Introduction: T cell activation and function are regulated by calcium signaling, a well-documented orchestrator of immune response mechanisms. Activation signaling is primarily initiated by the interaction between the T cell receptor (TCR) and peptide-MHC complexes on antigen-presenting cells, leading to calcium release from intracellular stores and sustained influx through store-operated calcium entry (SOCE). The amplitude and duration of calcium signaling are pivotal in determining the extent of T cell activation, as it directly influences signals necessary for T cell proliferation, differentiation, and effector functions. Central to these pathways is the transcription of activation genes such as NF-κB, NFAT, and the AP-1 complex components cJun N-terminal (JNK) and cFos. A large body of research has recently investigated the ways that mechanical stimuli, such as fluid shear stress (FSS), influences calcium signaling. In mechanically active environments such as the bloodstream or lymphatic system, T cells are subjected to varying levels of shear stress, which can alter the dynamics of calcium signaling through the activation of mechanosensitive channels such as Piezo1. The activation of Piezo1 by FSS leads to additional calcium influx and has been shown to significantly enhance T cell activation responses. This integration of mechanical and biochemical cues is essential for the effective function of T cells in physiological and pathophysiological conditions, highlighting the complexity of immune regulation and the potential for novel therapeutic strategies targeting these mechanoresponsive pathways.
Materials and
Methods: The present study introduces a MATLAB-based simulation to model T cell calcium dynamics in response to FSS. The simulation comprises multiple calcium storage compartments within the cell, including the endoplasmic reticulum (ER), cytosol, and mitochondria, and models the dynamics of calcium exchange among them. Critical components simulated include the inositol trisphosphate (IP3) receptors on the ER for calcium release, the sarco/endoplasmic reticulum Ca²⁺-ATPases (SERCAs) for reuptake, and various channels and pumps such as the Calcium Release-Activated Calcium (CRAC) channels and plasmalemmal Ca²⁺-ATPases (PMCA). This model incorporates the passive influx of calcium ions through mechanosensitive ion channels (MSCs), such as Piezo1, in response to mechanical stimuli. According to steady state ion flux across the cell membrane, as described by the Goldman–Hodgkin–Katz equation, the gating behavior of MSCs is modelled in this study as the product of the membrane permeability coefficient and the probability fraction of open channels. Therefore, the probability of open channels increases in response to mechanical stimuli and results in a calcium influx across the cell membrane. The integration of this function allows the model to dynamically reflect the response of T cells to mechanical forces, offering insights into the mechanotransduction pathways of immune cells under varying FSS conditions.
Results, Conclusions, and Discussions: The MATLAB-based model successfully simulates calcium signaling in T cells, incorporating 77 differential equations to describe various biochemical processes and downstream gene transcription (Figure 1A). In this simulation, the application of FSS is represented by increasing the probability function of opened MSCs (0%, 25%, 50%, 75%, and 100%) over 5000 seconds. Under these conditions, the membrane potential remained relatively constant (-59.7 mV) in conditions with 0%-75% of MSCs opened (Figure 1B). When 100% of MSCs were opened, there is a pronounced hyperpolarization (-62.9 mV) of the membrane potential, agreeing with observed cellular responses to FSS in the literature. Increased levels of FSS led to enhanced and sustained calcium influx from the extracellular space into the cytosol reaching steady state in approximately 300 seconds (Figure 1C). Simultaneously, calcium efflux from the ER (Figure 1D) into the cytosol decreased, representing the release of ER calcium stores via SOCE. Furthermore, calcium flux from the mitochondria into the cytoplasm decreased, indicating enhanced calcium retention as FSS stimulation increased (Figure 1E). This model also demonstrates that FSS significantly enhances the phosphorylation of NF-κB in the cytoplasm and levels of active NFAT and active JNK in the nucleus (Figure 1F-H). This demonstrates that higher FSS levels robustly activate key signaling pathways involved in T cell response. These results agree with experimental data that demonstrates rapid phosphorylation of NF-κB in the cytoplasm and translocation of active NFAT and JNK into the nucleus with increased levels of intracellular calcium. Ongoing work aims to increase the physiological relevance of the FSS function by including T cell and Piezo1 specific parameters, as well as a defined magnitude of fluid shear stress in dyne/cm². To accomplish this, the probability of a MSC in the open state (Figure I) will follow a Boltzmann dependence on the strain energy density within the plasma membrane (Figure J). Overall, this model provides a computational framework to explore the impact of mechanical stimuli on immune cell function. This approach could lead to novel strategies for manipulating T cell activity in various therapeutic contexts, including autoimmunity and cancer.
