ARTICLE IN PRESS

Immunobiology 214 (2009) 737–747

0171-2985/$ - se

doi:10.1016/j.im

Abbreviations

cells; Tregs, reg�CorrespondE-mail addr

1Present addr

Cedex 9, Franc

www.elsevier.de/imbio

REVIEW

Understanding the role of monocytic cells in liver inflammation using

parasite infection as a model

Tom Bosschaertsa,b, Martin Guilliamsa,b,1, Benoit Stijlemansa,b,Patrick De Baetseliera,b, Alain Beschina,b,�

aDepartment of Molecular and Cellular Interactions, VIB, 1050 Brussel, BelgiumbLaboratory of Cellular and Molecular Immunology, Vrije Universiteit Brussel, 1050 Brussels, Belgium

Abstract

Uncontrolled inflammation is a major cause of pathogenicity during chronic parasite infections. Novel therapiesshould therefore aim at re-establishing the balance between pro- and anti-inflammatory signals during disease to avoidtissue damage and ensure survival of the host. In this context, we are intending to identify strategies capable ofinducing counter-inflammatory activity in injured liver and thereby increasing the resistance of the host to Africantrypanosomiasis as a model for parasite infection. Here, recent evidence is summarized revealing how monocytic cellsrecruited to the liver of African trypanosome-infected mice develop an M1 or M2 activation status, therebymaintaining the capacity of the host to control parasite growth while avoiding the development of liver damage, whichotherwise culminates in early death of the host.r 2009 Elsevier GmbH. All rights reserved.

Keywords: Cell differentiation; Inflammation; Liver; Monocytes; Parasite infection

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738

Classically activated monocytic cells (M1) and T cells oriented towards a type 1 immune response are crucial for the control of

parasitemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 739

Skewing towards an alternative type 2 immune response is essential to limit type 1 inflammation-associated pathogenicity 740

TNF and iNOS producing dendritic cells (TIP-DCs) are a major M1 monocytic cell population involved in parasite control

and pathogenicity development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 741

Dampening pathogenicity of African trypanosome infection through expansion of regulatory T cells (Tregs) or delivery

of IL-10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 741

Proposed model for resistance to African trypanosome infection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 742

e front matter r 2009 Elsevier GmbH. All rights reserved.

bio.2009.06.010

: TIP-DCs, TNF and iNOS producing dendritic cells; M1, classically activated monocytic cells; M2, alternatively activated monocytic

ulatory T cells.

ing author at: Laboratory of Cellular and Molecular Immunology, Building E, floor 8, Pleinlaan 2, 1050 Brussel, Belgium.

ess: Alain.Beschin@imol.vub.ac.be (A. Beschin).

ess: Centre d’Immunologie de Marseille-Luminy (CIML), Parc Scientifique et Technologique de Luminy-Case 906, 13288 Marseille,

e.

ARTICLE IN PRESST. Bosschaerts et al. / Immunobiology 214 (2009) 737–747738

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 743

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 744

Introduction

African trypanosomes are blood-borne unicellularprotozoan parasites that are transmitted through thebite of their vector, the tsetse fly. They infect bothhuman and livestock from north of South Africa tosouth of Algeria, Libya and Egypt (the ‘‘tsetse belt’’).Human African trypanosomiasis (sleeping sickness) is are-emergent disease with an underestimated incidence ofat least 300,000 cases within 60 million people living inrisk areas leading to 50,000 deaths each year (Steverding2008). Two types of sleeping sickness exist: the T. brucei

gambiense form is chronic, with individuals carrying theparasite without symptom for long periods, and whensymptoms appear, the patient is often already at anadvanced stage. This form occurs in west and centralAfrica and represents more than 90% of the reportedcases of sleeping sickness. The Trypanosoma brucei

rhodesiense form, found in eastern and southern Africaand representing less than 10% of the reported cases, isacute with symptoms revealing soon after the infection(within weeks) and the disease developing rapidly.Uganda is the only country where both forms exist.Animal African trypanosomiasis (Nagana disease,caused by T. congolense, T. vivax and T. b. brucei) isa major constraint on cattle production, which is themain source of income for smallholder farmers, killing1 million cattle each year. The estimated economiccost of the sole animal diseases is $1.2 billions each year.Both, in humans and animals untreated cases areeventually fatal (Steverding 2008).

Several phenomena greatly complicate both theclinical and pathogenic features of African trypanoso-miasis. First, all African trypanosomes use antigenicvariation of their variant specific glycoprotein (VSG)coat protein to evade the antibody response of theirmammalian host (Taylor and Rudenko 2006). Second,the parasite species infecting humans (and African apes)have evolved mechanisms allowing them to resist to lysiscaused by an innate immune factor from the blood, theapolipoprotein L1 present in a subset of HDL particles(Pays and Vanhollebeke 2008). Third, a profoundimmunosuppression occurs following infection by theseparasites, which lowers the host’s resistance to otherinfections and results in often fatal secondary disease(Greenwood et al. 1973; Mansfield and Paulnock 2005).Fourth, immunologic lesions to tissues including thespleen, lymph nodes, myocardium, brain, kidneys andliver are common in natural African trypanosomiasis,which prevent the normal functioning of the affected

organ and thereby compromise both the host survivaland the parasite transmission (Losos and Ikede 1972).The ever changing VSG coat as well as immunosuppres-sion hamper vaccination against trypanosomes andleave chemotherapy as the only disease-control method.However, most of the currently available drugs thatwere developed empirically before 1950 remain expen-sive for the affected populations and have toxic effects.Moreover, there is a lack of commercial interest in thedevelopment of new drugs. Thus, the human and animalAfrican trypanosomiases remain as neglected diseaseswith a high negative socio-economical impact. It istherefore important to identify the cellular and mole-cular players that govern the delicate balance betweeneradication of the pathogen by the host’s immunesystem and limitation of undesirable secondary effects.

Cells of the monocytic lineage, in particular macro-phages, have long been recognized as the first hostimmune cells to encounter infecting trypanosomes and/or their products during natural as well as experimentalinfection, thereby orchestrating the onset of infection(Askonas 1985). It is now well established that thesecells can play a protective or pathogenic role dependingon their state of activation during the course of infection(Noel et al. 2004). Due to ethical and practicalconsiderations as well as prohibitive cost and difficultiesassociated with bovine challenge experiments, the easilyaccessible mouse models have currently become analternative to the identification of the contributionof cells from the monocytic lineage in the clearanceof parasites from the circulation as well as onthe inflammatory (pathogenic)/anti-inflammatory (anti-pathogenic) mechanisms. In mice, symptoms reflectingpathogenicity include liver injury and anemia. Althoughliver monocytic cells have long been recognized asessential contributors to parasite clearance, it is only inthe recent years that they have received a particularattention. Accordingly, depending on their activationstatus, liver monocytic cells can contribute to theprotection or induction of liver damage, therebymaintaining or impairing the parasite clearance capacityand survival of the host. The studies addressing the roleof liver monocytic cells in liver destruction are here-insummarized. For readers interested, the role of mono-cytic cells in the development of infection-associatedanemia has been recently reviewed (Naessens 2006;Stijlemans et al. 2008). It should be mentioned that mostof these murine studies have been performed with twoAfrican trypanosome species: T. congolense and T. b.

brucei. The results obtained show some differences,

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which could be due to differences in the biology of thesetwo parasites: T. brucei (subgenus Trypanozoon) travelsin the blood but principally resides in the lymphand peripheral organs while T. congolense (subgenusNannomonas) localizes strictly to the blood vessels andcapillaries of the animals it infects (Naessens 2006).

Classically activated monocytic cells (M1) and

T cells oriented towards a type 1 immune

response are crucial for the control of

parasitemia

African trypanosomes remain extracellular parasitesduring the whole course of infection, exposing them toimmune attacks by antibody responses and microbicidalcompounds released by monocytic cells. Binding of IgGantibodies to exposed epitopes of the VSG coat isbelieved to lead to opsonisation and phagocytosis of theparasites mainly by the Kupffer cells, i.e. the residentmonocytic cells from the liver (Dempsey and Mansfield1983; Morrison et al. 1982; Muller et al. 1996; Theodosand Mansfield 1990; Theodos et al. 1990; Williams et al.,1996). In accordance, depletion of liver monocytic cellsby treating T. congolense-infected mice with gadoliniumchloride, or due to increased apoptosis of liver mono-cytic cells during infection in Sepp1 KO mice or in anti-IL-10R antibody-treated mice, abolishes the ability ofthe host to control the parasite growth and severelyshorten its survival (Bosschaerts et al. 2008; Shi et al.2003, 2004).

Besides antibody-mediated opsonisation and phago-cytosis of the parasites, other effector molecules arerequired to control African trypanosome growth amongwhich IFN-g, TNF and NO are the best studied. IFN-gis mainly produced by T cells during T. congolense- andT. brucei-infection via an IL-12p70-dependent butIL-12p80 homodimer or IL-23-independent mechanisminducing the development of classically activated mono-cytic cells (M1) producing trypanostatic or trypanotoxiccomponents: TNF, ROS and NO (Barkhuizen et al.2007, 2008; Hertz and Mansfield 1999; Iraqi et al. 2001;Kaushik et al. 2000; Magez et al. 1999, 2006;Namangala et al. 2001b; Schleifer and Mansfield 1993;Shi et al. 2006; Taiwo et al. 2002). MHC-II signaling isalso important for the activation of T cells to produceIFN-g and the subsequent activation of monocyticcells to produce TNF and NO in mice infected withT. congolense (Magez et al. 2006).

Infection of iNOS KO mice with T. congolense hasshown that NO is absolutely required for controlling thefirst of parasitemia (Magez et al. 2006). On the otherside, TNF KO or TNF-R1 KO mice (p55TNF-R1�X�)infected with T. congolense cannot control the first peakof parasitemia and succumb earlier than wild-type mice

(Iraqi et al. 2001; Magez et al. 2007). It has beenproposed that TNF-R1 signaling triggers iNOS activa-tion resulting in the production of trypanostatic NO,which would facilitate the opsonisation and phagocy-tosis of trypanosomes by Kupffer cells (Magez et al.2007). In T. brucei infection, NO appears to be lessimportant for parasite control than in T. congolense

infection, parasitemia and survival being similar iniNOS KO and wild-type mice (Hertz and Mansfield1999). In contrast, TNF seems to be involved in thecontrol of parasitemia (Magez et al. 1999). Moreover,NO and TNF have been shown to exhibit a direct lyticactivity in vitro on T. brucei but not on T. congolense

(Daulouede et al. 2001; Magez et al. 1999, 2007;Vincendeau et al. 1992).

Hence, in both T. congolense- and T. brucei-infection,an IFN-g driven M1 activation of monocytic cells isrequired to induce the production of trypanotoxiccompounds controlling the parasite growth, i.e. TNFand/or NO. Taking into consideration the role ofantibodies in the clearance of parasites, the currentpostulate states that three distinct effector mechanismsare necessary for efficient clearance of African trypano-somes from the host circulation: (i) production oftrypanotoxic molecules by M1 (TNF and/or NO) todecrease the in vivo fitness of the parasites, (ii) therecognition of the parasite by VSG-specific antibodiesand (iii) opsonisation and phagocytosis of theseintoxicated, damaged and antibodies covered parasitesby (liver) monocytic cells.

One predominant mechanism for the innate activationof monocytic cells towards an M1 activation status,including TNF, NO, IL-12, and type I IFN production,involves the recognition of PAMPs on microorganisms(Coller et al. 2003; Dagenais et al. 2009; Drennan et al.2005; Harris et al., 2006 2007; Kaushik et al. 2000;Leppert et al. 2007; Lopez et al. 2008; Schleifer andMansfield 1993; Tabel et al. 1999). Two PAMPstriggering the development of M1 have been identifiedso far in African trypanosomes, namely the GPI anchorsof the VSG (the most abundant trypanosome molecule,107/trypanosome) and unmethylated DNA motifs(Drennan et al. 2005; Harris et al. 2006, 2007). Wheninfected with T. brucei, MyD88 KO mice control theparasitemia less efficiently, succumb earlier and producereduced levels of IFN-g and TNF indicating thatMyD88 signaling is required for the development ofM1 and parasite control. The TLRs involved in therecognition of the GPI moiety from VSG T. brucei couldnot be identified while TLR9 has been shown to beinvolved in the recognition of trypanosome DNA and inthe control of parasitemia (Drennan et al. 2005; Harriset al. 2006). Besides TLRs, other innate receptors,including scavenger receptor, have been proposed torecognize VSG (Leppert et al. 2007). Other molecules,the T lymphocyte triggering factor (TLTF) that induces

ARTICLE IN PRESST. Bosschaerts et al. / Immunobiology 214 (2009) 737–747740

CD8+ T cells to secrete IFN-g (Olsson et al. 1993) andthe trypanosome suppression inducing factor (TSIF)whose recombinant form triggers the synthesis of TNFand NO by monocyte-derived cells (Gomez-Rodrıguezet al. 2008), could represent additional T. brucei M1triggers, although this has not been demonstrated.

Skewing towards an alternative type 2 immune

response is essential to limit type 1

inflammation-associated pathogenicity

The large repertoire of VSG genes results in theperiodic expression of unique antigenic determinantsthrough antigenic variation that prevents the completeelimination of African trypanosomes despite the induc-tion of VSG-specific immune responses, thereby ensur-ing chronic infection of the host. This chronic infectionresults in the development of pathogenic features forwhich the severity depends on the host–trypanosomecombination, with some hosts being able to limit theinduction of pathogenicity, thereby living with theparasite for months or years in absence of significantdisease symptoms. The hosts undergoing an infectionwith low pathogenicity have been termed trypanotoler-ant/resistant, as opposed to hosts displaying severedisease symptoms and pathogenicity which have beencalled trypanosusceptible/susceptible (Pinder et al. 1988;Tabel et al. 2000; Taylor 1998). As for natural hosts(Naessens 2006), mouse models also display phenotypicdifferences in trypanotolerance/susceptibility wheninfected with various trypanosome species (Antoine-Moussiaux et al. 2008). BALB/c and CBA mice arehighly susceptible to T. congolense infection while C3Hmice show an intermediate phenotype and C57BL/6mice display a trypanotolerant phenotype characterizedby long survival and attenuated pathogenicity. ForT. brucei infection, C3H and BALB/c mice are shown tobe more trypanosusceptible as compared to C57BL/6and CBA mice. Susceptibility patterns between mousestrains have been used to identify candidate genesassociated with trypanotolerance/susceptibility throughgenetic cross-breeding schemes. Most studies have beenconducted in T. congolense models (d’Ieteren et al. 1998;Hill et al. 2005; Iraqi et al. 2000; Kemp et al. 1997;MacLean et al. 2004; Maillard et al. 2005; Trail et al.1994). These studies indicate that the difference insusceptibility observed between parental mouse strainsis accounted by three loci, named Tir1, Tir2 and Tir3mapped to chromosomes 17, 5 and 1, respectively.Interestingly, the TNF and IL-10 genes and some oftheir regulatory elements were found in Tir1 and Tir3,respectively (Tabel et al., 2000) suggesting an importantrole for both cytokines in the outcome of the disease.Accordingly, the absence or limited production of TNF

during T. congolense- or T. brucei-infection preventsexcessive liver damage and correlates with prolongedsurvival (Guilliams et al. 2008, 2007; Magez et al. 2004).As TNF, NO is also implicated in induction ofpathogenicity since iNOS KO mice show decreasedpathogenicity to T. brucei infection and survive at leastas long as wild-type mice (Hertz and Mansfield 1999;Magez et al. 1999). Together, these data highlight thecontribution of M1 to the pathogenicity of Africantrypanosomiasis. Thus, M1 by producing TNF andNO play a dual role during African trypanosomiasisbeing involved in parasite control (see point 2) and ininduction of disease pathogenicity.

On the other hand, IL-10 is shown to be crucial forhost survival by limiting the IFN-g-driven M1 activa-tion of monocytic cells avoiding excessive productionof pathogenic TNF and NO molecules during bothT. congolense- and T. brucei-infection (Namangala et al.2001b; Shi et al. 2003, 2006). In agreement, the increasedpathogenicity observed in T. congolense-infected micetreated with an anti-IL10R antibody could be counter-acted by treating mice in the early infection with an anti-IFN-g antibody (Shi et al. 2003, 2006). IL-10 is thusessential to maintain the balance between pro-inflam-matory (IFN-g, TNF and NO) signals governing theability of the host to control parasite growth and anti-inflammatory/anti-pathogenic signals avoiding collat-eral tissue damage during African trypanosomiasis. Ofimportance, the IFN-g, TNF and NO production aswell as the IL-10 production in C57BL/6 mice infectedwith T. congolense must be timely regulated; the IL-10-mediated immune response driving monocytic cellstowards an alternative activation status (M2) occurringin the chronic stage of infection, i.e. once the first andmost aggressive peak of parasitemia has been controlledby the IFN-g-dependent M1 activation (Noel et al.2002). These observations have been confirmed duringT. brucei infection using attenuated phospholipase-C-deleted (PLC�X�) T. brucei parasites (Namangalaet al.2000, 2001a). Indeed, mice infected with wild-typeT. brucei, which only survive 5 weeks post infection, arelocked in a type 1 immune response and cannot sustainIL-10 secretion. On the other hand, mice infected withT. brucei PLC�X� strain survive for about 30 weeks andexhibit a sequential switch from a IFN-g-dependentM1 activation of monocytic cells in the early stage ofinfection to an IL-10-driven M2 activation of monocyticcells, which is beneficial for the host by protecting fromcollateral tissue damage (Bosschaerts et al. 2008;Guilliams et al. 2007; Namangala et al. 2001b).

It can be mentioned that although IL-4 and IL-13,which are known to induce M2 activation of monocyticcells (Martinez et al. 2008), are produced in the chronicstage of T. congolense- or T. brucei-PLC�X� infection,they do not, in contrast to IL-10, contribute tothe decrease pathogenicity of T. congolense infection

ARTICLE IN PRESST. Bosschaerts et al. / Immunobiology 214 (2009) 737–747 741

((Namangala et al. 2000; Noel et al. 2002) and Beschinet al. unpublished). Thus, IL-10-driven M2 are crucialfor protecting the host against African trypanosomiasispathogenicity.

These M2 cells have been found to express genes withpotential wound healing and anti-inflammatory proper-ties and have been proposed to play an active role inprotection against pathogenicity (Ghassabeh et al. 2006;Noel et al. 2004). In this context, our lab is currentlyinvestigating the role of IL-10-dependent M2 associatedgenes in protection against pathogenicity during Africantrypanosome infection. A number of genes preferentiallyexpressed by M2 elicited during T. congolense infectionas well as in other pathological situations associatedwith the emergence of M2 (helminth infection, cancerprogression), have been identified. Within this pool ofgenes, 8 were either IL-10 inducible in vitro in monocyticcells from non-infected mice (Arg1, Sepp1, Mgl1, Mrc1

and Folr2; Ghassabeh et al. 2006) or IL-10 dependentduring T. congolense infection (Sepp1, Ctss, F13a1 andNgfb) since these latter genes were induced in monocyticcells from T. congolense-infected wild-type mice, butnot in IL-10 KO mice (Bosschaerts et al. 2008). Inthis context, we have initiated experiments to addressthe functional implications of M2-associated genes inT. congolense infection starting with the gene product ofSepp1 (coding for selenoprotein P). Current data showthat selenoprotein P favors the resistance to Africantrypanosomiasis by reducing the pathogenicity of thedisease. Through anti-oxidant properties, selenoproteinP limits the production of ROS via its N-terminaldomain-mediated oxidoreductase enzymatic function,therefore preserving parenchymal and monocytic cellsfrom the liver from type 1 inflammation mediated theapoptotic/necrotic damage.

TNF and iNOS producing dendritic cells (TIP-

DCs) are a major M1 monocytic cell population

involved in parasite control and pathogenicity

development

Recent efforts have focused on the identificationof cellular sources of pathogenic molecules in Africantrypanosome-infected hosts. A particular subset ofMHC-II-restricted, so called ‘‘pathogenic’’ CD4+ Tcells, producing IFN-g has been suggested to contributeto the early mortality of infected hosts (Shi et al. 2006).Although cells from the monocytic lineage have longbeen shown to be the major source of TNF duringT. congolense- and T. brucei-infection (Magez et al.1999; Mansfield and Paulnock 2005; Namangala et al.2001a; Noel et al. 2002; Schleifer and Mansfield 1993;Sternberg et al. 2005), their phenotype/identity hasnever been formally recognized. In this context, we have

recently reported that CD11b+Ly6chiLy6 g� monocytesgradually accumulate in the liver of mice infected withT. brucei from which a significant proportion differ-entiate to CD11b+Ly6chiCD11c+ inflammatory den-dritic cells (DCs) (Similar observation was made in thespleen and the lymph nodes). In addition, monocyte-derived CD11b+Ly6chiCD11c+ inflammatory DCs ex-hibit a mature phenotype expressing elevated levels ofCD80/CD86 and MHC-I/MHC-II molecules, and beinga major source of TNF and iNOS, classifying these cellsas TIP-DCs (Guilliams et al. 2009). This phenotype isreminiscent of the cell-type that has been described asessential to control the growth of intracellular pathogens(Serbina et al. 2008). TIP-DCs also represent a majorsource of TNF and NO in T. congolense infection(Bosschaerts et al. unpublished).

Considering the essential role for TNF in T. brucei

clearance (Magez et al. 1997, 1999), we suggest that TIP-DCs, as TNF (and iNOS) producing cells, participate tothe control of trypanosome growth. On the other hand,considering the known role for TNF and NO in thepathogenicity of T. brucei infection (Hertz and Mans-field 1999; Magez et al. 1999), TIP-DCs are mostprobably involved in the induction of pathogenicity.Accordingly, we have observed that the enhancedmaturation of TIP-DCs in T. brucei-infected IL-10 KOmice associates with increased tissue damage andreduced survival, but does not influence the control ofthe parasite growth (Guilliams et al. 2009).

Dampening pathogenicity of African

trypanosome infection through expansion of

regulatory T cells (Tregs) or delivery of IL-10

Concerning the cellular sources of anti-pathogenicmolecules, M2 and CD4+CD25+Foxp3+ naturallyoccurring Tregs have been found to be major sourcesof IL-10 during T. congolense infection in C57BL/6 mice(Bosschaerts et al. 2008; Guilliams et al. 2007). On theother hand, as compared to T. congolense-infected mice,T. brucei infection is associated with low IL-10 produc-tion in the absence of Treg expansion and M2development (Guilliams et al. 2008). Therefore, wewondered whether we should increase the resistance ofT. brucei-infected host by stimulating Treg and/or M2expansion through CD28 superagonist antibody treat-ment (that has been documented to induce the expan-sion and activation of Tregs (Dennehy et al. 2006;Guilliams et al. 2008) and/or by inducing the IL-10production through AAV gene delivery (Guilliams et al.2009)). Both treatments down-regulate the productionof IFN-g (by CD4+ and CD8+ T cells) and reducethe differentiation and maturation of monocytes toTIP-DCs in the liver (as well as in the spleen and the

ARTICLE IN PRESST. Bosschaerts et al. / Immunobiology 214 (2009) 737–747742

lymph nodes). Monocytes from anti-CD28 superagonistantibody and IL-10/AAV-treated infected mice expresslower CD80, CD86, MHC-I and MHC-II levels anddisplay a decreased TNF and iNOS production. Thereduced differentiation and maturation of monocytes toTIP-DCs in CD28 superagonist antibody and IL-10/AAV-treated mice associates with the protection againstliver injury and prolonged survival.

In parallel, in view of the strongM1 activating potentialof T. brucei VSG–GPI (see point 2.), strategies have beendesigned to counteract the GPI-mediated M1 activation(Stijlemans et al. 2007). The GPI-based treatment resultsin a significantly prolonged survival and substantialprotection against infection-associated liver damage.

Although the GPI-based treatment alleviates T. brucei-mediated pathogenicity, it somehow differs from theprotective mechanisms elicited by the CD28 super-agonist antibody treatment or by the IL-10/AAV treat-ment. Indeed, both the CD28 superagonist antibody andthe IL-10/AAV treatment block the acquisition of CD11cby CD11b+Ly6chi inflammatory monocytes and theirmaturation into TNF and NO producing TIP-DCS, whilethe GPI-based treatment solely limits the maturation stepof inflammatory DCs (Guilliams et al. unpublished).Moreover, the GPI-based treatment, as the IL-10/AAVtreatment, does no elicit the development of Tregs incontrast to CD28 superagonist antibody treatment(Guilliams et al. unpublished).

Notably, neither the CD28 superagonist antibody northe IL-10/AAV nor the GPI-based treatment in T. brucei-infected mice exacerbate parasite growth despite reducingthe TIP-DCs maturation required for parasite clearance(Guilliams et al. 2008; Stijlemans et al. 2007). Thus, theTIP-DC maturation can be dampened to a certain extentwithout impairing the control of T. brucei growth. Inagreement with this notion, IL-10 KO mice do not havean enhanced parasite clearance capacity despite theincreased level of TIP-DCs (Guilliams et al. 2009). Thus,the major factor determining the survival of the host is itsability to limit the tissue injury. Accordingly, the survivalof IL-10 KO mice is shorter than the survival of IFN-gRKO mice (Guilliams et al. 2009; Mabbott et al. 1998).Importantly, IL-10-mediated suppression of monocytedifferentiation is reversible since monocytes fromT. brucei-infected IL-10-treated mice transferred ininfected recipient mice in absence of IL-10 differentiaterapidly into inflammatory DCs (Guilliams et al. 2009).This suggests that IL-10 is constantly required to controlthe long-term TIP-DC-mediated inflammation.

Fig. 1. Proposed model for the role of liver monocyte-derived

cells in resistance to African trypanosome infection. See text

for details.

Proposed model for resistance to African

trypanosome infection

Based on the here-above summarized data, a modelaccounting for resistance to African trypanosome

infection in mice can be tentatively integrated (Fig. 1).IFN-g-dependent classically activated monocytic cells(M1) producing TNF, NO and ROS have been longrecognized to be involved in parasite clearance as well asin the induction of pathogenicity during Africantrypanosomiasis. In mice, liver destruction represents amajor sign of pathogenicity, which results in loss ofparasite control capacity of the host culminating in earlydeath. However, IL-10 produced by Foxp3+ Tregs, M2and other sources (potentially including Tr1 cells orregulatory B cells; (Anderson et al. 2007; Bosschaertset al. 2008; Jankovic et al. 2007; Kaushik et al. 2000;Lund et al. 2005; Roncarolo et al. 2006) contribute todecrease the pathogenicity of African trypanosomiasisby limiting the IFN-g production by CD4+ and CD8+

T cells, thereby limiting M1 activation and theirsecretion of TNF, NO and ROS. Expansion of Tregsand increased IL-10 production also associate with M2activation. In this context, by analyzing the gene profilein M2-oriented monocytic cells in T. congolense- andT. brucei-infected mice, a number of IL-10-dependentgenes potentially involved in the control of inflamma-tory processes, that could prevent liver pathogenicityand thus contribute to the resistance of the host to thedisease, have been identified. One of these genes codingfor selenoprotein P was found essential to control thepathogenic effect of the parasite including excessiveproduction of ROS as well as destruction by apoptosis/necrosis of liver monocytic cells and hepatocytes(Bosschaerts et al. 2008). Recent findings suggestthat overexpression of genes from the bile acid andcholesterol synthesis pathways in the liver could also beassociated with T. congolense trypanotolerance in mice

ARTICLE IN PRESST. Bosschaerts et al. / Immunobiology 214 (2009) 737–747 743

(Kierstein et al. 2006). Since the regulation of lipidmetabolism can affect cytokine production and phago-cytosis by monocytic cells (Joseph et al. 2004; Kay et al.2006; Manley et al. 2006), it is of interest to investigatethe effect of lipid homeostasis on monocytic cellactivation and liver immunopathogenicity during Afri-can trypanosome infection.

TIP-DCs are a major source of TNF and NO, actingboth in trypanosome clearance as well as in inductionof pathogenicity. TIP-DCs likely originated fromCD11b+Ly6chiCD11c� monocytes recruited from thebone marrow, which differentiate and mature intoCD11b+Ly6chiCD11c+ TIP-DCs upon IFN-g-mediated activation in the spleen, the lymph nodesand the liver of infected mice. These TIP-DCs can beconsidered as a subpopulation of classically activatedM1 monocytic cells that have long been recognized intrypanosome infection (Magez et al. 1999; Namangalaet al 2001b; Schleifer and Mansfield 1993; Tabel et al.2000). Interestingly, the M1-like TIP-DC activation ofmonocytes is impaired by Tregs and IL-10 and leads tothe expression of M2-associated genes. Thus, dependingon the immune environment, recruited monocytes couldadopt an M1-like TIP-DC phenotype in conditionswith high IFN-g and low IL-10 production, or an M2phenotype in an environment with high IL-10 and lowIFN-g production. Efforts are currently devoted toidentify the chemokines mediating the recruitment ofpathogenic M1 to the liver of infected mice in order todecrease the inflammation and increase the resistance tothe disease. Up to now, works have focused mainly onthe role of recruited CD11b+Ly6chi monocyte-derivedcells in the control of African trypanosome infection,while the role of residential (CD11b+Ly6clo) monocyticcells, including liver Kupffer cells, still has to beaddressed. In this context the finding that recruitedmonocyte-derived cells produce high levels of patho-genic compounds TNF and iNOS calls for a thoroughevaluation of the contribution of resident monocyticcells in this process. We cannot exclude that residentand recruited monocytic cells in the liver cooperatedifferently to the elimination of African trypanosomesthrough phagocytic activity and production of trypano-toxic compounds TNF and NO, respectively, asreported during Listeria monocytogenes infection(Serbina et al. 2008). In addition, it is currently unclearif and how resident and/or recruited monocyticcell populations from the liver contribute to the T cellimmunosuppression occurring during African trypano-some.

Considering the anti-inflammatory activity of M2,M2 present in the liver of trypanotolerant hosts protectthe organ from destruction by the early stage-associatedtype 1 immune response. However, even these animalsfinally die within 5 months post-infection. Since M2 cancontribute to liver injury (including fibrosis) through the

prototypic M2-associated arginase-1 activity (Pesceet al. 2009), M2 treatment may contribute to liverdamage.

Conclusion

The present model is based on findings from murinemodels of tolerance and susceptibility to Africantrypanosomiasis, mainly based on 2 parasite species –T. congolense and T. brucei – infecting the same C57BL/6 mice, which are well-developed and offer an accessiblemeans to study the underlying mechanisms. Althoughthe comparison between T. congolense- and T. brucei-infected C57BL/6 mice has proven to be a fruitfulapproach to discover essential immunoregulatory me-chanisms, T. congolense- and T. brucei-parasites displaya different infection pattern and distribution in thebody of the host. As such, one might argue thatthe comparison between T. congolense- and T. brucei-infected mice is not optimal. However, the generalconclusions regarding the association between main-tained IFN-g-mediated inflammation, induction ofpathogenicity and short survival hold true inT. congolense- and T. brucei-infected mice. Moreover,the immunoregulatory mechanisms identified duringT. congolense infection could successfully be used toincrease the trypanotolerance/resistance of T. brucei-infected mice. Thus, so far, the general conclusionsremain applicable in both models. However, it hasbecome clear in the two last years that even in murinemodels, the mechanisms of parasite control duringAfrican trypanosomiasis can differ. Indeed, T. evansi

causes the induction of IFN-g, TNF and NO asT. brucei or T. congolense. However, in contrast toT. brucei- and T. congolense-infection, none of thesemolecules is crucial for parasitemia control and survivalof T. evansi-infected mice; only IgM antibodies con-tribute significantly to the control of the latter parasite(Barkhuizen et al. 2007, 2008; Magez et al. 2008). Thus,the regulatory mechanisms underlying parasite controlas well as the induction of immunopathogenicity maydepend on specific host–parasite interactions. In thesame vein, it can be mentioned that IL-10 producingTregs are crucial for limitation of infection-associatedpathogenicity in T. congolense trypanoresistant C57Bl/6mice (Guilliams et al. 2007) while Tregs impede parasiteclearance during T. congolense infection in trypanosus-ceptible BALB/c mice (Wei and Tabel 2008).

Mice are not natural hosts of trypanosomes. There-fore, more works are needed before extrapolating thefindings from murine infection to human or bovinenatural infection since there are important differencesbetween murine, human and bovine trypanotolerance(Kennedy 2007; Naessens 2006; Sternberg 2004).

ARTICLE IN PRESST. Bosschaerts et al. / Immunobiology 214 (2009) 737–747744

Anemia is considered as the main pathogenic feature ofbovine trypanosomiasis initiated by T. congolense- andT. b. brucei-subspecies. Moreover, decreased pathogeni-city of bovine strains associates with decreased secretionof NO by IFN-g-activated monocytes and increasedtranscription of IL-10 (Taylor 1998). On the other hand,human African trypanosomiasis is caused by T. b.

rhodesiense and T. b. gambiense subspecies, which differfrom T. b. brucei and do not cause extensive (liver) tissuedestruction. Rather, human lethality results frominfiltration of parasites through the blood–brain barrierresulting from changes in endothelial cell properties(Kennedy 2007). Interestingly, a role for TNF and NOin increasing the permeability of the blood–brain barrierhas been proposed (Ballabh et al. 2004) and high IL-10levels in the brain are associated with reduced braindamage during human trypanosomiasis (Sternberg et al.2005). Thus, whether M1, including TIP-DCs, andIL-10 modulate the pathogenic features of bovine orhuman natural infections deserves investigation.

There is an unmet medical need to develop newmodalities for the therapy of liver damage, a commoninfliction that affects millions worldwide. Inflammatoryresponses play an important role in exacerbating liverinjury with often lethal consequences. Liver injury,independent of the etiology (i.e. chemicals, virus,parasite, autoimmunity, etc.), includes common char-acteristics (necrosis/apoptosis, cellular changes andscarring) that result from uncontrolled activation ofliver monocyte-derived cells that can be alleviated by IL-10. Yet, IL-10 inducible genes represent novel and morespecific therapeutic options to intervene in the host’sability to mount a protective immune response duringinflammatory/anti-inflammatory processes. Liver immu-nopathogenicity induced by parasitic infections maythus serve as an ideal model to unravel these mechan-isms and to identify new therapeutic leads (including IL-10-induced M2-associated genes that contribute to thelimitation of severity of African trypanosome infection)to treat hepatic inflammation in general, and parasite-induced hepatic injury in particular.

Acknowledgments

This work, performed in frame of an InteruniversityAttraction Pole Program, was supported by grants from the‘‘Institute for Promotion of Innovation by Science andTechnology in Flanders’’ (IWT-Vlaanderen) and the ‘‘Fundfor Scientific Research Flanders’’ (FWO-Vlaanderen).

References

Anderson, C.F., Oukka, M., Kuchroo, V.J., Sacks, D., 2007.

CD4(+)CD25(�)Foxp3(�) Th1 cells are the source of IL-

10-mediated immune suppression in chronic cutaneous

leishmaniasis. J. Exp. Med. 204, 285–297.

Antoine-Moussiaux, N., Magez, S., Desmecht, D., 2008.

Contributions of experimental mouse models to the under-

standing of African trypanosomiasis. Trends Parasitol. 24,

411–418.

Askonas, B.A., 1985. Macrophages as mediators of immuno-

suppression in murine African trypanosomiasis. Curr. Top.

Microbiol. Immunol. 117, 119–127.

Ballabh, P., Braun, A., Nedergaard, M., 2004. The blood-

brain barrier: an overview: structure, regulation, and

clinical implications. Neurobiol. Dis. 16, 1–13.

Barkhuizen, M., Magez, S., Atkinson, R.A., Brombacher, F.,

2007. Interleukin-12p70-dependent interferon- gamma pro-

duction is crucial for resistance in African trypanosomiasis.

J. Infect. Dis. 196, 1253–1260.

Barkhuizen, M., Magez, S., Ryffel, B., Brombacher, F., 2008.

Interleukin-12p70 deficiency increases survival and

diminishes pathology in Trypanosoma congolense infection.

J. Infect. Dis. 198, 1284–1291.

Bosschaerts, T., Guilliams, M., Noel, W., Herin, M., Burk,

R.F., Hill, K.E., Brys, L., Raes, G., Ghassabeh, G.H., De

Baetselier, P., Beschin, A., 2008. Alternatively activated

myeloid cells limit pathogenicity associated with African

trypanosomiasis through the IL-10 inducible gene seleno-

protein P. J. Immunol. 180, 6168–6175.

Coller, S.P., Mansfield, J.M., Paulnock, D.M., 2003. Glyco-

sylinositolphosphate soluble variant surface glycoprotein

inhibits IFN-gamma-induced nitric oxide production via

reduction in STAT1 phosphorylation in African trypano-

somiasis. J. Immunol. 171, 1466–1472.

Dagenais, T.R., Demick, K.P., Bangs, J.D., Forest, K.T.,

Paulnock, D.M., Mansfield, J.M., 2009. T-cell responses

to the trypanosome variant surface glycoprotein are not

limited to hypervariable subregions. Infect. Immun. 77,

141–151.

Daulouede, S., Bouteille, B., Moynet, D., De Baetselier, P.,

Courtois, P., Lemesre, J.L., Buguet, A., Cespuglio, R.,

Vincendeau, P., 2001. Human macrophage tumor necrosis

factor (TNF)-alpha production induced by Trypanosoma

brucei gambiense and the role of TNF-alpha in parasite

control. J. Infect. Dis. 183, 988–991.

Dempsey, W.L., Mansfield, J.M., 1983. Lymphocyte function

in experimental African trypanosomiasis. V. Role of

antibody and the mononuclear phagocyte system in

variant-specific immunity. J. Immunol. 130, 405–411.

Dennehy, K.M., Elias, F., Zeder-Lutz, G., Ding, X., Altschuh,

D., Luhder, F., Hunig, T., 2006. Cutting edge: mono-

valency of CD28 maintains the antigen dependence of T cell

costimulatory responses. J. Immunol. 176, 5725–5729.

d’Ieteren, G.D., Authie, E., Wissocq, N., Murray, M., 1998.

Trypanotolerance, an option for sustainable livestock

production in areas at risk from trypanosomosis. Rev.

Sci. Technol. 17, 154–175.

Drennan, M.B., Stijlemans, B., Van den Abbeele, J.,

Quesniaux, V.J., Barkhuizen, M., Brombacher, F., De

Baetselier, P., Ryffel, B., Magez, S., 2005. The induction

of a type 1 immune response following a Trypanosoma

brucei infection is MyD88 dependent. J. Immunol. 175,

2501–2509.

ARTICLE IN PRESST. Bosschaerts et al. / Immunobiology 214 (2009) 737–747 745

Ghassabeh, G.H., De Baetselier, P., Brys, L., Noel, W., Van

Ginderachter, J.A., Meerschaut, S., Beschin, A., Bromba-

cher, F., Raes, G., 2006. Identification of a common gene

signature for type II cytokine-associated myeloid cells

elicited in vivo in different pathologic conditions. Blood

108, 575–583.

Gomez-Rodrıguez, J., Stijlemans, B., De Muylder, G., Korf,

H., Brys, L., Berberof, M., Darji, A., Pays, E., De

Baetselier, P., Beschin, A., 2008. Identification of a parasitic

immunomodulatory protein triggering the development of

suppressive M1 macrophages during African trypanoso-

miasis. J. Infect. Dis., in press.

Greenwood, B.M., Whittle, H.C., Molyneux, D.H., 1973.

Immunosuppression in Gambian trypanosomiasis. Trans. R.

Soc. Trop. Med. Hyg. 67, 846–850.

Guilliams, M., Bosschaerts, T., Herin, M., Hunig, T., Loi, P.,

Flamand, V., De Baetselier, P., Beschin, A., 2008. Experi-

mental expansion of the regulatory T cell population

increases resistance to African trypanosomiasis. J. Infect.

Dis. 198, 781–791.

Guilliams, M., Movahedi, K., Bosschaerts, T., Vanden-

Driessche, T., Chuah, M.K., Herin, M., Acosta-Sanchez,

A., Ma, L., Moser, M., Van Ginderachter, J.A., Brys, L.,

De Baetselier, P., Beschin, A., 2009. IL-10 dampens

TNF/inducible nitric oxide synthase-producing dendritic

cell-mediated pathogenicity during parasitic infection.

J. Immunol. 182, 1107–1118.

Guilliams, M., Oldenhove, G., Noel, W., Herin, M., Brys, L.,

Loi, P., Flamand, V., Moser, M., De Baetselier, P., Beschin,

A., 2007. African trypanosomiasis: naturally occurring

regulatory T cells favor trypanotolerance by limiting

pathology associated with sustained type 1 inflammation.

J. Immunol. 179, 2748–2757.

Harris, T.H., Cooney, N.M., Mansfield, J.M., Paulnock,

D.M., 2006. Signal transduction, gene transcription, and

cytokine production triggered in macrophages by exposure

to trypanosome DNA. Infect. Immun. 74, 4530–4537.

Harris, T.H., Mansfield, J.M., Paulnock, D.M., 2007. CpG

oligodeoxynucleotide treatment enhances innate resistance

and acquired immunity to African trypanosomes. Infect.

Immun. 75, 2366–2373.

Hertz, C.J., Mansfield, J.M., 1999. IFN-gamma-dependent

nitric oxide production is not linked to resistance in

experimental African trypanosomiasis. Cell Immunol. 192,

24–32.

Hill, E.W., O’Gorman, G.M., Agaba, M., Gibson, J.P.,

Hanotte, O., Kemp, S.J., Naessens, J., Coussens, P.M.,

MacHugh, D.E., 2005. Understanding bovine trypanoso-

miasis and trypanotolerance: the promise of functional

genomics. Vet. Immunol. Immunopathol. 105, 247–258.

Iraqi, F., Clapcott, S.J., Kumari, P., Haley, C.S., Kemp, S.J.,

Teale, A.J., 2000. Fine mapping of trypanosomiasis

resistance loci in murine advanced intercross lines. Mamm.

Genome 11, 645–648.

Iraqi, F., Sekikawa, K., Rowlands, J., Teale, A., 2001.

Susceptibility of tumour necrosis factor-alpha genetically

deficient mice to Trypanosoma congolense infection. Para-

site Immunol. 23, 445–451.

Jankovic, D., Kullberg, M.C., Feng, C.G., Goldszmid, R.S.,

Collazo, C.M., Wilson, M., Wynn, T.A., Kamanaka, M.,

Flavell, R.A., Sher, A., 2007. Conventional T-

bet(+)Foxp3(�) Th1 cells are the major source of host-

protective regulatory IL-10 during intracellular protozoan

infection. J. Exp. Med. 204, 273–283.

Joseph, S.B., Bradley, M.N., Castrillo, A., Bruhn, K.W., Mak,

P.A., Pei, L., Hogenesch, J., O’Connell R, M., Cheng, G.,

Saez, E., Miller, J.F., Tontonoz, P., 2004. LXR-dependent

gene expression is important for macrophage survival and

the innate immune response. Cell 119, 299–309.

Kaushik, R.S., Uzonna, J.E., Zhang, Y., Gordon, J.R., Tabel,

H., 2000. Innate resistance to experimental African

trypanosomiasis: differences in cytokine (TNF-alpha,

IL-6, IL-10 and IL-12) production by bone marrow-derived

macrophages from resistant and susceptible mice. Cytokine

12, 1024–1034.

Kay, J.G., Murray, R.Z., Pagan, J.K., Stow, J.L., 2006.

Cytokine secretion via cholesterol-rich lipid raft-associated

SNAREs at the phagocytic cup. J. Biol. Chem. 281,

11949–11954.

Kemp, S.J., Iraqi, F., Darvasi, A., Soller, M., Teale, A.J.,

1997. Localization of genes controlling resistance to

trypanosomiasis in mice. Nat. Genet. 16, 194–196.

Kennedy, P.G., 2007. Animal models of human African

trypanosomiasis — very useful or too far removed? Trans.

R. Soc. Trop. Med. Hyg. 101, 1061–1062.

Kierstein, S., Noyes, H., Naessens, J., Nakamura, Y.,

Pritchard, C., Gibson, J., Kemp, S., Brass, A., 2006. Gene

expression profiling in a mouse model for African

trypanosomiasis. Genes Immun. 7, 667–679.

Leppert, B.J., Mansfield, J.M., Paulnock, D.M., 2007. The

soluble variant surface glycoprotein of African trypano-

somes is recognized by a macrophage scavenger receptor

and induces I kappa B alpha degradation independently of

TRAF6-mediated TLR signaling. J. Immunol. 179,

548–556.

Lopez, R., Demick, K.P., Mansfield, J.M., Paulnock, D.M.,

2008. Type I IFNs play a role in early resistance, but

subsequent susceptibility, to the African trypanosomes.

J. Immunol. 181, 4908–4917.

Losos, G.J., Ikede, B.O., 1972. Review of the pathology of

disease in domestic and laboratory animals caused by

Trypanosoma congolense, T. vivax, T. brucei, T. rhodesiense,

and T. gambiense. Vet. Pathol. 9, 1–71.

Lund, F.E., Garvy, B.A., Randall, T.D., Harris, D.P., 2005.

Regulatory roles for cytokine-producing B cells in infection

and autoimmune disease. Curr. Dir. Autoimmun. 8, 25–54.

Mabbott, N.A., Coulson, P.S., Smythies, L.E., Wilson, R.A.,

Sternberg, J.M., 1998. African trypanosome infections in

mice that lack the interferon-gamma receptor gene: nitric

oxide-dependent and -independent suppression of T-cell

proliferative responses and the development of anaemia.

Immunology 94, 476–480.

MacLean, L., Chisi, J.E., Odiit, M., Gibson, W.C., Ferris, V.,

Picozzi, K., Sternberg, J.M., 2004. Severity of human

african trypanosomiasis in East Africa is associated with

geographic location, parasite genotype, and host inflam-

matory cytokine response profile. Infect. Immun. 72,

7040–7044.

Magez, S., Geuskens, M., Beschin, A., del Favero, H.,

Verschueren, H., Lucas, R., Pays, E., de Baetselier, P.,

ARTICLE IN PRESST. Bosschaerts et al. / Immunobiology 214 (2009) 737–747746

1997. Specific uptake of tumor necrosis factor-alpha is

involved in growth control of Trypanosoma brucei. J. Cell

Biol. 137, 715–727.

Magez, S., Radwanska, M., Beschin, A., Sekikawa, K., De

Baetselier, P., 1999. Tumor necrosis factor alpha is a key

mediator in the regulation of experimental Trypanosoma

brucei infections. Infect. Immun. 67, 3128–3132.

Magez, S., Radwanska, M., Drennan, M., Fick, L., Baral,

T.N., Allie, N., Jacobs, M., Nedospasov, S., Brombacher,

F., Ryffel, B., De Baetselier, P., 2007. Tumor necrosis

factor (TNF) receptor-1 (TNFp55) signal transduction and

macrophage-derived soluble TNF are crucial for nitric

oxide-mediated Trypanosoma congolense parasite killing.

J. Infect. Dis. 196, 954–962.

Magez, S., Radwanska, M., Drennan, M., Fick, L., Baral,

T.N., Brombacher, F., De Baetselier, P., 2006. Interferon-

gamma and nitric oxide in combination with antibodies are

key protective host immune factors during trypanosoma

congolense Tc13 Infections. J. Infect. Dis. 193, 1575–1583.

Magez, S., Schwegmann, A., Atkinson, R., Claes, F.,

Drennan, M., De Baetselier, P., Brombacher, F., 2008.

The role of B-cells and IgM antibodies in parasitemia,

anemia, and VSG switching in Trypanosoma brucei-infected

mice. PLoS Pathog. 4, e1000122.

Magez, S., Truyens, C., Merimi, M., Radwanska, M., Stijle-

mans, B., Brouckaert, P., Brombacher, F., Pays, E., De

Baetselier, P., 2004. P75 tumor necrosis factor-receptor

shedding occurs as a protective host response during

African trypanosomiasis. J. Infect. Dis. 189, 527–539.

Maillard, J.C., Berthier, D., Thevenon, S., Piquemal, D.,

Chantal, I., Marti, J., 2005. Efficiency and limits of the

Serial Analysis of Gene Expression (SAGE) method:

discussions based on first results in bovine trypanotoler-

ance. Vet. Immunol. Immunopathol. 108, 59–69.

Manley, P.N., Ancsin, J.B., Kisilevsky, R., 2006. Rapid

recycling of cholesterol: the joint biologic role of C-reactive

protein and serum amyloid A. Med. Hypotheses 66,

784–792.

Mansfield, J.M., Paulnock, D.M., 2005. Regulation of innate

and acquired immunity in African trypanosomiasis. Para-

site Immunol. 27, 361–371.

Martinez, F.O., Helming, L., Gordon, S., 2008. Alternative

activation of macrophages: an immunologic functional

perspective. Annu. Rev. Immunol.

Morrison, W.I., Black, S.J., Paris, J., Hinson, C.A., Wells,

P.W., 1982. Protective immunity and specificity of antibody

responses elicited in cattle by irradiated Trypanosoma

brucei. Parasite Immunol. 4, 395–407.

Muller, N., Mansfield, J.M., Seebeck, T., 1996. Trypanosome

variant surface glycoproteins are recognized by self-reactive

antibodies in uninfected hosts. Infect. Immun. 64,

4593–4597.

Naessens, J., 2006. Bovine trypanotolerance: a natural ability

to prevent severe anaemia and haemophagocytic syndrome?

Int. J. Parasitol. 36, 521–528.

Namangala, B., de Baetselier, P., Brijs, L., Stijlemans, B.,

Noel, W., Pays, E., Carrington, M., Beschin, A., 2000.

Attenuation of Trypanosoma brucei is associated with

reduced immunosuppression and concomitant production

of Th2 lymphokines. J. Infect. Dis. 181, 1110–1120.

Namangala, B., De Baetselier, P., Noel, W., Brys, L.,

Beschin, A., 2001a. Alternative versus classical macrophage

activation during experimental African trypanosomosis.

J. Leukoc. Biol. 69, 387–396.

Namangala, B., Noel, W., De Baetselier, P., Brys, L., Beschin,

A., 2001b. Relative contribution of interferon-gamma and

interleukin-10 to resistance to murine African trypanoso-

mosis. J. Infect. Dis. 183, 1794–1800.

Noel, W., Hassanzadeh, G., Raes, G., Namangala, B., Daems,

I., Brys, L., Brombacher, F., Baetselier, P.D., Beschin, A.,

2002. Infection stage-dependent modulation of macrophage

activation in Trypanosoma congolense-resistant and -sus-

ceptible mice. Infect. Immun. 70, 6180–6187.

Noel, W., Raes, G., Hassanzadeh Ghassabeh, G., De Baetselier,

P., Beschin, A., 2004. Alternatively activated macrophages

during parasite infections. Trends Parasitol. 20, 126–133.

Olsson, T., Bakhiet, M., Hojeberg, B., Ljungdahl, A., Edlund,

C., Andersson, G., Ekre, H.P., Fung-Leung, W.P., Mak,

T., Wigzell, H., et al., 1993. CD8 is critically involved in

lymphocyte activation by a T. brucei brucei-released

molecule. Cell 72, 715–727.

Pays, E., Vanhollebeke, B., 2008. Mutual self-defence: the

trypanolytic factor story. Microbes Infect. 10, 985–989.

Pesce, J.T., Ramalingam, T.R., Mentink-Kane, M.M., Wilson,

M.S., El Kasmi, K.C., Smith, A.M., Thompson, R.W.,

Cheever, A.W., Murray, P.J., Wynn, T.A., 2009. Arginase-

1-expressing macrophages suppress Th2 cytokine-driven

inflammation and fibrosis. PLoS Pathog. 5, e1000371.

Pinder, M., Bauer, J., Van Melick, A., Fumoux, F., 1988.

Immune responses of trypanoresistant and trypanosuscep-

tible cattle after cyclic infection with Trypanosoma con-

golense. Vet. Immunol. Immunopathol. 18, 245–257.

Roncarolo, M.G., Gregori, S., Battaglia, M., Bacchetta, R.,

Fleischhauer, K., Levings, M.K., 2006. Interleukin-10-

secreting type 1 regulatory T cells in rodents and humans.

Immunol. Rev. 212, 28–50.

Schleifer, K.W., Mansfield, J.M., 1993. Suppressor macro-

phages in African trypanosomiasis inhibit T cell prolifera-

tive responses by nitric oxide and prostaglandins. J.

Immunol. 151, 5492–5503.

Serbina, N.V., Jia, T., Hohl, T.M., Pamer, E.G., 2008.

Monocyte-mediated defense against microbial pathogens.

Annu. Rev. Immunol. 26, 421–452.

Shi, M., Pan, W., Tabel, H., 2003. Experimental African

trypanosomiasis: IFN-gamma mediates early mortality.

Eur. J. Immunol. 33, 108–118.

Shi, M., Wei, G., Pan, W., Tabel, H., 2004. Trypanosoma

congolense infections: antibody-mediated phagocytosis by

Kupffer cells. J. Leukoc. Biol. 76, 399–405.

Shi, M., Wei, G., Pan, W., Tabel, H., 2006. Experimental

African trypanosomiasis: a subset of pathogenic,

IFN-gamma-producing, MHC class II-restricted CD4+ T

cells mediates early mortality in highly susceptible mice.

J. Immunol. 176, 1724–1732.

Sternberg, J.M., 2004. Human African trypanosomiasis:

clinical presentation and immune response. Parasite Im-

munol. 26, 469–476.

Sternberg, J.M., Rodgers, J., Bradley, B., Maclean, L.,

Murray, M., Kennedy, P.G., 2005. Meningoencephalitic

African trypanosomiasis: Brain IL-10 and IL-6 are

ARTICLE IN PRESST. Bosschaerts et al. / Immunobiology 214 (2009) 737–747 747

associated with protection from neuro-inflammatory

pathology. J. Neuroimmunol. 167, 81–89.

Steverding, D., 2008. The history of African trypanosomiasis.

Parasit Vectors 1, 3.

Stijlemans, B., Baral, T.N., Guilliams, M., Brys, L., Korf, J.,

Drennan, M., Van Den Abbeele, J., De Baetselier, P.,

Magez, S., 2007. A glycosylphosphatidylinositol-based

treatment alleviates trypanosomiasis-associated immuno-

pathology. J. Immunol. 179, 4003–4014.

Stijlemans, B., Vankrunkelsven, A., Brys, L., Magez, S., De

Baetselier, P., 2008. Role of iron homeostasis in trypano-

somiasis-associated anemia. Immunobiology 213, 823–835.

Tabel, H., Kaushik, R.S., Uzonna, J., 1999. Experimental

African trypanosomiasis: differences in cytokine and nitric

oxide production by macrophages from resistant and

susceptible mice. Pathobiology 67, 273–276.

Tabel, H., Kaushik, R.S., Uzonna, J.E., 2000. Susceptibility

and resistance to Trypanosoma congolense infections.

Microbes Infect. 2, 1619–1629.

Taiwo, V.O., Adejinmi, J.O., Oluwaniyi, J.O., 2002. Non-

immune control of trypanosomosis: in vitro oxidative burst

of PMA- and trypanosome-stimulated neutrophils of Boran

and N’Dama cattle. Onderstepoort J. Vet. Res. 69, 155–161.

Taylor, J.E., Rudenko, G., 2006. Switching trypanosome

coats: what’s in the wardrobe? Trends Genet. 22, 614–620.

Taylor, K.A., 1998. Immune responses of cattle to African

trypanosomes: protective or pathogenic? Int. J. Parasitol.

28, 219–240.

Theodos, C.M., Mansfield, J.M., 1990. Regulation of B cell

responses to the variant surface glycoprotein molecule in

trypanosomiasis. II. Down-regulation of idiotype expres-

sion is associated with the appearance of lymphocytes

expressing antiidiotypic receptors. J. Immunol. 144,

4022–4029.

Theodos, C.M., Reinitz, D.M., Mansfield, J.M., 1990.

Regulation of B cell responses to the variant surface

glycoprotein (VSG) molecule in trypanosomiasis. I. Epi-

tope specificity and idiotypic profile of monoclonal anti-

bodies to the VSG of Trypanosoma brucei rhodesiense.

J. Immunol. 144, 4011–4021.

Trail, J.C., Wissocq, N., d’Ieteren, G.D., Kakiese, O., Murray,

M., 1994. Quantitative phenotyping of N’Dama cattle for

aspects of trypanotolerance under field tsetse challenge.

Vet. Parasitol. 55, 185–195.

Vincendeau, P., Daulouede, S., Veyret, B., Darde, M.L.,

Bouteille, B., Lemesre, J.L., 1992. Nitric oxide-

mediated cytostatic activity on Trypanosoma brucei gam-

biense and Trypanosoma brucei brucei. Exp. Parasitol. 75,

353–360.

Wei, G., Tabel, H., 2008. Regulatory T cells prevent control of

experimental African trypanosomiasis. J. Immunol. 180,

2514–2521.

Williams, D.J., Taylor, K., Newson, J., Gichuki, B., Naessens,

J., 1996. The role of anti-variable surface glycoprotein

antibody responses in bovine trypanotolerance. Parasite

Immunol. 18, 209–218.