CD8 T cell tolerance to tumors is multi faceted and includes both extrinsic and intrinsic factors. Extrinsically, the tumor microenvironment seems to play a large role in the functional inertness of CD8 T cells. In particular, persistent antigen exposure in the environment as well as T regulatory cells are major contributors. Intrinsically, lack of costimulation by secondary signal seems to play a role (anergy), and often, the presence of inhibitory checkpoints such as CTLA-4, PD-1, and LAG-3 can also facilitate this down regulation of CD8 T cell function. In a model for prostate cancer, we have shown that persistent antigen in the tumor microenvironment paralyzes antigen specific CD8 T cells, and cytolytic activity is only restored when the cells are removed from the site of tumor. In human prostate cancer, we have found that a major CD4 T helper subset are IL-17 producing cells, which correlate with disease state, and we have further shown that T regulatory cells are uniformly present in patients, independent of tumor burden. In the CD8 prostate infiltrating lymphocytes (PILS), we observe a refractoriness to TCR stimulation accompanied by reduced cytokine production that cannot be reversed by addition of IL-2. Thus, in trying to decipher the exact intrinsic factor that could play a direct role on the CD8 PILS, we measured inhibitory checkpoints on the cell surface. PD-1 was a major checkpoint, and blockade with anti PD-1 restored proliferation in the CD8 PILS. Other checkpoints such as CTLA-4, LAG-3 and 41BB had significantly minimal expression compared to PD-1, however, further studies on LAG-3 in tumor infiltrating lymphocytes has garnered significant interest. At the expression level, we observed variable levels of LAG-3 expression on patient CD8 tumor infiltrating lymphocytes (TILs). At a functional level, we have shown that a human anti LAG-3 antibody enhances T cell proliferation and cytokine function. In addition, an increased amount of death in CD8 T cells that are LAG-3 positive has introduced a different function for the molecule. Taken together, these studies have offered further insight on CD8 T cell tolerance to tumors.