The effect of vaccination with a Salmonella enteritidis aroA mutant on early cellular responses in caecal lamina propria of newly-hatched chickens.

When newly hatched chicks are inoculated with a Salmonella strain, they induce a rapid onset of resistance to intestinal colonization by other Salmonella strains. The exact mechanism of this early colonization-inhibition is not known. To study host-related contributions to this phenomenon, the kinetics of immune cell infiltration in the caecal wall was analyzed during the first 10 days after vaccination of newly hatched chickens with a Salmonella enterica serovar Enteritidis aroA mutant, and infection 1 day later with a virulent S. enterica serovar Enteritidis strain. These data were correlated with bacterial colonization and clearance of the Salmonella Enteritidis challenge strain. Bacteriological data showed that vaccinated animals had a much lower number of challenge bacteria in their organs and caecal contents the first days post-challenge, relative to unvaccinated animals. Immunohistochemical analysis of the caecal lamina propria revealed that heterophils started infiltrating the caecal lamina propria from 12 h post-vaccination. Macrophages and T-lymphocytes started infiltrating from 20 h and B-lymphocytes from 24 h post-vaccination. These data imply that immune cells already colonized the caecal wall at the time of challenge in vaccinated animals. The presence and activity of these cells in the caecal wall shortly after administration of a Salmonella Enteritidis aroA mutant might contribute to the inhibition of colonization of a virulent Salmonella strain, subsequently administered.