IDH1 MUTATIONS AS A IMMUNOTHERAPEUTIC TARGET FOR BRAIN TUMORS

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Abstract

BACKGROUND: Immunotherapy promises an exquisitely precise therapeutic approach, and substantial evidence suggests that activated T cells can eradicate large tumors, even within the “immunologically privileged” brain. EGFRvIII and isocitrate dehydrogenase 1 (IDH1) mutations are two of the only consistent tumor-specific mutations that have been described. IDH1 is an evolutionarily-conserved enzyme essential to cell function. Greater than 90% of all IDH1 mutations occur from a substitution of histidine for arginine at codon 132, resulting in the highly conserved and tumor-specific mutation, IDH1R132H. IDH1R132H is homogeneously expressed in all tumor cells in tumors that express this mutation, including single infiltrating tumor cells but is absent in normal cells. The high frequency, specificity, and homogeneous expression of the IDH1 mutation make it an ideal target for therapeutic intervention. METHODS: To assess IDH1R132H as an immunotherapeutic target, we have developed a series of peptides of various lengths that span the IDH1R123H mutation and that are homologous in mice and humans. The murine homologue of IDH1R123H was cloned and transfected into murine glioma lines to be used as a therapeutic target. For immunogenicity experiments mice received 3 weekly intradermal immunizations with 100 µg of peptide. One week following the third vaccination spleen cells were analyzed for interferon gamma (IFN-γ) production by ELISPOT. Adjuvants investigated included CFA/IFA/IFA, IFA x3, IFA + GM-CSF, and IFA x3 following pretreatment with Imiquimod. For efficacy experiments C57BL/6 mice received three i.d. vaccinations with PEPIDH1M in IFA every seven days or no treatment. Seven days after the third vaccination, mice received IC implantation of 5x104 IDH1R132H positive KR158B tumor cells. RESULTS: Lymphocytes from mice immunized with peptides of various lengths containing the IDH1R123H mutation produced IFN-γ by ELISPOT in response to stimulation with the mutant peptide, PEPIDH1M but not wild type. This was true for four different strains of mice and transgenic HLA-A2 mice. The 25mer PEPIDH1M induced the most robust immune response. Lymphocytes derived from PEPIDH1M immunized mice co-cultured with IDH1R132H positive glioma secrete significantly higher levels of IFN-γ than lymphocytes co-cultured with IDH1R132H negative tumor cells. Mice immunized with the 25mer PEPIDH1M mixed with GM-CSF or after treating the skin site with Imiquimod produce a greater IFN-γ response to PEPIDH1M alone. Mice immunized with 25mer PEPIDH1M and challenged 7 days later with KR158B-IDH1R132H tumors showed a statically significant increase in median survival (P = 0.0385, Wilcoxon). CONCLUSIONS: The 25mer PEPIDH1M is capable of inducing a therapeutically effective immune response against intracranial tumors. SECONDARY CATEGORY: Preclinical Experimental Therapeutics.

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