Enhancing CAR T Cell Efficacy By Modulating PI3K Signaling Via a Synthetic CD28 Rheostat

• Published in: Blood 2024-11, Vol.144, p.4802-4802
• Main Authors: Shahid, Sanam, Cai, Winson, Burns, Erin R., Um, Jasmine S., Mathew, Serena, Souness, Sydney, Yoo, Sarah, Chen, Bing-Yi, Fujino, Takeshi, Brentjens, Renier, Daniyan, Anthony F.
• Format: Article
• Language: English
• ISSN: 0006-4971
• DOI: 10.1182/blood-2024-209964

Introduction: CD28 is a costimulatory protein incorporated into two FDA-approved second-generation CD19-directed CAR T cell products and utilized extensively preclinically. Within CAR T cells, CD28 induces complex signaling including activation of the PI3K/AKT/mTOR pathway, with ingrained signaling redundancies. Although PI3K signaling enhances proliferation, excessive growth signals may lead to T cell exhaustion and terminal differentiation, correlating with suboptimal clinical responses. Importantly, PI3K and AKT inhibitors have been used to modulate CAR T cell signaling, enriching memory-like and reducing highly differentiated T cells, leading to a more potent cellular product. We aimed to attenuate CD28 signaling redundancies through the development of a “synthetic rheostat” that tempers PI3K/AKT/mTOR pathway signaling. Methods:We mutated CD28 to reduce PI3K signaling and assessed the function of this mutant (referred to as SAVVY) compared to a native CD28 in second-generation CD19-targeting CAR T cells against NALM6, a B cell acute lymphoblastic leukemia (B-ALL) cell line. CD19-targeting CAR T cells were produced by gammaretroviral transduction of peripheral blood-derived primary T cells. In vitro, we evaluated CAR T cell proliferation, target lysis, CAR recycling, T cell mitochondrial mass/function, T cell immunophenotype (memory/exhaustion), and T cell cytokine secretion. In vivo, we evaluated tumor control and overall survival in cell-line derived xenograft murine models. In vitro and in vivo, models of normal antigen density, low antigen expression, and PD-L1 overexpression were evaluated given the clinical relevance and prevalence of antigen-positive relapses, as well as CD19 antigen-low/negative relapses post-treatment with FDA-approved CAR T cell therapies. Results:Although CD19-targetingSAVVY-CAR T cells demonstrated no significant difference in long-term anti-tumor efficacy compared to native CD28-CAR T cells against NALM6 (effector:target (E:T) ranging from 1:5 to 1:40), they exhibited superior proliferation in vitro. When cocultured long-term in vitro with low antigen density tumor (NALM6-CD19low) and PD-L1-overexpressing tumor (NALM6-PD-L1+), SAVVY-CAR T cells demonstrated comparable cytotoxicity and proliferation at an E:T of 1:5 but superior tumor control at a lower E:T of 1:40. The SAVVY modification did not perturb blastogenesis or expression of early activation markers (CD69), but it protracted late activation marker expression (CD25,