Omicron-specific mRNA vaccine induced cross-protective immunity against ancestral SARS-CoV-2 infection with low neutralizing antibodies

Abstract

The major challenge in COVID-19 vaccine effectiveness is immune escape by SARS-CoV-2 variants. To overcome this, an Omicron-specific messenger RNA (mRNA) vaccine was designed. The extracellular domain of the spike of the Omicron variant was fused with a modified GCN4 trimerization domain with low immunogenicity (TSomi). After immunization with TSomi mRNA in hamsters, animals were challenged with SARS-CoV-2 virus. The raised nonneutralizing antibodies or cytokine secretion responses can recognize both Wuhan S and Omicron S. However, the raised antibodies neutralized SARS-CoV-2 Omicron virus infection but failed to generate Wuhan virus neutralizing antibodies. Surprisingly, TSomi mRNA immunization protected animals from Wuhan virus challenge. These data indicated that non-neutralizing antibodies or cellular immunity may play a more important role in vaccine-induced protection than previously believed. Next-generation COVID-19 vaccines using the Omicron S antigen may provide sufficient protection against ancestral or current SARS-CoV-2 variants.

Keywords: COVID-19; Omicron; SARS-CoV-2; mRNA vaccine.

Figure 1

SARS‐CoV‐2 TSomi RNA vaccine immunization induced a protective effect against Wuhan variant challenge in a hamster model. (A) Hamsters were immunized with 10 μg of TSomi mRNA, and serum samples were collected at Weeks 2, 4, and 6 after the first immunization. Total anti‐spike protein IgG titers were determined by (A) SARS‐CoV‐2 Wuhan TS protein or (B) SARS‐CoV‐2 Omicron TS protein antigen‐coating ELISA. 6‐week immunized hamster sera were used to determine neutralizing antibody titers against (C) the Wuhan SARS‐CoV‐2 or (D) the SARS‐CoV‐2 Omicron variant. (E) The TSomi mRNA‐vaccinated hamsters were challenged with Wuhan SARS‐CoV‐2 at Week 6 after immunization. The figure presents the percentage body weight change after virus challenge. (F) Viral titers in the lungs of infected hamsters at DPI 3 and DPI 6 were determined by TCID50 assay. (G) Histopathology of lungs from infected hamsters at 6 DPI was performed by hematoxylin and eosin staining. Scale bar for the low‐magnification image: 1 mm, scale bar for the high‐magnification image: 500 μm. Pathological severity scores of infected hamsters. The p value was calculated by t‐test. **p < 0.01 and ***p < 0. mRNA, messenger RNA; SARS‐CoV‐2, Severe acute respiratory syndrome coronavirus 2.

Figure 2

TSomi mRNA vaccination induced protective immunity against Wuhan SARS‐CoV‐2 challenge through the CD8⁺ T‐cell response. C57BL/6 mice were immunized with two doses of 2 μg of TSomi mRNA vaccine at two‐week intervals. (A) At the 4th week after immunization, splenocytes were pulsed with the indicated peptide pool derived from the ancestral Wuhan strain spike protein to induce IFN‐γ production. (B) TSomi mRNA vaccine‐immunized K18‐hACE2 transgenic mice were injected with 200 μg of isotype control IgG2a, anti‐CD8 antibodies (clone 53‐6.7) or anti‐NK1.1 (clone PK136) 7 and 3 days before virus challenge. The K18‐hACE2 transgenic mice were infected with 10³ SARS‐CoV‐2 TW04 (Wuhan variant) at 6 weeks after immunization. The percent body weight change in each group was monitored every day. (C) The survival rate of (B) was monitored until 11 DPI (n = 4). (D) K18‐hACE2 transgenic mice were immunized with 50 μL of vehicle or 2 μg of TSomi mRNA vaccine at 2‐week intervals. The immunized mice were injected with 200 μg of anti‐CD4 antibodies (clone GK1.5) at 7 and 3 days before virus challenge and 3 and 7 days after virus challenge. The percent body weight change in each group was monitored every day until 11 DPI (n = 5). The p value was calculated by two‐way ANOVA. (E) The survival rate of (D) was monitored until 11 DPI. The p value was calculated by the log‐rank (Mantel‒Cox) test. *p < 0.05 and ***p < 0.001. mRNA, messenger RNA; SARS‐CoV‐2, Severe acute respiratory syndrome coronavirus 2.