IL-15 and IL-21: Better when membrane-tethered together on antitumor T cells

Summary:

Systemic administration of homeostatic γ-chain cytokines mediates antitumor responses in some patients treated with adoptive immunotherapy. Yet many patients experience toxic side effects. New work presented herein suggests these limitations can be overcome by membrane-tethering IL-15 and IL-21 to T cell products. This finding has major implications in advancing medicine.

In this issue of Clinical Cancer Research, Nguyen and colleagues report a broad, safe, and effective new way to augment adoptive cancer immunotherapy¹. This finding is significant and has immediate implications for translation into the clinic. The team reports a novel genetic approach to specifically target homeostatic γ-chain cytokines Interluekin-15 (IL-15) and IL-21 to the tumor by tethering them to infused antigen-specific T cells cells during the manufacture process. IL-15 has long been appreciated as a promising cytokine for use in adoptive T cell transfer (ACT) therapy, largely due to its potent immunostimulatory abilities and preferential expansion of stem and central memory T cells^2–5. IL-15 also potentiates the function and expansion of natural kill (NK) cells.^{6, 7} Consequently, this teams unique technology of co-tethering IL-15 and IL-21 to any desired immune cell could be easily incorpated into the generation of naturally arising TIL produces, as well as CAR engineered NK cells or macropahges for the treatment of patients. With regards to the promise of IL-21, previous work by this team (last author of this report) discovered that IL-21 treatment could dramatically enhance the antitumor efficacy of adoptively transferred CD8⁺ T cells against large-established melanoma in vivo. Conversely, IL-2 impaired the generation of potent anti-melanoma T cells for adoptive immunotherapy⁸. While both IL-15 and IL-21 have emerged as promising in the field of cancer immunotherapy⁹, they can unfortunately mediate toxicity when administered directly to patients. For example, there are side effects associated with systemic administration of IL-15, including fever, rigor, and autoimmune toxicities, in turn leading to some limitations in implementing this cytokine treatment into current clinical trials¹⁰. IL-21 has similarly been tested in patients, with overall positive safety data and doses of 30 ug/kg tolerated in most individuals. Alas, it is important to note that both cytokines IL-15 and IL-21 can induce a variety of immune cells and can have dose-limiting toxicities such as hepatotoxicity, neutropenia, and lightheadedness¹¹. Of vast clinical importance, the innovative approach proposed herein to membrane tether IL-15 and IL-21 to T cells and deliver them directly to the hostile tumor microenvironment (TME) could mitigate some of these limitations and appears safer and likely more cost effective than traditional means of systemic administration of cytokines.

As displayed visually in Figure 1, Nguyen and team in the Hinrichs laboratory at Rutgers University conducted a systematic comparison of membrane-tethered IL-15 and IL-21 in generating durable memory T cells for cellular therapy¹. The team, together with their collaborators at the National Institutes of Health, discovered that that tethering both IL-15 and IL-21 to the membrane of antitumor T cells could greatly potentiate their function and bolster their engraftment when infused into animals. Moreover, IL-15/IL-21-tethered T cells persisted better than T cells only membrane-tethered to either IL-15 or IL-21. Tethering of IL-15/IL-21 to the membrane ot T cells resulted in their increased expansion in the tumor microenvironment, thereby resulting in enhanced anti-tumor activity as compared to T cells tethered to a single cytokine. Importantly for translatability, when membrane-tethered, these cytokines could not be detected in the serum of animals to the same degree as cells engineered to secrete soluble versions of these cytokines. This finding implies that membrane-tethered IL-15/IL-21 could overcome the side effects seen in patients administered with systemic cytokines. These studies were performed in several established malignancies including animals bearing epithelial tumors and pediatric neuroblastomas, suggesting broader applicability of this innovative technology to multiple cell therapy platforms.

Figure 1:

IL-15 and IL-12-tethered T cells are more functional and less likely to differentiate into dysfunction NK-like T cells (CAR) than those tethered with only IL-15 or IL-21. (Figure adapted from Knochelmann et al., CAR T Cells in Solid Tumors: Blueprints for Building Effective Therapies, Front Immunol, 27 July 2018; © 2018 Knochelmann, Smith, Dwyer, Wyatt, Mehrotra, and Paulos; https://creativecommons.org/licenses/by/4.0/)

One question emerges from this work: Why does the combination of cytokines tethered to antitumor T cells improve adoptive immunotherapy? Nguyen and colleagues¹ used a series of elegant in vitro and in vivo experiments to uncover this answer. Perhaps most interesting, data from single cell RNA sequencing uncovered that if T cells were membrane-tethered to only IL-15, that they were more likely to differentiate into exhausted-like T cells during their ex vivo manufacturing process, as a cluster of lymphocytes with a NK-cell features emerged. Specifically, and as depicted in Figure 1, more NK-like cells expanded from membrane-tethered IL-15 expanded T cell cultures. At the gene level, these NK-like T-cells expressed CD3E, but additionally expressed some typical NK genes, such as KLRB1, KLRD1, and GNLY. This finding that combining IL-15 and IL-21 on T cells reduced this NK-like exhausted profile may imply that these two cytokines may trigger unique downstream signals that wire T cells to somehow resist exhaustion. It is important to appreciate however that T cells tethered to only membrane IL-21 were not included in this analysis, and thus could also play a role in help T cells resist undergoing exhaustive features during their manufacturing. Thus, further characterization of the biology of these T cells in vivo is warranted to fully understand the mechanism of how IL-15 and IL-21 lead to reduced tumor regression by antigen-specific T cells. Nonetheless, this work is important, especially given a recent report showing that dysfunctional T cells with NK-like profile emerge during exhaustion and can be detected in patients after CAR T-cell infusion¹².

Overall, this discovery uncovers a clever way to promote antitumor T cell activity using membrane-tethered IL-15 and IL-21. While this study focused on the actions of the membrane-tethered IL-15/IL-21 on the engineered T cells, these two cytokines are directed to the tumor via a “trojan horse T cell” that could also augment host immune cells in TME (including cytotoxic endogenous CD8 T cells, NK, helper CD4 T cells, and even B cells to the tumor), in turn contributing to durable tumor immunity. Indeed, IL-15/IL-21 membrane-tethered T cells could be a desirable tool for advancing the cancer immunotherapy field. These findings have immediate implications in the clinic, as directly administrating recombinant homeostatic cytokines, such as IL-2, IL-15, and IL-21, to patients with cancer, while promising when combined with checkpoint blockade or adoptive immunotherapy^{3, 13, 14}, can increase unwanted toxic side effects to the patient. Such side effects can compromise the quality of the patient’s daily life. The Hinrich’s team has found a potential way to overcome this hurdle and ultimately unlocking a new way to deliver these cytokines directly to the oppressive TME while reducing their systemic release to the entire body.