Gastrointestinal Cell Mediated Immunity and the Microsporidia

The microsporidia are obligate intracellular parasites that were previously thought to be ‘‘primitive’’ eukaryotes, but which are now recognized to be either related to or a sister group to the Fungi [1]. These protists are relatively common enteric pathogens that usually result in self-limited or asymptomatic infections in humans. Microsporidia were initially recognized as pathogens causing pébrine in silkworms in the late 19th century by Louis Pasteur. The cause of pébrine was named Nosema bombycis, providing the initial organism in the phylum Microsporidia that now includes approximately 170 genera containing over 1,400 species, many of which are of medical, veterinary, and agricultural importance [2]. All of the microsporidia form characteristic spores containing a specialized invasion organelle consisting of an internal polar tube that is attached to the inside of the anterior end of the spore by an anchoring disc and coils around the sporoplasm in the spore. During germination, the polar tube rapidly everts, forming a hollow tube that brings the sporoplasm into intimate contact with the host cell. The polar tube provides a bridge to deliver the sporoplasm to the host cell. The mechanism by which the polar tube interacts with the host cell membrane is not fully known, but this may require the participation of the host cell. Glycosylation is probably important in polar tube structure and function. This is supported by studies demonstrating carbohydrate residues on intact polar tubes, e.g., concanavalin A binds to the polar tube of several different microsporidia and there is biochemical evidence that major polar tube protein (PTP1) has O-linked mannosylation [3]. Pretreatment of host cells with mannose decreased infection by Encephalitozoon hellem consistent with an interaction between polar tube mannosylation and some unknown host cell mannose-binding molecule [3]. The spore coat consists of an electron-dense, proteinaceous exospore, an electron-lucent endospore composed of chitin and protein, and an inner membrane or plasmalemma. Several spore wall proteins are also modified by post-translational glycosylation involving mannosylation (L. Weiss, unpublished data). These modifications are likely important in adherence of the spore wall to mucin or to host cells during passage of the spores in the gastrointestinal tract, facilitating invasion; e.g., exogenous glycosaminoglycans decrease the adherence of spores to a host cell monolayer [4]. Spores are resistant to environmental conditions and this allows these organisms to persist in the environment, facilitating their transmission between hosts. These resistant spores may also allow these organisms to persist in infected tissues. The genome size of microsporidia varies from 2.3 to 19.5 Mb [2], with Encephalitozoon cuniculi being 2.9 Mb, E. hellem 2.5 Mb, and Encephalitozoon intestinalis 2.3 Mb, which are among the smallest eukaryotic nuclear genomes identified. These organisms are probably diploid, there are almost no introns in these genomes, the gene density is high, and proteins are shorter than corresponding yeast genes. These intracellular pathogens have lost many metabolic pathways, becoming dependent on their host cells [5]. A collaboration between Patrick Keeling (University of British Columbia), Saul Tzipori (Tufts University), Elizabeth Didier (Tulane University), Louis M. Weiss (Albert Einstein), and the Broad Institute (MIT) has resulted in the sequencing of the genomes of Enterocytozoon bieneusi, E. intestinalis, E. cuniculi (types 1, 2, and 3), and E. hellem, providing genomic data on these human pathogenic microsporidia [5]. In addition, the genome of Anncaliia (Brachiola) algerae is currently undergoing genome annotation by this same collaborative group [6]. Other groups have completed the genomes of several insect and invertebrate microsporidian pathogens, including Nosema ceranae, Octosporea bayeri and Vavria culicis floridensis, Nosema bombycis, and the soil nematode pathogen Nematocida parisii. Genome data on these various microsporidia is being made available at EuPathDB under MicrosporidiaDB (http://microsporidiadb.org/micro/).