Cross-Reactive Profile Against Two Conserved Coronavirus Antigens in Sera from SARS-CoV-2 Hybrid and Vaccinated Immune Donors.

Since the beginning of the pandemic, the pre-existing immunity against SARS-CoV-2 has been postulated as one possible cause of asymptomatic infections. Later, various works reported that pre-existing immune response against the two structural conserved antigens: S2 subunit and the nucleocapsid protein, were associated to some level of asymptomatic profile in infected individuals. To explore the Ab background against these two antigens, in the context of vaccine-elicited and hybrid (natural infection plus vaccination induced) immunity of SARS-CoV-2, in this work, we tested sera from inactivated vaccine-immunized donors and from vaccinated and subsequent natural infected donors upon the Omicron variant wave in Guangdong province, China. Serum samples were collected from 27 COVID-19 convalescent, 25 SARS-CoV-2 vaccinated, and 10 negative donors. The IgG cross-reactivity response against these two antigens from another relevant human coronavirus (HCoV) was also evaluated. The findings indicate that IgG response against S2 and N protein was particularly higher in sera with hybrid immunity. The cross-reactive Abs were more significant against SARS-CoV-1, while a wide cross-reactivity was detected for N antigen for one human Alpha coronavirus HCoV-229E even in the negative control samples. The presence of cross-reactive Abs against the two conserved antigens N and S2, particularly in the context of hybrid immunity, could pave the way for future boosted vaccines carrying these conserved regions.

Broad Humoral Immunity Generated in Mice by a Formulation Composed by Two Antigens From Delta Variant of SARS-CoV-2 (preprint)

Due to the rapid development of new variants of SARS-CoV-2 virus, as well as the real threat of new coronavirus zoonosis events, the development of a preventive vaccine with a broader scope of functionality is highly desirable. Previously, we reported the functionality of a nasal formulation based on the preparation of the nucleocapsid protein with the ODN-39M and combined with RBD, both antigens from Delta variant of SARSCoV-2. This combination induces a cross-reactive immunity in mucosal and systemic compartments until sarbecovirus level. In the present study, we explored the magnitude of the immunity generated in Balb/C mice by the same formulation, but adding alum as adjuvant, so as to enhance the humoral immunity against the two antigens. Animals were immunized with three doses of the bivalent formulation, administered by subcutaneous route. The humoral immunity was tested by ELISA and by a Surrogate of Viral Neutralization test. The cell-mediated immunity was also explored. High levels of antibodies against both antigens (N and RBD) were obtained upon immunization. Additionally, the anti-RBD Abs with neutralizing capacity reacted against the three SARS-CoV-2 variants of RBD assayed, including Omicron. At the same time, the Abs also recognized the nucleocapsid proteins from: SARS-CoV-1 and SARS-CoV-2 Delta and Omicron. Taken together, these results make the bivalent formulation tested, an attractive component of a pancorona vaccine able to broaden the scope of humoral immunity against both antigens. This will be particularly important in the reinforcement of immunity from previously vaccinated and/or infected populations.