Calcium Imaging in T Lymphocytes: a Protocol for Use with Genetically Encoded or Chemical Ca2+ Indicators

Elevations in cytosolic calcium (Ca2+) drive a wide array of immune cell functions, including cytokine production, gene expression, and cell motility. Live-cell imaging of cells loaded with ratiometric chemical Ca2+ indicators remains the gold standard for visualization and quantification of intracellular Ca2+ signals; ratiometric imaging can be accomplished with dyes such as Fura-2, the combination of Fluo-4 and Fura-Red, or, alternatively, by expressing genetically-encoded Ca2+ indicators (GECI) such as GCaMPs. Here, we describe a detailed protocol for Ca2+ imaging of T cells in vitro using genetically encoded or chemical indicators that can also be applied to a wide variety of cell types. The protocol addresses the challenge of facilitating T cell attachment on various substrates prepared on glass-bottom dishes to enable T cell imaging on an inverted microscope. The protocol also emphasizes cell preparation steps that ensure optimal cell viability - an essential requirement for recording dynamic changes in cytosolic Ca2+ levels - and that ensure reproducibility between multiple samples. Finally, we describe a simple algorithm to analyze single-cell Ca2+ signals over time using Fiji (ImageJ) software.