Clec9A-RBD immunization could trigger robust and sustained systemic and mucosal immune responses against rapidly evolving SARS-CoV-2 variants

About the study

In the present study, researchers investigated the cellular and humoral immune responses induced by a single systemic dosage of Clec9A-SARS-CoV-2 receptor-binding domain (RBD) antibody construct in mice.

The team determined that 2 µg of poly I:C-adjuvanted Clec9A-RBD elicited the strongest anti-RBD neutralizing titers among adult Balb/c mice after a single dose. A longitudinal examination of the RBD-specific humoral response in mice inoculated with Clec9A-RBD was performed. The neutralizing activity of Clec9A-RBD immunological sera was then evaluated against a panel of 20 sarbecoviruses, including SARS-CoV-2 variants of concern (VOCs), SARS-CoV, and animal CoV belonging to pangolin and bat lineages.

RBD-specific T follicular helper (TFH) cells and germinal centers (GC) B cells elicited were measured. The team then examined the RBD-specific T-cell responses following a single dose of Clec9A-RBD vaccination. The team tested whether a single dosage of Clec9A-RBD administered systemically could induce antigen-specific humoral and cellular responses in lung tissue.

Results

The study results showed that vaccination with an identical antigen dose of poly I:C-adjuvanted non-targeting purified recombinant SARS-CoV-2 RBD did not elicit a considerable anti-RBD immunoglobulin (Ig)-G response. On the other hand, Clec9A-RBD-vaccinated mice elicited detectable antibody titers seven days after immunization, highlighting the efficacy of this targeting technique.

Anti-RBD IgG titers persisted for up to 21 months after vaccination, with a distinct T-helper 1 (TH1) isotype profile. In addition, the antibody-dependent cell-mediated cytotoxicity (ADCC) and neutralizing properties, along with the binding affinity of the immunological sera against the corresponding SARS-CoV-2 ancestral strain, remained over the course of 21 months while gradually increasing over time. These findings strongly imply that Clec9A-RBD-vaccinated animals underwent a prolonged affinity maturation.

The neutralizing activity elicited against most SARS-CoV-2 VOCs rose over time and remained for the duration of the 21-month monitoring timespan, except for the SARS-CoV-2 Omicron variant. Intriguingly, the neutralizing activity observed against clade-1B sarbecoviruses belonging to pangolin and bat species was detectable at all tested time points and grew significantly throughout the 21-month monitoring period. On the other hand, the neutralizing activity against clade-1A sarbecoviruses was low.

The binding affinity of Clec9A-RBD immunological sera against SARS-CoV-2 VOCs rose over time and persisted for 21 months after vaccination. Likewise, ADCC activity reported against SARS-CoV-2 VOCs rose with time and remained elevated 21 months after vaccination. Furthermore, the ADCC activity of immunological sera against Omicron was equivalent to that evaluated against other VOCs.

Antigen-specific spleen TFH cell responses were elicited as early as seven days after Clec9A-RBD vaccination and were still detectable six to 12 months later. These responses were cross-reactive with most SARS-CoV-2 VOCs, such as Omicron. Also, almost two weeks and seven days post-immunization, the team detected GCs in the brachial lymph nodes and spleen, respectively, which survived even after six to 12 months.

Clec9A-RBD-inoculated mice produced considerably more interferon (IFN)-Ɣ-secreting cells when splenocytes were restimulated with RBD peptides. This demonstrated the superior ability to target Clec9A to trigger cellular immunological responses. The comparative abundance of T cells expressing IFN-Ɣ and interleukin (IL)-5 in animals immunized with Clec9A-RBD demonstrated significant TH1-biased cellular responses against corresponding SARS-CoV-2 VOCs and strain, respectively.

Anti-RBD IgG titers with a robust TH1 isotype profile were found in bronchoalveolar lavage (BAL) fluids collected two weeks after Clec9A-RBD vaccination. In addition, the BAL samples exhibited neutralizing as well as ADCC activity against the corresponding ancestral SARS-CoV-2 strain. The detection of RBD-specific GC B cells in the lung is compatible with the RBD-specific antibody production in this tissue.

Conclusion

The study findings showed that targeting SARS-CoV-2 RBD to cDC1s led to robust and extraordinarily long-lasting immune responses after a single vaccination dose. Significantly, the study revealed evidence of affinity development through time, which led to enhanced efficacy and magnitude of the humoral response against a wide range of sarbecovirus family members. The Clec9A targeting technology offers a flexible, plug-and-play vaccine platform that can be swiftly updated to combat rapidly developing variations circulating worldwide. The researchers believe that the human application of this vaccine approach is a feasible and interesting avenue.

*Important notice

bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

*Important notice: bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.