Single-Cell Investigation of the Role of Calcium Bursts in Human Immune Cells

Abstract

The intracellular calcium concentration of immune cells is known to exhibit dramatic bursts, during which the calcium level peaks throughout the whole cell on a timescale of seconds. Despite the well-established importance of calcium ions in numerous cellular processes, the physiological role of these bursts remains unclear. What causes calcium bursts? What do they cause in turn? When do they occur, and why do they occur at those particular times? By addressing questions like these, our ongoing work aims to contribute to a deeper mechanistic understanding of the behavior of immune cells, in particular human neutrophils. Our single-cell experiments combine micropipette manipulation with fluorescence imaging of the intracellular calcium concentration. The use of micropipettes enables us to separately inspect chemotaxis, adhesion, and phagocytosis during one-on-one encounters between neutrophils and pathogenic particles, while simultaneously monitoring the cortical tension of the cells. Surprisingly, we find that cell polarization and actin remodeling during complement-mediated pure (i.e., adhesion-free) chemotaxis neither require nor cause calcium bursts. On the other hand, our experiments confirm that calcium bursts consistently accompany phagocytosis and cell spreading. Furthermore, the occurrence of calcium bursts during phagocytosis appears to roughly correlate with a drastic increase of the cortical tension. Although treatment of the cells with 100 μM caffeine, or removal of calcium from the extracellular medium, modulates the strength and dynamics of calcium bursts, the bursts still occur during all phagocytosis experiments. These findings show that adhesion, but not extracellular calcium, is essential to stimulate calcium bursts. Moreover, the concurrent rise of the cortical tension indicates that in addition to adhesion, other mechanical processes may be closely related to these bursts.