Inducing Fat to Feed a Natural Killer of Malignancy

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In recent years there have been major advances in our knowledge of tumor pathophysiology and novel drug development. Nevertheless, cancer-related mortality remains a major challenge.¹ Cellular immunotherapy represents a promising tool against cancer, harnessing the ability of the immune system to recognize, respond to, and kill neoplastic cells. Monoclonal antibodies, checkpoint inhibitors, and, more recently, chimeric antigen receptor (CAR) T cells represent significant advances. Recently, CAR T cells were approved for the treatment of certain forms of acute lymphoblastic leukemia (ALL) and lymphoma, and this approach is now being tested in other malignances.² However, the utility of CAR T cells is limited by the lack of tumor-specific targets in a majority of cancers, loss of tumor target antigen as an immune escape mechanism, poor persistence following adoptive transfer, severe toxicity, relatively long manufacturing time, and high costs.³

Natural killer (NK) cells are innate lymphocytes that recognize targets through a wide array of germline coded receptors and have a potential to provide effective anti-tumor responses without inducing graft versus host disease (GVHD).⁴ Under physiological conditions, NK cells are stimulated by different cytokines, including interleukin-15 (IL-15), which is required for their normal development and survival.⁵ IL-15 binds the alpha chain of its receptor (IL-15Rα) on the cell surface of antigen-presenting cells (APCs), and this IL-15/IL-15Rα complex stimulates neighboring effector cells (NK cells and CD8⁺ T cells) by binding to the IL2/IL-15Rβ/γc receptor across the immunologic synapse. This type of cytokine stimulation, called trans-presentation, requires cell-cell contact and ensures context-dependent effector cell activation.⁶ Recombinant human IL-15 has been shown to activate and increase NK cell proliferation, leading to enhanced anti-tumor activity in vivo.⁷ However, IL-15 administration was associated with a short half-life and poor tolerability in a first-in-human clinical trial.⁸ Recent studies have shown IL-15/IL-15Rα complexes, such as ALT-803, to be superior to IL-15 alone, primarily owing to their extended half-life and longer persistence in tissues, resulting in enhanced anti-tumor effects.9, 10 Moreover, treatment with ALT-803 was found to be safe and induced several clinical responses in two recent clinical trials.11, 12 However, despite a longer half-life, ALT-803 needed to be given every 5–7 days and many patients developed skin rashes mediated by gamma delta T cell infiltration at the subcutaneous administration site.¹¹

The current study by Xiao et al.¹³ in this issue of Molecular Therapy demonstrates a novel approach aimed at more sustained IL-15 delivery in a preclinical mouse model. They used a dual-cassette recombinant adeno-associated virus (rAAV) vector system to selectively transduce visceral adipose tissue (VAT), and they observed minimal off-target transgene expression in the liver. After a single intraperitoneal (i.p.) injection, adipose IL-15/IL-15Rα complex gene delivery resulted in a 2-fold increase of the complex protein in the serum, inducing an increase of NK cells in the spleen, visceral VAT, tumor site, and peripheral blood, leading to anti-tumor effects without any adverse events. In the treated mice, there was a preferential decrease in immature CD11b^lowCD27^high NK cells and an increase in the more cytotoxic CD11b^highCD27^low NK cells. There was a slight decrease in T cell levels in the spleen, without affecting the CD4⁺/CD8⁺ T cell ratio in the treated animals. Moreover, this stimulation did not affect the regulatory T cell (Treg) compartment in the spleen and VAT in these animals. This novel alternative IL-15/IL-15Rα delivery approach resulted in lower peak concentrations of serum IL-15, translating into less toxicity in these animals. They then tested the anti-tumor efficacy of this approach in the Lewis lung carcinoma (LLC) and metastatic melanoma mouse models. Interestingly, in mice with LLC, IL-15/IL-15Rα complex treatment induced around 50% tumor reduction and was associated with increased NK infiltration into the tumor site, without any significant change in T cell levels. NK cells—particularly the mature subset—were also increased in the spleen, peripheral blood, and VAT. In treated animals, there was also a decrease in the CD4⁺/CD8⁺ T cell ratio in both spleen and VAT. Furthermore, mice injected with B16-F10 melanoma cells 5 weeks after treatment with the IL-15/IL-15Rα complex showed prolonged survival compared to the untreated animals. NK cells were increased similar to what was observed in the LLC model. However, in the melanoma model, CD8⁺ T cell tumor infiltration did increase in the IL-15/IL-15Rα complex-treated animals and was associated with a decrease in the CD8⁺ T cell levels in the VAT. Finally, NK cell interferon gamma (IFNγ) secretion was increased in treated mice only in the presence of the tumor, suggesting a dual component in the optimal NK activation in these mice.

This approach provides an efficient and potentially well-tolerated strategy to enhance NK expansion and NK antitumor activity in patients with advanced malignancies. However, before being translated into an early-phase clinical trial, a few key questions would need to be addressed. In a recently published study, prolonged stimulation with IL-15 caused a significant metabolic defect, leading to NK cell exhaustion and functional impairment.¹⁴ It remains to be seen whether the current approach would lead to similar NK cell exhaustion. In addition, there are also some concerns about tumor formation in the context of prolonged IL-15 stimulation, which would be critical to address before this approach is tested in patients.¹⁵ Lastly, advanced cancer patients often have low fat reserves and it is not clear at this point whether successful IL-15/15Rα gene transfer would be dependent on the VAT or whether this approach would also work using subcutaneous fat deposits instead of the VAT in patients with advanced malignancies.

Gene therapy approaches for genetic disorders have developed with the aim of permanently restoring mutant nonfunctional or dysfunctional genes. However, it is unclear at this point whether this paradigm will also hold true for gene therapy approaches for cancer. The capacity to manipulate the immune system so as to target advanced malignancies in a specific fashion is an exciting area of research, and the current work by Xiao et al.¹³ represents a potentially promising new direction in cancer immunotherapy. The study nicely blends gene and immune therapy approaches to enhance the activity of NK cells in vivo for improved tumor control. However, more studies are warranted in pre-clinical animal models before testing IL-15/15Rα gene transfer in cancer patients.