Adeno-associated virus-based malaria booster vaccine following attenuated replication-competent vaccinia virus LC16m8Δ priming.

We previously demonstrated that boosting with adeno-associated virus (AAV) type 1 (AAV1) can induce highly effective and long-lasting protective immune responses against malaria parasites when combined with replication-deficient adenovirus priming in a rodent model. In the present study, we compared the efficacy of two different AAV serotypes, AAV1 and AAV5, as malaria booster vaccines following priming with the attenuated replication-competent vaccinia virus strain LC16m8Δ (m8Δ), which harbors the fusion gene encoding both the pre-erythrocytic stage protein, Plasmodium falciparum circumsporozoite (PfCSP) and the sexual stage protein (Pfs25) in a two-dose heterologous prime-boost immunization regimen. Both regimens, m8Δ/AAV1 and m8Δ/AAV5, induced robust anti-PfCSP and anti-Pfs25 antibodies. To evaluate the protective efficacy, the mice were challenged with sporozoites twice after immunization. At the first sporozoite challenge, m8Δ/AAV5 achieved 100% sterile protection whereas m8Δ/AAV1 achieved 70% protection. However, at the second challenge, 100% of the surviving mice from the first challenge were protected in the m8Δ/AAV1 group whereas only 55.6% of those in the m8Δ/AAV5 group were protected. Regarding the transmission-blocking efficacy, we found that both immunization regimens induced high levels of transmission-reducing activity (>99%) and transmission-blocking activity (>95%). Our data indicate that the AAV5-based multistage malaria vaccine is as effective as the AAV1-based vaccine when administered following an m8Δ-based vaccine. These results suggest that AAV5 could be a viable alternate vaccine vector as a malaria booster vaccine.

Sterile protection and transmission blockade by a multistage anti-malarial vaccine in the pre-clinical study.

The Malaria Vaccine Technology Roadmap 2013 (World Health Organization) aims to develop safe and effective vaccines by 2030 that will offer at least 75% protective efficacy against clinical malaria and reduce parasite transmission. Here, we demonstrate a highly effective multistage vaccine against both the pre-erythrocytic and sexual stages of Plasmodium falciparum that protects and reduces transmission in a murine model. The vaccine is based on a viral-vectored vaccine platform, comprising a highly-attenuated vaccinia virus strain, LC16m8Δ (m8Δ), a genetically stable variant of a licensed and highly effective Japanese smallpox vaccine LC16m8, and an adeno-associated virus (AAV), a viral vector for human gene therapy. The genes encoding P. falciparum circumsporozoite protein (PfCSP) and the ookinete protein P25 (Pfs25) are expressed as a Pfs25-PfCSP fusion protein, and the heterologous m8Δ-prime/AAV-boost immunization regimen in mice provided both 100% protection against PfCSP-transgenic P. berghei sporozoites and up to 100% transmission blocking efficacy, as determined by a direct membrane feeding assay using parasites from P. falciparum-positive, naturally-infected donors from endemic settings. Remarkably, the persistence of vaccine-induced immune responses were over 7 months and additionally provided complete protection against repeated parasite challenge in a murine model. We propose that application of the m8Δ/AAV malaria multistage vaccine platform has the potential to contribute to the landmark goals of the malaria vaccine technology roadmap, to achieve life-long sterile protection and high-level transmission blocking efficacy.

A replication-competent smallpox vaccine LC16m8Δ-based COVID-19 vaccine.

Viral vectors are a potent vaccine platform for inducing humoral and T-cell immune responses. Among the various viral vectors, replication-competent ones are less commonly used for coronavirus disease 2019 (COVID-19) vaccine development compared with replication-deficient ones. Here, we show the availability of a smallpox vaccine LC16m8Δ (m8Δ) as a replication-competent viral vector for a COVID-19 vaccine. M8Δ is a genetically stable variant of the licensed and highly effective Japanese smallpox vaccine LC16m8. Here, we generated two m8Δ recombinants: one harbouring a gene cassette encoding the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S) glycoprotein, named m8Δ-SARS2(P7.5-S)-HA; and one encoding the S protein with a highly polybasic motif at the S1/S2 cleavage site, named m8Δ-SARS2(P7.5-SHN)-HA. M8Δ-SARS2(P7.5-S)-HA induced S-specific antibodies in mice that persisted for at least six weeks after a homologous boost immunization. All eight analysed serum samples displayed neutralizing activity against an S-pseudotyped virus at a level similar to that of serum samples from patients with COVID-19, and more than half (5/8) also had neutralizing activity against the Delta/B.1.617.2 variant of concern. Importantly, most serum samples also neutralized the infectious SARS-CoV-2 Wuhan and Delta/B.1.617.2 strains. In contrast, immunization with m8Δ-SARS2(P7.5-SHN)-HA elicited significantly lower antibody titres, and the induced antibodies had less neutralizing activity. Regarding T-cell immunity, both m8Δ recombinants elicited S-specific multifunctional CD8+ and CD4+ T-cell responses even after just a primary immunization. Thus, m8Δ provides an alternative method for developing a novel COVID-19 vaccine.