MAJOR ARTICLE
First-Trimester Plasmodium falciparum Infections Display a Typical “Placental” Phenotype Justin Doritchamou,1,2,3 Gwladys Bertin,1,2 Azizath Moussiliou,3 Pascal Bigey,1,4 Firmine Viwami,3 Sem Ezinmegnon,3 Nadine Fievet,1,2,3 Achille Massougbodji,3 Philippe Deloron,1,2 and Nicaise Tuikue Ndam1,2,3 1
PRES Sorbonne Paris Cité, Faculté de Pharmacie, Université Paris Descartes, and 2Institut de Recherche pour le Développement, UMR216 Mère et enfant face aux infections tropicales, Paris, France; 3Centre d’Etude et de Recherche sur le paludisme associé à la Grossesse et à l’Enfance, Université d’Abomey-Calavi, Cotonou, Benin; and 4Unité de pharmacologie chimique et génétique, Université Paris Descartes; ENSCP Chimie ParisTech, CNRS UMR8151; Inserm U 1022, Paris, France
Background. Plasmodium falciparum–infected erythrocytes (IEs) adhere to host cell receptors, allowing parasites to sequester into deep vascular beds of various organs. This defining phenomenon of malaria pathogenesis is key to the severe clinical complications associated with cerebral and placental malaria. The principal ligand associated with the binding to chondroitin sulfate A (CSA) that allows placental sequestration of IEs is a P. falciparum erythrocyte membrane protein 1 (PfEMP1) family member encoded by the var2csa gene. Methods. Here, we investigated the transcription pattern of var genes by real-time polymerase chain reaction, the expression of VAR2CSA, protein by flow cytometry, and the CSA-binding ability of IEs collected at different stages of pregnancy using a static-based Petri dish assay. Results. Through comparison with the profiles of isolates from nonpregnant hosts, we report several lines of evidence showing that parasites infecting women during pregnancy preferentially express VAR2CSA protein, and that selection for the capacity to adhere to CSA via VAR2CSA expression occurs early in pregnancy. Conclusions. Our data suggest that the placental tropism of P. falciparum is already established in the first trimester of pregnancy, with consequent implications for the development of the pathology associated with placental malaria. Malaria in pregnancy is an important cause of maternal anemia, stillbirth, and delivery of babies with low birth weight, the latter representing a major risk factor for infant mortality in Africa [1]. It is a wellestablished fact that pregnant women are more susceptible to infection with Plasmodium falciparum from the first trimester onward, with peaks of peripheral parasite density between 13 and 16 weeks of pregnancy [1], but the characteristics of these infections are poorly described. Relationships between parasitemia occurring in the first trimester and poor pregnancy
Received 25 May 2012; accepted 30 July 2012; electronically published 8 October 2012. Correspondence: Nicaise Tuikue Ndam, PhD, UMR216UPD-IRD, Faculté des sciences biologiques et pharmaceutiques; 4, avenue de l’observatoire, Paris 75006, France (
[email protected]). The Journal of Infectious Diseases 2012;206:1911–9 © The Author 2012. Published by Oxford University Press on behalf of the Infectious Diseases Society of America. All rights reserved. For Permissions, please e-mail:
[email protected]. DOI: 10.1093/infdis/jis629
outcomes, both in terms of birth weight and maternal anemia, have been reported [2, 3], highlighting the risk to which both the mother and fetus are exposed in early gestation. Paradoxically, current preventive measures for malaria during pregnancy do not cover this period, first because attendance rates at antenatal clinics by women in their first trimester are generally low with commensurately low rates of detection and treatment of infections, and second because, for reasons of safety, the currently recommended intermittent preventive treatment for malaria during pregnancy is implemented only after the first trimester. Potentially pathogenic infections arising in early pregnancy thus merit more detailed study. The increased susceptibility to infection with P. falciparum during pregnancy of women who have previously acquired a degree of immunity is associated with sequestration within the placenta of infected erythrocytes (IEs) that express unique variant surface antigens (VSAs) mediating adherence to chondroitin Placental Malaria in Early Pregnancy
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sulfate A (CSA) (reviewed in [4]). Plasmodium falciparum erythrocyte membrane protein-1 (PfEMP1) molecules coded by var genes are expressed on the surface of the IEs and are the main VSAs that mediate adhesion of the IEs to various host receptors [5, 6]. Association between var gene expression and clinical presentation of malaria and/or outcome has been investigated in several studies that revealed their crucial role in malaria pathogenesis [7–10]. The transcription of var genes belonging to groups A and/or B has been associated with symptomatic and severe malaria [8, 10, 11], whereas var group C expression is associated with asymptomatic malaria [11]. The PfEMP1 variant encoded by the var2csa gene was identified as the particular parasite ligand for CSA [12–15], and is associated with malaria in pregnancy [7, 9, 16]. Parasites isolated from placental blood selectively transcribe the var2csa gene, as do parasites selected for IE adhesion to CSA [7, 9, 17, 18]. High plasma levels of antibodies against VAR2CSA are associated with a lower frequency of the poor birth outcomes associated with P. falciparum infection during pregnancy [13, 19]. Furthermore, VAR2CSA-specific immunoglobulin G (IgG) can efficiently block IE adhesion to CSA [20, 21]. Given this knowledge, the VAR2CSA protein has become the major candidate antigen for the development of a vaccine designed to protect pregnant women from placental malaria. Because there are still uncertainties about the onset of var2csa-specific transcription in pregnancy, we assessed the abundance of var gene transcripts in parasites collected from women at different times during pregnancy, as well as in patients with uncomplicated malaria and in children with cerebral malaria. Simultaneously, we investigated the expression of VAR2CSA on the surface of IEs and their binding properties using a standardized in vitro CSA-binding assay.
description of the area has been reported elsewhere [2]. In brief, southern Benin is a high malaria transmission area with peaks during the 2 rainy seasons. Plasmodium falciparum is the predominant malaria-causing parasite, and the entomological inoculation rate ranges from 35 to 60 infective bites per person per year [22]. Pregnant women were enrolled during their first or second trimester of pregnancy (gestational age
