In-vitro generated memory-like CD8+ T cells elicited pronounced cytotoxic effect upon antigen specific TCR gene transduction
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Cytotoxic T cells are the important arm of immune system against malignant and virus infected cells. APC activated cytotoxic T cells survey body for threats, recognize peptide-MHC class I molecules by TCR and eliminate targets by inducing apoptosis. Following a successful clearance of danger, these cells undergo a contraction phase which eliminates 90-95% of antigen specific CD8+ T cells. At this stage, surviving cells are named as memory T cells. Maintenance of these cells are primarily depends on homeostatic cytokines, IL-7 and IL-15. Surviving memory CD8+ T cells are especially important against recurrence of malignancies and reinfections due to their quick and potent immune response. Similar memory functions and characteristic can be achieved under lymphopenic environment where abnormally low levels of lymphocytes present. This phenomenon is called homeostatic proliferation and has been shown to promote tumor clearance, reverse anergy and improve immune response against pathogens. Similar to natural generation of memory T cells, homeostatic proliferation fundamentally depends on homeostatic cytokines. Recent studies demonstrated memory-like phenotype (CD44hiCD62Lhi) can be acquired in vitro by culturing naive CD8+ T cell in the presence of anti-CD3/28 and recombinant IL-15. These in vitro generated phenotype when transferred to a mice proved to have an augmented antitumor response in vivo. In the light of these findings, we aimed to improve in vitro generation of memory-like phenotype and make it more practical and use identified improvements in tumor therapy. Our first approach is the utilization of DCs, known to be one of the most potent IL-15 secreting cells, in a co-culture setup with naive T cells. Identification of correct PRRs to trigger innate immunity and hence providing an environment suitable for transferred CD8+ T cells, should improve overall immune response and antitumor potential of CD8+ T cell. To test this idea, we made an in vitro co-culture setup. Stimulating DCs with potent IL-15 inducing ligands, c-GAMP and LPS, we achieved to improve expression of IL-15 but failed to induce memory-like phenotype in co-culture. Still we observed CD8+ T cells acquire a memory-like phenotype with an increased cytotoxic potential in the presence of recombinant IL-15. As an alternative, CD8+ T cells retrovirally transduced with an IL-15 vector to enable them to produce their own IL-15. Transduction managed to induce a memory-like progression without any additional supplementation. Similar to supplementation, IL-15 transduction improved cytotoxic potential of CD8+ T cell. Our approach can be used against any antigen with an improved immune response, eliminate the necessity of cytokine supplementation and enhance in vivo persistence of the transferred cells.