Abstract 1603: NKTR-255 engages the IL-15 pathway driving CD8 T cell survival and CD8 memory T cell proliferation
Authors: Kuo Peiwen , Maiti Mekhala , Quach Phi , Addepalli Murali , Lanka Arunasree , Mathamsetti Poornachandra , Fanton Christie , Zhang Ping , Kirk Peter , Miyazaki Takahiro , Zalevsky Jonathan 
Published: July 2017

Abstract
Background: IL-15, an immunostimulatory cytokine, plays an important role in both the innate and adaptive immune system. Notably, this pleiotropic cytokine is a key regulator of homeostasis and survival of CD8 and CD8 memory T cells. Targeting the IL-15 pathway has therefore become a promising therapeutic approach in oncology through the induction of long-term T cell activation and durable memory responses. NKTR-255 is a polymer-engineered IL-15 that as a single-agent reduces tumor burden in various tumor models. Here, we show that the anti-tumor effects of NKTR-255 can be attributed to its modulation of survival and proliferation of CD8 and memory CD8 T cells.
Methods: Immunophenotyping of CD8 T cell subpopulations was performed in naïve and tumor-bearing Balb/c mice treated with NKTR-255. Cell surface staining of CD3, CD8, CD44, CD62L and Sca-1 was conducted to identify effector (Tem), central (Tcm) and stem (Tscm) memory T cells. Intracellular staining of Ki67 and Bcl-2 were also analyzed by flow cytometry. Human whole blood and PBMCs were stimulated with IL-15 (0.0001-1000ng/ml) or NKTR-255 (0.001-10,000ng/ml). At various time points pSTAT5 response in CD3, CD4, CD8 and NK (CD56 bright and dim) cells was monitored by flow cytometry.
Results: In naive mice, single dose NKTR-255 (dose range 0.06-1mg/kg) increased the proliferation of Tem, Tcm and Tscm CD8 T cells in a dose-dependent manner. Treatment at 0.3 and 1mg/kg dose levels increased proliferation at least 4 fold across the CD8 memory populations. The abundance of CD8 and CD8 memory T cells was still apparent 6 days post-treatment unlike single dose IL-15 which was ineffective. In a CT-26 lung metastasis model, NKTR-255 reduced the number of lung nodules in a dose-dependent manner. A 0.3mg/kg dose level increased Bcl-2 MFI 1.5 fold in CD8 T cells. Furthermore, administration of NKTR-255 at 0.3, 1 or 3mg/kg significantly increased CD8 proliferation in a dose-dependent manner in blood (1.7, 4.6 and 5.3 fold) and spleen (2.5, 5.7 and 6.9 fold) compared to vehicle. The enhanced Bcl-2 expression and CD8 proliferation were accompanied by elevated CD8 T cells in blood (1.5, 2.4, and 3.4 fold increase) and spleen (1.3, 1.7 and 2.3 fold increase). Consistent with increased proliferation and increased Bcl-2 levels observed in vivo, both IL-15 and NKTR-255 showed dose-dependent induction of pSTAT5, a modulator of Bcl-2 expression, in CD4 and CD8 T cells from human whole blood and PBMCs.
Conclusions: NKTR-255 effectively engages the IL-15 pathway as evidenced by its strong induction of CD8 and memory CD8 T cell proliferation and promotion of survival. Combined with sustained activity and potency in human blood and PBMCs, our results support NKTR-255 as a novel tumor immunotherapeutic with great potential.
Citation Format: Peiwen Kuo, Mekhala Maiti, Phi Quach, Murali Addepalli, Arunasree Lanka, Poornachandra Mathamsetti, Christie Fanton, Ping Zhang, Peter Kirk, Takahiro Miyazaki, Jonathan Zalevsky. NKTR-255 engages the IL-15 pathway driving CD8 T cell survival and CD8 memory T cell proliferation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1603. doi:10.1158/1538-7445.AM2017-1603