Predominance of Duplicative VSG Gene Conversion in Antigenic Variation in African Trypanosomes

Abstract

A number of mechanisms have been described by which African trypanosomes undergo the genetic switches that differentially activate their variant surface glycoprotein genes (VSGs) and bring about antigenic variation. These mechanisms have been observed mainly in trypanosome lines adapted, by rapid syringe passaging, to laboratory conditions. Such "monomorphic" lines, which routinely yield only the proliferative bloodstream form and do not develop through their life cycle, have VSG switch rates up to 4 or 5 orders of magnitude lower than those of nonadapted lines. We have proposed that nonadapted, or pleomorphic, trypanosomes normally have an active VSG switch mechanism, involving gene duplication, that is depressed, or from which a component is absent, in monomorphic lines. We have characterized 88 trypanosome clones from the first two relapse peaks of a single rabbit infection with pleomorphic trypanosomes and shown that they represent 11 different variable antigen types (VATs). The pattern of appearance in the first relapse peak was generally reproducible in three more rabbit infections. Nine of these VATs had activated VSGs by gene duplication, the tenth possibly also had done so, and only one had activated a VSG by the transcriptional switch mechanism that predominates in monomorphic lines. At least 10 of the donor genes have telomeric silent copies, and many reside on minichromosomes. It appears that trypanosome antigenic variation is dominated by one, relatively highly active, mechanism rather than by the plethora of pathways described before.

Parasite-intrinsic factors can explain ordered progression of trypanosome antigenic variation

“…In our analysis, the experimental set of 30 VSGs was expended in only two or three peaks. This is more profligate than generally has been concluded from empirical clonal studies, but those studies screened only a few selected VSGs, in unusually slowly switching strains (14,22,36). Nevertheless, profligacy of similar magnitude has been recorded in the first relapse after the more natural situation, tsetse fly transmission (56).…”

“…Indeed, the bacterium Anaplasma marginale, using an archival system, achieves differential switching by way of homologies in both gene flank and coding sequences (6). There is empirical support for this general trypanosome molecular mechanism (7)(8)(9)(10)(11)(12)(13)(14), and recent quantitative analysis has revealed a higher degree of order than previously supposed (15). Three studies in particular, together involving Ͼ100 variants in Ͼ50 infections, have firmly established order in VSG expression (15)(16)(17).…”

“…Whether trypanosomes can resist antibodies during switching or at low antibody levels (24,48) would be included in these empirical profiles of immunity and do not need to be factored. Another generality on the basis of evidence (14,15) is that immune responses are very persistent.…”

Mismatch Repair Regulates Homologous Recombination, but Has Little Influence on Antigenic Variation, in Trypanosoma brucei

“…2). A higher switch rate appears to result from a greater use of recombinational events (15), but the underlying cause(s) remains unknown. From this work, we can exclude that impairment or down-regulation of MMR in pleomorphic T. brucei cells T. brucei unconstrains recombination and gives rise to a high VSG switching rate.…”

“…T. brucei also contains hundreds of silent VSG genes, both in multigene arrays in the megabase chromosomes and at the telomeres of minichromosomes. This silent reservoir is activated by recombination reactions that move the genes into the ES, normally by a gene conversion process (13)(14)(15). The available evidence suggests that recombinational VSG switching occurs by homologous recombination.…”