IL15/IL15Ra complex induces an anti-tumor immune response following radiation therapy only in the absence of Tregs and fails to expand progenitor TCF1+ CD8 T cells.

In this work, we show that the combination of radiation therapy (RT) and an IL15/IL15Rα fusion complex (denoted IL15c) fails to confer anti-tumor efficacy; however, a CD8-driven anti-tumor immune response can be elicited with the concurrent administration of an aCD25 Treg-depleting antibody. Using IL15-/- and Rag1-/- knockout mouse models, we show that the response to RT + IL15c + aCD25 is dependent on both IL15 and CTLs. Furthermore, despite an equivalent survival benefit following treatment with RT + IL15c + aCD25 and combination RT and PD1-IL2v, a novel immunocytokine with PD-1 and IL2Rbg binding domains, CTL immunophenotyping and phospho-proteomic analysis of intracellular metabolites showed a significant upregulation of activation and functionality in CD8 T cells in the RT + PD1-IL2v regimen. Finally, we show that in the absence of functional IL15 signaling, the immunostimulatory response to RT + PD1-IL2v is significantly diminished with a concurrent lack of TCF+ CD8 T cell generation, suggesting a necessity of IL15 for CD8 stem cells in mediating a durable response to treatment. Together, our results are illustrative of a mechanism wherein unimpeded effector T cell activation through IL2Rb signaling and Treg inhibition are necessary in mediating an anti-tumor immune response.