ABSTRACT
Background: Following natural infection or vaccination, the generation of stem cell-like memory T (Tscm) cells is essential for long-term protective immunity to the virus. Tscm cells have the capacity for self-renewal and multipotency. In SARS-CoV2 infection, the emergence of CD8+ Tscm cells is correlated with the number of symptom-free days. The development of a COVID-19 vaccine able to generate CD8+ Tscm cells is of the utmost importance since the emergence of SARS-CoV2 variants of concerns requires maintaining strong and long-lasting immune responses, 2) as an efficient alternative in immunocompromised people who have difficulties raising humoral immune responses. Methods: We have developed a new Dendritic Cell-based vaccine composed of a humanized αCD40 monoclonal antibody fused to the RBD protein in its C-terminal Fc-domains and three T cell epitopes spanning sequences from S and N proteins in its light chains (αCD40-CoV2). Previous studies have shown that this platform elicited durable and robust T-and B-cell responses and is currently in phase I clinical development in HIV. We tested the capacity of two injections of the vaccine (10υg, i.p) given with or without poly(IC) (50υg, i.p) at 3 weeks apart to i) elicit human (hu) B-, and huT-cell responses in NSG mice reconstituted with a Human Immune System (HIS mice), ii) protect against SARS-CoV2 infection in the hCD40xK18hACE2 transgenic mice. Results: We performed AIM assays and intracellular staining on spleen cells of HIS mice stimulated with overlapping peptide pools spanning the sequences of vaccine antigens. We found that both non-adjuvanted and adjuvanted vaccine efficiently induced SARS-CoV2-specific Th1 huCD4+ and huCD8+ T cells in all vaccinees compared to mock animals. SARS-CoV2-specific huCD4+ T cells were polyfunctional. We confirmed the presence of RBD-specific huCD8+ T cells in the vaccinated animals using HLA-I tetramers. A significant proportion of the multimer+ huCD8+ T cells were Tscm (CD45RA+ CD62L+ CD95+) cells in both vaccinated groups. Besides, we detected significant amounts of spike-IgG+ switched huB cells in all vaccinees. In SARS-CoV2 challenge experiments, we further showed that both vaccination settings significantly protected animals with a survival rate of 100%. Conclusion: We demonstrate that the targeting of SARS-CoV-2 epitopes to CD40 induces significant B and T cells with a long-term memory phenotype in HIS mice and the ability of the vaccine to ensure complete protection against SARS-CoV2 infection.