Mwenda J M
Institute of Primate Research, National Museums of Kenya, Karen, Nairobi.
Cell Mol Biol (Noisy-le-grand). 1993 May;39(3):317-28.
The presence of budding type-C retroviral-like particles in normal placental trophoblast, particularly at the basal surface of the placental syncytiotrophoblast, is well documented. Retroviral-like particles were isolated from human placental villous tissues using isopycnic sucrose gradient centrifugation. Reverse transcriptase activity (RTase) associated with isolated retroviral-like particles was characterised using a combination of synthetic template-primers. These studies showed that RTase activity was more specific with poly(rC).oligo(dG)12-18 than poly(dC).oligo(dG)12-18. Furthermore, activity was detected with poly(rCm).oligo(dG)12-18, a template-primer which has previously been shown to be specific for retroviral RTase. Maximum activity appeared at a sucrose density between 1.15-1.17 g/ml, characteristic of enveloped retroviral particles. Electron microscopy examination of the gradient purified particles revealed morphology and size similar to other retroviruses. Endogenous retroviral particles were isolated from 26 out of 32 (81%) first-trimester placental villous tissue extracts. These particles are likely to be product of endogenous proviral sequences present in the germline of humans. Although these studies showed presence of intact retroviral particles in placental tissues, it was not possible to propagate the isolated particles in vitro. All attempts to propagate placental retroviral particles by co-cultivation with human cells (U937 and JAr choriocarcinoma cells) and long term placental villous tissue explant cultures were unsuccessful. Subsequently, there was no evidence of retroviral-like particles or RTase activity in these cell cultures, including after stimulation with 5'-azacytidine or dexamethasone, chemical agents known to stimulate particle production in virus-infected lines.
正常胎盘滋养层细胞中，尤其是胎盘合体滋养层细胞的基底面，存在芽生型C型逆转录病毒样颗粒，这一点已有充分记录。使用等密度蔗糖梯度离心法从人胎盘绒毛组织中分离出逆转录病毒样颗粒。结合合成模板引物，对分离出的逆转录病毒样颗粒相关的逆转录酶活性（RTase）进行了表征。这些研究表明，与聚（dC）·寡聚（dG）12 - 18相比，聚（rC）·寡聚（dG）12 - 18对RTase活性的特异性更高。此外，用聚（rCm）·寡聚（dG）12 - 18检测到了活性，该模板引物先前已被证明对逆转录病毒RTase具有特异性。最大活性出现在蔗糖密度为1.15 - 1.17 g/ml之间，这是包膜逆转录病毒颗粒的特征。对梯度纯化颗粒的电子显微镜检查显示，其形态和大小与其他逆转录病毒相似。从32份孕早期胎盘绒毛组织提取物中的26份（81%）中分离出了内源性逆转录病毒颗粒。这些颗粒可能是人类种系中存在的内源性前病毒序列的产物。尽管这些研究表明胎盘组织中存在完整的逆转录病毒颗粒，但无法在体外繁殖分离出的颗粒。通过与人细胞（U937和JAr绒毛膜癌细胞）共培养以及长期胎盘绒毛组织外植体培养来繁殖胎盘逆转录病毒颗粒的所有尝试均未成功。随后，在这些细胞培养物中没有逆转录病毒样颗粒或RTase活性的证据，包括在用5'-氮杂胞苷或地塞米松刺激后，这两种化学试剂已知可刺激病毒感染细胞系中的颗粒产生。
