Authors

D O. Escrivani¹; V Scheidt¹; M Tinti¹; J Faria¹; D Horn¹;  ¹ The Wellcome Trust Centre for Anti-Infectives Research, School of Life Sciences, University of Dundee, UK

Discussion

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 (ES) at a time, with strict silencing of other telomeric ESs. VSG switching occurs at low frequency, primarily exploiting a reservoir of (sub)telomeric VSG templates, including >60 minichromosomal VSGs in Trypanosoma brucei, to replace the active VSG. It has been known for over fifty years that VSGs emerge in a hierarchical order, and VSGs from silent ESs (ES-VSGs) and from minichromosomes (MC-VSGs) are now known to dominate the early phase of infection. Here, we quantitatively assess factors that contribute to the hierarchy, independent of host immune selection. We triggered high-frequency VSG gene recombination and switching in in vitro culture using inducible CRISPR-Cas9 to target the active VSG. VSG dynamics were then assessed using RNA-seq. Activation rates were highly variable, but typically higher for polycistronic ES-VSGs. MC-VSGs subsequently displayed a competitive advantage over ES-VSGs, however, and came to dominate the population, a feature that was particularly pronounced when only longer VSGs were considered. Slower growth associated with ES-VSG activation was linked to widespread transcriptome differences, including increased expression of ES-associated genes. We conclude that VSG dynamics are underpinned by differential VSG activation, and subsequent competitive growth, both of which are VSG template location-dependent, and which is also VSG length-dependent in the latter case. These features may prolong parasite immune evasion using a limited set of variant antigens.