Biodegradable nanomaterials have been widely used in numerous medical fields. To further improve such efforts, this study focused on the intracellular disposition of chitosan nanoparticles (CsNPs) in macrophages, a primary cell of the mononuclear phagocyte system (MPS). Such interactions with the MPS determine the nanoparticle retention time in the body and consequently play a significant role in their own clinical safety. In this study, various dye-labeled CsNPs (about 250 nm) were prepared, and a murine macrophage cell line (RAW 264.7) was selected as a model macrophage. The results showed two mechanisms of macrophage incorporation of CsNPs, ie, a clathrin-mediated endocytosis pathway (the primary) and phagocytosis. Following internalization, the particles partly dissociated in the cells, indicating cellular digestion of the nanoparticles. It was proved that, after intracellular uptake, a large proportion of CsNPs were exocytosed within 24 h; this excretion induced a decrease in fluorescence intensity in cells by 69%, with the remaining particles possessing difficulty being cleared. Exocytosis could be inhibited by both wortmannin and vacuolin-1, indicating that CsNP uptake was mediated by lysosomal and multivesicular body pathways, and after exocytosis, the reuptake of CsNPs by neighboring cells was verified by further experiments. This study, thus, elucidated the fate of CsNPs in macrophages as well as identified cellular disposition mechanisms, providing the basis for how CsNPs are recognized by the MPS; such information is crucial to numerous medical applications of CsNPs.

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