Rapid Protection of Gnotobiotic Pigs against Experimental Salmonellosis following Induction of Polymorphonuclear Leukocytes by AvirulentSalmonella enterica

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Abstract
Oral inoculation of 5-day-old gnotobiotic pigs withSalmonella entericaserovar Typhimurium strain F98 resulted in severe enteritis and invasive disease. Preinoculation 24 h earlier with an avirulent mutant ofSalmonella entericaserovar Infantis (1326/28) completely prevented disease for up to 14 days (when the experiment was terminated).S. entericaserovar Infantis colonized the alimentary tract well, with high bacterial counts in the intestinal lumen but with almost no invasion into the tissues. Unprotected pigs had highS. entericaserovar Typhimurium counts in the intestines, blood, and major nonintestinal organs. Recovery of this strain from the blood and major organs inS. entericaserovar Infantis-protected pigs was substantially reduced despite the fact that intestinal counts were also very high. Protection against disease thus did not involve a colonization exclusion phenomenon. Significant (P< 0.05) infiltration of monocytes/macrophages was observed in the submucosal regions of the intestines of bothS. entericaserovar Infantis-protectedS. entericaserovar Typhimurium-challenged pigs and unprotectedS. entericaserovar Typhimurium-challenged pigs. However, only polymorphonuclear neutrophils (PMNs) were observed throughout the villus, where significant (P< 0.05) numbers infiltrated the lamina propria and the subnuclear and supranuclear regions of the epithelia, indicating that PMN induction and positioning followingS. entericaserovar Infantis inoculation was consistent with rapid protection against the challenge strain. Similarly, in vitro experiments using a human fetal intestinal epithelial cell line (INT 407) demonstrated that, although significantly (P< 0.05) fewerS. entericaserovar Infantis thanS. entericaserovar Typhimurium organisms invaded the monolayers,S. entericaserovar Infantis induced an NF-κB response and significantly (P< 0.05) raised interleukin 8 levels and transmigration of porcine PMN. The results of this study suggest that attenuatedSalmonellastrains can protect the immature intestine against clinical salmonellosis by PMN induction. They also demonstrate that PMN induction is not necessarily associated with clinical symptoms and/or intestinal pathology.

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