Exposure to fine particulate matter (PM2.5) might favour SARS-CoV-2 infection in host lung cells

The link between exposure to air pollution and COVID-19 has been postulated since the early stages of the pandemic. Air pollution could have several effects on the pathology and prognosis of the infection. Exposure to fine particulate matter (PM2.5) has been correlated with increases in SARS-CoV-2 infectivity, COVID-19 severity, and mortality rates, but whether this is a causal association has been debated (Katoto et al., 2021). One hypothesis in support of a causal role is that PM2.5 may induce direct/indirect systemic effects, ultimately leading to a reduction of cellular defences, making people more prone to SARS-CoV-2 infection (Baron, 2022; Zhang et al., 2020). Exposure to PM induces a state of pulmonary inflammation, resulting in the over-expression of the membrane receptor ACE2, for which SARS-CoV-2 has a high affinity. However, to date there is a lack of toxicological studies investigating this directly. Understanding how PM might play a crucial role in inducing ACE2 expression and lung inflammation, and possibly facilitating virus entry and spread in target cells, may help suggest new strategies to prevent/mitigate the effects of COVID-19 and future pandemic. This project aims to evaluate the in vitro interactions between SARS-CoV-2 and PM2.5, using human alveolar cells (A549) to provide evidence on the biological mechanisms by which PM facilitates virus infection and to verify if PM can exacerbate the virus-mediated pulmonary inflammatory response. PM2.5 was collected in Milan during winter 2021. A549 cells were primed with PM2.5 (2.5 μg/cm2) for 72 hours (to assess PM-mediated ACE2 activation), afterward SARS-CoV-2 (2.4 x 103 genome copies) was added for additional 2 or 24 h to evaluate the endosomal pathway as a parameter for viral particles internalization, and the activation of inflammatory response. Results demonstrated that after 72 h of exposure PM2.5 significantly increased ACE2 expression, which was not seen with SARS-CoV-2 alone. The ability of SARS-CoV-2 to enter host cells through ACE2 binding was investigated by studying the cell membrane-to-cytoplasm trafficking through the expression of the early endosome marker Rab5. The cells primed with PM2.5 show a significant increase in Rab5 level already after 2 h of exposure to the virus (Figure 1) compared to cells exposed only to PM or SARS-CoV-2. Finally, we observed that only PM-primed cells co-exposed for additional 24 h to SARS-CoV-2 significantly showed an altered inflammatory pathway, with increased NF-kB expression and IL-8 release. Taken together these results support a mechanism by which PM is able, at sub-acute exposure, to facilitate virus entry into the endosomal route (increased infection) and to induce an activation of the inflammatory pathway (increased responsiveness), possibly leading to an exacerbation and severity of the disease.