Seyfang A, Mecke D, Duszenko M
Physiologisch-chemisches Institut, Tübingen, Federal Republic of Germany.
J Protozool. 1990 Nov-Dec;37(6):546-52. doi: 10.1111/j.1550-7408.1990.tb01263.x.
Trypanosoma brucei bloodstream forms express a densely packed surface coat consisting of identical variant surface glycoprotein (VSG) molecules. This surface coat is subject to antigenic variation by sequential expression of different VSG genes and thus enables the cells to escape the mammalian host's specific immune response. VSG turnover was investigated and compared with the antigen switching rate. Living cells were radiochemically labeled with either 125I-Bolton-Hunter reagent or 35S-methionine, and immunogold-surface labeled for electron microscopy studies. The fate of labeled VSG was studied during subsequent incubation or cultivation of labeled trypanosomes. Our data show that living cells slowly released VSG into the medium with a shedding rate of 2.2 +/- 0.6% h-1 (t1/2 = 33 +/- 9 h). In contrast, VSG degradation accounted for only 0.3 +/- 0.06% h-1 (t1/2 = 237 +/- 45 h) and followed the classical lysosomal pathway as judged by electron microscopy. Since VSG uptake by endocytosis was rather high, our data suggest that most of the endocytosed VSG was recycled to the surface membrane. These results indicate that shedding of VSG at a regular turnover rate is sufficient to remove the old VSG coat within one week, and no increase of the VSG turnover rate seems to be necessary during antigenic variation.
布氏锥虫血流型表达一种由相同的可变表面糖蛋白（VSG）分子组成的密集表面被膜。这种表面被膜通过不同VSG基因的顺序表达而发生抗原变异，从而使细胞能够逃避哺乳动物宿主的特异性免疫反应。对VSG的周转进行了研究，并与抗原转换率进行了比较。用125I-博尔顿-亨特试剂或35S-甲硫氨酸对活细胞进行放射化学标记，并进行免疫金表面标记用于电子显微镜研究。在标记的锥虫随后的孵育或培养过程中研究标记VSG的命运。我们的数据表明，活细胞以2.2±0.6% h-1的脱落率（t1/2 = 33±9 h）缓慢地将VSG释放到培养基中。相比之下，VSG降解仅占0.3±0.06% h-1（t1/2 = 237±45 h），并且通过电子显微镜判断遵循经典的溶酶体途径。由于通过内吞作用摄取VSG相当高，我们的数据表明大多数内吞的VSG被循环到表面膜。这些结果表明，以恒定的周转率脱落VSG足以在一周内去除旧的VSG被膜，并且在抗原变异期间似乎没有必要增加VSG的周转率。
