NKT cells as an ideal anti-tumor immunotherapeutic

Abstract

Human natural killer T (NKT) cells are characterized by their expression of an invariant T cell antigen receptor α chain variable region encoded by a Vα24Jα18 rearrangement. These NKT cells recognize α-galactosylceramide (α-GalCer) in conjunction with the MHC class I-like CD1d molecule and bridge the innate and acquired immune systems to mediate efficient and augmented immune responses. A prime example of one such function is adjuvant activity: NKT cells augment anti-tumor responses because they can rapidly produce large amounts of IFN-γ, which acts on NK cells to eliminate MHC negative tumors and also on CD8 cytotoxic T cells to kill MHC positive tumors. Thus, upon administration of α-GalCer-pulsed DCs, both MHC negative and positive tumor cells can be effectively eliminated, resulting in complete tumor eradication without tumor recurrence. Clinical trials have been completed in a cohort of 17 patients with advanced non-small cell lung cancers and 10 cases of head and neck tumors. Sixty percent of advanced lung cancer patients with high IFN-γ production had significantly prolonged median survival times of 29.3 months with only the primary treatment. In the case of head and neck tumors, 10 patients who completed the trial all had stable disease or partial responses 5 weeks after the combination therapy of α-GalCer-DCs and activated NKT cells. We now focus on two potential powerful treatment options for the future. One is to establish artificial adjuvant vector cells containing tumor mRNA and α-GalCer/CD1d. This stimulates host NKT cells followed by DC maturation and NK cell activation but also induces tumor-specific long-term memory CD8 killer T cell responses, suppressing tumor metastasis even 1 year after the initial single injection. The other approach is to establish induced pluripotent stem (iPS) cells that can generate unlimited numbers of NKT cells with adjuvant activity. Such iPS-derived NKT cells produce IFN-γ in vitro and in vivo upon stimulation with α-GalCer/DCs, and mediated adjuvant effects, suppressing tumor growth in vivo.

Keywords: NKT cells; adjuvant effects; artificial adjuvant vector cells; clinical trial; induced pluripotent stem cells.

Figure 1

Natural killer T cell-mediated adjuvant effects on anti-tumor protective responses and clinical trial outcomes. (A) Mechanisms of NKT cell-targeted adjuvant cell therapy: upon NKT cell activation in patients by α-GalCer/DCs, immature DC become mature, and both MHC positive and negative tumor cells will be killed by CD8 killer T cells and NK cells, respectively. (B) Clinical trials of NKT cell-targeted adjuvant cell therapy on advanced non-small lung cancer: 60% of patients (**) showed significant prolonged median survival time of 29.3 months compared with best supportive care group with a MST of 4.6 months. The response to NKT cell therapy correlated with clinical efficacy (median survival time) and IFN-γ levels; patients with high (**H) levels responded significantly better than those with low (*L) levels. (C) Clinical trials of NKT cell-targeted adjuvant cell therapy for head and neck tumors: all 10 cases treated with the combination therapy of α-GalCer/DCs and activated NKT cells showed significant clinical efficacy (SD or PR). (D) Correlation between clinical efficacy (PR in red, SD in black) of head and neck tumors and NKT cell numbers in the tumor in situ.

Figure 2

Future directions for NKT cell-mediated cancer therapy. (A) Experimental model using OVA as an artificial tumor antigen to demonstrate NKT cell-mediated adjuvant activity (OVA model): the NKT-KO mice that had received iPS-derived NKT cells were immunized with OVA-loaded TAP-deficient spleen cells permeabilized by osmotic shock. A week later, the CD8 killer T cells in these immunized mice were analyzed for IFN-γ production after restimulation with OVA antigen in vitro. A significant increase in the number of antigen-specific IFN-γ producing CD8 killer T cells was detected in mice transferred with iPS-derived NKT cells. (B) Inhibition of the growth of OVA-expressing EL4 (EG7) tumor cells by NKT cell-mediated adjuvant therapy using iPS-derived NKT cells in vivo in the OVA model. A significant suppression of tumor growth in vivo was detected. (C) Generation of allogeneic artificial adjuvant vector cells. Artificial adjuvant vector cells were loaded with α-GalCer/CD1d and transfected with tumor mRNA. (D) Detection of long-term memory antigen-specific CD8 killer T cells even 1 year after a single injection of artificial adjuvant vector cells. Antigen-specific CD8 T cell responses in mice immunized with artificial adjuvant vector cells were analyzed using tetramer staining 1 year later. OVA was used in these experiments. (E) Suppression of melanoma lung metastasis after treatment with artificial adjuvant vector cells. Mice were intravenously injected with B16 melanoma cells to induce lung metastasis and, then 3 h later, intravenously with artificial adjuvant vector cells without tumor mRNA. The formation of metastatic nodules analyzed 2 weeks after melanoma cell injection was significantly suppressed according to the mechanisms of the activation of both NKT and NK cells but not that of CD8 killer T cells induced by artificial adjuvant vector cells carrying only α-GalCer/CD1d without tumor mRNA.