It was the early 1990s, and there was an unsolved mystery in innate immunity: what cytokine was responsible for supporting the development and survival of NK cells? We understood that the common γ-chain (γc) cytokine receptor was critical because patients with SCID due to IL-2RG mutations had markedly reduced NK cell function (1, 2). The prevailing thought in the field was that IL-2, which signals via a high-affinity heterotrimeric receptor, IL-2Rαβγ, was the most likely candidate. After all, IL-2 resulted in potent NK cell activation, proliferation, and survival in vitro. However, all of the facts did not add up; IL-2 is produced mainly by activated T cells, which are spatially and temporally disconnected from normal NK cell development and homoeostasis. Moreover, mice that had disrupted genes for IL-2 (3) or the IL-2Rα (4) did not have a defect in NK cells. What clandestine cytokine used the IL-2R to support NK cells?

The mystery was unraveled in landmark reports by two independent groups that employed a simple strategy to search for alternative IL-2R ligands: examining the composition of cell line supernatants capable of supporting IL-2R–dependent growth signals in the presence of anti-IL-2–neutralizing Abs (5, 6). Scientists at Immunex Corporation used chromatography to purify a 14-kDa IL-2–functional mimic from the simian CV-1/EBNA cell line supernatant, sequenced NH2-terminal residues, and then used degenerate oligonucleotide primers to clone a full-length cDNA. This approach ultimately revealed a novel cytokine designated IL-15 (this article is available at https://science.sciencemag.org/content/264/5161/965) (5). IL-15 used the IL-2Rβ and γc (7) but did not require the IL-2Rα, and IL-15 promoted NK cell activation and proliferation (8), matching the profile of the mystery cytokine. Concurrently, scientists at the National Institutes of Health, led by Thomas Waldmann, identified functional IL-2–mimicry activity in HTLV-1–transformed HuT-102 cell line supernatant and isolated a cytokine that was initially designated IL-T (this article is available at https://www.pnas.org/content/pnas/91/11/4935.full.pdf) (6). IL-T stimulated T cell proliferation and generated lymphokine-activated killer cells, required the IL-2Rβ and γc but not IL-2Rα, and thus also fit the cytokine puzzle. Reconciling IL-T and IL-15, mature IL-T was found to be identical to IL-15. Indeed, the gene encoding IL-T/IL-15 within HuT-102 was fused to the HTLV-1 LTR that had inserted into the 5′UTR of IL-T. This resulted in enhanced transcription, translation, and secretion of IL-T/IL-15 (9). IL-15 mRNA was found in a wide variety of tissues with an expression profile distinct from IL-2, also allowing for greater cytokine access to NK cells and nonclassical T cells. Further analysis revealed disparate IL-2 and IL-15 primary sequences, but protein modeling indicated that IL-15 folded into a 4-α helix bundle cytokine, similar to IL-2. Mystery solved, and a new cytokine supporting NK cell homeostasis was discovered!

Definitive evidence of IL-15’s nonredundant role in NK cell development and survival followed from IL-15 gene loss-of-function studies in mice (10). The importance of IL-15 was identified not only for NK cells but also several nonclassical T cell lineages and memory CD8 T cells. Gain-of-function studies using IL-15 transgenic mice complemented these findings, demonstrating an expansion of NK cells and memory-phenotype CD8 T cells but cautioning that chronic, unrestrained IL-15 can contribute to the development of T/NK leukemias (11). Thus, IL-15 is the primary cytokine responsible for IL-2/15Rβγc signals that promote and sustain NK cell development and survival.

Although the similarities between IL-2 and IL-15, including shared signaling receptor subunits, were instrumental in the discovery of IL-15, these same parallels influenced our initial understanding of IL-15R biology. After IL-15’s discovery, mice with genetic deletions of the γc (12) and IL-2/15Rβ (13) were shown to lack NK cells, solidifying the importance of IL-2/15Rβγc signaling in NK cell development. IL-2 interacts with several forms of the IL-2R with differing affinities; it has intermediate (nanomolar) affinity for the heterodimeric IL-2Rβγ and high (picomolar) affinity for the heterotrimeric IL-2αβγ, which includes a “private” α receptor subunit. However, IL-2Rα was not involved in IL-15 signaling, and a new search was on for additional IL-15R components responsible for high-affinity IL-15 binding. Almost immediately, IL-15Rα was identified and cloned based on structural homology to the IL-2Rα (14, 15). However, unlike IL-2Rα, which has low affinity for IL-2 on its own, IL-15Rα bound IL-15 with high affinity. This receptor biology leads to a situation in which limiting amounts of IL-2 are preferentially sensed by immune cells expressing IL-2Rα in the context of the IL-2Rαβγ. Indeed, ultra-low dose IL-2 therapy in humans expanded subsets of NK cells that expressed the IL-2Rαβγ (16) and was later recognized to also expand IL-2Rα+ regulatory T cells (17). In contrast, IL-15Rα on its own has high affinity for IL-15, which provides the opportunity to operate in cis when interacting with IL-15Rβγc on the same cell or in trans with IL-15Rα presenting IL-15 to IL-15Rβγc on a neighboring cell. A new mystery: which was the primary mode of IL-15 interaction with its signaling receptor components? In elegant studies using IL-15−/− and IL-15Rα−/− mice and bone marrow chimera approaches, Averil Ma’s group provided clear in vivo evidence that IL-15 trans-presentation by IL-15Rα to NK cells expressing the IL-2/15Rβγc was the major mode supporting NK cell development and homeostasis (1820). Thus, the initial similarity in biology between IL-2R and IL-15R has been revised, and we now know that these cytokines mediate fundamentally different functions in vivo, in part because of this distinct receptor biology.

Twenty-five years after the discovery of IL-15, the importance of solving these mysteries is more evident than ever with the clinical translation of IL-15 to treat human disease. Initially, the biology of IL-15 was thought to be similar to FDA-approved human rIL-2 and was therefore not pursued as a therapeutic drug. With more appreciation of the distinct biology between IL-2 and IL-15, IL-15R agonists now represent an emerging and exciting immunotherapy strategy to promote NK and CD8 T cell responses in the context of infection and cancer. In 2015, the first clinical trial demonstrated NK and CD8 T cell expansion and activation in response to human rIL-15 (21). Armed with a deeper understanding of IL-15R biology, new therapeutic agents have been developed to improve in vivo pharmacokinetics and mimic physiologic IL-15:IL-15Rα trans-presentation. These therapeutics have been shown to be safe and promote expansion and activation of NK and CD8 T cells without expanding regulatory T cells (22). Thus, in human patients, IL-15 has now been shown to possess distinct immunomodulatory properties compared with IL-2. These IL-15R agonists hold particular promise as combination agents with other forms of immunotherapy, including cellular therapies, tumor-targeting therapeutic Abs, inhibitory checkpoint blockade, and vaccines. Thus, although IL-15’s discovery solved an immunologic mystery in 1994, its recent application as a cancer immunotherapy appears promising, and only time will tell us the story of IL-15’s full impact on human health and disease.

T.A.F., through Washington University School of Medicine, has received funding to support research and clinical trials from Altor BioScience.

This work was supported by National Institutes of Health Grants R01CA205239 and P50CA171963.

Abbreviation used in this article:

γc

common γ-chain.

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