Calcium and Hydrogen Ion Concentrations in the Parasitophorous Vacuoles of Epithelial Cells Infected with the Microsporidian Encephalitozoon hellem

ABSTRACT. Microsporidia of the genus Encephalitozoon undergo merogony and sporogony in a parasitophorous vacuole within the host cell. Cultured green monkey kidney cells infected with Encephalitozoon hellem were loaded with the fluorescent dyes fura-2 or BCECF in order to measure intracellular concentrations of calcium and hydrogen ions respectively. Both the parasitophorous vacuole calcium concentration and pH values resembled those of the host cell cytoplasm in infected cells. Calcein entered the parasitophorous vacuole but not other host cell vacuoles or parasite stages within the parasitophorous vacuole. The lack of a pH or calcium concentration gradient across the parasitophorous vacuole membrane and the permeability of this membrane to a large anion such as calcein suggest that the vacuole membrane surrounding E. hellem resembles that surrounding some other intracellular parasites such as Toxoplasma gondii. A potential role is discussed for the parasitophorous vacuole calcium concentration in germination in situ.     

Antimalarial drugs disrupt ion homeostasis in malarial parasites

Plasmodium chabaudi malaria parasite organelles are major elements for ion homeostasis and cellular signaling and also target for antimalarial drugs. By using confocal imaging of intraerythrocytic parasites we demonstrated that the dye acridine orange (AO) is accumulated into P. chabaudi subcellular compartments. The AO could be released from the parasite organelles by collapsing the pH gradient with the K+/H+ ionophore nigericin (20 microM), or by inhibiting the H+-pump with bafilomycin (4 microM). Similarly, in isolated parasites loaded with calcium indicator Fluo 3-AM, bafilomycin caused calcium mobilization of the acidic calcium pool that could also be release with nigericin. Interestingly after complete release of the acidic compartments, addition of thapsigargin at 10 microM was still effective in releasing parasite intracellular calcium stores in parasites at trophozoite stage. The addition of antimalarial drugs chloroquine and artemisinin resulted in AO release from acidic compartments and also affected maintenance of calcium in ER store by using different drug concentrations.     

Trypanosoma cruzi infection of BSC-1 fibroblast cells causes cytoskeletal disruption and changes in intracellular calcium levels.

The disruption of vimentin and actin filaments of host BSC-1 fibroblast cells by Trypanosoma cruzi was investigated using a mouse monoclonal anti-vimentin antibody and rhodamine phalloidin, respectively. Indirect immunofluorescence microscopy demonstrated that infection of BSC-1 cells by T. cruzi caused disruption of both cytoskeletal components. The disruption was greater as infection progressed. Mechanisms other than mechanical ones may play a role in the disruption since disrupted cytoskeletal elements were well removed from the parasites. In the determination of intracellular calcium concentrations using Fura-2 AM, infected and uninfected cells both showed an initial increase in intracellular calcium levels. At later times of infection (3 to 5 days), intracellular calcium levels of infected cells were significantly lower than those of control cells. There was no specific localization of intracellular calcium in the infected host cells as determined by image analysis.     

Trypanosoma cruzi Infection of BSC-1 Fibroblast Cells Causes Cytoskeletal Disruption and Changes in Intracellular Calcium Levels

ABSTRACT The disruption of vimentin and actin filaments of host BSC-1 fibroblast cells by Trypanosoma cruzi was investigated using a mouse monoclonal anti-vimentin antibody and rhodamine phalloidin, respectively. Indirect immunofluorescence microscopy demonstrated that infection of BSC-1 cells by T. cruzi caused disruption of both cytoskeletal components. The disruption was greater as infection progressed. Mechanisms other than mechanical ones may play a role in the disruption since disrupted cytoskelelal elements were well removed from the parasites. In the determination of intracellular calcium concentrations using Fura-2 AM, infected and uninfected cells both showed an initial increase in intracellular calcium levels. At later times of infection (3 to 5 days), intracellular calcium levels of infected cells were significantly lower than those of control cells. There was no specific localization of intracellular calcium in the infected host cells as determined by image analysis.     

Calcium signal regulates temperature-dependent transformation of sporozoites in malaria parasite development

The infection by the malaria parasite of its mammalian host is initiated by the asexual reproduction of the parasite within the host hepatocyte. Before the reproduction, the elongated sporozoites undergo a depolarizing morphogenesis to the spherical exo-erythrocytic form (EEF). This change can be induced in vitro by shifting the environmental conditions, in the absence of host hepatocytes. Using rodent malaria parasites expressing a FRET-based calcium sensor, YC3.60, we observed that the intracellular calcium increased at the center of the bulbous structure during sporozoite transformation. Modulators of intracellular calcium signaling (A23187 and W-7) accelerated the sporozoite-rounding process. These data suggest that calcium signaling regulates the morphological development of the malaria parasite sporozoite to the EEF, and support a fundamental role for calcium as a universal transducer of external stimuli in the parasitic life cycle.     

Neuron-specific modulation by serotonin of regenerative outgrowth and intracellular calcium within the CNS of Helisoma trivolvis

We have investigated the cell-specific effect of serotonin (5-HT) on regenerating neurons within the adult central nervous system of the pond snail, Helisoma trivolvis. In culture, 5-HT arrests outgrowth of buccal neurons B19 but not neurons B5 (Haydon, McCobb, and Kater, 1984). After axotomy, neurons within the Helisoma nervous system typically exhibit profuse regenerative outgrowth. This study, on neurons within the CNS, shows that 5-HT selectively inhibits the outgrowth of specific identified neurons, and also causes significant elevations in intracellular calcium concentrations as measured by the calcium indicator dye, Fura-2. The outgrowth of neurons B19 and C1 was selectively inhibited when ganglia were incubated in 5 X 10(-5) M 5-HT. The outgrowth of buccal neurons B5, however, was not affected. Moreover, 5-HT caused significant transient elevations of calcium concentrations in neurons B19 over 30 minutes, but neurons B5 did not show any increases in calcium concentrations with the addition of 5-HT. These results suggest that the effect of 5-HT upon outgrowth of regenerating neurons may be due to an increase in the intracellular calcium concentration.     

Hormonal regulation of free intracellular calcium concentrations in small and large ovine luteal cells

The second messengers mediating hormonal regulation of the corpus luteum are incompletely defined, particularly for the primary luteolytic hormone prostaglandin F2 alpha (PGF2 alpha). In this study, hormonally induced changes in free intracellular calcium concentrations were measured in individual small and large ovine luteal cells by using computer-assisted microscopic imaging of fura-2 fluorescence. This technique could readily detect transient increases in free calcium concentrations within both small and large luteal cells after treatment with 1 microM of the calcium ionophore, A23187. Treatment with PGF2 alpha (1 microM) caused a dramatic increase in free calcium concentrations in large (before = 73 +/- 2 nM; 2 min after PGF2 alpha = 370 +/- 21 nM; n = 33 cells) but not in small (before = 66 +/- 4 nM; 2 min after PGF2 alpha = 69 +/- 8 nM; n = 12 cells) luteal cells. The magnitude and timing of the calcium response was dose- and time-dependent. The PGF2 alpha-induced increase in free intracellular calcium is probably due to influx of extracellular calcium, since inclusion of inorganic calcium channel blockers (100 microM manganese or cobalt) attenuated the response to PGF2 alpha and removal of extracellular calcium eliminated the response. In contrast to PGF2 alpha, luteinizing hormone (LH) (100 ng/ml) caused no change in intracellular levels of free calcium in small or large luteal cells, even though this dose of LH stimulated (p less than 0.01) progesterone production by small luteal cells. Therefore, alterations in free calcium concentrations could be the intracellular second message mediating the luteolytic action of PGF2 alpha in the large ovine luteal cell.     

Trypanosoma brucei: the tumor promoter thapsigargin stimulates calcium release from an intracellular compartment in slender bloodstream forms.

Maintenance of calcium homeostasis is a critical activity of eukaryotic cells. Homeostatic pathways stabilize intracellular free calcium concentrations ([Ca2+]i) at the resting level and provide the source of mobilized calcium for cellular activation. We have measured calcium release from intracellular pools within bloodstream forms of Trypanosoma brucei to better understand homeostatic pathways which operate in these organisms. Fura-2 and 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein were used to quantitate [Ca2+]i and intracellular pH (pHi), respectively. We report that the tumor promoter, thapsigargin, elevated [Ca2+]i by 50-75 nM. Mn2+ quench experiments demonstrated that the source of calcium was intracellular. No change in pHi was associated with the release of calcium from this compartment. In contrast, nigericin released approximately three-fold more calcium than thapsigargin from a pH-sensitive, intracellular pool. The nigericin-sensitive pool was nonmitochondrial. The effects of thapsigargin and nigericin on [Ca2+]i were additive, regardless of the order in which the treatment was given. We conclude that at least two pools of exchangeable calcium occur in bloodstream forms of T. brucei. One pool is sensitive to thapsigargin and apparently resides within the endoplasmic reticulum, while the nigericin-sensitive pool is nonmitochondrial and is of unknown origin.     

Artemisinin induces calcium-dependent protein secretion in the protozoan parasite Toxoplasma gondii

Intracellular calcium controls several crucial cellular events in apicomplexan parasites, including protein secretion, motility, and invasion into and egress from host cells. The plant compound thapsigargin inhibits the sarcoplasmic-endoplasmic reticulum calcium ATPase (SERCA), resulting in elevated calcium and induction of protein secretion in Toxoplasma gondii. Artemisinins are natural products that show potent and selective activity against parasites, making them useful for the treatment of malaria. While the mechanism of action is uncertain, previous studies have suggested that artemisinin may inhibit SERCA, thus disrupting calcium homeostasis. We cloned the single-copy gene encoding SERCA in T. gondii (TgSERCA) and demonstrate that the protein localizes to the endoplasmic reticulum in the parasite. In extracellular parasites, TgSERCA partially relocalized to the apical pole, a highly active site for regulated secretion of micronemes. TgSERCA complemented a calcium ATPase-defective yeast mutant, and this activity was inhibited by either thapsigargin or artemisinin. Treatment of T. gondii with artemisinin triggered calcium-dependent secretion of microneme proteins, similar to the SERCA inhibitor thapsigargin. Artemisinin treatment also altered intracellular calcium in parasites by increasing the periodicity of calcium oscillations and inducing recurrent, strong calcium spikes, as imaged using Fluo-4 labeling. Collectively, these results demonstrate that artemisinin perturbs calcium homeostasis in T. gondii, supporting the idea that Ca²⁺-ATPases are potential drug targets in parasites.     

The influence of calcium on the deformability of human granulocytes

Experiments were carried out to determine the importance of extra- and intracellular calcium for the deformability of granulocytes during filtration tests. At low calcium concentration (0.1 mM), granulocytes are more deformable than at the physiological free-calcium concentration of 1.25 mM. Increasing calcium concentrations up to 10 mM do not further impair the deformability. Parallel measurements of the intracellular calcium concentration by means of the fura fluorescence method were performed to explain this. Extracellular calcium concentrations between 1.25 mM and 10 mM had no influence on the intracellular calcium level. A lower extracellular calcium concentration (0.1 mM), however, decreased the intracellular calcium level. Therefore, the measurements of the intracellular calcium concentrations are consistent with the deformability results. Studies with the calcium entry blocker nifedipine suggested that a low intracellular calcium improves the deformability of granulocytes. It is concluded; (i) the physiological calcium concentration of 1.25 mM is stressful for isolated granulocytes, and (ii) the intracellular calcium level plays a crucial role in granulocyte deformability, i.e. the lower the intracellular calcium concentration the greater the deformability.     

Antimonial-induced increase in intracellular Ca2+ through non-selective cation channels in the host and the parasite is responsible for apoptosis of intracellular Leishmania donovani amastigotes

The capability of the obligate intracellular parasites like Leishmania donovani to survive within the host cell parasitophorous vacuoles as nonmotile amastigotes determines disease pathogenesis, but the mechanism of elimination of the parasites from these vacuoles are not well understood. By using the anti-leishmanial drug potassium antimony tartrate, we demonstrate that, upon drug exposure, intracellular L. donovani amastigotes undergo apoptotic death characterized by nuclear DNA fragmentation and externalization of phosphatidylserine. Changes upstream of DNA fragmentation included generation of reactive oxygen species like superoxide, nitric oxide, and hydrogen peroxide that were primarily concentrated in the parasitophorous vacuoles. In the presence of antioxidants like N-acetylcysteine or Mn(III) tetrakis(4-benzoic acid)porphyrin chloride, an inhibitor of inducible nitric-oxide synthase, a diminution of reactive oxygen species generation and improvement of amastigote survival were observed, suggesting a close link between drug-induced oxidative stress and amastigote death. Changes downstream to reactive oxygen species increase involved elevation of intracellular Ca2+ concentrations in both the parasite and the host that was preventable by antioxidants. Flufenamic acid, a non-selective cation channel blocker, decreased the elevation of Ca2+ in both the cell types and reduced amastigote death, thus establishing a central role of Ca2+ in intracellular parasite clearance. This influx of Ca2+ was preceded by a fall in the amastigote mitochondrial membrane potential. Therefore, this study projects the importance of flufenamic acid-sensitive non-selective cation channels as important modulators of antimonial efficacy and lends credence to the suggestion that, within the host cell, apoptosis is the preferred mode of death for the parasites.     

Effects and mechanisms of action of ionophorous antibiotics valinomycin and salinomycin-Na on Babesia gibsoni in vitro

Valinomycin and salinomycin-Na, 2 ionophorous antibiotics, exhibited in vitro antibabesial activities against Babesia gibsoni that infected normal canine erythrocytes containing low potassium (LK) and high sodium concentrations, i.e., LK erythrocytes, which completely lack Na,K-ATPase activity. The level of parasitemia of B. gibsoni was significantly decreased when the parasites were incubated in culture medium containing either 10(-1) ng/ml valinomycin or 10(2) ng/ml salinomycin-Na for 24 hr. Four-hour incubation in the culture medium containing 5 μg/ml salinomycin-Na led to the destruction of most parasites. In contrast, when the parasites infected canine erythrocytes containing high potassium (HK) and low sodium concentrations, i.e., HK erythrocytes, the in vitro antibabesial activities of both ionophorous antibiotics seemed to be weakened, apparently due to the protection by the host cells. Therefore, differential influences of ionophorous antibiotics on LK and HK erythrocytes were observed. In LK erythrocytes, the intracellular concentrations of potassium, sodium, and adenosine triphosphate (ATP) were not modified, and hemolysis was not observed after incubation in the medium containing each ionophorous antibiotic. These results suggested that these ionophorous antibiotics did not affect cells without Na,K-ATPase, and directly affected B. gibsoni. In HK erythrocytes, the ionophorous antibiotics increased the intracellular sodium concentration, and decreased the intracellular potassium and ATP concentrations, causing obvious hemolysis. Additionally, the decrease of the intracellular ATP concentration and the hemolysis in HK erythrocytes caused by valinomycin disappeared when the activity of Na,K-ATPase was inhibited by ouabain. These results indicate that modification of the intracellular cation concentrations by the ionophorous antibiotics led to the activation of Na,K-ATPase and increased consumption of intracellular ATP, and that the depletion of intracellular ATP resulted in hemolysis in HK erythrocytes. Moreover, the antibabesial activity of valinomycin disappeared when B. gibsoni in LK erythrocytes were incubated in culture media containing high potassium concentrations. This showed that the intracellular cation concentration in the parasites was not modified in those media and would remain the same.     

Intracellular calcium levels in the Plasmodium berghei ookinete

Ookinetes are the motile and invasive stages of Plasmodium parasites in the mosquito host. Here we explore the role of intracellular Ca2+ in ookinete survival and motility as well as in the formation of oocysts in vitro in the rodent malaria parasite Plasmodium berghei. Treatment with the Ca2+ ionophore A23187 induced death of the parasite, an effect that could be prevented if the ookinetes were co-incubated with insect cells before incubation with the ionophore. Treatment with the intracellular calcium chelator BAPTA/AM resulted in increased formation of oocysts in vitro. Calcium imaging in the ookinete using fluorescent calcium indicators revealed that the purified ookinetes have an intracellular calcium concentration in the range of 100 nm. Intracellular calcium levels decreased substantially when the ookinetes were incubated with insect cells and their motility was concomitantly increased. Our results suggest a pleiotropic role for intracellular calcium in the ookinete.     

The effects of dynorphin A (1-13) and U50, 488H on free intracellular calcium in guinea pig cerebellar synaptosomes.

The effects of the kappa-selective ligands dynorphin A (1-13) (DYN) and U50, 488H (U50) on free intracellular calcium were evaluated using synaptosomes prepared from the cerebellum of the guinea pig, an area with a high density of kappa receptors. DYN (10 microM) produced small nonsignificant decreases in basal free intracellular calcium (5-7%). U50 (10 microM) produced significant 15-20% decreases in basal free intracellular calcium which were reversed by nor-BNI (1 microM). When intracellular calcium levels were increased 8-10% by the administration of c-AMP or forskolin, DYN (10 microM) produced significant decreases in intracellular calcium of 10%. The effects of U50, 488H were not enhanced by increasing the synaptosomal levels of c-AMP. Neither DYN nor U50 (1 microM) significantly blocked the rise in free intracellular calcium induced by 50 mM KCI. When intracellular calcium concentrations were increased by the administration of 50 mM KCI prior to the administration of DYN or U50 (10 microM), the kappa ligands decreased intracellular calcium concentrations. These data indicate that DYN and U50 interact with kappa receptors resulting in a decrease in free intracellular calcium possibly via an enhancement of the efflux of calcium. The modulation of intracellular free calcium by the kappa opioids may be a mechanism by which these opioids produce their biological effects.     

Antithetic effects of ryanodine and ruthenium red on osteclast-mediated bone resorption and intracellular calcium concentrations

In the process of bone remodeling, osteoclasts are responsible for resorption of bone. High levels of intracellular calcium decrease the bone resorbing activity of osteoclasts and increase detachment of osteoclasts from the bone surface. The regulatory role of intracellular calcium in bone resorption is not clearly understood. To understand this phenomenon, we studied the effects of the intracellular calcium modulators ryanodine and ruthenium red on bone resorption using the disaggregated osteoclast pit assay. Changes in intracellular calcium concentrations after treatment with these compounds were detected with the fluoroprobe fura2. With ryanodine, a significant, dose-dependent decrease in bone resorption was detected. This inhibition of bone resorption was reversible upon the removal of ryanodine. Ryanodine increased intracellular calcium concentrations, suggesting that the mechanism of inhibition by ryanodine was via alterations in intracellular stores of calcium. After treatment with ruthenium red, osteoclasts resorbed significantly more bone compared to vehicle-treated cells. This increase in bone resorption correlated with a decrease in intracellular calcium concentrations. The addition of parathyroid hormone or ruthenium red to osteoclast cultures containing ryanodine did not attenuate the decrease in bone resorption caused by ryanodine, suggesting that the mechanism of ryanodine inhibition of bone resorption may involve the “locking” of a calcium channel in an open position. © 1995 Wiley-Liss, Inc.     

Sub-second quenched-flow/X-ray microanalysis shows rapid Ca2+ mobilization from cortical stores paralleled by Ca2+ influx during synchronous exocytosis in Paramecium cells

Though only actual local free Ca2+ concentrations, [Ca2+], rather than total Ca concentrations, [Ca], govern cellular responses, analysis of total calcium fluxes would be important to fully understand the very complex Ca2+ dynamics during cell stimulation. Using Paramecium cells we analyzed Ca2+ mobilization from cortical stores during synchronous (< or = 80 ms) exocytosis stimulation, by quenched-flow/cryofixation, freeze-substitution (modified for Ca retention) and X-ray microanalysis which registers total calcium concentrations, [Ca]. When the extracellular free calcium concentration, [Ca2+]e, is adjusted to approximately 30 nM, i.e. slightly below the normal free intracellular calcium concentration, [Ca2+]i = 65 nM, exocytosis stimulation causes release of 52% of calcium from stores within 80 ms. At higher extracellular calcium concentration, [Ca2+]e = 500 microM, Ca2+ release is counterbalanced by influx into stores within the first 80 ms, followed by decline of total calcium, [Ca], in stores to 21% of basal values within 1 s. This includes the time required for endocytosis coupling (350 ms), another Ca2+-dependent process. To confirm that Ca2+ mobilization from stores is superimposed by rapid Ca2+ influx and/or uptake into stores, we substituted Sr2+ for Ca2+ in the medium for 500 ms, followed by 80 ms stimulation. This reveals reduced Ca signals, but strong Sr signals in stores. During stimulation, Ca2+ is spilled over preformed exocytosis sites, particularly with increasing extracellular free calcium, [Ca2+]e. Cortically enriched mitochondria rapidly gain Ca signals during stimulation. Balance calculations indicate that total Ca2+ flux largely exceeds values of intracellular free calcium concentrations locally required for exocytosis (as determined previously). Our approach and some of our findings appear relevant also for some other secretory systems.     

Effect of hepcidin on intracellular calcium in human osteoblasts

Hepcidin is known to increase intracellular iron through binding to and degrading ferroportin, which is a transmembrane protein that transports iron from the intracellular to the outside. However, it is not clear whether hepcidin has a similar effect on intracellular calcium. Here, we investigated the influence of hepcidin on intracellular calcium in human osteoblasts, with or without high environmental iron concentrations. Our data showed that hepcidin (<100 nmol/L) could increase intracellular calcium, and this effect was more significant when cells were exposed to high environmental iron concentrations. To further explore its underlying mechanisms, we pretreated human osteoblasts with Nimodipine, a L-type calcium channel blocker, and Dantrolene, a ryanodine receptor antagonist to inhibit abnormal calcium release from the sarco-endoplasmic reticulum. These treatments had not resulted in any alteration of intracellular calcium in human osteoblasts. Thus, these findings indicate that the increase of intracellular calcium induced by hepcidin is probably due to calcium release from endoplasmic reticulum, which is triggered by calcium influx.     

Evidence against a Role of Elevated Intracellular Ca2+ during Plasmodium falciparum Preinvasion

Severe malaria is primarily caused by Plasmodium falciparum parasites during their asexual reproduction cycle within red blood cells. One of the least understood stages in this cycle is the brief preinvasion period during which merozoite-red cell contacts lead to apical alignment of the merozoite in readiness for penetration, a stage of major relevance in the control of invasion efficiency. Red blood cell deformations associated with this process were suggested to be active plasma membrane responses mediated by transients of elevated intracellular calcium. Few studies have addressed this hypothesis because of technical challenges, and the results remained inconclusive. Here, Fluo-4 was used as a fluorescent calcium indicator with optimized protocols to investigate the distribution of the dye in red blood cell populations used as P. falciparum invasion targets in egress-invasion assays. Preinvasion dynamics was observed simultaneously under bright-field and fluorescence microscopy by recording egress-invasion events. All the egress-invasion sequences showed red blood cell deformations of varied intensities during the preinvasion period and the echinocytic changes that follow during invasion. Intraerythrocytic calcium signals were absent throughout this interval in over half the records and totally absent during the preinvasion period, regardless of deformation strength. When present, calcium signals were of a punctate modality, initiated within merozoites already poised for invasion. These results argue against a role of elevated intracellular calcium during the preinvasion stage. We suggest an alternative mechanism of merozoite-induced preinvasion deformations based on passive red cell responses to transient agonist-receptor interactions associated with the formation of adhesive coat filaments.     

Salmonella typhimurium induces an inositol phosphate flux in infected epithelial cells

Salmonella typhimurium, like many other intracellular pathogens, is capable of inducing its own uptake into non-phagocytic cells by a process termed invasion, and residing within a membrane-bound inclusion. During invasion it causes significant rearrangement of the host cytoskeleton, indicating that signals are transduced between the bacterium and the host cell cytoplasm, across the eukaryotic cell membrane. We found that intracellular inositol phosphate concentrations in HeLa cells increased during S. typhimurium entry and returned to normal levels after bacterial internalization. A chelator of intracellular calcium (BAPTA/AM) blocked S. typhimurium uptake into HeLa epithelial cells, but extracellular calcium chelators (BAPTA, EGTA, EDTA) had no effect on bacterial invasion. These results indicate that S. typhimurium may activate host cell phospholipase C activity to form inositol phosphates which in turn stimulate release of intracellular calcium stores to facilitate bacterial uptake.     

Death receptor ligation or exposure to perforin trigger rapid egress of the intracellular parasite Toxoplasma gondii

The obligate intracellular parasite Toxoplasma gondii chronically infects up to one-third of the global population, can result in severe disease in immunocompromised individuals, and can be teratogenic. In this study, we demonstrate that death receptor ligation in T. gondii-infected cells leads to rapid egress of infectious parasites and lytic necrosis of the host cell, an active process mediated through the release of intracellular calcium as a consequence of caspase activation early in the apoptotic cascade. Upon acting on infected cells via death receptor- or perforin-dependent pathways, T cells induce rapid egress of infectious parasites able to infect surrounding cells, including the Ag-specific effector cells.