Antigenic variation as adaptive process: the case of Trypanosoma brucei

Antigenic variation as adaptive process: the case of Trypanosoma brucei

African trypanosomes infect a wide spectrum of mammalian hosts, including humans

Mechanisms of adaptation: I. Antigenic variation

A dense coat of Variant Surface Glycoprotein (VSG) covers the entire surface of T. brucei bloodstream forms Bloodstream form Procyclic form

N-terminal VSG domains

VSG coat: protection antibody VSG dimers phospholipids - Tightly packed array of 10 7 molecules organized in dimers - Impenetrable to macromolecules of the host, including antibodies - Only surface loops recognizable by the host

VSG coat: antigenic variation The coat changes every 10 3 to 10 5 cell divisions Anti-1 Anti-2 Anti-3 Parasite number 1 2 3 Time Significance: participation in the control of the parasite burden by attracting the lytic immune response and subsequently allowing new antigenic variants to prolong the infection.

Genetic mechanisms of antigenic variation in Trypanosoma brucei ~1,500 genes DNA recombination (either gene conversion or reciprocal recombination) VSG VS VSG VSG V G VSG 7 6 5 4 8 8 3 2 1 VSG In situ (in)activation 7 6 5 4 8 8 3 2 1 VSG ~15 telomericvsg expression sites

Mechanisms of adaptation: II. Generation of adaptive proteins

Parasites must escape the defenses of their hosts, but they also need to communicate with host cells and internalize vital host components antibodies transferrin

The flagellar pocket: the unique accessible site Surface receptors: - invariant - accessible - vital Vaccination targets??

Endocytosis in T. brucei out in digestive vacuole lysosome - only limited to 0.5% of the cell surface - highly efficient, probably due to special features (pnal lectin?)

Most ESAGs encode surface proteins, including a heterodimeric receptor for transferrin and a homodimeric receptor-like adenylyl cyclase TF 76 2 1 4 4 4? 4 4 4 plasma membrane plasma membrane = VSG N-terminal domains!!! AC? AC ATP AC AC camp Flagellar pocket Flagellum

The use of different ESs, thus, the expression of different sets of ESAGs, allows a better adaptation to a variety of different hosts: efficient uptake of transferrin from various mammalian species, hence, colonization of a wide spectrum of mammals resistance to lysis by human serum, hence, colonization of man

The use of different BES allows a better adaptation to different hosts : efficient uptake of transferrin ( ) ( 10 7 6 5 4 8 8 3 2 11 1 9 ) VSG ( ) ( 10 7 6 5 4 8 8 3 2 11 1 9 ) VSG

The use of different BES allows a better adaptation to different hosts: resistance to lysis by human serum T.b.brucei T.b.rhodesiense

Lysis by human serum requires endocytosis of the trypanosome lytic factor (TLF) TLF (HDL-linked) out in?? endocytosis lysosome

In T. b. rhodesiense, resistance to human serum is linked to activation of a specific VSG expression site (R-ES) in non-human serum R-ES VSG VSG result: trypanosomes sensitive to human serum (S clones) in human serum R-ES VSG VSG result: trypanosomes resistant to human serum (R clones) Xong et al (1998) Cell 95, 839-846

The R-ES site is severely truncated and contains the Serum Resistance-Associated gene (SRA) B-ES ( ) 10 7 6 5 4 8 8 3 2 11 1 9 ( ) VSG R-ES 6 7 5 SRA VSG Expression of SRA in the R-ES appears to be a general feature of T. b. rhodesiense strains ; SRA is the best available diagnostic tool of this subspecies

SRA is necessary and sufficient to confer full resistance to human serum 45 Trypanosoma b. b. w.t. FCS NHS 45 SRA transformants FCS NHS 40 40 35 35 parasitemia 30 25 20 15 parasitemia 30 25 20 15 10 10 5 5 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 days days Xong et al (1998) Cell 95, 839-846

SRA is a VSG-like glycoprotein devoid of surface loops N-term surface loops α-helix A α-helix B VSG MiTat 1.2 SRA VSG WaTat 1.2 Models of the N-terminal domain of SRA and VSGs

The study of the SRA moiety necessary to confer resistance to human serum has uncovered: the essential role of the N-terminal α-helix A, which is interactive in VSGs; the ability of this helix to interact with apolipoprotein L-I (antiparallel to C-terminal α-helix) control SRA (serumalbumin) apol1 Coomassie blue

ApoL1 is the trypanosome lytic factor of human serum. Interactions between the N-terminal α-helix of SRA and the C-terminal α-helix of apol1, which occur within the lysosome, prevent trypanolysis by human serum. SRA apol1 Vanhamme et al (2003) Nature 422, 83-87

Activity of apol1 The colicin-like anion-selective pore-forming domain is responsible for lytic activity. The membrane-addressing domain is responsible for both binding to HDL and addressing to a membrane. The C-terminal region is not required for either activity, but is the target for neutralisation by the trypanosome immunity protein SRA of T. b. rhodesiense. Pérez-Morga et al (2005) Science 309, 469-472

apol-i HDL flagellar Pocket lysosome ph 5.3 endosome Trafficking of apol1 to the lysosome of T. brucei: a model

0 h 1 h 2 h 3 h 4 h 5 h (1 µg/ml apol1; 33 C) NHS/apoL1 triggers swelling of the lysosome

Cl - Cl - DIDS apol1 ApoL1-driven effect on the lysosome: a model Pérez-Morga et al (2005) Science 309, 469-472

apoa-i Lipids apol1 Hpr 91% identity to haptoglobin (hemoglobin scavenger) ApoL1 is associated with Haptoglobin-related protein (Hpr) on the same subset of HDL particles (HDL3); Hpr is involved in the binding of the particles to the trypanosome surface. Vanhollebeke et al (2007) PNAS 104, 4118-4123

Haptoglobin(r) Alexa 488 hemoglobin Alexa 488 + hemoglobin + haptoglobin(r) k The haptoglobin(r)-hemoglobin complex is a ligand for T.brucei

The trypanosome receptor for Hp(r)-Hb was recently identified. In mouse serum, this receptor appears to be responsible for the uptake of heme, which is incorporated in hemoproteins that confer resistance of the parasite to the oxidative response of host macrophages. In human serum, this receptor also triggers the uptake of the trypanolytic HDL particles through recognition of the Hpr-Hb complex. Vanhollebeke et al (2008) Science 320, 677-681

Mutual adaptations between T. brucei and man Human infection Mφ Intravascular hemolysis T.b.rhodesiense SRA CD163 ROS RNS Hp-Hb Lipids TbHpHbR Hb-Hpr HDL3 apol1 humans Trypanolysis

Conclusions (I) * The telomeric VSG ESs are powerful genetic workshops for the adaptation of the parasite: - their high homologous recombination rate, due to both high level of sequence identity with other loci and high level of DNA accessibility to recombinases, allows the continuous creation of new antigens to cope with the immune system - their diversity allows the variation of surface receptors - their high recombination rate leads to the generation of new adaptive proteins

Conclusions (II) * The VSG gene seems to have been used as a major tool to construct various adaptive components (transferrin receptors, SRA, other VSG-like proteins..??) * Allelic exclusion is the key to adaptive variation of T. brucei

ESAG7/6 as VSG-like transferrin receptor: Didier Salmon SRA as inhibitor of trypanolysis: Huang Van Xong, Luc Vanhamme ApoL1 as trypanolytic factor: Luc Vanhamme, Françoise Paturiaux-Hanocq, Philippe Poelvoorde Mechanism of trypanolytic activity of apol1: David Pérez-Morga, Benoit Vanhollebeke Hpr as ligand of trypanolytic HDLs: Benoit Vanhollebeke Identification of the trypanosome Hp-Hb receptor: Benoit Vanhollebeke Annette Pays, Patricia Tebabi, Géraldine De Muylder, Laurence Lecordier, Derek Nolan