NK Receptor Signaling Lowers TCR Activation Threshold, Enhancing Selective Recognition of Cancer Cells by TAA-Specific CTLs.

Cytotoxic CD8+ T lymphocyte (CTL) recognition of non-mutated tumor-associated antigens (TAA), present on cancer cells and also in healthy tissues, is an important element of cancer immunity, but the mechanism of its selectivity for cancer cells and opportunities for its enhancement remain elusive. In this study, we found that CTL expression of the NK receptors (NKR) DNAM1 and NKG2D was associated with the effector status of CD8+ tumor-infiltrating lymphocytes and long-term survival of patients with melanoma. Using MART1 and NY-ESO-1 as model TAAs, we demonstrated that DNAM1 and NKG2D regulate T-cell receptor (TCR) functional avidity and set the threshold for TCR activation of human TAA-specific CTLs. Superior co-stimulatory effects of DNAM1 over CD28 involved enhanced TCR signaling, CTL killer function, and polyfunctionality. Double transduction of human CTLs with TAA-specific TCR and NKRs resulted in strongly enhanced antigen sensitivity, without a reduction in antigen specificity and selectivity of killer function. In addition, the elevation of NKR ligand expression on cancer cells due to chemotherapy also increased CTL recognition of cancer cells expressing low levels of TAAs. Our data help explain the ability of self-antigens to mediate tumor rejection in the absence of autoimmunity and support the development of dual-targeting adoptive T-cell therapies that use NKRs to enhance the potency and selectivity of recognition of TAA-expressing cancer cells.

NK Receptors Replace CD28 As the Dominant Source of Signal 2 for Cognate Recognition of Cancer Cells by TAA-specific Effector CD8+ T Cells.

CD28-driven "signal 2" is critical for naïve CD8+ T cell responses to dendritic cell (DC)-presented weak antigens, including non-mutated tumor-associated antigens (TAAs). However, it is unclear how DC-primed cytotoxic T lymphocytes (CTLs) respond to the same TAAs presented by cancer cells which lack CD28 ligands. Here, we show that NK receptors (NKRs) DNAM-1 and NKG2D replace CD28 during CTL re-activation by cancer cells presenting low levels of MHC I/TAA complexes, leading to enhanced proximal TCR signaling, immune synapse formation, CTL polyfunctionality, release of cytolytic granules and antigen-specific cancer cell killing. Double-transduction of T cells with recombinant TCR and NKR constructs or upregulation of NKR-ligand expression on cancer cells by chemotherapy enabled effective recognition and killing of poorly immunogenic tumor cells by CTLs. Operational synergy between TCR and NKRs in CTL recognition explains the ability of cancer-expressed self-antigens to serve as tumor rejection antigens, helping to develop more effective therapies.