Fusion protein-based COVID-19 vaccines exemplified by a chimeric vaccine based on a single fusion protein (W-PreS-O)

Abstract

In this article we discuss characteristics of fusion protein-based SARS-CoV-2 vaccines. We focus on recombinant vaccine antigens comprising fusion proteins consisting of combinations of SARS-CoV-2-derived antigens or peptides or combinations of SARS-CoV-2 antigens/peptides with SARS-CoV-2-unrelated proteins/peptides. These fusion proteins are made to increase the immunogenicity of the vaccine antigens and/or to enable special targeting of the immune system. The protein-based vaccine approach is exemplified solely in a proof of concept study by using W-PreS-O, a chimeric vaccine based on a single fusion protein (W-PreS-O), combining RBDs from Wuhan hu-1 wild-type and Omicron BA.1 with the hepatitis B virus (HBV)-derived PreS surface antigen adsorbed to aluminum hydroxide. The W-PreS-O vaccine was evaluated in Syrian hamsters which were immunized three times at three-week intervals with W-PreS-O or with aluminum hydroxide (placebo) before they were infected with Omicron BA.1. Neutralizing antibody (nAB) titers, weight, lung symptoms, and viral loads, as measured using RT-PCR in the upper and lower respiratory tracts, were determined. In addition, infectious virus titers from the lungs were measured using a plaque-forming assay. We found that W-PreS-O-vaccinated hamsters developed robust nABs against Omicron BA.1, showed almost no development of pneumonia, and had significantly reduced infectious virus titers in the lungs. Importantly, the viral loads in the nasal cavities of W-PreS-O-vaccinated hamsters were close to or above the PCR cycle threshold considered to be non-infectious. The data of our proof-of-concept study provides compelling evidence that the W-PreS-O vaccine has protective effect against Omicron BA.1 in a Syrian hamster in vivo infection model and thus support the promising results obtained also for other fusion protein-based SARS-CoV-2 vaccines.

Keywords: COVID-19; SARS-CoV-2; Syrian hamster; fusion protein-based vaccine; infection model; neutralizing antibodies; omicron; vaccine.

Figure 1

Neutralizing antibodies, body weight, and viral RNA load in oropharyngeal samples in Syrian hamsters. (A) Neutralizing antibody titers for the SARS-CoV-2 variant Omicron in sera (shown as a serum dilution, y-axis) were obtained at the indicated time points during immunization and after infection with virus. Titers below 1:8 were considered negative. Horizontal bars represent GMT values for each group (Green: vaccinated; blue: placebo; black: not infected=intact). The dots are results for each animal and the whiskers indicate standard deviations. Significant differences (p < 0.05) were determined with the Mann–Whitney test and are indicated. (B) Weight curves are presented as percentages of body weight change (y-axis) and (C) viral RNA contents in oropharyngeal swabs as cycle threshold C(t) values (y-axis) on the indicated days post-infection. C(t) values >40 were considered negative. The results are shown as mean values per group and standard deviations. Significant differences (p < 0.05) were determined and are indicated at observed time points.

Figure 2

Effect of vaccination with recombinant fusion protein W-PreS-O on SARS-CoV-2 RNA loads in the lungs, infectious virus titers in the lungs, and SARS-CoV-2 RNA loads in the nasal cavities. (A) Viral RNA loads and (B) infectious virus titers in the lungs or (C) viral RNA loads in the nasal cavities of Syrian hamsters (y-axes) immunized three times with the W-PreS-O vaccine (green) or alum (placebo) (blue) at the indicated time points after infection (x-axes) with SARS-CoV-2 variant Omicron. Untreated and non-infected animals (intact) (black) served as controls. Viral loads are given as cycle threshold C(t) values, and were considered negative if >40. Infectious virus titers, determined via plaque titration assay, are presented as logPFU/g values. The results are shown for individual animals, with horizontal bars representing mean values and whiskers indicating standard deviations per group (n = 8). Significant differences (p < 0.05) were determined with the Mann–Whitney test and are indicated. n.s., not significant. Severity of lung lesions in the different groups of infected animals by histological assessment of pneumonia severity is presented in (D) as representative microphotographs (lines indicate 3 mm) and (E) pneumonia indices (y-axis). The results are shown for individual sections and were calculated from semi-quantitative scores of the lesion area and pneumonia intensity for each section of investigated lung (two sections per animal). Results are shown for individual animals, with horizontal bars representing mean values and whiskers corresponding to standard deviations per group. Significant differences (p < 0.05) were determined with the Mann–Whitney test and are indicated. n.s., not significant.