Polystyrene Microplastics Impair Cellular Immune Responses in Galleria mellonella Following Controlled Internal Exposure.

Microplastics (MPs) are emerging environmental contaminants with increasing evidence of immunotoxic effects in invertebrates; however, their underlying cellular mechanisms remain insufficiently understood. This study investigated whether short-term internal exposure to 1 μm polystyrene microplastics affects immune and oxidative responses in Galleria mellonella. Larvae were injected with 1, 5, 20, 40, and 80 mg/mL PS-MPs (n = 16 per parameter), and total hemocyte count, encapsulation and melanization responses, phenoloxidase (PO) activity, antioxidant enzymes (SOD and CAT), malondialdehyde (MDA), and total hemolymph protein levels were evaluated. Microplastic exposure significantly reduced circulating hemocyte abundance across all concentrations (control: 232.75 × 105 cells/mL; lowest at 1 mg/mL: 148.44 × 105 cells/mL). Encapsulation and melanization responses were markedly suppressed, with strong encapsulation declining from 42.49% in controls to 4.31% at 20 mg/mL, and strong melanization decreasing from 19.38% to 0.77% at 5 mg/mL. Despite the pronounced impairment of cellular immune responses, systemic PO, SOD, and CAT activities showed no robust concentration-specific changes. In contrast, measured MDA levels decreased at ≥ 20 mg/mL, while total hemolymph protein levels increased at selected concentrations. The concurrent reduction in circulating hemocyte abundance and impairment of encapsulation and melanization indicate substantial disruption of hemocyte-mediated cellular immune function. Because hemocyte subpopulations, particle internalization, and cell viability were not directly assessed, the present data do not identify a specific hemocyte subtype or intracellular mechanism responsible for these effects. Overall, these findings demonstrate that controlled internal PS-MP exposure impairs cellular immune responses in G. mellonella, while systemic enzymatic responses are comparatively less affected.