Circulating in the oyster immune system: multi-compartment modeling reveals dynamic intercellular trafficking of biodegradable nanoplastics

Studies of the cellular fate of nanoplastics (NPs) in bivalves commonly overlook the dynamic trafficking of particles among immune cells. Here, we established a multi-compartment kinetic framework using time-resolved measurements of hemocyte populations for 2-h following a 1-h in vitro exposure to polylactic acid biodegradable nanoplastics (PLA-NPs). The framework quantitatively described how the accumulated NPs trafficked among different hemocyte subpopulations of oyster Crassostrea hongkongensis by incorporating modeled extracellular release, re-internalization, and net direct intercellular transfer through tunneling nanotubes. The model suggested that NP trafficking was a concentration-driven process. At the two lowest concentrations only slight fluorescence changes were observed, while at the intermediate concentration the model supported a coordinated trafficking pathway involving re-internalization and direct transfer from semigranulocytes to granulocytes (SG-to-G). At high concentration, the model indicated a shift toward extracellular recycling, with sustained particle release and recapture but reduced intracellular sequestration. Excessive loading redirected NPs to a more exchangeable state, and particle aggregation may have contributed to this pattern. A further model application using hemolymph serum captured the microenvironment-dependent NP redistribution pattern. In the serum microenvironment, granulocytes displayed reduced re-internalization, accompanied by greater SG participation. Single-cell imaging further indicated that the granulocyte compartment contained functional heterogeneity, with eosinophilic granulocytes acting as the major subtype involved in secondary accumulation and retention of redistributed NPs. These findings demonstrated that NPs were relatively dynamic and exchangeable in the coordinated oyster immune system. The proposed multi-compartment model provided a quantitative framework to resolve how exposure concentration and biological microenvironment regulated NP trafficking, providing a quantitative basis for assessing the fates of NPs in marine invertebrate immunity. • A multi-compartmental model quantitatively how PLA nanoplastic trafficking in oyster hemocytes. • Rising exposure concentrations shift NP fate from retention to dynamic transfer. • Hemolymph serum alters NP trafficking and prolongs extracellular persistence. • Eosinophilic granulocytes preferentially retain redistributed nanoplastics.