Characterisation of Salmonella enterica Typhimurium iron uptake mutants in human dendritic cells

Dendritic cells are antigen presenting cells which play an important role in the initiation of immune responses. This study analysed plastic adherence methods of differentiating human DCs from precursors in peripheral blood. It was determined that this is an effective method of producing DCs however if levels of GM-CSF and IL-4 are lowered differentiation appears to be slowed. FoxA is an iron-regulated outer membrane protein of Salmonella enterica Typhimurium required for the utilisation of ferrioxamines in iron uptake. Using an in vitro assay a mutant foxA strain was recovered from cultured human DCs at a higher percentage than its wild type parental strain or mutant strains carrying complementing plasmids; confirming foxA has a role in the interaction of S. enterica Typhimurium and human DCs The outer membrane proteins FepA and IroN are receptors for the siderophore enterobactin and DHBS which can also use the receptor Cir. Using strains with mutations preventing the utilisation of enterobactin and DHBS as well as in the production of the siderophore salmochelin, it was shown that only the inability to utilise DHBS (fepA iroN cir) affects recovery of S. enterica Typhimurium from within human dendritic cells. The cell surface marker expression of dendritic cells exposed to; the foxA mutant, the fepA iroN cir triple mutant and their respective parental strain were compared to mature dendritic cells. Results showed that infection by S. enterica Typhimurium causes maturation of dendritic cells. Furthermore no differences were seen between the mutants and their parental strains suggesting these mutations do not affect dendritic cell maturation. Collectively this data indicates that FoxA and DHBS utilisation play a role during Salmonella infection of human dendritic cells however they do not affect their maturation.