Abstract 3971: MTA-mediated inhibition of human T cells: Mechanism and MTAP overexpression as putative overcoming strategy
Authors: Strobl Carolin D. , Henrich Frederik , Singer Katrin , Peter Katrin , Kreutz Marina , Kremer Anita , Mackensen Andreas , Aigner Michael 
Published: July 2017

Abstract
Metabolic changes of malignant cells lead to the secretion of tumor metabolites which contribute to the shaping of a favorable milieu for tumor immune escape facilitating cancer development and resistance to anti-tumor immunotherapy. Recently, disrupted methylthioadenosine metabolism drew interest as further putative immunoinhibitory metabolic dysregulation. A broad spectrum of tumor entities was reported to lack methylthioadenosine phosphorylase (MTAP) expression leading to elevated levels of its substrate 5’-deoxy-5’-methylthioadenosine (MTA). MTAP deficiency is reported to correlate with worse response to adjuvant interferon-alpha therapy and a higher risk for metastatic disease in malignant melanoma providing evidence for immunobiological relevance. We therefore examined the effect of the tumor metabolite MTA on human T cell responses. We could show that MTA has a strong inhibitory impact on human T cell function regarding proliferation, viability, activation, differentiation, tumor antigen-specific expansion and cytotoxicity. We therefore aimed to overexpress MTAP in human T cells by retroviral transduction as a putative strategy to overcome MTA-mediated inhibition of T cells. In addition, we investigated the effect of MTA on T cell metabolism and maturation of human monocyte derived mDC. We successfully generated stable MTAP-overexpressing primary human T cells. First studies were performed to examine the resistance to MTA after equipping the T cells with higher levels of its metabolizing enzyme MTAP. MTAPhigh T cells revealed less sensitivity against inhibitory effects of MTA on proliferation, viability or cytotoxicity compared to mock transduced controls suggesting a promising approach to strengthen the performance of T cells facing the MTA-rich milieu of MTAP-deficient tumors. Mechanistically, we found MTA to interfere with several signaling pathways. Of most importance, MTA impaired Akt phosphorylation as well as intracellular protein methylation, both crucial processes for proper T cell function. In addition, we studied the combined effect of MTA and PRMT5 inhibition on T cells. Several metabolic studies focusing on glycolysis and fatty acid metabolism could confirm our findings that MTA keeps T cells in a rather inactive, naïve state. Finally, we found MTA to interfere with DC maturation and their potential to induce cytokine secretion of T cells. Our data emphasizes the crucial role of tumor metabolites such as MTA in the tumor milieu for tumor immune escape. Additionally, the identification of potential molecular mechanisms of MTA-induced immunosuppression offer the opportunity to pharmacologically tackle MTA-mediated immunoinhibition and will help to develop more effective immune-based therapies against MTAP-deficient tumors.
Citation Format: Carolin D. Strobl, Frederik Henrich, Katrin Singer, Katrin Peter, Marina Kreutz, Anita Kremer, Andreas Mackensen, Michael Aigner. MTA-mediated inhibition of human T cells: Mechanism and MTAP overexpression as putative overcoming strategy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3971. doi:10.1158/1538-7445.AM2017-3971