Long-term analysis of humoral responses and spike-specific T cell memory to Omicron variants after different COVID-19 vaccine regimens.

Background: The emergence of SARS-CoV-2 variants has raised concerns about the sustainability of vaccine-induced immunity. Little is known about the long-term humoral responses and spike-specific T cell memory to Omicron variants, with specific attention to BA.4/5, BQ.1.1, and XBB.1. Methods: We assessed immune responses in 50 uninfected individuals who received varying three-dose vaccination combinations (2X AstraZeneca + 1X Moderna, 1X AstraZeneca + 2X Moderna, and 3X Moderna) against wild-type (WT) and Omicron variants at eight months post-vaccination. The serum antibody titers were analyzed by enzyme-linked immunosorbent assays (ELISA), and neutralizing activities were examined by pseudovirus and infectious SARS-CoV-2 neutralization assays. T cell reactivities and their memory phenotypes were determined by flow cytometry. Results: We found that RBD-specific antibody titers, neutralizing activities, and CD4+ T cell reactivities were reduced against Omicron variants compared to WT. In contrast, CD8+ T cell responses, central memory, effector memory, and CD45RA+ effector memory T cells remained unaffected upon stimulation with the Omicron peptide pool. Notably, CD4+ effector memory T cells even exhibited a higher proportion of reactivity against Omicron variants. Furthermore, participants who received three doses of the Moderna showed a more robust response regarding neutralization and CD8+ T cell reactions than other three-dose vaccination groups. Conclusion: Reduction of humoral and CD4+ T cell responses against Omicron variants in vaccinees suggested that vaccine effectiveness after eight months may not have sufficient protection against the new emerging variants, which provides valuable information for future vaccination strategies such as receiving BA.4/5 or XBB.1-based bivalent vaccines.

Neutralizing antibodies targeting a novel epitope on envelope protein exhibited broad protection against flavivirus without risk of disease enhancement.

BACKGROUND: Flavivirus causes many serious public health problems worldwide. However, licensed DENV vaccine has restrictions on its use, and there is currently no approved ZIKV vaccine. Development of a potent and safe flavivirus vaccine is urgently needed. As a previous study revealed the epitope, RCPTQGE, located on the bc loop in the E protein domain II of DENV, in this study, we rationally designed and synthesized a series of peptides based on the sequence of JEV epitope RCPTTGE and DENV/ZIKV epitope RCPTQGE. METHODS: Immune sera were generated by immunization with the peptides which were synthesized by using five copies of RCPTTGE or RCPTQGE and named as JEV-NTE and DV/ZV-NTE. Immunogenicity and neutralizing abilities of JEV-NTE or DV/ZV-NTE-immune sera against flavivirus were evaluated by ELISA and neutralization tests, respectively. Protective efficacy in vivo were determined by passive transfer the immune sera into JEV-infected ICR or DENV- and ZIKV-challenged AG129 mice. In vitro and in vivo ADE assays were used to examine whether JEV-NTE or DV/ZV-NTE-immune sera would induce ADE. RESULTS: Passive immunization with JEV-NTE-immunized sera or DV/ZV-NTE-immunized sera could increase the survival rate or prolong the survival time in JEV-challenged ICR mice and reduce the viremia levels significantly in DENV- or ZIKV-infected AG129 mice. Furthermore, neither JEV -NTE- nor DV/ZV-NTE-immune sera induced antibody-dependent enhancement (ADE) as compared with the control mAb 4G2 both in vitro and in vivo. CONCLUSIONS: We showed for the first time that novel bc loop epitope RCPTQGE located on the amino acids 73 to 79 of DENV/ZIKV E protein could elicit cross-neutralizing antibodies and reduced the viremia level in DENV- and ZIKV-challenged AG129 mice. Our results highlighted that the bc loop epitope could be a promising target for flavivirus vaccine development.