Genetic detoxification of an unencapsulated Neisseria meningitidis strain enhances potency of outer membrane vesicle vaccines and cross-reactivity of vaccine-specific antibodies

ABSTRACT Meningococcal serogroup B (MenB) detergent-detoxified outer membrane vesicle (dOMV) vaccines demonstrate high effectiveness against the homologous production strain but confer limited cross-protection to heterologous strains. We previously reported that dOMVs isolated from a MenB strain deleted for the proteins PorA, PorB, and RmpM (ΔABR) elicited bactericidal antibodies with a greater breadth of coverage, but of lower overall titer, compared to wild-type (WT) dOMV-induced antibodies. Low ΔABR dOMV potency was associated with pronounced vesicular deterioration and lower levels of lipooligosaccharide (LOS) and capsular polysaccharide (Cps) relative to WT dOMVs, phenotypes resulting from detergent detoxification procedures required to limit LOS-induced toxicity. In this study, we engineered genetically detoxified ΔABR strains that were deleted singly for the lauroyl acyltransferase LpxL1 (ΔABRL) or in tandem for the Cps polysialyltransferase SiaD (ΔABRSL). Genetically detoxified, detergent-extracted OMVs (eOMVs) from ΔABRL and ΔABRSL retained vesicular structure and elicited functional antibodies with increased cross-reactivity compared to ΔABR and ΔABRL dOMVs when administered to rabbits. Antibody seropositivity and cross-reactivity were similarly increased in mice vaccinated with ΔABRL eOMVs vs ΔABRL dOMVs. ΔABRSL eOMV potency was higher than that of ΔABRL eOMVs, suggesting a role for Cps in suppressing antibody responses. Immunization with eOMVs was associated with trends toward improved murine T helper cell responses and elicitation of higher proportions of conformational antibodies that may have contributed to improved bactericidal activity. ΔABRSL eOMVs additionally showed decreased toxicity compared to WT and ΔABR dOMVs when tested in vitro , supporting further study as a vaccine candidate. IMPORTANCE The ability to target multiple serogroup B Neisseria meningitidis strains with a single vesicle vaccine is complicated by the diversity of antigens that span the bacterial surface. We previously reported that vesicles isolated from meningococcal strains deleted for the major antigenically diverse surface proteins elicited functional antibodies that could kill a large number of N. meningitidis strains though at relatively low titers. Low potency of the mutant vaccines was associated with a breakdown of the vesicular membrane upon detergent treatment, a process necessary to diminish endogenous endotoxin levels and ensure vaccine safety. In this study, we demonstrate that we can retain high cross-reactivity of anti-vesicle antibodies without decreasing the potency of the host immune response through genetic detoxification of the meningococcal vaccine strain. By preventing the formation of the complete endotoxin structure, we are able to isolate vesicles that are safe and immunogenic without the requirement for detergent detoxification.