High-Dimensional Analyses Reveal IL15 Enhances Activation of Sipuleucel-T Lymphocyte Subsets and Reverses Immunoresistance

Sipuleucel-T (sip-T) is the only FDA-approved autologous cellular immunotherapy for metastatic castration-resistant prostate cancer (mCRPC). Here, we report the first high-dimensional analyses of sip-T using mass cytometry (CyTOF) and show a lymphoid predominance, with CD3+ T cells constituting the highest proportion (median ~60%) of sip-T, followed by B-cells, and natural killer (NK) and NKT cells. We hypothesized that treatment of sip-T with homeostatic cytokines known to activate/expand effector lymphocytes could augment efficacy against prostate cancer. Of cytokines tested, IL-15 treated sip-T showed the most significant activation/proliferation of effector lymphocytes in an antigen-specific manner, as well as augmentation of tumor cytotoxicity in-vitro. Adoptive transfer of IL-15 treated sip-T into NSG mice resulted in potent prostate tumor growth inhibition compared with control sip-T. Evaluation of tumor-infiltrating lymphocytes revealed a two-to-fourteen-fold higher influx of sip-T and a significant increase in interferon (IFN)-γ producing CD8+ T and NKT cells within the tumor microenvironment (TME) in the IL-15 group. To evaluate the effect of IL-15 treated sip-T on in vivo tumor growth, LNCaP tumor cells were inoculated subcutaneously in mice. Once average tumor sizes reached 50-100 mm2, mice received four injections of IL-15 treated, or control sip-T (5x10^6/100 µl in PBS, intraperitoneally) with one-week intervals. rhIL-15 (1 µg) was also injected subcutaneously at the time of sip-T injection. All control mice were sham dosed with vehicle (PBS).