Abstract An immunosuppressive tumor microenvironment is considered one of the main problems in the treatment of different malignancies with immunotherapies like adoptive T cell transfer. Metabolites like lactate - which is produced by tumor cells even in the presence of sufficient oxygen ("Warburg-effect") - contribute to this setting. Another metabolic alteration found in many malignancies is a high level of 5‘-deoxy-5‘methylthioadenosine (MTA), due to a reduction of the catabolizing enzyme methylthioadenosine phosphorylase (MTAP). In case of malignant melanoma, MTAP-deficiency has been demonstrated to result in a higher invasive potential as well as in a worse response of melanoma cells to an interferon-α therapy. Additionally, studies showed an anti-inflammatory activity on macrophages and lymphocytes, emphasizing a tumor-promoting effect of MTA. We hypothesized that MTA has a direct suppressive effect on the induction of adaptive immunity. Indeed, polyclonally stimulated CD8+ and CD4+ T cells showed a strongly diminished proliferation in the presence of MTA (50μM). MTA pre-treated T cells remain in the G0 Phase of the cell cycle. Furthermore, in a co-culture-system of antigenic peptide-pulsed DCs with autologous CD8+ T cells, addition of 25μM MTA led to an inhibition of proliferation, activation and consequently to lower antigen-specificity. A potential toxic effect of MTA on resting T cells could be excluded as apoptosis was not observed until day seven of culture. In contrast, antigen-stimulated, activated CD8+ T cells that were incubated in co-culture with DCs in presence of 25μM MTA showed high apoptosis levels and decreasing cell numbers. Since MTA has been described as a protein methylation inhibitor, we conducted western blot analyses of polyclonally stimulated CD8+ T cells and found that presence of MTA lead to decreased protein methylation in CD8+ T cells. To prove the hypothesis that MTA exerts the effects on T cells by protein methylation inhibition, we compared MTA to another known inhibitor of protein methylation (adenosine-2,3-dialdehyde, AdOx) and found that AdOx as well inhibited antigen-specificity, proliferation and activation of T cells at comparable concentrations. In addition to direct effects of MTA on T cells, monocytes cultured in the presence of higher doses MTA (150μM) differentiated to DCs that showed a smaller phenotype as well as impaired expression of co-stimulatory and maturation markers when compared to DCs generated without MTA. MTA-DCs were still capable of activating T cells and inducing antigen-specificity in co-cultures, however, cytokine secretion of MTA-DCs stimulated CD8+ T cell was strongly impaired. In summary we provide in vitro evidence of a potential new immune evasion mechanism via secretion of MTA from tumor cells as well as an interesting new mechanism of non-toxic suppression of T cells via interfering with protein methylation. Citation Format: Frederik C. Henrich, Katrin Singer, Kerstin Förster-Poller, Katrin Peter, Bernadette Neueder, Dimitrios Mougiakakos, Marina Kreutz, Andreas Mackensen, Michael Aigner. Suppression of human adaptive immunity by the tumor metabolite 5′-deoxy-5′-methylthioadenosine. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1898. doi:10.1158/1538-7445.AM2013-1898