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is a natural host for many bacteria. Some are digested, others are not, and lead to the death of the host after their proliferation. Dictyostelium discoideum develops defence mechanism against the intruder. It alters the protein set expressed. Dictyostelium discoideum may constitute a model organism for macrophage. The results may contribute to better understanding of mechanisms how macrophage defence itself against intruders. An innate immune response against Brucella in humans was published in 2002 
Immune-like phagocyte activity in the social amoeba 
Social amoebae feed on bacteria in the soil but aggregate when starved to form a migrating slug. This article describes a previously unknown cell type in the social amoeba, which appears to provide detoxification and immune-like functions and which is termed sentinel (S) cells. S cells were observed to engulf bacteria and sequester toxins while circulating within the slug. A Toll/interleukin-1 receptor (TIR) domain protein, TirA, was also required for some S cell functions and for vegetative amoebae to feed on live bacteria. This apparent innate immune function in social amoebae, and the use of TirA for bacterial feeding, suggest an ancient cellular foraging mechanism that may have been adapted to defense functions well before the diversification of the animals. This properties stress the appropriateness for using Dictyostelium as a model organism to study macrophages.
The killing machinery
The transcriptomics array
After the completion of the Dictyostelium discoideum Genome Project it is possible to construct a whole cell transcriptomics or proteomics . However, only a transcriptomics arrays of about 6.500 transcripts of Dictyostelium is available. With this subset we investigated the cellular response upon infection by Legionella . Other projects are underway.
Legionnaires' disease (LD) and Pontiac Fever
Legionellosis is an infectious disease caused by bacteria belonging to the genus Legionella, mainly of the species pneumophila, an ubiquitous aquatic organism. Their natural host are aquatic amoeba.
Legionellosis takes two distinct forms:
Legionnaires' disease is the more severe form of the infection and produces pneumonia.
Pontiac fever produces a milder respiratory illness without pneumonia.
Legionnaires' disease acquired its name in 1976 when an outbreak of pneumonia occurred among people attending a convention of the American Legion in Philadelphia. On January 18, 1977 the causative agent was identified as a previously unknown bacterium, subsequently named Legionella.
Legionella pneumophila infects macrophages by phagocytosis. Instead of being digested, the bacteria is able to proliferate by enslaving the cell machinery. In Dictyostelium discoideum, L. pneumophila behaves similarly. Therefore, Dictyostelium resembles a good model of macrophages.
Figure 3: Simplified model of differential gene expression upon infection. Only a selection of those gene products influenced by the infection is shown. Red means up regulated and green down regulated genes .
Differential gene expression of Dictyostelium discoideum after infection with Legionella pneumophila was investigated using DNA microarrays. Investigation of a 48 h time course of infection revealed several clusters of co-regulated genes, an enrichment of preferentially up- or down regulated genes in distinct functional categories and also showed that most of the transcriptional changes occurred 24 h after infection. A detailed analysis of the 24 h time point post infection was performed in comparison to three controls, uninfected cells and co-incubation with Legionella hackeliae and L. pneumophilaDdotA. One hundred and thirty-one differentially expressed D. discoideum genes were identified as common to all three experiments and are thought to be involved in the pathogenic response. Functional annotation of the differentially regulated genes revealed that apart from triggering a stress response Legionella apparently not only interferes with intracellular vesicle fusion and destination but also profoundly influences and exploits the metabolism of its host. For some of the identified genes, e.g. rtoA involvement in the host response has been demonstrated in a recent study, for others such a role appears plausible. The results provide the basis for a better understanding of the complex host-pathogen interactions and for further studies on the Dictyostelium response to Legionella infection .
August 13, 2010
Institute for Biochemistry I, Joseph-Stelzmann-Strasse 52, D50931 Cologne
Suggestions and wishes: Budi Tunggal