The "amoebiasis program" of the Institute constitutes the world’s largest program dealing with the various aspects concerning the biology and pathogenicity of the protozoan parasite Entamoeba histolytica. Since its initiation in 1988, the program has produced more than 90 scientific publications and members of the program have received various awards. In 1997, Dr. Matthias Leippe was honored by the German Society for Tropical Medicine as well as by the German Society for Hygiene and Microbiology particularly for his outstanding scientific contributions on ‘amoebapores’, pore-forming peptides of the amoebae, which are considered responsible for killing of host cells.

During the last year the program has concentrated on three major topics: (1) characterization of E. histolytica evasion strategies and of proteins considered important for host tissue destruction, (2) generation of molecular tools for more comprehensive analyses of the parasite genome, and (3) development of a vaccine to prevent amoebic diseases.

(1) Entamoeba histolytica is characterized by its extraordinary capacity to destroy human tissues leading to massive and sometimes lethal pathological alterations such as ulcerative colitis or abscesses of various organs, most commonly of the liver. Amoebapores, a family of three different pore-forming peptides are the main effector molecules of the amoeba for killing of host cells. Comparison of this class of molecules between E. histolytica and the closely related but nonpathogenic species Entamoeba dispar revealed minor differences in primary structure but considerably higher expression of all three pore-forming peptides in E. histolytica.

The mechanism that confers resistance of E. histolytica trophozoites against attack by its own cytolytic effector peptides was elucidated, indicating that the specific phospholipid composition together with a relatively high cholesterol content of amoebic membranes are responsible for protection.

After tissue invasion E. histolytica is exposed to elevated levels of reactive oxygen species, which are highly toxic and therefore have to be inactivated. Proteins involved in protection, such as an iron-containing superoxide dismutase (EhFeSOD), an NADPH oxidase (Eh34) and a thiol-dependent peroxidase (Eh29) were further characterized. Interestingly, at least two of these molecules (EhFeSOD, Eh29) were found to be overexpressed in amoebae made resistant to metronidazole, the drug of choice for the treatment of invasive amoebiasis.

(2) At present, a number of questions concerning gene organization of E. histolytica as well as structure and ploidy of chromosomes have not been resolved. To facilitate the analysis of the E. histolytica genome, YAC- and cosmid-libraries containing genomic DNA of the amoebae were constructed. In addition, conditions for the separation of amoeba chromosomes were optimized, and more than 300 expressed sequence tags (ESTs) from an E. histolytica cDNA library were isolated and sequenced.

(3) Progress was made in the development of an amoebiasis vaccine. Immunization with an epitope of 25 amino acid residues only, derived from the 170 kDa amoeba surface lectin, was found to be sufficient to confer protection against amoebic liver abscesses formation in gerbils.