Naturally acquired immunity to plasmodium falciparum malaria : antibody responses and immunological memory

Plasmodium falciparum malaria is a significant public health concern particularly in Sub-Saharan Africa. Effective malaria vaccines will contribute significantly towards controlling the disease but their development is hampered by the incomplete understanding of immunity to malaria. Whereas naturally acquired immunity is known to have an important antibody mediated component, the targets and functional correlates as well longevity of these responses are largely unknown and merit further understanding. The studies presented in this thesis, investigate several aspects of naturally acquired immunity to some of the major merozoite vaccine candidate antigens. In longitudinally monitored children in Tanzania, antibody responses against seven merozoite surface antigens were investigated in relation to the genetic diversity of P. falciparum infections, as determined by genotyping of one of the merozoite surface protein genes. The breadth of anti-merozoite antibody responses was positively correlated with the number of concurrent P. falciparum clones in asymptomatic children. Further, broad antibody responses and genetically diverse infections, in combination, were more strongly associated with protection against malaria than they were individually suggesting that multicomponent malaria vaccines mimicking naturally acquired immunity should ideally induce antibody responses that can be boosted by natural infections. The inhibitory activity of naturally acquired antibodies on the in vitro growth of P. falciparum in relation to merozoite invasion phenotype was investigated in a case-control study in Tanzanian children. The growth-inhibitory activities (GIA) of plasma were different when tested on different parasite lines. The association between GIA and protection against clinical malaria was also parasite line-dependent thus emphasizing the importance of invasion phenotypes as well as the need to consider the choice of parasite lines in the use of GIA as a correlate of protection against clinical malaria in epidemiological and vaccine studies. Within a longitudinally monitored population in Kenya, temporal dynamics of anti-merozoite antibody responses were investigated in children with different susceptibilities to malaria. Overall, antibody levels were similar in children experiencing multiple episodes or only single episodes suggesting that differences in disease susceptibility are not attributable to differences in the acquisition of anti-merozoite antibody responses, and may be explained by other factors, such as differences in the intensity of exposure to the parasite in this setting of low-moderate malaria transmission. To investigate the longevity of immune responses induced by natural P. falciparum infections, circulating merozoite antigen-specific antibodies and memory B-cells (MBCs) were studied in travelers who had been diagnosed and treated for malaria in Stockholm 1-16 years previously. P. falciparum-specific MBCs, but not antibodies, were found to have been maintained for up to 16 years without re-exposure to the parasite. In conclusion, single natural P. falciparum infections induce long-lived memory-B cell responses to merozoite antigens, however, broad and protective antibody responses require repeated exposure and preferably persistent genetically diverse infections to confer clinical immunity to malaria. Taken together, these studies advance the understanding of naturally acquired immunity to malaria and have important implications for the development of malaria vaccines

Artemether-Lumefantrine Treatment Failure despite adequate lumefantrine day 7 Concentration in a Traveller with Plasmodium Falciparum Malaria after Returning from Tanzania.

Artemether-lumefantrine is currently first-line therapy of Plasmodium falciparum malaria in many countries. This report describes a treatment failure despite adequate drug concentrations in a traveller returning from sub-Saharan Africa. Genotyping confirmed recrudescence and suggested reduced sensitivity. Potential sub-optimal effect of artemether-lumefantrine highlights the need to follow non-immune individuals the weeks after treatment

Antibody acquisition models: a new tool for serological surveillance of malaria transmission intensity

Serology has become an increasingly important tool for the surveillance of a wide range of infectious diseases. It has been particularly useful to monitor malaria transmission in elimination settings where existing metrics such as parasite prevalence and incidence of clinical cases are less sensitive. Seroconversion rates, based on antibody prevalence to Plasmodium falciparum asexual blood-stage antigens, provide estimates of transmission intensity that correlate with entomological inoculation rates but lack precision in settings where seroprevalence is still high. Here we present a new and widely applicable method, based on cross-sectional data on individual antibody levels. We evaluate its use as a sero-surveillance tool in a Tanzanian setting with declining malaria prevalence. We find that the newly developed mathematical models produce more precise estimates of transmission patterns, are robust in high transmission settings and when sample sizes are small, and provide a powerful tool for serological evaluation of malaria transmission intensity

Completeness of malaria indicator data reporting via the District Health Information Software 2 in Kenya, 2011–2015

Abstract Background Health facility-based data reported through routine health information systems form the primary data source for programmatic monitoring and evaluation in most developing countries. The adoption of District Health Information Software (DHIS2) has contributed to improved availability of routine health facility-based data in many low-income countries. An assessment of malaria indicators data reported by health facilities in Kenya during the first 5 years of implementation of DHIS2, from January 2011 to December 2015, was conducted. Methods Data on 19 malaria indicators reported monthly by health facilities were extracted from the online Kenya DHIS2 database. Completeness of reporting was analysed for each of the 19 malaria indicators and expressed as the percentage of data values actually reported over the expected number; all health facilities were expected to report data for each indicator for all 12 months in a year. Results Malaria indicators data were analysed for 6235 public and 3143 private health facilities. Between 2011 and 2015, completeness of reporting in the public sector increased significantly for confirmed malaria cases across all age categories (26.5–41.9%, p < 0.0001, in children aged <5 years; 30.6–51.4%, p < 0.0001, in persons aged ≥5 years). Completeness of reporting of new antenatal care (ANC) clients increased from 53.7 to 70.5%, p < 0.0001). Completeness of reporting of intermittent preventive treatment in pregnancy (IPTp) decreased from 64.8 to 53.7%, p < 0.0001 for dose 1 and from 64.6 to 53.4%, p < 0.0001 for dose 2. Data on malaria tests performed and test results were not available in DHIS2 from 2011 to 2014. In 2015, sparse data on microscopy (11.5% for children aged <5 years; 11.8% for persons aged ≥5 years) and malaria rapid diagnostic tests (RDTs) (8.1% for all ages) were reported. In the private sector, completeness of reporting increased significantly for confirmed malaria cases across all age categories (16.7–23.1%, p < 0.0001, in children aged <5 years; 19.4–28.6%, p < 0.0001, in persons aged ≥5 years). Completeness of reporting also improved for new ANC clients (16.2–23.6%, p < 0.0001), and for IPTp doses 1 and 2 (16.6–20.2%, p < 0.0001 and 15.5–20.5%, p < 0.0001, respectively). In 2015, less than 3% of data values for malaria tests performed were reported in DHIS2 from the private sector. Conclusions There have been sustained improvements in the completeness of data reported for most key malaria indicators since the adoption of DHIS2 in Kenya in 2011. However, major data gaps were identified for the malaria-test indicator and overall low reporting across all indicators from private health facilities. A package of proven DHIS2 implementation interventions and performance-based incentives should be considered to improve private-sector data reporting

In Vitro Activities of Quinine and Other Antimalarials and pfnhe Polymorphisms in Plasmodium Isolates from Kenya▿

Resistance to the amino alcohol quinine has been associated with polymorphisms in pfnhe, a sodium hydrogen exchanger. We investigated the role of this gene in quinine resistance in vitro in isolates from Kenya. We analyzed pfnhe whole-gene polymorphisms, using capillary sequencing, and pfcrt at codon 76 (pfcrt-76) and pfmdr1 at codon 86 (pfmdr1-86), using PCR-enzyme restriction methodology, in 29 isolates from Kilifi, Kenya, for association with the in vitro activities of quinine and 2 amino alcohols, mefloquine and halofantrine. In vitro activity was assessed as the drug concentration that inhibits 50% of parasite growth (IC50). The median IC50s of quinine, halofantrine, and mefloquine were 92, 22, and 18 nM, respectively. The presence of 2 DNNND repeats in microsatellite ms4760 of pfnhe was associated with reduced susceptibility to quinine (60 versus 227 nM for 1 and 2 repeats, respectively; P < 0.05), while 3 repeats were associated with restoration of susceptibility. The decrease in susceptibility conferred by the 2 DNNND repeats was more pronounced in parasites harboring the pfmdr1-86 mutation. No association was found between susceptibility to quinine and the pfcrt-76 mutation or between susceptibility to mefloquine or halofantrine and the pfnhe gene and the pfcrt-76 and pfmdr1-86 mutations. Using previously published data on the in vitro activities of chloroquine, lumefantrine, piperaquine, and dihydroartemisinin, we investigated the association of their activities with pfnhe polymorphism. With the exception of a modulation of the activity of lumefantrine by a mutation at position 1437, pfnhe did not modulate their activities. Two DNNND repeats combined with the pfmdr1-86 mutation could be used as an indicator of reduced susceptibility to quinine

Risk of severe malaria associated with parasite positivity, number of clones and antibodies to schizont extract in three-monthly visits before admission.

<p>Conditional logistic regression adjusted for log number of visits including 42 risk sets where the case was no older than 2 years and 3 months at the time of severe malaria episode and thus had samples until three months before the admission^a.</p><p>a. excluding 13 risk sets with children with longer time since their last visit (4–47 months).</p><p>b. adjusted for log number of visits in the period before the severe admission of the cases and respective periods in the controls.</p><p>c. including PCR and microscopy results.</p><p>d. including only risk sets where PCR results were available (n = 41).</p><p>e. antibodies to schizont extract detected in at least one of the three most recent visits before admission in the cases and corresponding time in the controls, including 40 risk sets where samples for antibody analysis were available.</p

Characteristics of the study population.

<p>a. age at case admission of the cases and respective controls.</p><p>b. restricted to the 42 risk sets where case was younger than 2 years and 3 months at the severe malaria admission and thus had complete follow up periods before admission,</p><p>c. including PCR and microscopy results.</p><p>d. antibodies to schizont extract detected in at least one of the three most recent visits before admission in the cases and corresponding time in the controls.</p

Risk of impaired consciousness in relation to parasite exposure in three-monthly samples among 61 cases with severe malaria.

<p>Unmatched case-control analysis performed with exact logistic regression adjusted for age and number of samples, defining patients with impaired consciousness as “cases” (n = 32) and those with non-cerebral severe malaria as “controls” (n = 29).</p><p>a. including PCR and/or microscopy results.</p><p>b. including only PCR results.</p><p>c. antibodies to schizont extract detected in at least one of the three most recent visits before admission in the cases and corresponding time in the controls.</p

Syndromes overlap and mortality among the 61 cohort children admitted with severe malaria to Kilifi District Hospital.

<p>Four children died (marked *).</p

Matched case-control study profile.

<p>Matched case-control study profile.</p