“…One advantage to the use of mice for modeling of SARS-CoV-2 infection and COVID-19 is the availability of immunodeficient, human immune-reconstituted, and transgenic strains, making it possible to assess the importance of both viral and host factors in infection and disease development ( Kenney et al, 2022 ; Fu et al, 2021 ; Shang et al, 2021 ; Wahl et al, 2021 ; Sefik et al, 2021 , 2022 ; Mao et al, 2021 ). In the present study, we employed human ACE2 knock-in (NOD/ShiLtJGpt- Prkdc em26Cd52 IL2rg em26Cd22 Ace2 em1Cin(hACE2) /GptCRL) or mouse ACE2 (NOD- Prkdc em26Cd52 Il2rg em26Cd22 /NjuCrl) triple-immunodeficient (NCG) mice, which lack functional mouse T-, B-, and natural killer (NK) cells, and have reduced numbers of dendritic cells and macrophages ( Eberle et al, 2020 ; Tarpinian et al, 2020 ). Myeloablation of NCG mice followed by reconstitution with human umbilical cord blood-derived CD34 + stem cells generates double-humanized huCD34 + -hACE2-NCG or huCD34 + -mACE2-NCG mice, which have a broad range of circulating and tissue-associated human immune cells ( Shultz et al, 2007 ).…”