HI, a fusion protein that consists of the alpha-toxin (Hla) and the N2 domain of iron surface determinant B (IsdB), is one of the antigens in the previously reported S. aureus vaccine rFSAV and has already entered phase II clinical trials. Previous studies revealed that HI is highly immunogenic in both mice and healthy volunteers, and the humoral immune response plays key roles in HI-mediated protection. In this study, we further investigated the protective efficacy of immunization with HI plus four different adjuvants in a mouse bacteremia model. Results showed that HI-mediated protection was altered in response to different adjuvants. Using antisera from immunized mice, we identified seven B-cell immunodominant epitopes on Hla and IsdB, including 6 novel epitopes (Hla1-18, Hla84-101, Hla186-203, IsdB342-359, IsdB366-383, and IsdB384-401). The immunodominance of B-cell epitopes, total IgG titers and the levels of IFN-γ and IL-17A from mice immunized with HI plus different adjuvants were different from each other, which may explain the difference in protective immunity observed in each immunized group. Thus, our results indicate that adjuvants largely affected the immunodominance of epitopes and the protective efficacy of HI, which may guide further adjuvant screening for vaccine development and optimization.

We previously reported that monoclonal antibody (mAb) cocktail improves survival in Staphylococcus aureus infection. In this study, we used acute pneumonia model and lethal sepsis model to investigate the efficacy of antibiotic combined with epitopespecific mAb cocktail in treating MRSA252 infection. Methods: MRSA252 was challenged by tail vein injection or tracheal intubation to establish sepsis model or pneumonia model. One hour after infection, the mice received a single intravenous injection of normal saline, vancomycin, and vancomycin combined monoclonal antibody, linezolid alone or linezolid combined monoclonal antibody. Daily record survival rate (total 7 days), bacterial load, histology, cytokine analysis of serum and alveolar lavage fluid, and in vitro determination of the neutralizing ability of antibodies to SEB toxin and Hla toxin explained the mechanism of antibody action. Results: The mAb cocktail combined with low doses of vancomycin or linezolid improved survival rates in acute pneumonia model (70%, 80%) and lethal sepsis model (80%, 80%). Epitopespecific monoclonal antibodies reduced bacterial colonization in the kidneys and lungs of mice and inhibited the biological functions of the toxins Hla and SEB in vitro. Compared to the antibiotic alone or PBS groups, the combination group had higher levels of IL-1α, IL-1β and IFN-γ and lower levels of IL-6, IL-10, TNF-α. Further, the combination of antibiotic and mAb cocktail improved infection survival against the clinical MRSA isolates in a lethal sepsis model. Conclusion:This study demonstrates a novel method to treat people with low immunity against drug-resistant S. aureus infections.

The increase in confirmed COVID-19 cases and SARS-CoV-2 variants calls for the development of safe and broad cross-protective vaccines. The RBD of the spike protein was considered to be a safe and effective candidate antigen. However, the low immunogenicity limited its application in vaccine development. Herein, we designed and obtained an RBD heptamer (mHla-RBD) based on a carrier protein-aided assembly strategy. The molecular weight of mHla-RBD is up to 450 kDa, approximately 10 times higher than that of the RBD monomer. When formulated with alum adjuvant, mHla-RBD immunization significantly increased the immunogenicity of RBD, as indicated by increased titers of RBD-specific antibodies, neutralizing antibodies, Th2 cellular immune response, and pseudovirus neutralization activity, when compared to RBD monomer. Furthermore, we confirmed that RBD-specific antibodies predominantly target conformational epitopes, which was approximately 200 times that targeting linear epitopes. Finally, a pseudovirus neutralization assay revealed that neutralizing antibodies induced by mHla-RBD against different SARS-CoV-2 variants were comparable to those against the wild-type virus and showed broad-spectrum neutralizing activity toward different SARS-CoV-2 variants. Our results demonstrated that mHla-RBD is a promising candidate antigen for development of SARS-CoV-2 vaccines and the mHla could serve as a universal carrier protein for antigen design.

Background Previous studies have revealed that the receptor-binding domain (RBD) of the spike protein is immunogenic in both mice and healthy volunteers, and the humoral immune response plays key roles in RBD-mediated protection. In this study, we evaluated the immunodominant humoral response of RBD with different adjuvants and different immune routes in inducing neutralizing antibodies and immunodominant epitopes in RBD. Methods In this study, we investigated the protective efficacy of immunization with RBD plus three different adjuvants (Al(OH)3, ASO3 or AddaVax) and two different routes (intramuscular immunity or intranasal immunity) in a mouse model. Results The results showed that RBD-mediated protection was altered in response to different adjuvants; even with the same adjuvant, RBD-mediated protection was altered in different immune routes. Using antisera from immunized mice, we identified six B-cell immunodominant epitopes in the RBD, including 2 novel epitopes (RBD1 − 18 and RBD49 − 66) in intramuscular immunity and 3 novel epitopes (RBD31 − 48, RBD61 − 78, RBD97 − 114) in intranasal immunity. The B-cell immunodominant epitopes identified from mice immunized with RBD plus different adjuvants were also different from each other, which may explain the differences in protective immunity observed in each immunized group. Conclusions This study indicate that adjuvants and immune routes largely affect the immunodominance of epitopes and the protective efficacy of RBD, which may guide further adjuvant screening for vaccine development and optimization.