Frβ-Directed Car T Cells for Immunotherapy of Cancer

T-cells expressing chimeric antigen receptors (CARs) can elicit dramatic clinical responses in CD19+ malignancies; however, therapeutic options for acute myeloid leukemia (AML) and solid tumors remain limited. Folate receptor beta (FRβ) is a myeloid-lineage antigen expressed in 70% of AML. Here, we tested the hypothesis that FRβ could be an effective new CAR T-cell target in AML. Low affinity FRβ CAR T-cells displayed specific reactivity for FRβ+ cells in vitro and delayed human AML outgrowth in mice but were unable to completely eliminate the tumor. An optimized higher affinity CAR exhibited drastically increased anti-leukemic activity in vitro and in vivo. Further, we assessed the potential for toxicity caused by recognition of non-tumor tissues, the primary concern for new CAR target antigens. As many cell-surface markers are shared between AML blasts and healthy hematopoietic stem cells (HSCs), we measured CAR recognition of FRβ on HSCs. Neither low nor high affinity CAR T-cells showed reactivity against bone marrow HSCs. However, we noted high surface FRβ on mature monocytes and found that they were lysed when cultured with high affinity CAR T-cells. To minimize the potential for toxicity, we investigated transient CAR expression via mRNA electroporation. High affinity mRNA CAR T-cells retained effective anti-tumor activity while posing a decreased risk for long-term myeloid toxicity. In addition to AML, FRβ is also expressed on tumor associated macrophages (TAMs) in human patients and mouse models of cancer. Since TAMs help promote tumor growth and correlate with worse prognosis, we hypothesized that redirecting CAR T-cells to target FRβ+ TAMs could be an effective way to improve CAR T-cell therapy in epithelial cancers. We developed a mouse FRβ-specific CAR for use in implantable murine ovarian tumor models. Our preliminary data suggest that adoptive transfer of FRβ CAR T-cells results in destruction of FRβ+ TAMs, a mild delay in tumor growth, and systemic immune activation. However, transient toxicity and on-target depletion of macrophages in non-tumor tissues was also observed, warranting caution when translating FRβ CARs for clinical use. Our results suggest that safe application of FRβ CAR T-cells could contribute to tumor elimination in AML and solid tumors