Grimm et al. BMC Cancer 2010, 10:82 http://www.biomedcentral.com/1471-2407/10/82 study obviously does not reflect the human situation. The fact that most CD4+ T cells were also positive for Foxp3 shows that at different points of time during car- cinogenesis in this model, an increasing number of phe- notypical regulatory T cells (Tregs) were detectable at the tumor site demonstrating a potential protumoral effect. This is in accordance with data showing that nat- ural CD4+CD25+ Tregs may play a critical role in the progression of a number of cancers by suppressing anti tumor immune response effects [31-33]. In this regard we previously demonstrated that an increasing number of Tregs specific genes at the tumor site correlate with the UICC stage of human colon cancer supporting the protumoral effect of Tregs [10]. By contrast, however, it was shown recently that a high density of Foxp3+ Tregs in tumor tissue of patients with colon cancer was asso- ciated with an improved survival [34]. Thus, the final assessment of a potential anti- or protumoral effect of Tregs in colon cancer can still not be served. Recently a new CD8+CD25+Foxp3+ (T8reg) T cell subset was described in patients with colon cancer [35]. Although in small absolute number, T8regs were more abundant in the blood and tumor tissue of tumor patients com- pared to healthy volunteers. The study provides indirect evidence for an inhibitory effect of T8regs on the antitu- mor immune response which seems to be similar to the effects mediated by CD4+CD25+ Tregs. In our study we did not analyze the presence of T8regs which however should be of interest in further studies. CD4+ T cell response for anti-tumor immunity can be divided into different types depending upon their cyto- kine profile [36]. Th1 cells are characterized especially by the production of IFN-g, whereas Th2, Th3 and Tr1 cells secret cytokines IL-4, IL-5, IL-10 and TGF-b. The balance between Th1 and Th2/Th3/Tr1 cytokines has definite influence on the outcome of various immune responses, as Th1 preferentially induces cellular immu- nity whereas the others tend to elicit humoral immunity. Basically a Th1 cytokine profile mediates an anti-tumor immunity whereas a Th2/Th3/Tr1 cytokine profile sup- ports a pro-tumor activity. The change from a Th1 pro- file to a Th2/Th3/Tr1 profile, the so called Th1-/Th2- shift, is an important feature of an insufficient cellular immunity during carcinogenesis [37,38]. In colon cancer the expression of Th1-associated genes like IFN-g, which exerts antiproliferative, antiangiogenic, and proa- poptotic effects on a variety of tumor cells [39,40], was significantly associated with a more favourable clinical course of the disease [41]. In our tumor animal model described in this study cytokines IL-10 and TGF-b were expressed increasingly during carcinogenesis at different points of time in metastatic liver tissue following intraportal CT26.WT colon carcinoma cell injection whereas the expression of Page 9 of 11 IFN-g decreased beginning from day 10 after tumor cell injection. These results are in accordance to previous studies [4] and reflect the above mentioned Th1-/Th2 shift which could be observed in our model in the later stages of carcinogenesis (day 10 to 20). TNF-a, which is an important proinflammatory cytokine produced mainly by macrophages and dendritic cells, is known to support the development of IL-10 producing Tr1 cells [42]. Thereby the protumoral cytokine profile is aug- mented additionally by increasing levels of TNF-a, which also has been demonstrated in a murine mela- noma model [43]. Thus, in this model the pathophysio- logical change of cytokine profiles seen in human reality seems to be imitated properly. It has been shown that the interaction between FAS and its ligand (FAS-L) induced FAS-positive cell apopto- sis in colorectal tumor cell lines [44]. Different strategies of tumors to escape cancer immunosurveillance invol- ving these molecules have been studied and led to the so called “counterattack” hypothesis [45,46]. We observed that FAS gene expression was down regulated in liver metastases beginning from day 10 after tumor cell injection following an initially increased expression. FAS-L expression was increasingly detected at all points of time during carcinogenesis. In addition, FAS-L expression was clearly attributed to the tumor cell sur- face in liver metastases shown by immunohistochemis- try, while FAS expression was observed predominantly on CD8+ T cells infiltrating the perimetastatic site. Proper to this end the number of apoptotic CD8+ T cells was increased time-dependent during growth of metastases. This might contribute to the impairment of the cellular anti-tumor immune response although ele- vated gene expression and immunohistological confir- mation of an increasing number of CD8+ T cells was demonstrated during carcinogenesis in our model. This result supports the “counterattack” hypothesis morpho- logically and implies that tumor cells possibly circum- vent immunological surveillance by up-regulation of FAS-L expression and down-regulation of FAS, at least at later stages of carcinogenesis. The “counterattack” hypothesis, however, is hard to prove in vivo, since we cannot demonstrate the functional significance of these findings by mere morphological observation. Conclusion In conclusion, this study describes immunological escape mechanisms during metastatic tumor growth in a colorectal liver metastases mouse model at different points in time. We have shown that tumor growth induced an extensive cellular immune response of pre- dominantly regulatory T cells with a presumably protu- moral activity. Furthermore we could demonstrate a shift from Th1- to Th2-associated cytokine profiles