Salmonella enterica serovar Enteritidis dam mutant induces low NOS-2 and COX-2 expression in macrophages via attenuation of MAPK and NF-κB pathways

Although dam mutants of Salmonella have been proposed as live vaccines, their capacity to trigger cell inflammatory cascades has not been fully elucidated. We investigated in detail the ability of Salmonella enterica dam mutant to activate the signalling pathways of the inflammatory response in RAW 264.7 cells. Apoptosis in macrophages treated with Salmonella dam mutant was low. Similarly, the expression of both NOS-2 and COX-2 and subsequently the production of NO and PGE2 was significantly reduced. Also, Salmonella dam mutant induced an attenuated activation of the inflammatory signalling pathway as indicated by the reduced degradation of IκBα and IκBβ and the low IκBα phosphorylation found. In addition, translocation of p65 to the nucleus was notably impaired and the amount of phosphorylated p44, p42 and p38 MAPKs was clearly reduced in extracts from dam-infected macrophages. These results indicate that the lack of ERK and p38 phosphorylation at the proper time in dam-infected cells notably reduces the engagement of subsequent signalling pathways involved in the full activation of NF-κB in response to infection. Taken together, these results suggest that Salmonella activation of both signalling cascades in the inflammatory response is a mechanism requiring Dam protein participation.