Selective Accumulation of Mature Asexual Stages of Plasmodium falciparum-Infected Erythrocytes in the Placenta

A feature of malaria in pregnancy is accumulation of P. falciparum-infected erythrocytes (IEs) in the placenta, which is associated with adverse outcomes for mothers and infants. Infection appears to involve parasite adhesion to molecules such as chondroitin sulfate A, hyaluronic acid, and immunoglobulins. In vitro, adhesion is predominantly a property of mature asexual forms of IEs; however, adhesion of immature or ring forms has recently been reported. We have assessed the parasitemia and developmental stages of IEs in the placenta by examination of placental blood and histological sections with comparison to parasites in the peripheral blood from the same individuals. Approximately 90% of IEs in the placenta were mature forms. Compared to peripheral blood, the placental parasitemia was 10-fold higher and the density of mature IEs was over 200-fold higher. By contrast, the average peripheral and placental ring-stage parasitemias were not significantly different. In 2 of 14 cases, the density of ring forms was higher in placental than in peripheral blood. These findings demonstrate prominent selective accumulation of mature asexual-stage IEs but infrequent accumulation of ring stages in the placental blood spaces, consistent with an important role for mature-stage IE adhesion.

妊娠疟疾的典型特征为胎盘内聚集恶性疟原虫（Plasmodium falciparum）感染的红细胞（infected erythrocytes，简称IEs），该现象与母婴的不良妊娠结局密切相关。该感染过程似乎涉及疟原虫与硫酸软骨素A（chondroitin sulfate A）、透明质酸（hyaluronic acid）、免疫球蛋白（immunoglobulins）等分子的黏附作用。体外实验显示，IEs的黏附能力主要源于其成熟无性体（mature asexual forms）阶段；但近期已有研究报道未成熟体或环状体（ring forms）亦可发生黏附。本研究通过检测胎盘血液与组织切片，并与同一受试者外周血中的疟原虫进行对照，评估了胎盘内IEs的原虫血症（parasitemia）水平与发育阶段。结果显示，胎盘内约90%的IEs为成熟无性体。与外周血相比，胎盘血液中的原虫血症水平升高10倍，成熟IEs的密度更是高出200倍以上。与之相反，外周血与胎盘血液中环状体阶段的原虫血症平均水平并无显著差异；在14例受试者中，仅2例的胎盘血液内环状体密度高于外周血。本研究结果证实，胎盘血窦内成熟无性体阶段IEs存在显著的选择性聚集，而环状体的聚集则较为少见，这与成熟阶段IEs黏附作用的关键功能相符。