A SCID Mouse Model To Evaluate the Efficacy of Antivirals against SARS-CoV-2 Infection

Abstract

Ancestral severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) lacks the intrinsic ability to bind to the mouse ACE2 receptor, and therefore establishment of SARS-CoV-2 mouse models has been limited to the use of mouse-adapted viruses or genetically modified mice. Interestingly, some of the variants of concern, such as the Beta B.1.351 variant, show an improved binding to the mouse receptor and hence better replication in different wild-type (WT) mouse species. Here, we describe the establishment of a SARS-CoV-2 Beta B.1.351 variant infection model in male SCID mice as a tool to assess the antiviral efficacy of potential SARS-CoV-2 small-molecule inhibitors. Intranasal infection of male SCID mice with 10⁵ 50% tissue culture infective doses (TCID₅₀) of the Beta B.1.351 variant resulted in high viral loads in the lungs and moderate signs of lung pathology on day 3 postinfection. Treatment of infected mice with the antiviral drugs molnupiravir (200 mg/kg, twice a day [BID]) or nirmatrelvir (300 mg/kg, BID) for 3 consecutive days significantly reduced the infectious virus titers in the lungs by 2 and 3.9 log₁₀ TCID₅₀/mg of tissue, respectively, and significantly improved lung pathology. Together, these data demonstrate the validity of this SCID mouse Beta B.1.351 variant infection model as a convenient preclinical model for assessment of potential activity of antivirals against SARS-CoV-2. IMPORTANCE Unlike the ancestral SARS-CoV-2 strain, the Beta (B.1.351) variant of concern has been reported to replicate to some extent in WT mice (C57BL/6 and BALB/c). We demonstrate here that infection of SCID mice with the Beta variant resulted in high viral loads in the lungs on day 3 postinfection. Treatment of infected mice with molnupiravir or nirmatrelvir for 3 consecutive days markedly reduced the infectious virus titers in the lungs and improved lung pathology. The SARS-CoV2 SCID mouse infection model, which is ideally suited for antiviral studies, offers an advantage in comparison to other SARS-CoV2 mouse models, in that there is no need for the use of mouse-adapted virus strains or genetically modified mice. Mouse models also have advantages over hamster models because (i) lower amounts of test drugs are needed, (ii) more animals can be housed in a cage, and (iii) reagents to analyze mouse samples are more readily available than those for hamsters.

Keywords: Beta variant; SARS-CoV-2; antivirals; molnupiravir; mouse model; nirmatrelvir.

FIG 1

Replication of Beta (B.1.351) SARS-CoV-2 in different mouse strains. (A) Infectious viral titers in the lungs of male SCID, male BALB/c, and male C57BL/6 mice infected with 10⁵ TCID₅₀ of Beta SARS-CoV-2 variants at 3 days p.i. are expressed in log₁₀ TCID₅₀ per milligram of lung tissue. Individual data and median values are presented. (B) Weight change at day 3 p.i. as a percentage, normalized to the body weight at the time of infection. Bars represent means ± standard deviations (SD). (C) Cumulative severity score from H&E-stained slides of lungs from infected mice at day 3 p.i. Individual data are presented, and bars represent means ± SD. The dotted line represents the mean score of untreated noninfected mice. Data were analyzed with the Kruskal-Wallis test. ns, nonsignificant; **, P < 0.01. Data are from two independent experiment with n = 9 per group, d3 pi = day 3 postinfection.

FIG 2

Replication kinetics of Beta (B.1.351) SARS-CoV-2 in male SCID mice. (A) Infectious viral loads in the lungs of male SCID mice infected with 10⁵ TCID₅₀ of Beta SARS-CoV-2 variants at different days postinfection were expressed as the log₁₀ TCID₅₀ per milligram of lung tissue. Individual data and median values are presented. (B) Weight change at different days postinfection as a percentage, normalized to the body weight at the time of infection. Bars represent means ± SD. (C) Cumulative severity score from H&E-stained slides of lungs from infected mice at different days postinfection. Individual data are presented, and bars represent means ± SD. The dotted line represents the mean score of untreated noninfected mice. (D) Representative H&E image of lung from SCID mouse infected with the Beta variant at day 3 p.i., showing limited peribronchial inflammation (blue arrows), significant perivascular inflammation (red arrows), and intra-alveolar hemorhage (green arrow). Bar, 100 μm. Data were analyzed with the Kruskal-Wallis test. ns, nonsignificant; *, P < 0.05; ***, P < 0.001. All data are from 2 independent experiments with 10 animals per group, except for day 1 data in panel A and the data in panel C.

FIG 3

Molnupiravir (EIDD-2801) and nirmatrelvir (PF-332) reduced viral loads in the lungs of Beta (B.1.351) SARS-CoV-2–infected SCID mice. (A) Setup of the study. (B) Viral RNA levels in the lungs of control (vehicle-treated), EIDD-2801-treated (200 mg/kg, BID), and PF-332-treated (300 mg/kg, BID) SCID mice infected with SARS-CoV-2 (B.1.351) at day 3 p.i. are expressed as log₁₀ SARS-CoV-2 RNA copies per milligram of lung tissue. Individual data and median values are presented. (C) Infectious viral loads in the lungs of control (vehicle-treated), EIDD-2801-treated, and PF-332-treated beta SARS-CoV-2−infected SCID mice at day 3 p.i. are expressed as the log₁₀ TCID₅₀ per milligram of lung tissue. Individual data and median values are presented. (D) Cumulative severity scores from H&E-stained slides of lungs from control (vehicle-treated), EIDD-2801-treated (200 mg/kg, BID), and PF-332-treated (300 mg/kg, BID) SARS-CoV-2−infected SCID mice at day 3 p.i. Individual data are presented, and bars represent means ± SD. The dotted line represents the mean score of untreated noninfected mice. (E) Weight change at day 3 p.i. as a percentage, normalized to the body weight at the time of infection. Bars represent means ± SD. Data were analyzed with the Kruskal-Wallis test. *, P < 0.05; ***, P < 0.001; ****, P < 0.0001; ns, nonsignificant. All data are from two independent experiments with 14 animals per group, except for the vehicle group (n = 18).