pathogens Review The Cytological Events and Molecular Control of Life Cycle Development of Trypanosoma brucei in the Mammalian Bloodstream Eleanor Silvester †, Kirsty R. McWilliam † and Keith R. Matthews * Institute for Immunology and Infection Research, Centre for Immunity, Infection and Evolution, School of Biological Sciences, King’s Buildings, University of Edinburgh, Charlotte Auerbach Road, Edinburgh EH9 3FL, UK; [email protected] (E.S.); [email protected] (K.R.McW.) * Correspondence: [email protected]; Tel.: +44-131-651-3639 † These authors contributed equally to this work. Received: 23 May 2017; Accepted: 22 June 2017; Published: 28 June 2017 Abstract: African trypanosomes cause devastating disease in sub-Saharan Africa in humans and livestock. The parasite lives extracellularly within the bloodstream of mammalian hosts and is transmitted by blood-feeding tsetse flies. In the blood, trypanosomes exhibit two developmental forms: the slender form and the stumpy form. The slender form proliferates in the bloodstream, establishes the parasite numbers and avoids host immunity through antigenic variation. The stumpy form, in contrast, is non-proliferative and is adapted for transmission. Here, we overview the features of slender and stumpy form parasites in terms of their cytological and molecular characteristics and discuss how these contribute to their distinct biological functions. Thereafter, we describe the technical developments that have enabled recent discoveries that uncover how the slender to stumpy transition is enacted in molecular terms. Finally, we highlight new understanding of how control of the balance between slender and stumpy form parasites interfaces with other components of the infection dynamic of trypanosomes in their mammalian hosts. This interplay between the host environment and the parasite’s developmental biology may expose new vulnerabilities to therapeutic attack or reveal where drug control may be thwarted by the biological complexity of the parasite’s lifestyle. Keywords: Trypanosoma brucei; transmission; stumpy form; differentiation; life-cycle; quorum sensing 1. Introduction Persistence in regularly changing environments demands that vector-transmitted pathogens rapidly adapt to fluctuating conditions. Trypanosoma brucei, similar to many eukaryotic parasites, encounters different environments as it transitions between its mammalian host and the tsetse fly vector, Glossina spp. In these different environments, distinct T. brucei life cycle forms, specialised for the various niches encountered, have evolved to optimise parasite survival and to ensure transmission to the next host (Figure1). Pathogens 2017, 6, 29; doi:10.3390/pathogens6030029 www.mdpi.com/journal/pathogens Pathogens 2017, 6, 29 2 of 19 Pathogens 2017, 6, 29 2 of 18 salivary glands Epimastigote 3 To salivary glands via Metacyclic 2 proventriculus Tsetse immune Proteases response Procyclic Tsetse midgut 1 Changing pH Changing nutrient availability Temperature Stumpy change Metacyclic Presence of symbionts 4 5 Immune attack Mammalian Drug Slender bloodstream treatment Stumpy Depletion of Competitor Intermediate parasites host resources Figure 1. TheFigure changing 1. The changing environments environments of theof theT. T. brucei brucei lifelife cycle. cycle. (1) (1) Following Following a tsetse a tsetsefly blood fly meal blood meal on an infected mammal (blue arrow), T. brucei stumpy cells reaching the tsetse midgut differentiate on an infected mammal (blue arrow), T. brucei stumpy cells reaching the tsetse midgut differentiate to to procyclic cells. (2) Development of T. brucei proceeds through the tsetse midgut to the procyclic cells.proventriculus (2) Development. (3) Differentia of T.tion brucei toproceeds epimastigote through cells theis tsetsefollowed midgut by toproduction the proventriculus. of (3) Differentiationmammalian to-infective epimastigote metacyclic cells cells is in followed the salivary by gland production. (4) On transfer of mammalian-infective to a new mammalian host metacyclic cells in thevia salivary a tsetse gland.fly bite (4)(blue On arrow), transfer metacyclic to a new cells mammalian differentiate hostto bloodstream via a tsetse forms fly. bite(5) In (blue the arrow), metacyclicbloodstream, cells differentiate proliferative to bloodstream slender cells forms. elevate (5) the In theparasitaemia bloodstream, until proliferativeaccumulation of slender a cells density-dependent signal triggers differentiation to the cell-cycle arrested stumpy form (via an elevate the parasitaemia until accumulation of a density-dependent signal triggers differentiation to intermediate stage). The stumpy form of T. brucei faces a particular challenge in that it must survive the cell-cyclelong arrestedenough in stumpythe bloodstream form to (via be transmitted, an intermediate but it must stage). also survive The stumpy long enough form in oftheT. tsetse brucei faces a particularmidgut challenge to differentiate in that it to must the survivenext life longcycle enoughstage and in theestablish bloodstream infection. toThese be transmitted,different but it must alsoenviron survivements long pose enoughdifferent inchallenges the tsetse that midgut must be toovercome differentiate to enable to the continuation next life cycle of the stage and establish infection.parasite life Thesecycle. For different example, environments in the mammalian pose host different (red section), challenges parasites thatmust mustevade beimmune overcome to attack to ensure parasite survival and avoid excessive exploitation of host resources to ensure the enable the continuation of the parasite life cycle. For example, in the mammalian host (red section), host survives long enough to enable successful transmission to the tsetse fly vector. On uptake to the parasites musttsetse fly evade midgut immune (orange attacksection), to the ensure parasites parasite face a shift survival in nutrient and availability avoid excessive and temperature exploitation of host resourcesamong to other ensure challe thenges. host For survivesa review of long factors enough effecting to establishment enable successful of tsetse transmissionfly midgut infection to the tsetse fly vector.see On [1] uptake. to the tsetse fly midgut (orange section), the parasites face a shift in nutrient availability and temperature among other challenges. For a review of factors effecting establishment of The challenges presented by the distinct environments encountered during the life cycle differ. tsetse fly midgut infection see [1]. In the mammalian bloodstream, for example, the parasite must face a host immune response that is constantly trying to control the infection. To evade this control and to thrive extracellularly within The challengesthe mammalian presented host bloodstream, by the distinct trypanosomes environments have evolved encountered the extraordinary during thecapacity life cycleto differ. In the mammalianovercome a bloodstream,barrage of host forinnate example, and adaptive the parasite immunological must facedefences a host via immunethe expression, response and that is switching, of their expressed Variant Surface Glycoprotein (VSG) coat, a process termed antigenic constantlyvariation trying to[2]. control The periodic the infection. destruction To evadeof antigenic this control variants, and followed to thrive by extracellularlythe outgrowth withinof the mammalianantige hostnically bloodstream, distinct variants, trypanosomes contributes haveto the evolvedundulating the waves extraordinary of parasitaemia capacity typical to of overcome a barragetrypanosome of host innate infection. and adaptive immunological defences via the expression, and switching, of their expressedWithin Variant each Surfaceof these cyclical Glycoprotein waves, a (VSG)density- coat,dependent a process developmental termed switch antigenic represents variation [2]. another important evolutionary adaptation, both regulating parasite virulence and pre-adapting the The periodic destruction of antigenic variants, followed by the outgrowth of antigenically distinct parasite for uptake by the tsetse fly. Infections are maintained in the host bloodstream by rapidly variants, contributesproliferating morphologically to the undulating “slender waves” cells. of parasitaemiaAllowed to replicate typical unchecked, of trypanosome slender parasites infection. Withinwould each rapidly of these kill the cyclical host [3]. Consequently waves, a density-dependent, it is important that parasites developmental regulate their switch growth representsin another important evolutionary adaptation, both regulating parasite virulence and pre-adapting the parasite for uptake by the tsetse fly. Infections are maintained in the host bloodstream by rapidly proliferating morphologically “slender” cells. Allowed to replicate unchecked, slender parasites would rapidly kill the host [3]. Consequently, it is important that parasites regulate their growth in vivo to promote longevity of the host and maximize their transmission potential [4,5]. Such regulation is achieved by a developmental switch initiated upon reaching a critical parasite density. Specifically, proliferative slender cells respond to the accumulation of a small soluble parasite-derived factor, stumpy induction factor (or factors) (SIF), and differentiate to specialized cell-cycle arrested “stumpy” Pathogens 2017, 6, 29 3 of 19 cells [6,7]. As growth-arrested stumpy cells accumulate during the ascending parasitaemia, the parasite population growth