Submitted to: Molecular and Biochemical Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 30, 1995
Publication Date: N/A
Interpretive Summary: Infection by the protozoan (single celled) parasite Toxoplasma gondii causes mental retardation and loss of vision in congenitally infected children and abortion in livestock. Humans become infected by ingesting the parasite encysted in meat or by ingesting food and water contaminated with feces of infected cats containing the resistant form (oocyst) of T. gondii. Although a lot is known of the biology of the stages of the parasite present in tissues (tachyzoites and bradyzoites) little is known of the sporozoites in oocysts. Scientists at the Beltsville Agricultural Research Center and the Montana State University have found for the first time that after the entry of the sporozoites in cells, the sporozoites lack a predominant antigen found in tachyzoites and the sporozoites were surrounded by an unusually large vacuole. The results will be useful in elucidating the pathogenic mechanism because oocyst induced infections are more serious than the infections acquired by ingesting tissue cysts from meat.
The invasion of host cells by sporozoites of Toxoplasma gondii leads to the formation of parasitophorous vacuoles that are distinctly different from those surrounding tachyzoites. In sporozoite-infected cells, the fluid-filled space surrounding the sporozoite is many times larger in volume than the sporozoite, essentially lacks granular or tubular structures, and has no detectable continuous parasitophorous vacuolar membrane when prepared by conventional electron microscopic methods. Consistent with the ultrastructural differences, dense-granule protein GRA3, which associates with the parasitophorous vacuolar membrane of tachyzoites, was not detected by indirect immunofluorescence in sporozoite-infected cells 2-12 hr post-inoculation or by Western blot analysis of sporozoite extracts. Western blots incubated with the aROP/DG antiserum, which recognizes tachyzoite rhoptry and dense-granule proteins, revealed numerous other antigenic differences between sporozoites and tachyzoites. Cell cultures inoculated with sporozoites were monitored at various intervals for the expression of GRA3 and the developmentally- regulated tachyzoite surface protein SAG1. Expression of SAG1 and GRA3 was first observed in 30 % of the sporozoite-infected cells at 12 and 15 hr post-inoculation, respectively, and in all intracellular parasites at 24 hr. Parasite replication was only observed in sporozoite-infected cells that were positive for GRA3 and SAG1. Thus, these data indicate that sporozoites and their interaction with host cells differ substantially from tachyzoites and the expression of tachyzoite-specific proteins is likely required for parasite replication.