The role of TSPO in T-cell immune control of glioblastoma

胶质母细胞瘤（GB）是最恶性和最常见的原发性脑肿瘤类型，中位生存期不到21个月。除了分子改变之外，GB的发病机制在很大程度上依赖于免疫逃逸机制。转运蛋白18 kDa（TSPO）已被用作GB的预后生物标志物，因为与正常大脑相比，其表达上调，并且与恶性肿瘤正相关。基于初步研究结果，我们在第一个资助期假设TSPO通过保护脑肿瘤起始细胞（BTIC）免受免疫系统的排斥而促进GB进展。我们证明，TSPO是诱导BTIC接触肿瘤反应性T细胞和T细胞衍生因子。结果表明，T细胞来源的IFNγ和TNFα是BTIC中TSPO的主要诱导因子。我们还可以通过分析来自原代GB样品的单细胞RNA-Seq数据集来证实IFNγ和TNFα介导的途径是原代人GB原位中TSPO表达的主要原因。关于其在GB免疫抵抗中的作用，我们证明了TSPO的下调使GB细胞系和BTIC对T细胞介导的细胞毒性敏感，并损害BTIC的迁移能力。我们还发现，TSPO保护BTIC选择性地对抗TRAIL诱导的凋亡。与此一致，通过合成TSPO配体对TSPO活性的实验调节一致地增加了TRAIL的促凋亡作用并损害了线粒体活性。对TSPO+/-BTIC的转录组表达分析揭示了由TSPO控制的差异表达的基因，这些基因与BTIC中的抗肿瘤性、干性特征和治疗抗性间充质特征的诱导相关。在第二个资助期内，我们的第一个目标是阐明TSPO如何协调干细胞和抵抗GB中免疫细胞诱导的细胞凋亡，从而诱导形成治疗抗性GB表型的机制。在这里，我们将特别研究相对影响的干细胞和抗凋亡驱动基因的TSPO调节相比，直接影响TSPO对线粒体和外源性凋亡调控。为此，我们还将研究TSPO如何影响DNA内调控元件的可及性。第二，我们将通过在两种同基因小鼠模型中研究肿瘤排斥和免疫浸润以及对CAR T细胞实验性免疫治疗的抗性来研究TSPO在体内GB免疫抗性中的相关性。最后，我们将使用我们的体外和体内模型中产生的结果来开发翻译策略，以逆转GB中TSPO产生的免疫抗性。总的来说，我们将继续建立TSPO抑制作为一种新的手段，以提高针对GB的免疫治疗的效率。

Glioblastoma (GB) is the most malignant and common type of primary brain tumor with a median survival of less than 21 months. Apart from molecular alterations, the pathogenesis of GB is largely dependent on immune escape mechanisms. Translocator protein 18 kDa (TSPO) has been used as a prognostic biomarker for GB, due to its up-regulated expression compared to normal brain and positive correlation with malignancy. Based on preliminary findings we hypothesized in the first funding period that TSPO contributes to GB progression by protecting brain tumor initiating cells (BTICs) against rejection by the immune system. We demonstrated that TSPO is induced in BTICs upon their contact with tumor-reactive T-cells and T-cell derived factors. We could identify T-cell derived IFNγ and TNFα as the main inducers of TSPO in BTICs. We could also confirm IFNγ- and TNFα-mediated pathways as major causes of TSPO expression in primary human GB situ by analyzing single-cell RNA-Seq data set from primary GB samples. In regard to its` role in immune resistance of GB we demonstrated that downregulation of TSPO sensitized GB cell lines and BTICs against T cell-mediated cytotoxicity and impaired the migratory capacity of BTICs. We also found that TSPO protects BTICs selectively against TRAIL-induced apoptosis. In accordance, experimental regulation of TSPO activity by synthetic TSPO ligands consistently increased the pro-apoptotic effects of TRAIL and impaired mitochondrial activity. Transcriptome expression analysis on TSPO+/- BTICs revealed differentially expressed genes controlled by TSPO that were associated with apoptosis-resistance, stemness features, and induction of therapy-resistant mesenchymal characteristics in BTICs. Within the second funding period, our first aim is to unravel the mechanisms how TSPO orchestrates stemness and resistance against immune cell induced apoptosis in GB and thereby induces the formation of a treatment-resistant GB phenotype. Here, we will particularly study the relative impact of stemness- and anti-apoptotic driving genes regulated by TSPO compared to direct effects of TSPO on mitochondrial and extrinsic apoptosis regulation. To this end, we will also investigate how TSPO affects the accessibility of regulatory elements within the DNA. Second, we will study the relevance of TSPO in immune resistance of GB in vivo by investigating tumor rejection and immune infiltration as well as resistance against experimental immunotherapy with CAR T-cells in two syngeneic mouse models. Finally, we will use the results generated in our in vitro and in vivo models to develop translational strategies to revert immune resistance exerted by TSPO in GB. Overall, we will continue to establish TSPO inhibition as a new means to improve efficiency of immunotherapy against GB.