Limited antigenic diversity of Plasmodium falciparum apical membrane antigen 1 supports the development of effective multi-allele vaccines

BackgroundPolymorphism in antigens is a common mechanism for immune evasion used by many important pathogens, and presents major challenges in vaccine development. In malaria, many key immune targets and vaccine candidates show substantial polymorphism. However, knowledge on antigenic diversity of key antigens, the impact of polymorphism on potential vaccine escape, and how sequence polymorphism relates to antigenic differences is very limited, yet crucial for vaccine development. Plasmodium falciparum apical membrane antigen 1 (AMA1) is an important target of naturally-acquired antibodies in malaria immunity and a leading vaccine candidate. However, AMA1 has extensive allelic diversity with more than 60 polymorphic amino acid residues and more than 200 haplotypes in a single population. Therefore, AMA1 serves as an excellent model to assess antigenic diversity in malaria vaccine antigens and the feasibility of multi-allele vaccine approaches. While most previous research has focused on sequence diversity and antibody responses in laboratory animals, little has been done on the cross-reactivity of human antibodies.MethodsWe aimed to determine the extent of antigenic diversity of AMA1, defined by reactivity with human antibodies, and to aid the identification of specific alleles for potential inclusion in a multi-allele vaccine. We developed an approach using a multiple-antigen-competition enzyme-linked immunosorbent assay (ELISA) to examine cross-reactivity of naturally-acquired antibodies in Papua New Guinea and Kenya, and related this to differences in AMA1 sequence.ResultsWe found that adults had greater cross-reactivity of antibodies than children, although the patterns of cross-reactivity to alleles were the same. Patterns of antibody cross-reactivity were very similar between populations (Papua New Guinea and Kenya), and over time. Further, our results show that antigenic diversity of AMA1 alleles is surprisingly restricted, despite extensive sequence polymorphism. Our findings suggest that a combination of three different alleles, if selected appropriately, may be sufficient to cover the majority of antigenic diversity in polymorphic AMA1 antigens. Antigenic properties were not strongly related to existing haplotype groupings based on sequence analysis.ConclusionsAntigenic diversity of AMA1 is limited and a vaccine including a small number of alleles might be sufficient for coverage against naturally-circulating strains, supporting a multi-allele approach for developing polymorphic antigens as malaria vaccines

Quantifying the Importance of MSP1-19 as a Target of Growth-Inhibitory and Protective Antibodies against Plasmodium falciparum in Humans

BACKGROUND: Antibodies targeting blood stage antigens are important in protection against malaria, but the key targets and mechanisms of immunity are not well understood. Merozoite surface protein 1 (MSP1) is an abundant and essential protein. The C-terminal 19 kDa region (MSP1-19) is regarded as a promising vaccine candidate and may also be an important target of immunity. METHODOLOGY/FINDINGS: Growth inhibitory antibodies against asexual-stage parasites and IgG to recombinant MSP1-19 were measured in plasma samples from a longitudinal cohort of 206 children in Papua New Guinea. Differential inhibition by samples of mutant P. falciparum lines that expressed either the P. falciparum or P. chabaudi form of MSP1-19 were used to quantify MSP1-19 specific growth-inhibitory antibodies. The great majority of children had detectable IgG to MSP1-19, and high levels of IgG were significantly associated with a reduced risk of symptomatic P. falciparum malaria during the 6-month follow-up period. However, there was little evidence of PfMSP1-19 specific growth inhibition by plasma samples from children. Similar results were found when testing non-dialysed or dialysed plasma, or purified antibodies, or when measuring growth inhibition in flow cytometry or microscopy-based assays. Rabbit antisera generated by immunization with recombinant MSP1-19 demonstrated strong MSP1-19 specific growth-inhibitory activity, which appeared to be due to much higher antibody levels than human samples; antibody avidity was similar between rabbit antisera and human plasma. CONCLUSIONS/SIGNIFICANCE: These data suggest that MSP1-19 is not a major target of growth inhibitory antibodies and that the protective effects of antibodies to MSP1-19 are not due to growth inhibitory activity, but may instead be mediated by other mechanisms. Alternatively, antibodies to MSP1-19 may act as a marker of protective immunity

Lymphocyte expression of costimulator molecules in early life / Salenna R. Elliott.

Bibliography: leaves 170-206.vi, 206 leaves : ill. (some col.) ; 30 cm.In T-dependent antibody responses, costimulator molecules provide contact-medited signals during interactions between T cells and B cells that regulate lymphocyte activation. This study investigated the hypothesis that costimulator molecules are differentially expressed on lymphocytes from neonates and young children compared with adults, contributing to limitations of T-dependent antibody responses in early life. Results suggest that lymphocytes from young children should be able to deliver and respond to costimulatory signals. The differences in lymphocyte expression of these costimulator molecules in young children are unlikely to fully account for limitations in humoral immunity in early life, and may even represent a specialised adaptation for this stage of immune development.Thesis (Ph.D.)--University of Adelaide, Dept. of Paediatrics, 200

Heterologous Immunity in the Absence of Variant-Specific Antibodies after Exposure to Subpatent Infection with Blood-Stage Malaria

We examined immunity induced by subpatent blood-stage malaria (undetectable by microscopy) using the rodent malaria parasite, Plasmodium chabaudi chabaudi, postulating that limited infection may allow expansion of antigen-specific T cells that are normally deleted by apoptosis. After three infections drug cured at 48 h, mice were protected against high-dose challenge with homologous or heterologous parasites (different strain or variant). Immunity differed from that generated by three untreated, patent infections. Subpatently infected mice lacked immunoglobulin G (IgG) to variant surface antigens, despite producing similar titers of total malaria-specific IgG to those produced by patently infected mice, including antibodies specific for merozoite surface antigens conserved between heterologous strains. Antigen-specific proliferation of splenocytes harvested prechallenge was significantly higher in subpatently infected mice than in patently infected or naive mice. In subpatently infected mice, lymphoproliferation was similar in response to homologous and heterologous parasites, suggesting that antigenic targets of cell-mediated immunity were conserved. A Th1 cytokine response was evident during challenge. Apoptosis of CD4(+) and CD8(+) splenic lymphocytes occurred during patent but not subpatent infection, suggesting a reason for the relative prominence of cell-mediated immunity after subpatent infection. In conclusion, subpatent infection with blood stage malaria parasites induced protective immunity, which differed from that induced by patent infection and targeted conserved antigens. These findings suggest that alternative vaccine strategies based on delivery of multiple parasite antigens at low dose may induce effective immunity targeting conserved determinants

A rapid global review of strategies to improve influenza vaccination uptake in Australia

This study aimed to identify effective strategies for improving the uptake of influenza vaccination and to inform recommendations for influenza vaccination programs in Australia. A rapid systematic review was conducted to assimilate and synthesize peer-reviewed articles identified in PubMed. The National Health and Medical Research Council (NHMRC) Hierarchy of Evidence was used to appraise the quality of evidence. A systematic search identified 4373 articles and 52 that met the inclusion criteria were included. The evidence suggests influenza vaccination uptake may be improved by interventions that (1) increase community/patient demand and access to influenza vaccine and overcome practice-related barriers; (2) reinforce the critical role healthcare providers play in driving influenza vaccination uptake. Strategies such as standing orders, reminder and recall efforts were successful in improving influenza vaccination rates. Community pharmacies, particularly in regional/remote areas, are well positioned to improve influenza vaccine coverage. The findings of this rapid review can be utilized to improve the performance of influenza immunization programs in Australia and other countries with comparable programs; and recommend priorities for future evaluation of interventions to improve influenza vaccination uptake

Analysis of diagnostic data for sexually transmissible infections in South Australian Aboriginal Community Controlled Health Services (2008-16)

Background: Australian Aboriginal communities experience a high burden of sexually transmissible infections (STIs). Since 2009, a comprehensive sexual health program has been implemented at nine Aboriginal Community Controlled Health Services in South Australia. This study assessed trends in STI testing and positivity using deidentified diagnostic data from this period (2008-16). Methods: Testing data for Chlamydia trachomatis (CT), Neisseria gonorrhoeae (NG) and Trichomonas vaginalis (TV) from one urban, three regional and five remote Aboriginal health services were analysed using logistic regression. Results: From 2008 to 2016, testing increased for CT (twofold), NG (threefold) and TV (sixfold). On average, 30% of testing occurred during an annual 6-week screen. Fewer males were tested (range 27-38% annually). Mean annual STI testing coverage was 28% for 16-to 30-year-old clients attending regional or remote services (2013-16). Positivity at first testing episode for all three infections declined during the study period. From 2013 to 2016, when testing was stable and changes in positivity were more likely to indicate changes in prevalence, there were significant reductions in CT positivity (adjusted odds ratio (aOR) 0.4; 95% confidence interval (CI) 0.2-0.5) and TV positivity (aOR 0.6, 95% CI 0.4-0.9), although declines were statistically significant for females only. There was no significant decrease in NG positivity (aOR 0.9; 95% CI 0.5-1.5). Conclusions: Since the sexual health program began, STI testing increased and STI positivity declined, but significant reductions observed in CT and TV positivity were confined to females. These findings suggest evidence of benefit from sustained, comprehensive sexual health programs in Aboriginal communities with a high STI prevalence, but highlight the need to increase STI testing among men in these communities

Serum Lipoproteins Promote Efficient Presentation of the Malaria Virulence Protein PfEMP1 at the Erythrocyte Surface▿

The virulence of the malaria parasite Plasmodium falciparum is related to its ability to express a family of adhesive proteins known as P. falciparum erythrocyte membrane protein 1 (PfEMP1) at the infected red blood cell surface. The mechanism for the transport and delivery of these adhesins to the erythrocyte membrane is only poorly understood. In this work, we have used specific immune reagents in a flow cytometric assay to monitor the effects of serum components on the surface presentation of PfEMP1. We show that efficient presentation of the A4 and VAR2CSA variants of PfEMP1 is dependent on the presence of serum in the bathing medium during parasite maturation. Lipid-loaded albumin supports parasite growth but allows much less efficient presentation of PfEMP1 at the red blood cell surface. Analysis of the serum components reveals that lipoproteins, especially those of the low-density lipoprotein fraction, promote PfEMP1 presentation. Cytoadhesion of infected erythrocytes to the host cell receptors CD36 and ICAM-1 is also decreased in infected erythrocytes cultured in the absence of serum. The defect appears to be in the transfer of PfEMP1 from parasite-derived structures known as the Maurer's clefts to the erythrocyte membrane or in surface conformation rather than a down-regulation or switching of particular PfEMP1 variants

Cross-Reactive Surface Epitopes on Chondroitin Sulfate A-Adherent Plasmodium falciparum-Infected Erythrocytes Are Associated with Transcription of var2csa

Malaria in pregnancy is associated with placental accumulation of Plasmodium falciparum-infected erythrocytes (IE) that adhere to chondroitin sulfate A (CSA). Adhesion is mediated by P. falciparum erythrocyte membrane protein 1 (PfEMP1), a variant parasite protein expressed on the surface of IE and encoded by var genes. Rabbit antiserum was generated against the CSA-adherent P. falciparum line CS2, in which the dominant var transcribed is var2csa, a relatively conserved var gene that has been associated with CSA adhesion. Anti-CS2 recognized genetically distinct CSA-adherent P. falciparum lines but not CD36-adherent parent lines. Reactivity with anti-CS2 correlated with the level of adhesion to CSA. Fluorescence-activated cell sorting according to binding of anti-CS2 showed reactivity was associated with CSA adhesion and transcription of var2csa. These data are consistent with the hypothesis that var2csa encodes a PfEMP1 expressed on the surface of IE, which mediates adhesion to CSA and is relatively conserved between genetically distinct strains of P. falciparum