Apoptosis Governs the Elimination of Schistosoma japonicum from the Non-Permissive Host Microtus fortis

The reed vole, Microtus fortis, is the only known mammalian host in which schistosomes of Schistosoma japonicum are unable to mature and cause significant pathogenesis. However, little is known about how Schistosoma japonicum maturation (and, therefore, the development of schistosomiasis) is prevented in M. fortis. In the present study, the ultrastructure of 10 days post infection schistosomula from BALB/c mice and M. fortis were first compared using scanning electron microscopy and transmission electron microscopy. Electron microscopic investigations showed growth retardation and ultrastructural differences in the tegument and sub-tegumental tissues as well as in the parenchymal cells of schistosomula from M. fortis compared with those in BALB/c mice. Then, microarray analysis revealed significant differential expression between the schistosomula from the two rodents, with 3,293 down-regulated (by ≥2-fold) and 71 up-regulated (≥2 fold) genes in schistosomula from the former. The up-regulated genes included a proliferation-related gene encoding granulin (Grn) and tropomyosin. Genes that were down-regulated in schistosomula from M. fortis included apoptosis-inhibited genes encoding a baculoviral IAP repeat-containing protein (SjIAP) and cytokine-induced apoptosis inhibitor (SjCIAP), genes encoding molecules involved in insulin metabolism, long-chain fatty acid metabolism, signal transduction, the transforming growth factor (TGF) pathway, the Wnt pathway and in development. TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) and PI/Annexin V-FITC assays, caspase 3/7 activity analysis, and flow cytometry revealed that the percentages of early apoptotic and late apoptotic and/or necrotic cells, as well as the level of caspase activity, in schistosomula from M. fortis were all significantly higher than in those from BALB/c mice.

东方田鼠（Microtus fortis, M. fortis）是目前已知的唯一一种哺乳动物宿主，日本血吸虫（Schistosoma japonicum）在其体内无法发育成熟并引发显著病理损伤。然而，学界对东方田鼠如何阻断日本血吸虫发育成熟（进而抑制血吸虫病发生）的机制仍知之甚少。

本研究首先利用扫描电子显微镜（scanning electron microscopy, SEM）与透射电子显微镜（transmission electron microscopy, TEM），对比了感染后10天的日本血吸虫童虫在BALB/c小鼠与东方田鼠体内的超微结构。电镜观察结果显示，与BALB/c小鼠体内的童虫相比，东方田鼠来源的童虫存在生长发育迟缓现象，且其体被、体被下层组织以及实质细胞均存在超微结构差异。

随后，基因芯片（microarray）分析显示，两种宿主来源的童虫间存在显著的基因差异表达：东方田鼠来源的童虫中，共有3293个基因表达下调（下调倍数≥2）、71个基因表达上调（上调倍数≥2）。其中上调基因包括编码颗粒蛋白（granulin, Grn）的增殖相关基因以及原肌球蛋白（tropomyosin）；下调基因则涵盖编码杆状病毒IAP重复序列蛋白（SjIAP）与细胞因子诱导凋亡抑制剂（SjCIAP）的凋亡抑制基因，以及参与胰岛素代谢、长链脂肪酸代谢、信号转导、转化生长因子（transforming growth factor, TGF）通路、Wnt通路调控与发育过程的功能基因。

最后，通过TUNEL（脱氧核糖核苷酸末端转移酶介导的dUTP缺口末端标记，terminal deoxynucleotidyl transferase dUTP nick end labeling）法、PI/Annexin V-FITC双染法、半胱天冬氨酸蛋白酶3/7（caspase 3/7）活性检测以及流式细胞术分析，结果表明：东方田鼠来源的童虫中，早期凋亡细胞、晚期凋亡细胞和/或坏死细胞的占比，以及半胱天冬氨酸蛋白酶的活性水平，均显著高于BALB/c小鼠来源的童虫。