Fetal hemoglobin enables malaria parasite growth in sickle cells but augments production of transmission stage parasites

Abstract

Sickle cell trait is the quintessential example of the human evolutionary response to malaria, providing protection against severe disease, but leading to sickle cell disease (SCD) in the homozygous state. Fetal Hemoglobin (HbF) reduces the pathology of SCD and several mutations lead to the prolonged production of HbF into childhood and adult life. HbF has been suggested to contribute to protection against malaria. Two long-term cohorts were genotyped for three quantitative trait loci associated with HbF production and analyzed for HbF titers, malaria clinical episodes and the production of parasite stages infectious to mosquitoes, gametocytes, in asymptomatic infections. Plasmodium falciparum parasites were also grown in vitro in HbSS cells with measured levels of HbF. The genetic determinants of prolonged HbF production were associated with increased HbF titers and that increased HbF afforded protection from malaria disease but increased the production of gametocytes. The presence of HbF in sickled red cells was also shown in in vitro culture to enable parasite persistence in conditions otherwise deleterious for the parasite and enabled complete maturation of gametocytes. The beneficial personal effect of HbF, whether through protection against malaria or alleviating effects of SCD, is seemingly offset by increased parasite transmissibility and potential disease burden for the community. These individuals represent a potentially important reservoir of infection and could be targeted in elimination strategies.

Fig 1. The effect of number of fetal hemoglobin – associated mutant genotypes at one locus and at two or three loci of any of the three SNPs on HbF titre in (A) Dielmo and (B) Ndiop, Number of P. falciparum clinical episodes (C) Dielmo and (D) Ndiop and the Gametocyte prevalence rate (E) Dielmo and (F) Ndiop.

WT – Wild Type. Shown are the means and standard of errors from the loglinear regression analyses on the residual values for each of three variables (see Supplementary materials). Blue histograms include individuals with sickle cell trait (HbS), red histograms have excluded the HbS individuals prior to the analyses.

Fig 2. Development of P. falciparum in HbSS erythrocytes containing HbF.

A. Increase in asexual parasitemia after 8 days of culture in HbAA or HbSS erythrocytes under an atmosphere of 1% to 2% O₂. Circles show the number of independent experiments in blood from 8 HbAA donors and 14 HbSS donors containing HbF (from 2.1 to 21%). Parasitemia increased in blood of 11 out of 14 HbSS donors. B. Giemsa-stained images of ring-infected (top panel) or schizont-infected (bottom panel) or HbSS erythrocytes containing HbF. C, D. The presence of GFP-expressing gametocytes (green) in HbAA or HbSS erythrocytes containing 56% HbF-positive cells was analyzed by immunofluorescence analysis (C) and by flow cytometry (D). Erythrocyte staining with fetal hemoglobin monoclonal antibody highlighted that gametocytes preferentially develop in HbF-positive cells. DIC: differential interference contrast. The bars represent 2 µm. E, F. Follow-up of gametocytemia on Giemsa-stained smears show that gametocytes are able to complete their maturation in HbSS erythrocytes containing 12% HbF. Cultures were grown during 17 days under an atmosphere of 5% CO₂, 1% O₂ and 94% N₂. Fig 1A: each dot represents a biological replicate. Figs 1C–1D: the experiment was done only once with the blood from one patient. Figs 1E–1F: the follow-up of gametocytemia was performed in technical triplicate with the blood from one patient containing 12% HbF.