An RFLP map of the Plasmodium falciparum genome, recombination rates and favored linkage groups in a genetic cross.

We report a genetic linkage map of the Plasmodium falciparum genome, using the inheritance patterns of nearly 90 RFLP markers in a genetic cross. Markers were assigned to polymorphic loci on all 14 nuclear chromosomes. Genetic recombination between parental markers was detected in each of the progeny, indicating that progeny from cross-fertilization events were favored over progeny from self-fertilization of either parent alone. Inheritance patterns among the markers suggested that certain parental linkage groups on chromosomes 2, 3, 12 and 13 were favored in the cross. Recombination frequencies on five chromosomes indicated an approximate map unit size of 15-30 kb per centiMorgan for P. falciparum.

A test for genetic exchange in mixed infections of Leishmania major in the sand fly Phlebotomus papatasi.

We tested if genetic exchange was observable between two strains of Leishmania major (Trypanosomatidae) during mixed infection of the sand fly Phlebotomus papatasi. Previous studies suggested that genetic exchange may occur in natural populations of Leishmania at a low frequency, but experimental crosses examining small numbers of progeny (less than 60) did not reveal hybrid parasites. Accordingly, a strategy was devised to increase the number of progeny that could be screened by 100-fold. Clonal derivatives from two strains that were infective to flies and contained numerous restriction fragment length polymorphisms were characterized and selected for resistance to methotrexate or tunicamycin by gene amplification. A successfully mixed infection of P. papatasi was obtained, and a method was developed for directly plating promastigotes from the gut contents of infected flies onto selective media. Twenty-five hundred independent progeny were scored for the presence of both drug resistance markers. No hybrid parasites were observed, indicating that the frequency of genetic exchange in this cross must be less than 4 x 10(-4). The lines and methods established in this work may prove useful in future studies of the mechanism and frequency of gene exchange in Leishmania.

Genetic mapping of the chloroquine-resistance locus on Plasmodium falciparum chromosome 7.

The resurgence of malaria in recent decades has been accompanied by the worldwide spread of resistance to chloroquine, a drug once uncontested as the first-line antimalarial agent because of its efficacy and low toxicity. Chloroquine-resistant strains of Plasmodium falciparum counter the drug by expelling it rapidly via an unknown mechanism. In the absence of explicit biochemical knowledge of this efflux mechanism, reverse genetics provides a powerful approach to the molecular basis of chloroquine resistance. Here we report genetic linkage analysis in which 85 restriction fragment length polymorphism markers were used to examine inheritance of the 14 P. falciparum chromosomes in a laboratory cross between a chloroquine-resistant and a chloroquine-sensitive parasite. Inheritance data from 16 independent recombinant progeny show that the rapid efflux, chloroquine-resistant phenotype is governed by a single locus within an approximately 400-kilobase region of chromosome 7. Identification and characterization of genes within this region should lead to an understanding of the chloroquine-resistance mechanism.

Chloroquine resistance not linked to mdr-like genes in a Plasmodium falciparum cross.

Chloroquine is thought to act against falciparum malaria by accumulating in the acid vesicles of the parasite and interfering with their function. Parasites resistant to chloroquine expel the drug rapidly in an unaltered form, thereby reducing levels of accumulation in the vesicles. The discovery that verapamil partially reverses chloroquine resistance in vitro led to the proposal that efflux may involve an ATP-driven P-glycoprotein pump similar to that in mammalian multidrug-resistant (mdr) tumor cell lines. Indeed, Plasmodium falciparum contains at least two mdr-like genes, one of which has been suggested to confer the chloroquine resistant (CQR) phenotype. To determine if either of these genes is linked to chloroquine resistance, we performed a genetic cross between CQR and chloroquine-susceptible (CQS) clones of P. falciparum. Examination of 16 independent recombinant progeny indicated that the rapid efflux phenotype is controlled by a single gene or a closely linked group of genes. But, there was no linkage between the rapid efflux, CQR phenotype and either of the mdr-like P. falciparum genes or amplification of those genes. These data indicate that the genetic locus governing chloroquine efflux and resistance is independent of the known mdr-like genes.

Purification and partial characterization of an unusual protein of Plasmodium falciparum: histidine-rich protein II.

The human malarial parasite Plasmodium falciparum secretes a histidine-rich protein (HRP-II) from infected erythrocytes. HRP-II has a very high content of histidine (H) (34%), alanine (A) (37%) and aspartic acid (D) (10%) and many contiguous repeats of the sequences AHH and AHHAAD. The histidine content of the protein suggested the potential to bind metal ions. We have demonstrated by metal chelate chromatography an extraordinary capacity of HRP-II to bind zinc ions (Zn2+) and employed this characteristic to isolate the extracellular protein. The HRP-II was further purified by antibody affinity chromatography. The identity of the purified protein was verified by relative molecular weight on denaturing polyacrylamide gels, by reactivity with monoclonal antibodies and monospecific rabbit antiserum, and by comparison of the amino-acid analysis with that derived from the cloned gene sequence. Analysis of the sequence for periodicities using the hydrophobic moment method indicated that HRP-II may potentially form a 3/10 helix. Immunoprecipitation of HRP-II from culture supernatants of parasites metabolically labeled with tritiated sugars showed that the extracellular form of HRP-II is a glycoprotein containing galactose.

In vitro and in vivo studies of the effects of halogenated histidine analogs on Plasmodium falciparum.

The effects of four halogenated analogs of histidine on in vitro growth of Plasmodium falciparum malaria parasites were monitored by measurement of the incorporation of 3H-labeled amino acids into parasite proteins and by light and electron microscopy. The uptake of [3H]isoleucine was reduced to 50% of the control value by addition of 70 microM 2-fluoro-L-histidine (2-F-HIS) or 420 microM 2-iodo-L-histidine (2-I-HIS). [3H]histidine uptake into acid-insoluble material was affected equally by these two compounds, 50% inhibition resulting at 200 microM concentration. Morphological analysis of parasite development proved a sensitive assay, since development of mature trophozoites was inhibited 50% by 25 microM 2-F-HIS or 100 2-I-HIS. Electron microscopy studies suggested different mechanisms of action of 2-F-HIS and 2-I-HIS on P. falciparum. 2-F-HIS produced a decrease in knob number at the erythrocyte surface and accumulation of electron-dense material under the parasite membrane. 2-I-HIS had no obvious effect on knobs or electron-dense material but affected parasite morphology. Surprisingly, 2-chloro-L-histidine and 2-bromo-L-histidine did not inhibit P. falciparum in vitro, even though their halogen atom substituents are intermediate in size between F and I atoms. 2-F-HIS and 2-I-HIS were tested in vivo against P. falciparum in owl monkeys (Aotus sp.) but were ineffective at doses that were nontoxic.

Cytoadherence of Plasmodium falciparum-infected erythrocytes to human melanoma cell lines correlates with surface OKM5 antigen.

OKM5 antigen and thrombospondin are currently under investigation as potential receptors on the surface of human monocytes, endothelial cells, and melanomas responsible for the cytoadherence of Plasmodium falciparum-infected erythrocytes. We have studies the binding capacity of six human melanoma cell lines and related this property to the cytoplasmic and surface expression of the OKM5 antigen and thrombospondin by using indirect immunofluorescence assays on methanol-fixed and nonfixed melanomas. The presence of OKM5 antigen was detectable only in the melanoma lines which bound P. falciparum-infected erythrocytes. Thrombospondin was present in the cytoplasm of all the melanoma lines but was not detectable on the surface of any cells. Our work demonstrates a direct correlation between surface OKM5 antigen and cytoadherence in vitro. While our results do not exclude thrombospondin as a mediator of cytoadherence to endothelial cells in vivo, they showed no correlation between the presence of thrombospondin and the ability of melanoma cell lines to cytoadhere in vitro.

Localization of Plasmodium falciparum histidine-rich protein 1 in the erythrocyte skeleton under knobs.

Plasmodium falciparum parasites that induce knobs in the host erythrocyte membrane (K+ phenotype) synthesize a 90 kDa histidine-rich protein (PfHRP-1), whereas knobless variants do not. A monoclonal antibody (mAb 89) to PfHRP-1, in combination with cryo-thin section immunoelectron microscopy, localized the antigen in the parasitophorous vacuolar space and vesicles within the erythrocyte cytosol. Additional immunoelectron microscopic studies showed that PfHRP-1 was also associated with submembranous electron-dense material under knobs and with microfilaments of the host erythrocyte skeletal network. Immunofluorescence and immunoelectron microscopy of intact, non-fixed K+ infected erythrocytes using mAb 89 and a rabbit antiserum raised against purified PfHRP-1, failed to identify any surface exposed epitopes. These antibodies also failed to block cytoadherence of infected erythrocytes to C32 melanoma cells or to affect macrophage phagocytosis of infected erythrocytes.

Transport of an Mr approximately 300,000 Plasmodium falciparum protein (Pf EMP 2) from the intraerythrocytic asexual parasite to the cytoplasmic face of the host cell membrane.

The profound changes in the morphology, antigenicity, and functional properties of the host erythrocyte membrane induced by intraerythrocytic parasites of the human malaria Plasmodium falciparum are poorly understood at the molecular level. We have used mouse mAbs to identify a very large malarial protein (Mr approximately 300,000) that is exported from the parasite and deposited on the cytoplasmic face of the erythrocyte membrane. This protein is denoted P. falciparum erythrocyte membrane protein 2 (Pf EMP 2). The mAbs did not react with the surface of intact infected erythrocytes, nor was Pf EMP 2 accessible to exogenous proteases or lactoperoxidase-catalyzed radioiodination of intact cells. The mAbs also had no effect on in vitro cytoadherence of infected cells to the C32 amelanotic melanoma cell line. These properties distinguish Pf EMP 2 from Pf EMP 1, the cell surface malarial protein of similar size that is associated with the cytoadherent property of P. falciparum-infected erythrocytes. The mAbs did not react with Pf EMP 1. In one strain of parasite there was a significant difference in relative mobility of the 125I-surface-labeled Pf EMP 1 and the biosynthetically labeled Pf EMP 2, further distinguishing these proteins. By cryo-thin-section immunoelectron microscopy we identified organelles involved in the transit of Pf EMP through the erythrocyte cytoplasm to the internal face of the erythrocyte membrane where the protein is associated with electron-dense material under knobs. These results show that the intraerythrocytic malaria parasite has evolved a novel system for transporting malarial proteins beyond its own plasma membrane, through a vacuolar membrane and the host erythrocyte cytoplasm to the erythrocyte membrane, where they become membrane bound and presumably alter the properties of this membrane to the parasite's advantage.

Antigenic diversity and size diversity of Plasmodium falciparum antigens in isolates from Gambian patients. I. S-antigens.

Ring-stage asexual parasites of P. falciparum were collected from six Gambian children and the S-antigens radiolabelled by 3H-glycine uptake during in vitro culture up to rupture of infected cells and merozoite release. Ouchterlony double diffusion of boiled culture supernatants against a panel of adult Gambian sera identified one S-antigen precipitin arc for five isolates and two precipitin arcs for one isolate. Five of the six isolates were serologically distinct. Analysis of S-antigens by comparison of SDS-polyacrylamide gel electrophoresis patterns of heat-treated soluble proteins revealed a more complex pattern of 3H-labelled S-antigens that was different for each isolate. There were between two and six different 3H-labelled bands for each isolate in the size range of molecular weight 137 000 to 285 000. This result confirms the large size range of S-antigens identified with culture adapted P. falciparum. Several bands were relatively weakly labelled with 3H-glycine, suggesting that natural isolates contain one or two predominant S-antigen phenotypes and several other S-antigen phenotypes expressed by minor parasite subpopulations. Immunoprecipitation was performed using a panel of sera from Gambian adults, or, acute and 3 week convalescent sera from the same patients used for S-antigen radiolabelling. Adult sera generally immunoprecipitated some of the S-antigens in each isolate, including antigens that must represent extremely minor parasite subpopulations since they could not be seen in the patterns of non-immunoprecipitated heat-stable proteins. Sera from convalescent children were generally negative on immunoprecipitation, even with the homologous isolate. In one case we observed the acquisition of specific immunoprecipitating antibody to one of the homologous S-antigens during the convalescent period. The antigenic and structural complexity of S-antigens in natural isolates that have not been submitted to the selection pressure of adaptation for in vitro culture is clearly greater than for culture adapted P. falciparum.

Thrombospondin binds falciparum malaria parasitized erythrocytes and may mediate cytoadherence.

Plasmodium falciparum infected erythrocytes containing mature trophozoites and schizonts sequester along venular endothelium and are not in the peripheral circulation of patients with malaria. Knobs appear on infected erythrocytes and are the points of attachment to endothelium. Sequestration may protect the parasite from splenic destruction and may play a role in the pathogenesis of cerebral malaria. Correlates of sequestration have been developed in vitro using cultured human endothelium and an amelanotic melanoma cell line. Knobless strains (K-) of P. falciparum fail to sequester in vivo and to bind to cells in vitro. We now present evidence that the receptor for cytoadherence is the glycoprotein, thrombospondin. Aotus monkey or human erythrocytes containing knobby (K+) but not Aotus erythrocytes containing knobless strains of P. falciparum bind to immobilized thrombospondin. Neither binds to the adhesive proteins laminin, fibronectin, factor VIII/von Willebrand factor or vitronectin. Both soluble thrombospondin and anti-thrombospondin antibodies inhibit binding of parasitized Aotus erythrocytes to immobilize thrombospondin and to melanoma cells which secrete thrombospondin.

In vitro sensitivity of Plasmodium falciparum to chlorcycloguanil in The Gambia.

Chlorcycloguanil (10732), the active metabolite of the antifolate chlorproguanil, has been tested in vitro against 17 isolates of Plasmodium falciparum in The Gambia. Minimum inhibitory concentrations were 10(-9) molar or less. 11 isolates simultaneously tested with pyrimethamine were sensitive to 10(-8) molar concentrations of that drug.

Susceptibility of Plasmodium falciparum in The Gambia to pyrimethamine, Maloprim and chloroquine.

A five-year malaria chemoprophylaxis study has begun with Maloprim in children aged three months to five years and pregnant women in a population of 13,000 in the area of Farafenni, The Gambia. Sensitivity of Plasmodium falciparum to pyrimethamine, Maloprim and chloroquine was assessed in vivo and in vitro in rural Gambian villages before drug intervention. 569 children aged one to seven years inclusive were sampled at the end of the wet season of 1982; 46% had positive blood films. All afebrile children were treated with a single dose of one of the antimalarials under study. Febrile children were treated with chloroquine. 109 infected children were retested 7 to 10 days after treatment and none showed asexual parasitaemia. 83 micro in vitro tests were successfully performed from fingerprick blood samples and the results confirmed the in vivo study. Pyrimethamine in combination with dapsone, in the proportion present in Maloprim, i.e., 1:8, showed a synergistic effect, the mean effective dose of pyrimethamine being reduced 13 times at the 50% inhibitory level.

Studies of antigens in Plasmodium yoelii. I. Antigenic differences between parasite lines detected by crossed immunoelectrophoresis.

Antigens of three lines of the rodent malaria parasite Plasmodium yoelii have been studied using crossed immunoelectrophoresis. P. y. yoelii line A1 is a mild line which is restricted to reticulocytes. P. y. yoelii line YM and P. y. nigeriensis line D1 are virulent infections which multiply in both immature and mature erythrocytes. One antigen, designated Py-1, was found to differ in its electrophoretic mobility between the lines, being fast (F) in lines A1 and YM and slow (S) in line D1. Antigen Py-1 also varied in quantity among the three lines; greater amounts were detected in parasites inhabiting mature erythrocytes than in those in reticulocytes. These characters were stable during blood and mosquito passage.

Studies of antigens in Plasmodium yoelii. II. Inheritance and recombination of antigenic characters.

The inheritance of an antigen designated Py-1 in the rodent malaria parasite Plasmodium yoelii has been investigated. A cross was made between 2 lines differing in the electrophoretic mobility and quantity of Py-1 detected by crossed immunoelectrophoresis. In 10 clones isolated from the progeny of the cross the level of Py-1 always correlated with the virulence of the infection and it was concluded that these characters were different phenotypic effects of the same gene mutation. The electrophoretic mobility of Py-1 segregated independently of the virulence character and was therefore controlled by a different gene. These two antigenic markers also recombined with isoenzyme and drug-sensitivity characters distinguishing the parent lines.

Leishmaniasis in The Gambia. 3. Is its incidence increasing?

During 1982 a further case of visceral leishmaniasis and six cases of cutaneous leishmaniasis were seen at the Medical Research Council Laboratories in The Gambia, suggesting that the incidence of this infection in The Gambia is increasing.