Emerging role of complement in COVID-19 and other respiratory virus diseases.

The complement system, a key component of innate immunity, provides the first line of defense against bacterial infection; however, the COVID-19 pandemic has revealed that it may also engender severe complications in the context of viral respiratory disease. Here, we review the mechanisms of complement activation and regulation and explore their roles in both protecting against infection and exacerbating disease. We discuss emerging evidence related to complement-targeted therapeutics in COVID-19 and compare the role of the complement in other respiratory viral diseases like influenza and respiratory syncytial virus. We review recent mechanistic studies and animal models that can be used for further investigation. Novel knockout studies are proposed to better understand the nuances of the activation of the complement system in respiratory viral diseases.

Enhanced complement activation and MAC formation accelerates severe COVID-19.

Emerging evidence indicates that activation of complement system leading to the formation of the membrane attack complex (MAC) plays a detrimental role in COVID-19. However, their pathogenic roles have never been experimentally investigated before. We used three knock out mice strains (1. C3-/-; 2. C7-/-; and 3. Cd59ab-/-) to evaluate the role of complement in severe COVID-19 pathogenesis. C3 deficient mice lack a key common component of all three complement activation pathways and are unable to generate C3 and C5 convertases. C7 deficient mice lack a complement protein needed for MAC formation. Cd59ab deficient mice lack an important inhibitor of MAC formation. We also used anti-C5 antibody to block and evaluate the therapeutic potential of inhibiting MAC formation. We demonstrate that inhibition of complement activation (in C3-/-) and MAC formation (in C3-/-. C7-/-, and anti-C5 antibody) attenuates severe COVID-19; whereas enhancement of MAC formation (Cd59ab-/-) accelerates severe COVID-19. The degree of MAC but not C3 deposits in the lungs of C3-/-, C7-/- mice, and Cd59ab-/- mice as compared to their control mice is associated with the attenuation or acceleration of SARS-CoV-2-induced disease. Further, the lack of terminal complement activation for the formation of MAC in C7 deficient mice protects endothelial dysfunction, which is associated with the attenuation of diseases and pathologic changes. Our results demonstrated the causative effect of MAC in severe COVID-19 and indicate a potential avenue for modulating the complement system and MAC formation in the treatment of severe COVID-19.

Deficiency of Tlr7 and Irf7 in mice increases the severity of COVID-19 through the reduced interferon production.

Toll-like receptor 7 (Tlr7) deficiency-accelerated severe COVID-19 is associated with reduced production of interferons (IFNs). However, the underlying mechanisms remain elusive. To address these questions, we utilize Tlr7 and Irf7 deficiency mice, single-cell RNA analysis together with bone marrow transplantation approaches. We demonstrate that at the early phase of infection, SARS-CoV-2 causes the upregulation of Tlr7, Irf7, and IFN pathways in the lungs of the infected mice. The deficiency of Tlr7 and Irf7 globally and/or in immune cells in mice increases the severity of COVID-19 via impaired IFN activation in both immune and/or non-immune cells, leading to increased lung viral loads. These effects are associated with reduced IFN alpha and gamma levels in the circulation. The deficiency of Tlr7 tends to cause the reduced production and nuclear translocation of interferon regulatory factor 7 (IRF7) in the lungs of the infected mice, indicative of reduced IRF7 activation. Despite higher amounts of lung viral antigen, Tlr7 or Irf7 deficiency resulted in substantially reduced production of antibodies against SARS-CoV-2, thereby delaying the viral clearance. These results highlight the importance of the activation of TLR7 and IRF7 leading to IFN production on the development of innate and adaptive immunity against COVID-19.

Natural Killer Cells Do Not Attenuate a Mouse-Adapted SARS-CoV-2-Induced Disease in Rag2-/- Mice.

This study investigates the roles of T, B, and Natural Killer (NK) cells in the pathogenesis of severe COVID-19, utilizing mouse-adapted SARS-CoV-2-MA30 (MA30). To evaluate this MA30 mouse model, we characterized MA30-infected C57BL/6 mice (B6) and compared them with SARS-CoV-2-WA1 (an original SARS-CoV-2 strain) infected K18-human ACE2 (K18-hACE2) mice. We found that the infected B6 mice developed severe peribronchial inflammation and rapid severe pulmonary edema, but less lung interstitial inflammation than the infected K18-hACE2 mice. These pathological findings recapitulate some pathological changes seen in severe COVID-19 patients. Using this MA30-infected mouse model, we further demonstrate that T and/or B cells are essential in mounting an effective immune response against SARS-CoV-2. This was evident as Rag2-/- showed heightened vulnerability to infection and inhibited viral clearance. Conversely, the depletion of NK cells did not significantly alter the disease course in Rag2-/- mice, underscoring the minimal role of NK cells in the acute phase of MA30-induced disease. Together, our results indicate that T and/or B cells, but not NK cells, mitigate MA30-induced disease in mice and the infected mouse model can be used for dissecting the pathogenesis and immunology of severe COVID-19.

COVID-19 and influenza infections mediate distinct pulmonary cellular and transcriptomic changes.

SARS-CoV-2 infection can cause persistent respiratory sequelae. However, the underlying mechanisms remain unclear. Here we report that sub-lethally infected K18-human ACE2 mice show patchy pneumonia associated with histiocytic inflammation and collagen deposition at 21 and 45 days post infection (DPI). Transcriptomic analyses revealed that compared to influenza-infected mice, SARS-CoV-2-infected mice had reduced interferon-gamma/alpha responses at 4 DPI and failed to induce keratin 5 (Krt5) at 6 DPI in lung, a marker of nascent pulmonary progenitor cells. Histologically, influenza- but not SARS-CoV-2-infected mice showed extensive Krt5+ "pods" structure co-stained with stem cell markers Trp63/NGFR proliferated in the pulmonary consolidation area at both 7 and 14 DPI, with regression at 21 DPI. These Krt5+ "pods" structures were not observed in the lungs of SARS-CoV-2-infected humans or nonhuman primates. These results suggest that SARS-CoV-2 infection fails to induce nascent Krt5+ cell proliferation in consolidated regions, leading to incomplete repair of the injured lung.