A review: Competence, compromise, and concomitance-reaction of the host cell to Toxoplasma gondii infection and development.

Abstract

Toxoplasma gondii is an important zoonotic parasite with a worldwide distribution. It infects about one-third of the world's population, causing serious illness in immunosuppressed individuals, fetuses, and infants. Toxoplasma gondii biology within the host cell includes several important phases: (1) active invasion and establishment of a nonfusogenic parasitophorous vacuole in the host cell, (2) extensive modification of the parasitophorous vacuolar membrane for nutrient acquisition, (3) intracellular proliferation by endodyogeny, (4) egress and invasion of new host cells, and (5) stage conversion from tachyzoite to bradyzoite and establishment of chronic infection. During these processes, T. gondii regulates the host cell by modulating morphological, physiological, immunological, genetic, and cellular biological aspects of the host cell. Overall, the infection/development predispositions of T. gondii -host cell interactions overtakes the infection resistance aspects. Upon invasion and development, host cells are modulated to keep a delicate balance between facilitating and eliminating the infection.

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A REVIEW: COMPETENCE, COMPROMISE, AND CONCOMITANCE—REACTION OF THE
HOST CELL TO TOXOPLASMA GONDII INFECTION AND DEVELOPMENT
Hong-Juan Peng*, Xiao-Guang Chen*, and David S. Lindsay? ?
Department of Etio-biology, School of Public Health and Tropical Medicine, Southern Medical University, 1838 North Guangzhou Rd, Guangzhou,
Guangdong Province 510515, People’s Republic of China. e-mail: lindsayd@vt.edu
ABSTRACT:
world’s population, causing serious illness in immunosuppressed individuals, fetuses, and infants. Toxoplasma gondii biology within the
host cell includes several important phases: (1) active invasion and establishment of a nonfusogenic parasitophorous vacuole in the
host cell, (2) extensive modification of the parasitophorous vacuolar membrane for nutrient acquisition, (3) intracellular proliferation
by endodyogeny, (4) egress and invasion of new host cells, and (5) stage conversion from tachyzoite to bradyzoite and establishment of
chronic infection. During these processes, T. gondii regulates the host cell by modulating morphological, physiological, immunological,
genetic, and cellular biological aspects of the host cell. Overall, the infection/development predispositions of T. gondii–host cell
interactions overtakes the infection resistance aspects. Upon invasion and development, host cells are modulated to keep a delicate
balance between facilitating and eliminating the infection.
Toxoplasma gondii is an important zoonotic parasite with a worldwide distribution. It infects about one-third of the
Toxoplasma gondii is an apicomplexan parasite capable of
propagating asexually by endodyogeny in most nucleated cells.
Approximately one-third of the world’s human population is
estimated to be chronically infected and have T. gondii tissue cysts
containing bradyzoites, making it one of the most successful
parasites infecting humans (Persson et al., 2007; Laliberte ´ and
Carruthers, 2008). Most postnatally acquired infections of T.
gondii are not symptomatic in immunocompetent individuals.
However, primary T. gondii infections may become widely
disseminated, causing damage to the brain, eyes, liver, lungs, and
lymph nodes and even causing death in immunecompromised
individuals such as Acquired Immune Deficiency Syndrome patients
and cancer patients undergoing immunosuppressive therapy. In
pregnant women, T. gondii may be transmitted to the fetus
transplacentally, resulting in tissue destruction and hence develop-
mental defects in the fetus or newborn. Toxoplasma gondii is also an
economically important parasite of livestock (Dubey et al., 1998).
Infection by T. gondii consists of an acute phase followed by a
chronic phase. Infectious stages consist of sporozoites, tachy-
zoites, and bradyzoites. Felines, including domestic cats, act as
definitive hosts of T. gondii within which the parasite completes
the sexual phase of its lifecycle. Oocysts released in the feces
undergo sporulation to form sporocysts that become a source of
infection for intermediate hosts that include a wide range of
domestic animals and humans. When oocysts are ingested by the
intermediate host through contamination of water or food,
sporocysts pass through the stomach and excyst in the intestine
to release sporozoites. Sporozoites released from sporocysts in the
intestine invade epithelial cells and differentiate into tachyzoites,
an actively replicating form of T. gondii. Tachyzoites, due to their
active multiplication, cause extensive tissue damage, may
disseminate to different tissues of the host, and are also
transplacentally (5horizontally) transmitted. Eventually, tachy-
zoites undergo stage conversion and become slowly multiplying
bradyzoites within the tissue cyst. If for some reason the tissue
cyst becomes activated, then bradyzoites can undergo stage
conversion and become tachyzoites (bradyzoites are the only stage
that can infect feline enterocytes and lead to oocyst production,
but that is beyond the scope of this review; Dubey et al., 1998).
Acute infection consists of a cycle of host cell invasion by
tachyzoites, replication of tachyzoites inside the parasitophorous
vacuole (PV), and lytic egress from host cells. Chronic infection
encompasses stage conversion of tachyzoites to bradyzoites and
modification of the PV membrane (PVM) into a tissue cyst wall
and maintenance of the tissue cyst with enclosed latent
bradyzoites. During these processes, the host cells are competent
to T. gondii attachment and invasion. Finally, the host cells then
compromise with T. gondii, establishing a balance between
facilitating and eliminating infection. Last, the host cells and the
parasite are concomitant by the establishment of a chronic
infection, resulting in tissue cysts. The present review highlights
how host cells respond to the acute infection with T. gondii
tachyzoites during invasion, intracellular proliferation, egress,
and the establishment of chronic infection with bradyzoites in a
competent/susceptible, compromised, and concomitant way.
Toxoplasma gondii actively invades nonphagocytic and
phagocytic cells
The process of T. gondii invasion into host cells is different from
phagocytosis as it involves active penetration assisted by the
actomyosin motor system of the parasite (Dobrowolski and Sibley,
1996;BlackandBoothroyd,2000)(Figs. 1and2).Differencesoccur
between nonphagocytic and phagocytic cells, but the mechanisms
are the same except for some modifications, which occur on the cell
membrane of phagocytes. Nevertheless, T. gondii can be actively
phagocytosed by competent phagocytic cells (Pacheco-Soares and
De Souza, 2000; Li et al., 2008; Walker et al., 2008).
Using cultured cells, neutrophils, even those not in contact with
parasite, are activated to form membrane ruffles, filopodia (these
filopodia migrate over the parasite surface), or tunnel-like
invaginations on the membrane near the site of penetration; these
reactions are helpful for T. gondii invasion (MacLaren et al.,
2004). There are several kinds of interactions between T. gondii
tachyzoites and neutrophils before internalization. First, many
fingerlike filopodial projections of the host cell make contact with
the lateroposterior portion of the parasite, and roughly 59% of
tachyzoites interact with neutrophils in this manner. Second,
about 25% of tachyzoites attach to the surface of neutrophils with
Received 3 December 2010; revised 29 January 2011; accepted 11
February 2011.
*These authors contributed equally to this paper.
{To whom correspondence should be addressed: Department of
Biomedical Sciences and Pathobiology, Center for Molecular Medicine,
Virginia-Maryland Regional College of Veterinary Medicine, 1410
Prices Fork Road, Virginia Tech, Blacksburg, Virginia 24061.
DOI: 10.1645/GE-2712.1
J. Parasitol., 97(4), 2011, pp. 620–628
F American Society of Parasitologists 2011
620

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a protruded conoid, through the filopodia, or directly through the
conoid-host cell interface, where secretion seems to be occurring.
The tachyzoite enters a neutrophil by the anterior (conoidal) end,
which embraces it with filopodia or by forming a glovelike tunnel
around the anterior portion of the tachyzoite, and by the odd
formation of a cavity in a lymphocyte. Third, approximately 15%
of tachyzoites are internalized by the neutrophils with the
posterior end first (MacLaren et al., 2004).
Toxoplasma gondii contains 3 special sets of secretory
organelles that help the parasite to gain entry into host cells.
These include cigar-shaped micronemes (MICs), bulb-shaped
rhoptries, and spherical dense granules. Micronemes and rhop-
tries are located near the apical end, and dense granules are
scattered throughout the parasite. The contents of these
organelles are released at specific time points during invasion,
and hence host cell entry by T. gondii is a well-orchestrated event.
The process of host cell invasion by T. gondii begins with transient
attachment of the parasite to the host cell. Then, a calcium-
mediated signaling event results in secretion of MIC to the
parasite surface that enables the parasite to establish a firm
interaction with the host cell. However, many of the host cell
receptors involved in parasite entry into host cell remain
undefined (Saffer et al., 1992; Dubremetz and Schwartzman,
1993; Boothroyd and Dubremetz, 2008; Breinich et al., 2009).
FIGURE 1.
actively invades nonphagocytic and phagocytic cells. (II) The parasitophorous vacuole (PV) protects T. gondii from being eliminated by host cells,
especially from endosomal acidification and lysosomal fusion. (III) The host cell is induced to be competent to T. gondii infection and allow its
proliferation by controlling the host cell cycle. The host cell is induced to arrest at S-phase for T. gondii invasion and induced into entering G2/M phase
for T. gondii proliferation. (IV) The host cell provides a nutrient pool for T. gondii proliferation and replication. Toxoplasma gondii elaborates pores in
the PV membrane (PVM) to change its permeability, allowing small molecules to diffuse across the PVM. The other nutrients such as cholesterol are
translocated along host microtubules (MTs) to the PV, and invaginated intact into PVM through HOST structures containing host MTs. After nutrients
enter the PV, they likely enter into the parasite by membrane transporters. (V) The host cell is exploited by T. gondii to establish an invasion, replication,
and egress competent chemical (Ion) environment for the parasite. (VI) The host cell cytoskeleton is reorganized to facilitate PV formation and T. gondii
invasion. Microtubules form circular, basket-like structures that surround the PV in infected cells. The intimate interactions between T. gondii and host
microtubules result in suppression of cell division and/or cause a mitotic defect, thus providing a larger space for parasite multiplication. (VII) Host cell
anti-apoptotic reactions are induced by T. gondii for both self-protection and parasite-protection, mainly through regulating the death receptor pathway,
the NF-kB pathway, and the PI3K pathway. (VIII) Toxoplasma gondii infection induces several successful immune evasion mechanisms by the host cell.
Toxoplasma gondii induces blockage of IFN-c signaling pathway to control chronic and latent infection or induces the infected host cells to be less
responsive to IFN-c-induced upregulation of many genes, including MHC Class II, iNOS, and the p47 GTPases. (IX) Concomitance of host cell and T.
gondii appears with the conversion of tachyzoites into bradyzoites in immunocompetent hosts. Abbreviations: T: tachyzoite; B: bradyzoite; HN: host
nucleus; PVM: parasitophorous vacuole membrane; MT: microtubule; MF: microfilament; mt: mitochondria; SM: small molecule; H.O.S.T: host
organelle-sequestering tubulo structures.
Diagram of a host cell being hijacked to be competent, compromised, and concomitant to Toxoplasma gondii. (I) Toxoplasma gondii
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The parasitophorous vacuole (PV) protects T. gondii from
being eliminated by the host cell
Toxoplasma gondii dwells inside a PV once it gains entry into a
host cell (Figs. 3 and 4). The formation of the PV is closely linked
to the establishment of a moving junction (MJ) between the host
cell and tachyzoites during invasion. The MJ forms when T.
gondii attaches to receptors on the host cell and secretes MIC and
rhoptry neck proteins (RONs), which assemble at the interface of
the parasite and the host cell surface (Alexander et al., 2005; Cao
et al., 2009). Recent studies have identified some key components
of MJ that include microneme protein AMA1 and RON proteins
RON 2, RON4, RON5, and RON8 (Besteiro et al., 2009). After
establishing the moving junction, the parasite pushes itself
forward, and the invagination of the plasma membrane around
the invading parasite results in the formation of the PV (Fig. 2).
Through the MJ, the parasite actively excludes incorporation of
many of the host transmembrane proteins into the PV membrane,
while allowing GPI anchored proteins to remain. Toward the end
of the invasion process, the pinching off of the PVM results in
separation from the host cell plasma membrane; at this point, the
intracellular parasite is enclosed within the PVM within which it
spends its intracellular life (Figs. 3–5). After invasion, the parasite
further modifies the PVM, by inserting novel proteins derived
from its secretory organelles, the rhoptries and the dense granules
(Joiner and Roos, 2002; Straub et al., 2009). During penetration,
the parasite injects many rhoptry proteins into the host cell
cytosol that appear like small satellite vesicles and are referred to
as evacuoles that eventually fuse with the PVM (Hakansson et al.,
2001). As the invading parasite effectively excludes many of the
transmembrane proteins of the host cell that are involved in
fusion with lysosomes and most of the PVM is derived from the
host cell, the PVM is a nonfusogenic compartment that is resistant
to acidification by the endosome-lysosomal system of the host cell
(Mordue, Desai, and Dustin, 1999; Mordue, Hakansson, et al.,
1999; Charron and Sibley, 2004). After entry into the host cell, the
PVM closely associates with host mitochondria (Figs. 3, 4) and
endoplasmic reticulum (ER) and migrates toward the nucleus
using the host microtubule network (Sibley et al., 1994).
FIGURE 3.
parasitophorous vacuole (PV) surrounded by a parasitophorous vacuole
membrane (white arrows). Host cell mitochondria (open arrow) are
associated with the parasitophorous vacuole membrane. The nucleus (n)
of the tachyzoites is labeled, and the conoid (arrow) is visible in 1
tachyzoite. Bar 5 1.0 mm.
Group of 4 T. gondii tachyzoites developing in a
FIGURE 4.
developing tissue cyst of T. gondii. A bradyzoite in the parasitophorous
vacuole (PV) showing rhoptries (R), dense bodies (D), and amylopectin
granules (A). Bar 5 0.25 mm.
Mitochondrion (arrow) in close association with a
FIGURE 2.
human fibroblast cell and establishing a developing parasitophorous
vacuole (DPV). Note the association of the host cell membrane (black
arrows) with the conoidal end of the tachyzoite. Rhoptry sacks (R) and a
rhoptry neck (white arrow) are visible. Bar 5 1.0 mm.
Tachyzoite of the RH strain of T. gondii penetrating into a
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Toxoplasma gondii manipulates the cell cycle of the host
cell for effective infection and intracellular proliferation
Toxoplasma gondii can hijack host cells by modulating their cell
cycle. Thus, T. gondii infection induces human fibroblast monolay-
ers to transit from the G0/G1 to S-phase and remain arrested in the
S-phase of the cell cycle (Molestina et al., 2008). This transition
allows efficientinvasion by the parasite,asT. gondiihasbeenshown
to attach and invade S-phase host cells more readily than cells in
other phases of the cell cycle (Lavine and Arrizabalaga, 2009).
Interestingly, growth media from T. gondii infected cells have also
been shown to induce noninfected cells to enter S-phase. This
suggests that T. gondii directly releases or stimulates infected host
cells to release a soluble factor into the culture medium that induces
host cells to enter S-phase and thus facilitate efficient infection
(Lavine and Arrizabalaga, 2009). There is a preferential attachment
of T. gondii to S-phase cells compared to G1-phase, and it is not
dependent on the cells being in synchronous cycles at the time of
infection (Grimwood et al., 1996).
Once inside the host cell, tachyzoites induce the host cell to arrest
at the G2/M phase of the cell cycle to enable parasite proliferation.
Toxoplasma gondii can divide only in host cells arrested in G2 phase
in which mitosis has been prevented (Brunet et al., 2008; Molestina
etal.,2008).AcellcycleregulatinghostgeneUHRF1isrequired for
intracellular T. gondii proliferation and induces host cell transition
fromG1/SphasetoG2phase.RNAi-mediatedsilencing of UHRF1
causes the cell cycle to arrest in G1 phase, resulting in a significant
reduction in parasite proliferation (Brunet et al., 2008).
The host cell provides a nutrient pool for T. gondii
during proliferation
Transcription profiles and genome sequencing have revealed
that T. gondii is auxotrophic for some metabolites (Blader et al.,
2001; Gail et al., 2001; Crawford et al., 2006). Being an obligate
intracellular pathogen, T. gondii must acquire nutrients inside the
host cell, which cannot be obtained from extracellular sources.
The PVM not only surrounds the intracellular T. gondii and
provides a stable environment for parasite proliferation but also is
an exchange interface between host cell cytosol and the parasite.
Toxoplasma gondii modifies PVM permeability by making pores
in the PVM, allowing some small molecules that cannot be
synthesized de novo by the parasite, such as glucose, arginine,
iron, tryptophan, phospholipids, purine nucleosides, and cofac-
tors, to diffuse across the PVM. Then these molecules enter the
parasite by membrane transporters (Schwab et al., 1994, Charron
and Sibley, 2002; Sehgal et al., 2005; Blader and Saeij, 2009).
In contrast to these small molecules that passively diffuse across
the PVM, other nutrients are obtained by more active mecha-
nisms. Toxoplasma gondii is deficient in the ability to synthesize
sterol; blockade of exogenous and endogenous sources of host
cholesterol reduces parasite replication. Intracellular proliferation
of parasites is dependent on cholesterol captured from host cell
lysosomes, through low-density lipoprotein (LDL)–mediated PV
endocytosis, which is independent of vesicular fusion and requires
parasite viability. The host microtubules (MTs) are reorganized
upon T. gondii invasion, and the parasite is wrapped in a network
of host MTs. The endo-lysosomes containing LDL-loaded
cholesterol are translocated along host MTs to the PV and are
placed into the PVM through host MTs containing a specific
structure: host organelle-sequestering tubule structures (HOSTs).
At this point, elongated membranous tubules containing LDL-
loaded cholesterol are formed. These membrane tubules are
stabilized by the formation of a protein coat of parasite origin,
including the dense granule protein GRA7 (Coppens et al., 2000,
2006).
The exact mechanism for entry of nutrients into the vacuole is
unclear. The PV containing T. gondii is surrounded by an
extensive tubular network system accumulated within the PV and
in contact with the PVM. It has been speculated that the tubulo-
vesicular network in the vacuolar space delivers these nutrients
from the PVM or PV to the parasite, or reversely, from parasite to
PV or PVM (Sibley et al., 1995; Lingelbach and Joiner, 1998).
Toxoplasma gondii can also recruit host mitochondria and ER
in synthesizing and transporting some nutrients the parasite
needs; this recruitment happens quickly after the parasite invades
a cell (Sinai et al., 1997; Crawford et al., 2006). Even though
lipoate can be synthesized de novo and used in the T. gondii
apicoplast, the parasite can also scavenge lipoate from the host
cell, and the host-derived lipoate can be metabolized in the
parasite’s mitochondria (Crawford et al., 2006). This phenome-
non may indicate a deficiency in lipoate metabolism in T. gondii,
which may be due to the lack of a lipoate transporter required to
transfer the nutrient out of the apicoplast (Crawford et al., 2006).
The PVM has a close association with, or is tightly enshrouded
by, host cell mitochondrial membranes and ER, and even endo-
lysosomes (Lindsay, Mitschler et al., 1993). It is speculated that
host cell mitochondria and ER provide lipids and products of
intermediary metabolism to the intracellular parasite (Sinai et al.,
1997; Sinai and Joiner, 2001; Coppens et al., 2006). The tight
association of the PVM with the mitochondria and ER also
provides a key source of new phospholipids incorporated into the
PVM, which plays an important role in the PVM enlargement
(Sinai et al., 1997; Sinai and Joiner, 2001).
FIGURE 5.
decoquinate demonstrating bradyzoites (B) and tachyzoites (T). The
arrowhead points to a daughter tachyzoite developing by endodyogeny
within a tachyzoite. Note the tissue cyst wall (arrow) surrounding the
organisms. Bar 5 1.0 mm.
Young tissue cyst of the RH strain of T. gondii induced with
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The host cell is exploited by T. gondii to establish an
invasion-, replication-, and egress-competent chemical-
ion environment
Cellular chemical changes induce the host cell to become
competent to invasion and egress by T. gondii. Mobilization of
intracellular Ca2+plays an important role in facilitating parasite
invasion and egress events. When tachyzoites are attaching to
host cells, an undefined signaling event results in mobilization of
Ca2+from the parasite ER that triggers the invasion process
(Pingret et al., 1996; Lovett and Sibley, 2003; Li et al., 2008). An
elevation of [Ca2+]i is also associated with rapid egress of parasites
from the host cells at the end of intracellular proliferation.
Calcium ionophores have been used to induce the release of the
parasites from infected cells even in the early periods (2 hr) post-
invasion (Caldas et al., 2007; Kafsack et al., 2009)
Toxoplasma gondii invasion causes elevations of [Ca2+]i and
arachidonic acid (AA), which are from both host cells and
parasites. [Ca2+]i is an essential factor for protozoan parasite
differentiation, cytoskeleton dynamics, motility, and cell growth
(Li et al., 2008). During T. gondii invasion, the rise of [Ca2+]i
concentration in host cells depends on the activation of
phospholipase C (PLC) on the host cell membrane, consequently
triggering the PLC-PKC signaling pathway, which results in the
flowing of extracellular Ca2+and the releasing of intracellular
Ca2+pools. Elevated [Ca2+]i induces agglutination of host cell
microfilaments (Li et al., 2008). Phospholipase C hydrolyzes
phosphatidylinositol bisphosphate (PIP2), producing inositol tri-
phosphateand2-diacylglycerol.Inositoltriphosphateisasecondary
messenger, which also can regulate the releasing of the Ca2+in cells
(Lovett et al., 2002). Two-diacylglycerol will induce the release of
Ca2+attached to the ER membrane (Rodriguez et al., 1995; Salter
and Hicks, 1995). Arachidonic acid is a second messenger, which is
produced by phospholipase A and mediates decomposition of
phospholipids on the cell membrane (Ronco et al., 2002).
Arachidonic acid mediates multiple cellular activities such as cell
apoptosis and inflammation. The concentration of AA is 8.44-fold
and 5.02-fold of the basal concentration in the supernatant of
phagocytic (J774A.1) and nonphagocytic cells (L929), respectively,
after T. gondii infection (Li et al., 2008). Toxoplasma gondii can
produce secretory and cytosolic Phospholipase A2 (PLA2), and the
production is closely related to T. gondii virulence (Buitrago-Rey et
al., 2002). PLA2 inhibitors can significantly decrease the intracel-
lular AA concentration and the infectivity of T. gondii tachyzoites.
The host cell ER fuses with the PVM prior to PV disruption.
Fusion of ER with the PVM triggers the release of calcium into
the PV, which may also be the mechanism underlying intrava-
cuolar parasite movement and IFN-c–induced parasite egress
(Melzer et al., 2008).
Two recently published papers demonstrate the role of parasite-
encoded perforin-like protein 1 and host-derived calpain in parasite
escape from the host cell (Chandramohandas et al., 2009; Kafsack
et al., 2009). Both of these egress mechanisms are regulated by
calcium. Perforin-like protein 1 localizes to T. gondii micronemes
and is secreted in a calcium-dependent manner. Perforin-like
protein 1–deficient T. gondii loses the ability to rapidly permeabilize
the PVM and host cell membrane during egress. This results in a
failure to exit normally after proliferation in PV, resulting in
entrapment of tachyzoites within host cells (Kafsack et al., 2009).
However, it also has been shown that in addition to parasite
proteins, host-derived factor calpain plays an essential role in T.
gondii egress. A calcium signal triggered late during T. gondii
infection activates host cell calpain, which re-localizes to the host
cell plasma membrane, cleaving cytoskeletal proteins to facilitate
parasite egress (Chandramohanadas et al., 2009)
The host cell cytoskeleton is reorganized fitting to T. gondii
invasion and PV formation
Invasion of T. gondii can activate the reorganization of
cytoskeleton elements such as microfilaments and microtubles
of the host cell (da Silva et al., 2009). Interference of host cell actin
dynamics with cytochalasins and jasplakinolide partially blocks
T. gondii entrance into cells (Ferreira et al., 2003). Microfilament
adherence and accumulation has also been observed during T.
gondii invasion of phagocytic cells, but not non-phagocytic cells
(Li et al., 2008). Apparently, microfilaments function differently
in phagocytic and non-phagocytic cells during T. gondii invasion.
The agglutination of microfilaments in phagocytic cells plays an
important role for the successful invasion by T. gondii (Li et al.,
2008). Some studies have suggested that host MTs may not be
involved in T. gondii invasion (Dobrowolski and Sibley, 1996; Li
et al., 2008). It has been reported that host cell MTs are involved
in the development and enlargement of the PV (Andrade et al.,
2001). A recent report has shown that there is indeed reorgani-
zation of MTs during parasite entry (Sweeney et al., 2010).
Treatment of host cells with colchicine (a MT inhibitor) changes the
shape of the PV (Melo et al., 2001). Recruitment of T. gondii is
greatly reduced in cells treated with colchicum and cytochalasin D,
including phagocytic and non-phagocytic cells. This result may due
to the inhibition of MT movement, resulting in the impairment of
resistance of phagocytic lysosome fusion of the PVM enclosing
intracellular T. gondii (Andrade et al., 2001; Li et al., 2008).
Remodeling of the host cell MT network upon T. gondii
invasion in several different host cells has been reported. The
phenomenon of remodeling is not cell type specific. Microtubules
in uninfected host cells radiate from the host nucleus and form an
ordered network throughout the cytoplasm. Nevertheless, in
infected cells, microtubules form a circular basket-like structure
that surrounds the PV. The remodeling of MTs is dependent on
parasite viability and infection time. The intimate interactions
between T. gondii and host MTs results in suppression of host cell
division or causes a mitotic defect, or both, thus providing a
larger space for parasite multiplication (Walker et al., 2008).
The small GTPase of ARF6 from the host cell is recruited to
the site of entry and plays an important role in T. gondii invasion
through the activation of PI3-kinase, with the involvement of
PIP2 and PIP3. RNAi-mediated ARF6 gene knockdown greatly
reduces the recruitment of tachyzoites in Vero cells. This indicates
that the host cell cytoskeleton is involved in parasite invasion and
plays an important role, as ARF6 regulates membrane trafficking
and actin cytoskeleton reorganization (da Silva et al., 2009).
Maintenance of the integrity of the host cell actin cytoskeleton is
important in parasite invasion (da Silva et al., 2009).
The host cell anti-apoptotic reactions are induced by
T. gondii for both self-protection and parasite protection
Toxoplasma gondii can infect all nucleated cells and induces
little obvious disturbance during parasite multiplication prior to
host cell rupture. The ability to extend the life of infected host
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cells is an anti-apoptosis–induced reaction triggered by T. gondii
infection. Interference of T. gondii with host cell apoptosis is
related to virulence factors of T. gondii strains (Angeloni et al.,
2009) and depends on the cell types being infected (Vutova et al.,
2007; Hippe et al., 2008). The mechanisms are mainly through
regulating the death receptor (Hippe et al., 2008) and the NF-kB
(Payne et al., 2003) and the PI3K pathways (Yang et al., 2004).
Toxoplasma gondii inhibits death receptor-mediated apoptosis
in host cells (Goebel et al., 2001; Payne et al., 2003; Vutova et al.,
2007). Fas/CD95 triggers an apoptotic cascade that is crucial for
immunity and the outcome of infectious diseases. However, T.
gondii counteracts this death receptor-mediated cell death–Fas/
CD95 cascade differently in type I and type II host cells. In type I
host cells (like SKW6.4 cells), T. gondii blocks this apoptotic
cascade directly through interference with caspase 8. In type II
host cells (like HeLa cells), T. gondii significantly reduces Fas/
CD95–triggered apoptosis by inhibiting the activities of initiator
caspases 8 and 9 and effector caspase 3/7, by decreasing the
apoptogenic function of mitochondria at the mitochondrial
amplification loop (Vutova et al., 2007; Hippe et al., 2008).
The anti-apoptotic reaction induced by T. gondii infection is
also associated with upregulation of a series of anti-apoptotic
genes, such as the activation of NFkB-dependent anti-apoptotic
genes (Molestina et al., 2003; Payne et al., 2003). PKB inactivates
the Forkhead family transcription factor FKHR1, which regu-
lates apoptosis-inducing genes, reduces activation of caspase
molecules, and downregulates poly ADP–ribose polymerase
expression (Goebel et al., 2001). PKB has also been implicated
in positive regulation of the NFkB signaling pathway that leads to
induction of several survival-promoting genes (Patra et al., 2004).
The anti-apoptotic function of PKB occurs in part through
phosphorylative inactivation of the proapoptotic proteins Bad
and caspase-9 (Blume-Jensen et al., 1998).
The phosphoinositide 3-kinase (PI 3-kinase) pathway and
immediate downstream effector protein kinase B (PKB/Akt) play
important roles in cell survival and apoptosis inhibition (Scheid
and Woodget, 2001; Yang et al., 2004). Toxoplasma gondii
activates PI3-kinase through the signal pathway transmitted by
the heterotrimer of Gi protein, resulting in the phosphorylation of
PKB/Akt and ERK1/2 MAPK. Both in vitro and in vivo
experiments have demonstrated that after mouse phagocytes are
infected by T. gondii, the PKB/Akt pathway is activated, and the
induction of apoptosis in infected macrophages is prevented (Kim
and Denkers, 2006).
Toxoplasma gondii appears to block apoptosis in its host cells at
different points and to co-opt host apoptotic signaling pathways
in an environment-sensing mechanism (Persson et al., 2007). The
cells infected by T. gondii are resistant to multiple inducers of
apoptosis, including cytotoxic T lymphocyte (Fas-dependent or
-independent), IL-2 deprivation, gamma irradiation, UV irradi-
ation, and the calcium ionophore beauvericin (Nash et al., 1998;
Luder and Gross, 2005). However, T. gondii blocks host cell
apoptosis, including the inhibition of cytochrome C released from
host cell mitochondria, upregulation of anti-apoptotic proteins of
the Bcl-2 and IAP families, interference with proapoptotic Bax
and Bad, modulation of cell death-regulating kinase activities,
inhibition of caspase, and possibly modulation of cytochrome C–
induced caspase 3/7 activation (Hippe et al., 2008).
Usually T. gondii triggers host cell rupture and egress of
parasites before induction of host cell apoptosis (Persson et al.,
2007). Death receptor–induced egress of T. gondii depends on
caspase-mediated release of intracellular calcium early in the
apoptotic cascade. T cells induce rapid egress of infectious
parasites by acting on infected cells via death receptor- or
perforin-dependent pathways (Persson et al., 2007).
Toxoplasma gondii infection induces a series of successful
immune evasion mechanisms in the host cell
Successful parasitic infection with host and parasite survival
necessitates a balance between parasite immune evasion and host
immunesurveillance.Uponinvasion,T.gondiiinducesa strong cell-
mediated and IFN-c–driven immune response in its mammalian
hosts to resist acute infection and maintain latent infection (Suzuki
et al., 1989). However, T. gondii also adjusts the host immune
response by inducing infected cells to be less responsive to IFN-c
signaling. This ability may be the most important mechanism for
immune evasion from a robust IFN-c mediated cellular immunity
employed by T. gondii (Kim et al., 2007). For example, host cells
infected by T. gondii are significantly less responsive to IFN-c–
induced upregulation of many genes, including MHC Class II,
iNOS, and the p47 GTPases. IFN-c–inducible gene expression of
iNOS, MIG, IIGP1, and IFN-c–induced NO is inhibited in the
infected cells (Lang et al., 2006; Zimmermann et al., 2006; Kim et
al., 2007). The ability and inability of host cells to inhibit IFN-c
responses is correlated with differences in host range, T. gondii
virulence, and persistence (Kim et al., 2007). Toxoplasma gondii
infection-induced IFN-c signaling is inhibited by blocking STAT1
transcriptionalactivity(Kimetal.,2007)orviaadecreaseinSTAT1
tyrosine phosphorylation, or both (Zimmermann et al., 2006).
There is some evidence that T. gondii inhibits STAT1 by
upregulating levels of the endogenous suppressor of cytokine
signaling protein (SOCS) in the host cells, which contributes to
the parasite’s inhibition of IFN-c. SOCS represents a protein
family, including SOCS1-7 and CIS, which are considered as
attenuators of IFN-c signaling through either inhibiting the
catalytic activity of JAKs or by inhibiting recruitment of STATs
(Zimmermann et al., 2006). These findings show the inhibition of
IFN-c signaling will result in immune evasion of T. gondii, but the
mechanisms of these kinds of inhibitions are still not clear.
Concomitant status between the host cell and T. gondii
appears with the conversion of tachyzoites into
bradyzoites in immunocompetent hosts
After initial acquisition of infection in immunocompetent hosts,
T. gondii tachyzoites soon convert into bradyzoites to establish a
chronic, asymptomatic infection (Dubey et al., 1998). Bradyzoites
normally cannot be cleared by the host because of the weak
immune response that they elicit; elimination is also difficult
because of their refractoriness to existing drugs (Guimara ˜es et al.,
2008). Thus, a concomitant status between the host and the
parasite is established.
The mechanism for conversion from the acute to the chronic
phase of infection is still largely unknown. Most isolates of T.
gondii will produce tissue cysts in vitro (Lindsay, Toivio-
Kinnucan, and Blagburn, 1993) despite the infective stage
inoculated (Lindsay et al., 1991). A variety of environmental
stresses will induce this conversion. Stress results in phosphory-
lation of T. gondii eukaryotic initiation factor-2a (TgIF2a), and
TgIF2a phosphorylation is employed by cells to maintain a latent
PENG ET AL.—A REVIEW: REACTION OF HOST CELLS TO T. GONDII 625

Page 7

state. Phosphorylated TgIF2a in bradyzoites might be part of the
explanation for the manner in which these parasites maintain their
quiescent state in the host cells (Narasimhan et al., 2008).
The tachyzoite-bradyzoite conversion models in vitro can be
achieved using various stress conditions that mimic immune-
derived stressors, like high pH, IFN-c, TNF-a, nitric oxide, high
temperature, nutrient starvation, or by some drugs used to treat
T. gondii infections (Bohne et al., 1993; Soete et al., 1994; Lindsay
et al., 1998; Fox et al., 2004). Stress-induced elevation of cyclic
nucleotides may play an important role in the conversion of T.
gondii tachyzoites to bradyzoites (Kirkman et al., 2001).
At the beginning of conversion, the expression of tachyzoite-
specific genes such as SAG1, SAG2A, SAG2B, LDH1, ENO2,
PtdIns(t), and SRS1–SRS3 are turned off and replaced by the
expression of bradyzoite-specific genes such as SAG2C, SAG2D,
SAG4, BSR4, MAG1, LDH2, ENO1, BAG1, PtdIns(b), and p-
ATPase (Lyons et al., 2002). During stage conversion, there is a
decrease in the expression of immunogenic surface proteins, and
metabolic enzymes and an increase in the abundance of genes that
act to facilitate entry into host cells (Lyons et al., 2002). Changes
in host cell transcription can directly influence the molecular
environment to enable bradyzoite development. Human cell
division autoantigen-1 upregulation causes the inhibition of
parasite replication and subsequently leads to tachyzoite-brady-
zoite conversion. How the parasite modulates the biochemical
environment of the cell is unknown, but, apparently, bradyzoites
are primarily distributed in differentiated, long-lived cells such as
mature skeletal muscle and brain neurons (Radke et al., 2003).
Interferon-c also drives the conversion of T. gondii tachyzoites
to bradyzoites, resulting in the conversion of acute infection to
chronic infection, and, at the same time, suppresses the
reactivation of bradyzoites to tachyzoites in immune-competent
hosts (Bohne et al., 1993).
The intracellular T. gondii extensively modulates its host cell so
as to efficiently grow and divide. In doing so, it is one of the most
successful parasites of humans, infecting almost one-third of the
world’s population. In immunocompetent hosts, T. gondii induces
the host cell to be competent, compromised, and concomitant to
the parasite to help in every way for T. gondii invasion,
replication, and persistence. It remains mostly a mystery how
the host cell is induced to be competent for T. gondii invasion and
egress, compromised to T. gondii in parasitizing, proliferation,
and multiplication, and, eventually, concomitant in the relation-
ship between the parasite and host cell when tachyzoites
successfully convert to bradyzoites.
ACKNOWLEDGMENTS
We thank Drs. R. Y. Gaji, J. Strobl, and Q. Han for comments on this
manuscript. This work was supported by grants from the National
Scientific Foundation of China (Key program: 31030066), National
Special Research Programs for
(200803017), Guangdong Province Universities and Colleges Pearl River
Scholar Funded Scheme (2009) to X.-G.C., National Scientific Founda-
tion of China (81071377), Guangdong Provincial Natural Science
Foundation (10151051501000033), Research Fund for the Doctoral
Program of Higher Education of China (20104433120014), and Guang-
dong Provincial Medical Research Foundation (A2010356) to H.-J.P.
Non-ProfitTrades (Agriculture)
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