Th2-Oriented Immune Serum After SARS-CoV-2 Vaccination Does Not Enhance Infection In Vitro

Abstract

The relatively lower protection rate of the alum-adjuvanted inactivated severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines reminds us of the antibody-dependent enhancement (ADE) phenomenon observed in preclinical studies during the development of vaccines for Middle East respiratory syndrome coronavirus (MERS-CoV) and severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1). In this study, using the S1 segment of the SARS-CoV-2 spike protein or inactivated whole SARS-CoV-2 virus as an antigen and aluminum as an adjuvant, the risk of ADE of infection with T helper 2 (Th2)-oriented immune serum from mice (N=6) and humans (N=5) was examined in immune cell lines, which show different expression patterns of Fc receptors. Neither the immune serum from alum-adjuvanted S1 subunit vaccines nor inactivated SARS-CoV-2 vaccination enhanced SARS-CoV-2 S pseudotyped virus infection in any of the tested cell lines in vitro. Because both of these Th2-oriented immune sera could block SARS-CoV-2 infection without ADE of infection, we speculate that the lower protection rate of the inactivated SARS-CoV-2 vaccine may be attributed to the lower neutralizing antibody titers induced or the pulmonary eosinophilic immunopathology accompanied by eosinophilic infiltration in the lungs upon virus exposure. Adjustment of the immunization schedule to elevate the neutralizing antibody levels and skew adjuvants toward Th1-oriented responses may be considered to increase the efficacies of both inactivated and spike protein-based subunit SARS-CoV-2 vaccines.

Trial registration: ClinicalTrials.gov NCT04510207 NCT04456595 NCT04582344 NCT04651790.

Keywords: Th2; aluminum adjuvant; antibody-dependent enhancement of infection; inactivated SARS-CoV-2 vaccine; spike protein subunit vaccine.

Figure 1

Alum induced a Th2-oriented immune response, whereas nucleic acids induced a Th1-oriented response when adjuvanted with the S1 protein. (A) Immunization schedule, N=6 in each group; (B) S1-specific IgG titers of immune serum; (C) S1-specific IgG1 and IgG2a titers in immune serum; (D) IgG1/IgG2a ratio of immune serum. All of the above titers were detected in pooled serum from each immunized group, and the means and standard deviations of duplicates are shown.

Figure 2

Fc receptor expression patterns in immune cell lines. (A) The protein expression levels of ACE2, FcγR1 and FcγR2 were detected by Western blot. (B) The gene expression of ACE2 and more specific FcγR subtypes, including FcγR1A, FcγR2A, and FcγR2B, was detected by RT-PCR.

Figure 3

Th2-oriented immune serum after S1-based subunit vaccine administration does not enhance infection in vitro. (A) Pseudovirus-based neutralization assay of immune serum in Vero cells that express ACE2. (B–E) Assay of pseudovirus-based antibody-dependent enhancement of infection with immune serum in different FcγR-expressing cells. Poly(I:C)+CpG, nucleic acid-adjuvanted plus S1-purified protein immune serum; Alum, alum-adjuvanted plus S1-purified protein immune serum; Sham, serum from mice intramuscularly administered PBS instead of immunogens. All of the above analyses were performed using pooled serum from each immunized group. The means and standard deviations of duplicates are shown.

Figure 4

Th2-oriented immune serum after SARS-CoV-2 inactivated vaccine administration does not enhance infection in vitro. (A) Pseudovirus-based neutralization assay of human serum with low to high authentic SARS-CoV-2 neutralization titers (from 4 to 256) in Vero cells that express ACE2. The sample ID and authentic SARS-CoV-2 neutralization titer are shown in the table on the right. (B–E) Assay of pseudovirus-based antibody-dependent enhancement of infection with human serum in cells with different FcγR expression patterns. V1-V5, human sera after inactivated SARS-CoV-2 vaccination; HSC1-6: human convalescent serum. Each sample is shown separately.