In vivo Inhalation Toxicity of Polystyrene Nanoplastic Particles: PSLT-Like Responses of Apical Endpoints and Transcriptomic Changes

<title>Abstract</title> <p> Overall, the concordance between exposure concentration, retained lung burden, apical responses, and transcriptomic changes supports a PSLT-like, overload-associated interpretation of the findings. The study did not provide evidence for a PS-specific mode of inhalation toxicity under the conditions tested and demonstrates that integrating transcriptomics with apical endpoints can refine the mechanistic interpretation of particle-induced lung responses. <bold>Background</bold> Inhalation is a relevant route of exposure to micro- and nanoplastics (MNPs), yet it remains unclear whether inhaled polymer particles represent a distinct inhalation hazard or conform to established principles of poorly soluble low-toxicity particles (PSLTs). This study tested whether low-reactivity polystyrene (PS) particles elicit responses consistent with the PSLT paradigm, in which adverse pulmonary effects are expected primarily under conditions of excessive retained lung burden and impaired clearance. <bold>Methods</bold> Male Wistar rats were exposed nose-only to respirable agglomerates of PS particles at 5 and 50 mg/m ³ for 28 days, 6 hours/day and 5 days/week, followed by post-exposure periods of approximately five and 13 weeks. Apical endpoints, including bronchoalveolar lavage fluid parameters and histopathology, were evaluated together with transcriptomic analyses of lung and liver tissues. <bold>Results</bold> At 50 mg/m ³ , pulmonary effects including neutrophilic infiltration, type II pneumocyte and bronchiolo-alveolar hyperplasia, increased lung weight, and persistent particle-associated lung changes were observed, consistent with overload-associated responses. In contrast, no adverse apical effects occurred at 5 mg/m ³ , although transcriptomic alterations involving complement, coagulation, and lipid metabolism pathways were detected. These molecular changes occurred in the absence of adverse apical findings and are therefore interpreted as early exposure-related responses below the threshold for overt toxicity. In the liver, transcriptomic changes were limited, providing no evidence for relevant systemic toxicity. <bold>Conclusions</bold> </p>