Background: The expression of indoleamine 2,3-dioxygenase (IDO) by tumor cells has been considered as a major tumor immune escape mechanism. The aim of this study was to investigate the expression of IDO in lung cancer cell lines as well as in surgically resected lung cancer specimens comparing the latter, to the expression in autologous samples from the corresponding non malignant lung tissue. Correlations of IDO expression with clinicopathological parameters of the disease were performed.
Methods: Nine human lung cancer cell lines and 28 patients with various types of primary lung cancer were enrolled in the study. IDO expression was determined by quantitative real-time PCR using a sample of lung hamartoma as reference.
Results: IDO expression was detected in all but three patients' tumor samples, in all but four autologous non malignant lung tissues and in three out of the nine cell lines that were examined. The relative expression of IDO in lung cancer cell lines (4.7 +/- 11.1) was significantly lower than that of all patients' tumor samples (p = 0.006) as well as than that of the autologous non affected lung tissues (p = 0.027). No statistically significant differences were noted between ADC and SCC regarding either the tumor samples or the autologous non affected samples. No significant correlations between IDO expression and clinicopathological parameters were found.
Conclusion: Direct evidence is provided demonstrating that IDO mRNA can be constitutively expressed by lung cancer cells. The higher IDO expression observed in patients' samples can be attributed to the production of the enzyme by other cells recruited in the tumor microenvironment and the peri-tumoral lung area and/or to its induction by soluble factors of tumor origin.