Cell-associated flagella enhance the protection conferred by mucosally-administered attenuated Salmonella Paratyphi A vaccines
Authors: William C Blackwelder
Orit Gat
Sharon Tennant
David J Silverman
Marcela F Pasetti
Myron M Levine
James E Galen
Raphael Simon
Journal: PLoS Neglected Tropical Diseases (2011)
Discipline: IMMUNOLOGY
Keywords: Antibiotic-resistant; Salmonella enterica serovar Paratyphi A; vaccine; flagella; protective antigen
Affiliations: Center for Vaccine Development, University of Maryland School of Medicine, Baltimore, Maryland, United States of America.

Abstract:  Background: Antibiotic-resistant Salmonella enterica serovar Paratyphi A, the agent of paratyphoid A fever, poses an emerging public health dilemma in endemic areas of Asia and among travelers, as there is no licensed vaccine. Integral to our efforts to develop a S. Paratyphi A vaccine, we addressed the role of flagella as a potential protective antigen by comparing cell-associated flagella with exported flagellin subunits expressed by attenuated strains. Methodology: S. Paratyphi A strain ATCC 9150 was first deleted for the chromosomal guaBA locus, creating CVD 1901. Further chromosomal deletions in fliD (CVD 1901D) or flgK (CVD 1901K) were then engineered, resulting in the export of unpolymerized FliC, without impairing its overall expression. The virulence of the resulting isogenic strains was examined using a novel mouse LD(50) model to accommodate the human-host restricted S. Paratyphi A. The immunogenicity of the attenuated strains was then tested using a mouse intranasal model, followed by intraperitoneal challenge with wildtype ATCC 9150. Results: Mucosal (intranasal) immunization of mice with strain CVD 1901 expressing cell-associated flagella conferred superior protection (vaccine efficacy [VE], 90%) against a lethal intraperitoneal challenge, compared with the flagellin monomer-exporting mutants CVD 1901K (30% VE) or CVD 1901D (47% VE). The superior protection induced by CVD 1901 with its cell-attached flagella was associated with an increased IgG2a:IgG1 ratio of FliC-specific antibodies with enhanced opsonophagocytic capacity. Conclusions: Our results clearly suggest that enhanced anti-FliC antibody-mediated clearance of S. Paratyphi A by phagocytic cells, induced by vaccines expressing cell-associated rather than exported FliC, might be contributing to the vaccine-induced protection from S. Paratyphi A challenge in vivo. We speculate that an excess of IgG1 anti-FliC antibodies induced by the exported FliC may compete with the IgG2a subtype and block binding to specific phagocyte Fc receptors that are critical for clearing an S. Paratyphi A infection. 

Retracted: No

PubPeer comments:
 Figure 3A: In the top panel, the 1901 and 1901K bands appear to look unexpectedly similar. Shown with pink boxes. In the bottom panel, sharp vertical transitions appear to be visible making it hard to judge sizes and intensities of the bands. Shown with blue arrows. Image made darker to bring out details. Could the authors please check? ![file] , 
 Figure 3B. Two panels that represent different experiments appear to look unexpectedly similar. Shown with red boxes. ![file] ,