CD8⁺T cell-specific induction of NKG2D receptor by doxorubicin plus interleukin-12 and its contribution to CD8⁺T cell accumulation in tumors

Background: Increased infiltration of CD8⁺T cells into tumors has a positive impact on survival. Our previous study showed that doxorubicin (Dox) plus interleukin-12 (IL-12) boosted the accumulation of CD8⁺T cells in tumors and had a greater antitumor effect than did either agent alone. The purpose of this study was to determine the impact of NKG2D expression on CD8⁺T cell infiltration and antitumor efficacy.Methods: Tumor-bearing mice were administered Dox, IL-12 plasmid DNA, or both via intraperitoneal injection or intramuscular electroporation. The induction of NKG2D on CD8⁺T cells and other lymphocytes was analyzed via flow cytometry, and NKG2D-positive CD8⁺T cell-specific localization in tumors was determined by using immunofluorescence staining in various types of immune cell-depleted mice.Results: The combination of Dox plus IL-12 specifically increased expression of NKG2D in CD8⁺T cells but not in other types of immune cells, including NK cells, which naturally express NKG2D. This induced NKG2D expression in CD8⁺T cells was associated with increased accumulation of CD8⁺T cells in murine tumors. Administration of NKG2D-blocking antibody or CD8⁺T cell-depletion antibody abrogated the NKG2D⁺CD8⁺T cell detection in tumors, whereas administration of NK cell-depletion antibody had no effect. Increased NKG2D expression in CD8⁺T cells was associated with increased antitumor efficacy in vivo.Conclusion: We conclude that Dox plus IL-12 induces NKG2D in CD8⁺T cells in vivo and boosts NKG2D⁺CD8⁺T-dependent antitumor immune surveillance. This discovery reveals a novel mechanism for how chemoimmunotherapy synergistically promotes T cell-mediated antitumor immune surveillance.