Involvement of Apoptosis in Host-Parasite Interactions in the Zebra Mussel

The question of whether cell death by apoptosis plays a biological function during infection is key to understanding host-parasite interactions. We investigated the involvement of apoptosis in several host-parasite systems, using zebra mussels Dreissena polymorpha as test organisms and their micro- and macroparasites. As a stress response associated with parasitism, heat shock proteins (Hsp) can be induced. In this protein family, Hsp70 are known to be apoptosis inhibitors. Mussels were diagnosed for their respective infections by standard histological methods; apoptosis was detected using the TUNEL methods on paraffin sections and Hsp70 by immunohistochemistry on cryosections. Circulating hemocytes were the main cells observed in apoptosis whereas infected tissues displayed no or few apoptotic cells. Parasitism by intracellular bacteria Rickettsiales-like and the trematode Bucephalus polymorphus were associated with the inhibition of apoptosis whereas ciliates Ophryoglena spp. or the trematode Phyllodistomum folium did not involve significant differences in apoptosis. Even if some parasites were able to modulate apoptosis in zebra mussels, we did not see evidence of any involvement of Hsp70 on this mechanism.细胞凋亡（apoptosis）介导的细胞死亡在感染过程中是否具有生物学功能，这一问题是解析宿主-寄生虫互作机制的核心所在。本研究以斑马贻贝（Dreissena polymorpha）作为模式生物，结合其体内的微寄生虫与宏寄生虫，探究了细胞凋亡在多种宿主-寄生虫互作体系中的参与情况。作为与寄生虫感染相关的应激反应，热休克蛋白（heat shock proteins, Hsp）可被诱导表达；在该蛋白家族中，Hsp70已被证实为细胞凋亡抑制剂。本研究采用标准组织学方法对贻贝的感染状态进行诊断；通过石蜡切片的TUNEL法检测细胞凋亡，通过冰冻切片的免疫组化法检测Hsp70的表达水平。循环血细胞是发生凋亡的主要细胞类型，而受感染的组织中几乎未出现或仅存在极少量凋亡细胞。胞内立克次体目样细菌以及多形皱尾吸虫（Bucephalus polymorphus）的寄生会抑制细胞凋亡；而纤毛虫Ophryoglena属（Ophryoglena spp.）或叶形 Phyllodistomum folium 吸虫的寄生并未导致细胞凋亡水平出现显著变化。尽管部分寄生虫能够调控斑马贻贝的细胞凋亡，但本研究未发现Hsp70参与该调控过程的相关证据。