Tumor Specific T Cells Modified to Secrete IL-12 Eradicate Systemic Tumors in the Absence of Prior Toxic Chemotherapy Conditioning Regimens

Abstract 3120

T cells may be genetically modified to target tumor-associated antigens through the retroviral transduction of genes encoding chimeric antigen receptors (CARs). We have previously generated a series of CARs specific to the CD19 antigen expressed on most B cell tumors. In xenotransplant models of disease we have shown that human T cells expressing CD19 targeted CARs successfully eradicate established B cell tumors in immune compromised SCID-Beige mice. To further our understanding of the in vivo biology of CAR modified T cells, we generated a more clinically relevant syngeneic, immune competent tumor model utilizing CD19 knock out (mCD19^−/−) human CD19 knock in (hCD19^+/−) C57BL6 transgenic mice (C57BL6(mCD19^−/− hCD19^+/−)) bearing systemic syngeneic EL4(hCD19⁺) thymoma tumors. Treatment of tumor bearing mice with syngeneic T cells modified to express the CD19 targeted 19m ζ CAR alone failed to either eradicate tumor or induce predicted B cell aplasias. However, prior lymphodepletion with cyclophosphamide followed by infusion of 19m ζ⁺ T cells successfully eradicated tumor in 83% of treated mice and induced long term B cell aplasias. Translation of this therapy to the clinic has also revealed that optimal responses require pre-conditioning regimens. Given the toxicity of pre-conditioning treatments, these therapies are largely restricted to younger, healthier patients able to tolerate such intensive regimens. To further understand the mechanisms of action of improved therapy following prior cyclophosphamide therapy, we demonstrated markedly enhanced serum levels of the IL-12 and IFN γ cytokines as well as a marked reduction of endogenous CD4⁺ regulatory T cells. We postulated that IL-12, which induces IFN γ secretion, may in part explain the enhanced anti-tumor efficacy following prior lymphodepletion, and may potentially obviate the need for toxic conditioning pre-treatments. To address this hypothesis, we modified CAR⁺ T cells to constitutively secrete IL-12. Subsequent adoptive therapy of EL4(hCD19⁺) tumor bearing C57BL6(mCD19^−/− hCD19^+/−) mice with hCD19 targeted IL-12 secreting T cells successfully eradicated disease in 75% of treated mice and induced predicted B cell aplasias in the absence of prior lymphodepletion. Significantly, we found that this enhanced hCD19 targeted T cell activity required the infusion of both CD4⁺ and CD8⁺ gene modified T cells, and was further dependent upon autocrine IL-12 stimulation of the modified tumor targeted T cells as well as modified T cell IFN γ secretion and stimulation. To our knowledge, this is the first time adoptive T cell transfer has been demonstrated to successfully eradicate tumor in the absence of prior conditioning regimens. Therefore, these data support the rapid translation of this adoptive T cell therapy to the clinic, as it may enhance the anti-tumor efficacy of this therapy and further expand the patient population eligible for adoptive T cells therapy.