The article presents a comprehensive study on the reverse genetics system for SARS-CoV-2, which was used to generate reporter viruses to investigate the virus's pathogenesis and tropism in the human respiratory tract. Key findings include: 1. **Reverse Genetics System**: A full-length infectious cDNA clone of SARS-CoV-2 was generated, and two reporter viruses (GFP and luciferase) were constructed to study viral replication and neutralization. 2. **Viral Replication**: The recombinant viruses replicated similarly to the clinical isolate in vitro, with similar plaque sizes and serial passage capabilities. 3. **Protease Influence**: Serine proteases TMPRSS2 and furin enhanced viral replication, while trypsin did not. 4. **Neutralization Assays**: Neutralizing antibodies from SARS and MERS patients showed limited cross-neutralization against SARS-CoV-2, and convalescent COVID-19 sera had variable neutralization capabilities. 5. **ACE2 and TMPRSS2 Expression**: RNA in situ hybridization revealed that ACE2 expression was highest in the nose and decreased in the lower respiratory tract, with a gradient of SARS-CoV-2 infection. 6. **In Vivo Infection**: Autopsy studies showed patchy, segmental, and peripheral infection in the lungs, with ciliated cells in the nose and AT2 cells in the lungs being primary targets. 7. **Cystic Fibrosis (CF)**: CF lungs showed upregulation of ACE2 and TMPRSS2, suggesting potential vulnerability to SARS-CoV-2 infection. 8. **Pathogenesis Hypotheses**: The study supports the hypothesis that the nose is the initial site of infection, followed by aspiration into the lungs, and discusses the implications for disease severity and long-term outcomes. 9. **Reagents and Strategies**: The study provides valuable reagents for future research, including a high-throughput neutralization assay and insights into potential therapeutic strategies. Overall, the research highlights the nasal susceptibility to SARS-CoV-2 and the importance of understanding virus-host interactions in protective immunity, host susceptibility, and virus pathogenesis.The article presents a comprehensive study on the reverse genetics system for SARS-CoV-2, which was used to generate reporter viruses to investigate the virus's pathogenesis and tropism in the human respiratory tract. Key findings include: 1. **Reverse Genetics System**: A full-length infectious cDNA clone of SARS-CoV-2 was generated, and two reporter viruses (GFP and luciferase) were constructed to study viral replication and neutralization. 2. **Viral Replication**: The recombinant viruses replicated similarly to the clinical isolate in vitro, with similar plaque sizes and serial passage capabilities. 3. **Protease Influence**: Serine proteases TMPRSS2 and furin enhanced viral replication, while trypsin did not. 4. **Neutralization Assays**: Neutralizing antibodies from SARS and MERS patients showed limited cross-neutralization against SARS-CoV-2, and convalescent COVID-19 sera had variable neutralization capabilities. 5. **ACE2 and TMPRSS2 Expression**: RNA in situ hybridization revealed that ACE2 expression was highest in the nose and decreased in the lower respiratory tract, with a gradient of SARS-CoV-2 infection. 6. **In Vivo Infection**: Autopsy studies showed patchy, segmental, and peripheral infection in the lungs, with ciliated cells in the nose and AT2 cells in the lungs being primary targets. 7. **Cystic Fibrosis (CF)**: CF lungs showed upregulation of ACE2 and TMPRSS2, suggesting potential vulnerability to SARS-CoV-2 infection. 8. **Pathogenesis Hypotheses**: The study supports the hypothesis that the nose is the initial site of infection, followed by aspiration into the lungs, and discusses the implications for disease severity and long-term outcomes. 9. **Reagents and Strategies**: The study provides valuable reagents for future research, including a high-throughput neutralization assay and insights into potential therapeutic strategies. Overall, the research highlights the nasal susceptibility to SARS-CoV-2 and the importance of understanding virus-host interactions in protective immunity, host susceptibility, and virus pathogenesis.