Lethal autoimmunity associated with IL-2Rbeta-deficient mice is prevented after thymic transgenic expression of wild-type IL-2Rbeta in IL-2Rbeta(-/-) mice (Tg -/- mice). Here, we show that CD4(+)CD25(+) regulatory T cells were not readily detected in IL-2Rbeta(-/-) mice, but the production of functional CD4(+)CD25(+) T cells was reconstituted in Tg -/- mice. Adoptive transfer of normal CD4(+)CD25(+) T cells into neonatal IL-2Rbeta-deficient mice prevented this lethal autoimmune syndrome. The CD4(+)CD25(+) T cells in disease-free adult IL-2Rbeta-deficient recipient mice were present at a near normal frequency, were solely donor-derived, and depended on IL-2 for expansion. These observations indicate that the essential function of the IL-2/IL-2R system primarily lies at the level of the production of CD4(+)CD25(+) regulatory T cells.

Interleukin-15 (IL-15), a potent stimulant of CD8+ T and NK cells, is a promising cancer immunotherapeutic. ALT-803 is a complex of an IL-15 superagonist mutant and a dimeric IL-15 receptor αSu/Fc fusion protein that was found to exhibit enhanced biologic activity in vivo with a substantially longer serum half-life than recombinant IL-15. A single intravenous dose of ALT-803, but not IL-15, eliminated well-established tumors and prolonged survival of mice bearing multiple myeloma. In this study, we extended these findings to demonstrate the superior antitumor activity of ALT-803 over IL-15 in mice bearing subcutaneous B16F10 melanoma tumors and CT26 colon carcinoma metastases. Tissue biodistribution studies in mice also showed much greater retention of ALT-803 in the lymphoid organs compared to IL-15, consistent with its highly potent immunostimulatory and antitumor activities in vivo. Weekly dosing with 1 mg/kg ALT-803 in C57BL/6 mice was well-tolerated, yet capable of increasing peripheral blood lymphocyte, neutrophil and monocyte counts by >8-fold. ALT-803 dose-dependent stimulation of immune cell infiltration into the lymphoid organs was also observed. Similarly, cynomolgus monkeys treated weekly with ALT-803 showed dose-dependent increases of peripheral blood lymphocyte counts, including NK, CD4+, and CD8+ memory T cell subsets. In vitro studies demonstrated ALT-803-mediated stimulation of mouse and human immune cell proliferation and IFN-γ production without inducing a broad-based release of other proinflammatory cytokines (i.e., cytokine storm). Based on these results, a weekly dosing regimen of ALT-803 has been implemented in multiple clinical studies to evaluate the dose required for effective immune cell stimulation in humans.

ALT-803, a complex of an interleukin-15 (IL-15) superagonist mutant and a dimeric IL-15 receptor α/Fc fusion protein, was found to exhibit significantly stronger in vivo biological activity on NK and T cells than IL-15. In this study, we show that a single dose of ALT-803, but not IL-15 alone, eliminated well-established 5T33P and MOPC-315P myeloma cells in the bone marrow of tumor-bearing mice. ALT-803 treatment also significantly prolonged survival of myeloma-bearing mice and provided resistance to rechallenge with the same tumor cells through a CD8+ T cell-dependent mechanism. ALT-803 treatment stimulated CD8+ T cells to secrete large amounts of interferon-γ (IFN-γ) and promoted rapid expansion of CD8+CD44high memory T cells in vivo. These memory CD8+ T cells exhibited ALT-803-mediated up-regulation of NKG2D (KLRK1) but not PD-1 (PDCD1) or CD25 (IL2RA) on their cell surfaces. ALT-803-activated CD8+ memory T cells also exhibited non-specific cytotoxicity against myeloma and other tumor cells in vitro, whereas IFN-γ had no direct effect on myeloma cell growth. ALT-803 lost its anti-myeloma activity in tumor-bearing IFN-γ knockout mice but retained the ability to promote CD8+CD44high memory T cell proliferation, indicating that ALT-803-mediated stimulation of CD8+CD44high memory T cells is IFN-γ-independent. Thus, besides well-known IL-15 biological functions in host immunity, this study demonstrates that IL-15-based ALT-803 could activate CD8+CD44high memory T cells to acquire a unique innate-like phenotype and secrete IFN-γ for non-specific tumor cell killing. This unique immune modulatory property of ALT-803 strongly supports its clinical development as a novel immunotherapeutic agent against cancer and viral infections.

IL-15, a promising cytokine for treating cancer and viral diseases, is presented in trans by the IL-15 receptor (IL-15R) alpha-chain to the IL-15Rβγc complex displayed on the surface of T cells and natural killer (NK) cells. We previously reported that an asparagine to aspartic acid substitution at amino acid 72 (N72D) of IL-15 provides a 4–5 fold increase in biological activity compared to the native molecule. In this report, we describe Chinese hamster ovary (CHO) cell expression of a soluble complex (IL-15N72D:IL-15RαSu/Fc) consisting of the IL-15 N72D superagonist and a dimeric IL-15Rα sushi domain-IgG1 Fc fusion protein. A simple but readily scalable affinity and ion exchange chromatography method was developed to highly purify the complex having both IL-15 binding sites fully occupied. The immunostimulatory effects of this complex were confirmed using cell proliferation assays. Treatment of mice with a single intravenous dose of IL-15N72D:IL-15RαSu/Fc resulted in a significant increase in CD8+ T cells and NK cells that was not observed following IL-15 treatment. Pharmacokinetic analysis indicated that the complex has a 25-hour half-life in mice which is considerably longer than <40-minute half-life of IL-15. Thus, the enhanced activity of the IL-15N72D:IL-15RαSu/Fc complex is likely the result of the increased binding activity of IL-15N72D to IL-15Rβγc, optimized cytokine trans-presentation by the IL-15RαSu domain, the dimeric nature of the cytokine domain and its increased in vivo half-life compared to IL-15. These findings indicate that this IL-15 superagonist complex could serve as a superior immunostimulatory therapeutic agent.