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PubMed1999

The permeability properties of the parasite cell membrane.

The asexual development of the malaria parasite takes place inside the host's erythrocyte, an environment that is different from that of most other eukaryotic organisms. The intense and rapid development of the parasite, as well as the homeostatic regulation of its cellular composition, require an extensive exchange of material between the parasite and its immediate surroundings. Studies on free murine parasite species suggest that a plasma membrane H+ pump is responsible for the maintenance of membrane potential and pH gradient, which are used as driving forces for the uptake of glucose and extrusion of Ca2+ by means of a symporter and an antiporter, respectively. In Plasmodium falciparum, a similar transport of Ca2+ may prevail. Several other transporters have been assigned to the plasma membrane of this parasite, either by direct measurements or by inference: D-glucose, nucleosides, L-amino acids, L-lactate and pantothenic acid. A Na+/H+ antiporter has been demonstrated, and implicated in the regulation of pH, and an ATP/ADP antiporter, whose function remains controversial, has been characterized. The presence of Mg2+ and Na+/K+ pumps and an active extrusion of oxidized glutathione can be inferred from the composition of the parasite cytosol vs. that of the host cell. Several genes coding for cation pumps have been cloned and their functions await characterization.

Amino Acid Sequence↗
PubMed1983

Metabolism and surface transport of parasitized erythrocytes in malaria.

Plasmodium requires a living cell for growth and reproduction. Intraerythrocytically the parasite stores no reserve carbohydrate, relying entirely on host-supplied glucose and certain amino acids (glutamic acid) for its energy. Plasmodia are microaerophiles degrading glucose primarily to lactate rather than to CO2. The limited amounts of oxygen utilized may serve for biosynthetic purposes (e.g. pyrimidine biosynthesis) rather than being involved in an energy-yielding electron transport chain. Evidence for a parasite pentose pathway is weak since glucose-6-phosphate dehydrogenase has rarely been found; paradoxically, activity for 6-phosphogluconate dehydrogenase, the next enzyme in the pathway, is consistently identified. The parasites synthesize pyrimidines de novo, but being incapable of de novo purine biosynthesis they require preformed purines. Exogenously supplied purine, notably hypoxanthine derived from catabolism of erythrocytic ATP, is taken up and incorporated whereas pyrimidines are not. The capacity for de novo amino acid biosynthesis is limited and presumably haemoglobin supplies most of the amino acids required by the parasite. Degradation of haemoglobin, involving parasite proteases, notably a cathepsin D-like enzyme, leaves a characteristic golden-brown residue, haemozoin. Haemozoin consists of dimers of ferriprotoporphyrin IX, methaemoglobin and plasmodial proteins. For some species, isoleucine and methionine must be supplied exogenously for good plasmodial growth. Infected erythrocytes characteristically show altered permeability properties, changes which in large part contribute to parasite growth while at the same time impairing red cell function.

Amino Acids↗
PubMed1983

Maturation of the intracellular parasite and antigenicity.

The protein antigens synthesized by the malarial parasite change as the parasite matures, with a number of proteins showing strict stage-specificity. A detailed correlation between the stage-specificity of protein synthesis and parasite structure has yet to be established, but a number of proteins synthesized in the cycle are lost selectively during merozoite escape and reinvasion. These antigens are presumably associated with structures utilized and ultimately lost during this process. Particular interest has focused on some greater than 200K proteins identified as being on the surface of infected erythrocytes and internally and on the surface of merozoites. Smaller parasite proteins have also been identified in the erythrocyte membrane. The erythrocyte itself, including its membrane, is much modified by parasite growth. Changes include the presence of new cytoplasmic structures and differences in the surface labelling and isoantigenic characteristics of the surface membrane. An appreciation of the variability and specificity of exposed parasite antigens, and their relationship to newly exposed isoantigens, is central to our understanding of protective immunity to malaria.

Animals↗
PubMed1983

Influence of the spleen on the expression of surface antigens on parasitized erythrocytes.

Two malaria parasites, Plasmodium knowlesi and P. falciparum, when passaged in splenectomized hosts alter or fail to express parasite-dependent antigens on the surface membrane of erythrocytes infected with mature parasites. Experiments with cloned populations of P.knowlesi show this change to be phenotypic and to be modulated by the spleen of the host. In addition, the induction of antigen variation in P. knowlesi malaria apparently requires two factors: specific antibody and the spleen. Along with the altered expression of P. knowlesi variant antigen on the infected erythrocyte surface, there is a decrease in parasite virulence in non-splenectomized monkeys. It is suggested that the spleen-dependent expression of malarial antigens on the parasitized erythrocyte may be an adaptation of the malaria parasite for survival in the presence of a potentially destructive spleen-mediated host immunity.

Animals↗
PubMed1988

Cultural ecology of prehistoric parasitism on the Colorado Plateau as evidenced by coprology.

The study of coprolites (desiccated feces) is recognized as a viable method for analyzing parasitism of prehistoric peoples. Eight species of helminth parasites, including nematodes, cestodes, and acanthcephalans, have been recovered from archaeological sites on the Colorado Plateau. The comparative analysis of parasitological findings illustrates the effects of changing subsistence strategies and varying life-style on prehistoric human parasitism. This comparative study is based on the analysis of coprolites recovered from one Archaic hunter-gatherer site and two Anasazi agricultural villages. Hunter-gatherers are represented by coprolites from Dust Devil Cave in south-eastern Utah. Coprolites of prehistoric agriculturalists from Antelope House in Canyon de Chelly, Arizona, and from Salmon Ruin in northwestern New Mexico were studied. The results demonstrate that helminth parasitism increased with the advent of agriculture. Between the agricultural sites, differences in patterns of excreta disposal, foraging behavior, and local ecology resulted in pronounced variations in both percentage of coprolites containing parasite remains and the number of parasite species represented.

Animals↗
PubMed1995

Accumulation of neuropeptides in the cerebral neurosecretory system of Manduca sexta larvae parasitized by the braconid wasp Cotesia congregata.

Fifth instar larvae of Manduca sexta that were parasitized by the braconid wasp Cotesia congregata failed to develop after the parasitoid larvae emerged, and these host larvae lingered for 2-3 weeks in a quiescent, nonfeeding state without initiating a larval molt or metamorphosis. This study was focused on the neuroendocrine changes associated with the host's developmental arrest. Immunohistochemical studies suggested that the host brain neurosecretory cells as well as their axon terminals in the corpora cardiaca-corpora allata complex accumulated multiple neuropeptides. The extent of accumulation in cells and axons increased with time, so that hosts examined 7-14 days after the wasps emerged showed the most intense staining with antibodies against prothoracicotropic hormone, bombyxin, allatotropin, allatostatin, diuretic hormone, eclosion hormone, proctolin, and FMRFamide. Increased levels of prothoracicotropic hormone and FMRFamide-like peptides in the brains of parasitized larvae were confirmed using Western blots and enzyme-linked immunosorbent assay (ELISA), respectively. Starvation of the unparasitized larvae induced some accumulation of the neuropeptides; however, the intensity of staining and number of immunopositive cells and axons were in most cases clearly higher in the parasitized larvae. Our results suggest that accumulation of the neuropeptides is associated with developmental arrest of parasitized larvae. Because a similar developmental arrest occurs in a wide range of parasitized insects, our findings may have relevance for many other species. Moreover, these data illustrate the potential value of using parasitized M. sexta larvae as a model for studying the mechanisms governing the rates of neuropeptide expression, processing, packaging, and release, as well as providing a rich source of neuropeptides, thus facilitating their isolation and characterization.

Animals↗
PubMed1987

Plasmodium falciparum-specific human T cell clones: recognition of different parasite antigens.

T lymphocyte clones specific for malarial (Plasmodium falciparum) blood stage antigens were obtained from acutely infected patients or from donors living in a malaria-endemic area of West Africa. Thirty-four clones carrying the CD4 antigen, and one CD8+ clone, were tested in a proliferation assay for their capacity to recognize P. falciparum isolates of different geographical origins. Only one clone distinguished between different parasite isolates (it failed to react with a parasite isolate originating from East Africa, but did recognize West African and Asian isolates). All of the clones responded well to intact erythrocytes containing viable parasites, but some responded poorly to extracts of parasitized cells. Eight of 19 clones studied (all CD4+) recognized parasite antigens which had characteristic mobilities in sodium dodecyl sulfate-containing polyacrylamide gels. The antigens had apparent molecular weights of about 20,000, 35,000, 40,000, 120,000, 150,000-200,000 and 200,000. These results (together with a previous report of two clones recognizing an antigen of molecular weight about 50,000, Sinigaglia and Pink, EMBO J. 1985. 4:3819) show that T cells in infected individuals react with at least 6 different parasite proteins.

Animals↗
PubMed1995

Dendritic cells in Leishmania major-immune mice harbor persistent parasites and mediate an antigen-specific T cell immune response.

Upon infection with Leishmania major, a cause of human cutaneous leishmaniasis, mice of resistant strains are able to control the infection, with lesions resolving spontaneously. A long-lasting cell-mediated immunity protects them from reinfection. Nevertheless, small numbers of viable parasites persist in the lymph nodes of these mice. We have recently documented that, in addition to macrophages, epidermal Langerhans cells can ingest L. major. Furthermore, Langerhans cells have the unique ability to transport viable parasites from the infected skin to the draining lymph node for presentation to antigen-specific T cells and initiation of the cellular immune response. During migration, Langerhans cells develop into dendritic cells. In the present study, we analyzed whether dendritic cells support the persistence of parasites in immune hosts. Immunohistological studies and assays in vitro showed that in the lymph nodes of mice that have recovered from infection with L. major, both macrophages and dendritic cells harbor viable parasites. However, only dendritic cells were able to induce a vigorous T-cell immune response to L. major in vitro in the absence of exogenous antigen. Tracking experiments conducted in vivo suggested that the infected dendritic cells in the lymph nodes are derived from Langerhans cells that have emigrated from the skin. The data demonstrate that L. major-infected dendritic cells and macrophages in lymph nodes of immune animals represent long-term host cells, but only dendritic cells have the ability to present endogenous parasite antigen to T cells. Long-term infected dendritic cells may thus allow the sustained stimulation of a population of parasite-specific T cells, protecting the mice from reinfection. Our results favor the hypothesis that the persistence of antigen supports the maintenance of T cell memory and that dendritic cells are critically involved in this process.

Animals↗
PubMed1998

Parasites influence social rank and morphology, but not mate choice, in female red junglefowl, Gallus gallus.

Parasites can profoundly affect host morphology and behaviour, but previous work has focused on the effects of parasites on males. In the present study, we assessed the effects of infection with the nematode Ascaridia galli on the morphology and behaviour, including mate choice, of female red junglefowl. Hens infected with A. galli had lower mass and smaller combs than unparasitized birds when sexually mature. Parasite status had a significant effect on social rank in all-female flocks, with high-ranking birds being less likely to be parasitized. Larger females had higher social rank, but comb size was unrelated to social status. Neither parasite status nor social rank had any effect on mate choice. These results differ from those found for male red junglefowl, and suggest that males and females may allocate resources differently to comb versus growth. The low cost of choice in the red junglefowl mating system may also contribute to the lack of an effect of parasites and social status on mate choice Copyright 1998 The Association for the Study of Animal Behaviour

Journal Article↗
PubMed1999

Parasitized female guppies do not prefer showy males.

In many species male sexual characteristics are known to be affected negatively by parasites, which render their hosts unattractive to females, but how parasites affect the mating decisions of their female hosts has received little attention. The monogenean parasite Gyrodactylus turnbulli reduces the sexual display and colour intensity of male guppies, Poecilia reticulata, which makes them less attractive to females. Here, I examine how these parasites affect the mate choice behaviour of their female hosts. Virgin females were experimentally exposed to G. turnbulli and allowed to choose between an attractive and an unattractive male in a simultaneous choice test. Infected females were significantly less discriminatory than healthy ones and their level of activity during choice trials was reduced with increasing parasite load, suggesting an energetic constraint imposed by the parasites. This result implies that sexual selection pressure for male showiness is diminished, which is consistent with recent theoretical models. Copyright 1999 The Association for the Study of Animal Behaviour.

Journal Article↗
PubMed1994

Plasmodium ovale: observations on the parasite development in Saimiri monkey hepatocytes in vivo and in vitro in contrast with its inability to induce parasitemia.

Exoerythrocytic stage parasites of the human malaria parasite Plasmodium ovale were cultured in vitro by inoculating primary cultures of hepatocytes from Saimiri sciureus boliviensis monkeys with sporozoites. Morphology and size of the liver stages were similar to previous in vivo descriptions in humans and chimpanzees. Saimiri monkeys did not develop parasitemia after repeated inoculations with P. ovale sporozoites. However, liver-stage parasites were observed in liver biopsies performed 7 days after sporozoite inoculation. Together with observations on other parasite development, these results demonstrate that host specificity for many malaria parasites occurs at the blood-stage level. Lack of host specificity of primary malaria parasite species for the liver forms the basis for the close relationship existing between human and nonhuman primate malaria species.

Animals↗
PubMed1995

Plasmodium falciparum: effects of proteinase inhibitors on globin hydrolysis by cultured malaria parasites.

The effects of peptide proteinase inhibitors on globin hydrolysis by cultured malaria parasites were studied. All of the four cysteine proteinase inhibitors evaluated blocked globin hydrolysis, as documented by the development of a morphological abnormality in which parasite food vacuoles filled with undegraded globin and by SDS-PAGE showing that the cysteine proteinase inhibitor-treated parasites accumulated large quantities of globin. The aspartic proteinase inhibitor pepstatin did not block globin hydrolysis by cultured parasites. None of seven antimalarial drugs tested elicited the food vacuole abnormality caused by cysteine proteinase inhibitors, indicating that this morphological alteration was not simply a sign of nonspecific parasite toxicity. Our results indicate that a trophozoite cysteine proteinase is required for initial cleavages of globin by intact malaria parasites.

Animals↗
PubMed1995

Plasmodium falciparum: parasites defective in early stages of gametocytogenesis.

Some molecular characteristics of Plasmodium falciparum lines which do not produce gametocytes are described. Parasites carrying a subtelomerically deleted chromosome 9 cannot form even the earliest forms of gametocytes, detectable with antibodies against the gametocyte-specific antigen Pfg27. In a parasite culture of clone HB3, in which both intact and deleted forms of chromosome 9 are present, full-length chromosome 9 molecules are retained mainly in gametocytes. These data suggest that the subtelomeric portion of chromosome 9 is required at an early stage of gametocytogenesis. Parasite subclones derived from gametocyte producing clone 3D7, which completely lost ability to produce gametocytes, are also described. Unlike the previous gametocyteless lines, these parasites stably maintain a full-length chromosome 9 and the ability to cytoadhere to C32 melanoma cells after prolonged asexual propagation. Their defect in sexual development is therefore genetically and functionally distinct from that of parasites carrying a deleted chromosome 9. Gametocyteless subclones derived from 3D7 do not produce any Pfg27 mRNA, while this gene is anomalously expressed in asexual stage parasites of two lines of a different genetic background, 1776sel8 and C10, one able and the other unable to produce gametocytes.

Animals↗
PubMed1997

Plasmodium falciparum: an epitope within a highly conserved region of the 47-kDa amino-terminal domain of the serine repeat antigen is a target of parasite-inhibitory antibodies.

Previously, the Plasmodium falciparum serine repeat antigen has been shown to be protective in primate models of malaria immunity and also to be a target of in vitro parasite-inhibitory antibodies. To further define parasite-inhibitory epitopes a series of deletions from the amino-terminal 47-kDa domain of the serine repeat antigen (SERA) were constructed as glutathione-S-transferase fusion proteins. Several GST-SERA fusion proteins were used to vaccinate mice with Freund's adjuvant and the resulting immune sera were used to assay for the inhibition of P. falciparum invasion of erythrocytes in vitro. The minimal epitope shown to be the target of invasion-blocking antibodies was SERA amino acids 17-165. Additional GST-SERA deletion constructs of the 47-kDa domain were developed and evaluated for reactivity, by Western immunoblot analysis, with a parasite-inhibitory murine monoclonal antibody (mAb 43E5), a parasite-inhibitory pooled goat polyclonal sera, and a pooled human Nigerian immune serum. The parasite-inhibitory epitope defined by mAb 43E5 was mapped to SERA amino acids 17-110 and, at least, part of the epitope was defined to include amino acids in the region of amino acids 59-72. The parasite-inhibitory epitope recognized by mAb 43E5 appears to be well conserved between diverse geographical isolates of P. falciparum. The results have relevance for malaria vaccine development and suggest that an appropriately designed recombinant SERA antigen produced from a synthetic gene in Escherichia coli may be an effective component of a candidate malaria vaccine.

Amino Acid Sequence↗
PubMed1997

Toxoplasma gondii: dithiol-induced Ca2+ flux causes egress of parasites from the parasitophorous vacuole.

Ca2+ is an essential activator of motility in the obligate intracellular parasite Toxoplasma gondii. Ca2+ ionophore A23187 and intracellular microinjection of Ca2+ initiate motility of parasites residing in parasitophorous vacuoles (PV). The source of Ca2+ and the mechanism by which it activates motility in vivo remain uncertain. Exposure of the parasites to dithiothreitol (DTT) can activate egress of previously nonmotile intravacuolar parasites within 60 sec. DTT is also known to activate both isoforms of the highly concentrated nucleoside triphosphate hydrolase (NTPase) produced by T. gondii. Using an adherent cell analysis system (ACAS) for Ca2+ imaging, a brief 15-50% increase in intra-PV fluorescence ratio was observed after exposure of infected fibroblasts to 5 mM DTT. Chelation of intracellular Ca2+ with BAPTA-AM and extracellular Ca2+ with EGTA blocked the DTT effect; however, this chelation did not prevent the activation of parasites nor the Ca2+ response to the Ca2+ ionophore ionomycin, suggesting that the Ca2+ that activates motility may reside near or within the parasite itself. This result demonstrates that an increase in Ca2+ within the vacuole precedes the onset of motility and the correlation of the DTT effect on motility and tachyzoite NTPase suggests that NTPase activation may be involved in the Ca2+ flux.

Animals↗
PubMed1999

Plasmodium: immunization with carboxyl-terminal regions of MSP-1 protects against homologous but not heterologous blood-stage parasite challenge.

A leading candidate for a vaccine targeted at the erythrocytic stages of plasmodial parasite development is the merozoite surface protein-1 (MSP-1). We have previously shown that the carboxyl-terminal region of MSP-1 derived from Plasmodium yoelii yoelii 17XL, expressed as a fusion protein with glutathione S-transferase (GST-PYC2), can immunize mice against an otherwise lethal homologous challenge infection. This protection has been shown to be predominantly mediated by antibodies. We report here on the efficacy of immunization with MSP-1 carboxyl regions when the challenge is a heterologous rodent parasite species. The course of parasitemia was not altered in mice immunized with GST-PYC2 and challenged with 10(4) heterologous Plasmodium chabaudi adami parasites, as both control and immunized mice developed infections that peaked at day 7 and then rapidly declined. Similarly, mice immunized with GST-PYC2 and challenged with 10(5) Plasmodium berghei ANKA parasites displayed virulence similar to that seen in infection control mice. The homologous region of the P. chabaudi adami MSP-1 gene was similarly expressed as a fusion protein with GST. Mice immunized with GST-PCC2 and challenged with 10(4) parasites showed significant protection against homologous P. chabaudi adami infection but no protection whatsoever against heterologous P. yoelii yoelii 17XL infection. These in vivo results correlate with the observation that sera generated by immunization with the carboxyl region of MSP-1 recognizes this protein from homologous, but not heterologous, radiolabeled parasite protein preparations.

Amino Acid Sequence↗
PubMed2000

Functional significance of parasitism-induced suppression of juvenile hormone esterase activity in developmentally delayed Choristoneura fumiferana larvae.

The parasitic wasp Tranosema rostrale transmits a polydnavirus (PDV) to its host, Choristoneura fumiferana, during oviposition. Last-instar C. fumiferana larvae parasitized by T. rostrale early in the stadium fail to undergo metamorphosis, and injection of the wasp's calyx fluid (CxF; contains PDV) into healthy caterpillars induces a dose-dependent delay in initiation of metamorphosis (D. Doucet and M. Cusson, 1996, Entomol. Exp. Appl. 81, 21-30). In the present work, parasitization and injection of CxF (0.5 female equivalent) on the first day of the last stadium both prevented the rise in hemolymph 20-hydroxyecdysone (20HE) titer observed between day 4 and day 7 in control and saline-injected larvae. Similarly, juvenile hormone esterase (JHE) activity was depressed following parasitization or CxF injection, whereas control larvae displayed a peak on day 4. However, neither parasitism nor injection of CxF on day 1 prevented the JH-producing glands from turning off during the first half of the last stadium. Likewise, low but clearly detectable JH titers were observed in the first hours following the molt but very low titers, at or near the detection limit of our radioimmunoassay, were seen in both control and parasitized larvae on day 4. Prothoracic glands showed no apparent sign of degeneration 4 days after injection of CxF but had significantly smaller cells than saline-injected larvae 7 days postinjection. It is not clear whether this was a direct effect of T. rostrale PDV. Thus, disruption of spruce budworm metamorphosis by T. rostrale CxF involves depression of 20HE titers but is not associated with a measurable increase in the level of JH, as shown for some other host-parasitoid systems. In view of the latter observation, we put forward three hypotheses regarding the functional significance of the observed suppression of JHE activity in developmentally arrested C. fumiferana larvae.

Animals↗
PubMed1997

Regulation and trafficking of three distinct 18 S ribosomal RNAs during development of the malaria parasite.

The human malaria parasite Plasmodium vivax has been shown to regulate the transcription of two distinct 18 RNAs during development. Here we show a third and distinctive type of ribosome that is present shortly after zygote formation, a transcriptional pattern of ribosome types that relates closely to the developmental state of the parasite and a phenomenon that separates ribosomal types at a critical phase of maturation. The A-type ribosome is predominantly found in infected erythrocytes of the vertebrate and the mosquito blood meal. Transcripts from the A gene are replaced by transcripts from another locus, the O gene, shortly after fertilization and increase in number as the parasite develops on the mosquito midgut. Transcripts from another locus, the S gene, begins as the oocyst form of the parasite matures. RNA transcripts from the S gene are preferentially included in sporozoites that bud off from the oocyst and migrate to the salivary gland while the O gene transcripts are left within the oocyst. Although all three genes are typically eukaryotic in structure, the O gene transcript, described here, varies from the other two in core regions of the rRNA that are involved in mRNA decoding and translational termination. We now can correlate developmental progression of the parasite with changes in regions of rRNA sequence that are broadly conserved, where sequence alterations have been related to function in other systems and whose effects can be studied outside of Plasmodium. This should allow assessment of the role of translational control in parasite development.

Animals↗