Wellcome Centre for Anti-Infectives Research, School of Life Sciences, University of Dundee, Dow Street, Dundee, United Kingdom.
PLoS Pathog. 2023 Jul 17;19(7):e1011530. doi: 10.1371/journal.ppat.1011530. eCollection 2023 Jul.
Several persistent pathogens employ antigenic variation to continually evade mammalian host adaptive immune responses. African trypanosomes use variant surface glycoproteins (VSGs) for this purpose, transcribing one telomeric VSG expression-site at a time, and exploiting a reservoir of (sub)telomeric VSG templates to switch the active VSG. It has been known for over fifty years that new VSGs emerge in a predictable order in Trypanosoma brucei, and differential activation frequencies are now known to contribute to the hierarchy. Switching of approximately 0.01% of dividing cells to many new VSGs, in the absence of post-switching competition, suggests that VSGs are deployed in a highly profligate manner, however. Here, we report that switched trypanosomes do indeed compete, in a highly predictable manner that is dependent upon the activated VSG. We induced VSG gene recombination and switching in in vitro culture using CRISPR-Cas9 nuclease to target the active VSG. VSG dynamics, that were independent of host immune selection, were subsequently assessed using RNA-seq. Although trypanosomes activated VSGs from repressed expression-sites at relatively higher frequencies, the population of cells that activated minichromosomal VSGs subsequently displayed a competitive advantage and came to dominate. Furthermore, the advantage appeared to be more pronounced for longer VSGs. Differential growth of switched clones was also associated with wider differences, affecting transcripts involved in nucleolar function, translation, and energy metabolism. We conclude that antigenic variants compete, and that the population of cells that activates minichromosome derived VSGs displays a competitive advantage. Thus, competition among variants impacts antigenic variation dynamics in African trypanosomes and likely prolongs immune evasion with a limited set of antigens.
几种持久性病原体利用抗原变异来不断逃避哺乳动物宿主的适应性免疫反应。非洲锥虫为此使用变异表面糖蛋白（VSG），一次转录一个端粒 VSG 表达位点，并利用（亚）端粒 VSG 模板库来切换活性 VSG。五十多年来，人们已经知道在布氏锥虫中，新的 VSG 会按照可预测的顺序出现，并且现在已知差异激活频率有助于这种层次结构。在没有切换后竞争的情况下，大约 0.01%的分裂细胞切换到许多新的 VSG，表明 VSG 的部署方式非常浪费。然而，在这里，我们报告说切换的锥虫确实会竞争，而且是以高度可预测的方式，这取决于激活的 VSG。我们使用 CRISPR-Cas9 核酸酶在体外培养中诱导 VSG 基因重组和切换，以靶向活性 VSG。随后使用 RNA-seq 评估与宿主免疫选择无关的 VSG 动力学。尽管锥虫以相对较高的频率从受抑制的表达位点激活 VSG，但激活微小染色体 VSG 的细胞群体随后表现出竞争优势并占据主导地位。此外，对于较长的 VSG，优势似乎更为明显。切换克隆的差异生长也与更广泛的差异相关，影响涉及核仁功能、翻译和能量代谢的转录本。我们得出结论，抗原变体相互竞争，并且激活微小染色体衍生 VSG 的细胞群体显示出竞争优势。因此，变体之间的竞争会影响非洲锥虫的抗原变异动态，并可能通过有限的一组抗原延长免疫逃避。
