Trypanosoma congolense Variant Surface Glycoprotein gene expression occurs in the absence of monoallelic transcription control

Abstract

Antigenic variation is a very widespread process for pathogen evasion of mammalian adaptive immunity, involving the continuous change of exposed antigens. In many single-celled pathogens, antigen expression during antigenic variation is monoallelic: just a single gene from a large family is expressed in one cell at a time. In African trypanosomes, antigenic variation relies on expression of Variant Surface Glycoprotein (VSG), and in Trypanosoma brucei there is detailed understanding of the machinery that dictates that only one of approximately 15 VSG expression sites is actively transcribed at a time. In the closely related African trypanosome, T. congolense, which remains a significant blight on agriculture productivity in sub-Sharan Africa, we have no such understanding of VSG gene expression control or dynamics. Here, we have investigated the mechanics of antigenic variation in T. congolense, first examining the patterns of VSG expression at the transcript and protein level in small parasite populations in vitro. Surprisingly, this analysis revealed much greater VSG diversity than seen in T. brucei, with such expression diversity unaltered in mutants that impair homologous recombination. Using single cell transcriptomics, we explain this diversity, since we find no evidence for monoallelic transcription of T. congolense VSGs, but show instead that each parasite can dynamically express up to [~]40 different VSGs in a single cell both in vitro and in vivo. VSG co-expression occurs from VSG genes distributed across the genome, indicating the lack of a dedicated locus for VSG transcription. Thus, comparing two trypanosome species that rely on the same class of surface antigen for immune evasion has revealed highly distinct mechanisms for controlling antigen gene expression, challenging the assumed common operation of antigenic variation across African trypanosome species.