PROJECT SUMMARY Recent advances using CAR T cells in lymphoid malignancies have made it clear that manipulation of the host immune system can radically alter the course of a cancer. Although there have been lifesaving responses in some patients, all too many patients have inadequate responses. Some patients and tumor types have been more amenable to CAR T cell therapies than others, despite similar levels of antigen expression, uniformity, and density. The exact mechanisms by which CAR T cells induce tumor cell death are unknown, and may be due to a combination of multiple T cell effector functions that ultimately result in cell death, potentially by triggering programmed cell death in tumor cells. Our overall hypothesis is that CAR T cells mediate tumor cell death by inducing programmed cell death (PCD) pathways in target cells. A logical corollary of this hypothesis is that one potential mechanism of resistance, which has received scant attention, is genetic or functional resistance to PCD in the target tumor cells. Furthermore, we hypothesize that the state of PCD constituents in tumor cells confers sensitivity or resistance to T-cell mediated killing, and that this relative resistance can be overcome with drugs that enhance PCD signaling in tumor cells. Finally, because systemically administered drugs that enhance PCD signaling may also affect CAR T cells, we propose genetic engineering approaches to render CAR T cells resistant to candidate PCD-enhancing drugs. This is a collaborative project between an expert in CAR T cells and immunology and an expert in programmed cell pathways and tumor biology. Together we aim to define the effector functions of T cells that induce tumor cell death (Aim 1), define the programmed cell death pathways in tumors that confer sensitivity or resistance to CAR T cell mediated killing (Aim 2), and use innovative strategies to enhance CAR T cell killing of tumor cells by manipulating PCD pathways using small molecule drugs in combination with genetic engineering of the CAR T cells.