This trial is to compare the efficacy and safety of modified FOLFOX6 [mFOLFOX6, a specific chemotherapy regimen of Oxaliplatin ,5-Fluorouracil and Leucovorin] chemotherapy plus Antigen Pulsed Dendritic Cells (APDC，a kind of autologous tumor lysates pulsed human dendritic cells vaccine) with modified chemotherapy alone in patients with metastatic colorectal cancer.