Abstract
Development of effective preventative interventions against SARS-CoV-2, the etiologic agent of COVID-19 is urgently needed. The viral surface spike (S) protein of SARS-CoV-2 is a key target for prophylactic measures as it is critical for the viral replication cycle and the primary target of neutralizing antibodies. We evaluated design elements previously shown for other coronavirus S protein-based vaccines to be successful, e.g., prefusion-stabilizing substitutions and heterologous signal peptides, for selection of a S-based SARS-CoV-2 vaccine candidate. In vitro characterization demonstrated that the introduction of stabilizing substitutions (i.e., furin cleavage site mutations and two consecutive prolines in the hinge region of S2) increased the ratio of neutralizing versus non-neutralizing antibody binding, suggestive for a prefusion conformation of the S protein. Furthermore, the wild-type signal peptide was best suited for the correct cleavage needed for a natively folded protein. These observations translated into superior immunogenicity in mice where the Ad26 vector encoding for a membrane-bound stabilized S protein with a wild-type signal peptide elicited potent neutralizing humoral immunity and cellular immunity that was polarized towards Th1 IFN-γ. This optimized Ad26 vector-based vaccine for SARS-CoV-2, termed Ad26.COV2.S, is currently being evaluated in a phase I clinical trial (ClinicalTrials.gov Identifier: NCT04436276).
Highlights
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) belongs to the Betacoronavirus genus, the Sarbecovirus subgenus, and is a member of the species SARS-related coronavirus, and the causative agent of coronavirus disease 2019 (COVID-19)[1]
SP, (3) full-length S with the tissue plasminogen activator (tPA) SP positioned upstream of after position 13, that tPA SP leads to a lower number of correct Nthe wt SP13, (4) full-length S with the tPA SP, the furin termini and that tPA.WT.S is cleaved predominantly after position cleavage site mutations (R682S, R685G) and proline substitutions 13 of the wt SP, but that a small fraction of incorrectly (K986P, V987P), (5) full-length S with wild-type SP, with processed signal peptide was observed with the wt SP still furin cleavage site mutations and proline substitutions (S.PP), (6) attached (Fig. 3)
Our data demonstrate that the design of the S protein is critical for the characteristics of the immunogen and the resulting immune responses elicited by the vaccine
Summary
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) belongs to the Betacoronavirus genus, the Sarbecovirus subgenus, and is a member of the species SARS-related coronavirus, and the causative agent of coronavirus disease 2019 (COVID-19)[1]. Several vaccine approaches using different designs of the S protein (e.g., with or without stabilizing substitutions, using a wild-type (wt) signal peptide (SP) or Tissue Plasminogen Activator (tPA) SP13, have been described, which induce neutralizing antibodies (NAbs) and protection in animal challenge models against SARS-CoV14, MERS-CoV13, and SARS-CoV-2 infections[15,16,17,18]. For SARS-CoV prototype vaccines that were generated in response to the 2003 SARS-CoV outbreak, a theoretical risk of vaccine-associated enhanced disease induction has been associated with a Th2type immune response in animal models[19,20,21]. Different immunogen design elements and vaccine platforms can influence these vaccine characteristics and should be evaluated
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