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Abstract

Antigen presentation on major histocompatibility complex (MHC) plays an important role in anti-tumor immunity. In addition to epitope presentation on MHC class I that activates cytolytic T cells, antigen presentation on MHC class II and a resulting CD4+ T cell immune response have gained increasing relevance. Preclinical and clinical studies have shown that a successful immunotherapeutic intervention depends on the presentation of neoantigens on MHC class II. However, the exact mechanisms that drive an MHC class II-restricted immune response as well as the relevance of different antigen-presenting cell subsets are not fully understood. Brain tumors such as glioma are characterized by a low mutational load and therefore a reduced abundance of potential neoepitopes. For some glioma associated tumor-driver mutations immunogenicity has been shown; however, the number of targetable epitopes and efficacy of neoantigen specific immunotherapies remains low.

In the present thesis several mutations frequently present in low grade gliomas have been screened for immunogenicity. The point mutation CICR215W has been shown to elicit mutation specific immune responses in an MHC-humanized mouse model. This neoepitope was shown to be presented on MHC class II and to lead to a CD4+ T cell-driven immune response. Using single cell T cell receptor sequencing, CICR215W-specific TCRs could be retrieved und functionality was validated in vitro. For in vivo validation, a novel brain tumor model was established using CRISPR-Cas9 mediated genetic manipulations. Locoregionally adoptive transfer of T cell receptor-transgenic T cells led to a reduced tumor growth in CICR215W-mutant brain tumor-bearing mice. Overall, a novel shared therapeutic target in gliomas could be established, that can be targeted via adoptive transfer of T cell receptor transgenic T cells in order to elicit anti-tumor immune responses.

Furthermore, the relevance of MHC class II presentation in gliomas has been investigated. Using a novel mouse model allowing for depletion of MHC class II on tumor infiltrating macrophages, it could be shown that prevalence of MHC class II is essential for the response to immunotherapy due to necessary activation of CD4+ T cells. Using antigen-specific tumor models and single-cell transcriptomics it could be shown that the lack of activated CD4+ T cells leads to an exhausted phenotype of tumor-reactive CD8+ T cells that thereupon lose their ability to lyse tumor cells. This effect was only apparent during early stages of T cell activation. A late depletion of MHC class II did not result in any differences in CD8+ T cell activation. In summary, the present thesis describes a novel target for T cell mediated immunotherapy and demonstrates the necessity of MHC class II-restricted antigen presentation on tumor-infiltrating macrophages for successful anti-tumor activity of cytotoxic CD8+ T cells.