The role of intestinal dendritic cells and the microbiota during oral Salmonella infection

Abstract

The intestinal pathogen Salmonella causes millions of infections per year worldwide. The immune response to these bacteria involves interactions between several cell types via specific molecules and is under the influence of the intestinal microbiota. Dendritic cells (DC) initiate immune responses including those to Salmonella. Toll-like receptors and CD40 can act synergistically on DC activation but their cooperativity during bacterial infection had not been addressed. Salmonella-infected mice lacking MyD88, CD40 or both (DKO) showed that synergistic effects of CD40 and MyD88 do not influence host survival, bacterial burden in intestinal tissues or serum levels of IFN-γ and IL-10 during infection. However, cooperativity between CD40 and MyD88 influenced IL-10 production in DC-T cell co-cultures using killed Salmonella as the antigen. Moreover, cooperative effects of CD40 and MyD88 on T cell effector functions such as proliferation and IFN-γ production were influenced by the complexity of the antigen. Although some studies had addressed the role of DC subsets in infection, the influence of the CD103+CD11b+ DC in Salmonella infection was unknown. Studies using mice with a reduced CD103+CD11b+ DC population in mesenteric lymph nodes (MLN) and small intestine lamina propria showed no alterations in Salmonella colonization of intestinal tissues or spleen. Moreover, mechanisms important in host survival to Salmonella infection such as IFN-γ production analyzed by flow cytometry and antibody production analyzed by ELISA were not altered. This suggests that the absence of CD103+CD11b+ DC has a limited effect on the host response to Salmonella infection. Interactions between Salmonella and the microbiota at an early phase of colonization have been reported, but the role of the microbiota later during infection was poorly understood. Salmonella-infected germ-free (GF) and antibiotic treated mice (ABX) revealed a higher bacterial burden in the MLN, which seems to be due to increased intestinal bacterial translocation to MLN caused by the lack of the microbiota. Furthermore, higher IFN-γ in MLN of GF and ABX relative to controls was detected by flow cytometry despite similar IL-12 levels six days post infection. While the higher IFN-γ in MLN of ABX mice correlated to the severity of infection, a lack of immune signals provided by the microbiota from birth may influence IFN-γ production in GF mice. These studies provide further information about the role of DC and the microbiota during Salmonella infection, which could be used for the generation of vaccines or treatments for this infection.