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 during prophylaxis and therapy of K18-hACE2 mice intranasally infected with SARS-CoV-2-nanoluciferase. Real time imaging revealed that the virus spread sequentially from the nasal cavity to the lungs in mice and thereafter systemically 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. In addition to direct neutralization, depletion studies indicated that Fc effector interactions of NAbs with monocytes, neutrophils and natural killer cells were required to effectively dampen inflammatory responses and limit immunopathology. Our study highlights that both Fab and Fc effector functions of NAbs are essential for optimal in vivo efficacy against SARS-CoV-2.

Protein functions are often revealed by their localization to specialized cellular sites. Recent reports demonstrated that swiprosin-1 is found together with actin and actin-binding proteins in the cytoskeleton fraction of human mast cells and NK-like cells. However, direct evidence of whether swiprosin-1 regulates actin dynamics is currently lacking. We found that swiprosin-1 localizes to microvilli-like membrane protrusions and lamellipodia and exhibits actin-binding activity. Overexpression of swiprosin-1 enhanced lamellipodia formation and cell spreading. In contrast, swiprosin-1 knockdown showed reduced cell spreading and migration. Swiprosin-1 induced actin bundling in the presence of Ca2+, and deletion of the EF-hand motifs partially reduced bundling activity. Swiprosin-1 dimerized in the presence of Ca2+ via its coiled-coil domain, and a lysine (Lys)-rich region in the coiled-coil domain was essential for regulation of actin bundling. Consistent with these observations, mutations of the EF-hand motifs and coiled-coil region significantly reduced cell spreading and lamellipodia formation. We provide new evidence of how swiprosin-1 influences cytoskeleton reorganization and cell spreading.

The implementation of connection boundary for the hybrid implicit-explicit finite-difference time-domain (HIE-FDTD) method is discussed in this article. It shows that the incident field of the HIE-FDTD method must be split into two time steps. Compared with the implementation method commonly used in the conventionalThe finite-difference time-domain (FDTD) method [1] has been proven to be an effective means that provides accurate predictions of field behaviors for varieties of electromagnetic interaction problems. However, as it is based on an explicit finitedifference algorithm, the Courant-Friedrich-Levy (CFL) condition [2] must be satisfied when this method is used. Therefore, a maximum time-step size is limited by minimum cell size in a computational domain, which makes this method inefficient for the problems where fine scale dimensions are used.To overcome the Courant limit on the time-step size of the FDTD method, unconditionally stable methods such as the alternating-direction implicit FDTD (ADI-FDTD) scheme [3][4][5] have been studied extensively. Although the time-step size in the ADI-FDTD simulation is no longer bounded by the CFL criterion, the method exhibits a splitting error [6,7] that is proportional to the square of the time-step size and the spatial derivatives of the field. When field variation and/or the timestep size is large, the splitting error becomes pronounced. The accuracy of the ADI-FDTD method is limited. Besides, in the ADI-FDTD scheme, three time steps are used for defining the field components, and two subiterations are required for field advancement. It must solve six tridiagonal matrices and six explicit update for one full update cycle, which makes the ADI-FDTD computationally inefficient.Based on the theory of ADI-FDTD method, a novel threedimensional hybrid implicit-explicit finite-difference time-domain (HIE-FDTD) method has been developed recently [8][9][10][11][12][13]. In this method, the CFL condition is not removed totally, being weaker than that of the conventional FDTD method. The time step in this scheme is only determined by two space discretizations, which is extremely useful for problems where a very fine mesh is needed in one direction. In the HIE-FDTD method, only a single iteration (with two tridiagonal matrices and four explicit update) is needed for the field development. The HIE-FDTD method has better accuracy and higher computation efficiency than the ADI-FDTD method, especially for larger field variation. When the same timestep size is maintained, the CPU time for the HIE-FDTD method can be reduced to about half of that for the ADI-FDTD method [10].Because of the important impact of HIE-FDTD method on the electromagnetic computation, an accurate and efficient connection boundary condition must be developed to simulate electromagnetic interaction. This article gives two implementation strategies of the connection boundary, one uses split incident field and the other does not. By comparing the two implementation strategies, it shows that the split in...

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.

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