Up-regulation of the MyD88-dependent pathway of TLR signaling in spleen and caecum of young chickens infected with Salmonella serovar Pullorum

TLR signaling plays a role in Salmonella infection, but less information is available in chickens infected with Salmonella serovar Pullorum. The present study with young chickens, experimentally infected with S. Pullorum, has used real-time quantitative RT-PCR to investigate the relative expression of genes of the TLR4 signaling pathway (TLR4, MyD88, TRAF6 and NF-κB) in the spleen and caecum at 1, 3, 7 and 14 days post-infection (dpi). Three-day-old specific-pathogen-free chickens were orally infected with S. Pullorum or saline (controls). In addition to gene expression, the bacterial burden of spleen and caecum was assessed and serum concentrations and total IgG were measured. Significantly more bacteria were found in the caecum than in the spleen. Concentrations of IgG were significantly higher in infected birds at 1 dpi, reached a peak at 3 dpi (P < 0.01), and then gradually decreased with the time. Infected young chickens had significant up-regulation of the expression of TLR4, MyD88, TRAF6 and NF-κB in the spleen at 3 dpi compared to controls (P < 0.05). In the caecum, only increased expression of NF-κB mRNA (P < 0.05) was observed at 1 dpi. The pattern of immune response to infection with S. Pullorum differed between spleen and caecum and did not directly correspond with pathogen burden. The observed changes indicate that the MyD88-dependent pathway of TLR4 signaling plays a role in young chickens infected with S. Pullorum.