with bone marrow (BM)-derived DC in terms of in vivo anti-leukaemia effect. We used the murine L1210 B lymphocytic leukaemia genetically modified to express on the cell surface of human CD4 (hCD4) (L1210/hCD4) as a model tumour-associated antigen (TAA). DC cultures were loaded with either purified soluble hCD4 (shCD4) protein or unfractionated L1210/hCD4 extracts and injected as vaccine into mice. The efficacy of these vaccinations was compared with that of vaccination with shCD4 protein emulsified in Freund's adjuvant (FA). We evaluated the immune responses generated after these vaccinal protocols and the survival rate of vaccinated mice subsequently challenged with a lethal injection of L1210/hCD4 cells. Our results demonstrated that vaccination with shCD4 protein or tumour extract-loaded DC mainly generated an hCD4 antigen-specific cell-mediated cytotoxic immune response that was associated with a specific protection against leukaemia. In contrast, vaccination with the protein emulsified in FA only generated potent humoral immune responses that were not protective against leukaemia. Altogether, our results indicate that the unique property of loaded DC to trigger an anti-leukaemia protective effect is mainly associated with cellular immune responses.

Dendritic cell immunotherapy for cancer: application to low-grade lymphoma and multiple myeloma

The confirmation that most cancers express one or more molecular changes, which may act as tumour-associated antigens (TAA), combined with the knowledge that T lymphocytes recognize even single amino acid differences in MHC presented peptides has stimulated renewed clinical interest in immunotherapeutic strategies. Dendritic cells (DC) are now recognized as specialist antigen-presenting cells, which initiate, direct and regulate immune responses. Recent data suggest that DC are not recruited into, or activated by, cancers and that other abnormalities in DC function are associated with malignancy, including multiple myeloma. This provides a rationale for designing immunotherapeutic strategies, which exploit DC as nature's adjuvant either in vivo or in vitro. Low-grade lymphoma and multiple myeloma are slowly progressive malignancies, which generally express a unique immunoglobulin idiotype as a potential TAA. Data from animal models and clinical studies suggest that DC-based immunotherapy strategies, applied when the patient has minimal residual disease, may improve the long-term prognosis in these diseases.

Identification of Peptide Superagonists for a Self-K-ras-Reactive CD4+ T Cell Clone Using Combinatorial Peptide Libraries and Mass Spectrometry

The proliferative responses of a human CD4+ T cell clone 29.15.2, reactive with a self-K-ras-derived peptide (3EYKLVVVGAGGVGKSALT20), were tested using a set of X9 combinatorial peptide libraries containing the flanking residues (EYKLVXXXXXXXXXSALT, where X indicates random amino acids). Certain peptide libraries, such as EYKLVXXXXXXMXXSALT and EYKLVXXXXXXXHXSALT, stimulated a marked proliferation of 29.15.2. However, no combinations of substitutions tested, such as EYKLVXXXXXXMHXSALT, exhibited additive effects. We subsequently synthesized peptides with degenerate sequences (a mixture of 480 species), where each position is composed of the wild-type (wt) residue or of amino acids that induced the proliferation of 29.15.2, in positional scanning. Interestingly, one fraction of degenerate peptides, separated by reverse-phase HPLC, stimulated much higher proliferation than did the wt; in addition, the retention time of this fraction was distinct from that of the wt. Mass spectrometry analysis of this fraction and flanking fractions identified five peptide species that exhibit strong signals in a manner that parallels the antigenic activity. Finally, 17 candidate peptide sequences were deduced from mass spectrometry and hydrophobicity scoring results, of which two peptides (EYKLVVVGAGGMLKSALT and EYKLVVVGAGGMIKSALT) did induce 52- and 61-fold stronger proliferation, respectively, compared with the wt. These findings indicate that: 1) synthetic peptides that carry “the best” residue substitution at each position of combinatorial peptide libraries do not always exhibit superagonism, and 2) such a drawback can be overcome with the use of mass spectrometry. This approach provides new perspectives for the accurate and efficient identification of peptide superagonists.

T cell activation by monoclonal antibodies bound to tumor cells by a cell surface displayed single-chain antibody

Tumor cells often lack the costimulatory molecules necessary for T cell activation. However, the transformation of cells with more than one stimulatory molecule is a difficult procedure. We therefore developed a retroviral vector for the expression of a cell membrane anchored single-chain antibody fragment (scFv) directed against the hapten 4-ethoxymethylene-2-phenyl-2-oxazoline-5-one (phOx). Proteins and peptides can be readily modified with this hapten, thus, enabling them to be bound to cells with the cell surface displayed anti-phOx scFv. To test combinations of surface-bound stimulatory molecules on T cell activation, SK-Mel63 human melanoma cells expressing the membrane anchored anti-phOx scFv were incubated with phOx-labeled mAbs against CD3, CD28 and CD5. Cells presenting a given mixture of modified anti-CD3 and anti-CD28 molecules stimulated T cell activation better than any single antibody and a given mixture of anti-CD3, anti-CD28 and anti-CD5 provided a stimulatory response higher than the best double combination. However, the relative concentrations are very important and must be carefully chosen. Concentrations of antibodies giving good T cell responses when used alone can block synergistic effects.

Efficient induction of human CD4+ T cell lines reactive with a self-K-ras-derived peptide in vitro, using a mAb to CD29

In recent studies, we analyzed effects of a large series of mAbs submitted to T-cell and adhesion structure sections of 6th International Workshop on Human Leukocyte Differentiation Antigens, on proliferative responses of a human CD4+ T cell clone. We found that certain mAbs to CD27, CD28, CD29, CD43 and CD44 markedly restore T cell responsiveness, only in the presence of the natural peptide ligand at low concentrations and even in the absence IL-2. To set up efficient strategies for developing T cell lines and clones reactive with self-peptides in vitro, we stimulated PBMC with a K-ras-derived peptide in the presence of these mAbs and found that: (a) anti-CD29 mAb MAR4 is effective for in vitro expansion of K-ras (residue 3-20)-reactive T cells from PBMC; (b) indeed, DR8 (DRB*0802)-restricted and DR51 (DRB5*0102)-restricted CD4+ T cell clones were established from the T cell lines; (c) MAR4 inhibits anti-CD3-induced apoptosis of PBMC; and (d) MAR4 is effective in enhancing anti-CD3- and antigen-induced proliferative responses of PBMC. Therefore, mAb MAR4 increased efficiency in establishing T cell clones from PBMC, both by suppressing antigen-driven activation-induced cell death and by enhancing the T cell proliferation, only in the presence of TCR/CD3-mediated stimulation. Possible application of MAR4 for establishing self-reactive T cell lines and expanding T cells ex vivo for anti-cancer immunotherapy, is discussed.

Augmentation of immune response by an analog of the antigenic peptide in a human T-cell clone recognizing mutated Ras-derived peptides

T-cells that recognize mutated p21 Ras are relevant to immune surveillance systems against cancer. We report here evidence that immune responses of a T-cell clone recognizing mutated p21 Ras can be augmented by an analog peptide. Using spleen cells from a gastric cancer patient, we established the CD4+ alpha beta Th1-like clone C27 that recognizes wild-type (3EYKLVVVGAGGVGKS17) and mutated p21 Ras protein molecules and peptides, in an HLA-DR1-restricted manner. C27 responded prominently to mutated Ras peptides carrying Val or Ala at position 12, as compared to wild-type and other mutated peptides. C27 also exhibited a much stronger response to a mutated p21 Ras whole-protein molecule-carrying Val at position 12, as compared with the wild-type protein. The proliferative response and production of GM-CSF, TNF-alpha, and IFN-gamma by C27 were further augmented by replacing the possible first DR anchor 4Tyr of the mutated Ras peptide with Trp, a more potent anchor residue for the DR1 molecule. Enhancement of peptide antigenicity by substituting the HLA anchor residue of an antigenic peptide recognized by tumor-reactive T-cells may prove to be a novel strategy for antigen-specific cancer immunotherapy.

Characterisation of immune responses in pancreatic carcinoma patients after mutant p21 ras peptide vaccination

This is a study of immune responses generated by mutant ras peptide vaccination of patients with pancreatic adenocarcinoma. Responding T cells from one patient were cloned and two CD4+ T-lymphocyte clones (TLC) specific for the 12 Val peptide and restricted by HLA-DR6 or DQ2 were obtained. These class II molecules have not previously been found to bind or present mutant ras peptides to T cells. The DR6-restricted TLC showed marked cytotoxicity against autologous target cells pulsed with the 12 Val peptide. Target cells pulsed with the control peptide were not killed. Responding T cells from another patient showed cross-reactivity towards the homologous ras peptides. Investigation by limiting dilution analysis (LDA) revealed different T-cell precursor frequencies for the immunising, mutant ras peptide (1:28000), compared with the normal ras peptide (1:110000).

Cytotoxic T-lymphocyte responses against mutated p21 ras peptides: an analysis of specific T-cell-receptor gene usage

Generation of cytotoxic-T-lymphocyte (CTL) responses against mutated ras peptides from peripheral-blood mononuclear cells (PBMC) was attempted in a group of HLA-A2.1+ healthy donors. Bulk PBMC cultures were stimulated in vitro with a mixture of peptides encompassing 12 Gly --> Val, 61 Gln --> Lys or 61 Gln --> Leu ras mutations and displaying HLA-A2.1 binding motifs, selected by a computer program. A promiscuous tetanus toxoid peptide was also added. Weekly thereafter, PBMC were re-stimulated with peptide pulsed autologous Epstein-Barr virus (EBV)-transformed B cells. After 8 rounds of re-stimulation, reproducible cytotoxic activity against peptide-pulsed target cells was detectable in one donor. The CTL line recognized 2 nonamers encompassing ras 61 Gln --> Leu mutation. Killing was mediated by CD8+ T cells displaying alphabeta TCR and was inhibited by anti-HLA-A2.1 monoclonal antibodies. No killing of tumor cells expressing the specific mutation could be observed. More than 60 CTL clones were generated. Fine specificity studies revealed effective, though differing cytotoxic activity against both 53-LDILDTAGL-61 and 55-ILDTAGLEE-63, but not against 54-DILDTAGLE-62 mutated peptides, in all but one of the clones. None was able to exert effective cytotoxic activity against tumor cells expressing the specific mutation. T-cell-receptor (TCR) usage was then analyzed phenotypically, by reverse-transcription-polymerase-chain-reaction (RT-PCR) and by sequence analysis. This study revealed the monoclonal nature of the CTL response against mutated nonamers, with TCR expressing Vbeta14 gene product in combination with, Jbeta2.7 and Cbeta2.

K-ras mutations and HLA-DR expression in large bowel adenomas

A total of 72 sporadic colorectal adenomas in 56 patients were studied for the presence of point mutations in codons 12 and 13 of the K-ras gene and for HLA-DR antigen expression related to clinicopathological variables. Forty K-ras mutations in 39 adenomas were found (54%): 31 (77%) in codon 12 and nine (23%) in codon 13. There was a strong relationship between the incidence of K-ras mutations and adenoma type, degree of dysplasia and sex. The highest frequency of K-ras mutations was seen in large adenomas of the villous type with high-grade dysplasia. Fourteen out of 15 adenomas obtained from 14 women above 65 years of age carried mutations. HLA-DR positivity was found in 38% of the adenomas, large tumours and those with high-grade dysplasia having the strongest staining. Coexpression of K-ras mutations and HLA-DR was found significantly more frequently in large and highly dysplastic adenomas, although two-way analysis of variance showing size and grade of dysplasia to be the most important variable. None of the adenomas with low-grade dysplasia showed both K-ras mutation and HLA-DR positivity (P = 0.004). K-ras mutation is recognised as an early event in colorectal carcinogenesis. The mutation might give rise to peptides that may be presented on the tumour cell surface by class II molecules, and thereby induce immune responses against neoplastic cells.

Development of immunogenic colorectal cancer cell lines for vaccination: expression of CD80 (B7.1) is not sufficient to restore impaired primary T cell activation in vitro

The capacity of colorectal carcinoma and melanoma cell lines to induce primary versus effector T lymphocyte activation in vitro was investigated. Established epithelial tumour cell lines derived from colorectal carcinoma and melanoma did not activate a primary proliferative response of resting T lymphocytes in allogeneic mixed lymphocyte tumour cell cultures (MLTCs). In contrast, the same tumour cells were effectively lysed by preactivated cytolytic T cell clones. This demonstrates that tumour cells are impaired in inducing a primary immune response but are susceptible to effector immune responses. Attempts at improving primary T cell activation revealed that exogenous cytokines, including interferon-gamma (IFN-gamma), tumour necrosis factor-alpha (TNF-alpha) and interleukin-2 (IL-2), were not effective. Expression of CD80 (B7.1), by transfecting a CD80 cDNA into the melanoma cell line SkMel63, improved T cell proliferation considerably. In contrast, CD80 expression in two colorectal carcinoma cell lines (SW480, SW707) did not result in T cell activation. This was not due to lack of class II MHC expression on SW480 since coexpression of a HLA-DR3 alloantigen and CD80 had no effect. Our data suggest that de novo CD80 expression is not, in general, sufficient to improve primary T cell activation by human tumour cells.

Vaccination with mutant ras peptides and induction of T-cell responsiveness in pancreatic carcinoma patients carrying the corresponding RAS mutation

Mutations in codon 12 of K-RAS are frequently found in pancreatic adenocarcinomas. T-cell responses specific for individual RAS mutations can be elicited in vitro by stimulation of peripheral blood mononuclear cells with synthetic peptides. Mutant ras peptides are therefore a candidate vaccine for specific immunotherapy in pancreatic carcinoma patients. When vaccinated with a synthetic ras peptide representing the K-RAS mutation in their tumours, a transient ras-specific T-cell response was induced in two of five patients treated. The vaccination protocol involved multiple infusions of large amounts of peptide-pulsed antigen-presenting-cells obtained by leucapheresis. These results indicate that specific T-cell responses against mutations uniquely harboured in tumour cells can be induced in cancer patients by vaccination.

Antigens recognized on human tumors by cytolytic T lymphocytes: towards vaccination?

It has been known for many years that cytolytic T lymphocytes that specifically recognize the tumor cells of the same patient can be derived from the blood of melanoma patients. Several of the antigens recognized by these antitumor T lymphocytes have now been completely identified. Some of them are sufficiently tumor-specific to envision their use as antitumor vaccines in selected cancer patients.

Induction and characterization of cytotoxic T-lymphocytes recognizing a mutated p21ras peptide presented by HLA-A*0201

The ras oncogene is frequently found to be activated in human cancer through point mutations at codons 12, 13 or 61. We explored whether these altered p21ras protein sequences contain peptide sequences that can activate naive CD8+ cytotoxic T lymphocytes (CTL). Several wild-type and mutated p21ras peptides were identified that carry a binding motif for human leukocyte antigen (HLA)-A*0201. Two peptides were found to bind strongly to this allele. CD8+ CTL bulk cultures specifically reacting with one of these peptides could be induced, using processing-defective T2 cells loaded with peptide CLLDILDTAGL as stimulators. The peptide is derived from p21ras, position 51-61, and carries a 61 Gln-->Leu mutation. In contrast, a 9-mer peptide CLLDILDTA corresponding to amino acid sequence 51-59 of wild-type p21ras did not yield reactive CTL cultures. T-cell clones with low affinity for the 11-mer peptide were isolated from CLLDILDTAGL-reactive bulk cultures. These T cells did not lyse melanoma cells transfected with 61-Leu N-ras, although lysis was found when these transfectants were pulsed with the 11-mer peptide. Possibly, T cells of higher affinity may be required to demonstrate processed peptide on the cell surface. The combined experiments suggest that a peptide derived from mutated p21ras can be recognized by HLA class I-restricted CTL, whereas an analogous wild-type p21ras peptide may not be immunogenic.

CD8+ T cells from a patient with colon carcinoma, specific for a mutant p21-Ras-derived peptide (Gly13-->Asp), are cytotoxic towards a carcinoma cell line harbouring the same mutation

Several T lymphocyte clones (TLC), specific for a p21-Ras-derived peptide expressing a Gly13-->Asp mutation and of the CD8+ subtype, were generated from peripheral blood of a colon carcinoma patient. The TLC exerted cytotoxicity against an interferon-gamma (IFN gamma)-pretreated colon carcinoma cell line, HCT116, which harbours the Gly13-->Asp mutation and shares both HLA-A2 and HLA-B12(44) with the patient. This cytotoxic effect could be blocked by a monoclonal antibody (mAb) against CD8 molecules, as well as with a mAb against HLA class I molecules and a polyclonal antiserum against HLA-B12, identifying B12(44) as the antigen-presenting molecule. In growth-inhibition experiments, the growth of both IFN gamma-pretreated and untreated target cells were strongly inhibited by the presence of the CD8+ TLC. Together these data indicate that human cancer cells harbouring a spontaneous ras mutation can process aberrant p21 Ras and express peptide/HLA-class-I complexes on their surface in sufficient density to be recognized by Ras-specific cytotoxic T lymphocytes.

A K-ras 13Gly-->Asp mutation is recognized by HLA-DQ7 restricted T cells in a patient with colorectal cancer. Modifying effect of DQ7 on established cancers harbouring this mutation?

We have characterized and described in detail 2 CD4+ T-lymphocyte clones (TLC) from a colonic cancer patient. These TLC specifically recognize a K-ras-derived peptide carrying the 13Asp mutation commonly found in adenocarcinomas of the colon. The TLC were independently derived, as they carried 2 different T-cell receptors. The TLC recognized partly overlapping epitopes within the 13Asp peptide, presented by HLA-DQ7 molecules, suggesting that this molecule might confer some protective immunity against the mutation. On the basis of analysis of 251 colonic carcinomas, the presence of HLA-DQ7 did not seem to protect against the establishment of carcinomas carrying the 13Asp mutation, since the frequency of the DQ7 haplotype was not decreased among patients having this mutation. A modifying effect of DQ7 on the development of carcinomas with a 13Asp mutation was, however, observed, resulting in fewer tumours reaching advanced Dukes stages when DQ7 was present.

Binding of ras oncogene peptides to purified HLA-DQ(alpha 1*0102,beta 1*0602) and -DR(alpha,beta 1*0101) molecules

Mutated oncogene peptides may be presented to T cells by HLA molecules. To be able to design the optimal peptides for stimulation of T cells in individuals with different HLA molecules, it is important to analyse the binding characteristics of oncogene peptides to HLA. HLA-DQ6 (DQ(alpha 1*0102,beta 1*0602)) and HLA-DR1 (DR(alpha,beta 1*0101)) molecules were purified from lysates of homozygous EBV-transformed cell lines. Purified HLA molecules were then tested for their ability to bind synthetic peptides in gel filtration assays. A p21 ras oncogene peptide (previously found to stimulate DQ6-restricted T-cell clones) and an influenza matrix peptide were labelled with 125I and served as indicator peptides for binding to DQ6 and DR1 respectively. Binding of homologous truncated and mutated p21 ras peptides and unrelated peptides was then evaluated by their capacity to inhibit binding of the indicator peptides. p21 ras-derived peptides were found to bind to both DQ6 and DR1 molecules indicating the existence of a promiscuous binding motif in these peptides. The binding affinities seemed to vary between the different peptides, but the amino acid substitutions resulting from natural mutations were not critical for binding. Notably, the results obtained for DQ6 in the biochemical peptide binding assay correlated well with results obtained in a functional assay using T-cell clones as probes.

T cell epitopes encompassing the mutational hot spot position 61 of p21 ras. Promiscuity in ras peptide binding to HLA

Activated ras carry a point mutation either in codon 12, 13 or 61 which is tumor specific. Peptides derived from this oncoprotein are therefore potential tumor antigens. Essential for the feasibility of using ras-derived peptides in therapy of cancer is whether p21 ras-derived peptides can be processed, bind to human histocompatibility leukocyte antigen (HLA) and be recognized by T cells. Here we report the fine specificity and HLA restriction of several T lymphocyte clones (TLC) specific for a peptide which is derived from the second mutational hot spot in ras encoding residue 61. These TLC were generated from memory T cells present in the blood of a cancer patient and recognized a ras-derived peptide carrying Leu instead of Gln at residue 61. By sequencing of the T cell receptor (TcR) genes three sets of "sister" TLC carrying highly different TcR were identified. Two of the TLC recognized a peptide carrying the 61 Leu mutation presented by HLA-DQ8 and one recognized the same peptide presented by HLA-DQ4. By using truncated peptides derived from residues 51 to 69 of p21 ras, partially overlapping minimal epitopes could be defined. All three TLC recognized the corresponding recombinant mutant p21 ras oncoprotein carrying Leu at residue 61 presented by autologous B-lymphoblastoid cell lines (B-LCL). This demonstrates that naturally derived ras peptides from this region of p21 ras encompass the three epitopes recognized by the TLC. These results indicate that immunogenic ras-derived peptides may be used in immunotherapy of cancer where transforming ras oncoproteins are involved.

p21-ras-peptide-specific T-cell responses in a patient with colorectal cancer. CD4+ and CD8+ T cells recognize a peptide corresponding to a common mutation (13Gly-->Asp)

Peptides derived from mutated ras are immunogenic in mice and humans, and represent a group of specific tumor antigens that are potential targets for immunotherapy. T-cell responses against mutant p21 ras can be initiated in vitro by repeated stimulation of peripheral-blood mononuclear cells with mutant ras-derived peptides. Patients with tumors commonly harbouring ras mutations may therefore show evidence of in vivo reactivity against such mutations. Peripheral-blood mononuclear cells from 10 patients with colorectal adenocarcinoma were screened for reactivity against synthetic ras-derived peptides corresponding to the most commonly found mutations in this type of cancer. In one patient, T-cell reactivity against the 1-25,13Gly-->Asp peptide was detected. From this patient, both CD4+ and CD8+ T-cell clones specific for the 1-25,13Gly-->Asp mutation could be raised. We were not, however, able to detect the corresponding mutation in the cancer. The 13Gly-->Asp mutation in the ras oncogene is frequent and constitutes 9 to 27% of all K ras mutations found in biopsies from patients with colorectal carcinomas. Our study demonstrates a mutant ras-specific T-cell response of both the CD4+ and the CD8+ phenotype in a cancer patient. We speculate that in this patient a specific T-cell response resulted in eradication of tumor cells harboring the 13Gly-->Asp mutation.

Heterogeneity of T cells specific for a particular peptide/HLA-DQ complex

Whether T cells specific for a particular peptide/HLA-DQ complex are restricted with respect to TCR usage has not been fully established. TCR usage of T cells specific for a peptide presented by a given HLA-DQ molecule has not been studied before. We therefore sequenced the TCR genes of five different TCCs derived from the same donor, which were specific for a p21 ras-derived synthetic peptide presented by the HLA-DQ(alpha 1*0102,beta*0602) (DQ6) molecule. We found that these T cells which recognized the same peptide/HLA-DQ complex used highly diverse TCRs. However, dose-response experiments using various truncations of the p21 ras-derived peptide revealed that the peptide fine specificities of the five TCCs were not completely identical. This may explain the heterogeneity in TCR usage.

Overlapping epitopes encompassing a point mutation (12 Gly-->Arg) in p21 ras can be recognized by HLA-DR, -DP and -DQ restricted T cells

Mutations in ras genes which result in transforming gene products carrying amino acid substitutions in position 12, 13 or 61 are common in human cancer. Peptides encompassing these mutations in ras are shown to be immunogenic in both mice and humans. The potential usefulness of such peptides in cancer therapy, depends on their ability to bind to HLA molecules. We therefore stimulated T cells from healthy donors with mutated ras-derived peptides. By repeated in vitro stimulation of peripheral blood mononuclear cells, several T cells clones could be generated which recognized a p21 ras derived peptide carrying a position 12 Gly-->Arg substitution. This peptide (1-25,12 Arg) could be specifically recognized by T cells restricted by either HLA-DQ7 or -DP3. Previously, we showed that this peptide is also recognized by a T cell clone restricted by HLA-DR2. The core region of the peptide was determined to span positions 9-16 for all three HLA restriction elements, and accordingly contains the mutational hot spots in position 12 and 13. The observation that the mutant 1-25,12 Arg ras-derived peptide may contain a promiscuous epitope encompassing the Gly-->Arg mutation in position 12 indicates that lack of peptide presentation by given HLA molecules may not be a major constraint in responsiveness against ras mutations.

Potential immunogenicity of oncogene and tumor suppressor gene products

The immunogenicity of viral oncoproteins has been established beyond doubt. Cytotoxic T lymphocytes directed against viral oncogene products can eradicate large established tumor masses. This stage has not yet been reached for cellular oncogene and tumor suppressor gene products, but T cells have been raised against MHC-binding peptides encoded by both mutant and wild-type alleles of the ras oncogene and the p53 tumor suppressor gene. In addition, T cells specific for joining region peptides of abnormal fusion proteins resulting from chromosome translocation in tumor cells have been generated. Some of these peptides are processed in cells infected with, for example, vaccinia-ras, but direct anti-tumor effects of peptide specific T lymphocytes remain to be demonstrated.

Tumor-rejection antigens recognized by T lymphocytes

Generations of immunologists have been searching for evidence to confirm the tantalizing notion that tumor-rejection antigens exist. If found, the ultimate reward was the possibility that these molecules might be used to induce tumor-specific immunity and effect tumor rejection. Until recently rewards have been few and far between. That is changing. The immediate rewards have become more satisfying and the ultimate reward almost palpable.