Adaptive responses during giardia-host interactions

Abstract

Giardia lamblia is a major cause of water-borne enteric disease. The parasite is a noninvasive, flagellated protozoan that inhabits the upper part of the small intestine of its vertebrate hosts. Differentiation is central to its success as a pathogen since transmission to a new host is dependent on a rapid developmental metamorphosis between the infectious cyst and the disease-causing trophozoite. To investigate this process the genome content throughout the parasite's life cycle was determined.

During vegetative growth of the binucleated trophozoites, each nucleus cycle between a diploid (2N) and tetraploid (4N) genome content. Fully differentiated cysts contain four nuclei, each with a ploidy of 4N, resulting in a cyst ploidy of 16N. The newly excysted cell has a ploidy of 16N, but divide twice to form four trophozoites containing two diploid nuclei. The polypIoidy of the cyst is likely to be of importance in infection, since it will de facto increase the infectious dose with a factor of four.

During an infection, the trophozoite attaches to the small intestinal cells using a unique cytoskeletal organelle, the adhesive disk. The structure and composition of this organelle was determined throughout the life cycle of the parasite. A new disk protein, SALP-1, was identified.

During encystation the levels of the mRNAs of the disk proteins were down regulated but the protein levels were constant. The disk proteins are stored in fragments within the cyst and assembled again during excystation. Transcriptional regulation during differentiation was further studied by characterization of a Myb-related transcription factor found in the Giardia genome. This protein was shown to localize to both nuclei, be upregulated during encystation and bind to a target sequence found in the promoters of genes induced during encystation. The target sequence of the Myb protein was shown to be crucial for stage-specific expression. This implies that this transcription factor is important for gene regulation during differentiation. Taken together, these data show that regulation at both protein and transcriptional level are important during differentiation.

Infection is not accompanied by heavy inflammation, although a humoral immune response is triggered that is associated to clearance of the parasite and reduction of symptoms. A large-scale proteomic study to identify what proteins the Immoral immune system targets during infection was undertaken. Sixteen immunoreactive proteins were identified using serum samples from patients with acute giardiasis. Most of the identified proteins were associated with the cytoskeleton (tubulins, alpha- and beta-giardins) or metabolism (ADI, OCT, enolase). The proteins localized to the adhesive disk were also shown to be immunogenic.

In addition, the major supernatant proteins after in vitro interaction between Giardia and intestinal epithelial cells were identified. The secreted Giardia proteins were shown to be the immunogenic metabolic enzymes (ADI, OCT and enolase). These proteins are also secreted from other pathogens living on mucosal surfaces. We hypothesize that the identified proteins are essential in the pathogenesis related to Giardia infection.