Table 3.

Immune evasion mechanisms employed by tumor cells and the tumor microenvironment

Tumor cell-mediated mechanism(s)
TGF-β production	Inhibition of effector T cell functions	123
Indoleamine 2,3-dioxygenase expression	Depletion of tryptophan pool against T cells	124,125
PD-L1 expression	Suppression of effector T cells	126,127
p53 mutation	Deregulation of cell cycles and survival	130
Serine protease inhibitor PI-9/SPI-6 expression	Inhibition of granzyme B function	131
cFLIP over-expression	Blocking of death receptor-dependent tumor cell lysis pathway	132
Antigen epitope alteration	Escape from effector T cell recognition	63,133
Antigen loss	Escape from effector T cell recognition	134,135
Downregulation of MHC class I	Escape from effector T cell recognition	136
Overexpression of anti-apoptotic molecules	Survival from CTL-mediated apoptosis	137
Immune cell-mediated mechanism(s)
Tumor-associated macrophages	Suppression of effector T cells	Reviewed in138
Regulatory T cells	Suppression of effector T cells	Reviewed in139
Myeloid-derived suppressor cells	Suppression of effector T cells	Reviewed in140