Camus D, Zalis M G, Vannier-Santos M A, Banic D M
INSERM U42, Biologie et Biochimie Parasitaires et Fongiques, Villeneuve d'Ascq, France.
Braz J Med Biol Res. 1995 Apr;28(4):399-413.
Parasites develop and survive in an environment which is often hostile to them. When facing aggressive conditions parasites are able to use various and complex strategies. Echinococcus granulosus, Toxocara canis, Pneumocystis carinii, Entamoeba or Toxoplasma gondii are able to seclude from the environment when stressed by surrounding (immunologic or non-immunologic) aggressive factors. Specific antigens which exert a functional activity during a short period of time appear to be concealed from the immune attack at this crucial moment. This is the case for rhoptry or dense granule antigens of Plasmodium or Toxoplasma sporozoa involved in the formation of the parasitophorous vacuole which are released in a space perfectly isolated from the outside and therefore from antibodies. Some parasites like Schistosoma mansoni or Trypanosoma brucei reveal an amazing opportunistic behavior when they use cytokines of host origin induced by the infectious process for their own development. Leishmania, Toxoplasma and Trypanosoma cruzi are able to invade immunologically competent macrophages and to avoid the triggering of killing mechanisms of these cells. Parasites also take advantage of the genetic restriction of the immune response and it has been observed for Plasmodia that some high molecular weight antigens are unable to induce an immune response in particular strains of mice. Parasite receptors involved in the invasion of host cells by parasites can function in the presence of antibodies which can explain the failure of vaccination attempts targeting this type of molecules. Among the mechanisms developed by parasites to resist to drugs it appears that transmembrane transporters described in many protozoa or helminth parasites could play a role. Moreover, the description of parasite-specific enzymes able to protect them against the damaging effects of oxygen radicals suggests that parasites are potentially able to develop a resistance phenomenon against drugs acting via an oxidative burst.
寄生虫在通常对它们不利的环境中发育和生存。面对恶劣条件时，寄生虫能够采用各种复杂的策略。细粒棘球绦虫、犬弓首蛔虫、卡氏肺孢子虫、溶组织内阿米巴或刚地弓形虫在受到周围（免疫或非免疫）攻击因素的压力时，能够与环境隔离开来。在短时间内发挥功能活性的特定抗原，在这个关键时刻似乎能躲避免疫攻击。疟原虫或弓形虫子孢子的棒状体或致密颗粒抗原参与寄生泡的形成，它们释放到一个与外界完全隔离的空间，因此也与抗体隔离，就是这种情况。一些寄生虫，如曼氏血吸虫或布氏锥虫，在利用感染过程诱导的宿主来源的细胞因子促进自身发育时，表现出惊人的机会主义行为。利什曼原虫、弓形虫和克氏锥虫能够侵入具有免疫活性的巨噬细胞，并避免触发这些细胞的杀伤机制。寄生虫还利用免疫反应的遗传限制，据观察，疟原虫的一些高分子量抗原无法在特定品系的小鼠中诱导免疫反应。参与寄生虫侵入宿主细胞的寄生虫受体在抗体存在的情况下仍能发挥作用，这可以解释针对这类分子的疫苗接种尝试为何失败。在寄生虫为抵抗药物而发展出的机制中，许多原生动物或蠕虫寄生虫中描述的跨膜转运蛋白似乎可能发挥作用。此外，对能够保护寄生虫免受氧自由基破坏作用的寄生虫特异性酶的描述表明寄生虫可能会对通过氧化爆发起作用的药物产生耐药现象。
