IL15, but Not IL2, Supports Long-Term Survival and Function of Human and Macaque Antigen-Specific CD8⁺ T Cell Clones.

The adoptive transfer of antigen-specific CD8⁺ cytotoxic T lymphocyte (CTL) clones that have been isolated and expanded in vitro is a promising treatment modality for both human malignancies and infections. However, establishing immunity of sufficient magnitude and persistence for sustained efficacy is a limitation of this approach. Recent studies have identified a critical role for cytokine signaling including that mediated by IL15 in the establishment and maintenance of CD8⁺ T cell memory, suggesting that protocols for generating and transferring antigen-specific T cells might be improved. Interleukin-2 (IL2) is the T cell growth factor that has been widely used in vitro and in vivo for promoting T cell proliferation and persistence, but prolonged exposure of T cells to IL2 can enhance susceptibility to cell death and limit CD8⁺ memory T cell survival. IL15 is a novel cytokine that shares some activities with IL2 such as the induction of T cell proliferation, but exerts contrasting effects on the homeostasis of CD8⁺ T cell memory in experimental models. Here, we study the utility of IL15 to enhance the long-term survival and function of human and macaque antigen-specific CD8⁺ CTL clones in vitro. Human and macaque CD8⁺ CTL clones reactive against CMV were isolated by limiting dilution, expanded over 14 days in the presence of IL2 or IL15 (1–10 ng/ml), and then rested for >4 weeks in media alone and with IL2 or IL15 at 0.01–10 ng/ml. Surviving T cells were enumerated at intervals, monitored for cell surface phenotype, and assayed for cytotoxicity by chromium release assay. CTL expanded in IL2 or IL15 proliferated equivalently over 14 days with a median of 1100 and 1400 fold increase in number, displayed surface markers consistent with an effector memory phenotype (CD45RA⁻CD62L⁻CCR7⁻CD28⁻), and showed comparable cytotoxicity (n=4). However, exposure after 14 days to IL15 at doses as little as 0.05-0.1 ng/ml greatly enhanced the survival of the CD8⁺ CTL as determined by Annexin V staining. By contrast, cells cultured without cytokines or with IL2 declined >80% in number over 3 or 11 days, respectively. Of note, IL15 at higher doses (>0.5 ng/ml), but not IL2, efficiently promoted sustained cell growth illustrated by labeling cells with CFSE. Cells cultured with IL15 displayed 1.5-fold increased expression of antiapoptotic molecules such as Bcl-x_L and Bcl-2 over those plated in IL2 (n=4), indicating IL15 mediated its effects at least in part by preventing apoptosis. Of note, the cytotoxicity of CTL rested in IL2 was markedly reduced (>60%, n=3), while the presence of IL15 permitted for sustained CTL function and expansion after restimulation. The responses of human and macaque CTL clones to IL15 were equivalent suggesting in vivo studies of T cell transfer in macaques may be predictive of results in humans. We have constructed retroviral vectors encoding intracytoplasmic truncated macaque CD34 or CD19 genes that could serve as nonimmunogenic selectable marker to track macaque T cells after transfer. Macaque T cells were efficiently transduced to express CD34t and CD19t (>50%), and enriched to high purity by immunomagnetic selection. Studies to examine the safety and utility of IL15 on the survival of adoptively transferred CTL in macaques are in progress. Collectively, our data support that novel cytokines such as IL15 may prove useful to augment the long-term survival and effector function of ex vivo expanded antigen-specific CD8⁺ CTL clones after transfer.