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Purpose: Indoleamine 2,3-dioxygenase (IDO) is activated by IFN-γ and via local tryptophan depletion modulates T-cell function and promotes immune tolerance. IDO has been demonstrated to be critically involved in maternal tolerance and recent attention has turned towards its role in immune evasion of certain tumors. However, if IDO expression is involved in tumorigenesis and tumor growth control still remains controversial.

Material and Methods: Murine CT26 colon adenocarcinoma cells were transfected with the human IDO gene and stably expressing cells (CT26-IDO+ and CT26-vector control) were then injected subcutaneously into BALB/c mice. Macroscopic tumor growth was assessed after 12 days. Concentrations of kynurenine and tryptophan in culture supernatants and serum were analyzed by HPLC. Kynurenine to tryptophan ratio (kyn/trp) was calculated to estimate IDO-activity. IDO gene expression in cell lines and tumor tissue was assessed by quantitative PCR and Western blot analysis.

Results: CT26-IDO+ cells exhibit high IDO enzyme activity as determined by HPLC and expressed as the kynurenine to tryptophan ratio (μmol/mmol) after 24 hours. In vivo, IDO activity (kyn/trp) determined in serum of tumor bearing mice was significantly higher (75.25±10.21µmol/mmol) in animals with CT26-IDO+ tumors as compared to 37.29±7.85µmol/mmol in animals with IDO negative tumors. All mice developed clinically evident tumors after six days, as confirmed by histology. Tumor size of CT26-IDO+ tumors was significantly smaller than that of CT26 vector controls (136.2 ± 28.46 mm3 vs. 237.0 ± 63.02 mm3). However, only mice with CT26-IDO+ tumors developed peritoneal carcinosis, malignant aszites and distant metastases, whereas mice with CT26-IDO negative tumors showed locally restricted tumor growth.

Conclusion: This is the first preclinical model to demonstrate that IDO expression by colorectal tumor cells significantly contributes to disease progression, frequency of metastases and overall survival. Interfering with the IDO pathway by the use of IDO inhibitors might add a novel tool in the panel of cancer therapeutics and may enhance T cell-dependent antitumor immunitiy.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA