Th1 Dominant Nucleocapsid and Spike Antigen-Specific CD4+ and CD8+ Memory T Cell Recall Induced by hAd5 S-Fusion + N-ETSD Infection of Autologous Dendritic Cells from Patients Previously Infected with SARS-CoV-2

To address the need for a safe, efficacious vaccine against SARS-CoV-2 infection with the critical properties of enabling both blocking viral entry into cells and clearing virus from cells already infected, we have developed a bivalent, human adenovirus serotype 5 (hAd5) SARS-CoV- 2 S-Fusion + N-ETSD vaccine that is currently in clinical testing. This vaccine uses the next- generation hAd5 [E1-, E2b-, E3-] platform previously used successfully in cancer patients with pre-existing adenovirus immunity, engineered to express both SARS-CoV-2 spike (S) protein modified to improve the generation of neutralizing antibodies to block entry of the virus, and nucleocapsid (N) protein with an Enhanced T cell Stimulation Domain (ETSD) to activate CD4+ and CD8+ T cells to clear the virus and block replication by killing infected cells. The targeting of N to endosomes and lysosomes to enhance CD4+ and CD8+ T-cell responses distinguishes our vaccine. In our previously reported pre-clinical studies we showed that in mice, the hAd5 S-Fusion + N-ETSD vaccine elicits both humoral and T-cell responses that are robust and T helper cell 1 (Th1) dominant. Here we report that the hAd5 S-Fusion + N-ETSD vaccine is recognized by anti-sera and T cells from previously SARS-CoV-2 infected patients, and that the presence of N is vital for T-cell recall. The findings presented herein: (i) demonstrate specific recognition of hAd5 S-Fusion + N-ETSD infected cells by plasma antibodies from previously SARS-CoV-2 infected patients, but not antibodies from virus-naive subjects; (ii) show enhanced binding of plasma SARS-CoV-2 antibodies from previously infected patients to monocyte-derived dendritic cells (MoDCs) expressing the hAd5 S-Fusion + N-ETSD vaccine as compared to hAd5 S-Fusion alone; (iii) reveal N-ETSD localizes to vesicles associated with MHC class II antigen presentation, including endosomes, lysosomes and autophagosomes in MoDCs; (iv) demonstrate endosome/lysosome-targeted N-ETSD elicits higher interferon-{gamma} T-cell responses than cytoplasm-localized N; and (v) N-ETSD alone or in the hAd5 S-Fusion + N-ETSD construct induces both CD4+ and CD8+ T cell memory recall. This recognition of hAd5 S-Fusion + N-ETSD vaccine antigens by T cells from previously SARS-CoV-2 infected patients, together with the ability of this vaccine candidate to elicit de novo immune responses in naive mice suggests that it re-capitulates the natural immune response to SARS-CoV-2 to activate both B and T cells towards viral neutralization and recognition of infected cells, critical for prevention of COVID-19 disease. Intriguingly, our hAd5 S-Fusion + N-ETSD T-cell biased vaccine has the potential to not only provide protection for uninfected individuals, but also to be utilized as a therapeutic for already infected patients to induce rapid clearance of the virus by activating T cells to kill the virus-infected cells, thereby reducing viral replication and lateral transmission.