(485c) Mechanistic Model of Chimeric Antigen Receptor (CAR) Signaling Predicts T Cell Response Time

Chimeric antigen receptors (CARs) are engineered proteins that contain an extracellular antibody-like region linked to a selection of intracellular signaling domains involved in T cell activation.  CAR engineered T cells have shown promise in treating certain types of cancer, but the effects of different signaling domain combinations and their role in T cell activation are not well understood. We have developed a mechanistic computational model to explore the signaling properties of different CAR domains to better understand how to control T cell activation. The model will be paired with experimental data of CAR signaling to help guide the development of T cells for adoptive cell therapy.

The model network was constructed based on previous models and experimental data in the literature. When available, initial parameter estimates were taken from experimentally measured values, the rest were based on a model and data by Altan-Bonnet.  Due to the complexity of the multiprotein interactions in this pathway, a rule-based model was written in BioNetGen and implemented in MatLab. The model is valid for short times after antigen binding (<120 seconds), allowing protein degradation to be neglected.

The first version of the model is comprised of 184 rules, encoding 7,647 reactions between 545 species.  It compares signaling differences between CD3ζ-only and CD3ζ-CD28 CARs, but can be updated to simulate a variety of different signaling domains. In the model, the kinase LCK is recruited to the membrane and activates a CAR upon its binding to an antigen.  LCK also triggers competing negative and positive feedback loops through SHP1 and ERK, respectively. The signaling cascade ultimately activates a variety of proteins that can be linked to specific cellular processes, including SOS (proliferation), PLCγ (Ca²⁺ signaling), Vav1 (cytoskeletal reorganization), and PI3K (cell survival). The active levels of these four proteins as well as ERK and SHP1 can be directly compared to experimental data. Having distinct markers of cellular functions that relate to T cell activation will allow us to more accurately understand the role of each CAR signaling domain and how they are integrated during T cell activation. 

While collecting CAR-specific experimental data, we have fit the CD3ζ-CD28 CAR model to data for endogenous T cell signaling from Altan-Bonnet. The model accurately mimics how the response time of doubly phosphorylated ERK changes with increasing antigen concentration, and shows different dynamics with or without the CD28 signaling domain.  The connections specified in the signaling network are able to recreate many of the known events of endogenous T cell signaling; thus, the model provides a framework to study the effects of CAR signaling. 

The model shows promise toward being able to accurately represent the effects of CAR signaling to better understand how different signaling domains control the strength and persistence of activated T cells.  This understanding can be used to optimize CAR engineered T cells to target cancer cells in a manner that is both sensitive and specific. 

References:

Altan-Bonnet G, Germain RN (2005) Modeling T cell antigen discrimination based on feedback control of digital ERK responses. PLoS Biol 3(11): e356.