ACE2 and infection: As initial immune responses to SARS-CoV-2 infection in the lungs play a pivotal role in the early development and severity of COVID-19 in patients [31,32], we hypothesized that the increase in disease severity may be due to a more pronounced pro-inflammatory response to infection, and that Dp16;ACE2 mice may exhibit increases in neutrophil infiltration and pro-inflammatory cytokine induction compared with ACE2 mice.