The effects of varying exposure to malaria transmission on development of antimalarial antibody responses in preschool children. XVI. Asembo Bay Cohort Project

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.

Malaria hotspots defined by clinical malaria, asymptomatic carriage, PCR and vector numbers in a low transmission area on the Kenyan Coast

Background

Targeted malaria control interventions are expected to be cost-effective. Clinical, parasitological and serological markers of malaria transmission have been used to detect malaria transmission hotspots, but few studies have examined the relationship between the different potential markers in low transmission areas. The present study reports on the relationships between clinical, parasitological, serological and entomological markers of malaria transmission in an area of low transmission intensity in Coastal Kenya.

Methods

Longitudinal data collected from 831 children aged 5–17 months, cross-sectional survey data from 800 older children and adults, and entomological survey data collected in Ganze on the Kenyan Coast were used in the present study. The spatial scan statistic test used to detect malaria transmission hotspots was based on incidence of clinical malaria episodes, prevalence of asymptomatic asexual parasites carriage detected by microscopy and polymerase chain reaction (PCR), seroprevalence of antibodies to two Plasmodium falciparum merozoite antigens (AMA1 and MSP1-19) and densities of Anopheles mosquitoes in CDC light-trap catches.

Results

There was considerable overlapping of hotspots by these different markers, but only weak to moderate correlation between parasitological and serological markers. PCR prevalence and seroprevalence of antibodies to AMA1 or MSP1-19 appeared to be more sensitive markers of hotspots at very low transmission intensity.

Conclusion

These findings may support the choice of either serology or PCR as markers in the detection of malaria transmission hotspots for targeted interventions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1260-3) contains supplementary material, which is available to authorized users.

Acquired immune responses to three malaria vaccine candidates and their relationship to invasion inhibition in two populations naturally exposed to malaria

Background

Malaria still represents a major cause of morbidity and mortality predominantly in several developing countries, and remains a priority in many public health programmes. Despite the enormous gains made in control and prevention the development of an effective vaccine represents a persisting challenge. Although several parasite antigens including pre-erythrocytic antigens and blood stage antigens have been thoroughly investigated, the identification of solid immune correlates of protection against infection by Plasmodium falciparum or clinical malaria remains a major hurdle. In this study, an immuno-epidemiological survey was carried out between two populations naturally exposed to P. falciparum malaria to determine the immune correlates of protection.

Methods

Plasma samples of immune adults from two countries (Ghana and Madagascar) were tested for their reactivity against the merozoite surface proteins MSP1-19, MSP3 and AMA1 by ELISA. The antigens had been selected on the basis of cumulative evidence of their role in anti-malarial immunity. Additionally, reactivity against crude P. falciparum lysate was investigated. Purified IgG from these samples were furthermore tested in an invasion inhibition assay for their antiparasitic activity.

Results

Significant intra- and inter- population variation of the reactivity of the samples to the tested antigens were found, as well as a significant positive correlation between MSP1-19 reactivity and invasion inhibition (p < 0.05). Interestingly, male donors showed a significantly higher antibody response to all tested antigens than their female counterparts. In vitro invasion inhibition assays comparing the purified antibodies from the donors from Ghana and Madagascar did not show any statistically significant difference. Although in vitro invasion inhibition increased with breadth of antibody response, the increase was not statistically significant.

Conclusions

The findings support the fact that the development of semi-immunity to malaria is probably contingent on the development of antibodies to not only one, but a range of antigens and that invasion inhibition in immune adults may be a function of antibodies to various antigens. This supports strategies of vaccination including multicomponent vaccines as well as passive vaccination strategies with antibody cocktails.

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.

Plasmodium falciparum malaria in the Peruvian Amazon, a region of low transmission, is associated with immunologic memory

The development of clinical immunity to Plasmodium falciparum malaria is thought to require years of parasite exposure, a delay often attributed to difficulties in developing protective antibody levels. In this study, we evaluated several P. falciparum vaccine candidate antigens, including apical membrane antigen 1 (AMA-1), circumsporozoite protein (CSP), erythrocyte binding antigen 175 (EBA-175), and the 19-kDa region of merozoite surface protein 1 (MSP1(19)). After observing a more robust antibody response to MSP1(19), we evaluated the magnitude and longevity of IgG responses specific to this antigen in Peruvian adults and children before, during, and after P. falciparum infection. In this low-transmission region, even one reported prior infection was sufficient to produce a positive anti-MSP1(19) IgG response for >5 months in the absence of reinfection. We also observed an expansion of the total plasmablast (CD19(+) CD27(+) CD38(high)) population in the majority of individuals shortly after infection and detected MSP1-specific memory B cells in a subset of individuals at various postinfection time points. This evidence supports our hypothesis that effective antimalaria humoral immunity can develop in low-transmission regions.

Effect of transmission reduction by insecticide-treated bednets (ITNs) on antimalarial drug resistance in western Kenya

Despite the clear public health benefit of insecticide-treated bednets (ITNs), the impact of malaria transmission-reduction by vector control on the spread of drug resistance is not well understood. In the present study, the effect of sustained transmission reduction by ITNs on the prevalence of Plasmodium falciparum gene mutations associated with resistance to the antimalarial drugs sulfadoxine-pyrimethamine (SP) and chloroquine (CQ) in children under the age of five years was investigated during an ITN trial in Asembo area, western Kenya. During the ITN trial, the national first line antimalarial treatment changed from CQ to SP. Smear-positive samples collected from cross sectional surveys prior to ITN introduction (baseline, n = 250) and five years post-ITN intervention (year 5 survey, n = 242) were genotyped for single nucleotide polymorphisms (SNPs) at dhfr-51, 59, 108, 164 and dhps-437, 540 (SP resistance), and pfcrt-76 and pfmdr1-86 (CQ resistance). The association between the drug resistance mutations and epidemiological variables was evaluated. There were significant increases in the prevalence of SP dhps mutations and the dhfr/dhps quintuple mutant, and a significant reduction in the proportion of mixed infections detected at dhfr-51, 59 and dhps-437, 540 SNPs from baseline to the year 5 survey. There was no change in the high prevalence of pfcrt-76 and pfmdr1-86 mutations. Multivariable regression analysis further showed that current antifolate use and year of survey were significantly associated with more SP drug resistance mutations. These results suggest that increased antifolate drug use due to drug policy change likely led to the high prevalence of SP mutations 5 years post-ITN intervention and reduced transmission had no apparent effect on the existing high prevalence of CQ mutations. There is no evidence from the current study that sustained transmission reduction by ITNs reduces the prevalence of genes associated with malaria drug resistance.

Effects of transmission reduction by insecticide-treated bed nets (ITNs) on parasite genetics population structure: I. The genetic diversity of Plasmodium falciparum parasites by microsatellite markers in western Kenya

Background

Insecticide-treated bed nets (ITNs) reduce malaria transmission and are an important prevention tool. However, there are still information gaps on how the reduction in malaria transmission by ITNs affects parasite genetics population structure. This study examined the relationship between transmission reduction from ITN use and the population genetic diversity of Plasmodium falciparum in an area of high ITN coverage in western Kenya.

Methods

Parasite genetic diversity was assessed by scoring eight single copy neutral multilocus microsatellite (MS) markers in samples collected from P. falciparum-infected children (< five years) before introduction of ITNs (1996, baseline, n = 69) and five years after intervention (2001, follow-up, n = 74).

Results

There were no significant changes in overall high mixed infections and unbiased expected heterozygosity between baseline (%M_A = 94% and H_e = 0.75) and follow up (%M_A = 95% and H_e = 0.79) years. However, locus specific analysis detected significant differences for some individual loci between the two time points. Pfg377 loci, a gametocyte-specific MS marker showed significant increase in mixed infections and H_e in the follow up survey (%M_A = 53% and H_e = 0.57) compared to the baseline (%M_A = 30% and H_e = 0.29). An opposite trend was observed in the erythrocyte binding protein (EBP) MS marker. There was moderate genetic differentiation at the Pfg377 and TAA60 loci (F_ST = 0.117 and 0.137 respectively) between the baseline and post-ITN parasite populations. Further analysis revealed linkage disequilibrium (LD) of the microsatellites in the baseline (14 significant pair-wise tests and I^S_A = 0.016) that was broken in the follow up parasite population (6 significant pairs and I^S_A = 0.0003). The locus specific change in H_e, the moderate population differentiation and break in LD between the baseline and follow up years suggest an underlying change in population sub-structure despite the stability in the overall genetic diversity and multiple infection levels.

Conclusions

The results from this study suggest that although P. falciparum population maintained an overall stability in genetic diversity after five years of high ITN coverage, there was significant locus specific change associated with gametocytes, marking these for further investigation.

Temporal stability of naturally acquired immunity to Merozoite Surface Protein-1 in Kenyan adults

Background

Naturally acquired immunity to blood-stage Plasmodium falciparum infection develops with age and after repeated infections. In order to identify immune surrogates that can inform vaccine trials conducted in malaria endemic populations and to better understand the basis of naturally acquired immunity it is important to appreciate the temporal stability of cellular and humoral immune responses to malaria antigens.

Methods

Blood samples from 16 adults living in a malaria holoendemic region of western Kenya were obtained at six time points over the course of 9 months. T cell immunity to the 42 kDa C-terminal fragment of Merozoite Surface Protein-1 (MSP-1₄₂) was determined by IFN-γ ELISPOT. Antibodies to the 42 kDa and 19 kDa C-terminal fragments of MSP-1 were determined by serology and by functional assays that measure MSP-1₁₉ invasion inhibition antibodies (IIA) to the E-TSR (3D7) allele and growth inhibitory activity (GIA). The haplotype of MSP-1₁₉ alleles circulating in the population was determined by PCR. The kappa test of agreement was used to determine stability of immunity over the specified time intervals of 3 weeks, 6 weeks, 6 months, and 9 months.

Results

MSP-1 IgG antibodies determined by serology were most consistent over time, followed by MSP-1 specific T cell IFN-γ responses and GIA. MSP-1₁₉ IIA showed the least stability over time. However, the level of MSP-1₁₉ specific IIA correlated with relatively higher rainfall and higher prevalence of P. falciparum infection with the MSP-1₁₉ E-TSR haplotype.

Conclusion

Variation in the stability of cellular and humoral immune responses to P. falciparum blood stage antigens needs to be considered when interpreting the significance of these measurements as immune endpoints in residents of malaria endemic regions.

Antibody response dynamics to the Plasmodium falciparum conserved vaccine candidate antigen, merozoite surface protein-1 C-terminal 19kD (MSP1-19kD), in Peruvians exposed to hypoendemic malaria transmission

Background

In high-transmission areas, developing immunity to symptomatic Plasmodium falciparum infections requires 2–10 years of uninterrupted exposure. Delayed malaria-immunity has been attributed to difficult-to-develop and then short-lived antibody responses.

Methods

In a study area with <0.5 P. falciparum infections/person/year, antibody responses to the MSP1-19kD antigen were evaluated and associations with P. falciparum infections in children and adults. In months surrounding and during the malaria seasons of 2003–2004, 1,772 participants received ≥6 active visits in one study-year. Community-wide surveys were conducted at the beginning and end of each malaria season, and weekly active visits were completed for randomly-selected individuals each month. There were 79 P. falciparum infections with serum samples collected during and approximately one month before and after infection. Anti-MSP1-19kD IgG levels were measured by ELISA.

Results

The infection prevalence during February-July was similar in children (0.02–0.12 infections/person/month) and adults (0.03–0.14 infections/person/month) and was negligible in the four-month dry season. In children and adults, the seroprevalence was maintained in the beginning (children = 28.9%, adults = 61.8%) versus ending malaria-season community survey (children = 26.7%, adults = 64.6%). Despite the four-month non-transmission season, the IgG levels in Plasmodium-negative adults were similar to P. falciparum-positive adults. Although children frequently responded upon infection, the transition from a negative/low level before infection to a high level during/after infection was slower in children. Adults and children IgG-positive before infection had reduced symptoms and parasite density.

Conclusion

Individuals in low transmission areas can rapidly develop and maintain αMSP1-19kD IgG responses for >4 months, unlike responses reported in high transmission study areas. A greater immune capacity might contribute to the frequent asymptomatic P. falciparum infections in this Peruvian population.

Impact of intermittent preventive treatment with sulfadoxine-pyrimethamine on antibody responses to erythrocytic-stage Plasmodium falciparum antigens in infants in Mozambique

We evaluated the impact of intermittent preventive treatment in infants (IPTi) with sulfadoxine-pyrimethamine (SP), which was given at ages 3, 4, and 9 months through the Expanded Program on Immunization (EPI), on the development of antibody responses to Plasmodium falciparum in Mozambique. Immunoglobulin M (IgM) and IgG subclass antibodies specific to whole asexual parasites and to recombinant MSP-1(19), AMA-1, and EBA-175 were measured at ages 5, 9, 12, and 24 months for 302 children by immunofluorescence antibody tests and by enzyme-linked immunosorbent assays. Antibody responses did not significantly differ between children receiving IPTi with SP and those receiving a placebo at any time point measured, with the exception of the responses of IgG and IgG1 to AMA-1 and/or MSP-1(19), which were significantly higher in the SP-treated group than in the placebo group at ages 5, 9, and/or 24 months. IPTi with SP given through the EPI reduces the frequency of malarial illness while allowing the development of naturally acquired antibody responses to P. falciparum antigens.

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.

An elaborated feeding cycle model for reductions in vectorial capacity of night-biting mosquitoes by insecticide-treated nets

Background

Insecticide Treated Nets (ITNs) are an important tool for malaria control. ITNs are effective because they work on several parts of the mosquito feeding cycle, including both adult killing and repelling effects.

Methods

Using an elaborated description of the classic feeding cycle model, simple formulas have been derived to describe how ITNs change mosquito behaviour and the intensity of malaria transmission, as summarized by vectorial capacity and EIR. The predicted changes are illustrated as a function of the frequency of ITN use for four different vector populations using parameter estimates from the literature.

Results

The model demonstrates that ITNs simultaneously reduce mosquitoes' lifespans, lengthen the feeding cycle, and by discouraging human biting divert more bites onto non-human hosts. ITNs can substantially reduce vectorial capacity through small changes to all of these quantities. The total reductions in vectorial capacity differ, moreover, depending on baseline behavior in the absence of ITNs. Reductions in lifespan and vectorial capacity are strongest for vector species with high baseline survival. Anthropophilic and zoophilic species are affected differently by ITNs; the feeding cycle is lengthened more for anthrophilic species, and the proportion of bites that are diverted onto non-human hosts is higher for zoophilic species.

Conclusion

This model suggests that the efficacy of ITNs should be measured as a total reduction in transmission intensity, and that the quantitative effects will differ by species and by transmission intensity. At very high rates of ITN use, ITNs can generate large reductions in transmission intensity that could provide very large reductions in transmission intensity, and effective malaria control in some areas, especially when used in combination with other control measures. At high EIR, ITNs will probably not substantially reduce the parasite rate, but when transmission intensity is low, reductions in vectorial capacity combine with reductions in the parasite rate to generate very large reductions in EIR.

Clustered local transmission and asymptomatic Plasmodium falciparum and Plasmodium vivax malaria infections in a recently emerged, hypoendemic Peruvian Amazon community

Background

There is a low incidence of malaria in Iquitos, Peru, suburbs detected by passive case-detection. This low incidence might be attributable to infections clustered in some households/regions and/or undetected asymptomatic infections.

Methods

Passive case-detection (PCD) during the malaria season (February-July) and an active case-detection (ACD) community-wide survey (March) surveyed 1,907 persons. Each month, April-July, 100-metre at-risk zones were defined by location of Plasmodium falciparum infections in the previous month. Longitudinal ACD and PCD (ACP+PCD) occurred within at-risk zones, where 137 houses (573 persons) were randomly selected as sentinels, each with one month of weekly active sampling. Entomological captures were conducted in the sentinel houses.

Results

The PCD incidence was 0.03 P. falciparum and 0.22 Plasmodium vivax infections/person/malaria-season. However, the ACD+PCD prevalence was 0.13 and 0.39, respectively. One explanation for this 4.33 and 1.77-fold increase, respectively, was infection clustering within at-risk zones and contiguous households. Clustering makes PCD, generalized to the entire population, artificially low. Another attributable-factor was that only 41% and 24% of the P. falciparum and P. vivax infections were associated with fever and 80% of the asymptomatic infections had low-density or absent parasitaemias the following week. After accounting for asymptomatic infections, a 2.6-fold increase in ACD+PCD versus PCD was attributable to clustered transmission in at-risk zones.

Conclusion

Even in low transmission, there are frequent highly-clustered asymptomatic infections, making PCD an inadequate measure of incidence. These findings support a strategy of concentrating ACD and insecticide campaigns in houses adjacent to houses were malaria was detected one month prior.

Modeling the development of acquired clinical immunity to Plasmodium falciparum malaria

Individuals living in regions where malaria is endemic develop an acquired immunity to malaria which enables them to remain asymptomatic while still carrying parasites. Field studies indicate that cumulative exposure to a variety of diverse Plasmodium parasites is required for the transition from symptomatic to asymptomatic malaria. This study used a simulation model of the within-host dynamics of P. falciparum to investigate the development of acquired clinical immunity under different transmission conditions and levels of parasite diversity. Antibodies developed to P. falciparum erythrocyte membrane protein 1 (PfEMP1), a clonally variant molecule, were assumed to be a key human immunological response to P. falciparum infection, along with responses to clonally conserved but polymorphic antigens. The time to the development of clinical immunity was found to be proportional to parasite diversity and inversely proportional to transmission intensity. The effect of early termination of symptomatic infections by chemotherapy was investigated and found not to inhibit the host's ability to develop acquired immunity. However, the time required to achieve this state was approximately double that compared to when no treatment was administered. This study demonstrates that an immune response primarily targeted against PfEMP1 has the ability to reduce clinical symptoms of infections irrespective of whether treatment is administered, supporting its role in the development of acquired clinical immunity. The results also illustrate a novel use for simulation models of P. falciparum infections, investigation of the influence of intervention strategies on the development of naturally acquired clinical immunity.