Live Imaging of SARS-CoV-2 Infection in Mice Reveals Neutralizing Antibodies Require Fc Function for Optimal Efficacy

Abstract

Neutralizing antibodies (NAbs) are effective in treating COVID-19 but the mechanism of immune protection is not fully understood. Here, we applied live bioluminescence imaging (BLI) to monitor the real-time effects of NAb treatment in prophylaxis and therapy of K18-hACE2 mice intranasally infected with SARS-CoV-2-nanoluciferase. We visualized sequential spread of virus from the nasal cavity to the lungs followed by systemic spread to various organs including the brain, culminating in death. Highly potent NAbs from a COVID-19 convalescent subject prevented, and also effectively resolved, established infection when administered within three days of infection. In addition to direct neutralization, in vivo efficacy required Fc effector functions of NAbs, with contributions from monocytes, neutrophils and natural killer cells, to dampen inflammatory responses and limit immunopathology. Thus, our study highlights the requirement of both Fab and Fc effector functions for an optimal in vivo efficacy afforded by NAbs against SARS-CoV-2.

Live Imaging of SARS-CoV-2 Infection in Mice Reveals Neutralizing Antibodies Require Fc Function for Optimal Efficacy

“…However, while mouse FcγRs bind human antibodies with similar affinities as mouse antibodies, their expression pattern on various mouse immune cells differs from those in humans. Therefore, additional investigations in other models are required to confirm definitive absence of ADE during human SARS-CoV-2 infection (Gorman et al, 2021). Moreover, elucidation of the major FcγR(s) (FcγRI, FcγRIII and/or FcγRIV) engaged by NAbs will help design ultrapotent SARS-CoV-2 Nab therapies (Smith et al, 2012).…”

“…Tomographic tilt-series and large-area montaged overviews were acquired automatically using the SerialEM software package (Mastronarde, 2005(Mastronarde, , 2008Mastronarde and Held, 2017). For tomography, samples were tilted +/-62° and images collected at 1° intervals.…”

“…The grid was then rotated 90° and a similar series taken about the orthogonal axis. Tomographic data was calculated, analyzed, and modeled using the IMOD software package (Mastronarde, 2005(Mastronarde, , 2008 Mastronarde and Held, 2017) on iMac Pro and MacPro computers (Apple, Inc., Cupertino, CA).…”

“…This allows for a rapid turnaround and a high bar for identifying effective prophylactic or therapeutic intervention strategies. In susceptible humans, SARS-CoV-2 infection disables innate immunity and elicits an imbalanced inflammatory cytokine response in the lungs leading to acute respiratory distress syndrome (ARDS) which is the major cause of death (Graham and Baric, 2020). Lethality in K18-hACE2 mice was initially associated with lung inflammation, cytokine storm and impaired respiratory function (Winkler et al, 2020).…”

“…A BLI-driven platform has not been harnessed for studying infectious respiratory pathogens like SARS-CoV-2 that require level 3 biosafety containment. Here, we have established a BLI-driven approach to study SARS-CoV-reporter in the place of the Orf7A gene (Xie et al, 2020a;Xie et al, 2020b). SARS-CoV-2-nLuc closely mimics the wildtype virus replication kinetics and stably maintains the nLuc reporter over five generations in vitro.…”