Curbside particulate matter and susceptibility to SARS–CoV-2 infection

BACKGROUND: Biologic plausibility for the association between exposure to particulate matter (PM) less than 10 μm in aerodynamic diameter (PM(10)) and coronavirus disease 2019 (COVID-19) morbidity in epidemiologic studies has not been determined. The upregulation of angiotensin-converting enzyme 2 (ACE2), the severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) entry receptor on host cells, by PM(10) is a putative mechanism. OBJECTIVE: We sought to assess the effect of PM(10) on SARS-CoV-2 infection of cells in vitro. METHODS: PM(10) from the curbside of London's Marylebone Road and from exhaust emissions was collected by cyclone. A549 cells, human primary nasal epithelial cells (HPNEpCs), SARS-CoV-2–susceptible Vero-E6 and Calu3 cells were cultured with PM(10). ACE2 expression (as determined by median fluorescent intensity) was assessed by flow cytometry, and ACE2 mRNA transcript level was assessed by PCR. The role of oxidative stress was determined by N-acetyl cysteine. The cytopathic effect of SARS-CoV-2 (percentage of infection enhancement) and expression of SARS-CoV-2 genes' open reading frame (ORF) 1ab, S protein, and N protein (focus-forming units/mL) were assessed in Vero-E6 cells. Data were analyzed by either the Mann-Whitney U test or Kruskal-Wallis test with the Dunn multiple comparisons test. RESULTS: Curbside PM(10) at concentrations of 10 μg/mL or more increased ACE2 expression in A549 cells (P = .0021). Both diesel PM(10) and curbside PM(10) in a concentration of 10 μg/mL increased ACE2 expression in HPNEpCs (P = .0022 and P = .0072, respectively). ACE2 expression simulated by curbside PM(10) was attenuated by N-acetyl cysteine in HPNEpCs (P = .0464). Curbside PM(10) increased ACE2 expression in Calu3 cells (P = .0256). In Vero-E6 cells, curbside PM(10) increased ACE2 expression (P = .0079), ACE2 transcript level (P = .0079), SARS-CoV-2 cytopathic effect (P = .0002), and expression of the SARS-CoV-2 genes' ORF1ab, S protein, and N protein (P = .0079). CONCLUSIONS: Curbside PM(10) increases susceptibility to SARS-COV-2 infection in vitro.