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111 Armored CAR T cells secreting 4–1BB ligand crosslinked to PD-1 checkpoint inhibitor for enhanced solid tumor efficacy
  1. Zachary Dunn,
  2. Yun Qu,
  3. Melanie MacMullan,
  4. Xianhui Chen,
  5. Gunce Cinay and
  6. Pin Wang
  1. University of Southern California, South Pasadena, CA, USA


Background Chimeric antigen receptor (CAR) T cell therapy has transformed the treatment of hematological malignancies but has yet to achieve similar success in solid tumors due to a lack of persistence and function in the tumor microenvironment. We previously reported the augmentation CAR T cell therapy in an engineered solid tumor model through the secretion of anti-PD-1 scFv, as shown by enhanced CAR T cell antitumor efficacy, expansion, and vitality.1 We have since matured the platform to create a superior cellular product – CAR T cells secreting single-chain trimeric 4-1BB ligand crosslinked to anti-PD-1 scFv (αPD1-41BBL). 4-1BB signaling promotes cytotoxic T lymphocytes proliferation and survival but targeting 4-1BB with agonist antibodies in the clinic has been hindered by low antitumor activity and high toxicity. CAR T cells using 4-1BB endodomain for costimulatory signals have demonstrated milder anti-tumor response and longer persistence compared to CAR T cells costimulated by CD28 endodomain. We have, for the first time, engineered CAR T cells to secrete a fusion protein containing the soluble trimeric 4-1BB ligand.

Methods We hypothesized that crosslinking the current anti-PD-1 scFv with 4-1BB ligand would provide additional benefits to CAR T cells and is potentially of translational value in the management of tumors resistant to PD-1 blockade due to lack of T cell function. Therefore, we engineered CAR T cells to secrete a novel immunomodulatory fusion protein consisting of anti-PD-1 scFv crosslinked to a single-chain format of trimeric 4-1BB ligand, in which three extracellular domain units of 41BBL are connected with polypeptide linkers. The CAR T cells were then characterized in vitro and subcutaneous tumor models.

Results In vitro and in vivo, CAR19.αPD1-41BBL T cells exhibited reduced inhibitory receptor upregulation, enhanced persistence and proliferation, and a less differentiated memory status compared to CAR T cells without additional 4-1BB:4-1BBL costimulation. Accordingly, CAR19.αPD1-41BBL T cell-treated mice displayed significantly improved tumor growth control and overall survival. Spurred on by our preclinical success targeting CD19 as a model antigen, we produced mesothelin-targeting CAR T cells and confirmed the enhanced solid tumor efficacy and persistence of αPD1-41BBL secreting CAR T cells.

Conclusions Given the significantly better therapeutic efficacy of αPD1-41BBL expressing T cells over αPD1 expressing T cells, we believe that it is of high translational value to adopt secretion of αPD1-41BBL fusion protein to improve CAR T cell solid tumor efficacy, especially given the large number of patients that are PD1/PD-L1 therapy resistant.


  1. Li S, Siriwon N, Zhang X, Yang S, Jin T, He F, et al. Enhanced cancer immunotherapy by chimeric antigen receptor–modified T cells engineered to secrete checkpoint inhibitors. Clin Cancer Res 2017;23(22):6982–92.

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