Chicken Cecum Immune Response toSalmonella entericaSerovars of Different Levels of Invasiveness

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Abstract
Day-old chicks are very susceptible to infections withSalmonella entericasubspecies. The gut mucosa is the initial site of host invasion and provides the first line of defense against the bacteria. To study the potential of differentS. entericaserovars to invade the gut mucosa and trigger an immune response, day-old chicks were infected orally withSalmonella entericaserovar Enteritidis,S. entericaserovar Typhimurium,S. entericaserovar Hadar, orS. entericaserovar Infantis, respectively. The localization ofSalmonellaorganisms in gut mucosa and the number of immune cells in cecum were determined by immunohistochemistry in the period between 4 h and 9 days after infection. Using quantitative real-time reverse transcription (RT)-PCR, mRNA expression of various cytokines, chemokines, and inducible nitric oxide synthase (iNOS) was examined in cecum. As a result, allS. entericaserovars were able to infect epithelial cells and the lamina propria. Notably, serovar Enteritidis showed the highest invasiveness of lamina propria tissue, whereas serovars Typhimurium and Hadar displayed moderate invasiveness and serovar Infantis hardly any invasion capabilities. Only a limited number of bacteria of all serovars were found within intestinal macrophages. Elevated numbers of granulocytes, CD8+cells, and TCR1+cells and mRNA expression rates for interleukin 12 (IL-12), IL-18, tumor necrosis factor alpha factor, and iNOS in cecum correlated well with the invasiveness of serovars in the lamina propria. In contrast, changes in numbers of TCR2+and CD4+cells and IL-2 mRNA expression seemed to be more dependent on infection of epithelial cells. The data indicate that the capability ofSalmonellaserovars to enter the cecal mucosa and invade lower regions affects both the level and character of the immune response in tissue.