A study of var gene transcription in vitro using universal var gene primers

Transcription of multiple var genes by individual, trophozoite-stage Plasmodium falciparum cells expressing a chondroitin sulphate A binding phenotype

In this study, we detected multiple var gene transcripts within single, mature trophozoite-infected red blood cells (iRBCs) bound to chondroitin sulphate A (CSA). Several of the var detected had previously been demonstrated to encode Plasmodium falciparum erythrocyte membrane protein-1 (PfEMP-1) variants with domains that mediated iRBC adhesion to receptors other than CSA. Parasites expressing the CSA-adherent phenotype transcribed far more of one var than of all others, but this gene was different from the two other var previously purported to encode adhesion to CSA. Previous work suggesting that only single var are transcribed by mature trophozoites needs re-examination in the light of these data from single, infected cells.

Antigenic variation at the infected red cell surface in malaria

Many pathogens that either rely on an insect vector to complete their life cycle (e.g., Trypanosoma spp. and Borrelia spp.) or exist in a unique ecological niche where transmission from host to host is sporadic (e.g., Neisseria spp.) have evolved strategies to maintain infection of their mammalian hosts for long periods of time in order to ensure their survival. Because they have to survive in the face of a fully functional immune system, a common feature of many of these organisms is their development of sophisticated strategies for immune evasion. For the above organisms and for malaria parasites of the genus Plasmodium, a common theme is the ability to undergo clonal antigenic variation. In all cases, surface molecules that are important targets of the humoral immune response are encoded in the genome as multicopy, nonallelic gene families. Antigenic variation is accomplished by the successive expression of members of these gene families that show little or no immunological cross-reactivity. In the case of malaria parasites, however, some of the molecules that undergo antigenic variation are also major virulence factors, adding an additional level of complication to the host-parasite interaction. In this review, we cover the history of antigenic variation in malaria and then summarize the more recent data with particular emphasis on Plasmodium falciparum, the etiological agent of the most severe form of human malaria.

Decoding the language of var genes and Plasmodium falciparum sequestration

Sequestration and rosetting are key determinants of Plasmodium falciparum pathogenesis. They are mediated by a large family of variant proteins called P. falciparum erythrocyte membrane protein 1 (PfEMP1). PfEMP1 proteins are multispecific binding receptors that are transported to parasite-induced, 'knob-like' binding structures at the erythrocyte surface. To evade immunity and extend infections, parasites clonally vary their expressed PfEMP1. Thus, PfEMP1 are functionally selected for binding while immune selection acts to diversify the family. Here, we describe a new way to analyse PfEMP1 sequence that provides insight into domain function and protein architecture with potential implications for malaria disease.

An update on antigenic variation in African trypanosomes

African trypanosomes can spend a long time in the blood of their mammalian host, where they are exposed to the immune system and are thought to take advantage of it to modulate their own numbers. Their major immunogenic protein is the variant surface glycoprotein (VSG), the gene for which must be in one of the 20--40 specialized telomeric expression sites in order to be transcribed. Trypanosomes escape antibody-mediated destruction through periodic changes of the expressed VSG gene from a repertoire of approximately 1000. How do trypanosomes exclusively express only one VSG and how do they switch between them?

Plasmodium falciparum homologue of the genes for Plasmodium vivax and Plasmodium yoelii adhesive proteins, which is transcribed but not translated

The 235-kDa family of rhoptry proteins in Plasmodium yoelii and the two reticulocyte binding proteins of P. vivax comprise a family of proteins involved in host cell selection and erythrocyte invasion. Here we described a member of the gene family found in P. falciparum (PfRH3) that is transcribed in its entirety, under stage-specific control, with correct splicing of the intron, but appears not to be translated, probably due to two reading frameshifts at the 5' end of the gene.

Variant antigens of Plasmodium falciparum encoded by the var multigenic family are multifunctional macromolecules

Cytoadhesion of parasitized red blood cells (PRBCs) to vascular endothelial cells (sequestration) and binding of unparasitized RBCs to PRBCs (rosetting) are virulence factors of Plasmodium falciparum, the species responsible for lethal human malaria. Variant antigens involved in both phenomena have been identified as products of the multicopy var gene family. In this review, progress in the understanding of molecular mechanisms of sequestration is summarized, in particular, concerning the structure of var gene products related to specificity of binding to endothelial receptors, and the origin of var gene diversity.

Molecular mechanisms of Plasmodium falciparum placental adhesion

In natural Plasmodium falciparum infections, parasitized erythrocytes (PEs) circulate in the peripheral blood for a period corresponding roughly to the first part of the erythrocytic life cycle (ring stage). Later, in blood-stage development, parasite-encoded adhesion molecules are inserted into the erythrocyte membrane, preventing the circulation of the PEs. The principal molecule mediating PE adhesion is P. falciparum erythrocyte membrane protein 1 (PfEMP1), encoded by the polymorphic var gene family. The population of parasites is subject to clonal antigenic variation through changes in var expression, and a single PfEMP1 variant is expressed at the PE surface in a mutually exclusive manner. In addition to its role in immune evasion, switches in PfEMP1 expression may be associated with fundamental changes in parasite tissue tropism in malaria patients. A switch from CD36 binding to chondroitin sulphate A (CSA) binding may lead to extensive sequestration of PEs in placenta syncytiotrophoblasts. This is probably a key event in malaria pathogenesis during pregnancy. The CSA-binding phenotype of mature PEs is linked to another distinct adhesive phenotype: the recently described CSA-independent cytoadhesion of ring-stage PEs. Thus, a subpopulation of PEs that sequentially displays these two different phenotypes may bind to an individual endothelial cell or syncytiotrophoblast throughout the asexual blood-stage cycle. This suggests that non-circulating (cryptic) parasite subpopulations are present in malaria patients.

Malaria vaccines

Although the possibility of a live attenuated malaria vaccine has been considered, current malaria vaccine development activities are dominated by attempts to develop a subunit vaccine. Hence, it is entirely appropriate that a session of the Molecular Approaches to Malaria conference, Lorne, Australia, 2-5 February 2000, was devoted to vaccine development. The oral presentations in this session and the relevant poster presentations are outlined here by Robin Anders and Allan Saul.