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Infect. Immun. doi:10.1128/IAI.00416-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Inhibition of Salmonella Motility and SPI-1 Mediated Entry into Epithelial Cells by a Protective Anti-Lipopolysaccharide Monoclonal IgA

Biomedical Sciences Program, University at Albany School of Public Health, Albany, NY 12201; Division of Infectious Diseases, Wadsworth Center, New York State Department of Health, Albany, NY 12208

^* To whom correspondence should be addressed. Email: nmantis{at}wadsworth.org.

	Abstract

Secretory IgA (SIgA) antibodies directed against the O-antigen of lipopolysaccharide (LPS) are the primary determinants of mucosal immunity to gram-negative enteric pathogens. However, the underlying mechanisms by which these antibodies interfere with bacterial colonization and invasion of intestinal epithelial cells are not well understood. In this study, we report that Sal4, a protective, anti-O5 specific monoclonal IgA, is a potent inhibitor of Salmonella enterica serovar Typhimurium (S. Typhimurium) flagellum-based motility. Using video light microscopy, we observed that Sal4 completely, and virtually instantaneously, "paralyzed" laboratory and clinical strains of S. Typhimurium. Sal4-mediated motility arrest preceded and occurred independently of agglutination. Polyclonal anti-LPS IgG antibodies and F(ab)₂ fragments were as potent as was Sal4 at impeding bacterial motility, whereas monovalent Fab fragments were 5-10 fold less effective. To determine whether motility arrest can fully account for Sal4's protective capacity in vitro, we performed epithelial cell infection assays in which the requirement for flagellar motility in adherence and invasion was by-passed by centrifugation. Under these conditions, Sal4-treated S.Typhimurium remained non-invasive, revealing that the monoclonal IgA, in addition to interfering with motility, has an effect on bacterial uptake into epithelial cells. Sal4 did not, however, inhibit bacterial uptake into mouse macrophages, indicating that the antibody interferes specifically with Salmonella pathogenicity island 1 (SPI-1) -dependent – but not SPI-1-independent – entry into host cells. These results reveal a previously unrecognized capacity of SIgA to "disarm" microbial pathogens on mucosal surfaces and prevent colonization and invasion of the intestinal epithelium.