Therapeutic administration of IL-15 superagonist complex ALT-803 leads to long-term survival and durable antitumor immune response in a murine glioblastoma model

Abstract

Glioblastoma is the most aggressive primary central nervous system malignancy with a poor prognosis in patients. Despite the need for better treatments against glioblastoma, very little progress has been made in discovering new therapies that exhibit superior survival benefit than the standard of care. Immunotherapy has been shown to be a promising treatment modality that could help improve clinical outcomes of glioblastoma patients by assisting the immune system to overcome the immunosuppressive tumor environment. Interleukin-15 (IL-15), a cytokine shown to activate several effector components of the immune system, may serve as an excellent immunotherapeutic candidate for the treatment of glioblastoma. Thus, we evaluated the efficacy of an IL-15 superagonist complex (IL-15N72D:IL-15RαSu-Fc; also known as ALT-803) in a murine GL261-luc glioblastoma model. We show that ALT-803, as a single treatment as well as in combination with anti-PD-1 antibody or stereotactic radiosurgery, exhibits a robust antitumor immune response resulting in a prolonged survival including complete remission in tumor bearing mice. In addition, ALT-803 treatment results in long-term immune memory against glioblastoma tumor rechallenge. Flow cytometric analysis of tumor infiltrating immune cells shows that ALT-803 leads to increased percentage of CD8+-cell infiltration, but not the NK cells, and IFN-γ production into the tumor microenvironment. Cell depletion studies, in accordance with the flow cytometric results, show that the ALT-803 therapeutic effect is dependent on CD4+ and CD8+ cells. These results provide a rationale for evaluating the therapeutic activity of ALT-803 against glioblastoma in the clinical setting.

Keywords: ALT-803; IL-15 superagonist; anti-PD-1; glioblastoma.

Figure 1

Experimental schedule and survival outcome of ALT-803 administration. (A) Two different therapeutic schedules were tested: 2 doses (day 7 and 14) and 3 doses (day 10, 12, and 14) of ALT-803. The progression of tumor burden was assessed via IVIS imaging. In both ALT-803 treatment groups, inhibition of tumor growth was observed compared to controls with 10–20% of the ALT-803-treated animals exhibiting complete tumor eradication. (B) Kaplan–Meier survival curves of ALT-803 treated mice bearing glioblastoma tumors. ALT-803 monotherapy (2× or 3×) exhibited a statistically significant increase in the survival of mice compared to the PBS control group. Three doses of ALT-803 provided the longest median overall survival and resulted in 22% of the mice as long-term survivors. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Figure 2

(A) Administration of ALT-803 (either 3 doses or 4 doses) or anti-PD-1 antibody (3 doses) produced 20% of long-term survivor mice, and their combination increased to 40% long-term survival. Anti-tumor immune memory was maintained upon tumor rechallenge. (B) Kaplan–Meier survival curves. ALT-803 (4×)+anti-PD-1 Ab therapy provided the longest median survival time and produced the greatest percentage of long-term survivors (40%) compared to monotherapies (ALT-803-20%, anti-PD-1-20%). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Figure 3

ALT-803 has similar survival benefit with stereotactic radiosurgery (SRS) alone but the combination of the treatments does not exhibit synergism. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Figure 4

The antitumor effect of ALT-803 against glioblastoma is mediated by CD4+ and CD8+ T cells. CD4 and CD8 depletion but not NK depletion abrogates the survival benefit of ALT-803. Mice were depleted of different immune cell populations before the initiation of ALT-803 treatment and the immune cell depletion was maintained by giving additional depletion antibody injections after the end of the ALT-803 treatment. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Figure 5

ALT-803 treated mice are exhibiting increased infiltration of immune cells with an activated phenotype. (A) ALT-803 increases the infiltration of brain tumor by tumor infiltrating lymphocytes and natural killer cells, while it is preferentially increasing the percentage of CD8+ T cells in the immune infiltrate compared to CD4+ T cells. ALT-803 is increasing the ratio of CD3+/NK cells (8 vs 2, p =0.1) in the tumor microenviroment and is increasing the percentage of tumor infiltrating CD8+ T cells (50% vs 62%, p =0.56). (B) ALT-803 increases the infiltration of CD8+/IFNγ+ effector T cells (T_eff) significantly (1.8% vs 0.8%, p = 0.05) and decreases the percentage of CD4+/FoxP3+ regulatory T cells (T_reg) (7.8 vs 5.3, p = 0.4). As a result, the ratio of T_eff/T_reg is higher in the ALT-803 treated compared to the control mice (6 vs 0.1, p =0.2). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]