Pathogenesis Host cell invasion

Early work on invasion of the host cell has been reviewed by Werk (1985), and the invasion process has recently been reviewed by Schwartzman and Saffer (1992), Dubremetz and Schwartzman (1993), and Kasper and Mineo (1994). The invasion of the host cell is an active process involving attachment, host cell membrane penetration, formation of a moving junction with the host cell membrane, formation of the PVM, and subsequent closure of the host cell membrane after entry. The invasion process is calcium dependent (Bonhomme et al. 1993).

The major T. gondii surface protein, SAG1, has been implicated in attachment and penetration (Grimwood and Smith 1992; Mineo et al. 1993), and host cell laminin is also involved in attachment (Joiner 1991b; Furtado et al. 1992a,b). There are higher concentrations of laminin-binding proteins (Joiner et al. 1989) and Fc-binding sites at one end of the tachyzoite (Budzko et al. 1989). Considering the wide range of animal hosts, it is likely that T. gondii has several molecules which can be important in cell adhesion (Dubremetz and Schwartzman 1993; Mineo et al. 1993).

After entry into a host cell, T. gondii is surrounded by the PVM, partly derived from the host cell membrane and partly containing parasite material. The PVM lacks host cell plasma membrane markers, and freeze fracture analysis indicates that the PVM may completely lack intramembranous particles and may consist of only a phospholipid bilayer. The dense granules probably contribute to intravacuolar tubules (Achbarou et al. 1991; Dubremetz et al. 1993). The main phospholipid of the rhoptries is phosphatidylcholine (Foussard et al. 1991), which may be identical to the phospholipids found in the PVM (Joiner 1991a). Toxoplasma gondii phospholipase has been suggested to play a role in invasion, possibly by softening of the host cell membrane after attachment (Saffer and Schwartzman 1991). The micropore is formed by an invagination of the outer membrane of the pellicle (Nichols and Chiappino 1987).

Toxoplasma gondii may infect almost any host cell, but it seems to have a preference for monocytes, macrophages, and muscle cells. In the central nervous system T. gondii can infect and grow in neurons, astrocytes, microglia, and oligodendrocytes (Fischer et al. 1997).

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