Safety assessment of compliant, highly invasive, lipid A-altered, O-antigen-defected Salmonella strains as prospective vaccine delivery systems

Abstract

In the present study, two prospective Salmonella delivery strains, JOL2782 and JOL2837, were developed by gene deletions of lon and cpxR, which are related to cellular adhesion and intracellular survival. Additionally, sifA deletion was introduced for JOL2782, which confers immune susceptibility and improves antigen delivery. Similarly, the rfaL deletion and lpxE substitution for pagL were accomplished in JOL2837 to reduce virulence and endotoxicity. Thus, enhanced adhesion and invasion and reduced intracellular survival were attained. Furthermore, aspartic acid auxotrophic (asd) was deleted to impose Darwinian selection on retention of the foreign antigen-expressing plasmid. Both delivery strains induced sufficient cytokine expression, but the level was significantly lower than that of the wild-type strain; the lowest cytokine expression was induced by the JOL2837 strain, indicating reduced endotoxicity. In parallel, IgG production was significantly enhanced by both delivery strains. Thus, the innate and adaptive immunogenicity of the strains was ensured. The environmental safety of these strains was ascertained through faecal dissemination assays. The nonpathogenicity of these strains to the host was confirmed by body weight monitoring, survival assays, and morphological and histological assessments of the vital organs. The in vitro assay in murine and human cell lines and in vivo safety assessments in mice suggest that these novel strains possess safety, invasiveness, and immunogenicity, making them ideal delivery strains. Overall, the results clearly showed that strain JOL2782 with sifA deletion had higher invasiveness, demonstrating superior vaccine deliverability, while JOL2837 with lpxE substitution for pagL and rfaL deletion had outstanding safety potential with drastically abridged endotoxicity.

Construction and Evaluation of an Efficient Live Attenuated Salmonella Choleraesuis Vaccine and Its Ability as a Vaccine Carrier to Deliver Heterologous Antigens

“…Therefore, it is essential for a live bacterial vaccine to reduce the endotoxic activity when the vaccine is injected. Our previous research indicated that the deletion of pagL , lpxR , and codon-optimized lpxE substitution for pagP to engineer Salmonella synthesizing 4′-monophosphoryl-hexa-acylated lipid A decreases the interaction of lipid A and TLR4 and enhances the ability of Salmonella to adapt to the host environment, and the Salmonella vaccine with monophosphorylated lipid A showed low endotoxic activity while maintaining its good immunogenicity [ 31 ], which was also demonstrated by recent research conducted by Lee groups [ 51 ]. Therefore, the lpxE gene was introduced into constructing S .…”

Salmonella-mediated oral delivery of multiple-target vaccine constructs with conserved and variable regions of SARS-CoV-2 protect against the Delta and Omicron variants in hamster

“…With the innate ability of Salmonella to invade and proliferate in professional antigen-presenting cells (APCs), it has the advantage to directly deliver the DNA cargo to these cells and subsequently result in induction of cellular and humoral response [ 49 ]. Also, as safety concern is one of the important considerations in vaccine development especially with live-attenuated bacteria, of the vaccine delivery strain JOL2500 used in this study with genotype of Δlon, ΔcpxR, ΔsifA , and Δasd has been proven to have optimal survival in the host without any virulence [ 14 , 50 ]. Furthermore, vaccine production for this unique strategy can be easily scaled up and prepared rapidly at lower cost.…”

Live-Attenuated Salmonella-Based Oral Vaccine Candidates Expressing PCV2d Cap and Rep by Novel Expression Plasmids as a Vaccination Strategy for Mucosal and Systemic Immune Responses against PCV2d

“…PCV2 causes several disease syndromes in pigs resulting in significant economic losses in the swine industry globally. In this study, the oral vaccination of the vaccine candidates elicited a broad spectrum of immunity, covering both systemic and mucosal sites through the safe and efficient vaccine delivery of the Salmonella vaccine strain to different organs such as liver, spleen, and lungs [ 29 , 35 , 36 ] and expression of the target antigens, Cap and Rep, in antigen-presenting cells ( Figure 2 ). Salmonella Typhimurium is one of several bacterial species that can transfer eukaryotic expression plasmids into host cells, which can serve as a carrier for vaccine antigens or its genetic material [ 37 , 38 ].…”

“…To diminish the impact of pre-existing immunity on the vaccine delivery efficiency and immunogenicity, an attenuated Salmonella vector strain was previously engineered by deleting the O-antigen ligase ( rfal ) from the Salmonella Typhimurium genome [ 70 ]. The optimized Salmonella vaccine strain JOL3000 with genotype Δlon, Δcpxr, Δrfal, ΔpagL::lpxE, Δasd [ 36 , 71 ] can be utilized for further studies to explore its efficient vaccine delivery by overcoming the pre-existing antibodies to Salmonella and potential application in the swine model. Aside from this, optimizing the dosage, vaccination schedule, and incorporation of stimulators for the host immune response could also be explored to further improve its immunogenicity.…”