New Interleukin-15 Superagonist (IL-15 SA) Significantly Enhances Graft-Versus-Tumor Activity After Allogeneic Hematopoietic Stem Cell Transplantation

Interleukin-15 (IL-15) is a pleiotropic cytokine, which plays various roles in the innate and adaptive immune system, including the development, activation, homing and survival of immune effector cells. IL-15 has been previously shown to increase CD8+ T and NK cells number and function in normal mice and recipients of stem cell transplantation. However, obstacles remain in using IL-15 therapeutically, specifically its low potency and short in vivo half-life. To overcome this, a new IL-15 mutant (IL-15N72D, J. Immunol, 2009; 183:3598) has been developed, with increased biological activity. Co-expressing IL-15N72D, in conjunction with IL-15RαSu/Fc produced a biologically active and highly potent IL-15 superagonist complex (IL-15SA, also known as ALT-803, Cytokine, 2011; 56:804). We evaluated the effects of IL-15-SA on immune reconstitution and graft-versus-tumor (GVT) activity in recipients of allogeneic hematopoietic stem cell transplantation (HSCT). Lethally irradiated BALB/c recipients were transplanted with T-cell depleted (TCD) bone marrow (BM) cells from B6 mice. IL-15 SA was administered via IP injection in two doses on days +17 and +24 after transplant. Animals were sacrificed at day 28. Administration of IL-15 significantly increased the numbers of CD8+ T cells and NK cells. IL-15 SA also augmented interferon-γ secretion from CD8+ T cells. We observed similar activity in B6CBA→CB6F1 transplant model. Interestingly IL-15 SA upregulates NKG2D and CD107a expression on CD8+ T cells. IL-15 SA administration also specifically increased slow-proliferative CD8+ T-cell proliferation in conjunction with robust IFN-γ and TNF-α secretion in CD8+ T cells in recipients of CFSE (carboxyfluorescein succinimidyl ester) labeled-T-cell infusion, whereas there was no effect on CD4+ T-cell proliferation. We then tested the anti-tumor activity of IL-15 SA in three different tumor models; murine mastocytoma (P815), murine B cell lymphoma (A20) and murine renal cell carcinoma (Renca). We found that IL-15 SA administration enhanced GVT activity against P815 and A20 in recipients of allogeneic HSCT though this activity required a low-dose T cell infusion with HSCT. Interestingly, augmented GVT activity against to Renca after IL-15 SA administration in recipients of allogeneic HSCT did not require T cell infusion.

We conclude that IL-15 SA is a very potent cytokine complex for enhancing CD8+ and NK cell reconstitution and function after HSCT, which would be a candidate for post-transplant immunotherapy.