Adoptive Therapy with Human CD19 (hCD19)-Targeted T Cells Further Modified to Express IL-12 Overcome the Need for Prior Lymphodepletion to Eradicate Established hCD19⁺ Tumors and Induce B Cell Aplasias In Immune Competent hCD19⁺ Transgenic Mice

Abstract 2097

Most patients with B cell malignancies, including acute and chronic B cell leukemias as well as most B cell lymphomas, are either incurable or will die from their disease. For this reason novel approaches are needed to treat these patients. To this end, we have previously generated genes encoding artificial T cell receptors, termed chimeric antigen receptors (CARs), specific to the CD19 antigen expressed on normal B cells as well as most B cell malignancies. Human T cells retrovirally modified to express hCD19 targeted CARs lyse hCD19⁺ tumor cells in vitro and eradicate established systemic hCD19⁺ tumors in SCID-Beige mice. In order to better assess the efficacy of CD19-targeted CAR modified T cells in a more clinically relevant setting, we have generated a novel syngeneic immune competent hCD19 tumor model utilizing transgenic C57BL6 mice (C57BL6 mCD19^-/- hCD19^+/−) which exclusively express hCD19 on normal B cells bearing systemic established syngeneic EL4(hCD19) thymoma tumors. Initial studies of EL4(hCD19) bearing C57BL6 mCD19^-/- hCD19^+/− mice treated with syngeneic T cells expressing the hCD19-targeted CAR, termed 19mz, failed both to eradicate tumor and to induce predicted B cell aplasias. In contrast, following initial lymphodepleting cyclophosphamide chemotherapy, mice treated with 19mz modified T cells both effectively eradicated established tumors as well as induced long-term persistent B cell aplasias. Studies on the effects of cyclophosphamide lymphodepletion in this model revealed elevations of both serum IL-12 and IFN-γ and a decrease in circulating CD4⁺ Foxp3⁺ regulatory T cells. In light of these findings we subsequently further modified 19mz⁺ T cells to express murine IL-12. Constitutive expression of IL-12 by 19mz⁺ T cells significantly enhanced both cytotoxicity and IFN-γ production as assessed by in vitro assays. Strikingly, we found that 19mz/IL-12 modified T cells no longer required prior lymphodepletion to either induce B cell aplasias or to successfully eradicate established EL4(hCD19) tumors in C57BL6 mCD19^-/- hCD19^+/− mice. Further in vivo mechanistic studies of these 19mz/IL-12 modified T cells demonstrated a requisite role of autocrine IL-12 mediated signaling of modified T cells through the IL12 receptor expressed on modified T cells, as well as a significant role for IL-12 induced IFN-γ production in mediating the observed in vivo B cell aplasias and anti-tumor effects seen in the absence of prior cyclophosphamide lymphodepletion. Significantly, all mice infused with IL-12 secreting tumor targeted T cells tolerated therapy well with no obvious side effects as assessed by direct observation. These data highlight an exciting and potentially significant advance in the emerging field of adoptive immunotherapy of cancer utilizing genetically modified autologous T cells. Further these studies provide the initial rationale for the clinical application of autologous CD19 CAR targeted IL-12 secreting T cells in the treatment of patients with advanced chemotherapy refractory B cell malignancies.