Variable expression of virulence in the rodent malaria parasite Plasmodium yoelii yoelii

Ex Vivo Maturation Assay for Testing Antimalarial Sensitivity of Rodent Malaria Parasites

Ex vivo assay systems provide a powerful approach to studying human malaria parasite biology and to testing antimalarials. For rodent malaria parasites, short-term in vitro culture and ex vivo antimalarial susceptibility assays are relatively cumbersome, relying on in vivo passage for synchronization, since ring-stage parasites are an essential starting material. Here, we describe a new approach based on the enrichment of ring-stage Plasmodium berghei, P. yoelii, and P. vinckei vinckei using a single-step Percoll gradient. Importantly, we demonstrate that the enriched ring-stage parasites develop synchronously regardless of the parasite strain or species used. Using a flow cytometry assay with Hoechst and ethidium or MitoTracker dye, we show that parasite development is easily and rapidly monitored. Finally, we demonstrate that this approach can be used to screen antimalarial drugs.

The role of immunity in mosquito-induced attenuation of malaria virulence

A recent study found that mosquito-transmitted (MT) lines of rodent malaria parasites elicit a more effective immune response than non-transmitted lines maintained by serial blood passage (non-MT), thereby causing lower parasite densities in the blood and less pathology to the host. The authors attribute these changes to higher diversity in expression of antigen-encoding genes in MT cf. non-MT lines. Alternative explanations that are equally parsimonious with these new data, and results from previous studies, suggest that this conclusion may be premature.

Vector transmission regulates immune control of Plasmodium virulence

Defining mechanisms by which Plasmodium virulence is regulated is central to understanding the pathogenesis of human malaria. Serial blood passage of Plasmodium through rodents^1-3, primates⁴ or humans⁵ increases parasite virulence, suggesting that vector transmission regulates Plasmodium virulence within the mammalian host. In agreement, disease severity can be modified by vector transmission^6-8, which is assumed to ‘reset’ Plasmodium to its original character³. However, direct evidence that vector transmission regulates Plasmodium virulence is lacking. Here we utilise mosquito transmission of serially blood passaged (SBP) Plasmodium chabaudi chabaudi⁹ to interrogate regulation of parasite virulence. Analysis of SBP P.c. chabaudi before and after mosquito transmission demonstrates that vector transmission intrinsically modifies the asexual blood-stage parasite, which in turn, modifies the elicited mammalian immune response, which in turn, attenuates parasite growth and associated pathology. Attenuated parasite virulence associates with modified expression of the pir multi-gene family. Vector transmission of Plasmodium therefore regulates gene expression of probable variant antigens in the erythrocytic cycle, modifies the elicited mammalian immune response, and thus regulates parasite virulence. These results place the mosquito at the centre of our efforts to dissect mechanisms of protective immunity to malaria for the development of an effective vaccine.

Variable expression of the 235 kDa rhoptry protein ofPlasmodium yoeliimediate host cell adaptation and immune evasion

The severity of infections caused by the malaria parasite Plasmodium is in part due to the rapid multiplication cycles in the blood of an infected individual. A fundamental step in this phenomenon is the invasion of selected erythrocytes of the host by the parasite. The py235 rhoptry protein multigene family of the rodent malaria parasite Plasmodium yoelii has been implicated in mediating host cell selection during erythrocyte invasion and virulence. Here we show using quantitative real-time polymerase chain reaction and Western blot analysis that variations in the amounts of py235 may be a mechanism that the parasite uses to define its host cell repertoire. High levels of py235 expression leads to a wider range of erythrocytes invaded and therefore increased virulence. In contrast, to evade PY235-specific immunity, the parasite downregulates py235 thereby decreasing the host cell repertoire and virulence. These results demonstrate a new mechanism where variations in the amounts of parasite ligand define the parasite host cell repertoire and enable it to evade host immunity.

The effects of mosquito transmission and population bottlenecking on virulence, multiplication rate and rosetting in rodent malaria

Malaria parasites vary in virulence, but the effects of mosquito transmission on virulence phenotypes have not been systematically analysed. Using six lines of malaria parasite that varied widely in virulence, three of which had been serially blood-stage passaged many times, we found that mosquito transmission led to a general reduction in malaria virulence. Despite that, the between-line variation in virulence remained. Forcing serially passaged lines through extreme population bottlenecks (<5 parasites) reduced virulence in only one of two lines. That reduction was to a level intermediate between that of the virulent parental and avirulent ancestral line. Mosquito transmission did not reverse the increased parasite replication rates that had accrued during serial passage, but it did increase rosetting frequencies. Re-setting of asexual stage genes during the sexual stages of the life cycle, coupled with stochastic sampling of parasites with variable virulence during population bottlenecks, could account for the virulence reductions and increased rosetting induced by mosquito transmission.

Immunity promotes virulence evolution in a malaria model

Evolutionary models predict that host immunity will shape the evolution of parasite virulence. While some assumptions of these models have been tested, the actual evolutionary outcome of immune selection on virulence has not. Using the mouse malaria model, Plasmodium chabaudi, we experimentally tested whether immune pressure promotes the evolution of more virulent pathogens by evolving parasite lines in immunized and nonimmunized ("naïve") mice using serial passage. We found that parasite lines evolved in immunized mice became more virulent to both naïve and immune mice than lines evolved in naïve mice. When these evolved lines were transmitted through mosquitoes, there was a general reduction in virulence across all lines. However, the immune-selected lines remained more virulent to naïve mice than the naïve-selected lines, though not to immunized mice. Thus, immune selection accelerated the rate of virulence evolution, rendering parasites more dangerous to naïve hosts. These results argue for further consideration of the evolutionary consequences for pathogen virulence of vaccination.

The 235 kDa rhoptry protein of Plasmodium (yoelii) yoelii: function at the junction

All malaria parasites are obligate intracellular organisms that must clearly recognise and discriminate between different cells during their life cycle. Invasion into a cell is a multi-step event that is marked by initial attachment proceeding to irreversible junction formation and penetration. A 235 kDa rhoptry protein (Py235) in the rodent malaria, Plasmodium yoelii yoelii has been shown to be involved in red blood cell (rbc) binding and is involved in a new mechanism of clonal phenotypic variation that may be important in adaptation and immune evasion. Immunisation studies using Py235 have also revealed a role for this protein in the virulence phenotype seen with P. y. yoelii in laboratory mice. Interestingly, the genes that encode this protein are present as a multi-gene family. In this paper, we examine Py235 at the level of DNA, transcription and expression, discussing the role of this protein during invasion, in virulence and in immune evasion.

Immunization with parasite-derived apical membrane antigen 1 or passive immunization with a specific monoclonal antibody protects BALB/c mice against lethal Plasmodium yoelii yoelii YM blood-stage infection

We have purified apical merozoite antigen 1 (AMA-1) from extracts of red blood cells infected with the rodent malaria parasite Plasmodium yoelii yoelii YM. When used to immunize mice, the protein induced a strong protective response against a challenge with the parasite. Monoclonal antibodies specific for P. yoelii yoelii AMA-1 were prepared, and one was very effective against the parasite on passive immunization. A second protein that appears to be located in the apical rhoptry organelles and associated with AMA-1 was identified.

Chromosomal organisation of a gene family encoding rhoptry proteins in Plasmodium yoelii

The genomic organisation of the genes coding for a group of high molecular mass rhoptry proteins of the rodent malaria parasite Plasmodium yoelii YM was investigated using blotting, two dimensional gel electrophoresis and restriction fragment length analysis. The genes were found on chromosomes 1, 5, 6 and 10, with the possibility that related genes were also present on chromosomes 3 and 4. On chromosome 1 the genes were located close to one end, whereas they were present at both ends of chromosome 5, 6 and 10. Two genes, e3 and e8, that had been partially characterised previously were present on chromosomes 5 and 1, respectively. Based on an analysis of the 3' end of the genes, three subfamilies present on chromosomes 1, 5 and 6, and 10, respectively, were identified.

Development of an in vitro cloning method for Cowdria ruminantium

Cowdria ruminantium is a tick-borne rickettsia which causes severe disease in ruminants. All studies with C. ruminantium reported so far were carried out with stocks consisting of infective blood collected from reacting animals or from the same stocks propagated in vitro. Cloned isolates are needed to conduct studies on immune response of the host, on genetic diversity of the parasite, and on mechanisms of attenuation and the development of vaccines. A method of cloning based on the particular chlamydia life cycle of Cowdria was developed. Instead of cloning extracellular elementary bodies, it appeared more convenient to clone endothelial cells infected by one morula resulting from the infection of the cell by one elementary body of Cowdria. Two hundred and sixteen clones were obtained by limiting dilution of infected cells. The method was experimentally validated by comparing randomly amplified polymorphic DNA fingerprints from individual clones obtained from endothelial cell cultures coinfected with two different stocks of C. ruminantium.

Multiple genes code for high-molecular-mass rhoptry proteins of Plasmodium yoelii

We have examined the number of genes coding for a group of high-molecular-mass rhoptry protein(s) in the malaria parasite Plasmodium yoelii, and studied variation in the gene family within the parasite's genome. A region of the genes was amplified using oligonucleotides based on conserved DNA sequences and the products cloned. The sequences could be divided into 7 groups by restriction-fragment-length polymorphism. Further variation was detected by sequence analysis; 11 different sequences were detected in the 16 clones analyzed. The genes in the family were distributed on 6 chromosomes probably at 9 or more loci.

A gene coding for a high-molecular mass rhoptry protein of Plasmodium yoelii

We describe the deduced amino acid sequence for a gene encoding a high molecular mass rhoptry protein of Plasmodium yoelii. The sequence was obtained from an EcoRI genomic clone that overlaps a short DraI fragment isolated previously. The open reading frame consists of 2294 codons and putative hydrophobic signal and membrane anchor sequences were identified. Similarity with sequence from a clone coding for part of a Plasmodium vivax reticulocyte-binding protein was noted. Based on the sequence and location of the protein and the biological properties of antibodies that react with it, we propose that this may be an erythrocyte-binding protein.

Study of virulence and vector transmission of Babesia bovis by use of cloned parasite lines

Cloned lines of Babesia bovis were prepared from the avirulent vaccine strain, Ka, by an in vivo limiting dilution procedure. The virulence of these clones for adult Bos taurus cattle varied from completely avirulent to highly virulent. This suggests that the parent strain, Ka, is composed of a mixture of subpopulations of varied virulence. Passage of the avirulent clone K-19-47 in intact (nonsplenectomized) cattle resulted in its full reversion to virulence. This suggests that two mechanisms are operating to enable virulence to be a readily modified characteristic in this parasite: differential gene expression and phenotypic selection of subpopulations. A series of experiments demonstrated that all clones were non-tick transmissible. This lack of vector transmission was a stable characteristic and could not be altered by the passage of K-19-47 in intact cattle, despite the fact that passage in intact animals caused this clone to revert to the fully virulent phenotype. A mechanism is suggested for the virulence and vector transmission variations observed in B. bovis.

Babesia bovis: molecular and biological characteristics of cloned parasite lines

An in vivo limiting dilution technique was used to produce several Babesia bovis cloned lines with which to study the basis of virulence and immunogenicity in this parasite. DNA hybridization using a cloned DNA fragment from the BabR locus demonstrated that the cloned lines were a more restricted genetic population than the parent strain. Biosynthetic labeling and immunoprecipitation studies indicated that the cloned lines differed from each other and from the parentals in the expression of a small number of polypeptides and antigens. Animal trials with three of the lines demonstrated that the parental line contains both virulent and avirulent parasite populations, at least three of which are not tick transmissible, and that while the lines do provide significant protection against heterologous challenge, they may not give as effective protection as the parental line. These experiments demonstrated the existence of subpopulations with distinctive molecular and biological properties, providing evidence that the attenuation process is based on the selection of preexisting parasite subpopulations combined with the ability of these parasites to vary genetically.

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.

DNA polymorphisms and subpopulations in Babesia bovis

Independent isolates of Babesia bovis differ by only a limited number of polypeptides, some of which may be important as host protective antigens. Avirulent derivatives of these parasites also differ from their virulent counterparts in only a few polypeptides. To identify genes encoding such polypeptides we have isolated cDNA clones corresponding to poly(A)+ RNAs that are expressed only in certain isolates. For this purpose a cDNA clone library was constructed from poly(A)+ RNA of the K-avirulent isolate (KA). These clones were screened by colony hybridization using [32P]cDNA complementary to poly(A)+ RNA from KA and from virulent isolates, in order to identify clones that selectively hybridize to one cDNA probe. Hybridization of DNA from three clones, designated pK4, pK5 and pK6 to poly(A)+ RNA from various isolates revealed different and complex patterns. The gene represented by clone pK5 appeared to be transcribed predominantly in avirulent parasites. Analysis of genomic DNA by the Southern procedure enabled each isolate to be distinguished and suggested that most isolates are comprised of a heterogeneous mixture of subpopulations. Analysis of genomic DNA from parasites obtained after passage of KA through the tick vector (Boophilus microplus) suggested that a subpopulation was being selected that more closely resembled KV than KA.

Babesia bovis: proteins of virulent and avirulent parasites passaged through ticks and splenectomized or intact calves

Passage of the avirulent vaccine (K) strain of Babesia bovis (KA) through either Boophilus microplus ticks, intact calves, or intact calves and then ticks, resulted in two distinct protein and protein antigen profiles as analyzed by two-dimensional gel electrophoresis of biosynthetically labeled proteins and immunoprecipitates. Different degrees of expression of two major acidic antigens of KA designated Ka1 (Mr 47,500) and Ka2 (Mr 43,000) were observed. Ka1 was apparently lost following passage of KA B. bovis through intact calves but was strongly represented in the parasite population following a single tick passage. In contrast, passage through ticks of the virulent KV B. bovis (from which KA was derived by passage in splenectomized calves) did not lead to strong representation of the Ka1 protein although there was increased representation of another major acidic protein antigen, designated KV (Mr 35,000). These data suggest that the previously recognized reversion to a strain-dependent basal antigenic type in the tick vector depends also on intrastrain characteristics such as virulence and strain heterogeneity. The data suggest that KA is a more heterogeneous population than KV although cloned isolates are required to establish this point. Comparable syringe passage of another strain of B. bovis, designated C strain, through splenectomized calves resulted in less marked differences between the putative CA and CV B. bovis. This may explain the less stable avirulence of CA compared to KA B. bovis. Various selection pressures must act, in either the tick or the vertebrate host, on subpopulations in heterogeneous isolates to produce the changes described in protein antigen profiles of B. bovis. The possible relevance of changes in representation of proteins to biological characteristics of B. bovis (such as virulence and tick transmissibility) is discussed.

The genetic basis of diversity in malaria parasites

The principal findings of the P. falciparum surveys are given below. Considerable diversity of enzymes, antigens, drug sensitivity and other characters is seen among P. falciparum isolates. Cloning studies show that certain isolates contain mixtures of parasites which may be diverse in one or more of these characters. No obvious regional distribution is seen in the enzymic and antigenic characters examined, although differences in the frequencies of certain enzymes appear to exist. Variations in drug sensitivity are seen among parasites from different regions, the occurrence of resistant forms usually being correlated with the extent of use of the drug concerned.