Naturally secreted bacterial outer membrane vesicles: potential platform for a vaccine against Campylobacter jejuni

Acute diarrheal illness and gastroenteritis caused by Campylobacter jejuni (C. jejuni) infection remain significant public health risks in developing countries with substantial mortality and morbidity in humans, particularly in children under the age of five. Despite improved global awareness in sanitation and hygiene practices, including food safety measures, C. jejuni infections continue to rise even across the developed nations and no vaccine is currently available for humans. Genetic diversities among C. jejuni strains as well as limited understanding of immunological correlates of host protection remain primary impediments for developing an effective vaccine against C. jejuni. Given the role of bacterial outer membrane-associated proteins in intestinal adherence and invasion as well as modulating dynamic interplay between host and pathogens, bacterial outer membrane vesicles (OMVs) have emerged as potential vaccine platforms against a number of enteric pathogens, including C. jejuni. In the present study, we describe a mucosal vaccine strategy using chitosan (CS) coated OMVs (CS-OMVs) to induce specific immune responses against C. jejuni in mice. However, considering the challenges of mucosal delivery of OMVs in terms of exposure to variable pH, risk of enzymatic degradation, rapid gut transit, and low permeability across the intestinal epithelium, we preferentially used CS as a non-toxic, mucoadhesive polymer to coat OMVs. Mucosal administration of CS-OMVs induced high titre of systemic (IgG) and local (secretory IgA) antibodies in mice. The neutralizing ability of secretory IgA (sIgA) produced in the intestine was confirmed by in vitro inhibition of cell adherence and invasion of C. jejuni while in vivo challenge study in OMVs immunized mice showed a significant reduction in cecal colonization of C. jejuni. Moreover, to investigate the immunological correlates of the observed protection, present data suggest OMVs driven T cell proliferation with an increased population of CD4+ and CD8+ T cells. In addition to antibody isotype profile, significant upregulation of IFN-{gamma} and IL-6 gene expression in mesenteric lymph nodes collected from OMVs immunized mice further suggests that mucosal delivery of OMVs promotes a Th1/Th2 mixed type immune responses. Together, we provide strong experimental evidence that as an acellular and non-replicating canonical end product of bacterial secretion, mucosal delivery of OMVs may represent a promising platform for developing an effective vaccine against C. jejuni. Author SummaryDespite the loss of 7.5 million disability-adjusted life years, which is over and above any other globally prevalent enteric or enterotoxigenic pathogens, C. jejuni remains a neglected foodborne pathogen, particularly in tropical countries. Even with the improved global awareness in sanitation and hygiene practices, including food safety measures C. jejuni infections continue to rise globally and no vaccine is currently available for humans. In light of the importance of the diverse cargo selection by bacterial OMVs, the present study describes a mucosal vaccine strategy using chitosan-coated OMVs to induce specific immune responses against C. jejuni in mice. We provide here strong experimental evidence that as a non-replicating canonical end product of bacterial secretion, mucosal delivery of OMVs represents an attractive vaccine platform against C. jejuni.