This study focuses on the clearance of Rose Bengal Acetate (RBAc)-PhotoDynamic Therapy (PDT)-generated apoptotic and autophagic HeLa cells by murine and human macrophages. Indeed, phagocytosis of dead cells drives the therapeutic efficacy of PDT through both efficient removal of dead/dying cells and macrophages response evoked during engulfment and, up to now, clearance of dying photosensitized cells has been less investigated than PDT mechanisms of cell death induction. RBAc-PDT ensures a long onset of cytotoxicity and a time-related cell death of HeLa cells by signals originating from or converging on almost all intracellular organelles. On this basis, to clarify whether the efficacious cell death commitment is followed by an efficient clearance mechanism, we primarily focused on the analysis of 'eat me' signals exposure and 'find me' signals release, and then investigated the migration, recognition, engulfment and response of murine Raw 264.7 and human blood isolated macrophages. Dead cells secreted 'find me' signals, i.e. fractalkine and Heat Shock Protein 70 (HSP 70), to recruit macrophages and promote their fast phagocytosis. Macrophages phagocytosed apoptotic and autophagic PDT-treated cells more efficiently than the respective positive controls, i.e. puromycin-induced apoptotic and Earle's balanced salt solution-starved autophagic cells. Phagocytosis depends on the glycans exposed on dead cells. The macrophages internalization of photokilled cells elicits the production of Interleukin-10, Transforming Growth Factor-β and Tumour Necrosis Factor-α by macrophages. TNFα production, along with HSP70 release and plasma membrane translocation on dead cells, suggest an immunogenic impact of RBAc-PDT. In fact, macrophages, activated fibroblasts and endothelial cells colonized the inoculum site of photosensitized cells in rat calf muscles, endorsing the hypothesis of immunogenic elicitation of RBAc-PDT.
Keywords: HSP70; cytokines; fractalkine; phagocytosis; photodynamic therapy; rose bengal acetate.