Prevalence and levels of antibodies to the circumsporozoite protein of Plasmodium falciparum in an endemic area and their relationship to resistance against malaria infection

Major transcriptional changes observed in the Fulani, an ethnic group less susceptible to malaria

The Fulani ethnic group has relatively better protection from Plasmodium falciparum malaria, as reflected by fewer symptomatic cases of malaria, lower infection rates, and lower parasite densities compared to sympatric ethnic groups. However, the basis for this lower susceptibility to malaria by the Fulani is unknown. The incidence of classic malaria resistance genes are lower in the Fulani than in other sympatric ethnic populations, and targeted SNP analyses of other candidate genes involved in the immune response to malaria have not been able to account for the observed difference in the Fulani susceptibility to P.falciparum. Therefore, we have performed a pilot study to examine global transcription and DNA methylation patterns in specific immune cell populations in the Fulani to elucidate the mechanisms that confer the lower susceptibility to P.falciparum malaria. When we compared uninfected and infected Fulani individuals, in contrast to uninfected and infected individuals from the sympatric ethnic group Mossi, we observed a key difference: a strong transcriptional response was only detected in the monocyte fraction of the Fulani, where over 1000 genes were significantly differentially expressed upon P.falciparum infection.

Naturally acquired antibody response to Plasmodium falciparum describes heterogeneity in transmission on islands in Lake Victoria

As markers of exposure anti-malaria antibody responses can help characterise heterogeneity in malaria transmission. In the present study antibody responses to Plasmodium falciparum AMA-1, MSP-1₁₉ and CSP were measured with the aim to describe transmission patterns in meso-endemic settings in Lake Victoria. Two cross-sectional surveys were conducted in Lake Victoria in January and August 2012. The study area comprised of three settings: mainland (Ungoye), large island (Mfangano) and small islands (Takawiri, Kibuogi, Ngodhe). Individuals provided a finger-blood sample to assess malaria infection by microscopy and PCR. Antibody response to P. falciparum was determined in 4,112 individuals by ELISA using eluted dried blood from filter paper. The overall seroprevalence was 64.0% for AMA-1, 39.5% for MSP-1₁₉, and 12.9% for CSP. Between settings, seroprevalences for merozoite antigens were similar between Ungoye and Mfangano, but higher when compared to the small islands. For AMA-1, the seroconversion rates (SCRs) ranged from 0.121 (Ngodhe) to 0.202 (Ungoye), and were strongly correlated to parasite prevalence. We observed heterogeneity in serological indices across study sites in Lake Victoria. These data suggest that AMA-1 and MSP-1₁₉ sero-epidemiological analysis may provide further evidence in assessing variation in malaria exposure and evaluating malaria control efforts in high endemic area.

Immune Responses in Malaria

Evidence accumulated through the years clearly indicates that antiparasite immune responses can efficiently control malaria parasite infection at all development stages, and under certain circumstances they can prevent parasite infection. Translating these findings into vaccines or immunotherapeutic interventions has been difficult in part because of the extraordinary biological complexity of this parasite, which has several developmental stages expressing unique sets of stage-specific genes and multiple antigens, most of which are antigenically diverse. Nevertheless, in the last 30 years major advances have resulted in characterization of a number of vaccine candidates, exploration of the repertoire of host immune responses to the various parasite stages, and also identification of significant hurdles that need to be overcome. Most important, these advances strengthened the concept that the induction of host immune responses that target all developmental stages of Plasmodium can efficiently control or abrogate Plasmodium infections and strongly support the notion that an effective vaccine can be developed. This vaccine would be a critical component for programs aimed at controlling or eradicating malaria.

High Sporozoite Antibody Titers in Conjunction with Microscopically Detectable Blood Infection Display Signatures of Protection from Clinical Malaria

Immunoepidemiological studies typically reveal slow, age-dependent acquisition of immune responses against Plasmodium falciparum sporozoites. Naturally acquired immunity against preerythrocytic stages is considered inadequate to confer protection against clinical malaria. To explore previously unrecognized antisporozoite responses, we measured serum levels of naturally acquired antibodies to whole Plasmodium falciparum sporozoites (Pfspz) and the immunodominant (NANP)₅ repeats of the major sporozoite surface protein, circumsporozoite protein, in a well-characterized Kenyan cohort. Sera were sampled at the start of the malaria transmission season, and all subjects were prospectively monitored for uncomplicated clinical malaria in the ensuing 6 months. We used Kaplan–Meier analysis and multivariable regression to investigate the association of antisporozoite immunity with incidence of clinical malaria. Although naturally acquired humoral responses against Pfspz and (NANP)₅ were strongly correlated (p < 0.0001), 37% of Pfspz responders did not recognize (NANP)₅. The prevalence and magnitude of antisporozoite responses increased with age, although some high Pfspz responders were identified among children. Survival analysis revealed a reduced risk of and increased time to first or only episode of clinical malaria among Pfspz or (NANP)₅ responders carrying microscopically detectable Plasmodium falciparum (Pf) parasitemia at the start of the transmission season (p < 0.03). Our Cox regression interaction models indicated a potentially protective interaction between high anti-Pfspz (p = 0.002) or anti-(NANP)₅ (p = 0.001) antibody levels and microscopically detectable Pf parasitemia on the risk of subsequent clinical malaria. Our findings indicate that robust antisporozoite immune responses can be naturally acquired already at an early age. A potentially protective role of high levels of anti-Pfspz antibodies against clinical episodes of uncomplicated malaria was detected, suggesting that antibody-mediated preerythrocytic immunity might indeed contribute to protection in nature.

Interferon-γ responses to Plasmodium falciparum vaccine candidate antigens decrease in the absence of malaria transmission

Background

Malaria elimination campaigns are planned or active in many countries. The effects of malaria elimination on immune responses such as antigen-specific IFN- γ responses are not well characterized.

Methods

IFN- γ responses to the P. falciparum antigens circumsporozoite protein, liver stage antigen-1, thrombospondin-related adhesive protein, apical membrane antigen-1, MB2, and merozoite surface protein-1 were tested by ELISA in 243 individuals in highland Kenya in April 2008, October 2008, and April 2009, after a one-year period of interrupted malaria transmission from April 2007 to March 2008.

Results

While one individual (0.4%) tested positive for P. falciparum by PCR inOctober 2008 and another two (0.9%) tested positive in April 2009, no clinical malaria cases were detected during weekly visits. Levels of IFN-γ to all antigens decreased significantly from April 2008 to April 2009 (all P < 0.001).

Discussion

Naturally acquired IFN- γ responses to P. falciparum antigensare short-lived in the absence of repeated P. falciparum infection. Even short periods of malaria interruption may significantly decrease IFN-γ responses to P. falciparum antigens.

Genetic Diversity and Protective Efficacy of the RTS,S/AS01 Malaria Vaccine

BACKGROUND

The RTS,S/AS01 vaccine targets the circumsporozoite protein of Plasmodium falciparum and has partial protective efficacy against clinical and severe malaria disease in infants and children. We investigated whether the vaccine efficacy was specific to certain parasite genotypes at the circumsporozoite protein locus.

METHODS

We used polymerase chain reaction-based next-generation sequencing of DNA extracted from samples from 4985 participants to survey circumsporozoite protein polymorphisms. We evaluated the effect that polymorphic positions and haplotypic regions within the circumsporozoite protein had on vaccine efficacy against first episodes of clinical malaria within 1 year after vaccination.

RESULTS

In the per-protocol group of 4577 RTS,S/AS01-vaccinated participants and 2335 control-vaccinated participants who were 5 to 17 months of age, the 1-year cumulative vaccine efficacy was 50.3% (95% confidence interval [CI], 34.6 to 62.3) against clinical malaria in which parasites matched the vaccine in the entire circumsporozoite protein C-terminal (139 infections), as compared with 33.4% (95% CI, 29.3 to 37.2) against mismatched malaria (1951 infections) (P=0.04 for differential vaccine efficacy). The vaccine efficacy based on the hazard ratio was 62.7% (95% CI, 51.6 to 71.3) against matched infections versus 54.2% (95% CI, 49.9 to 58.1) against mismatched infections (P=0.06). In the group of infants 6 to 12 weeks of age, there was no evidence of differential allele-specific vaccine efficacy.

CONCLUSIONS

These results suggest that among children 5 to 17 months of age, the RTS,S vaccine has greater activity against malaria parasites with the matched circumsporozoite protein allele than against mismatched malaria. The overall vaccine efficacy in this age category will depend on the proportion of matched alleles in the local parasite population; in this trial, less than 10% of parasites had matched alleles. (Funded by the National Institutes of Health and others.).

Novel serologic biomarkers provide accurate estimates of recent Plasmodium falciparum exposure for individuals and communities

Tools to reliably measure Plasmodium falciparum (Pf) exposure in individuals and communities are needed to guide and evaluate malaria control interventions. Serologic assays can potentially produce precise exposure estimates at low cost; however, current approaches based on responses to a few characterized antigens are not designed to estimate exposure in individuals. Pf-specific antibody responses differ by antigen, suggesting that selection of antigens with defined kinetic profiles will improve estimates of Pf exposure. To identify novel serologic biomarkers of malaria exposure, we evaluated responses to 856 Pf antigens by protein microarray in 186 Ugandan children, for whom detailed Pf exposure data were available. Using data-adaptive statistical methods, we identified combinations of antibody responses that maximized information on an individual's recent exposure. Responses to three novel Pf antigens accurately classified whether an individual had been infected within the last 30, 90, or 365 d (cross-validated area under the curve = 0.86-0.93), whereas responses to six antigens accurately estimated an individual's malaria incidence in the prior year. Cross-validated incidence predictions for individuals in different communities provided accurate stratification of exposure between populations and suggest that precise estimates of community exposure can be obtained from sampling a small subset of that community. In addition, serologic incidence predictions from cross-sectional samples characterized heterogeneity within a community similarly to 1 y of continuous passive surveillance. Development of simple ELISA-based assays derived from the successful selection strategy outlined here offers the potential to generate rich epidemiologic surveillance data that will be widely accessible to malaria control programs.

Serological markers for monitoring historical changes in malaria transmission intensity in a highly endemic region of Western Kenya, 1994-2009

Background

Monitoring local malaria transmission intensity is essential for planning evidence-based control strategies and evaluating their impact over time. Anti-malarial antibodies provide information on cumulative exposure and have proven useful, in areas where transmission has dropped to low sustained levels, for retrospectively reconstructing the timing and magnitude of transmission reduction. It is unclear whether serological markers are also informative in high transmission settings, where interventions may reduce transmission, but to a level where considerable exposure continues.

Methods

This study was conducted through ongoing KEMRI and CDC collaboration. Asembo, in Western Kenya, is an area where intense malaria transmission was drastically reduced during a 1997–1999 community-randomized, controlled insecticide-treated net (ITN) trial. Two approaches were taken to reconstruct malaria transmission history during the period from 1994 to 2009. First, point measurements were calculated for seroprevalence, mean antibody titre, and seroconversion rate (SCR) against three Plasmodium falciparum antigens (AMA-1, MSP-1₁₉, and CSP) at five time points for comparison against traditional malaria indices (parasite prevalence and entomological inoculation rate). Second, within individual post-ITN years, age-stratified seroprevalence data were analysed retrospectively for an abrupt drop in SCR by fitting alternative reversible catalytic conversion models that allowed for change in SCR.

Results

Generally, point measurements of seroprevalence, antibody titres and SCR produced consistent patterns indicating that a gradual but substantial drop in malaria transmission (46-70%) occurred from 1994 to 2007, followed by a marginal increase beginning in 2008 or 2009. In particular, proportionate changes in seroprevalence and SCR point estimates (relative to 1994 baseline values) for AMA-1 and CSP, but not MSP-1₁₉, correlated closely with trends in parasite prevalence throughout the entire 15-year study period. However, retrospective analyses using datasets from 2007, 2008 and 2009 failed to detect any abrupt drop in transmission coinciding with the timing of the 1997–1999 ITN trial.

Conclusions

In this highly endemic area, serological markers were useful for generating accurate point estimates of malaria transmission intensity, but not for retrospective analysis of historical changes. Further investigation, including exploration of different malaria antigens and/or alternative models of population seroconversion, may yield serological tools that are more informative in high transmission settings.

Electronic supplementary material

The online version of this article (doi:10.1186/1475-2875-13-451) contains supplementary material, which is available to authorized users.

Prevalence and Level of Antibodies Anti-Plasmodium spp. in Travellers with Clinical History of Imported Malaria

In this study, we show that 40.29% of travellers with a possible history of malaria exposure were positive for anti-Plasmodium spp. antibodies, while these individuals were negative by microscopy. The antibody test described here is useful to elucidate malaria exposure in microscopy-negative travellers from endemic countries.

Marked variation in MSP-119 antibody responses to malaria in western Kenyan highlands

Background

Assessment of malaria endemicity at different altitudes and transmission intensities, in the era of dwindling vector densities in the highlands, will provide valuable information for malaria control and surveillance. Measurement of serum anti-malarial antibodies is a useful marker of malaria exposure that indicates long-term transmission potential. We studied the serologic evidence of malaria endemicity at two highland sites along a transmission intensity cline. An improved understanding of the micro-geographic variation in malaria exposure in the highland ecosystems will be relevant in planning effective malaria control.

Methods

Total IgG levels to Plasmodium falciparum MSP-1₁₉ were measured in an age-stratified cohort (< 5, 5-14 and ≥ 15 years) in 795 participants from an uphill and valley bottom residents during low and high malaria transmission seasons. Antibody prevalence and level was compared between different localities. Regression analysis was performed to examine the association between antibody prevalence and parasite prevalence. Age-specific MSP-1₁₉ seroprevalence data was fitted to a simple reversible catalytic model to investigate the relationship between parasite exposure and age.

Results

Higher MSP-1₁₉ seroprevalence and density were observed in the valley residents than in the uphill dwellers. Adults (> 15 years) recorded high and stable immune response in spite of changing seasons. Lower responses were observed in children (≤ 15 years), which, fluctuated with changing seasons particularly in the valley residents. In the uphill population, annual seroconversion rate (SCR) was 8.3% and reversion rate was 3.0%, with seroprevalence reaching a plateau of 73.3% by age of 20. Contrary, in the valley bottom population, the annual SCR was 35.8% and the annual seroreversion rate was 3.5%, and seroprevalence in the population had reached 91.2% by age 10.

Conclusion

The study reveals the micro-geographic variation in malaria endemicity in the highland eco-system; this validates the usefulness of sero-epidemiological tools in assessing malaria endemicity in the era of decreasing sensitivity of conventional tools.

Wide cross-reactivity between Anopheles gambiae and Anopheles funestus SG6 salivary proteins supports exploitation of gSG6 as a marker of human exposure to major malaria vectors in tropical Africa

Background

The Anopheles gambiae gSG6 is an anopheline-specific salivary protein which helps female mosquitoes to efficiently feed on blood. Besides its role in haematophagy, gSG6 is immunogenic and elicits in exposed individuals an IgG response, which may be used as indicator of exposure to the main African malaria vector A. gambiae. However, malaria transmission in tropical Africa is sustained by three main vectors (A. gambiae, Anopheles arabiensis and Anopheles funestus) and a general marker, reflecting exposure to at least these three species, would be especially valuable. The SG6 protein is highly conserved within the A. gambiae species complex whereas the A. funestus homologue, fSG6, is more divergent (80% identity with gSG6). The aim of this study was to evaluate cross-reactivity of human sera to gSG6 and fSG6.

Methods

The A. funestus SG6 protein was expressed/purified and the humoral response to gSG6, fSG6 and a combination of the two antigens was compared in a population from a malaria hyperendemic area of Burkina Faso where both vectors were present, although with a large A. gambiae prevalence (>75%). Sera collected at the beginning and at the end of the high transmission/rainy season, as well as during the following low transmission/dry season, were analysed.

Results

According to previous observations, both anti-SG6 IgG level and prevalence decreased during the low transmission/dry season and showed a typical age-dependent pattern. No significant difference in the response to the two antigens was found, although their combined use yielded in most cases higher IgG level.

Conclusions

Comparative analysis of gSG6 and fSG6 immunogenicity to humans suggests the occurrence of a wide cross-reactivity, even though the two proteins carry species-specific epitopes. This study supports the use of gSG6 as reliable indicator of exposure to the three main African malaria vectors, a marker which may be useful to monitor malaria transmission and evaluate vector control measures, especially in conditions of low malaria transmission and/or reduced vector density. The Anopheles stephensi SG6 protein also shares 80% identity with gSG6, suggesting the attractive possibility that the A. gambiae protein may also be useful to assess human exposure to several Asian malaria vectors.

Humoral response to the Anopheles gambiae salivary protein gSG6: a serological indicator of exposure to Afrotropical malaria vectors

Salivary proteins injected by blood feeding arthropods into their hosts evoke a saliva-specific humoral response which can be useful to evaluate exposure to bites of disease vectors. However, saliva of hematophagous arthropods is a complex cocktail of bioactive factors and its use in immunoassays can be misleading because of potential cross-reactivity to other antigens. Toward the development of a serological marker of exposure to Afrotropical malaria vectors we expressed the Anopheles gambiae gSG6, a small anopheline-specific salivary protein, and we measured the anti-gSG6 IgG response in individuals from a malaria hyperendemic area of Burkina Faso, West Africa. The gSG6 protein was immunogenic and anti-gSG6 IgG levels and/or prevalence increased in exposed individuals during the malaria transmission/rainy season. Moreover, this response dropped during the intervening low transmission/dry season, suggesting it is sensitive enough to detect variation in vector density. Members of the Fulani ethnic group showed higher anti-gSG6 IgG response as compared to Mossi, a result consistent with the stronger immune reactivity reported in this group. Remarkably, anti-gSG6 IgG levels among responders were high in children and gradually declined with age. This unusual pattern, opposite to the one observed with Plasmodium antigens, is compatible with a progressive desensitization to mosquito saliva and may be linked to the continued exposure to bites of anopheline mosquitoes. Overall, the humoral anti-gSG6 IgG response appears a reliable serological indicator of exposure to bites of the main African malaria vectors (An. gambiae, Anopheles arabiensis and, possibly, Anopheles funestus) and it may be exploited for malaria epidemiological studies, development of risk maps and evaluation of anti-vector measures. In addition, the gSG6 protein may represent a powerful model system to get a deeper understanding of molecular and cellular mechanisms underlying the immune tolerance and progressive desensitization to insect salivary allergens.

Plasmodium falciparum genotypes diversity in symptomatic malaria of children living in an urban and a rural setting in Burkina Faso

BACKGROUND

The clinical presentation of malaria, considered as the result of a complex interaction between parasite and human genetics, is described to be different between rural and urban areas. The analysis of the Plasmodium falciparum genetic diversity in children with uncomplicated malaria, living in these two different areas, may help to understand the effect of urbanization on the distribution of P. falciparum genotypes.

METHODS

Isolates collected from 75 and 89 children with uncomplicated malaria infection living in a rural and an urban area of Burkina Faso, respectively, were analysed by a nested PCR amplification of msp1 and msp2 genes to compare P. falciparum diversity.

RESULTS

The K1 allelic family was widespread in children living in the two sites, compared to other msp1 allelic families (frequency >90%). The MAD 20 allelic family of msp1 was more prevalent (p = 0.0001) in the urban (85.3%) than the rural area (63.2%). In the urban area, the 3D7 alleles of msp2 were more prevalent compared to FC27 alleles, with a high frequency for the 3D7 300bp allele (>30%). The multiplicity of infection was in the range of one to six in the urban area and of one to seven in the rural area. There was no difference in the frequency of multiple infections (p = 0.6): 96.0% (95% C.I: 91.6-100) in urban versus 93.1% (95%C.I: 87.6-98.6) in rural areas. The complexity of infection increased with age [p = 0.04 (rural area), p = 0.06 (urban area)].

CONCLUSION

Urban-rural area differences were observed in some allelic families (MAD20, FC27, 3D7), suggesting a probable impact of urbanization on genetic variability of P. falciparum. This should be taken into account in the implementation of malaria control measures.

Acquisition of naturally occurring antibody responses to recombinant protein domains of Plasmodium falciparum erythrocyte membrane protein 1

Background

Antibodies targeting variant antigens expressed on the surface of Plasmodium falciparum infected erythrocytes have been associated with protection from clinical malaria. The precise target for these antibodies is unknown. The best characterized and most likely target is the erythrocyte surface-expressed variant protein family Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1).

Methods

Using recombinant proteins corresponding to five domains of the expressed A4 var gene, A4 PfEMP1, the naturally occurring antibody response was assessed, by ELISA, to each domain in serum samples obtained from individuals resident in two communities of differing malaria transmission intensity on the Kenyan coast. Using flow cytometry, the correlation in individual responses to each domain with responses to intact A4-infected erythrocytes expressing A4 PfEMP1 on their surface as well as responses to two alternative parasite clones and one clinical isolate was assessed.

Results

Marked variability in the prevalence of responses between each domain and between each transmission area was observed, as wasa strong correlation between age and reactivity with some but not all domains. Individual responses to each domain varied strikingly, with some individuals showing reactivity to all domains and others with no reactivity to any, this was apparent at all age groups. Evidence for possible cross-reactivity in responses to the domain DBL4γ was found.

Conclusion

Individuals acquire antibodies to surface expressed domains of a highly variant protein. The finding of potential cross-reactivity in responses to one of these domains is an important initial finding in the consideration of potential vaccine targets.

Do antibody responses to malaria vaccine candidates influenced by the level of malaria transmission protect from malaria?

OBJECTIVES

To examine whether the humoural response to malaria vaccine candidate antigens, Plasmodium falciparum [circumsporozoite repetitive sequence (NANP)(5) GLURP fragments (R0 and R2) and MSP3] varies with the level of malaria transmission and to determine whether the antibodies (IgG) present at the beginning of the malaria transmission season protect against clinical malaria.

METHODS

Cross-sectional surveys were conducted to measure antibody response before, at the peak and at the end of the transmission season in children aged 6 months to 10 years in two villages with different levels of malaria transmission. A cohort study was performed to estimate the incidence of clinical malaria.

RESULTS

Antibodies to these antigens showed different seasonal patterns. IgG concentrations to any of the four antigens were higher in the village with high entomological inoculation rate. Multivariate analysis of combined data from the two villages indicated that children who were classified as responders to the selected antigens were at lower risk of clinical malaria than children classified as non-responders [(NANP)(5) (incidence rate ratio (IRR) = 0.65, 95% CI: 0.46-0.92; P = 0.016), R0 (IRR = 0.69, 95% CI: 0.48-0.97; P = 0.032), R2 (IRR = 0.73, 95% CI: 0.50-1.06; P = 0.09), MSP3 (IRR = 0.52, 95% CI: 0.32-0.85; P = 0.009)]. Fitting a model with all four antibody responses showed that MSP3 looked the best malaria vaccine candidate (IRR = 0.63; 95% CI: 0.38-1.05; P = 0.08).

CONCLUSION

Antibody levels to the four antigens are affected by the intensity of malaria transmission and associated with protection against clinical malaria. It is worthwhile investing in the development of these antigens as potential malaria vaccine candidates.

Serology: a robust indicator of malaria transmission intensity?

To estimate the burden of malarial disease, and evaluate the likely effects of control strategies, requires reliable predictions of malaria transmission intensity. It has long been suggested that antimalarial antibody prevalences could provide a more accurate estimate of transmission intensity than traditional measures such as parasite prevalence or entomological inoculation rates, but there has been no systematic evaluation of this approach. Now, the availability of well characterized malarial antigens allows us to test whether serological measurements provide a practical method for estimating transmission. Here we present a suggested methodology, highlight the advantages and shortcomings of serological measurements of malaria transmission and identify areas in which further work is desirable.

Immune effector mechanisms in malaria

That humans in endemic areas become immune to malaria offers encouragement to the idea of developing protective vaccines. However natural immunity is relatively inefficient, being bought at the cost of substantial childhood mortality, and current vaccines are only partially protective. Understanding potential targets and mechanisms of protective immunity is important in the development and evaluation of future vaccines. Some of the problems in identifying such targets and mechanisms in humans naturally exposed to malaria may stem from conceptual and methodological issues related to defining who in a population is susceptible, problems in defining immune responsiveness at single time points and issues related to antigenic polymorphism, as well as the failure of many current approaches to examine functional aspects of the immune response. Protective immune responses may be directed to the pre erythrocytic parasite, to the free merozoite of the blood stage parasite or to new antigens induced on the infected red cell surface. Tackling the methodological issues of defining protection and immune response, together with studies that combine functional assays with new approaches such as allelic exchange and gene knock out offer opportunities for better defining key targets and mechanisms.

Distinct Interethnic Differences in Immunoglobulin G Class/Subclass and Immunoglobulin M Antibody Responses to Malaria Antigens but not in Immunoglobulin G Responses to Nonmalarial Antigens in Sympatric Tribes Living in West Africa

The well-established relative resistance to malaria observed in the Fulani as compared with other sympatric tribes in West Africa has been attributed to their higher levels of serum immunoglobulin (Ig) G antibodies to malarial antigens. In this study, we confirm and extend the previous findings by analyses of the levels of IgM, IgG and IgG subclasses of anti-malarial antibodies in asymptomatic individuals of different sympatric tribes in Burkina Faso (Fulani/Mossi) and Mali (Fulani/Dogon). The Fulani showed significantly higher median concentrations of anti-malarial IgG and IgM antibodies than the sympatric tribes at both locations. Although the overall subclass pattern of antibodies did not differ between the tribes, with IgG1 and IgG3 as dominant, the Fulani showed consistently significantly higher levels of these subclasses as compared with those of the non-Fulani individuals. No significant differences were seen in the levels of total IgG between the tribes, but the Fulani showed significantly higher levels of total IgM than their neighbours in both countries. While the antibody levels to some nonmalarial antigens showed the same pattern of differences seen for antibody levels to malaria antigens, no significant such differences were seen with antibodies to other nonmalarial antigens. In conclusion, our results show that the Fulani in two different countries show higher levels of anti-malarial antibodies than sympatric tribes, and this appears not to be a reflection of a general hyper-reactivity in the Fulani.

IL4-589C/T polymorphism and IgE levels in severe malaria

Previous studies identified an allelic variant of the IL4 promoter region (IL4-589T) that appears to enhance the transcriptional activity of IL4, and is associated with increased IgE levels. Total serum IgE levels are elevated in malaria endemic regions, and higher in children with severe malaria. Here, we investigated the relationship of the IL4-589C/T polymorphism with severity of the disease in a case-control study of severe malaria in Burkina Faso, West Africa. No association between the IL4-589T and severe malaria was observed. No difference in Plasmodium falciparum-specific IgE was detected between severe and uncomplicated malaria patients. Among children with severe malaria, total IgE levels were significantly elevated in those carrying the IL4-589T allele (P = 0.018). In children with uncomplicated malaria, no significant difference was found. These results raise the possibility that there is a relationship between susceptibility to severe malaria, IgE production and genetic variation in the IL4 region, which merits further investigation in other epidemiological settings.

Humoral responses to defined malaria antigens in children living since birth under insecticide treated curtains in Burkina Faso

Insecticide treated materials (ITM) are considered a useful malaria control measure for endemic countries, but whether they also delay the acquisition of immunity to malaria remains unclear. This study investigates plasma antibody levels in 160 children aged 3-6 years from five villages protected by insecticide treated curtains (ITC) over 6 years and in 184 children of the same age group from five villages in the same area never covered by ITC. The antigens to which antibodies were investigated were: the Plasmodium falciparum circumsporozoite protein (CSP) repetitive sequence (NANP)5; the C-terminal domain of the P. falciparum exported protein 1 (Cter-PfExp1); three fragments of the glutamate rich protein (GLURP), referred to as R0, R1 and R2; the merozoite surface protein 3 (MSP3). The level of antibodies was lower in children from the ITC area than in children from the non-ITC area for (NANP)5, R0, R2 and MSP3. Prevalence and intensity of P. falciparum infection were similar in the two groups of children. These findings suggest that reducing the level of malaria transmission over a long period may affect the level of antibodies in children to both sporozoite and blood stage malaria antigens.

Does infection with Human Immunodeficiency Virus affect the antibody responses to Plasmodium falciparum antigenic determinants in asymptomatic pregnant women?

OBJECTIVES

HIV-seropositive pregnant women are more susceptible to malaria than HIV-seronegative women. We assessed whether HIV infection alters maternal and cord plasma malarial antibody responses and the mother-to-infant transfer of malaria antibodies.

METHODS

We determined plasma levels of maternal and cord antibodies [Immunoglobulin (IgG)] to recombinant malarial proteins [merozoite surface protein 1 (MSP-1(19kD)), the erythrocyte binding antigen (EBA-175)], the synthetic peptides [MSP-2, MSP-3, rhoptry associated protein 1 (RAP-1), and the pre-erythrocytic stage, circumsporozoite protein (NANP)(5)] antigenic determinants of Plasmodium falciparum; and tetanus toxoid (TT) by ELISA among samples of 99 HIV-seropositive mothers, 69 of their infants, 102 HIV-seronegative mothers and 62 of their infants.

RESULTS

The prevalence of maternal antibodies to the malarial antigenic determinants ranged from 18% on MSP3 to 91% on EBA-175; in cord plasma it ranged from 13% to 91%, respectively. More than 97% of maternal and cord samples had antibodies to TT. In multivariate analysis, HIV infection was only associated with reduced antibodies to (NANP)(5) in maternal (P=0.001) and cord plasma (P=0.001); and reduced mother-to-infant antibody transfer to (NANP)(5) (P=0.012). This effect of HIV was independent of maternal age, gravidity and placental malaria. No consistent HIV-associated differences were observed for other antigenic determinants.

CONCLUSION

An effect of HIV infection was only observed on one malarial antigenic determinant, suggesting that the increased susceptibility to malaria among HIV-infected pregnant women may not be explained on the basis of their reduced antibody response to malaria antigens.

Plasmodium falciparum: use of a NANP19 antibody-test for the detection of infection in non-immune travellers

Circumsporozoite (CS) antibodies are a reliable serological marker for the infection of Plasmodium falciparum. The purpose of this investigation was to construct and evaluate an enzyme-linked immunosorbent assay test for the detection of CS antibodies. While the sensitivity of the newly developed test reached 78%, the specificity was 99%. In addition, the optimized kit was used to test for infection with P. falciparum in 1903 travellers that were recruited from a prospective study for malaria chemoprophylaxis. Sixty-six of the 1903 patients (3.5%) showed elevated CS antigen antibody titres. However, seroconversion could only be demonstrated in 18 (0.95%) patients. Among those seroconverting, there was a significantly higher percentage of male travellers (1.28%) than female travellers (0.56%). Positive reactions were more frequent among returnees from West and East Africa (1.49 and 1.14%, respectively) than among those from other endemic areas, e.g. South America (n=0). Despite its limited sensitivity, this newly developed kit for CS antibody testing may be a valuable tool for the estimation of the risk for travellers in malarious regions to acquire an infection with P. falciparum. It may also be useful for the determination of the efficacy of malaria chemoprophylaxis for inhibiting outbreak of disease.

The lower susceptibility to Plasmodium falciparum malaria of Fulani of Burkina Faso (west Africa) is associated with low frequencies of classic malaria-resistance genes

The gene frequencies in 1993-94 for haemoglobin S, haemoglobin C, alpha-3.7 deletional thalassaemia, G6PDA-, HLAB*5301 were estimated in Fulani, Mossi and Rimaibé ethnic groups of Burkina Faso, West Africa. The aim of the study was to verify whether the previously reported Fulani lower susceptibility to Plasmodium falciparum malaria was associated with any of these malaria-resistance genes. Similar frequencies for haemoglobin S were recorded in the 3 ethnic groups (0.024 +/- 0.008, 0.030 +/- 0.011, 0.022 +/- 0.013; in Mossi, Rimaibé and Fulani, respectively). The Mossi and Rimaibé showed higher frequencies when compared to Fulani for haemoglobin C (0.117 +/- 0.018, 0.127 +/- 0.020, 0.059 +/- 0.020), alpha-3.7 deletional thalassaemia (0.227 +/- 0.040, 0.134 +/- 0.032, 0.103 +/- 0.028), G6PDA- (0.196 +/- 0.025, 0.187 +/- 0.044, 0.069 +/- 0.025) and HLA B*5301 (0.189 +/- 0.038, 0.202 +/- 0.041, 0.061 +/- 0.024). Among Fulani the proportion of individuals not having any of these protective alleles was more than 3-fold greater than in the Mossi-Rimaibé group (56.8% vs 16.7%; P < 0.001). These findings exclude the involvement of these genetic factors of resistance to P. falciparum in the lower susceptibility to malaria of Fulani. This evidence, in association with the previously reported higher immune reactivity to malaria of Fulani, further supports the existence in this ethnic group of unknown genetic factor(s) of resistance to malaria probably involved in the regulation of humoral immune responses.

Analysis of Plasmodium falciparum infections in a village community in Northern Nigeria: determination of msp2 genotypes and parasite-specific IgG responses

The genetic diversity of P. falciparum and multiplicity of infection has been studied in a village in Northern Nigeria at the end of the rainy season, when transmission is high. We analysed blood samples from 104 individuals aged 5-70 years by polymerase chain reaction (PCR) amplifying the gene for the merozoite surface protein MSP2 followed by genotyping based on restriction fragment length polymorphism (RFLP). 94.2% of all samples were parasite positive by PCR and over 80% of those had multiple infections. The age distribution of the average number of parasite clones present in P. falciparum infections showed an initial increase, then reached a peak multiplicity in children 8-10 years of age, and afterwards decreased significantly with age. Mean multiplicity in those 8-10-year-old children was 5.4 clones per carrier. Peak multiplicity and parasite diversity in Nigerian individuals is compared to findings from other study sites in Africa and PNG. The prevalence of IgG antibodies against the circumsporozoite protein (CSP), an indicator for malaria exposure, was over 85% in all age groups showing a high exposure of villagers to P. falciparum. OD values in ELISA were positively correlated with age. There was no correlation between the level of IgG against CSP and the multiplicity of P. falciparum infections determined by PCR of msp2. These results imply that in highly endemic areas multiplicity of infection is not directly correlated with exposure to P. falciparum.

Wide-scale installation of insecticide-treated curtains confers high levels of protection against malaria transmission in a hyperendemic area of Burkina Faso

In a region of Sudanese savannah in Burkina Faso, insecticide-treated curtains were installed in 8 out of 16 zones, each covering an area of about 50 km2. Longitudinal entomological monitoring using CDC light traps was performed in 4 villages (2 intervention, 2 control) over a period of 3 years (including 1 year prior to intervention). In the 3rd year a cross-sectional entomological survey using spray catches was performed in 84 villages (40 intervention). Indoor vector densities in protected houses showed large reductions (P = 0.01). The available data were also consistent with an impact on outdoor and unprotected indoor densities. The proportion of mosquitoes carrying sporozoites was 4.1% in protected villages compared with 11.5% in unprotected villages (P = 0.07). Entomological inoculation rates fell substantially (P = 0.01), reflecting these reductions. The impact of this intervention on mosquito survival appears to have been greater than those in similar trials conducted in the Gambia, Ghana and Kenya in which the intervention was applied over smaller areas.

Different response to Plasmodium falciparum malaria in west African sympatric ethnic groups

The comparison of malaria indicators among populations that have different genetic backgrounds and are uniformly exposed to the same parasite strains is one approach to the study of human heterogeneties in the response to the infection. We report the results of comparative surveys on three sympatric West African ethnic groups, Fulani, Mossi, and Rimaibé, living in the same conditions of hyperendemic transmission in a Sudan savanna area northeast of Ouagadougou, Burkina Faso. The Mossi and Rimaibé are Sudanese negroid populations with a long tradition of sedentary farming, while the Fulani are nomadic pastoralists, partly settled and characterized by non-negroid features of possible caucasoid origin. Parasitological, clinical, and immunological investigations showed consistent interethnic differences in Plasmodium falciparum infection rates, malaria morbidity, and prevalence and levels of antibodies to various P. falciparum antigens. The data point to a remarkably similar response to malaria in the Mossi and Rimaibé, while the Fulani are clearly less parasitized, less affected by the disease, and more responsive to all antigens tested. No difference in the use of malaria protective measures was demonstrated that could account for these findings, and sociocultural or environmental factors do not seem to be involved. Known genetic factors of resistance to malaria did not show higher frequencies in the Fulani. The differences in the immune response were not explained by the entomological observations, which indicated substantially uniform exposure to infective bites. The available data support the existence of unknown genetic factors, possibly related to humoral immune responses, determining interethnic differences in the susceptibility to malaria.

Comparison of the number of IL-4 and IFN-gamma secreting cells in response to the malaria vaccine candidate antigen Pf155/RESA in two groups of naturally primed individuals living in a malaria endemic area in Burkina Faso

The enzyme-linked immunospot (ELISPOT) assay was used to enumerate the number of IFN-gamma and IL-4 producing cells after in vitro stimulation with a highly purified recombinant malaria vaccine candidate antigen (r-Pf155/RESA) or synthetic peptides corresponding to its major T-cell epitopes. Two groups of naturally primed individuals living in rural areas of Burkina Faso were studied. The donors comprised one group of healthy (non-parasitemic) mainly adult people and one parasitemic mainly younger people. IL-4 producing cells were detected in response to PHA but no such cells were detected in response to the malarial antigens. The most frequent IFN-gamma responses were seen with r-Pf155/RESA. Thus, after stimulation with this antigen 52% of the donors responded positively in the ELISPOT assay, while only 17% responded to the synthetic peptides, suggesting that the rPf155/RESA contained T-cell epitopes not covered by the peptides used in this study. The number of IFN-gamma producing cells in response to the malarial antigens did not differ between the two groups. However, IFN-gamma levels found in sera from the parasitemic individuals were significantly higher than in those from healthy donors. This latter finding and the lack of differences seen in the number of IFN-gamma producing spots in the two groups indicate that IFN-gamma producing cells may have sequestered to other organs in the parasitemic group.

Antibodies to ring-infected erythrocyte surface antigen (Pf155/RESA) protect against P. falciparum parasitemia in highly exposed multigravidas women in Malawi

To determine whether antibodies to defined B-cell epitopes of Plasmodium falciparum antigens were related to protection against parasitemic attacks in highly exposed pregnant women, two samples of 235 with no initial P. falciparum parasitemia (NP) and 89 multigravidas who presented initial P. falciparum parasitemia (IP) were enrolled in an antimalarial prophylaxis trial in the Mangochi District in Malawi. Sera were collected under effective prophylaxis and tested for antibody measurement using FAST-ELISA. Mean antibody titers to synthetic peptides reproducing the 3 major B-cell epitopes of the ring-infected erythrocyte surface antigen (Pf155/RESA), as (EENV)4, (EENVEHDA)4 and (DDEHVEEPTVA)3, were higher in the NP than in the IP multigravidas, and this remained consistent within the season of malaria transmission (all p < 0.05). All antibodies to Pf155/RESA were positively intercorrelated within each group. Mean antibody titers to peptide (PNAN)5 reproducing the major B-cell epitope of the circumsporozoite protein (CS protein) were similar between NP and IP multigravidas in both dry and rainy season. Antibodies to Pf155/RESA epitopes may contribute to immune protection against blood-stage parasite multiplication in these highly malaria-exposed pregnant women.

The level of anti-sporozoite antibodies in a highly endemic malaria area and its relationship with exposure to mosquitoes. Kilombero Malaria Project

The relationship between the humoral immune response to the conserved repeated epitope of the Plasmodium falciparum sporozoite and exposure to the mosquito vectors was examined in a study carried out in rural southern Tanzania, an area highly endemic for malaria. Considerable aggregation of the immune response between houses was observed. A statistically significant portion of this aggregation could be explained by differences in individual exposure to mosquitoes. However, two-thirds of the variance due to aggregation between households could not be accounted for, so that antibody level after controlling for exposure remained aggregated. Most of the variability in the development of the immune response was between individuals within households, and may be related to individual differences in behaviour and attractiveness to mosquitoes. The observed correlation of the immune response with exposure was due to continual exposure during several months, whereas recent exposure had almost no effect on the immune response observed in an endemic area. We concluded that in a highly endemic area the anti-sporozoite antibody level cannot be used as an indicator of recent infection and has only limited use as an indicator of continual infection.

High seasonal malaria transmission rates in the intermediate rainfall zone of Sri Lanka

Malaria transmission was studied at Nikawehera, a long-established farming village, located in the intermediate rainfall zone of Sri Lanka. Observations were made over a 12-month period (October-September) that included the main rainy season which occurred during the north-east monsoon in November-January. Anolpheles culicifacies, the recognized vector of malaria in Sri Lanka, was the predominant anopheline mosquito collected by human night baiting at Nikawehera. High entomological inoculation rates with An. culcifacies (0.12/hour for Plasmodium vivax) were observed during the height of the transmission season which occurs during, and immediately after, the north-east monsoon. Anolpheles subpictus was identified as a possible additional vector at Nikawehera. Anopheles annularis, a major vector at Weheragala, a site in a new irrigation development (the Mahaweli Scheme) located 70 km away in the dry zone, was not collected by human baiting at Nikawehera. Clinical, entomological and parasitological data suggest that malaria is hyperendemic at Nikawehera, with high seasonal transmission rates.

Is Immunity to malaria really short-lived?

Protection against Plasmodium falciparum malaria is usually considered to be the cumulative product of repeated exposure to parasites, and thus a function of age, in endemic areas. The recent outbreak of malaria in the central highlands of Madagascar gave Philippe Deloron and Claire Chougnet the opportunity to compare the incidence of malaria in children and young adults exposed to malaria for the first time, with that in older adults who spent their childhood in the study area before malaria control was introduced. Protection, as well as immune responses to two major P. falciparum antigens, was not related to age. Individuals older than 40 years were more protected than were younger adults. This increased protection was probably due to immunological memory.

Malaria and pregnancy in Cameroonian women. Naturally acquired antibody responses to asexual blood-stage antigens and the circumsporozoite protein of Plasmodium falciparum

Antibody responses to Plasmodium falciparum antigens in women during pregnancy were investigated in Mfou, a rural community in Cameroon. The study consisted of cross-sectional analyses involving 225 pregnant women and 75 non-pregnant controls. Blood samples were collected from each woman to determine serological reactivity to intraerythrocytic malarial antigens, ring-infected erythrocyte surface antigen (RESA) and circumsporozoite (CS) repeat peptide (NANP)5 by the indirect fluorescent antibody assay, modified immunofluorescent antibody assay, and enzyme-linked immunosorbent assay, respectively. Reactivity to intraerythrocytic asexual blood-stage antigens and to the CS repeat region was similar in both pregnant and non-pregnant women, and no correlation with parasitaemia was found. In contrast, anti-RESA antibody levels were significantly lower in pregnant than in non-pregnant women (P = 0.02) and in primigravidae than in multigravidae (P = 0.002), and were inversely correlated with parasitaemia (r = -0.36; P < 0.01). These data suggest that the increased susceptibility to malarial infection in pregnant women may be explained in part by their lower reactivity to RESA.

In vitro immune recognition of synthetic peptides from the Plasmodium falciparum CS protein by individuals naturally exposed to different sporozoite challenge

The impact of duration and intensity of sporozoite challenge on the in vitro cell immune response to synthetic peptides of the circumsporozoite (CS) protein of Plasmodium falciparum was investigated in residents of a malaria endemic area in Burkina Faso (West Africa). Lymphocyte proliferation and interferon-gamma (IFN-gamma) production were used to assess immune recognition of synthetic peptides corresponding to the polymorphic Th2R and Th3R regions, to the conserved CS.T3 sequence and to NANP and degenerate NVDP repeats. Immune responses were measured in adults and children from a village where they received more than 100 sporozoite inoculations per year and in adults living in a town, exposed to a 10-100 times lower challenge. A lifetime intense exposure apparently increased the ability to proliferate in response to most peptides in the rural adults, who all produced antibodies to NANP repeats. Surprisingly, cell cultures from these subjects seldom contained appreciable levels of IFN-gamma. In the urban adults, possibly due to the moderate challenge they are exposed to, significant differences in the proliferative potentials of the peptides could be detected. The highest stimulation indices were obtained with the genetically unrestricted CS.T3 peptide. Remarkably, proliferative responses to Th2R and Th3R appeared to be correlated with the humoral response to the CS protein, indicating a T helper significance of the epitopes. The differing proliferative potential of the polymorphic epitopes in the urban adults suggests that polymorphism might delay the development of immune responsiveness under conditions of sporadic transmission. The children from the highly malarious village displayed the lowest proliferative scores, accompanied by a high prevalence of antibodies to NANP repeats. On the basis of these findings, the hypothesis is proposed that a pure B cell reactivity to NANP repeats could ontogenetically precede the mounting of a conventional T-B cooperative immune response.

Anti-circumsporozoite protein antibodies measure age related exposure to malaria in Kataragama, Sri Lanka

Antibodies to two peptides DDAAD and (NANP)40 representing the repetitive sequence of circumsporozoite antigens (CS protein) of P. vivax and P. falciparum respectively were measured in a cohort of 149 and 107 individuals respectively at four, 6 monthly blood surveys performed on residents of Kataragama, a P. vivax malaria endemic region in southern Sri Lanka. The prevalence of antibodies to the CS protein of both species was relatively low being less than 20% to either peptide in the population as a whole, this being consistent with the low entomological inoculation rates in the area. A marked age related prevalence pattern was evident, with the prevalence of antibodies increasing with age to reach between 25 to 30% in the 25-50 year age group in both P. vivax and P. falciparum. The population had had a life long exposure to P. vivax malaria but not to P. falciparum, an epidemic of which occurred in this region a few months prior to the beginning of this study. Nevertheless, the age-related prevalence of these antibodies was identical with respect to the two species. This suggests that the age-related prevalence pattern reflected differences in inoculation rates between the age groups due to differences in exposure to inoculation rather than an age acquired response resulting from a cumulative experience over several years. An analysis of antibody prevalence in individuals showed first, that sporozoite inoculations must have been clustered rather than homogeneously distributed in the population and secondly, that sero-conversion did not correlate with malaria infections in these individuals.

Multiple antigen peptides for specific detection of antibodies to a malaria antigen in human sera

Multiple antigen peptides (MAP), consisting of a number of peptide copies synthesized on a branching lysyl core, offer a novel approach for rendering small peptides immunoreactive in solid-phase immunoassays. An octameric MAP, carrying 6 repeats of the sequence -N-A-A-G-, tandem repeated in the immunodominant region of the circumsporozoite (CS) protein of Plasmodium malariae, was used as a model to evaluate the suitability of the MAP system in an indirect enzyme-linked immunosorbent assay (ELISA) for detecting antibodies against a parasite antigen in individuals exposed to natural infection. The reaction of endemic sera in ELISA on MAP8-(NAAG)6 was related to that obtained in immunofluorescence on sporozoites, indicating the specificity of the antibody-MAP interaction. The reactivity of immune sera was found to be directed only against the (NAAG)6 moiety of the MAP and not against the lysyl core, since antibody binding to MAP8-(NAAG)6 was completely inhibited by (NAAG)6-NA monomer, but remained uninfluenced when lysyl core was used as competing ligand. The levels of antibodies to MAP8-(NAAG)6, in two groups of individuals naturally exposed to malaria infection, appeared to be related to their respective exposure to the parasite.

Permethrin-impregnated curtains in malaria control

The impact of permethrin-impregnated curtains on the incidence of malaria episodes, parasitaemia and splenomegaly was assessed during a 22 month period in 2 groups of children aged 0.5-6 years. One group lived in houses where permethrin-impregnated curtains had been installed, the other group lived in houses without curtains. A significant reduction of incidence of malaria episodes, mean parasite density, parasite prevalence and splenomegaly was consistently observed in the intervention group towards the end of the period of moderate transmission, whereas no clear-cut impact could be demonstrated during the high transmission period. The influence of malaria pressure and community utilization on the protective efficiency of curtains is discussed. Because of their acceptability and the ease of reimpregnation, curtains proved to be a suitable technique for integration into primary health care.

Development of natural immunity in Plasmodium falciparum malaria: study of antibody response by Western immunoblotting

A longitudinal study was carried out in Burkina Faso to investigate the natural development of the immune response to Plasmodium falciparum malaria. Three bleedings were carried out before, during, and after the seasonal peak of transmission. Detailed antigen mapping and antibody prevalence of the 248 collected serum samples were established by immunoblotting on the basis of several epidemiological and biological parameters. An improved Western immunoblotting system was used to analyze up to 67 serum samples on each nitrocellulose sheet. This system allowed us to perform the entire study with strictly comparable conditions. Two different blood-stage antigens (exoantigens and somatic antigens) were used to analyze the distribution of different classes and subclasses of immunoglobulins according to the age of the individuals, the presence or absence of a malarial attack, the transmission period, the origin of parasite isolates, and the response to intraerythrocytic stages. Although this analysis emphasizes strong individual variations, reactions with two major antigens of 115 and 103 kDa were especially noted. These antigens induced high antibody levels and prevalences but were probably not involved in protection. The prevalence of immunoglobulin G (IgG) antibodies differed by isotype. Most of antigens stimulating IgG production were also responsible for the IgM antibody response. The role played by these antibodies in the development of natural immunity against malaria is discussed.

Synthetic peptides for Plasmodium vivax malaria sero-epidemiology. Application of Fmoc-polyamide and displacement chromatography

The immunodominant epitope of Plasmodium vivax, one of the major causative agents of malaria in man, consists of the tandem repetitions of a nonapeptide sequence, AspArgAlaAsp/AlaGlyGlnProAlaGly, with Asp (variant d) or Ala (variant a), in the fourth position. Synthetic peptides corresponding to the P. vivax epitope, containing a different number of nonapeptide sequences, were prepared by solid-phase synthesis according to the Fmoc-polyamide method. Three peptides, containing 1, 2, and 4 copies of the d variant, were assembled on the gel polymer; none of these peptides, however, was suitable for P. vivax sero-epidemiology. A 45-peptide containing both the d and a variants, ddaad, was prepared by continuous-flow Fmoc-polyamide (flow-polyamide). Among the cleavage procedures evaluated for the removal of the five Mtr groups only TFMSA/TFA/1,2-ethanedithiol (1:89:10 by vol) brought deblocking to completion; a substantial level of impurities originated, however, from these procedures. The product was purified by reversed-phase displacement chromatography, a technique only recently applied to peptides, which shows distinct advantages over conventional, linear elution chromatography. In a single experiment, 107 mg of the crude mixture were loaded onto an analytical column (250 x 4 mm), obtaining in purified form 85% of the desired material present in the sample. An ELISA test base on the ddaad peptide was developed and is being applied to the sero-epidemiology of P. vivax malaria.

Effects of untreated bed nets on the transmission of Plasmodium falciparum, P. vivax and Wuchereria bancrofti in Papua New Guinea

The impact of untreated bed nets on the transmission of human malaria and filariasis in a village in a hyperendemic area of Papua New Guinea was studied. In anopheline mosquitoes, the Plasmodium falciparum sporozoite antigen positivity rate, filarial infection rates and human blood indices dropped significantly after bed nets were introduced. This reduction in human-vector contact did not affect mosquito density as no significant difference in either landing rates or indoor resting catches was found. The number of bed nets in a house and ownership of dogs were factors significantly associated with a reduction in the number of indoor resting mosquitoes. However, the reduction in the P. falciparum sporozoite antigen rate in mosquitoes was not accompanied by a reduction in either malaria parasite or antibody prevalences or titres against the P. falciparum circumsporozoite protein.

Prevalence of antibody to heterologous circumsporozoite protein of Plasmodium vivax in Thailand

The distribution in Thailand of antibody to a recently discovered variant of circumsporozoite proteins of Plasmodium vivax was determined by enzyme-linked immunosorbent assay (ELISA). The ELISA capture antigens were a synthetic peptide of the principal variant sequence ANGAGNQPG and a candidate P vivax vaccine that contained the predominant repeat sequence GDRAA/DGQPA. Serological evidence of recent inoculation with the variant was found throughout Thailand and in migrants from Cambodia, Malaysia, and Burma. IgG antibody to the two P vivax circumsporozoite proteins was detected in 217 of 804 test sera (27%). Within the regions studied the proportion of positive sera specific for the variant epitope ranged from 28% to 66%. A vaccine against the predominant repeat domain may rapidly select for the variant, which already appears to be widespread within Thailand.

Recognition of dominant T cell-stimulating epitopes from the circumsporozoite protein of Plasmodium falciparum and relationship to malaria morbidity in Gambian children

Cellular immune responses to the Plasmodium falciparum circumsporozoite (CS) protein were measured by proliferation and interferon-gamma production in a cohort of children aged 3 to 8 years, living in The Gambia. Anti-CS antibody titres, malariometric indices and sickle cell status were also determined. Malaria morbidity in the ensuing malaria transmission season was monitored by weekly health questionnaire, axillary temperature measurements and examination of blood films. Exposure to malaria was inferred from entomological data collected during the transmission season. Immunological and parasitological measurements were repeated at the end of the rainy season. Immunological findings were compared between children who experienced clinical malaria or asymptomatic infection and children who had no evidence of infection. No association was found between cellular immune responses to the CS protein at the beginning of the transmission season and subsequent susceptibility to infection except among children with high titres of antibody to (NANP)40. Seropositive children who did not become infected had a higher mean proliferative response to the Th3R epitope than seropositive children who did become infected. High titres of anti-(NANP)40 antibodies alone were not protective. Responses to the Th2R epitope were significantly higher at the end of the rainy season than at the beginning in children who experienced an asymptomatic infection. Responses to variant sequences of the 2 epitopes were highly correlated at an individual level but there was no correlation between proliferative and interferon responses to a particular epitope.

A longitudinal study of antibodies to the Plasmodium falciparum antigen Pf155/RESA and immunity to malaria infection in adult Liberians

118 adult Liberians from 2 villages were studied prospectively for one year with monthly blood examinations for malaria parasites. The crude parasite rate was 41.5% and the crude gametocyte rate was 6.1%. The inoculation rate varied between 0.075 in the dry season and almost 0.4 in the rainy season, which is in accordance with other data from holoendemic areas. 47.5% (56) had a titre to the Pf155/RESA antigen less than or equal to 1/50 ('low responders') and 52.5% (62) had a titre of greater than or equal to 1/250 ('high responders'). The response was not age-dependent in this adult population, which may suggest that genetic factors are determining whether the individual become a high or low responder. Antibodies against the Pf155/RESA antigen were measured in 2 surveys 8 months apart, and the mean antibody response to Pf155/RESA and its EENV sequence was constant without seasonal variation. Pf155/RESA high responders had lower parasite densities during all 3 seasons surveyed, and Pf155/RESA high responders, with high antibody reactivity against the (EENV)6 sequence from the 3' repeat region of Pf155/RESA, had significantly lower parasite densities in the rainy season of 1987. The data suggest that high titres of antibodies to the Pf155/RESA antigen, and especially to its EENV sequence, might play a role in protective immunity in adults.

Antibodies to the ring-infected erythrocyte surface antigen and the circumsporozoite protein of Plasmodium falciparum in a rural community from Burkina Faso

A population-based study in Burkina Faso was conducted to determine, under conditions of natural exposure to malaria parasites, the prevalence rate and seasonal variation of antibodies to the repeated amino acid sequences of the Plasmodium falciparum ring-infected erythrocyte antigen (RESA) and the circumsporozoite (CS) protein, and the relationship of the presence of malaria parasites in the blood with the level of these antibodies. A random sample of households was selected and visited twice during 1988, in April just before the rainy season and in September at the end of the rainy season. Serological testing included an immunofluorescent assay for total anti-blood stage antibodies; a modified immunofluorescent assay on glutaraldehyde-fixed parasitized erythrocytes to detect antibodies to RESA; and an enzyme-linked immunosorbent assay (ELISA) using synthetic peptides representing repeat amino acid sequences of RESA [(EENV)5, (EENVEHDA)4, and (DDEHVEEPTVA)2] and CS protein [(PNAN5)]. In April, at the end of the dry season, 98% of the individuals had total P. falciparum antibodies, and 39% had anti-RESA antibodies. 32%, 82%, 80%, and 41% of the individuals had antibodies to the respective peptides. All prevalence rates increased with age. After the rains, in September, prevalence rates and levels of reactivity increased in each serological assay, except in the ELISA with (DDEHVEEPTVA)2. Presence of P. falciparum parasitaemia was inversely related to antibody response to the CS protein (PNAN)5 peptide but not to any other assay.

The relationship between splenomegaly and antibody to the circumsporozoite protein of Plasmodium falciparum in two groups of women with high and low enlarged spleen rates in Madang, Papua New Guinea

The objective of this study was to determine the prevalence of antibodies recognizing the circumsporozoite (CS) protein of Plasmodium by an enzyme-linked immunosorbent assay, in 2 subpopulations of women with significantly different enlarged spleen rates but similar exposure to malaria, on the north coast of Papua New Guinea. Antibody levels of immunoglobulin G (IgG) antibody to CS protein in the high and low spleen rate groups were similar (56.2% and 55.1%) but there was a significant difference in IgM (29.6% and 16.7%). In neither group did antibodies increase with parity (age). In both groups a high level of either IgG or IgM antibody to CS protein was associated with a high spleen rate and women with hyper-reactive malarious splenomegaly were more likely to be positive for both. Lower parasite rates were associated only with increased IgM antibody titres. High levels of antibody to blood-stage parasites were also present in the high spleen rate group, suggesting that antibodies to the CS protein were not protective. It is considered that cell-mediated immunity may be deficient in women with persistent splenomegaly.

Evaluation of an ELISA kit for epidemiological detection of antibodies to Plasmodium falciparum sporozoites in human sera and bloodspot eluates

Antibodies to Plasmodium falciparum sporozoites represent a serological transmission indicator, which can be applied in epidemiological studies to estimate the intensity of malaria transmission. An ELISA method has been developed as an industrial kit to detect these antibodies, using a chemically synthesized (NANP)40 peptide as antigen. The results obtained with this kit are compared in the present paper with those obtained by an ELISA test already applied in epidemiological studies. In testing sera from individuals living in endemic areas, a high diagnostic concordance (92.1%) was obtained between the two assays. The absorbances of these sera correlated well, as shown by a correlation coefficient r = 0.877. Sera from individuals never exposed to malaria gave very low absorbances with the kit. This minimum non-specific binding increases the probability of comparable results in different studies. When the two ELISAs were evaluated for analytical sensitivity and precision, similar satisfactory results were achieved. The test can be performed not only with sera but also with eluates from filterpaper bloodspots. Modifications of the kit to reduce its cost and suggestions regarding distribution and funding are also proposed.

Sporozoite antibodies and malaria in children in a rural area of The Gambia

Sporozoite antibody levels were measured in a group of children aged one to nine years resident in a rural area of The Gambia, using an ELISA to the repeat peptide (NANP)40. The prevalence and titre of antibodies varied with age but not with sex or ethnic group. Significant variations in prevalence were recorded within a group of adjacent villages. Children who were seropositive at the beginning of the dry season had higher spleen and parasite rates both at this time and at the end of the subsequent rainy season than did seronegative children, suggesting that they were exposed more frequently to infection. However, seropositive children had fewer episodes of fever accompanied by high levels of parasitaemia than did seronegative children, suggesting that they had a greater degree of clinical immunity. No differences were found in seroprevalence rates or in mean antibody titres between children who slept under conventional or Permethrin treated bed nets and those who did not, even though bed nets significantly reduced the number of bites by vector mosquitoes.