PDCD1 and neoplasm: We modeled fundamental immune-tumor interactions in T cell therapy, for instance (1) PD-1+ T cells interaction with PD-L1 on the surface of PD-L1+ MHC-I+ tumor cells; (2) PD-1− T cells/PD1+ T cells recognition of tumor cells through interactions of the TCR with MHC-I on tumor cells wherein PD-1− T cells can be converted to PD-1+ T cells through repeated stimulation of their TCR; (3) CD8+ T cells’ release of IFNγ, which then converts tumor cells to PD-L1+ MHC-I+ tumor cells; and (4) CD8+ T cells’ release of cytotoxic granules that enable localized tumor killing (Figure 2D, right panel).