Early Induction of Apoptosis of Human Monocyte-derived Dendritic Cells upon Infection by Virulent Legionella pneumophila.

Legionella pneumophila is the causative agent of pneumonic illness called legionnaires’ disease. it is a gram-negative monopolar-flagellated bacterium, which was shown to be a facultative intracellular pathogen that replicate within amoeba and alveolar macrophages. to identify the interaction of both virulent and avirulent legionella pneumophila with human dcs and the ability of the bacterium to multiply within human dc, the uptake, intracellular growth of the bacillus and the induction of apoptosis were investigated. in this study human monocyte-derived dcs were infected in vitro by virulent or avirulent l. pneumophila. the infection, intracellular, and the induction of apoptosis were determined over a period of time using both strains. the results provide evidence that human monocyte-derived dcs can be infected by both virulent and avirulent strains of l. pneumophila. although, dcs were able to take-up both strains, only the virulent strain was able to multiply during 48h of infection as indicated by the viable count. such infection resulted in the reduction of the viability of dcs. in contrast, avirulent strain did not affect the viability of the latter cells. moreover, induction of apoptosis in human dcs infected by virulent l. pneumophila was evident after 2h of initial infection but the avirulent strain did not induce dc-apoptosis. significant induction of necrosis was also detected 24h post infection by the virulent strain. interestingly, intracellular multiplication of l. pneumophila inside dcs was not correlated with apoptosis whereas intracellular multiplication coincided with necrosis. in conclusion, the results suggest that the virulent l. pneumophila may subvert human dcs function as antigen presenting cells by the early induction of dc-apoptosis that may contribute to the pathogenicity of the bacterium.