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Availability of synthetic and recombinant peptides reproducing the repetitive regions of the circumsporozoite (CS) proteins of Plasmodium falciparum and P. vivax has allowed the development of assays for the detection of specific antibodies and of potential subunit vaccines. Knowledge of the immune responses to malaria sporozoites is a prerequisite for the optimal design of a sporozoite antigen-based vaccine. Studies carried out in areas with stable P. falciparum malaria (United Republic of Tanzania) have shown that antibodies against the synthetic peptide (NANP)40 increase as a function of age. Cluster analysis revealed marked inter-household variation of the anti-sporozoite antibody response, despite comparable risks of exposure to infectious bites. An age-related prevalence of anti-P. vivax sporozoite antibodies has been observed in an area of Sri Lanka with unstable malaria, using a 45-mer synthetic peptide reproducing a defined sequential array of the two main 9-mer variants of the P. vivax CS protein. In this area, anti-(NANP)40 antibodies became detectable after the first epidemic of P. falciparum malaria. Interestingly, their prevalence also increased with age. Since this population had not been exposed to P. falciparum malaria for at least 10 years previously, one can suggest that anti-sporozoite antibodies reflect the relative exposure to infectious bites in the different age groups, and, in turn, the transmission of the disease. This can be particularly useful in areas where entomological indices of transmission tend to be unreliable because of the low vectorial capacity and wide fluctuations in vector densities.

A new achievement in the immunodiagnosis of malaria has been reached after the knowledge of the molecular structure of some plasmodial antigens has become available. One example is given by the repetitive immunodominant epitope of Plasmodium falciparum circumsporozoite protein, which consists of 4 tandemly repeated aminoacids (Asn-Ala-Asn-Pro = NANP). A large synthetic peptide reproducing 40 NANP repeats, (NANP)40, has been shown to reproduce efficiently the native antigen in the CS protein and has been used to develop an enzyme-linked immunosorbent assay (ELISA) for the detection of antisporozoite antibodies in individuals from malaria-endemic countries. This (NANP)40 ELISA has been employed in a longitudinal study in a rural community in Tanzania. The results obtained have shown (i) that the presence of anti-(NANP) antibodies is associated with a certain degree of protective immunity; and (ii) that genetic factors could play a role in the host immune responsiveness to (NANP). Such an ELISA can be easily applied to field research and can be useful for monitoring the immune status of populations participating, in the future, to malaria vaccination trials employing P. falciparum sporozoite peptides.

From 1982 to 1984 170 children of Kikwawila village (Kilombero district, Tanzania) were followed for nutritional (anthropometric measures, hematocrit, serum retinol, prealbumin, and zinc concentrations), parasitological (malaria parasitemia, urinary schistosomiasis, intestinal parasites) and immunological characteristics. Between 2.9% and 12.4% had serum retinol levels less than 100 micrograms/l which indicate deficiency. Retinol concentrations were correlated with age, hematocrits, prealbumin levels and mid upperarm circumferences. The latter correlation may be useful in nutritional surveys and primary health care programs for the identification of populations at risk of retinol deficiency. No association was found between average retinol levels and the presence of parasites, with the exception of malaria. Retinol levels were inversely correlated with malaria parasitemia in 1982, and directly correlated with antibody titers to synthetic sporozoite peptide in 1984. Since retinol, malaria parasitemia, and antisporozoite antibodies increased with age, confounding by age could not be excluded. Six months after administration of ornidazole in a single oral dose of 10 mg/kg, a significant effect on the prevalence of Giardia lamblia was found. Following treatment, average retinol levels were increased in persons with confirmed G. lamblia infections, but not in uninfected or untreated controls.

An ELISA employing a novel synthetic peptide consisting of 40 (Asn-Ala-Asn-Pro) repeats of Plasmodium falciparum circumsporozoite protein, (NANP)40, was used to detect antibodies against P. falciparum circumsporozoite protein in 132 children, 1 month to 15 years old, from a rural community (Kikwawila village) of Tanzania, a region where malaria is hyperendemic. The children were surveyed comprehensively over 3 consecutive years for clinical, parasitological, and serological parameters. Entomological data were also gathered for selected households in this village. The following results were obtained: anti-(NANP)40 antibodies increased as a function of age; the majority of children over 10 years showed a stable positivity for such antibodies during the longitudinal study; a negative correlation was observed between the levels of anti-sporozoite antibodies and both spleen enlargement and the presence of parasites in thick smears; no relationship was found between anti-(NANP)40 antibodies and asexual blood stage antibodies; children living in two representative households with comparable indoor resting mosquito densities showed markedly different frequencies of anti-(NANP)40 antibodies, in spite of comparable clinical, parasitological, and serological parameters. Thus, in addition to the exposure to infectious mosquito bites, other (e.g., genetic) factors, may play a role in the ability of certain individuals to mount an antibody response against this immunodominant repetitive epitope. The results presented in this paper confirm that the (NANP)40-ELISA represents a simple, reliable means for the detection of anti-(NANP)40 circumsporozoite protein antibodies and suggest that such antibodies may contribute to the immune protection against malaria in humans.