Fig. 2.

Mechanisms of immune evasion by tumors. a In the early phase of tumor development tumors remain undetected by the immune system because of the lack of danger signals such as significant levels of apoptotic or necrotic cells or pro-inflammatory cytokines. b By secretion of soluble factors such as indoleamine 2,3-dioxygenase (IDO) by tumor cells the infiltration of lymphocytes is inhibited. c If moderate immune cell infiltration eventually occurs tumor cells downregulate the expression of components of the antigen presentation machinery including MHC class I and II which results in their impaired recognition by antigen-specific T cells. d In tumors with a larger extent of immune cell infiltration, tumor cell destruction by cytotoxic T cells is inhibited by high expression of immunosuppressive mechanisms such as IDO, PD-L1 and FoxP3+ Treg