Mosaic VSGs and the Scale of Trypanosoma brucei Antigenic Variation

Abstract

A main determinant of prolonged Trypanosoma brucei infection and transmission and success of the parasite is the interplay between host acquired immunity and antigenic variation of the parasite variant surface glycoprotein (VSG) coat. About 0.1% of trypanosome divisions produce a switch to a different VSG through differential expression of an archive of hundreds of silent VSG genes and pseudogenes, but the patterns and extent of the trypanosome diversity phenotype, particularly in chronic infection, are unclear. We applied longitudinal VSG cDNA sequencing to estimate variant richness and test whether pseudogenes contribute to antigenic variation. We show that individual growth peaks can contain at least 15 distinct variants, are estimated computationally to comprise many more, and that antigenically distinct ‘mosaic’ VSGs arise from segmental gene conversion between donor VSG genes or pseudogenes. The potential for trypanosome antigenic variation is probably much greater than VSG archive size; mosaic VSGs are core to antigenic variation and chronic infection.

Deoxyhypusine Modification of Eukaryotic Translation Initiation Factor 5A (eIF5A) Is Essential for Trypanosoma brucei Growth and for Expression of Polyprolyl-containing Proteins

“…VSGs are expressed in the bloodstream form of the parasite life cycle and form a dense surface coat. T. brucei contain more than 1000 VSG genes, which permit the antigenic variation that allows the parasite to evade the host adaptive immune system (32). During the insect stage, VSGs are silenced, and the parasite expresses a dense coat of procyclins.…”

DNA Recombination Strategies During Antigenic Variation in the African Trypanosome

“…Segmentally converted VSGs identified from infections share the structural features of other expressed VSGs: they are not markedly different in length to intact VSGs, they possess a similar domain structure, and appear to form functional surface coats (124,189). This is significant, because unlike gene conversion of intact, full-length VSGs, both 3′ donation and mosaicism readily use segments of pseudogene VSGs.…”

“…Second, segmental VSG conversion has the capacity to generate many-fold larger numbers of VSG coats than merely the number of VSG genes in the archive (123) and, in this sense, has a comparable purpose and amplification of scale to the rearrangements used to generate mature immunoglobulin genes (120). Recent studies have built upon the genome sequence of T. brucei to show that segmental VSG conversion comes to predominate as T. brucei infections progress and that VSG diversity appears enormous (37,124). These studies have confirmed far-sighted predictions by Kamper and Barbet, which predated whole genome sequencing and suggested that mosaic VSG formation is the key to infection chronicity and, most likely, even longer-term parasite-host interaction (125,126).…”

“…In addition, SGC can form a "mosaic" VSG, exchanging segments across the gene and introducing variation into the antigenically-important N-terminal domain. Mosaic VSGs identified from chronic infections can involve some quite complex rearrangements, involving up to four donors and more than ten segments (124). Both patterns of SGC-mosaicism and 3′ donationappear to have been facilitated by homology: donors in both cases shared at least 85% nucleotide identity at the boundary of conversion, and the donors for mosaic VSGs were at least 73% identical across their N-terminal domain-encoding regions (124).…”

“…Death may be immune-mediated, or may result from a cessation of protein synthesis following detection of nonfunctional VSG expression (190191). Mosaic VSGs tend to appear only relatively later in infection (after 3 weeks in mice, although the kinetics of antigenic variation are likely to vary significantly between hosts) (37,124). The "late" appearance of mosaic VSGs suggests either that they have a lower probability of becoming expressed than intact VSGs, perhaps because the SGC events are less efficient and tend not to occur earlier in infection, and/or that trypanosomes expressing mosaic VSGs grow more slowly.…”