A recombinant spike-XBB.1.5 protein vaccine induces broad-spectrum immune responses against XBB.1.5-included Omicron variants of SARS-CoV-2

Abstract

The XBB.1.5 subvariant has drawn great attention owing to its exceptionality in immune evasion and transmissibility. Therefore, it is essential to develop a universally protective coronavirus disease 2019 vaccine against various strains of Omicron, especially XBB.1.5. In this study, we evaluated and compared the immune responses induced by six different spike protein vaccines targeting the ancestral or various Omicron strains of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in mice. We found that spike-wild-type immunization induced high titers of neutralizing antibodies (NAbs) against ancestral SARS-CoV-2. However, its activity in neutralizing Omicron subvariants decreased sharply as the number of mutations in receptor-binding domain (RBD) of these viruses increased. Spike-BA.5, spike-BF.7, and spike-BQ.1.1 vaccines induced strong NAbs against BA.5, BF.7, BQ.1, and BQ.1.1 viruses but were poor in protecting against XBB and XBB.1.5, which have more RBD mutations. In sharp contrast, spike-XBB.1.5 vaccination can activate strong and broadly protective immune responses against XBB.1.5 and other common subvariants of Omicron. By performing correlation analysis, we found that the NAbs titers were negatively correlated with the number of RBD mutations in the Omicron subvariants. Vaccines with more RBD mutations can effectively overcome the immune resistance caused by the accumulation of RBD mutations, making spike-XBB.1.5 the most promising vaccine candidate against universal Omicron variants.

Keywords: Omicron; RBD mutations; SARS‐CoV‐2; XBB.1.5; neutralizing antibody; recombinant protein vaccine.

FIGURE 1

(A) Schematic illustration of the mutations in receptor‐binding domain (RBD) proteins of various Omicron subvariants. (B) Numbers of mutations in the RBD of various Omicron subvariants. (C) Scheme of immunization. Mice (n = 6/group) were immunized with PBS, spike‐WT, spike‐Omicron BQ.1.1, spike‐Omicron BA.5, spike‐Omicron XBB, spike‐Omicron BF.7, or spike‐Omicron XBB.1.5 on days 0, 21, and 42 intramuscularly. On day 56, serum samples were collected for further studies.

FIGURE 2

Immunization with recombinant spike protein vaccines induced strong immunoglobulin G (IgG) responses. Detection of spike‐WT (A), spike‐BA.5 (B), spike‐BF.7 (C), spike‐BQ.1.1 (D), spike‐XBB (E), and spike‐XBB.1.5 (F)‐specific IgG antibody titers in the serum of mice immunized with corresponding vaccines. Left: absorbance values at 450 nm indicating the binding to serially diluted antibodies; right: endpoint titers of IgG antibodies in the sera of vaccinated animals (n = 6). Data are presented as geometric mean values ± SD.

FIGURE 3

Immune sera from mice immunized with spike‐XBB and spike‐XBB.1.5 neutralized the infection of multiple Omicron variants. Fifty percent neutralizing titers of immune sera from mice immunized with spike‐XBB.1.5 (A) or spike‐XBB (B) against luciferase‐expressing pseudoviruses of the WT strain or various Omicron subvariants, including BA.5, BF.7, BQ.1, BQ.1.1, XBB, and XBB.1.5. Fifty percent neutralization is defined as the dilution titer that neutralized 50% infection (n = 6). Data are presented as geometric mean values ± SD.

FIGURE 4

Immune sera from mice immunized with spike‐WT, spike‐BA.5, spike‐BF.7, or spike‐BQ.1.1 showed limited efficacy in neutralizing XBB and XBB.1.5 subvariants. Upper: 50% neutralizing titers of immune sera from mice immunized with spike‐WT (A), spike‐BA.5 (B), spike‐BF.7 (C), or spike‐BQ.1.1 (D) against luciferase‐expressing pseudoviruses of the WT strain or various Omicron subvariants, including BA.5, BF.7, BQ.1, BQ.1.1, XBB, and XBB.1.5. Fifty percent neutralization is defined as the dilution titer that neutralized 50% infection. Lower: Pearson correlation analysis between 50% neutralizing antibody titers and mutation numbers of RBD in Omicron sublineages (n = 6). Data are presented as geometric mean values ± SD.

FIGURE 5

Splenic T‐cell responses against receptor‐binding domain (RBD)‐XBB.1.5 in immunized mice. Mice (n = 5/group) were intramuscularly immunized with PBS, spike‐WT, spike‐Omicron BQ.1.1, spike‐Omicron BA.5, spike‐Omicron XBB, spike‐Omicron BF.7, or spike‐Omicron XBB.1.5 on days 0, 21, and 42 intramuscularly. On day 56, splenic lymphocytes were isolated and stimulated with RBD‐XBB.1.5 antigens for 48 h and analyzed with flow cytometry. The frequencies of CD4⁺CD44⁺IL‐4⁺ (A), CD4⁺CD44⁺IFN‐γ⁺ (B), CD8⁺CD44⁺IL‐4⁺ (C), and CD8⁺CD44⁺IFN‐γ⁺ (D) T cells. (C and D) Representative flow cytometry plots (left) and quantification (right) of interleukin‐4 (IL‐4) (C) and interferon‐γ (IFN‐γ) (D) expression on CD8⁺CD44⁺ T cells (n = 5). Data are presented as mean ± SEM. ^* p‐Values compared with the Spike‐XBB.1.5 group. ^# p‐Values compared with the PBS group. ^*/# p < 0.05, ^**/## p < 0.01, ^***/### p < 0.001, ^****/#### p < 0.0001. Black ns represents comparison with the PBS group. Red ns represents comparison with the Spike‐XBB.1.5 group.

FIGURE 6

Splenic T‐cell responses against receptor‐binding domain (RBD)‐BA.5 in immunized mice. On day 56, splenic lymphocytes from immunized mice were isolated and stimulated with RBD‐BA.5 antigens for 48 h and analyzed with flow cytometry. The frequencies of CD4⁺CD44⁺IL‐4⁺ (A), CD4⁺CD44⁺IFN‐γ⁺ (B), CD8⁺CD44⁺IL‐4⁺ (C), and CD8⁺CD44⁺IFN‐γ⁺ (D) T cells. (C and D) Representative flow cytometry plots (left) and quantification (right) of interleukin‐4 (IL‐4) (C) and interferon‐γ (IFN‐γ) (D) expression on CD8⁺CD44⁺ T cells (n = 5). Data are presented as mean ± SEM. ^* p‐Values compared with the Spike‐BA.5 group. ^# p‐Values compared with the PBS group. ^*/# p < 0.05, ^**/## p < 0.01, ^***/### p < 0.001, ^****/#### p < 0.0001. Black ns represents comparison with the PBS group. Red ns represents comparison with the Spike‐BA.5 group.