Antigens

Two-dimensional gel electrophoresis has identified more then 1000 spots after 35S-methionine labelling of tachyzoites cultured in vitro (Handman et al. 1980), which probably reflects at least the number of different proteins present. The different stages of the parasite share common antigens and express stage-specific antigens, both between oocysts and other stages (Kasper and Ware 1985) and between bradyzoites and tachyzoites (Woodison and Smith 1990).

The antigens involved in the attachment of T. gondii to the host cell, and antigens involved in penetration of the host cell and formation of the parasitophorous vacuole have received special attention (Cesbron-Delauw et al. 1989; Charif et al. 1990; Achbarou et al. 1991; Leriche and Dubremetz 1991; Saavedra et al. 1991; Ossorio et al. 1992).

The most abundant surface antigen is a 30-kDa protein (Burg et al. 1988). The P30-antigen (surface antigen 1, SAG1) appears to be conserved between different T. gondii isolates (Bülow and Boothroyd 1991; Sibley and Boothroyd 1992b). The SAG1 is found only in tachyzoites, where it constitutes up to about 5% of the total tachyzoite protein. It is distributed evenly on the surface of the tachyzoite and in the tubular network of the parasitophorous vacuole and is shed from the surface of the parasite at the moving junction between the parasite and the host cell during the invasion process (Dubremetz et al. 1985). A T. gondii mutant of the RH strain lacking the SAG1 surface antigen has been described (Kasper 1987), which is able to grow in continuous in vitro culture. Surface neoglycoproteins have been identified (Robert et al. 1991), but their role in antigenicity and attachment/ invasion is still unknown. A 22-kDa SAG2 has been sequenced (Prince et al. 1990), and a 43-kDa SAG3 has recently been cloned and sequenced and has been found to have structural similarities to SAG1 (Cesbron-Delauw et al. 1994). SAG4, is a 18-kDa bradyzoite-specific antigen (Ödberg-Ferragut et al. 1996). The first-sequenced bradyzoite-specific antigen was a 30-kDa antigen belonging to the heat shock protein family (Bohne et al. 1995).

The major surface proteins of 43-, 35-, 30-, and 22 kDa are anchored by a glycosylphosphatidyl-inositol anchor (Nagel and Boothroyd 1988; Tomavo et al. 1989). Early work suggested the presence of a

'penetration-enhancing factor' secreted from the rhoptries (Lycke and Norrby 1966); several rhoptry proteins (ROP) have been identified later: ROP1, which is a 61-kDa rhoptry protein (Schwartzman and Krug 1989; Ossorio et al. 1992),

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ROP2, 3, and 4 (Gelder et al. 1993), ROP5 (Leriche and Dubremetz 1991), ROP6 (Dubremetz et al. 1987), and ROP7 (Leriche and Dubremetz 1991). ROP2 has recently been cloned and sequenced as a 54-kDa protein (Saavedra et al. 1991).

Five dense granule proteins have been identified, and have been shown to associate with the intravacuolar network: GRA1, 2, and 4 (Prince et al. 1989) and the PVM's, GRA1 and 3 (Achbarou et al. 1991).

Excreted-secreted antigens (ESA) (Decoster et al. 1988; Cazabonne et al. 1994), are secreted from the rhoptries and the dense granules. At least three ESA's are located in the dense granules, GRA1(P23), GRA2(P28.5) and GRA3(P21), and are released inside the parasitophorous vacuole (Leriche and Dubremetz 1990, 1991). The GRA3 antigen is inserted into the PVM after secretion (Ossorio et al. 1994), and GRA2 and GRA5 are associated with the cyst wall (Torpier et al. 1993). The 23-kDa calcium-binding protein is also found in the parasitophorous vacuole of the host cell (Cesbron-Delauw et al. 1989). The ESA's were reviewed by Cesbron-Delauw and Capron (1993). Different alleles have been demonstrated for the SAG1, GRA2, and GRA4 antigens thus far (Meisel et al. 1996). Bradyzoite-specific antigens are just beginning to be described. One of the first was bradyzoite antigen 1, BAG1, belonging to the heat shock protein family (Bohne et al. 1995); the first surface antigen expressed exclusively in brazyzoites was named SAG4 (Odberg-Ferragut et al. 1996).

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