Antigen B from Echinococcus granulosus enters mammalian cells by endocytic pathways

BackgroundCystic hydatid disease is a zoonosis caused by the larval stage (hydatid) of Echinococcus granulosus (Cestoda, Taeniidae). The hydatid develops in the viscera of intermediate host as a unilocular structure filled by the hydatid fluid, which contains parasitic excretory/secretory products. The lipoprotein Antigen B (AgB) is the major component of E. granulosus metacestode hydatid fluid. Functionally, AgB has been implicated in immunomodulation and lipid transport. However, the mechanisms underlying AgB functions are not completely known.Methodology/Principal findingsIn this study, we investigated AgB interactions with different mammalian cell types and the pathways involved in its internalization. AgB uptake was observed in four different cell lines, NIH-3T3, A549, J774 and RH. Inhibition of caveolae/raft-mediated endocytosis causes about 50 and 69% decrease in AgB internalization by RH and A549 cells, respectively. Interestingly, AgB colocalized with the raft endocytic marker, but also showed a partial colocalization with the clathrin endocytic marker. Finally, AgB colocalized with an endolysosomal tracker, providing evidence for a possible AgB destination after endocytosis.Conclusions/SignificanceThe results indicate that caveolae/raft-mediated endocytosis is the main route to AgB internalization, and that a clathrin-mediated entry may also occur at a lower frequency. A possible fate for AgB after endocytosis seems to be the endolysosomal system. Cellular internalization and further access to subcellular compartments could be a requirement for AgB functions as a lipid carrier and/or immunomodulatory molecule, contributing to create a more permissive microenvironment to metacestode development and survival.

背景
囊型包虫病（cystic hydatid disease）是由细粒棘球绦虫（Echinococcus granulosus，带科带绦虫属）的幼虫期（棘球蚴）引发的人兽共患病。棘球蚴在中间宿主的内脏器官中发育为单房状结构，内部充满棘球蚴液，其中含有寄生虫的排泄/分泌产物。脂蛋白抗原B（AgB）是细粒棘球绦虫续绦期（metacestode）棘球蚴液的主要组成成分。功能层面，AgB被证实参与免疫调节与脂质转运过程，但目前其发挥功能的分子机制尚未完全明确。

材料与方法/主要结果
本研究探究了AgB与不同哺乳动物细胞类型的相互作用，以及其内化过程所涉及的信号通路。研究在NIH-3T3、A549、J774及RH四种不同细胞系中均观测到了AgB的摄取现象。抑制小窝/脂筏介导的内吞（caveolae/raft-mediated endocytosis）作用后，RH细胞与A549细胞的AgB内化水平分别下降约50%与69%。值得注意的是，AgB不仅与脂筏内吞标志物存在共定位，还与网格蛋白（clathrin）内吞标志物呈现部分共定位。最终，AgB与内溶酶体追踪剂发生共定位，为内吞后AgB的潜在归宿提供了实验证据。

结论与意义
本研究结果表明，小窝/脂筏介导的内吞作用是AgB内化的主要途径，而网格蛋白介导的内吞可能以较低频率存在。内吞后的AgB潜在归宿似乎为内溶酶体系统。细胞内化及进一步进入亚细胞区室，可能是AgB作为脂质载体和/或免疫调节分子发挥功能的必要条件，有助于为细粒棘球绦虫续绦期的发育与存活营造更适宜的微环境。