Persistence, immune specificity, and functional ability of murine mutant ras epitope-specific CD4(+) and CD8(+) T lymphocytes following in vivo adoptive transfer

Facts and Hopes in Immunotherapy Strategies Targeting Antigens Derived from KRAS Mutations

In this commentary, we advance the notion that mutant KRAS (mKRAS) is an ideal tumor neoantigen that is amenable for targeting by the adaptive immune system. Recent progress highlights key advances on various fronts that validate mKRAS as a molecular target and support further pursuit as an immunological target. Because mKRAS is an intracellular membrane localized protein and not normally expressed on the cell surface, we surmise that proteasome degradation will generate short peptides that bind to HLA class I (HLA-I) molecules in the endoplasmic reticulum for transport through the Golgi for display on the cell surface. T-cell receptors (TCR)αβ and antibodies have been isolated that specifically recognize mKRAS encoded epitope(s) or haptenated-mKRAS peptides in the context of HLA-I on tumor cells. Case reports using adoptive T-cell therapy provide proof of principle that KRAS G12D can be successfully targeted by the immune system in patients with cancer. Among the challenges facing investigators is the requirement of precision medicine to identify and match patients to available mKRAS peptide/HLA therapeutics and to increase the population coverage by targeting additional mKRAS epitopes. Ultimately, we envision mKRAS-directed immunotherapy as an effective treatment option for selected patients that will complement and perhaps synergize with small-molecule mKRAS inhibitors and targeted mKRAS degraders.

Clinical evaluation of TRICOM vector therapeutic cancer vaccines

We have developed an "off-the-shelf" vector-based vaccine platform containing transgenes for carcinoma-associated antigens and multiple costimulatory molecules (designated TRICOM). Two TRICOM platforms have been evaluated both preclinically and in clinical trials. PROSTVAC consists of rV, rF-PSA-TRICOM and is being used in prostate cancer therapy trials. PANVAC consists of rV, rF-CEA-MUC1-TRICOM; the expression of the two pan-carcinoma transgenes CEA and MUC-1 renders PANVAC vaccination applicable for therapeutic applications for a range of human carcinomas. Many new paradigms have emerged as a consequence of completed and ongoing TRICOM vaccine trials, including (1) clinical evidence of patient benefit may be delayed, because multiple vaccinations may be necessary to induce a sufficient anti-tumor immune response; (2) survival, and not strict adherence to RECIST criteria or time-to-progression, may be the most appropriate trial endpoint when TRICOM vaccines are used as monotherapy; (3) certain patient populations are more likely to benefit from vaccine therapy as compared to other therapeutics; and (4) TRICOM vaccines combined with standard-of-care therapeutics, either concomitantly or sequentially, are feasible because of the limited toxicity of vaccines.

Development of anti-PAX3 immune responses; a target for cancer immunotherapy

PAX3 is overexpressed in several human cancers and is absent from normal adult human tissues. It is known to have an oncogenic function in human malignancy, and is therefore a promising target for cancer immunotherapy. We screened the murine and human PAX3 amino acid sequences for peptides that bind common MHC class I types, and identified murine GVFINGRPL and human KLTEARVQV sequences. Mice immunised with either a selected PAX3 peptide, or with a PAX3 expressing DNA vector, developed specific anti-PAX3 immune responses that inhibited tumour growth. The intensity of the immune response was significantly enhanced by pulsing of the peptide onto dendritic cells. Anti-PAX3 T cell lines were established from splenocytes of immunised mice. Intravenous administration of anti-PAX3 T cells caused regression of established tumours indicating a promising clinical application for anti-PAX3 immunotherapy. The human peptide stimulated growth of similar T cell lines from peripheral blood of three out of three normal human blood donors. These showed specific cytotoxicity against a range of human PAX3+ and HLA-A2+ cancer cell lines. Moreover, an anti-PAX3 response was detected as a component of the anti-tumour immune response in a patient treated with lysate pulsed dendritic cell vaccination. The ability to generate strong and specific anti PAX3 immune responses from the T cell repertoire in both mice and humans, provides evidence for PAX3 as a promising target for immunotherapy of cancer.

Viral Vaccines for Cancer Immunotherapy

The understanding that tumor cells can be recognized and eliminated by the immune system has led to intense interest in the development of cancer vaccines. Viruses are naturally occurring agents that cause human disease but have the potential to prevent disease when attenuated forms or subunits are used as vaccines before exposure. A large number of viruses have been engineered as attenuated vaccines for the expression of tumor antigens, immunomodulatory molecules, and as vehicles for direct destruction of tumor cells or expression of highly specific gene products. This article focuses on the major viruses that are under development as cancer vaccines, including the poxviruses, adenoviruses, adeno-associated viruses, herpesviruses, retroviruses, and lentiviruses. The biology supporting these viruses as vaccines is reviewed and clinical progress is reported.

Toward Personalized Immunotherapy for Non-Hodgkin Lymphoma

The idiotypic determinants of B-cell lymphomas, formed by cell-specific rearrangement of the immunoglobulin genes, are unique and are therefore a suitable target against which to direct immunotherapy. Recent advances in our understanding of the fundamental mechanisms behind an effective immune response, coupled with advances in genetic engineering techniques, have led to a renewed interest in immunotherapy. Early clinical studies have confirmed the immunogenicity of the idiotypic antigen in patients with lymphoma. This review discusses the different methods of idiotypic vaccination currently under investigation in the clinic, including protein, genetic, and cellular vaccines. Protein vaccines are the most clinically advanced, with phase III trials of idiotypic protein linked to GM-CSF currently underway. DNA vaccines are easier to produce but to date only appear to be weakly immunogenic in man. Dendritic cell vaccines have shown promise but their use may be limited by the complexity of this approach. This review also highlights other approaches not yet in the clinic but that have shown promise in the laboratory, such as viral vaccines and T-cell therapy.

Adoptive Transfer of Anti-idiotypic T Cells Cure Mice of Disseminated B Cell Lymphoma

There is extensive interest in idiotypic vaccination as a treatment of lymphoma. An alternative approach is the adoptive transfer of in vitro generated T cells. This strategy has been used to treat posttransplantation EBV-related diseases. The ability to generate in vitro T cells to peptides derived from immunoglobulin idiotypes raises the possibility of directly using such cells as a treatment of lymphoma. Investigating the adoptive transfer of specific T cells to idiotype derived peptides in a murine lymphoma model is therefore an important part of the clinical translation of this alternative approach. We have generated an idiotype-specific T cell line, able to recognise a defined, naturally processed idiotype-derived epitope. This line has been used to successfully treat mice with disseminated lymphoma supporting the clinical use of idiotype specific T cells.

Adoptive-cell-transfer therapy for the treatment of patients with cancer

Adoptive immunotherapy--the isolation of antigen-specific cells, their ex vivo expansion and activation, and subsequent autologous administration--is a promising approach to inducing antitumour immune responses. The molecular identification of tumour antigens and the ability to monitor the persistence and transport of transferred cells has provided new insights into the mechanisms of tumour immunotherapy. Recent studies have shown the effectiveness of cell-transfer therapies for the treatment of patients with selected metastatic cancers. These studies provide a blueprint for the wider application of adoptive-cell-transfer therapy, and emphasize the requirement for in vivo persistence of the cells for therapeutic efficacy.

Ras as a target in cancer therapy

Ras proteins are guanine nucleotide-binding proteins that are central to the control of normal and transformed cell growth and that are mutated in approximately 30% of human cancers. Binding of ligands to various growth factor receptors activates Ras and subsequently a plethora of downstream effectors including the Raf-1/mitogen-activated protein kinase pathway. For effective ras functioning and for transformation, Ras proteins must undergo post-translational modifications that facilitate their attachment to the plasma membrane. Farnesylation, catalysed by farnesyl protein transferase (FPT), is the first and the most important of these modifications; inhibition of which ablates ras activity, resulting in significant anti-proliferative effect in vitro and in human cancer xenograft models. FPT inhibitors are being assessed in a range of phase I and phase II trials, which incorporate both pharmacokinetic and dynamic end-points. In addition, ras mutations can also generate neo-epitopes for cytotoxic and helper T-cell recognition, rendering ras-mutated tumours a potential target for immunotherapy. Though their clinical evaluation is still in infancy, these two modes of ras targeting represent rational therapeutic strategies that can undergo mechanistic evaluation in the clinic.

Regression of extensive pulmonary metastases in mice by adoptive transfer of antigen-specific CD8(+) CTL reactive against tumor cells expressing a naturally occurring rejection epitope

In this study, we developed a mouse model of adoptive immunotherapy reflecting immune recognition of syngeneic tumor cells naturally expressing an endogenous rejection Ag. Specifically, in a pulmonary metastases model, we examined the potency and maintenance of an antitumor CD8(+) CTL response in vivo, as well as its effectiveness against an "extensive" tumor burden. The approach taken was to first generate tumor-specific CTL from mice challenged with the CMS4 sarcoma coadministered with anti-CTLA4 mAb, which has been shown to facilitate the induction of Ag-specific T cell responses in vivo. An H-2L(d)-restricted nonamer peptide, derived from an endogenous murine leukemia provirus was identified as a CMS4-reactive CTL epitope based upon the following: CTL cross-recognition of another syngeneic tumor cell line (CT26 colon carcinoma) previously characterized to express that gene product; sensitization of Ag-negative lymphoblasts or P815 targets with the peptide; and by cold target inhibition assays. In vivo, the adoptive transfer of CMS4-reactive CTL (> or =1 x 10(6)) resulted in nearly the complete regression of 3-day established lung metastases. Furthermore, mice that rejected CMS4 following a single adoptive transfer of CTL displayed antitumor activity to a rechallenge 45 days later, not only in the lung, but also at a s.c. distal site. Lastly, the adoptive transfer of CTL to mice harboring extensive pulmonary metastases (> 150 nodules) led to a substantial reduction in tumor burden. Overall, these data suggest that the adoptive transfer of tumor-specific CTL may have therapeutic potential for malignancies that proliferate in or metastasize to the lung.

Identification of a ras oncogene peptide that contains both CD4(+) and CD8(+) T cell epitopes in a nested configuration and elicits both T cell subset responses by peptide or DNA immunization

Mutations in ras proto-oncogenes are commonly found in a diversity of malignancies and may encode unique, non-self epitopes for T cell-mediated antitumor activity. In a BALB/c (H-2(d)) murine model, we have identified a single peptide sequence derived from the ras oncogenes that contained both CD8(+) and CD4(+) T cell epitopes in a nested configuration. This peptide reflected ras sequence 4-16, and contained the substitution of Gly to Val at position 12 ¿i.e., 4-16(Val12)¿. Mice immunized with this 13-mer peptide induced a strong antigen (Ag)-specific CD4(+) proliferative response in vitro. In contrast, mice inoculated with the wild-type ras sequence failed to generate a peptide-specific T cell response. Additionally, mice immunized with the ras 4-16(Val12) peptide concomitantly displayed an Ag-specific CD8(+) cytotoxic T lymphocyte (CTL) response, as determined by lysis of syngeneic tumor target cells incubated with the nominal 9-mer nested epitope peptide ¿i.e., 4-12(Val12)¿, as well as lysis of tumor target cells expressing the corresponding ras codon 12 mutation. Analysis of the Valpha- and Vbeta-chains of the T cell receptor (TCR) expressed by these CTL revealed usage of the Valpha1 and Vbeta9 subunits, consistent with the TCR phenotype of anti-ras Val12 CTL lines produced by in vivo immunization with the nominal peptide epitope alone. Moreover, immunization with the nested epitope peptide, as compared to immunization with either the 9-mer CTL peptide alone or an admixture of the 9-mer CTL peptide with an overlapping 13-mer CD4(+) T cell helper peptide ¿i.e., 5-17(Val12)¿ lacking the class I N-terminus anchor site, enhanced the production of the CD8(+) T cell response. Finally, immunization with plasmid DNA encoding the ras 4-16(Val12) sequence led to the induction of both Ag-specific proliferative and cytotoxic responses. Overall, these results suggested that a single peptide immunogen containing nested mutant ras-specific CD4(+) and CD8(+) T cell epitopes: (1) can be processed in vivo to induce both subset-specific T lymphocyte responses; and (2) leads to the generation of a quantitatively enhanced CD8(+) CTL response, likely due to the intimate coexistence of CD4(+) help, which may have implications in peptide- or DNA-based immunotherapies.

Enhanced growth of primary tumors in cancer-prone mice after immunization against the mutant region of an inherited oncoprotein

One major objective of tumor immunologists is to prevent cancer development in individuals at high risk. (TG.AC x C57BL/6)F1 mice serve as a model for testing the feasibility of this objective. The mice carry in the germline a mutant ras oncogene that has an arginine at codon 12 instead of glycine present in the wild-type, and after physical (wounding) or chemical promotion, these mice have a high probability for developing papillomas that progress to cancer. Furthermore, F1 mice immunized with Arg(12) mutant ras peptide in complete Freund's adjuvant (CFA) develop T cells within 10 d that proliferate in vitro on stimulation with the Arg(12) mutant ras peptide. Within 14 d, these mice have delayed-type hypersensitivity to the peptide. Immunization with CFA alone or with a different Arg(12) mutant ras peptide in CFA induced neither response. To determine the effect of immunization on development of tumors, mice immunized 3 wk earlier were painted on the back with phorbol 12-myristate 13-acetate every 3 d for 8 wk. The time of appearance and the number of papillomas were about the same in immunized and control mice, but the tumors grew faster and became much larger in the mice immunized with the Arg(12) mutant ras peptide. Thus, the immunization failed to protect against growth of papillomas. The peptide-induced CD4(+) T cells preferentially recognized the peptide but not the native mutant ras protein. On the other hand, mice immunized with Arg(12) mutant ras peptide and bearing papillomas had serum antibodies that did bind native mutant ras protein. Together, these studies indicate that active immunization of cancer-prone individuals may result in immune responses that fail to eradicate mutant oncogene-expressing tumor cells, but rather induce a remarkable enhancement of tumor growth.

Rational antigen modification as a strategy to upregulate or downregulate antigen recognition

Recent and rapid advances in our understanding of the cellular and molecular mechanisms of antigen recognition by CD8(+) and CD4(+) T lymphocytes have led to the birth of possibilities for site-directed, rational modification of cognate antigenic determinants. This immunologic concept has vast biomedical implications for regulation of host immunity against the pathogenesis of diverse disease processes. The upregulation of antigen-specific T-cell responses by 'agonistic' peptides would be most desirable in response to invasive pathogenic challenges, such as infectious and neoplastic disease, while the downregulation of antigen-specific T-cell responses by 'antagonistic' peptides would be most efficacious during inappropriate pathologic consequences, such as autoimmunity. The capacity to experimentally manipulate intrinsic properties of cognate peptide ligands to appropriately alter the nature, course and potency of cellular immune interactions has important potential in both preventive and therapeutic clinical paradigms.