Cellular invasion by avian Eimeria species

Invasion of an appropriate host cell by sporozoites of avian Eimeria species is the first step in the establishment of coccidial infection, The invasion process is a multi-step event that begins with the contact of the apical end of the initial invasive stage (sporozoite) with the host cell membrane and culminates with the sealing of the sporozoite in a vacuole within the cell, Sporozoite motility appears to be essential for cell invasion because chilling or metabolic poisoning of the host cell has little effect on numbers of intracellular sporozoites, whereas similar treatment of the sporozoite markedly reduces the numbers that invade. Sporozoites of each species of Eimeria invade narrowly defined, specific sites in the intestines of the natural and foreign host birds. The predilection for different sites by Eimeria species has been attributed to a number of traits including (1) excystation time and (2) the presence of common antigens in the sporozoite and the host cell, possibly incorporated into reciprocal receptor or recognition molecules that mediate the invasion process. Evidence for the existence of recognition molecules on the host cell is provided by extensive studies that show that sporozoite invasion can be inhibited with a variety of compounds that selectively neutralize, bind, or cleave specific host cell surface moieties. The existence of sporozoite receptors is suggested by their possession of lectins with sugar affinities that correlate with specific glycoconjugates in the intestine, and by the ability of monoclonal antibodies, developed against putative receptors, to inhibit sporozoite invasion. Sporozoites also secrete a variety of molecules, including proteinases and thrombospondin-like compounds, which may aid in attachment to the host cell or to invagination of the host cell membrane which contributes to the formation of the parasitophorous vacuole.