Salmonella flagellin induces bystander activation of splenic dendritic cells and hinders bacterial replication in vivo

Bacterial flagellin is a target of innate and adaptive immune responses during Salmonella infection. Intravenous injection of Salmonella flagellin into C57BL/6 mice induced rapid IL-6 production and increased expression of activation markers by splenic dendritic cells. CD11b⁺, CD8α⁺, and plasmacytoid dendritic cells each increased expression of CD86 and CD40 in response to flagellin stimulation, although CD11b⁺ dendritic cells were more sensitive than the other subsets. In addition, flagellin caused the rapid redistribution of dendritic cells from the red pulp and marginal zone of the spleen into the T cell area of the white pulp. Purified splenic dendritic cells did not respond directly to flagellin, indicating that flagellin-mediated activation of splenic dendritic cells occurs via bystander activation. IL-6 production, increased expression of activation markers, and dendritic cell redistribution in the spleen were dependent on MyD88 expression by bone marrow-derived cells. Avoiding this innate immune response to flagellin is important for bacterial survival, because Salmonella- overexpressing recombinant flagellin was highly attenuated in vivo. These data indicate that flagellin-mediated activation of dendritic cells is rapid, mediated by bystander activation, and highly deleterious to bacterial survival.