Clark T G, Lin T L, Dickerson H W
Department of Medical Microbiology, College of Veterinary Medicine, University of Georgia, Athens 30602, USA.
Proc Natl Acad Sci U S A. 1996 Jun 25;93(13):6825-9. doi: 10.1073/pnas.93.13.6825.
We used the common fish pathogen Ichthyophthirius multifiliis as a model for studying interactions between parasitic ciliates and their vertebrate hosts. Although highly pathogenic, Ichthyophthirius can elicit a strong protective immune response in fish after exposure to controlled infections. To investigate the mechanisms underlying host resistance, a series of passive immunization experiments were carried out using mouse monoclonal antibodies against a class of surface membrane proteins, known as immobilization antigens (or i-antigens), thought to play a role in the protective response. Such antibodies bind to cilia and immobilize I. multifiliis in vitro. Surprisingly, we found that passive antibody transfer in vivo caused rapid exit of parasites from the host. The effect was highly specific for a given I. multifiliis serotype. F(ab)2 subfragments had the same effect as intact antibody, whereas monovalent Fab fragments failed to protect. The activity of Fab could, nevertheless, be restored after subsequent i.p. injection of bivalent goat anti-mouse IgG. Parasites that exit the host had detectable antibody on their surface and appeared viable in all respects. These findings represent a novel instance among protists in which protective immunity (and evasion of the host response) result from an effect of antibody on parasite behavior.
我们使用常见的鱼类病原体多子小瓜虫作为模型，来研究寄生纤毛虫与其脊椎动物宿主之间的相互作用。尽管多子小瓜虫具有高度致病性，但在鱼类受到可控感染后，它能引发强烈的保护性免疫反应。为了探究宿主抗性的潜在机制，我们使用了针对一类表面膜蛋白（称为固定抗原，或i抗原）的小鼠单克隆抗体进行了一系列被动免疫实验，这类蛋白被认为在保护性反应中发挥作用。此类抗体在体外能与纤毛结合并使多子小瓜虫固定不动。令人惊讶的是，我们发现体内被动抗体转移导致寄生虫迅速从宿主体内排出。这种效应对于特定的多子小瓜虫血清型具有高度特异性。F(ab)2亚片段与完整抗体具有相同的效果，而单价Fab片段则无法起到保护作用。不过，在随后腹腔注射二价山羊抗小鼠IgG后，Fab的活性可以恢复。从宿主体内排出的寄生虫表面可检测到抗体，并且在各方面看起来都具有活力。这些发现代表了原生生物中的一个新例子，即保护性免疫（以及对宿主反应的逃避）是由抗体对寄生虫行为的影响所导致的。
