Grimm et al. BMC Cancer 2010, 10:82 http://www.biomedcentral.com/1471-2407/10/82 Page 5 of 11 Figure 1 (a) Real Time PCR: Significantly increasing gene expression of CD4, Foxp3, CD8, and CD25 in metastatic liver tissue on day 5, 10, 15 and 20 after intraportal injection of 1 × 105 CT26.WT cells (each group n = 7). Analysis was measured to control tissue (liver). (b) Immunohistochemistry: Increasing number (in %) of CD4, Foxp3, CD8, and CD25 at metastatic sites in the liver over time (day 5, 10, 15 and 20) after intraportal injection of 1 × 105 CT26.WT cells (each group n = 7). Lines with asterisks show significant increase during metastatic growth. Discussion Based on the hypothesis of the immunogenic effect of malignant transformed cells, firstly announced in 1957 by Sir Frank MacFarlane Burnet, lymphocytes have a central role in cancer immunosurveillance [20]. Nowadays this hypothesis is proven although the interaction between malignant tumors and the immune system is much more complex than Mr Burnet thought more than fifty years ago. Increasing knowledge of the interplay between malignant cells and the innate and adaptive immune sys- tem is the basis for the development of modern immu- notherapeutic strategies. These strategies, however, need to be tested preclinically in reliable animal models. In this study we describe a colorectal liver metas- tases animal model showing tumor growth in an immunocompetent mouse. In such a model tumor induced immune responses similar to the situation in human beings can be analysed at different points of time during carcinogenesis. In this regard such a model can be an important tool for further immunotherapeutic intervention studies. CT26.WT, which has been used in this study, is a common murine colon cancer cell line, which has been used in several studies to investigate the mechanisms of tumor stromal interactions and anti-tumor immune responses [21,22]. Numerous animal models have been developed to gain detailed information on initiation, promotion, and progression of colorectal liver metas- tases. The morphology, biochemical alterations, and bio- logical behaviour of the malignant disease should be,