Optimized cardiac leukocyte isolation for flow cytometry enumeration and sorting

Accurate characterization of immune cell populations within the heart is critical for understanding inflammatory mechanisms that drive cardiac injury and repair. However, isolating viable leukocytes from cardiac tissue remains technically challenging due to the dense extracellular matrix, high collagen content, and susceptibility of immune subsets to enzymatic or mechanical stress. Here, we describe an optimized protocol for cardiac leukocyte isolation that maximizes cell yield, preserves viability, and maintains the integrity of surface markers for flow cytometric analysis and cell sorting. The method integrates controlled enzymatic digestion, gentle mechanical dissociation, and precise debris removal, thereby minimizing stromal contamination and reducing cellular activation artifacts. Using this approach, we demonstrate reproducible recovery of diverse immune populations, including neutrophils, monocytes, macrophages, and lymphocyte subsets from murine hearts. Furthermore, compatibility with high-dimensional flow cytometry and fluorescence-activated cell sorting enables robust downstream applications such as transcriptomic profiling and functional assays. This optimized isolation strategy provides a reliable and versatile tool for investigating the cardiac immune landscape, offering new opportunities to delineate the immunopathology of cardiovascular disease and to develop targeted immunomodulatory therapies.NEW & NOTEWORTHY Flow cytometry of immune cells is highly tissue-specific, requiring tailored enzymatic digestion and staining protocols. We optimized a protocol for cardiac immune cells isolation and enumeration using the following steps: 1) tissue digestion, 2) cell viability, 3) titration of live/dead dye, 4) antibody titration, and 5) compensation and blocking. The resulting protocol was used both for flow cytometry and FACS analysis of neutrophils, monocytes, and macrophages, but it could easily be extended to innate immune cells.