Fig. 1.

Immunosuppressive strategies used by tumors to evade immune responses. Tumors employ a plethora of immunosuppressive mechanisms, which may act in concert to counteract effective immune responses. These include tumor-induced impairment of the antigen presentation machinery, activation of negative costimulatory signals in the tumor microenvironment (CTLA-4/B7, PD-1/PD-L1, Fas/ FasL), elaboration of immunosuppressive factors (IL-10, TGF-β, galectin-1, gangliosides, PGE ₂), and overexpression of indoleamine 2,3 dioxygenase (IDO). In addition, different regulatory cell populations contribute to this immunosuppressive network including CD4 ⁺ CD25 ⁺ regulatory T-cells (Tregs) and inducible T regulatory (Tr1) cells which negatively impact on the fate of effector T cells. Abbreviations: Gal-1 galectin-1, PGE ₂ prostaglandin-E₂, TGF-β transforming growth factor-β, MHC-I major histocompatibility complex, TAP transporter-associated protein