Calicum microdomains form within neutrophils at the neutrophil–tumor cell synapse: role in antibody-dependent target cell apoptosis

Ca²⁺ messages are broadly important in cellular signal transduction. In immune cells, Ca²⁺ signaling is an essential step in many forms of activation. Neutrophil-mediated antibody-dependent cell-mediated cytotoxicity (ADCC) is one form of leukocyte activation that plays an important role in tumor cell killing in vitro and in patient care. Using fluorescence methodologies, we found that neutrophils exhibit Ca²⁺ signals during ADCC directed against breast fibrosarcoma cells. Importantly, these signals were localized to Ca²⁺ microdomains at the neutrophil-to-tumor cell interface where they display dynamic features such as movement, fusion, and fission. These signals were blocked by the intracellular Ca²⁺ buffer BAPTA. At the neutrophil–tumor cell synapse, the neutrophil’s cytoplasm was enriched in STIM1, a crucial mediator of Ca²⁺ signaling, whereas the Ca²⁺-binding proteins calbindin and parvalbumin were not affected. Our findings suggest that Ca²⁺ microdomains are due to an active signaling process. As Ca²⁺ signals within neutrophils were necessary for specific tumor cell apoptosis, a central role of microdomains in leukocyte-mediated tumor cell destruction is indicated.