Association of naturally acquired IgG antibodies against Plasmodium falciparum serine repeat antigen-5 with reduced placental parasitemia and normal birth weight in pregnant Ugandan women: A pilot study

Immune tolerance caused by repeated P. falciparum infection against SE36 malaria vaccine candidate antigen and the resulting limited polymorphism

The call for second generation malaria vaccines needs not only the identification of novel candidate antigens or adjuvants but also a better understanding of immune responses and the underlying protective processes. Plasmodium parasites have evolved a range of strategies to manipulate the host immune system to guarantee survival and establish parasitism. These immune evasion strategies hamper efforts to develop effective malaria vaccines. In the case of a malaria vaccine targeting the N-terminal domain of P. falciparum serine repeat antigen 5 (SE36), now in clinical trials, we observed reduced responsiveness (lowered immunogenicity) which may be attributed to immune tolerance/immune suppression. Here, immunogenicity data and insights into the immune responses to SE36 antigen from epidemiological studies and clinical trials are summarized. Documenting these observations is important to help identify gaps for SE36 continued development and engender hope that highly effective blood-stage/multi-stage vaccines can be achieved.

Plasmodium falciparum infection coinciding with the malaria vaccine candidate BK-SE36 administration interferes with the immune responses in Burkinabe children

Background

A vaccine targeting the erythrocyte stages of Plasmodium falciparum could play a role in preventing clinical disease. BK-SE36 is a promising malaria vaccine candidate that has shown a good safety profile and immunological responses during field evaluations. It was observed that repeated natural infections could result in immune tolerance against SE36 molecule.

Methods

The primary trial was conducted to assess the safety and immunogenicity of the BK-SE36 in two cohorts of children aged 25-60 months (Cohort 1) and 12-24 months (Cohort 2). Immunization was at full dose (1.0 mL) administered at 0, 1, and 6 months. Blood samples were collected before each vaccination for immunological assessments and detection of Plasmodium falciparum infection by microscopy. Blood samples were further collected one month post each vaccination to evaluate immunogenicity.

Results

Of seventy-two (72) subjects that have received BK-SE36 vaccination, 71 had available blood smears during vaccination days. One month post Dose 2, the geometric mean of SE36 antibodies was 263.2 (95% CI: 178.9-387.1) in uninfected individuals compared to 77.1 (95% CI: 47.3-125.7) in infected participants. The same trend was observed one-month post booster dose. Participants uninfected at the time of booster vaccination had significantly higher GMTs compared to those who were infected (424.1 (95% CI: 301.9-595.8) vs. 92.8 (95% CI: 34.9-246.6), p = 0.002. There was a 14.3 (95% CI: 9.7-21.1) and 2.4 (95% CI: 1.3-4.4) fold-change, respectively, in uninfected and infected participants between one-month post Dose 2 and booster. The difference was statistically significant (p < 0.001).

Conclusion

Concomitant infection by P. falciparum during BK-SE36 vaccine candidate administration is associated with reduced humoral responses. However, it is to be noted that the BK-SE36 primary trial was not designed to investigate the influence of concomitant infection on vaccine-induced immune response and should be interpreted cautiously.

Trial registration

WHO ICTRP, PACTR201411000934120.

African-specific polymorphisms in Plasmodium falciparum serine repeat antigen 5 in Uganda and Burkina Faso clinical samples do not interfere with antibody response to BK-SE36 vaccination

BK-SE36, based on Plasmodium falciparum serine repeat antigen 5 (SERA5), is a blood-stage malaria vaccine candidate currently being evaluated in clinical trials. Phase 1 trials in Uganda and Burkina Faso have demonstrated promising safety and immunogenicity profiles. However, the genetic diversity of sera5 in Africa and the role of allele/variant-specific immunity remain a major concern. Here, sequence analyses were done on 226 strains collected from the two clinical trial/follow-up studies and 88 strains from two cross-sectional studies in Africa. Compared to other highly polymorphic vaccine candidate antigens, polymorphisms in sera5 were largely confined to the repeat regions of the gene. Results also confirmed a SERA5 consensus sequence with African-specific polymorphisms. Mismatches with the vaccine-type SE36 (BK-SE36) in the octamer repeat, serine repeat, and flanking regions, and single-nucleotide polymorphisms in non-repeat regions could compromise vaccine response and efficacy. However, the haplotype diversity of SERA5 was similar between vaccinated and control participants. There was no marked bias or difference in the patterns of distribution of the SE36 haplotype and no statistically significant genetic differentiation among parasites infecting BK-SE36 vaccinees and controls. Results indicate that BK-SE36 does not elicit an allele-specific immune response.

Association of Antibodies to VAR2CSA and Merozoite Antigens with Pregnancy Outcomes in Women Living in Yaoundé, Cameroon

Plasmodium falciparum infections are serious in pregnant women, because VAR2CSA allows parasitized erythrocytes to sequester in the placenta, causing placental malaria (PM). In areas of endemicity, women have substantial malarial immunity prior to pregnancy, including antibodies to merozoite antigens, but produce antibodies to VAR2CSA only during pregnancy. The current study sought to determine the importance of antibodies to VAR2CSA and merozoite antigens in pregnant women in Yaoundé, Cameroon, where malaria transmission was relatively low. A total of 1,377 archival plasma samples collected at delivery were selected (at a 1:3 ratio of PM-positive [PM+] to PM-negative [PM-] women) and screened for antibodies to full-length VAR2CSA and 7 merozoite antigens. Results showed that many PM+ women and most PM- women lacked antibodies to VAR2CSA at delivery. Among PM+ women, antibodies to VAR2CSA were associated with a reduced risk of having high placental parasitemia (odds ratio [OR], 0.432; confidence interval [CI], 0.272, 0.687; P = 0.0004) and low-birth-weight (LBW) babies (OR = 0.444; CI, 0.247, 0.799; P = 0.0068), even during first pregnancies. Among antibodies to the 7 merozoite antigens, i.e., AMA1, EBA-175, MSP1₄₂, MSP2, MSP3, MSP11, and Pf41, only antibodies to MSP3, EBA-175, and Pf41 were associated with reduced risk for high placental parasitemias (P = 0.0389, 0.0291, and 0.0211, respectively) and antibodies to EBA-175 were associated with reduced risk of premature deliveries (P = 0.0211). However, after adjusting for multiple comparisons significance declined. Thus, in PM+ women, antibodies to VAR2CSA were associated with lower placental parasitemias and reduced prevalence of LBW babies in this low-transmission setting.

Identification and validation of a novel panel of Plasmodium knowlesi biomarkers of serological exposure

BACKGROUND

Plasmodium knowlesi is the most common cause of malaria in Malaysian Borneo, with reporting limited to clinical cases presenting to health facilities and scarce data on the true extent of transmission. Serological estimations of transmission have been used with other malaria species to garner information about epidemiological patterns. However, there are a distinct lack of suitable serosurveillance tools for this neglected disease.

METHODOLOGY/PRINCIPAL FINDINGS

Using in silico tools, we designed and expressed four novel P. knowlesi protein products to address the distinct lack of suitable serosurveillance tools: PkSERA3 antigens 1 and 2, PkSSP2/TRAP and PkTSERA2 antigen 1. Antibody prevalence to these antigens was determined by ELISA for three time-points post-treatment from a hospital-based clinical treatment trial in Sabah, East Malaysia (n = 97 individuals; 241 total samples for all time points). Higher responses were observed for the PkSERA3 antigen 2 (67%, 65/97) across all time-points (day 0: 36.9% 34/92; day 7: 63.8% 46/72; day 28: 58.4% 45/77) with significant differences between the clinical cases and controls (n = 55, mean plus 3 SD) (day 0 p<0.0001; day 7 p<0.0001; day 28 p<0.0001). Using boosted regression trees, we developed models to classify P. knowlesi exposure (cross-validated AUC 88.9%; IQR 86.1-91.3%) and identified the most predictive antibody responses.

CONCLUSIONS/SIGNIFICANCE

The PkSERA3 antigen 2 had the highest relative variable importance in all models. Further validation of these antigens is underway to determine the specificity of these tools in the context of multi-species infections at the population level.

In silico design of knowledge-based Plasmodium falciparum epitope ensemble vaccines

Malaria is a global health burden, and a major cause of mortality and morbidity in Africa. Here we designed a putative malaria epitope ensemble vaccine by selecting an optimal set of pathogen epitopes. From the IEDB database, 584 experimentally-verified CD8+ epitopes and 483 experimentally-verified CD4+ epitopes were collected; 89% of which were found in 8 proteins. Using the PVS server, highly conserved epitopes were identified from variability analysis of multiple alignments of Plasmodium falciparum protein sequences. The allele-dependent binding of epitopes was then assessed using IEDB analysis tools, from which the population protection coverage of single and combined epitopes was estimated. Ten conserved epitopes from four well-studied antigens were found to have a coverage of 97.9% of the world population: 7 CD8+ T cell epitopes (LLMDCSGSI, FLIFFDLFLV, LLACAGLAYK, TPYAGEPAPF, LLACAGLAY, SLKKNSRSL, and NEVVVKEEY) and 3 CD4+ T cell epitopes (MRKLAILSVSSFLFV, KSKYKLATSVLAGLL and GLAYKFVVPGAATPYE). The addition of four heteroclitic peptides - single point mutated epitopes - increased HLA binding affinity and raised the predicted world population coverage above 99%.

Safety, immunogenicity, and cross-species protection of a plasmid DNA encoding Plasmodium falciparum SERA5 polypeptide, microbial epitopes and chemokine genes in mice and olive baboons

Incorporation of biomolecular epitopes to malarial antigens should be explored in the development of strain-transcending malarial vaccines. The present study sought to determine safety, immunogenicity and cross-species efficacy ofPlasmodium falciparum serine repeat antigen 5 polypeptide co-expressed with epitopes of Bacille-Calmette Guerin (BCG), tetanus toxoid (TT) and a chemokine gene. Olive baboons and BALB/c mice were randomly assigned into vaccine and control groups. The vaccine group animals were primed and boosted twice with pIRES plasmids encoding the SERA5+ BCG+ TT alone, or with either CCL5 or CCL20 and the control group with pIRES plasmid vector backbone. Mice and baboons were challenged withP. berghei ANKA and P. knowlesi H strain parasites, respectively. Safety was determined by observing for injection sites reactogenicities, hematology and clinical chemistry. Parasitaemia and survivorship profiles were used to determine cross-species efficacy, and T cell phenotypes, Th1-, Th2-type, T-regulatory immune responses and antibody responses were assessed to determine vaccine immunogenicity. The pSeBCGTT plasmid DNA vaccines were safe and induced Th1-, Th2-type, and T-regulatory responses vaccinated animals showed enhanced CD4+ (P<0.01), CD 8+ T cells (P<0.001) activation and IgG anti-SE36 antibodies responses (P<0.001) at week 4 and 8 post vaccination compared to the control group. Vaccinated mice had a 31.45-68.69% cumulative parasite load reduction and 60% suppression in baboons (P<0.05) and enhanced survivorship (P<0.001) with no clinical signs of malaria compared to the control group. The results showed that the vaccines were safe, immunogenic and conferred partial cross-species protection.

IgG isotypic antibodies to crude Plasmodium falciparum blood-stage antigen associated with placental malaria infection in parturient Cameroonian women

BACKGROUND

Few studies have reported an association between placental malaria (PM) infection and levels of isotypic antibodies against non-pregnancy associated antigens.

OBJECTIVE

To determine and evaluate IgG isotypic antibody levels to crude P. falciparum blood stage in women with and without PM infection.

METHODS

Levels of IgG (IgG1-IgG4) and IgM to crude P. falciparum blood stage antigen were measured by ELISA in 271 parturient women. Placental malaria infection was determined by placental blood microscopy and placental histology. Age, parity and intermittent preventive treatment during pregnancy with sulphadoxine-pyrimethamine (IPTp-SP) usage were considered during analysis.

RESULTS

P. falciparum-specific IgG1 (96.5%) and IgG3 (96.7%) antibodies were predominant compared with IgG2 (64.6%) and IgG4 (49.1%). Active PM infection was associated with significant increased levels of IgG1, IgG4 and IgM while lower levels of these antibodies were associated with uptake of two or more IPTp-SP doses. PM infection was the only independent factor associated with IgG4 levels. Mean IgG1 + IgG3/IgG2 + IgG4 and IgG1 + IgG2 +IgG3/ IgG4 ratios were higher among the PM-uninfected group while IgG4/IgG2 ratio prevailed in the infected group.

CONCLUSION

PM infection and IPTp-SP dosage influenced P. falciparum-specific isotypic antibody responses to blood stage antigens. An increase in IgG4 levels in response to PM infection is of particular interest.

Association between malaria immunity and pregnancy outcomes among Malawian pregnant women receiving nutrient supplementation

Background

Malaria antibody responses measured at delivery have been associated with protection from maternal anaemia and low birth weight deliveries. Whether malarial antibodies present in the first half of pregnancy may protect from these or other poor birth outcomes is unclear. To determine whether malaria antibodies in the first half of pregnancy predict pregnancy outcomes, antibodies were measured to a range of merozoite antigens and to antigens expressed on the surface of parasitized red blood cells (pRBCs) in plasma samples collected at 14–20 weeks of gestation from Malawian women. The latter antibodies were measured as total IgG to pRBCs, and antibodies promoting opsonic phagocytosis of pRBCs. Associations between antibodies and maternal haemoglobin in late pregnancy or newborn size were investigated, after adjusting for potential covariates.

Results

Antibodies to pRBC surface antigens were associated with higher haemoglobin concentration at 36 weeks. Total IgG to pRBCs was associated with 0.4 g/l [(95% confidence interval (0.04, 0.8)] increase in haemoglobin, and opsonizing antibody with 0.5 (0.05, 0.9) increase in haemoglobin for each 10% increase in antibody. These antibodies were not associated with birthweight, placental malaria, or newborn anthropometrics. Antibodies to merozoite antigens and non-placental-binding IEs were not associated with decreased risk of any of these outcomes. In some instances, they were negatively associated with outcomes of interest.

Conclusion

Antibodies to placental-binding infected erythrocytes may be associated with higher haemoglobin levels in pregnancy, whereas antibodies to other malaria antigens may instead be markers of malaria exposure.

Trial registration clinicaltrials.gov NCT01239693. Registered Nov 10, 2010.