Gross U, Kempf M C, Seeber F, Lüder C G, Lugert R, Bohne W
Institute of Hygiene and Microbiology, University of Würzburg, Germany.
Behring Inst Mitt. 1997 Mar(99):97-106.
The protozoan parasite Toxoplasma gondii comprises three clonal lineages that are associated with the clinical outcome in infected individuals. Whereas group C strains are mainly found in animals, group A and B strains are associated with human disease (Howe and Sibley, 1995). An increased level of transcripts of the tachyzoite-specifically expressed gene SAG1 could be identified in group A T. gondii strains compared to group B strains. Since SAG1-mediated host-cell invasion seems to be important for parasite replication, the observed higher replication rate in group A T. gondii strains might explain the association with clinically overt symptoms at the acute stage in patients who are infected with this group of parasite strains. The presence of external stress factors, such as interferon-gamma (IFN-gamma)-mediated nitric oxide (NO) formation has been identified to stabilize the cyst stage, most likely by activation of promoter(s) which drive the expression of genes encoding bradyzoite-specific antigens. Reactivation of chronic toxoplasmosis thus might occur in the absence of external stress factors, as has been observed in AIDS patients with decreases levels of IFN-gamma. Since group B T. gondii strains might form more cysts in infected individuals due to an increased potential to convert into bradyzoites, reactivation with resulting toxoplasmic encephalitis could be a more common event in those AIDS patients who were infected with persistent cysts of this group of parasite strains.
原生动物寄生虫刚地弓形虫包含三个克隆谱系，它们与感染个体的临床结果相关。C组菌株主要存在于动物中，而A组和B组菌株与人类疾病有关（豪和西布利，1995年）。与B组菌株相比，在A组刚地弓形虫菌株中可检测到速殖子特异性表达基因SAG1的转录本水平升高。由于SAG1介导的宿主细胞入侵似乎对寄生虫复制很重要，在A组刚地弓形虫菌株中观察到的较高复制率可能解释了感染这组寄生虫菌株的患者在急性期与明显临床症状的关联。已确定外部应激因素的存在，如干扰素-γ（IFN-γ）介导的一氧化氮（NO）形成，可稳定包囊阶段，最有可能是通过激活驱动缓殖子特异性抗原编码基因表达的启动子。因此，慢性弓形虫病的再激活可能在没有外部应激因素的情况下发生，正如在IFN-γ水平降低的艾滋病患者中所观察到的那样。由于B组刚地弓形虫菌株可能由于转化为缓殖子的潜力增加而在感染个体中形成更多包囊，对于感染了这组寄生虫菌株持续性包囊的艾滋病患者，由此导致的弓形虫性脑炎再激活可能是更常见的事件。
