Authors

D O Escrivani¹; VS Scheidt²; M Tinti¹; J Faria³; D Horn¹;  ¹ The Wellcome Trust Centre for Anti-Infectives Research, School of Life Sciences, University of Dundee, UK;  ² Dr. Brill & KEBOS GmbH, Institute for Hygiene and Microbiology, Germany;  ³ York Biomedical Research Institute, Department of Biology, University of York, UK

Discussion

African trypanosomes employ antigenic variation of variant surface glycoproteins (VSGs) to evade mammalian host immune responses, 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, in vivo, new VSGs emerge from a reservoir of (sub)telomeric VSG templates 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 were subsequently assessed using RNA-seq, independent of host immune selection. Although trypanosomes activated VSGs from repressed expression-sites at relatively higher frequencies, cells that activated minichromosomal VSGs displayed a competitive advantage and came to dominate the population. Furthermore, the advantage correlated with VSG length. Differential growth was also associated with wider transcriptome differences, affecting transcripts involved in nucleolar function, translation, and energy metabolism. We conclude that both VSG template location and VSG length contribute to post-switching antigenic variation dynamics in African trypanosomes. Competition among variants likely prolongs immune evasion with a limited set of antigens.