Phenotypic Models of CAR T-Cell Activation Elucidate the Pivotal Regulatory Role of CAR Downmodulation

Abstract

Adoptive cell immunotherapy with chimeric antigen receptor (CAR) showed limited potency in solid tumors, despite durable remissions for hematopoietic malignancies. Therefore, an investigation of ways to enhance the efficacy of CARs' antitumor response has been engaged upon. We previously examined the interplay between the biophysical parameters of CAR binding (i.e., affinity, avidity, and antigen density), as regulators of CAR T-cell activity and detected nonmonotonic behaviors of affinity and antigen density and an interrelation between avidity and antigen density. Here, we built an evolving phenotypic model of CAR T-cell regulation, which suggested that receptor downmodulation is a key determinant of CAR T-cell function. We verified this assumption by measuring and manipulating receptor downmodulation and intracellular signaling processes. CAR downmodulation inhibition, via actin polymerization inhibition, but not inhibition of regulatory inhibitory phosphatases, was able to increase CAR T-cell responses. In addition, we documented trogocytosis in CAR T cells that depends on actin polymerization. In summary, our study modeled the parameters that govern CAR T-cell engagement and revealed an underappreciated mechanism of T-cell regulation. These results have a potential to predict and therefore advance the rational design of CAR T cells for adoptive cell treatments. See related article on p. 872

Receptor transfer between immune cells by autoantibody-enhanced, CD32-driven trogocytosis is hijacked by HIV-1 to infect resting CD4 T cells

“…In addition to representing an immune evasion strategy of HIV-1, the consequences of such trogocytosis need to be considered when assessing the efficacy and safety profiles of bNAbs for clinical application. 75 , 76 , 77 , 78 In contrast to CAR-based immunotherapies, in which adverse effects due to trogocytosis are triggered by Fc-mediated antibody binding to the acceptor cells, the process we describe elicits antibody-mediated trogocytosis upon binding to the FcγR on the donor cell. Immunotherapies will thus require optimization to circumvent both types of cell-cell communication.…”

Model‐informed drug development of autologous CAR‐T cell therapy: Strategies to optimize CAR‐T cell exposure leveraging cell kinetic/dynamic modeling

“…The activation threshold of a CAR depends on the antigen‐binding domain's affinity and avidity, as well as on the density of the expressed target antigen 25 . High affinity scFvs are often selected to assure sufficient activation; however, they might not always be the optimal choice.…”

“…The activation threshold of a CAR depends on the antigenbinding domain's affinity and avidity, as well as on the density of the expressed target antigen. 25 High affinity scFvs are often selected to assure sufficient activation; however, they might not always be the optimal choice. First, if the target antigen is not exclusive to the tumor but also expressed at low levels on healthy tissue, the use of a high affinity scFv may result in life-threatening on-target off-tumor toxicity.…”

Speed and Location Both Matter: Antigen Stimulus Dynamics Controls CAR-T Cell Response

“…Figure 1D (left) shows that cell-intrinsic mechanisms can include surface CAR internalization and negative regulation of key downstream signaling elements. Downregulation of surface CAR has been broadly reported in many studies, especially after prolonged challenges with target cells (14,29). CAR downregulation usually occurs immediately upon antigen binding, preventing CAR-T cell hyperactivation but also diminishing subsequent target-seeking ability.…”