Tumor microenvironment (TM) refers to the cellular environment surrounding a tumor, which includes blood vessels, normal cells and molecules. The interaction between a tumor and its microenvironment results in different physiological processes providing both beneficial and adverse consequences for tumorigenesis (Quail & Joyce, 2013). Gene expression is altered in tumor cells to secrete molecules such as cytokines and growth factors and cellular components for instance exosomes to recruit stroma and vascular cells (Quail & Joyce, 2013; Mittal et al., 2014). Regardless of different tumor types and their tissue of origin there are some general hallmarks that govern tumorigenesis. Various immune effector cells can also be found in tumor microenvironment, yet their anti-tumor functions are downregulated due to tumor-derived signals such as cytokines (TL Whiteside, 2013). Hypoxia is defined as the reduction or lack of oxygen in organs, tissues or cells (Wu D. & Yotnda P., 2010). Hypoxia is a constantly evolving participant in overall tumor growth (Patel & Sant, 2016; Kim Y et al.,2009) and remains the predominant regulator of angiogenesis in the tumor microenvironment (Mittal et al., 2014). Tumor angiogenesis is normally induced by a tumor to generate its dedicated blood supply for oxygen and other nutrients by secretion of various growth factors including vascular endothelial growth factor (VEGF). VEGF promotes growth of blood vessels of a tumor and is regarded as constituent element of the tumor microenvironment (Mittal et al. 2014). In addition, acidic extracellular pH is a major feature of tumor tissue due to lactate secretion from anaerobic glycolysis and CO2 from the pentose phosphate pathway (Kato et al. 2013). Those factors that determine the nature of tumor are considered to develop targeted therapies. In our project, three of the most common characteristics of tumor microenvironment are chosen to implement one AND-Gate and control the expression of CAR.

The oxygen supply provided by blood vessels is designed to support the regular amount of cells in the tissue. When a tumor is present in the tissue, the oxygen level is not sufficient to supply the cells in and around the tumor with enough oxygen. This oxygen depletion occurs due to the high metabolism and proliferation rate of tumor cells. The result is hypoxia in the microenvironment of tumors. It was shown that HIF1α, a transcription factor of HRE, gets stabilized under hypoxia.

As a result of the hypoxic conditions in the tissue around the tumor, the cells switch to anaerobic metabolism, which leads to the secretion of lactic acid. The effect is a lowering of the pH value. In our project we make use of the ability of cells to sense low pH. A low pH activates the TDAG8 receptor, thus triggering a signaling cascade which ends in the activation of pCRE.

VEGF is often found in high concentrations in the microenvironment of tumors. Tumor cells secrete VEGF in order to trigger the formation of new blood vessels thereby overcoming hypoxia. High VEGF concentrations are detected by the VEGFR2-receptor leading to a signaling cascade which forms the transcription factor NFAT. pCTLA4 is then activated via NFAT.