ABSTRACT The clinical benefits of cancer immunotherapies, including adoptive cell transfer (ACT) of chimeric antigen receptor (CAR) T cells, are limited when used against solid tumors. The immuno-suppressive tumor microenvironment (TME) is enriched in cellular components (regulatory T cells, myeloid derived suppressor cells, tumor-associated macrophages, etc) and acellular factors (hypoxia, deficit of nutrients, acidosis, adenosine, etc) that decrease viability and tumoricidal activities of anti-tumor native CD8+ cytotoxic T lymphocytes (CTL) and of therapeutic CAR T cells. Therapeutic neutralization of these factors and components is challenging because of their diversity and redundancy. Instead, we aim to identify and thwart the key mechanisms by which the TME-derived factors and conditions undermine viability and the anti-tumor activities of CAR T cells. We will focus on TME-triggered downregulation of type I interferon (IFN1) receptor IFNAR1, which normally supports viability and activity of native CTLs and CAR T cells. Our preliminary data show that MAPK Activated Protein Kinase 2 (MK2) and mono-ADP-ribosyl transferase PARP11 cooperate to downregulate IFNAR1 on intratumoral CAR-bearing T cells, leading to their inactivation and rapid cell death. IFNAR1 loss leads to downregulation of IFN1-inducible cholesterol 25-hydroxylase (CH25H). CH25H acts to limit the effector trogocytosis between malignant cells and specific CAR T cells. In the absence of sufficient levels of CH25H and its product 25-hydroxycholesterol (25HC), this trogocytosis undermines the activities of CAR T cells and exposes them to fratricidal killing. These and other exciting preliminary results suggest an overarching hypothesis that targeting TME-driven PARP11/MK2/CH25H-dependent mechanisms that regulate the viability and activity of CAR T cells should enhance their anti-tumor activities and increase the efficacy of CAR T ACT. To test this hypothesis, we will determine (i) the roles of TME-induced PARP11 in inactivation of CAR T cells, (ii) the importance of MK2 activity in suppression of intratumoral CAR T cells and (iii) the contribution of downregulation of CH25H in CAR T cells inactivation and decreased efficacy of CAR T ACT. Completion of these studies should gain insight into immunosuppression of intratumoral CAR T cells and help to develop novel CAR constructs and CAR T pre-treatments as well as combinatorial approaches to increase the efficacy of CAR T ACT.