C. Immune Escape and Tumor Progression
If complete elimination of the malignant cells fails, the continued immunologic pressure on the emerging tumor will instead lead to the selection of increasingly less immunogenic tumor cells (Figure 2C). is phase of immunoediting is driven largely by T cells and occurs over several years.[72] Malignant cells with high chromosomal instability escape antigen-speci c T-cell rec- ognition by antigenic loss or upregulation of immunosuppres- sive mediators. Accumulation of de novo mutations eventually results in an invasive phenotype that favors escape from immune surveillance, leading ultimately to an overt (detectable) tumor. [73] Traits acquired by tumor cells that enable them to avoid recognition by adaptive immune cells include downregulated surface expression of MHC class I, as well as the shedding of antigens and natural killer group 2D (NKG2D) ligands.[74-76] Moreover, tumors gradually become more resistant to elimina- tion by immune cells due to upregulation of immune-inhibitory molecules, such as ligands for the T-cell–inhibitory receptor pro- grammed death 1 (PD-1). e shedding of death receptors and the overexpression of anti-apoptotic proteins increase resistance to apoptosis.[77-80] Importantly, tumor cells also manipulate the cellular composition of the tumor microenvironment by se- creting chemokines and cytokines that can recruit and polarize myeloid cells and some lymphoid cells to an immunosuppres- sive, protumorigenic phenotype. Unfortunately, the role of exer- cise in modulating these processes remains unexplored.