Fawcett D W, Doxsey S, Stagg D A, Young A S
Eur J Cell Biol. 1982 Apr;27(1):10-21.
The entry of sporozoites of Theileria parva into bovine lymphoid cells in vitro was studied with the electron microscope. Endocytosis is completed in less than 10 min. No local mobilization of actin or other cytoskeletal elements is detected in the cytoplasm of the cell being invaded and no engulfing pseudopods are formed. At the site of initial contact, the membranes of parasite and host cell come into very close apposition. As the zippering up of the membranes spreads laterally, the sporozoite sinks into a progressively deepening recess in the surface of the host cell until the rim of the invagination closes and fuses over the parasite. The observation that sporozoites are interiorized at 1-2 degrees C as well as at 37 degrees C suggests that endocytosis depends mainly upon a ligand-receptor interaction of the parasite and host cell membranes and requires little energy. Sporozoites may enter in any orientation, unlike other sporozoan parasites in which the membrane overlying an apical complex is invariably the site of attachment. 24 h after entry, the sporozoite is located in the Golgi region and the investing host cell membrane acquired during endocytosis has disappeared. The Golgi complex has been activated to form small lysosomes which gather around the parasite but are ineffective for lack of a membrane which they can fuse. It is suggested that removal of the investing host-cell membrane permits the parasite to evade destruction by the phagolysosomal system of the host cell. Persistence of micronemes after entry of the sporozoite and their subsequent disappearances invites the speculation that these parasite organelles may play a role in dispersal of the invaginated host cell membrane.
用电子显微镜研究了小泰勒虫裂殖子体外进入牛淋巴细胞的过程。内吞作用在不到10分钟内完成。在被入侵细胞的细胞质中未检测到肌动蛋白或其他细胞骨架成分的局部动员，也未形成吞噬伪足。在初始接触部位，寄生虫和宿主细胞的膜紧密贴靠。随着膜的拉链式合拢横向扩展，裂殖子沉入宿主细胞表面逐渐加深的凹陷中，直到内陷边缘闭合并在寄生虫上方融合。裂殖子在1-2℃和37℃时都能内化，这一观察结果表明内吞作用主要依赖于寄生虫和宿主细胞膜的配体-受体相互作用，且所需能量很少。与其他孢子虫寄生虫不同，裂殖子可以以任何方向进入，其他孢子虫寄生虫中覆盖顶复合器的膜始终是附着部位。进入后24小时，裂殖子位于高尔基体区域，内吞过程中获得的包被宿主细胞膜已消失。高尔基体复合体被激活形成小溶酶体，这些小溶酶体聚集在寄生虫周围，但由于缺乏可融合的膜而无效。有人认为，去除包被宿主细胞膜可使寄生虫逃避宿主细胞吞噬溶酶体系统的破坏。裂殖子进入后微线体的持续存在及其随后的消失引发了这样的推测，即这些寄生虫细胞器可能在凹陷的宿主细胞膜的分散中起作用。
