Development and evaluation of candidate recombinant Salmonella-vectored Salmonella vaccines.

Attenuated Salmonella Enteriditis (ΔSE) recombinant vaccine vectors incorporating a Salmonella flagellar filament protein (fliC) subunit, a putative cell-mediated epitope, for expression of the lamB gene (encoding a maltose outer membrane porin), with or without co-expression of a putative immune-enhancing CD154 oligopeptide, were developed and compared with wild-type Salmonella Enteriditis (experiments 1 and 2) or the attenuated ΔSE empty vector (experiment 3) as initial vaccine candidates against Salmonella infection. A total of 3 experiments were performed to assess the infection and clearance rate of each of these constructs. Each construct or Salmonella Enteriditis was orally administered to broiler chicks at day of hatch by oral gavage (~10(8) cfu/chick). In experiments 1 to 3, liver-spleen and cecal tonsils were removed aseptically for recovery of wild-type Salmonella Enteriditis or ΔSE mutants. These experiments suggested that cell surface expression of fliC alone markedly increased the clearance rate of the vector at or before 21d postvaccination in all 3 experiments. In a fourth experiment, broilers were vaccinated with one of the vaccine constructs or the ΔSE empty vector and then challenged with wild-type Salmonella Typhimurium. At 19 d posthatch, 16 d postinfection, neither candidate protected against challenge significantly better than the ΔSE empty vector, although there was significantly less Salmonella recovered from vaccinated chickens as compared with nonvaccinated controls. These experiments indicate that these experimental vaccines did not protect against heterologous challenge or enhance clearance after Salmonella Typhimurium challenge; as such, their value as vaccines is limited. The increased clearance of the candidate vaccines, particularly the vector expressing fliC alone, may have value in that the fliC epitope may decrease the clearance time of other recombinant vectored Salmonella vaccines.

Effect of probiotic treatment in broiler chicks on intestinal macrophage numbers and phagocytosis of Salmonella enteritidis by abdominal exudate cells.

Previous data have indicated that a Lactobacillus-based probiotic culture (FM-B11) is efficacious in reducing Salmonella Enteritidis colonization within 24 h when administered within 1 h of challenge. We hypothesized that the innate immune system, specifically macrophages, may play a role in the observed reduction of Salmonella Enteritidis colonization with probiotic treatment. Day-of-hatch chicks were challenged with Salmonella Enteritidis and then treated with the probiotic culture 1 h later. Three other treatment groups were not treated (negative control), challenged only, or treated with probiotic only. In all experiments, probiotic treatment on the day of hatch reduced (P < 0.05) cecal Salmonella Enteritidis recovery as compared with the control treatment. In experiments 1 and 2, immunohistochemistry was used to evaluate the presence of macrophages (KUL01+) in the ileum and cecum of 7 to 10 chicks per group at 24 h posttreatment. In experiment 1, the number of macrophages observed per 10,000 microm(2) in the ileum of Salmonella Enteritidis-challenged chicks was higher (P < 0.05) than that of nonchallenged chicks (4.87 +/- 0.31 vs. 3.05 +/- 0.19). In the cecum, there were more (P < 0.05) macrophages per 10,000 microm(2) in chicks receiving probiotic treatment without challenge than in negative control chicks (5.32 +/- 0.41 vs. 3.66 +/- 0.35). However, in experiment 2 we found no differences among treatments in the numbers of macrophages for both the ileum and cecum. Experiments 3 and 4 were performed to evaluate the ability of Sephadex-elicited abdominal exudate cells (AEC) from chicks to phagocytose Salmonella Enteritidis in vitro. Abdominal exudate cells were isolated from the abdominal cavity, maintained in tissue culture plates overnight, and then assayed for phagocytic activity by coincubating with Salmonella Enteritidis. In experiment 3, more (P < 0.05) Salmonella Enteritidis was recovered from AEC derived from probiotic-treated chicks than in any other treatment. However, in experiment 4, all treatments resulted in similar levels of elicited AEC, and phagocytosis of Salmonella Enteritidis was at low levels in all groups. Although not conclusive, the modest differences detected in experiments 1 and 3, and the fact that those differences were not repeatedly detectable, suggest that these macrophage-related changes were not solely responsible for the reductions of Salmonella Enteritidis following probiotic treatment.