Smith Justine R, Franc Daniel T, Carter Nicola S, Zamora David, Planck Stephen R, Rosenbaum James T
Casey Eye Institute and the Department of Biochemistry, Oregon Health and Science University, Portland, Oregon 97239-4197, USA.
Invest Ophthalmol Vis Sci. 2004 Apr;45(4):1157-61. doi: 10.1167/iovs.03-1105.
Retinochoroidal infection with the protozoan parasite Toxoplasma gondii is the most common cause of posterior uveitis worldwide. Tachyzoites spread throughout the body through the blood stream and lymphatics, but preferentially encyst in the eye and other parts of the central nervous system (CNS). It is unknown whether T. gondii penetrates the CNS selectively or whether these sites of immune privilege have limited capacity to eradicate the parasite.
Human vascular endothelial cell lines, including retinal (three lines from three different donors), aortic, umbilical vein, and dermal microvascular endothelium, as well as human foreskin fibroblasts, were grown to confluence in 24-well plates. Cells were incubated with RH-strain T. gondii tachyzoites in the presence of [(3)H]-uracil. Trichloroacetic acid-insoluble radioactivity was measured as an index of T. gondii proliferation, because tachyzoites, but not human cells, incorporate uracil directly through pyrimidine salvage.
Tachyzoites showed higher [(3)H]-uracil incorporation after incubation with retinal vascular endothelial cells in comparison with aortic (55% more), umbilical vein (33% more) and dermal (34% more) endothelial cells. In eight separate assays, significantly greater radioactivity was measured for tachyzoites cultured with retinal versus other cell subtypes (P < 0.05), except for one assay in which differences reached only borderline significance (P <or= 0.07). In contrast, experiments comparing different retinal endothelial lines revealed no difference between any pair. Growth of the tachyzoites was approximately 2.8-fold higher in retinal endothelium than in foreskin fibroblasts, the cell subtype often used to investigate processes of T. gondii infection.
Enhanced susceptibility of retinal vascular endothelium to infection by T. gondii tachyzoites may explain, at least in part, preferential localization of T. gondii to the retina. Susceptibility may relate to preferential binding of tachyzoites to the retinal vascular endothelial surface, relative ease of penetration into the cell, rate of replication within the cell and/or cell response to infection.
原生动物寄生虫刚地弓形虫引起的视网膜脉络膜感染是全球后葡萄膜炎最常见的病因。速殖子通过血流和淋巴管在全身传播，但优先在眼睛和中枢神经系统（CNS）的其他部位形成包囊。尚不清楚刚地弓形虫是选择性地穿透中枢神经系统，还是这些免疫赦免部位清除寄生虫的能力有限。
将包括视网膜（来自三个不同供体的三株细胞系）、主动脉、脐静脉和真皮微血管内皮细胞系，以及人包皮成纤维细胞在内的人血管内皮细胞系在24孔板中培养至汇合。细胞在[³H] - 尿嘧啶存在的情况下与RH株刚地弓形虫速殖子一起孵育。由于速殖子而非人类细胞通过嘧啶补救途径直接掺入尿嘧啶，因此测量三氯乙酸不溶性放射性作为刚地弓形虫增殖的指标。
与主动脉内皮细胞（多55%）、脐静脉内皮细胞（多33%）和真皮内皮细胞（多34%）相比，速殖子与视网膜血管内皮细胞孵育后显示出更高的[³H] - 尿嘧啶掺入量。在八项独立试验中，与其他细胞亚型相比，培养有视网膜细胞的速殖子测得的放射性显著更高（P < 0.05），但有一项试验差异仅达到临界显著水平（P≤0.07）。相比之下，比较不同视网膜内皮细胞系的实验显示任何一对之间均无差异。速殖子在视网膜内皮中的生长比包皮成纤维细胞高约2.8倍，包皮成纤维细胞是常用于研究刚地弓形虫感染过程的细胞亚型。
视网膜血管内皮对刚地弓形虫速殖子感染的易感性增强可能至少部分解释了刚地弓形虫在视网膜中的优先定位。易感性可能与速殖子与视网膜血管内皮表面的优先结合、相对容易穿透细胞、细胞内复制速率和/或细胞对感染的反应有关。
