It has been shown that tumors have developed numerous ways to escape tumor specific immune responses. These mechanisms will ultimately not only enhance tumor growth, but also impair the effect of immune based therapies in cancer. Myeloid derived suppressor cells represent a recently identified cell population, which has been shown to impair tumor specific immune responses both in mice and human with liver cancer. Liver cancer occurs in the majority of cases in patients with an underlying chronic liver disease such as hepatitis. Chronic infections can lead to an increase in the frequency of MDSCs. We are investigating the specific role of MDSCs in the context of hepatitis induced HCC formation in mice. We have been able to demonstrate that MDSC accumulate in the livers of tumor bearing mice. Unexpectedly these cells cannot be depleted using conventional antibody treatment and upon activation cause liver damage.