Host age and expression of genes involved in red blood cell invasion in Plasmodium falciparum field isolates

RTS,S/AS01E immunization increases antibody responses to vaccine-unrelated Plasmodium falciparum antigens associated with protection against clinical malaria in African children: a case-control study

BACKGROUND

Vaccination and naturally acquired immunity against microbial pathogens may have complex interactions that influence disease outcomes. To date, only vaccine-specific immune responses have routinely been investigated in malaria vaccine trials conducted in endemic areas. We hypothesized that RTS,S/A01E immunization affects acquisition of antibodies to Plasmodium falciparum antigens not included in the vaccine and that such responses have an impact on overall malaria protective immunity.

METHODS

We evaluated IgM and IgG responses to 38 P. falciparum proteins putatively involved in naturally acquired immunity to malaria in 195 young children participating in a case-control study nested within the African phase 3 clinical trial of RTS,S/AS01E (MAL055 NCT00866619) in two sites of different transmission intensity (Kintampo high and Manhiça moderate/low). We measured antibody levels by quantitative suspension array technology and applied regression models, multimarker analysis, and machine learning techniques to analyze factors affecting their levels and correlates of protection.

RESULTS

RTS,S/AS01E immunization decreased antibody responses to parasite antigens considered as markers of exposure (MSP1₄₂, AMA1) and levels correlated with risk of clinical malaria over 1-year follow-up. In addition, we show for the first time that RTS,S vaccination increased IgG levels to a specific group of pre-erythrocytic and blood-stage antigens (MSP5, MSP1 block 2, RH4.2, EBA140, and SSP2/TRAP) which levels correlated with protection against clinical malaria (odds ratio [95% confidence interval] 0.53 [0.3-0.93], p = 0.03, for MSP1; 0.52 [0.26-0.98], p = 0.05, for SSP2) in multivariable logistic regression analyses.

CONCLUSIONS

Increased antibody responses to specific P. falciparum antigens in subjects immunized with this partially efficacious vaccine upon natural infection may contribute to overall protective immunity against malaria. Inclusion of such antigens in multivalent constructs could result in more efficacious second-generation multistage vaccines.

Parasite-Related Genetic and Epigenetic Aspects and Host Factors Influencing Plasmodium falciparum Invasion of Erythrocytes

Malaria, a disease caused by Plasmodium parasites, is widespread throughout tropical and sub-tropical regions worldwide; it mostly affects children and pregnant woman. Eradication has stalled despite effective prevention measures and medication being available for this disease; this has mainly been due to the parasite's resistance to medical treatment and the mosquito vector's resistance to insecticides. Tackling such resistance involves using renewed approaches and techniques for accruing a deep understanding of the parasite's biology, and developing new drugs and vaccines. Studying the parasite's invasion of erythrocytes should shed light on its ability to switch between invasion phenotypes related to the expression of gene sets encoding proteins acting as ligands during target cell invasion, thereby conferring mechanisms for evading a particular host's immune response and adapting to changes in target cell surface receptors. This review considers some factors influencing the expression of such phenotypes, such as Plasmodium's genetic, transcriptional and epigenetic characteristics, and explores some host-related aspects which could affect parasite phenotypes, aiming at integrating knowledge regarding this topic and the possible relationship between the parasite's biology and host factors playing a role in erythrocyte invasion.

Assessing naturally acquired immune response and malaria treatment outcomes in Lagos, Nigeria

Background: There are emerging reports of poor efficacy of artemisinin-based combination treatment (ACT). However, mutations on the Kelch-13 gene marking delayed parasite clearance have no clinically defined relationship with ACT resistance across Africa. With increasing malaria control efforts, declining acquired immunity could be responsible for varying drug response profiles that may be dependent on levels of exposure to infections. To examine antibody responses against malaria and the influence on the efficacy of artemether-lumefantrine (AL), plasma samples were collected, prior to treatment, from individuals presenting with uncomplicated malaria. Methods: Participants were stratified into two groups: early (within 24 hours, N = 20) and late (between 48 – 72 hours, N = 30) parasite clearance after treatment, as determined by var gene acidic terminal sequence (varATS) polymerase chain reaction. Magnetic bead-based luminex assay was used to profile antibody responses specific to a panel of 21 Plasmodium falciparum sporozoite, merozoite and An. gambiae salivary antigens. Results: Median fluorescence intensity (MFI) of the antibodies was highest against glutamate-rich protein (GLURP-R0) and lowest against merozoite surface protein (MSP2) antigen. Analysis showed a positive correlation between expression of immunity and age of individuals (P = 0.023). However, there was no association between parasite density and antibody responses, except a significant positive relationship with reticulocyte binding protein-like homologue 5 (Rh5), P = 0.047; Plasmodium exported protein (Hyp2), P = 0.037 and merozoite surface protein 11 (H103), P = 0.038. Though higher levels of antibodies against erythrocyte binding antigens (EBA 140 and 175), MSP1.19, GLURP, circumsporozoite protein (CSP) and Rh4.2 were observed in individuals who recorded early parasite clearance, there was no significant difference in antibody responses in the early and late parasitological response groups. Conclusions: Characterization of additional markers in larger populations is required to reveal potential immunological correlates of drug efficacy.