Acta Leidensia最新文献

There are two views on the origin and maintenance of the high levels of polymorphism found in antigenic Plasmodium proteins. Immune selectionists consider that mutations which avoid stimulating a host response are frequent and advantageous. Proponents of the random genetic drift of selectively equivalent mutations hold that Plasmodium antigens are relatively unconstrained and can tolerate considerable structural diversity. Both sides agree that antigenic diversity is advantageous although selectionists see benefits in individual mutations whereas the proponents of random genetic drift see the advantage in the parasite's capacity to tolerate diversity per se.

There are several mechanisms responsible for the extensive antigenic diversity found in the asexual blood stages of Plasmodium falciparum. Failure to express antigens is a feature of many isolates cultured in vitro but probably is not a major cause of antigenic diversity in vivo. Numerous point mutations occur in allelic forms of asexual blood stage antigens and are assumed to contribute to antigenic diversity but as yet few such mutations have been mapped to antigenic epitopes. A major cause of antigenic diversity is the expression of different repetitive sequences in allelic forms of several antigens including the S-antigen and the two merozoite surface antigens, MSA-1 and MSA-2. The sequencing data indicates that S-antigen genes fall into many allelic families whereas both MSA-1 and MSA-2 are dimorphic. Further diversity has arisen as a result of intragenic recombinations between the dimorphic forms of both MSA-1 and MSA-2. In addition to this diversity reflecting the expression of different allelic genes, asexual blood stages of malaria parasites undergo antigenic variation in that clonal parasite populations can vary the form of an antigen on the surface of infected erythrocytes. Antibodies or DNA probes directed against variable repeat sequences can be used to distinguish different isolates of P. falciparum. The use of antibodies to S-antigen repeats has been particularly useful for typing the parasites causing infections. The application of S-antigen typing to field studies in Papua New Guinea has demonstrated marked diversity in the parasites causing infections in one area.

By definition, the biology of a living organism must be characterized before its molecular biology can be interpreted. Malariologists are fortunate in that the malaria parasite was used as a well-controlled therapy for tens of thousands of hospital patients. During many of these treatments the opportunity was taken to study malaria and the behaviour of the parasite in detail. From these, and similar studies on volunteers, together with numerous epidemiological surveys, the operational characteristics of immunity to malaria in man have been well defined. Unfortunately this information, which exists in some detail in the older literature, does not seem to have been available to many investigators. This situation has led to interpretations of molecular data which are inconsistent with the known biology of the parasites and human-parasite relationships. This article considers how the structure of one of the best studied antigens, MSP1, can be viewed in the context of the host-parasite relationship. It postulates some testable hypotheses which aim to reconcile the molecular characteristics of the antigen with the biology and immunology of the asexual erythrocytic stage of the parasite.

All of the results of the various experiments support a role for living, proliferating parasites in the efficient induction of anti-parasitic as well as anti-disease (CM) immunity. Non-proliferating parasites or material from disrupted parasites are poor or non-antigens in this respect. Three possibilities as to why living parasites are important in immunity could be considered: 1. circulating parasites contain insufficient antigen to induce protective immunity, but sufficient antigen can be produced during proliferation; 2. only circulating parasites arrive at critical places (e.g. parts of the white pulp of the spleen) for the presentation of the important antigen or induction of appropriate signals. 3. Architectural changes are needed (i.e. formation of barrie-cell-complexes) for the immune response to be effective. The first possibility explains why exoantigens, as well as live, proliferating parasites are efficient inducers of anti-CM immunity. Since these immunizations have no effect on parasitemia, additional/other immune reaction(s) are needed for anti-parasitic immunity. The important role of the spleen in malaria and malaria immunity is well-known. The second possibility includes the idea that live, proliferating parasites circulate through the spleen continuously where unsatisfactory or infected erythrocytes are removed rather than in the liver. Injected killed parasites or material from them when present in the circulation is to a larger extent taken up by the Kupffer cells from the liver rather than the spleen. Presence and uptake of parasites in the spleen may provide the critical confrontation and/or delivery of signals necessary for the development of immunity.(ABSTRACT TRUNCATED AT 250 WORDS)

Chloroquine resistance in Plasmodium falciparum bears a striking similarity to the multi-drug resistance (MDR) phenotype of mammalian tumour cells which is mediated by P-glycoprotein. P. falciparum has two mdr-like genes (pfmdr 1 and pfmdr 2) and pfmdr 1 has been linked to the chloroquine resistance phenotype. We show that pfmdr 1 encodes a protein of 160,000 Daltons that is expressed at higher levels in a chloroquine resistant cloned isolate. The pfmdr 2 gene is located on chromosome 14 and it is in equal copy number in chloroquine resistant and sensitive isolates. Therefore amplification of pfmdr 2 is not linked to chloroquine resistance. This is in contrast to the pfmdr 1 gene which has been shown to be amplified in some chloroquine resistant isolates.

P. falciparum lacks a functional citric acid cycle. Unlike most tissues of the mammalian host, it is totally dependent on glycolysis for energy generation. A compound which selectively inhibits the parasite's ATP-generating machinery is therefore a potential antimalarial agent. Such a drug may interact in two ways: a) by inhibiting the activity of an enzyme or b) by disturbing the micro-organization of consecutive enzymes in a metabolic pathway. In mammalian tissues the glycolytic pathway involves the cytoskeleton as a matrix to keep phosphofructokinase, aldolase and glyceraldehyde-3-phosphate dehydrogenase in an optimal sterical position for rapid substrate conversion. For instance, these three enzymes bind to the band 3 protein in erythrocytes or to actin in muscle cells. P. falciparum aldolase binds with very high affinity to the band 3 protein of human erythrocyte ghosts. However, the true in vivo site of association is believed to be actin II of P. falciparum. This actin has a sequence element which is almost identical to that of the band 3 aldolase binding site. We therefore suppose that plasmodia exploit a similar matrix organization. If true, the association of these enzymes with the cytoskeleton is a target for novel antimalarials. In contrast to all vertebrate aldolases, P. falciparum and P. berghei aldolases have two neighbouring lysine residues near the carboxy-terminus. We show here that mutagenesis of these basic residues has an effect on the catalytic constants Vmax and KM and moreover, the ability to bind to band 3 is reduced.(ABSTRACT TRUNCATED AT 250 WORDS)