Authors

Discussion

Antigenic variation is a widespread process for pathogen evasion of mammalian adaptive immunity, involving the continuous change of exposed antigens. In African trypanosomes, antigenic variation relies on expression of Variant Surface Glycoprotein (VSG), and in Trypanosoma brucei we have detailed understanding of the machinery that dictates how just one of approximately 15 VSG expression sites is actively transcribed at a time. In the closely related African trypanosome, T. congolense, we have no such understanding of VSG gene expression control or dynamics. Here, we have examined the patterns of VSG expression at the transcript and protein level in populations of T. congolense, revealing much greater diversity of VSG expression than seen in T. brucei. This diversity is 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 30 different VSG transcripts in a single cell. VSG co-expression reflects the absence of dedicated VSG expression sites, with transcripts derived from VSGs distributed across the genome. Thus, comparing two trypanosomes that rely on the same class of surface antigen for immune evasion has revealed highly distinct mechanisms for generating antigen diversity, challenging the assumed generality of monoallelic antigen expression and common operation of antigenic variation across African trypanosome species.