10046 Background: Malignant tumors may evade immunological surveillance by an active downregulation of T cell activation by PD-1 (programmed death 1)/ PDL-1 signaling. This would result in a decreased immune response, thus, promoting tumor growth. Methods: We analyzed the expression of PD-1/PDL-1 genes, cytokines, and T cell subpopulations in tumor tissues of 81 consecutive patients who underwent surgery for primary colorectal cancer according to their UICC stage. Expression of PD-1/PDL-1, T cell subpopulations, and cytokines in tumor tissues were assessed using immunostaining, immunofluorescence, and Real Time PCR. Results: PDL-1 expression on tumor cells was significantly increased, whereas PD-1 expression on tumor cells was decreased at UICC stage III/IV. However, PD-1 expression on infiltrating CD4+ T cells was detectable in patients with advanced tumors whereas PDL-1 expression on CD4+ cells was particularly found at early stages. More regulatory T cells (CD4+CD25+CTLA-4+Foxp3+) were observed in tumors of stage III/IV patients compared to early stages. In addition, a remarkably increased amount of IL-10 was observed in the tumors of UICC III/IV patients. High expression of TGF-ß receptor II in tumors correlated with a progressive course of the disease and tumor relapses, i.e. TGF-ß RII in early tumor stages was indicative for early tumor progress. Conclusions: Our findings indicate that PDL-1 plays a key role during tumor progression and that regulatory T cells are involved in the process of evasion from immune surveillance. The elevated PDL-1 expression on T cells at early tumor stages suggests that PD1/PDL-1 interaction mediates inhibitory signals between T cells enabling the tumor to progress. PDL-1 blockade may prove to be a valuable approach for cancer immunotherapy. No significant financial relationships to disclose.