Development of humoral immunity to severe and uncomplicated malaria in children from different geographical locations

Abstract

Malaria is one of the leading causes of childhood morbidity and mortality, and Plasmodium falciparum infection is responsible for nearly all malaria deaths. Pregnant women and children under 5 years of age are mainly affected by its severe pathological complications. Although a considerable number of previous studies have focused on pregnancy related malaria, there is scarcity on the number of studies in infants and the mechanism of development of naturally acquired immunity to malaria in young children.

Naturally acquired immunity to malaria is developed by the continuous acquisition of antimalarial antibodies in response to exposure which may protect the children against severe complications. Moreover, immunity to severe malaria is attained quite early in life compared to uncomplicated malaria infections which require longer time to get immune to. The mechanisms of developing naturally acquired immunity to malaria in young children remain elusive and there are many factors (e.g. undernutrition, clinical infections etc.) to consider which may influence this acquisition. This thesis aims to understand the influence of nutritional supplements and symptomatic malaria infections on the development of naturally acquired immunity to malaria in infancy, and to identify antibody responses that may protect against severe malaria.

To address these research questions, antibodies to several blood stage merozoite antigens, schizont extract and variant surface antigens were measured by enzyme-linked immunosorbent assays and flow cytometry-based assays on plasma samples from young children residing in Malawi and Papua New Guinea.

The 1st part of the thesis evaluated antibodies in a subset of children enrolled to mothers in iLiNS-DYAD study. In this longitudinal study, children from rural Malawi and their mothers were enrolled in a randomized clinical trial, Antibody levels and seroprevalence were tested at 6 and 18 months of age and compared for nutrient supplementation groups lipid nutrient supplement (LNS), multiple micronutrient supplement (MMN) and iron and folic acid (IFA). The study found no association between intake of additional nutrient supplementation and improved malarial immunity in 6 and 18-month-old children.

Secondly, to understand whether early life exposure to malaria has an impact on the antibody acquisition in infants, weekly follow up was done for children from the same parent study from birth to 18 months. Febrile episodes were reported and confirmed cases of malarial episodes were related to the antibody measurements at 18 months. The results suggested that both antibody levels and seroprevalence to tested antimalarial antigens increased in early childhood following infection. But the levels and seroprevalence of antibodies at 18 months of age did not differ depending on the age of the child at the time of malaria episodes.

Finally, to understand whether the severity of previous malarial infections influence the antibody acquisition in young children, a subset of Papua New Guinean children (0.5 to 10 years) from a case control study were tested for antibody responses to circulating parasites by flow cytometry. Var gene transcription levels by the parasites were also determined by qRTPCR. Homologous boosting of antibody in convalescence was common in children regardless of the severity of infection or blood groups of children. Among children with non-O blood groups, in comparison to children on presentation (acute) with severe malaria, there were broadly higher levels of antibody to IE surface antigens in convalescence from severe malaria and in acute or convalescent samples from uncomplicated malaria. By contrast, in children with blood group O, only convalescent plasma from severe malaria showed higher recognition of tested isolates. These differences may relate to the differential susceptibility of children with blood group O to severe malaria. Several var genes were upregulated in severe malaria including mostly var type A, but also some var types B/A, B and B/C, while none were upregulated in uncomplicated relative to severe malaria. Increased transcription, predominantly of var genes associated with severe malaria in Africa, was common in severe malaria in PNG.

Overall, this thesis provides considerable understanding on the mechanism of acquisition of antimalarial antibodies in young children from birth to infancy and at an older age covering two separate geographical locations from Africa and Oceania. The results suggested that additional nutrient supplementation for both mothers and children didn’t improve the malaria antibody immunity at 6 and 18 months of age in Malawian children. But the subsequent malaria infections lead to the acquisition of antimalarial antibodies in 18 months of age indicating that some of this blood stage antigens might be used as a sero-surveillance tools for biomarkers of malaria exposure. Antibody responses to circulating parasite isolates in some Papua New Guinean children suggested that broad response to severe isolates is acquired in some children with severe malaria. Upregulation of mostly group A var genes were also observed in Papua New Guinean children with severe malaria. Findings from this study can be used as a basis for further clinical studies using larger sample size to provide useful information on the dynamics of naturally acquired immunity to malaria in children.