Fig. 1.

Mechanisms by which tumors avoid immune recognition. Mutations in the Ras signaling impair antigen presentation by decreased expression of MHC I and antigen processing molecules. Regulatory T-cells interact with tumor-associated dendritic cells and hamper their immunogenic function by inhibiting the expression of costimulatoiy ligands required for cytotoxic T-cell activity, thus, creating an immunosuppressive environment. Tumor cells also secrete chemokines such as CCL2 and create a physical barrier that prevents immune infiltration. Immunosuppressive immune cells secrete factors such as TGFβ, IL-10, and IL-35 to inhibit CTL activity.