We determined the efficacy of in vitro expanded P14 TCR transgenic CD8 T cells to mediate tumor cell elimination and to protect against viral infection in mice. Contrary to previous studies, an adoptive transfer model without lymphodepletion, vaccination or cytokine treatment was used. Antigen-activated P14 T cells cultured in IL-2-containing medium for 7 days (P14 IL-2 ) exhibited potent effector cell functions in vitro but did not confer protection against melanoma growth or viral infection. In contrast, P14 T cells cultured in IL-15 (P14 ) were highly effective in vivo although they displayed only moderate effector functions in vitro. Therapeutic efficacy correlated with the survival of the transferred T cells in the recipients: P14 IL-2 cells disappeared rapidly whereas P14 IL-15 cells persisted for prolonged time. Decreasing the IL-2 concentration in the culture media improved in vivo survival and efficacy but also lowered the cell yield of the cultures. Finally, we could extend the findings with monoclonal P14 T cells to polyclonal CD8 T cells. Thus, in vitro expansion of antigen-specific CD8 T cells in IL-15 allowed the generation of substantial numbers of T cells without inducing terminally differentiated effector cells that turned out to be unfavorable in the transfer model examined here.Key words: Cytotoxic T cells . Rodent . Tumor immunity . Viral
IntroductionAdoptive transfer of antigen-specific CD8 T cells into hosts represents a promising approach to treat tumors and viral infections [1][2][3][4]. To generate sufficient numbers of T cells for this therapy, T cells have to be stimulated and expanded in vitro. In most protocols used today, IL-2 is added to the culture medium to ensure T-cell proliferation, differentiation and survival. Besides IL-2, IL-15 also supports CD8 T-cell growth [5]; however, the phenotype and the functional activity of IL-2-versus IL-15-cultured CD8 T cells differ considerably. Manjunath et al. [6] were the first to show that CD8 T cells from P14 TCR-tg mice specific for gp33 epitope of lymphocytic choriomeningitis virus (LCMV) acquired an effector memory phenotype with high cytolytic activity when cultured in the presence of IL-2 whereas addition of IL-15 led to a central memory phenotype with low effector cell functions. Further analysis in the same TCR transgenic system revealed that these two cytokines were equivalent mitogens for antigen-stimulated CD8 T cells but that IL-2 was more potent in inducing amino acid uptake and protein synthesis [7].For adoptive T-cell therapy, it is important to generate CD8 T cells in vitro, which are efficient in inducing tumor regression or virus clearance in vivo. The degree of specific target cell lysis and the amount of antigen-triggered IFN-g production are frequently used to predict the efficacy of CD8 T cells in vivo. However, several recent studies in the pmel-1 TCR-tg model specific for the self/tumor antigen gp100 of B16 melanoma cells indicated that antigen-stimulated CD8 T cells with a less differentiated phenotype were sup...