Circumsporozoite protein gene from Plasmodium brasilianum. Animal reservoirs for human malaria parasites?

We describe here the sequence of the circumsporozoite protein gene of the monkey malaria parasite Plasmodium brasilianum and show that the immunodominant repeat domain is the same as that of the human malaria parasite, Plasmodium malariae. The immunodominant epitope on the surface of sporozoites of a third species of human malaria parasite has, therefore, been identified. This genetic based data and the biological similarities between P. brasilianum and P. malariae support their putative zoonotic/anthroponotic relationship. We also show that an ape malaria parasite, Plasmodium reichenowi, and the human malaria parasite, Plasmodium falciparum, have a similar relationship. The implications of these observations are discussed with respect to vaccine development.

Repeated immunogenic amino acid sequences of Plasmodium species share sequence homologies with proteins from humans and human viruses

The use of recombinant peptides based upon the repeated amino acid sequences of Plasmodium has been proposed for malaria vaccines. By reducing homologies of such peptide vaccines to host proteins, the possibility of autoimmune complications may be reduced, and the effective immune response may be enhanced. The Wilbur and Lipman Wordsearch algorithm was used to identify homologous amino acid sequences between tandemly repeated Plasmodium amino acid sequences and the human and human viral sequences compiled in the National Biomedical Research Foundation database. Six published repetitive immunogenic amino acid sequences from the circumsporozoite (CS) antigen, ring-infected erythrocyte surface antigen (RESA), soluble (S) antigen, and falciparum interspersed repetitive antigen (FIRA) of P. falciparum, and the CS protein of P. vivax, were analyzed by computer. Matches of at least 4 amino acids were found for all sequences. In the database, 29 matches were found for human proteins and 26 matches were found for human viruses with the 6 antigen sequences. Most of the matched proteins, and many of the matched human viruses, are found in blood. The biological significance of these matches remains to be clarified.

Plasmodium berghei: cloning of the circumsporozoite protein gene

A DNA fragment encoding the carboxy terminal 80% of the Plasmodium berghei circumsporozoite protein was selected from a genomic DNA expression library. Sequencing revealed that the P. berghei circumsporozoite protein was similar in overall structure to circumsporozoite proteins from other malaria species, although the central repeat region was unique in comprising two different blocks of tandem peptide repeats: 11 eight amino acid repeats with predominant sequence DPAPPNAN were followed by 16 two amino repeats, predominantly PQ. The P. berghei circumsporozoite protein exhibited limited, but about equal amino acid homology to circumsporozoite proteins from P. knowlesi, P. vivax, and P. falciparum, indicating that P. berghei is not closely related to any of these other malaria species. Cloning of the P. berghei circumsporozoite protein gene will allow direct testing of sporozoite vaccines in mice.

Evolution of the immunodominant domain of the circumsporozoite protein gene from Plasmodium vivax. Implications for vaccines

Recent work directed toward the development of a malarial vaccine has focused on the identification and production of the immunodominant repeating peptide of the circumsporozoite protein of the human malaria parasites as an antigen. An important factor which relates to the usefulness of this antigen in a vaccine is the rate at which the molecule changes in sequence. We have determined the sequence and arrangement of the repeating epitope of the circumsporozoite protein gene from a Plasmodium vivax isolate from La Paz, El Salvador (Sal-I). This is compared with a portion of the previously published sequence of the circumsporozoite protein gene from a P. vivax isolate from Belém, Brazil. The genes appear to be very similar in the repeat region. There are 20 similar repeating units in the El Salvador strain and only 19 units are conserved in the Brazilian strain. Following this there are degenerate repeats in both strains. Even the pattern of silent mutations in the repeat area are similar; however, they are not necessarily in the identical location and appear to have shifted. The data suggest that the repeat region of these genes may be evolving by an accelerated mechanism(s). Such a phenomenon could severely decrease the long-term efficacy of a repeat-based anti-sporozoite vaccine.

Leishmaniasis and malaria: new tools for epidemiologic analysis

Parasitic diseases are still prevalent in many parts of the world, causing both human suffering and economic loss. Recent developments in biotechnology, such as the use of monoclonal antibodies and recombinant DNA, have the potential for providing both more extensive and detailed information on the parasite in the infected human and in insect vectors. New methods of detection, both in man and insect vectors, have been developed for two parasitic diseases, leishmaniasis and malaria. These new methodologies will be important in epidemiologic studies on the prevalence and transmission of these parasitic diseases.

Specific DNA probe for the diagnosis of Plasmodium falciparum malaria

Malaria can be diagnosed either by direct microscopic examination of blood smears, which is time consuming and requires expertise, or by immunological techniques, which are effective but do not distinguish between past and present infections. In this study, a simple procedure was developed for spotting lysed blood from infected patients directly onto nitrocellulose paper and identifying the malaria species on the basis of hybridization of parasite DNA with a species-specific probe. A genomic DNA library of Plasmodium falciparum was screened to detect clones containing DNA sequences that are highly repeated within the parasite genome. Several such clones were further analyzed to identify those that hybridize specifically with P. falciparum DNA but not with DNA from humans, P. vivax, or P. cynomolgi. This technique appears to be sensitive enough to detect 10 picograms of purified P. falciparum DNA (equivalent to 100 parasites) and in field studies is able to detect approximately 40 parasites per microliter of blood.

Circumsporozoite protein of Plasmodium vivax: gene cloning and characterization of the immunodominant epitope

The gene encoding the circumsporozoite (CS) protein of the human malaria parasite Plasmodium vivax has been cloned. The deduced sequence of the protein consists of 373 amino acids with a central region of 19 tandem repeats of the nonapeptide Asp-Arg-Ala-Asp/Ala-Gly-Gln-Pro-Ala-Gly. A synthetic 18-amino acid peptide containing two tandem repeats binds to a monoclonal antibody directed to the CS protein of Plasmodium vivax and inhibits the interaction of this antibody with the native protein in sporozoite extracts. The portions of the CS gene that do not contain repeats are closely related to the corresponding regions of the CS genes of two simian malarias, Plasmodium cynomolgi and Plasmodium knowlesi. In contrast, the homology between the CS genes of Plasmodium vivax and Plasmodium falciparum, another malaria parasite of humans, is very limited.

Detection of Plasmodium falciparum using a synthetic DNA probe

A labeled synthetic polynucleotide representing a repetitive sequence from Plasmodium falciparum was hybridized with genomic DNA spotted on nitrocellulose. After an overnight exposure, 0.1 ng of P. falciparum DNA was specifically detected and 0.01 ng was detected after an exposure of 1 week. The synthetic probe showed no cross-hybridization with host DNA or with DNA isolated from other species in the phylum Apicomplexa, P. vivax and Babesia species. Since synthetic DNA is easily prepared, the observed sensitivity and specificity suggests that synthetic DNA probes would be generally useful in diagnosis.

Evolutionary relatedness of Plasmodium species as determined by the structure of DNA

Malaria parasites can be grouped evolutionarily by analysis of DNA composition and genome arrangement. Those that vary widely with regard to host range, morphology, and biological characteristics fit into only a small number of distinctive groups. The DNA of the human parasite Plasmodium falciparum fits into a group that includes rodent and avian malarias and is unlike the DNA of other primate malaria parasites. The DNA of Plasmodium vivax, which is also a human parasite, fits into a distinctly different group that includes Plasmodium cynomolgi, a parasite of monkeys. The evolutionary lines suggested here appear to be consistent with similarities seen among malaria parasites with regard to gene sequence.

[Plasmodium ovale malaria in France. Probability of of genetic control of the incubation period (author's transl)]

Plasmodium ovale as the causative parasite in attacks of malaria is rare. Nevertheless, the number of cases seen in France would seem to be on the increase. In 137 attacks of malaria collected between 1967 and 1978, Plasmodium ovale was found on 13 occasions. The main characteristics of this form of malaria are, apart from its benign nature, the usual absence of any recurrence and a very variable incubation period, ranging from fifteen days to several months or even a year (with an average of 3.2 months). It is thus important to bear this diagnosis in mind, even if a long period elapsed since return from an endemic area, and it will be realised that adequate chemoprophylaxis using a schizonticide does not offer protection from forms with a long incubation period. Genetic control of the duration of the period of maturation of exo-erythrocytic forms, i.e. the incubation period, is suggested, identical to that recently advanced in the case of Plasmodium vivax. This would account for the variability and possible long duration of incubation periods. A usual monoclonal character of these infections could explain the virtual absence of recurrences.

Population studies of Plasmodium vivax. 1. The theory of polymorphism of sporozoites and epidemiological phenomena of tertian malaria

The understanding of clinico-epidemiological phenomena of tertian malaria has been the subject of controversy. The authors suggest a system of postulates which give a non-contradictory explanation of the phenomena of relapses and long incubation. The main idea is that the duration of exoerythrocytic development of Plasmodium vivax is a polymorphic characteristic controlled by a set of genes. According to these postulates sporozoites may be subdivided into two groups designated as tachysporozoites and bradysporozoites, responsible for early and late manifestations, respectively. The logical analysis of the system suggests that it does not contradict the experimental facts. Moreover, the theory of polymorphic sporozoites permits an explanation and quantification of interrelations between different phenomena. The authors stress that the variation of genes is much greater in natural populations of parasites than in individual isolates and strains and therefore that the features of strains do not fully reflect the features of populations. Classical laboratory experiments should be combined with epidemiological experiments which allow a study of the population as a whole. The methodology of experiments to be undertaken in further investigations of the long latency period is discussed.

Population studies of Plasmodium vivax. 2. Distribution of manifestations in foci of tertian malaria

The authors investigate a mathematical model based on the theory they proposed in a previous publication. The model fits field data collected in re-established foci of tertian malaria. The patterns of distribution of manifestations of tertian malaria among the population may readily be explained on the basis of the theory of polymorphism of sporozoites.