Ca²⁺ puffs underlie adhesion-triggered Ca²⁺ microdomains in T cells

Ca²⁺ signalling is pivotal in T cell activation, an essential process in adaptive immune responses. Key to this activation are Ca²⁺ microdomains, which are transient increases in cytosolic Ca²⁺ concentration occurring within narrow regions between the endoplasmic reticulum (ER) and the plasma membrane (PM), lasting a few tens of milliseconds. Adhesion Dependent Ca²⁺ Microdomains (ADCM) rely on store-operated Ca²⁺ entry (SOCE) via the ORAI/STIM system. The nanometric scale at which these microdomains form poses challenges for direct experimental observation. Following the previous work of Gil et al. [1], which introduced a three-dimensional model of the ER-PM junction, this study combines a detailed description of the Ca²⁺ fluxes at the junction with stochastic dynamics of a cluster of D-myo-inositol 1,4,5 trisphosphate receptors (IP₃R) located in the ER surrounding the junction. Because the consideration of Ca²⁺ release through the IP₃R calls for the simulation of a portion of the cytoplasm considerably larger than the junction, our study also investigates the spatial distribution of PMCAs, revealing their likely localization outside the ER-PM junction. Simulations indicate that Ca²⁺ puffs implying the opening of 2-6 IP₃Rs create ADCMs by provoking local depletions of ER Ca²⁺ stimulating Ca²⁺ entry through the ORAI1 channels. Such conditions allow the reproduction of the amplitude, duration and spatial extent of the observed ADCMs. By integrating advanced computational techniques with insights from experimental studies, our approach provides valuable information on the mechanisms governing early Ca²⁺ signalling in T cell activation, paving the way for a deeper understanding of immune responses.