Purpose: Memory CD8+ T cells characterized the long survival after the strong activation followed by the appropriate antigenic stimulation have been believed to be an effective tumoricidal effectors in patients with cancer because they are possible to attack tumor cells repeatedly for a long time. Despite its importance, analysis of differentiation and function of memory CD8+ T cells is insufficient. In this study, we compared the memory CD8+ T cells induced by two murine vaccine systems to identify mechanism(s) participating in maintenance of effector function of tumoricidal long survival memory CTLs.

Methods and Results: Full-length human NY-ESO-1 cDNA or CTL epitope peptide of NY-ESO-1 (p81-88) was immunized subcutaneously (s.c.) in BALB/c mice twice at 2 weeks interval or 3 times at weekly interval, respectively. At 44 days after the final vaccination, NY-ESO-1-expressing CT26 cells (CT26/NY-ESO-1) were inoculated s.c. of immunized mice, and subsequent tumor growth was measured. In addition, IFN-γ-expressing CD8+ T cell ratio in spleen of the immunized mice was monitored to evaluate vaccination efficacy. As a result, the inhibition of tumor growth was surprisingly shown in the NY-ESO-1 cDNA vaccination group more vigorously than the NY-ESO-1 CTL epitope peptide-treated group in comparison with untreated group. In addition, significant elevation of vaccine-specific IFN-γ-expressing CD8+ T cell ratio was observed in spleen of the NY-ESO-1 cDNA-treated mice. These results indicated that quality of memory CTLs was fully maintained in DNA vaccination group, but not peptide-treated group. To address this potent issue of superiority of DNA vaccination on the generation of long survival memory CTLs, vaccine-specific CTLs from DNA and peptide vaccination groups were purified and subjected to the total RNA micro array analysis. Memory CTLs from DNA-treated mice expressed signal transducing adaptor protein 2 (STAP2) over a 200 fold change compared with peptide-induced CTLs. Moreover, we clearly demonstrated by the study using STAP2 siRNA that STAP2 is essential for the maintenance of the tumoricidal effector function and cytokine expression of DNA-induced long survival memory CTLs.

Conclusion: In this study, our murine vaccination system using human NY-ESO-1 gene and peptide clearly indicated that DNA vaccination is effective for the generation of tumoricidal long survival memory CTLs. In addition, we demonstrated for the first time that STAP2 plays as a crucial factor in the long-term maintenance of memory CTLs by probably the lasting care of cytokine expression and cytotoxicity. Since the maintenance of the effector function of the tumor antigen-specific memory CTLs has been thought to influence directly in the outcome of caner immunotherapy, our results regarding correlation between the generation of long survival memory CTLs and STAP2 have a possibility to be useful in the future treatment of patients with cancer.

Citation Format: Daisuke Muraoka, Naohiro Seo, Tae Hayashi, Naozumi Harada, Hiroshi Shiku. Signal transducing adaptor protein 2 (STAP2) has the crucial role in maintaining the CTL function of memory T cells. [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy: A New Chapter; December 1-4, 2014; Orlando, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2015;3(10 Suppl):Abstract nr B34.