(1) Background: Although the important role of dietary energy intake in regulating both cancer progression and host immunity has been widely recognized, it remains unclear whether dietary calorie restriction (CR) has any impact on anti-tumor immune responses. (2) Methods: Using an immunogenic B16 melanoma cell expressing ovalbumin (B16-OVA), we examined the effect of the CR diet on B16-OVA tumor growth and host immune responses. To further test whether the CR diet affects the efficacy of cancer immunotherapy, we examined the effect of CR against anti-PD-1 monoclonal antibody (anti-PD-1 Ab) treatment. (3) Results: The CR diet significantly slowed down the tumor growth of B16-OVA without affecting both CD4⁺ and CD8⁺ T cell infiltration into the tumor. Although in vivo depletion of CD8⁺ T cells facilitated B16-OVA tumor growth in the control diet group, there was no significant change in the tumor growth in the CR diet group with or without CD8⁺ T cell-depletion. Anti-PD-1 Ab treatment lost its efficacy to suppress tumor growth along with the activation and metabolic shift of CD8⁺ T cells under CR condition. (4) Conclusions: Our present results suggest that a physical condition restricted in energy intake in cancer patients may impair CD8⁺ T cell immune surveillance and the efficacy of immunotherapy.

Natural killer (NK) cells are important innate immune effector cells for controlling tumor growth and metastasis. Differentiated mature NK cells preferentially reside in the peripheral tissues and express higher levels of self-major histocompatibility complex class I (MHC-I)-recognizing inhibitory receptors. MHC-I recognition by NK cells are known to be important for their development and maturation processes, however, the role of homeostatic MHC-I recognition in maintaining effector functions of mature NK cells in the peripheral tissues needs to be elucidated. In this study, we utilized a pan anti-MHC-I blocking monoclonal antibody (anti-MHC-I) to examine the role of homeostatic MHC-I recognition in the response of pulmonary mature NK cells in an experimental lung metastasis model of B16F10 melanoma. Anti-MHC-I treatment showed significant inhibition of the lung metastasis of B16F10 melanoma in NK cell- and IFN-γ-dependent mechanisms. The blockade of homeostatic MHC-I recognition increased mature lung NK cell responsiveness, such as direct cytotoxicity and IFN-γ production, rather than the number of lung NK cells. Mechanistically, the gene expression of activating receptors including DNAX accessory molecule-1 (DNAM-1) was upregulated in NK cells treated with anti-MHC-I, and further the enhanced NK cell cytotoxicity against B16F10 cells was DNAM-1-dependent. Collectively, homeostatic self-MHC-I recognition regulates anti-metastatic function of mature lung NK cells by restraining the expression of activating receptors.

Asialo-GM1 (ASGM1) has been identified as a cell surface marker of murine NK cells. Although polyclonal anti-asialo-GM1 antibodies (anti-ASGM1 pAb) have been widely used for studying natural killer (NK) cell functions in vivo, the technical challenges have existed in their specificity for NK cell depletion. Furthermore, the exact expression of ASGM1 on the NK cell lineage and other immune cells has not been characterized due to the lack of appropriate reagents. In this context, we have recently established several clones of monoclonal antibodies against ASGM1 with different specificity to other structurally related gangliosides. In this study, we characterized the rabbit anti-ASGM1 mAb clone GA134 with both higher specificity to ASGM1 and efficacy in depleting NK cells, and the in vivo GA134 treatment significantly impaired NK cell-dependent anti-tumor effector function. We further determined the expression of ASGM1 on the NK cell lineage at distinct differentiation stages using fluorescent-labeled GA134 and found that both CD122⁺ NK1.1^- NKP and immature NK1.1⁺ CD11b^lo CD27^lo NK subsets were the earliest NK cell lineages acquiring ASGM1 expression on their cell surface. In addition to NK cells, we characterized the expression of ASGM1 on a subpopulation of T cells, mostly NK1.1⁺ T cells, and basophils.