Sudan virus (SUDV) causes severe lethal hemorrhagic fever in humans and nonhuman primates. The most effective and economical way to protect against Sudan ebolavirus disease is prophylactic vaccination. However, there are no licensed vaccines to prevent SUDV infections. In this study, a bacterium-like particle (BLP)-based vaccine displaying the extracellular domain of the SUDV glycoprotein (eGP) was developed based on a gram-positive enhancer matrix-protein anchor (GEM-PA) surface display system. Expression of the recombinant GEM-displayed eGP (eGP-PA-GEM) was verified by Western blotting and immunofluorescence assays. The SUDV BLPs (SBLPs), which were mixed with Montanide ISA 201VG plus Poly (I:C) combined adjuvant, could induce high SUDV GP-specific IgG titers of up to 1:40,960 and robust virus-neutralizing antibody titers reached 1:460. The SBLP also elicited T-helper 1 (Th1) and T-helper 2 (Th2) cell-mediated immunity. These data indicate that the SBLP subunit vaccine has the potential to be developed into a promising candidate vaccine against SUDV infections. Viruses 2019, 11, 1149 2 of 15 and receiving unprecedented attention worldwide [4]. Unfortunately, an EVD outbreak occurred again in 2018 in eastern Democratic Republic of Congo and, as of August 2019, a total of 2997 EVD cases had been reported, including 2892 confirmed cases with 1998 deaths (overall case fatality rate of 67%) [5]. Since the first outbreak was reported in 1976, five different species of ebolaviruses, including Ebola virus (EBOV), Sudan virus (SUDV), Reston virus (RESTV), Bundibugyo virus (BDBV), and Taï Forest virus (TAFV,) have been identified and their genomic sequences differ by~35-45% [2]. Excluding EBOV, SUDV has the highest mortality and outbreak rates among the species of ebolaviruses. SUDV has emerged at least six times with average mortality rates of up to 53.76% [6,7]. Multiple vaccines and monoclonal antibodies against EBOV have been developed, but very few of them can effectively prevent and control SUDV infection [8]. Therefore, there is an urgent need to develop a safe and efficacious vaccine against SUDV.The EBOV genome encodes nine structural proteins, including the surface envelope glycoprotein (GP) as the only membrane protein [9]. The mature GP protein forms homotrimers on the surface of infected cells and virions, which is crucial for receptor binding, viral entry, and host immunity induction, making GP an ideal vaccine target [10]. A number of candidate vaccines for EBOV have demonstrated protection against lethal EBOV challenge in animal models and progressed to clinical trials, and most of these vaccines, including DNA vaccines [11], vaccines based on viral vectors, such as recombinant adenovirus [12] and vesicular stomatitis virus (VSV) [13], and protein-based vaccines, such as virus-like particles (VLPs) [14], have been based on GP. Most notably, a number of the candidate vaccines currently under clinical phase evaluation are viral vector-based vaccines [15,16]. While recombinant vesicular stomatitis virus (rVS...

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The frequent emergence of SARS-CoV-2 variants thwarts the prophylactic and therapeutic countermeasures confronting COVID-19. Among them, the Delta variant attracts widespread attention due to its high pathogenicity and fatality rate compared with other variants. However, with the emergence of new variants, studies on Delta variants have been gradually weakened and ignored. In this study, a replication-competent recombinant virus carrying the S protein of the SARS-CoV-2 Delta variant was established based on the vesicular stomatitis virus (VSV), which presented a safe alternative model for studying the Delta variant. The recombinant virus showed a replication advantage in Vero E6 cells, and the viral titers reach 107.3 TCID50/mL at 36 h post-inoculation. In the VSV-vectored recombinant platform, the spike proteins of the Delta variant mediated higher fusion activity and syncytium formation than the wild-type strain. Notably, the recombinant virus was avirulent in BALB/c mice, Syrian hamsters, 3-day ICR suckling mice, and IFNAR/GR−/− mice. It induced protective neutralizing antibodies in rodents, and protected the Syrian hamsters against the SARS-CoV-2 Delta variant infection. Meanwhile, the eGFP reporter of recombinant virus enabled the visual assay of neutralizing antibodies. Therefore, the recombinant virus could be a safe and convenient surrogate tool for authentic SARS-CoV-2. This efficient and reliable model has significant potential for research on viral-host interactions, epidemiological investigation of serum-neutralizing antibodies, and vaccine development.