Labeling and exocytosis of secretory compartments in RBL mastocytes by polystyrene and mesoporous silica nanoparticles

Background: For a safe 'in vivo' biomedical utilization of nanoparticles, it is essential to assess not only biocompatibility, but also the potential to trigger unwanted side effects at both cellular and tissue levels. Mastocytes (cells having secretory granules containing cytokines, vasoactive amine, and proteases) play a pivotal role in the immune and inflammatory responses against exogenous toxins. Mastocytes are also recruited in the tumor stroma and are involved in tumor vascularization and growth. Aim and methods: In this work, mastocyte-like rat basophilic leukemia (RBL) cells were used to investigate whether carboxyl-modified 30 nm polystyrene (PS) nanoparticles (NPs) and naked mesoporous silica (MPS) 10 nm NPs are able to label the secretory inflammatory granules, and possibly induce exocytosis of these granules. Uptake, cellular retention and localization of fluorescent NPs were analyzed by cytofluorometry and microscope imaging. Results: Our findings were that: (1) secretory granules of mastocytes are accessible by NPs via endocytosis; (2) PS and MPS silica NPs label two distinct subpopulations of inflammatory granules in RBL mastocytes; and (3) PS NPs induce calcium-dependent exocytosis of inflammatory granules. Conclusion: These findings highlight the value of NPs for live imaging of inflammatory processes, and also have important implications for the clinical use of PS-based NPs, due to their potential to trigger the unwanted activation of mastocytes.