Infection of dendritic cells by enterobacteriaceae

Dendritic cells (DC) are potent antigen-presenting cells that play a crucial role in initiation and modulation of specific immune responses. Various pathogens like viruses or bacteria are able to persist inside DC. In this study we investigated the ability of the Gram-negative bacteria Salmonella typhimurium and Escherichia coli to infect DC. DC isolated from peripheral blood of healthy donors were infected with wild-type S. typhimurium and a nonpathogenic E. coli stool isolate. Association of bacteria with DC was assessed by labeling of the bacteria with green fluorescent protein. Both Gram-negative bacteria were associated with DC as evidenced by microscopy and flow cytometry. The intracellular location could be confirmed by lysis of DC and subsequent determination of colony-forming units on agar plates, which showed a rapid decline in viable Gram-negative bacteria 6 h after infection, being by far more pronounced for E. coli than for S. typhimurium. Testing the stimulation of T cells by infected versus uninfected but otherwise identically treated human immature DC in a mitogen-dependent T cell proliferation assay, we found that S. typhimurium. but not E. coli exhibited a suppressive effect on T cell stimulation, being most significant on days 3-5 after infection. Thus, suppression of dendritic cell function was associated with an enteropathogenic bacterium, S. typhimurium, which can cause severe forms of enteritis. The bacteria with normally mild or no gastric symptoms, E. coli, had no influence on stimulation of T cells by DC.