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Imaging biological processes become an issue when these biological processes involve a great number of targets;

Apoptosis is a genetically controlled event occurring during normal development, and its dysregulation is linked to diseases. Apoptotic cells end up as apoptotic bodies whose clearance is done by macrophages through phagocytosis. Key-steps in this process are the activation of macrophages by apoptotic cells and the identification of the latter by specific molecular recognition. This work aims at studying the interactions between apoptotic cells and macrophages at the single cell level so as to understand the process of phagocytosis. For that purpose, a few cells of each type are placed in a PDMS microchamber and observed using microscopy and time-lapse imaging.

However, apoptosis is not only a complex process that involves many events in the cell, but also a long process (> 10 h) while significant events are very short (<2 min). Subsequently, an issue for imaging apoptosis (or phagocytosis) arises from the optical properties of conventional organic dyes (low photostability, broad excitation/emission spectra) that prevent frequent and time-lapse imaging and the use of multiple staining agents.

We demonstrated here the use of Qdots-based stains for imaging phagocytosis as Qdots suppress the aforementioned problems. Macrophages were labeled with generic Qdots while specific Qdots were developed for apoptotic cells (Le Gac et al., NanoLetters, 2006). Besides, three types of interactions between apoptotic cells and macrophages were visualized; (i) activation of macrophages in presence of an apoptotic body, (ii) engulfment by macrophages of microparticles released by apoptotic cells, and (iii) eating of apoptotic bodies by macrophages after overnight incubation at 37°C.