How to create coats for all seasons: elucidating antigenic variation in African trypanosomes

Abstract

Extracellular parasites of the mammalian bloodstream face considerable challenges including incessant assault by the immune system. African trypanosomes are consummate survivors in this inclement environment and are renowned for their supremely sophisticated strategy of antigenic variation of their protective surface coat during the course of chronic infections. Recent developments are making us realize how complex this antigenic machinery is and are allowing us to tackle previously intractable problems. However, many of the simplest (and arguably the most important) questions still remain unanswered! How is the extraordinary heterogeneity of variant surface glycoprotein variants during a chronic infection generated?The bloodstream form of Trypanosoma brucei is coated with a dense covering of ∼10 7 variant surface glycoprotein (VSG) molecules, which extend as antigenically variable rods connected to the cell surface via glycosylphosphatidylinositol (GPI) anchors [1]. This GPI attachment allows VSGs to move freely over the cell surface [2]. Tight packing of these VSGs produces a protective barrier shielding invariant surface proteins from recognition by host antibodies and preventing trypanosome lysis by the alternative pathway of the complement system [3]. Extraordinarily high rates of recycling of surface VSG allow selective removal of host cell molecules including antibodies and complement from the trypanosome surface, thereby functioning as a 'coat-cleaning ' machine [4]. This prolongs trypanosome survival in rising antibody titres and facilitates escape from macrophages [5].Each trypanosome has a vast repertoire of VSG genes [6], differing in number and content between different strains, presumably as a consequence of diversifying forces. For example, the 'VSGnome' of T. brucei 427 contains more than 2500 different VSG genes, of which more than 80% are not immediately functional [7]. Switching the active VSG can entail transcriptional switches between the ∼15 telomeric VSG expression site (ES) transcription units (Figure 1). More importantly, DNA rearrangements and predominantly gene conversions can copy new VSGs (or segments of VSGs) into the active ES, thereby resulting in an antigenic switch [8] (Figure 1). This extraordinary ability to construct novel patchwork, or mosaic, VSGs is key for trypanosome survival [9]. The continuous recombination of various permutations and combinations of sections of VSG genes (or pseudogenes) allows the trypanosome to construct an almost infinite number of antigenically distinct VSG coat types.A given wave of trypanosome parasitaemia is normally predominantly composed of a major VSG variant, although it has long been known that minor VSG variants are also present within each wave. However, there is possibly phenomenal heterogeneity within these infection waves, as next-generation RNA sequencing of RNA transcripts has documented an extraordinary variety of minor VSG variants [10]. These experiments argue that a considerable percentage of the available VSG repertoire is disp...

Synthetic biology tools for engineering Goodwin oscillation in Trypanosoma brucei brucei

“…In trypanosomes, as in other eukaryotes, RNA polymerase I transcribes non-protein encoding RNA, such as ribosomal RNA (rRNA), RNA polymerase II transcribes mRNA, and RNA polymerase III transcribes tRNAs, 5S rRNA and snRNAs ( Palenchar and Bellofatto, 2006 ). Uniquely within studied eukaryotes, T. b. brucei RNA polymerase I also transcribes the mRNA for the surface coat proteins, procyclin and VSG ( Ooi and Rudenko, 2017 ). RNA polymerase I-mediated VSG mRNA transcription occurs in an extra-nucleolar structure ( Navarro and Gull 2001 ) known as the Expression Site Body (ESB).…”

Basic Biology of Trypanosoma brucei with Reference to the Development of Chemotherapies

“…VSG is a surface protein with key roles in innate and adaptive (complement) immune evasion. Reviewed [47] TLF1 and TLF2 Human complex that contains ApoL1, the pore-forming trypanocidal protein.…”

“…At high titre, antibody clearance from the cell surface is insufficient and they become a target for recognition by macrophages [46,47]. To avoid recognition by host antibodies, trypanosomes 'switch' expression to a new VSG isotype.…”

Evaluation of mechanisms that may generate DNA lesions triggering antigenic variation in African trypanosomes

“…Very recently, a protein called VSG exclusion-1 (VEX 1) was described and has been suggested to coordinate VSG expression through a “winner-takes-all” strategy [ 77 ]. Put simply, it is proposed that VEX 1 is recruited to a single ES by a positive feedback mechanism related to RNAP I, enhancing VSG transcription of the active ES, while VEX 1 also aids homology-dependent silencing of the other ES [ 77 , 78 ]. Other studies point to a crucial role of ES chromatin in regulating VSG expression, including through a modified nucleotide called base J (ß-D-glucosyl-hydroxymethyluracil) and a histone H3 variant (H3.V) [ 79 , 80 ].…”