The global prevalence of plastics is driven by their strength, light weight, durability, chemical resistance, and affordability [1]. As of 2022, 400 million tons were produced worldwide, with production increasing by around 5 % annually [2,3]. Despite their utility, the chemical stability of plastics has led to their persistent accumulation in the natural environment.

Microplastics (MPs) are ubiquitous particles formed by the fragmentation of plastic waste, and are typically defined as synthetic polymer particles with a diameter of ≤5 mm [4,5]. These particles are increasingly detected across various environmental compartments, including air, water, and soil [5,6]. In particular, MPs are now recognized as airborne contaminants present in both indoor and outdoor environments, originating from sources such as textile abrasion, urban dust, industrial emissions, and the resuspension of settled particles [7]. They are transported by wind and deposited via atmospheric fallout, and detected even in remote regions, indicating their extensive environmental distribution. Furthermore, MPs have been detected in human biological samples, including lung tissue [8,9], sputum [10], urine [11], and blood samples [12], suggesting the possibility of systemic exposure. However, their hazardous effects on human health remain poorly understood.

Asthma is a chronic respiratory disease characterized by airway inflammation, airway hyperresponsiveness (AHR), and airway remodeling [13], affecting around 300 million individuals globally, and contributing to approximately 1,000 deaths per day [14]. Environmental pollutants are recognized as key factors contributing to allergen sensitization, increased risk of exacerbation, and poor clinical outcomes [15]. Among airborne pollutants, MPs have recently been identified as potential detrimental factors influencing asthma outcomes in experimental studies [16-20]. However, a previous study reported conflicting findings, showing that airway resistance and Th2 cytokine levels did not differ significantly following MP exposure in a murine model of asthma [20].

In this study, we evaluated the impact of repeated MP exposure in an acute ovalbumin (OVA)-induced asthma murine model. We exposed OVA-sensitized mice and healthy controls to polystyrene MPs over the course of allergen challenge, and assessed resultant airway inflammation, cytokine profiles, and macrophage activation states.

In this study, we investigated the immunologic effects of repeated MP exposure in a murine model of acute OVA-induced asthma. In contrast to our initial expectation that MP exposure would exacerbate allergic airway inflammation, we found that MP treatment did not intensify Th2-mediated responses in asthmatic mice. Instead, MP exposure resulted in a reduction of eosinophilic inflammation and Th2 cytokines (IL-5, IL-13) in OVA-sensitized animals. Interestingly, in healthy control mice, MP exposure increased pro-inflammatory cytokines, such as TNF-α and IFN-γ, elevated epithelial alarmins (IL-25 and IL-33), and induced histological signs of airway inflammation. However, these effects were not observed in OVA-challenged mice, suggesting a context-dependent immune modulation by MPs. Furthermore, macrophage polarization analysis revealed that MP exposure shifted the balance toward M1-like activation in control mice, but not in asthmatic lungs. These results imply that MPs exert differential immune effects, depending on the presence or absence of pre-existing airway inflammation.

Previous studies have shown contradictory results regarding the impact of MP exposure on airway inflammation in asthma models. Several factors may explain these discrepancies. First, particle size of the MPs may have contributed to the differential outcomes. Lu et al. [20] employed spherical polystyrene MPs of 1−5 μm, identical to those used in our study, and demonstrated no significant changes in airway inflammation, IL-4 or IL-5 levels, or airway resistance in a house dust mite (HDM)-induced asthma model, which corresponds to our study. In contrast, Han et al. [19] used significantly smaller MPs of 0.2 to 0.5 μm and reported increased airway inflammation, elevated IL-17 and IgE levels in OVA-sensitized asthma model, and further exacerbation of allergic responses when co-administered with di(2-ethylhexyl) phthalate (DEHP), a commonly used plasticizer. Furthermore, Fan et al. [23] conducted a particle size comparison study in rats using 100 nm to 2.5 μm MPs, and demonstrated that smaller particles showed greater pulmonary deposition, whereas 2.5 μm particles showed minimal lung accumulation, supporting these size-dependent effects. These findings collectively suggest that the relatively larger particle size used in our study may have limited the degree of airway penetration and cellular interaction, potentially explaining the attenuated or neutral inflammatory responses observed in our asthma model.

Furthermore, the route of MP administration may have influenced the observed outcomes. Both our study and that of Lu et al. [20], which reported similar findings, employed intranasal instillation of relatively large MPs of 1-5 μm. In contrast, studies reporting adverse effects of MP exposure often utilized the intratracheal route or whole-body inhalation system [23-26]. Given the larger particle size and intranasal delivery in our study, the extent of MP deposition in the lower airways may have been limited, potentially insufficient to induce or exacerbate airway inflammation.

The observed increase in epithelial alarmins such as IL-25 and IL-33 following MP exposure in control mice suggests that MPs can act as local stressors at the airway epithelial barrier [27,28]. Alarmins are typically released in response to epithelial damage or irritation, and their upregulation in naïve lungs implies that MPs are recognized as foreign irritants that are capable of triggering innate immune signaling [27,28]. In contrast, in OVA-sensitized mice, alarmin levels were generally elevated compared to controls, reflecting the ongoing epithelial stress and type 2 inflammation induced by allergen exposure. However, the expression of IL-25 and IL-33 did not differ significantly between OVA/Veh and OVA/MP groups, indicating that additional MP exposure did not further enhance alarmin signaling in already inflamed lungs.

Our macrophage polarization analysis also revealed a divergence in immune response, depending on the airway status. In control mice, MP exposure promoted a shift toward M1-like polarization, as evidenced by increased iNOS expression and a higher M1/M2 index, indicating an innate pro-inflammatory reaction [29]. However, this polarization shift was not observed in OVA-sensitized mice, where despite MP exposure, the M1/M2 balance remained unchanged. Interestingly, the observed reduction in Th2 responses occurred without a concurrent shift in the M1/M2 macrophage balance in the OVA-sensitized groups. Although M2 macrophages are known contributors to eosinophilic inflammation, this finding suggests that the immunomodulatory effects of MPs on allergic inflammation in our model are likely independent of this specific macrophage activation pathway. These data suggest that the impact of MPs on innate immune cell activation is diverse by the pre-existing immune environment of the lung.

This study has several limitations. First, we used an acute asthma model, which does not reflect the chronic features of asthma, such as airway remodeling. Second, we tested only one type and size of MP, using spherical polystyrene particles ranging 1 to 5 μm. Other particle sizes, shapes, or chemical compositions were not included, so our findings cannot be generalized to all types of MP exposure. Third, our study did not include an assessment of AHR, a key functional hallmark of asthma. Therefore, the direct impact of MP exposure on airway physiology remains to be determined. Fourth, the intranasal instillation method may have limited the effective dose of MPs reaching the lower airways, particularly in the mucus-rich environment of inflamed asthmatic lungs. It is also possible that a ‘ceiling effect’ in the robustly inflamed OVA model could have masked any potential additive pro-inflammatory effects of the MPs. Further studies using chronic models and a broader range of particle characteristics are needed.

In conclusion, our study demonstrates that repeated exposure to polystyrene MPs does not aggravate allergic airway inflammation in an acute murine asthma model. The immunologic effects of MPs appear to vary depending on the underlying inflammatory status of the airway, with distinct responses observed in healthy versus allergen-sensitized lungs. Further studies using chronic exposure models and a broader range of particle sizes, compositions, and shapes are needed to clarify their impact on asthmatic airways.