| Abstract: | The current pandemic of COVID-19 caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) highlights an urgent need to develop a safe, efficacious, and durable vaccine. Using a measles virus (rMeV) vaccine strain as the backbone, we developed a series of recombinant attenuated vaccine candidates expressing various forms of the SARS-CoV-2 spike (S) protein and its receptor binding domain (RBD) and evaluated their efficacy in cotton rat, IFNAR−/−mice, IFNAR−/−-hCD46 mice, and golden Syrian hamsters. We found that rMeV expressing stabilized prefusion S protein (rMeV-preS) was more potent in inducing SARS-CoV-2–specific neutralizing antibodies than rMeV expressing full-length S protein (rMeV-S), while the rMeVs expressing different lengths of RBD (rMeV-RBD) were the least potent. Animals immunized with rMeV-preS produced higher levels of neutralizing antibody than found in convalescent sera from COVID-19 patients and a strong Th1-biased T cell response. The rMeV-preS also provided complete protection of hamsters from challenge with SARS-CoV-2, preventing replication in lungs and nasal turbinates, body weight loss, cytokine storm, and lung pathology. These data demonstrate that rMeV-preS is a safe and highly efficacious vaccine candidate, supporting its further development as a SARS-CoV-2 vaccine.In December 2019, a novel coronavirus disease (COVID-19) was first identified in Wuhan City, Hubei Province, People’s Republic of China. The causative agent was named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). On 11 March 2020 the World Health Organization (WHO) declared COVID-19 a global pandemic (1–3). It spread rapidly within China and swept into at least 200 countries within 3 mo. Symptoms are primarily pneumonia, as with two other important human coronaviruses (CoVs), SARS-CoV-1 and Middle East respiratory syndrome (MERS)-CoV (1–3). As of 1 February 2021, more than 102,399,513 cases had been reported worldwide, with 2,217,005 deaths (∼2.2% mortality). There is an urgent need to develop a safe and efficacious vaccine to protect the populace from this new virus. Globally, more than 300 SARS-CoV-2 vaccine candidates are in preclinical development (4–6) and at least 30 vaccine candidates have entered human clinical trials (4, 5, 7, 8). Among them, vaccines based on messenger RNA (mRNA), inactivated virus, and adenovirus vectors (Ad5-nCoV and ChAdOx1) are now in phase III clinical trials. Excitingly, preliminary results indicate that these vaccines are highly efficacious, reaching 90 to 95% effectiveness against SARS-CoV-2 infection in some cases. The durability of the protection conferred by these vaccine candidates is unknown. Although these vaccine candidates are highly promising, exploration of other vaccine platforms is needed.The CoV spike (S) protein is the main target for neutralizing antibodies that inhibit infection and prevent disease. As such, the S protein is the primary focus for CoV vaccine development (9, 10). The CoV S protein is a class I fusion protein trimer that is incorporated into virions as they bud into the endoplasmic reticulum–Golgi intermediate compartment. For SARS-CoV-2, S is cleaved into S1 and S2 subunits by furin before the virion is released. The S1 subunit contains the receptor-binding domain (RBD) that attaches to the hACE2 receptor on the surface of a target cell. The S2 subunit is further cleaved by TMPRSS2 (or cathepsin L/B) and possesses the membrane-fusing activity (9, 11, 12). Both S and its RBD have been shown to be immunogenic for many CoVs (13–15). The native S in the virion is in its “prefusion” form. Upon triggering, the prefusion S (preS) undergoes significant conformational changes to insert its fusion peptide into the target cell membrane and bring the virion and cell membranes together, arriving at its postfusion S form as it causes the membranes to fuse. For paramyxoviruses, pneumoviruses, and HIV, it has been shown that prefusion forms of glycoprotein are more potent in inducing neutralizing antibodies than their postfusion forms (16–20). Currently, whether the SARS-CoV-2 preS protein is more immunogenic than the postfusion S protein is unknown.Live attenuated measles virus (MeV) vaccine has been one of the safest and most efficient human vaccines and has been used in children since the 1960s (21, 22). Worldwide MeV vaccination campaigns have been very successful in controlling measles. MeV is an enveloped nonsegmented negative-sense RNA virus that belongs to the genus Morbillivirus within the Paramyxoviridae family. MeV is an excellent vector to deliver vaccines for human pathogens primarily because of its high safety, efficacy, and long-lived immunity (22, 23). MeV has previously been shown to be a highly efficacious vaccine vector for many viral diseases such as HIV (24, 25), SARS-CoV-1 (26, 27), MERS-CoV (28, 29), respiratory syncytial virus (30), hepatitis B and C viruses (31), influenza virus (30, 32), chikungunya virus (CHIKV) (33), and flaviviruses (Zika virus, dengue virus, West Nile virus, and yellow fever virus) (34–36). Recent human clinical trials have demonstrated that an recombinant MeV (rMeV)-based CHIKV vaccine is safe and highly immunogenic in healthy adults, even in the presence of preexisting anti-MeV vector immunity (33).In this study, we developed a series of rMeV-based vaccine candidates expressing different forms of the SARS-CoV-2 S protein and evaluated them in cotton rats, IFNAR−/−mice, IFNAR−/−-hCD46 mice, and golden Syrian hamsters. We found that all SARS-CoV-2 S antigens are highly expressed by the MeV vector. Among these vaccine candidates, rMeV expressing stabilized preS (rMeV-preS) and full-length S (rMeV-S) proteins were the most potent in triggering SARS-CoV-2–specific antibodies. Animals immunized with rMeV-preS induced the highest level of neutralizing antibodies that were higher than convalescent sera of patients recovered from COVID-19, and the highest Th1-biased T cell immune response. Furthermore, hamsters immunized with rMeV-preS provided complete protection against SARS-CoV-2 challenge and lung pathology. |
