IL15 and IL2 are similar cytokines that stimulate the proliferation and differentiation of B cells, T cells, and NK cells. Both cytokines exert their cell signaling function through binding to a trimeric complex consisting of two shared receptors, the common gamma chain and IL2Rβ, as well as an alpha chain receptor unique to each cytokine: IL2Rα or IL15Rα. IL2Rα is highly expressed on Tregs, and the therapeutic benefit of IL2 for cancer treatment has been limited for this reason. IL15 is produced by monocytes and dendritic cells and functions as a stabilized heterodimeric complex with membrane-bound IL15Rα present on the same cells. This IL15/IL15Rα complex is presented in trans to NK cells and CD8+ T cells expressing IL2Rβ and the common gamma chain. It has been shown that recombinant IL15/IL15Rα heterodimer is highly active. Currently there are no approved versions of recombinant IL15 although several clinical trials are ongoing. As potential drugs, cytokines suffer from a very fast clearance that hinders favorable dosing. Consequently, a more druggable version of IL15 would consist of the IL15/IL15Rα complex coupled with slower clearance. We created various IL15/IL15Rα heterodimeric Fc-fusions in an effort to facilitate production, promote FcRn-mediated recycling of the complex, and thus prolong half-life.

We engineered IL15/IL15Rα heterodimeric Fc-fusions by either fusing IL15 to one side of a heterodimeric Fc-region, and the sushi domain of IL15Rα to the other side, or by creating a single-chain IL15/IL15Rα that was attached to one side of a heterodimeric Fc-region. These Fc-fusions were tuned for optimal activity by engineering the linker regions between IL15/IL15Rα and the Fc and/or by engineering substitutions on IL15 at the IL15/IL2Rβ or IL15/gamma chain interface. In vitro proliferation of T and NK cells in healthy PBMCs was monitored by counting Ki67+ cells after incubation with Fc-fusions for 4 days. In vivo activity was evaluated using a mouse model in which human PBMCs are engrafted into NSG mice (huPBMC-NSG) and measuring the extent of T cell engraftment by flow cytometry as well as IFNγ. PK was evaluated in C57BL/6J mice.

IL15/IL15Rα heterodimeric Fc-fusions were successfully produced with favorable yields. The Fc-fusions enhanced proliferation of T and NK cells in vitro. Treatment of huPBMC-NSG mice with weekly doses of IL15/IL15Rα heterodimeric Fc-fusions promoted enhanced T cell engraftment and elevated levels of IFNγ in a dose dependent manner. Severe graft versus host disease was observed in treated mice. Little activity was seen with a comparable weekly dose of recombinant IL15. PK in C57BL/6J mice indicated half-lives of several days for the IL15/IL15Rα heterodimeric Fc-fusions, which are significantly longer than the <1 hr half-life reported for IL15.

Together, these data indicate that IL15/IL15Rα heterodimeric Fc-fusions demonstrate the high activity of IL15 with a more favorable PK profile.

Citation Format: Matthew J. Bernett, Christine Bonzon, Rumana Rashid, Rajat Varma, Kendra N. Avery, Irene W. Leung, Seung Y. Chu, Umesh S. Muchhal, Gregory L. Moore, John R. Desjarlais. IL15/IL15Rα heterodimeric Fc-fusions with extended half-lives [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1595. doi:10.1158/1538-7445.AM2017-1595