A self-amplifying mRNA COVID-19 vaccine drives potent and broad immune responses at low doses that protects non-human primates against SARS-CoV-2

The coronavirus disease 2019 (COVID-19) pandemic continues to spread globally, highlighting the urgent need for safe and effective vaccines that could be rapidly mobilized to immunize large populations. We report the preclinical development of a self-amplifying mRNA (SAM) vaccine encoding a prefusion stabilized severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike glycoprotein and demonstrate potent cellular and humoral immune responses at low doses in mice and rhesus macaques. The homologous prime-boost vaccination regimen of SAM at 3, 10 and 30 μg induced potent neutralizing antibody titers in rhesus macaques following two SAM vaccinations at all dose levels, with the 10 μg dose generating geometric mean titers (GMT) 48-fold greater than the GMT of a panel of SARS-CoV-2 convalescent human sera. Spike-specific T cell responses were observed at all dose levels. SAM vaccination provided protective efficacy against SARS-CoV-2 challenge as both a homologous prime-boost and as a single boost following ChAd prime, demonstrating reduction of viral replication in both the upper and lower airways. Protection was most effective with a SAM prime-boost vaccination regimen at 10 and 30 μg and with a ChAd/SAM heterologous prime-boost regimen. The SAM vaccine is currently being evaluated in clinical trials as both a homologous prime-boost regimen at low doses and as a boost following heterologous prime.

Marked enhancement of neutralizing antibody and IFN-γ T-cell responses by GX-19N DNA booster in mice primed with inactivated vaccine

In response to the COVID-19 pandemic, an unprecedented level of vaccine development has occurred. As a result, various COVID-19 vaccines have been approved for use. Among these, inactivated virus particle vaccines have been widely used worldwide, but additional vaccination strategies are needed because of the short duration of immune responses elicited by these vaccines. Here, we evaluated homologous and heterologous prime-boost regimens using inactivated virus particle vaccine and GX-19N DNA vaccine for their ability to enhance the protective immune response against SARS-CoV-2. We demonstrated that a heterologous prime-boost regimen with the inactivated virus particle vaccine and GX-19N DNA vaccine resulted in enhanced SRBD- and N-specific antibody responses, compared to the homologous inactivated virus particle vaccine prime-boost vaccination. In addition, the neutralizing antibody response was significantly improved with the heterologous inactivated virus particle prime DNA boost regimen, and the neutralizing antibody induced with the heterologous prime boost regimen did not decrease against the SARS-CoV-2 variant of concern. The heterologous inactivated virus particle prime-DNA boost regimen not only significantly increased S- and N-specific IFN-g T cell responses, but also induced an equivalent level of T cell response against SARS-CoV-2 variant of concerns. Our results provide new insights into prophylactic vaccination strategies for COVID-19 vaccination.

Liver-Directed AAV8 Booster Vaccine Expressing Plasmodium falciparum Antigen Following Adenovirus Vaccine Priming Elicits Sterile Protection in a Murine Model

Hepatocyte infection by malaria sporozoites is a bottleneck in the life-cycle of Plasmodium spp. including P. falciparum, which causes the most lethal form of malaria. Therefore, developing an effective vaccine capable of inducing the strong humoral and cellular immune responses necessary to block the pre-erythrocytic stage has potential to overcome the spatiotemporal hindrances pertaining to parasite biology and hepatic microanatomy. We recently showed that when combined with a human adenovirus type 5 (AdHu5)-priming vaccine, adeno-associated virus serotype 1 (AAV1) is a potent booster malaria vaccine vector capable of inducing strong and long-lasting protective immune responses in a rodent malaria model. Here, we evaluated the protective efficacy of a hepatotropic virus, adeno-associated virus serotype 8 (AAV8), as a booster vector because it can deliver a transgene potently and rapidly to the liver, the organ malaria sporozoites initially infect and multiply in following sporozoite injection by the bite of an infected mosquito. We first generated an AAV8-vectored vaccine expressing P. falciparum circumsporozoite protein (PfCSP). Intravenous (i.v.) administration of AAV8-PfCSP to mice initially primed with AdHu5-PfCSP resulted in a hepatocyte transduction rate ~2.5 times above that seen with intramuscular (i.m.) administration. This immunization regimen provided a better protection rate (100% sterile protection) than that of the i.m. AdHu5-prime/i.m. AAV8-boost regimen (60%, p < 0.05), i.m. AdHu5-prime/i.v. AAV1-boost (78%), or i.m. AdHu5-prime/i.m. AAV1-boost (80%) against challenge with transgenic PfCSP-expressing P. berghei sporozoites. Compared with the i.m. AdHu5-prime/i.v. AAV1-boost regimen, three other regimens induced higher levels of PfCSP-specific humoral immune responses. Importantly, a single i.v. dose of AAV8-PfCSP recruited CD8+ T cells, especially resident memory CD8+ T cells, in the liver. These data suggest that boost with i.v. AAV8-PfCSP can improve humoral and cellular immune responses in BALB/c mice. Therefore, this regimen holds great promise as a next-generation platform for the development of an effective malaria vaccine.

Mice Immunized with the Vaccine Candidate HexaPro Spike Produce Neutralizing Antibodies against SARS-CoV-2

Updated and revised versions of COVID-19 vaccines are vital due to genetic variations of the SARS-CoV-2 spike antigen. Furthermore, vaccines that are safe, cost-effective, and logistic-friendly are critically needed for global equity, especially for middle- to low-income countries. Recombinant protein-based subunit vaccines against SARS-CoV-2 have been reported using the receptor-binding domain (RBD) and the prefusion spike trimers (S-2P). Recently, a new version of prefusion spike trimers, named HexaPro, has been shown to possess two RBD in the “up” conformation, due to its physical property, as opposed to just one exposed RBD found in S-2P. Importantly, this HexaPro spike antigen is more stable than S-2P, raising its feasibility for global logistics and supply chain. Here, we report that the spike protein HexaPro offers a promising candidate for the SARS-CoV-2 vaccine. Mice immunized by the recombinant HexaPro adjuvanted with aluminum hydroxide using a prime-boost regimen produced high-titer neutralizing antibodies for up to 56 days after initial immunization against live SARS-CoV-2 infection. Also, the level of neutralization activity is comparable to that of convalescence sera. Our results indicate that the HexaPro subunit vaccine confers neutralization activity in sera collected from mice receiving the prime-boost regimen.

Mice immunized with the vaccine candidate HexaPro spike produce neutralizing antibodies against SARS-CoV-2

Updated and revised versions of COVID-19 vaccines are vital due to genetic variations of the SARS-CoV-2 spike antigen. Furthermore, vaccines that are safe, cost-effective, and logistically friendly are critically needed for global equity, especially for middle to low income countries. Recombinant protein-based subunit vaccines against SARS-CoV-2 have been reported with the use of the receptor binding domain (RBD) and the prefusion spike trimers (S-2P). Recently, a new version of prefusion spike trimers, so called "HexaPro", has been shown for its physical property to possess two RBD in the "up" conformation, as opposed to just one exposed RBD found in S-2P. Importantly, this HexaPro spike antigen is more stable than S-2P, raising its feasibility for global logistics and supply chain. Here, we report that the spike protein HexaPro offers a promising candidate for SARS-CoV-2 vaccine. Mice immunized by the recombinant HexaPro adjuvanted with aluminium hydroxide using a prime-boost regimen produced high-titer neutralizing antibodies for up to 56 days after initial immunization against live SARS-CoV-2 infection. In addition, the level of neutralization activity is comparable to that of convalescence sera. Our results indicate that the HexaPro subunit vaccine confers neutralization activity in sera collected from mice receiving the prime-boost regimen.

Parainfluenza Virus 5 Priming Followed by SIV/HIV Virus-Like-Particle Boosting Induces Potent and Durable Immune Responses in Nonhuman Primates

The search for a preventive vaccine against HIV infection remains an ongoing challenge, indicating the need for novel approaches. Parainfluenza virus 5 (PIV5) is a paramyxovirus replicating in the upper airways that is not associated with any animal or human pathology. In animal models, PIV5-vectored vaccines have shown protection against influenza, RSV, and other human pathogens. Here, we generated PIV5 vaccines expressing HIV envelope (Env) and SIV Gag and administered them intranasally to macaques, followed by boosting with virus-like particles (VLPs) containing trimeric HIV Env. Moreover, we compared the immune responses generated by PIV5-SHIV prime/VLPs boost regimen in naïve vs a control group in which pre-existing immunity to the PIV5 vector was established. We demonstrate for the first time that intranasal administration of PIV5-based HIV vaccines is safe, well-tolerated and immunogenic, and that boosting with adjuvanted trimeric Env VLPs enhances humoral and cellular immune responses. The PIV5 prime/VLPs boost regimen induced robust and durable systemic and mucosal Env-specific antibody titers with functional activities including ADCC and neutralization. This regimen also induced highly polyfunctional antigen-specific T cell responses. Importantly, we show that diminished responses due to PIV5 pre-existing immunity can be overcome in part with VLP protein boosts. Overall, these results establish that PIV5-based HIV vaccine candidates are promising and warrant further investigation including moving on to primate challenge studies.

Long-term protective efficacy with a BCG-prime ID93/GLA-SE boost regimen against the hyper-virulent Mycobacterium tuberculosis strain K in a mouse model

Since ID93/GLA-SE was developed as a targeted BCG-prime booster vaccine, in the present study, we evaluated the protective efficacy of ID93/GLA-SE as a boost to a BCG-prime against the hypervirulent Mycobacterium tuberculosis (Mtb) K challenge to provide further information on the development and application of this vaccine candidate. Boosting BCG with the ID93/GLA-SE vaccine significantly reduced bacterial burden at 16 weeks post-challenge while the BCG vaccine alone did not confer significant protection against Mtb K. The pathological analysis of the lung from the challenged mice also showed the remarkably protective boosting effect of ID93/GLA-SE on BCG-immunised animals. Moreover, qualitative and quantitative analysis of the immune responses following ID93/GLA-SE-immunisation demonstrated that ID93/GLA-SE was able to elicit robust and sustained Th1-biased antigen-specific multifunctional CD4+ T-cell responses up to 16 weeks post-challenge as well as a high magnitude of an antigen-specific IgG response. Our findings demonstrate that the ID93/GLA-SE vaccine candidate given as a BCG-prime boost regimen confers a high level of long-term protection against the hypervirulent Mtb Beijing infection. These findings will provide further and more feasible validation for the potential utility of this vaccine candidate particularly in East-Asian countries, with the predominance of the Beijing genotype, after BCG vaccination.

Optimized Mucosal MVA Prime/ Soluble gp120 Boost Vaccination Regimen Induces Similar Antibody Responses as an Intramuscular Regimen

ABSTRACTThe benefits of mucosal vaccines over injected vaccines are difficult to ascertain since mucosally administered vaccines often induce serum antibody responses of lower magnitude than those induced by injected vaccines. This study aimed to determine if mucosal vaccination using a modified vaccinia Ankara expressing HIV-1 gp120 (MVA-g120) prime and HIV-1 gp120 protein boost could be optimized to induce serum antibody responses similar to those induced by an intramuscularly (IM) administered MVA prime/gp120 boost to allow comparison of an IM immunization regimen to a mucosal vaccination regimen for their ability to protect against a low dose rectal SHIV challenge while inducing similar serum anti-HIV-1 antibody responses. A 3-fold higher antigen dose was required for intranasal (IN) immunization with gp120 to induce serum anti-gp120 IgG responses not significantly different than those induced by IM immunization. Gp120 fused to the Adenovirus type 2 fiber binding domain (gp120-Ad2F), a mucosal targeting ligand, exhibited enhanced IN immunogenicity when compared to gp120 alone. MVA-gp120 was more immunogenic after IN delivery than gastric or rectal delivery, although serum antibodies induced by IN immunization were lower than those induced by intramuscular immunization. Using these optimized vaccines, an IN MVA-gp120 prime, combined IM (gp120) and IN (gp120-Ad2F) boost regimen (IN/IM+IN) induced serum anti-gp120 antibody titers similar to those induced by the intramuscular prime/boost regimen (IM/IM) in rabbits and non-human primates. Despite the induction of similar systemic anti-HIV-1 antibody responses, neither the IM/IM nor the IN/IM+IN regimen induced elevated anti-HIV-1 mucosal IgA responses nor protected against a repeated low-dose rectal SHIV challenge. These results demonstrate that immunization regimens utilizing the IN route are able to induce serum antigen-specific antibody responses similar to those induced by systemic immunizationIMPORTANCEMucosal vaccination is proposed as a method of immunization able to induce protection against mucosal pathogens that is superior to protection provided by parenteral immunization. However, mucosal vaccination often induces serum antigen-specific immune responses of lower magnitude than those induced by parenteral immunization, making the comparison of mucosal and parenteral immunization difficult. We identified vaccine parameters that allowed an immunization regimen consisting of an IN prime followed with boosters administered by both IN and IM routes to induce serum antibody responses similar to those induced by IM prime/boost vaccination. Additional studies are needed to determine the potential benefit of mucosal immunization for HIV-1 and other mucosally-transmitted pathogens.