Interleukin-15 and cancer: some solved and many unsolved questions

Abstract

Soluble interleukin (IL)-15 exists under two forms: as monomer (sIL-15) or as heterodimeric complex in association with sIL-15Rα (sIL-15/IL-15Rα). Both forms have been successfully tested in experimental tumor murine models and are currently undergoing investigation in phase I/II clinical trials. Despite more than 20 years research on IL-15, some controversial issues remain to be addressed. A first point concerns the detection of the sIL-15/IL-15Rα in plasma of healthy donors or patients with cancer and its biological significance. The second and third unsolved question regards the protumorigenic role of the IL-15/IL-15Rα complex in human cancer and the detrimental immunological consequences associated to prolonged exposure of natural killer (NK) cells to both forms of soluble IL-15, respectively. Data suggest that in vivo prolonged or repeated exposure to monomeric sIL-15 or the soluble complex may lead to NK hypo-responsiveness through the expansion of the CD8⁺/CD44⁺ T cell subset that would suppress NK cell functions. In vitro experiments indicate that soluble complex and monomeric IL-15 may cause NK hyporesponsiveness through a direct effect caused by their prolonged stimulation, suggesting that this mechanism could also be effective in vivo. Therefore, a better knowledge of IL-15 and a more appropriate use of both its soluble forms, in terms of concentrations and time of exposure, are essential in order to improve their therapeutic use. In cancer, the overproduction of sIL-15/IL-15Rα could represent a novel mechanism of immune escape. The soluble complex may act as a decoy cytokine unable to efficiently foster NK cells, or could induce NK hyporesponsiveness through an excessive and prolonged stimulation depending on the type of IL-15Rα isoforms associated. All these unsolved questions are not merely limited to the knowledge of IL-15 pathophysiology, but are crucial also for the therapeutic use of this cytokine. Therefore, in this review, we will discuss key unanswered issues on the heterogeneity and biological significance of IL-15 isoforms, analyzing both their cancer-related biological functions and their therapeutic implications.

Keywords: cytokines; immunologic; killer cells; natural; receptors; tumor escape; tumor microenvironment.

Figure 1

Different membrane-bound IL-15 and sIL-15/IL-15Rα complexes defined by the IL-15Rα isoform present in the complex. (A) IL-15 exists under several functional forms: a soluble monomeric form (sIL-15), the soluble complex sIL-15/IL-15Rα (a), the tp-IL-15 and the tmb-IL-15 (b). sIL-15 binds either the intermediate affinity dimeric IL-2Rβ/γ_c receptor or the high affinity trimeric IL-15Rα/IL-2Rβ/γ_c receptor, whereas the sIL-15/IL-15Rα complex binds only the IL-2Rβ/γ_c receptor (a). The tp-IL-15 is bound to membrane IL-15Rα chains and the tmb-IL-15, is anchored to the cell membrane via an IL-15Rα–independent mechanism: both signals in trans to surrounding cells expressing the dimeric IL-2Rβ/γ_c receptor (transpresentation). tp-IL-15 is either assembled within the cells before emerging to the cell surface, or sIL-15 may bind cytokine-free membrane-bound IL-15Rα to promote consequently IL-15 trans-presentation to IL-2Rβ/γ_c receptor expressing cells (b). (B) Uncleavable tp-IL-15/IL-15Rα complex bearing the IL-15Rα EX2 isoforms, competent for signaling in cis and trans (a). sIL-15/IL-15Rα complex bearing the NPR-IL-15Rα without biological activity (b). Uncleavable IL-15/IL-15Rα complex bearing IL-15Rα-IC-EX2a-AID isoforms, competent for reverse signaling (c). sIL-15/IL-15Rα complex bearing the IL-15Rα-IC3 isoform provided of high biological activity (d). IL-15, interleukin-15; NPR, natural proteolytic release.

Figure 2

Detection of sIL-15/IL-15Rα complex in human plasmas and effect of long-lasting stimulation of NK cells with the hyperagonist RLI. (A) Detection of sIL-15/IL-15Rα complex in the plasma of HD and metastatic melanoma patients. Dot-Plot analysis of the levels of sIL-15/IL-15Rα complex in the plasma from 35 HD and from 35 metastatic melanoma patients before and after anti-BRAF treatment. Quantification was performed by ELISA assay (for additional information see online supplemental material). (B) Analysis of the long-term stimulation on the cytotoxic potential of activated NK (aNK) cells with the Hyperagonist RLI. Percentage of K-562 cell lysis resulted by ANK cell cytotoxicity assays. Allogeneic ANK cells were used as effector cells against cell Tracker green labeled K-562 cells used as targets at different E:T ratios. Freshly NK cells were amplified over 12 days with rhIL-2 at 600 UI/mL. Thereafter, NK cells were either continuously fed with rhIL-2 (Ctrl) or starved for 36 hours and then restimulated over additional 8 days with rhIL-2 at 600 UI/mL (starved-Ctrl) or with Hyperagonist IL-15clpx at 1 ng/mL. Data were expressed as mean±SD (n=3). HD, healthy donors; NK, natural killer; rhIL-2, human recombinant interleukin; sIL, soluble IL.