Ionophore-resistant mutants of Toxoplasma gondii reveal host cell permeabilization as an early event in egress

Toxoplasma gondii is an obligate intracellular pathogen within the phylum A picomplexa. Invasion and egress by this protozoan parasite are rapid events that are dependent upon parasite motility and appear to be directed by flu ctuations in intracellular [Ca2+]. Treatment of infected host cells with th e calcium ionophore A23187 causes the parasites to undergo rapid egress in a process termed ionophore-induced egress (IIE). In contrast, when extracel lular parasites are exposed to this ionophore, they quickly lose infectivit y (termed ionophore-induced death [IID]). From among several Iie(-) mutants described here, two were identified that differ in several attributes, mos t notably in their resistance to IID. The association between the Iie(-) an d Iid(-) phenotypes Is supported by the observation that two-thirds of muta nts selected as Iid(-) are also Iie(-). Characterization of three distinct classes of IIE and IID mutants revealed that the lie(-) phenotype is due to a defect in a parasite-dependent activity that normally causes infected ho st cells to be permeabilized just prior to egress. Iie- parasites underwent rapid egress when infected cells were artificially permeabilized by a mild saponin treatment, confirming that this step is deficient in the Iie- muta nts. A model is proposed that includes host: cell permeabilization as a cri tical part of the signaling pathway leading to parasite egress. The fact th at Iie(-) mutants are also defective in early stages of the lytic cycle ind icates some commonality between these normal processes and IIE.