The role of Plasmodium knowlesi in the history of malaria research.

In recent years, a malaria infection of humans in South East Asia, originally diagnosed as a known human-infecting species, Plasmodium malariae, has been identified as a simian parasite, Plasmodium knowlesi. This species had been subject to considerable investigation in monkeys since the 1930s. With the development of continuous culture of the erythrocytic stages of the human malarial parasite, Plasmodium falciparum in 1976, the emphasis in research shifted away from knowlesi. However, its importance as a human pathogen has provoked a renewed interest in P. knowlesi, not least because it too can be maintained in continuous culture and thus provides an experimental model. In fact, this parasite species has a long history in malaria research, and the purpose of this chapter is to outline approximately the first 50 years of this history.

Me, Myself, and Not-I: Self-Discrepancy Type Predicts Avatar Creation Style.

In video games, identification with avatars-virtual entities or characters driven by human behavior-has been shown to serve many interpersonal and intraindividual functions (like social connection, self-expression, or identity exploration) but our understanding of the psychological variables that influence players' avatar choices remains incomplete. The study presented in this paper tested whether players' preferred style of avatar creation is linked to the magnitude of self-perceived discrepancies between who they are, who they aspire to be, and who they think they should be. One-hundred-and-twenty-five undergraduate gamers indicated their preferred avatar creation style and completed a values measure from three different perspectives: their actual, ideal, and ought selves. The average actual/ideal values discrepancy was greater among those who preferred idealized avatars vs. those who preferred realistic avatars. The average actual/ought values discrepancy was greater among those who preferred completely different avatars (i.e., fantasy/role-players) vs. those who preferred realistic avatars. These results, therefore, offer additional evidence that self-discrepancy theory is a useful framework for understanding avatar preferences.

Diversity of circumsporozoite antigen genes from two strains of the malarial parasite Plasmodium knowlesi.

The complete nucleotide sequence of the coding region of the circumsporozoite antigen gene (CS gene) of the Nuri strain of the malarial parasite Plasmodium knowlesi is presented. The gene from the Nuri strain exhibits a novel form of sequence diversity when compared to the CS gene from the H strain. Instead of the 12 tandem repeating 36-base pair units of the H strain, the Nuri strain contains 16 tandem repeating 27-base pair units of a different nucleotide sequence that encodes a different repeating peptide. In contrast, the 5' and 3' coding and noncoding sequences flanking the repeats are 98 percent conserved in both strains.

The malaria merozoite, forty years on.

The invasive blood stage of malaria parasites, merozoites, are complex entities specialized for the capture and entry of red blood cells. Their potential for vaccination and other anti-malaria strategies have attracted much research attention over the last 40 years, and there is now a considerable body of data relating to their biology. In this article some of the major advances over this period and remaining challenges are reviewed.

Malaria parasite invasion: interactions with the red cell membrane.

The capacity to invade red cells is central to the biology of malaria parasites; both asexual multiplication and reinfection of the definitive mosquito host depend upon intraerythrocytic stages. The invasion process is complex. The briefly free merozoite specifically recognizes and adheres to ligands on the red cell surface, then alters the red cell membrane to produce an invagination into which it moves, and so becomes enclosed in a membrane-bound parasitophorous vacuole. Here we assess new evidence that bears on our understanding of this process. This has come from sources including biochemical and ultrastructural studies of the specialized surface and organelles of merozoites, from in vitro invasion studies using naturally refractory or artificially modified red cells, and from structural, chemical, and immunological analyses of the newly parasitized cell.

Monoclonal antibodies from Epstein-Barr virus-transformed lymphocytes of common marmosets (Callithrix jacchus) immune to malaria.

The B lymphocytes of the common marmoset Callithrix jacchus can be immortalized by infection with Epstein-Barr virus (EBV) in vitro (Desgranges et al., 1976). C. jacchus is susceptible to infection with the blood stages of several species of malaria parasite including the line designated MVF1 (Mitchell et al., 1988) from which it recovers and shows immunity to reinfection. By exploiting these two phenomena, EBV-transformed, marmoset lymphoblastoid cell lines secreting antibodies to malaria parasite antigens have been generated and cloned. We believe this to be the first time that monoclonal antibodies (MAbs) have been raised from common marmosets. Since numerous and diverse human pathogens can infect this small primate in the laboratory, these methods may prove generally applicable for the generation of MAbs whose specificities derive from immune responses to infection.

The Fab fragments of monoclonal IgG to a merozoite surface antigen inhibit Plasmodium knowlesi invasion of erythrocytes.

Two rat monoclonal antibodies (both IgG2a isotype and having closely related specificities) and a pool of rhesus immune IgG, all of which inhibit Plasmodium knowlesi merozoite invasion of rhesus erythrocytes, have been studied before and after proteolytic digestion. The F(ab')2 and Fab fragments of both rat monoclonal antibodies show considerably enhanced inhibition of merozoite invasion as compared with the intact IgG. Inhibition by monovalent fragments indicates that these antibodies are not dependent upon merozoite agglutination and may act by blocking merozoite attachment to the specific red cell receptor. The fact that the inhibitory activities of F(ab')2 and Fab are equally enhanced on a weight basis, as compared with IgG, suggests that the removal of Fc may reduce electrostatic repulsion between antibody and merozoite surface, both of which are negatively charged at neutral pH. By contrast, papain digestion of polyclonal IgG derived from an immunised rhesus pool markedly reduces its inhibitory activity. This suggests that much of the inhibition mediated by polyclonal IgG results from merozoite agglutination and that the specificity of the rat inhibitory monoclonal antibodies is poorly represented in the immune pool. The P. knowlesi antigen reactive with the inhibitory monoclonal antibodies is known to be synthesized as a minor 66 kDa polypeptide during the last 1.5 h. of schizont development and is processed to smaller products (44 and 42 kDa) present on the merozoite surface. The present results suggest that this antigen may have particular interest as a vaccine against P. knowlesi malaria.

Microtubule associated motor proteins of Plasmodium falciparum merozoites.

We have studied the occurrence, stage specificity and cellular location of key molecules associated with microtubules in Plasmodium falciparum merozoites. Antibodies to gamma tubulin, conventional kinesin and cytoplasmic dynein were used to determine the polarity of merozoite microtubules (mt), the stage specificity of the motor proteins and their location during merozoite development. We conclude that the minus ends of the mts are located at their apical pole. Kinesin was present throughout the lifecycle, appearing as a distinct crescent at the apex of developing merozoites. The vast majority of cytoplasmic dynein reactivity occurred in late merogony, also appearing at the merozoite apex. Destruction of mt with dinitroanilines did not affect the cellular location of kinesin or dynein. In invasion assays, dynein inhibitors reduced the number of ring stage parasites. Our results show that both conventional kinesin and cytoplasmic dynein are abundant, located at the negative pole of the merozoite mt and, intriguingly, appear there only in very late merogony, prior to merozoite release and invasion.

A review of metozoite vaccination against Plasmodium knowlesi malaria.

Techniques for the isolation of merozoites of Plasmodium knowlesi malaria have allowed their use in experimental vaccines. Rhesus monkeys were protected to a very great extent from otherwise lethal challenge with this malaria when Freund's Complete Adjuvant was a vaccine component.

Long term cultivation of a simian malaria parasite (Plasmodium knowlesi) in a semi-automated apparatus.

An apparatus is described and illustrated for the continuous semi-automated cultivation of Plasmodium knowlesi. The change of medium was automated and involved six operations. Parasites were maintained for 12 weeks, at which time the experiment ended. During this period, parasite density, morphology and serological specificity were monitored. Parasites retained synchromy and normal morphology only for the first cycle in culture. The maximum degree of morphological abnormality (90%) was reached at three weeks. Lysis of infected erythrocytes seemed to occur before full maturation of the parasites. Infected cells cultured for 31 days produced a normal, fulminating infection in a rhesus monkey.

Gamma delta T cells in the peripheral blood of individuals from an area of holoendemic Plasmodium falciparum transmission.

gamma delta T cells bearing V gamma 9 T cell receptors from unexposed Caucasian donors make large responses to Plasmodium falciparum in vitro. This finding, together with observations of others showing high levels of V gamma 9+ T cells in the blood of infected non-immune individuals, led us to hypothesize that the response of these cells might contribute to the pathology of P. falciparum malaria. Acquisition of immunity to disease in people naturally exposed to infection may therefore be due in part to down-regulation or alteration of the function of gamma delta T cells. Supporting this view, and in contrast to infection in non-immune individuals, V gamma 9+ T cells are not elevated in peripheral blood of children or adults living in an endemic area despite constant exposure to P. falciparum. After in vitro stimulation with P. falciparum, however, the expansion of V gamma 9+ cells from the African donors is of similar magnitude to that observed for non-exposed Europeans. Thus, although these cells are not elevated in peripheral blood, they are still able to respond to P. falciparum antigens. In adult European donors the major gamma delta T cell population in peripheral blood is V gamma 9+ (approximately 70% of all gamma delta cells), whereas in the majority of adult Africans V delta 1+ V gamma 9- T cells predominated (approximately 70% of total gamma delta cells).

Plasmodium knowlesi infections in a small number of non-immune natural hosts (Macaca fascicularis) and in rhesus monkeys (M. mulatta).

The natural host of Plasmodium knowlesi is the kra monkey, Macaca fascicularis, but this parasite, initially mistaken for P. malariae, is now infecting humans in some areas of Southeast Asia. Here we present data from experiments performed in the 1970s in which sera from a few naive M. fascicularis, taken in the course of a first infection, exhibited rapidly rising inhibition of in vitro replication of P. knowlesi. The results were compared with sera from P. knowlesi-infected rhesus monkeys that usually die if left untreated.

Vaccination against malaria: its plausibility and the present state of research.

A vaccine for public health use against malaria is urgently required and is being actively researched into. The present review outlines the biology of malaria parasites and the immune response to them, with an emphasis on the worst of the human diseases, and considers the current analytical and molecular biological work on malaria parasite surfaces and antigens.

PIP: The literature on the biology of malaria parasites and the associated immune responses is reviewed, with emphasis on malaria parasite surfaces and antigens. Although eradication of malaria is considered unrealistic, efforts are underway to develop an effective, safe, inexpensive, and long-lasting vaccine. It is unlikely that malaria parasites as such will be used in a vaccine given the difficulties of culture or purification from the host component. However, the use of merozoites, sporozoites, and gamete-containing experimental vaccines has provided important information on the immunology of malaria. Some functional merozoites, perhaps in particular the molecules concerned with final orientation to the red cell and invasion, may constitute promising candidates for vaccine components. Those investigating malaria antigens are working to identify and clone the major circumsporozoite proteins of Plasmodium falciparum. In addition, an S antigen fragment has shown interesting immunologic properties such as direct B cell stimulation. A major remaining task is to test the functional immunogenicity of bacterially expressed malaria antigens.