Route-dependent immune modulation and long-term lung retention after polypropylene microplastic exposure in mice

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Polypropylene microplastics (PP MPs) are among the most widely used synthetic polymers and represent a major component of global microplastic (MP) pollution, yet their immunological and environmental health effects remain poorly understood. We investigated immune responses to pristine PP MPs in mouse models using short-term airway and internal exposure models across defined dose ranges designed to assess hazard-relevant immune effects. Intranasally administered PP MPs induced pulmonary inflammation in naïve mice and further potentiated ragweed pollen-induced allergic airway inflammation, demonstrating that PP MPs possess intrinsic inflammatory activity while also modifying ongoing allergic immune responses. Following epithelial barrier bypass, intraperitoneal administration of PP MPs differentially modulated antigen-specific immune responses, producing allergen- and timing-dependent reductions in peritoneal leukocyte accumulation and altering antigen-specific cytokine and antibody responses. These data demonstrate that pristine PP MPs are biologically active and that their immunological effects depend on exposure route, biological compartment, allergen context, and the timing of exposure. Even short-term exposures at controlled doses were sufficient to elicit measurable immune modulation, suggesting the potential for biologically relevant effects following repeated or cumulative exposure. These findings highlight the importance of immunological endpoints and particle persistence when evaluating the hazard potential of micro- and nanoplastic particles across different exposure routes.