Dr
Francis Ndungu
Current Organisation
KEMRI - Wellcome Trust Research Prgramme
Current Job Title
Group Leader
Biography
Publications
Iron acquisition is critical for life. Ferroportin (FPN) exports iron from mature erythrocytes, and deletion of the Fpn gene results in hemolytic anemia and increased fatality in malaria-infected mice. The FPN Q248H mutation (glutamine to histidine at position 248) renders FPN partially resistant to hepcidin-induced degradation and was associated with protection from malaria in human studies of limited size. Using data from cohorts including over 18,000 African children, we show that the Q248H mutation is associated with modest protection against anemia, hemolysis, and iron deficiency, but we found little evidence of protection against severe malaria or bacteremia. We additionally observed no excess Plasmodium growth in Q248H erythrocytes ex vivo, nor evidence of selection driven by malaria exposure, suggesting that the Q248H mutation does not protect from malaria and is unlikely to deprive malaria parasites of iron essential for their growth.
B-cell and antibody responses to Plasmodium spp., the parasite that causes malaria, are critical for control of parasitemia and associated immunopathology. Antibodies also provide protection to reinfection. Long-lasting B-cell memory has been shown to occur in response to Plasmodium spp. in experimental model infections, and in human malaria. However, there are reports that antibody responses to several malaria antigens in young children living with malaria are not similarly long-lived, suggesting a dysfunction in the maintenance of circulating antibodies. Some studies attribute this to the expansion of atypical memory B cells (AMB), which express multiple inhibitory receptors and activation markers, and are hyporesponsive to B-cell receptor (BCR) restimulation in vitro. AMB are also expanded in other chronic infections such as tuberculosis, hepatitis B and C, and HIV, as well as in autoimmunity and old age, highlighting the importance of understanding their role in immunity. Whether AMB are dysfunctional remains controversial, as there are also studies in other infections showing that AMB can produce isotype-switched antibodies and in mouse can contribute to protection against infection. In light of these controversies, we review the most recent literature on either side of the debate and challenge some of the currently held views regarding B-cell responses to Plasmodium infections.
Project Title
Determining Correlates of Naturally Acquired Pre-Erythrocytic Immunity to Plasmodium falciparum Malaria in an Experimental Human Challenge Model (CoNAIPS)
EDCTP Project
TMA2016SF1513
EDCTP Program
EDCTP2
EDCTP Project Call
Senior Fellowship (SF)
Host Organisation
| Department | Institution | Country |
|---|---|---|
| Health Research Operations Kenya Limited | Health Research Operations Kenya Limited | KE |
Project Objectives
Hypothesis There are measurable levels and qualities of the Pf specific sporozoite and liver stage immune responses before the administration of live sporozoites during CHMI in CIA adults, which are associated with resistance to establishment of blood-stage infection following CHMI. Objectives 1) To characterize in detail Pf-pre-erythrocytic stage specific T cell responses prior to initiation of CHMI of CIA and non-CIA adults with intravenous injection of live sporozoites. 2) To comprehensively screen for Pf pre-erythrocytic stage antigen specific antibodies prior to initiation of CHMI in CIA and non-CIA adults with intravenous injection of live sporozoites. 3) To determine levels of inhibition of liver stage Pf development, in vitro, by serum antibodies taken prior to the initiation of CHMI in CIA and non-CIA adults with intravenous injection of live sporozoites (in vitro). 4) To compare the baseline levels and qualities of responses measured in 1), 2), and 3) between CIA and non-CIA adults.
Study Design
Study Design This is a prospective study correlating the pre-CHMI immune response to sporozoite and liver stage antigens with the complete resistance to the development of blood-stage infection following the experimental exposure of immune adults to live sporozoites. Controlled Human Malaria Infections (CHMI): Through a Wellcome Trust Funded strategic award, up to 200 healthy adults will undergo experimental malaria challenge. Participants will be recruited from areas with varying malaria endemicity in Kenya and will be screened for red blood cell abnormalities such as sickle cell, which might reduce parasite multiplication rates (40, 41). These individuals will be inoculated with 3200 Pf sporozoites via direct venous injection, which will lead to a blood-stage malaria infection after 7 days of incubation in the liver. Participants will then be admitted to a facility in Pwani University for the duration of the challenge. Daily blood tests will be done to monitor the density of the infection and anti-malarial treatment will be given either: a) when the density of infection rises to a threshold of 500 parasites per microlitre (a threshold substantially lower than 2,500 parasites per microlitre, at which clinical illness becomes more common in children in Kenya) (42); b) if a volunteer develops symptoms or signs of illness defined in our clinical monitoring procedure and an immediate blood film examination shows evidence of detectable malaria parasites; or c) the volunteer reaches day 21 of monitoring. We will monitor anti-malaria treatment and confirm clinical and parasitological cure before discharging the participant. Sampling: Venous blood samples for cellular and serological studies will be taken pre-CHMI (a day before challenge) and at 5, 7, 9, 14, and day of diagnosis or 21 days of CHMI, with 30 mls drawn per time point. PBMCs will be separated immediately after venesection and stored in liquid nitrogen, while serum aliquots will be snap frozen and stored at
Project Summary
Although two malaria vaccines were approved by WHO for use in infants during this study, malaria remains a major public health problem affecting the health and economic wellbeing of over 50% of sub-Saharan Africa’s population. However, the efficacies for these vaccines fall below WHO’s target and have only been approved for infants – as their immunity and efficacy among older age groups is much lower. Thus, whilst the new vaccines will advance control, malaria will remain a public health challenge, in the foreseeable future. The development of the next generation of better malaria vaccines would greatly benefit from a better understanding of the mechanisms that mediate naturally acquired immunity in adults with life-long exposure. Understanding naturally acquired immunity from field studies of malaria is very difficult as different study participants get infected at different times, with different parasite strains and with different doses of the parasite. In this study, we used experimental medicine, where adult participants with different levels of prior exposure, are deliberately infected with the same strain and dose of malaria parasites in a controlled and safe manner – and then, we studied their immune response to that experimental infection. Using this approach, we identified measurable immune responses to components of the P. falciparum parasite from the pre-erythrocytic and erythrocytic stages of its life cycle. We found that study volunteers who controlled the malaria infection had higher levels of antibodies before and after the experimental infection. We also identified highly immune individuals who have antibodies that block invasion of hepatic cells in cell cultures. We will now develop monoclonal antibodies these individuals and test their utility for passive immunisation. The data on the T cell responses associated with immunity and the antibody levels is still being analysed, after which we will publish a paper describing differences in the immune response of those will ability to control malaria and those who don’t. During the project, two MSC and one PhD students were successfully trained in immunology and research methods. The Senior Fellowship (SF) leading the study was appointed an Associate Professor of the University of Oxford, UK, increasing his impact in the development of science in the continent. Besides the publications of the three theses, we have so far published two peer reviewed articles with the EDCTP support, demonstrating that the anti-CS IgG antibodies induced by the RTS,S vaccine are long-lasting, though their levels are not boosted by natural exposure (https://pubmed.ncbi.nlm.nih.gov/34856981/ and https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0286117). Furthermore, natural exposure induces anti-CS IgM but not IgG antibodies, whose levels increase with age. We explored and reported methods for standardizing measurements of immune responses to the same vaccine but in different populations and laboratories (https://journals.plos.org/plosone/article/authors?id=10.1371/journal.pone.0286117) . This work on RTS, S immunology naturally extended the fellow’s work to supervising Sigrid Marie Dyrkorn (Norwegian University of Science and Technology), whose MSc thesis is also now published – “Safety and immunogenicity performance of RTS,S/AS01 and RTS,S/AS02 malaria vaccines in sub-Saharan African children: a systematic review and meta-analysis of randomized controlled trials”. This study found no differences in immunogenicity of RTS,S/AS01 from different geographical regions. However, the vaccine was more immunogenic in younger compared to older children. https://kemri-wellcome.org/programme/conaips/