The identification, purification, and characterization of two invariant surface glycoproteins located beneath the surface coat barrier of bloodstream forms of Trypanosoma brucei

Trypanosoma brucei Invariant Surface Glycoprotein 75 Is an Immunoglobulin Fc Receptor Inhibiting Complement Activation and Antibody-Mediated Cellular Phagocytosis

Various subspecies of the unicellular parasite Trypanosoma brucei cause sleeping sickness, a neglected tropical disease affecting millions of individuals and domestic animals. Immune evasion mechanisms play a pivotal role in parasite survival within the host and enable the parasite to establish a chronic infection. In particular, the rapid switching of variant surface glycoproteins covering a large proportion of the parasite's surface enables the parasite to avoid clearance by the adaptive immune system of the host. In this article, we present the crystal structure and discover an immune-evasive function of the extracellular region of the T. brucei invariant surface gp75 (ISG75). Structural analysis determined that the ISG75 ectodomain is organized as a globular head domain and a long slender coiled-coil domain. Subsequent ligand screening and binding analysis determined that the head domain of ISG75 confers interaction with the Fc region of all subclasses of human IgG. Importantly, the ISG75-IgG interaction strongly inhibits both activation of the classical complement pathway and Ab-dependent cellular phagocytosis by competing with C1q and host cell FcγR CD32. Our data reveal a novel immune evasion mechanism of T. brucei, with ISG75 able to inactivate the activities of Abs recognizing the parasite surface proteins.

Chemical Inhibition of Bromodomain Proteins in Insect-Stage African Trypanosomes Perturbs Silencing of the Variant Surface Glycoprotein Repertoire and Results in Widespread Changes in the Transcriptome

The eukaryotic protozoan parasite Trypanosoma brucei is transmitted by the tsetse fly to both humans and animals, where it causes a fatal disease called African trypanosomiasis. While the parasite lacks canonical DNA sequence-specific transcription factors, it does possess histones, histone modifications, and proteins that write, erase, and read histone marks. Chemical inhibition of chromatin-interacting bromodomain proteins has previously been shown to perturb bloodstream specific trypanosome processes, including silencing of the variant surface glycoprotein (VSG) genes and immune evasion. Transcriptomic changes that occur in bromodomain-inhibited bloodstream parasites mirror many of the changes that occur as parasites developmentally progress from the bloodstream to the insect stage. We performed transcriptome sequencing (RNA-seq) time courses to determine the effects of chemical bromodomain inhibition in insect-stage parasites using the compound I-BET151. We found that treatment with I-BET151 causes large changes in the transcriptome of insect-stage parasites and also perturbs silencing of VSG genes. The transcriptomes of bromodomain-inhibited parasites share some features with early metacyclic-stage parasites in the fly salivary gland, implicating bromodomain proteins as important for regulating transcript levels for developmentally relevant genes. However, the downregulation of surface procyclin protein that typically accompanies developmental progression is absent in bromodomain-inhibited insect-stage parasites. We conclude that chemical modulation of bromodomain proteins causes widespread transcriptomic changes in multiple trypanosome life cycle stages. Understanding the gene-regulatory processes that facilitate transcriptome remodeling in this highly diverged eukaryote may shed light on how these mechanisms evolved. IMPORTANCE The disease African trypanosomiasis imposes a severe human and economic burden for communities in sub-Saharan Africa. The parasite that causes the disease is transmitted to the bloodstream of a human or ungulate via the tsetse fly. Because the environments of the fly and the bloodstream differ, the parasite modulates the expression of its genes to accommodate two different lifestyles in these disparate niches. Perturbation of bromodomain proteins that interact with histone proteins around which DNA is wrapped (chromatin) causes profound changes in gene expression in bloodstream-stage parasites. This paper reports that gene expression is also affected by chemical bromodomain inhibition in insect-stage parasites but that the genes affected differ depending on life cycle stage. Because trypanosomes diverged early from model eukaryotes, an understanding of how trypanosomes regulate gene expression may lend insight into how gene-regulatory mechanisms evolved. This could also be leveraged to generate new therapeutic strategies.

The role of invariant surface glycoprotein 75 in xenobiotic acquisition by African trypanosomes

The surface proteins of parasitic protozoa mediate functions essential to survival within a host, including nutrient accumulation, environmental sensing and immune evasion. Several receptors involved in nutrient uptake and defence from the innate immune response have been described in African trypanosomes and, together with antigenic variation, contribute towards persistence within vertebrate hosts. Significantly, a superfamily of invariant surface glycoproteins (ISGs) populates the trypanosome surface, one of which, ISG75, is implicated in uptake of the century-old drug suramin. By CRISPR/Cas9 knockout and biophysical analysis, we show here that ISG75 directly binds suramin and mediates uptake of additional naphthol-related compounds, making ISG75 a conduit for entry of at least one structural class of trypanocidal compounds. However, ISG75 null cells present only modest attenuation of suramin sensitivity, have unaltered viability in vivo and in vitro and no alteration to suramin-invoked proteome responses. While ISG75 is demonstrated as a valid suramin cell entry pathway, we suggest the presence of additional mechanisms for suramin accumulation, further demonstrating the complexity of trypanosomatid drug interactions and potential for evolution of resistance.

Exploring the potential of invariable surface glycoprotein (ISG65) as promising antigen for diagnosis of Trypanosoma evansi infection

Trypanosoma evansi, a hemoflagellate protozoan, leads to wasting disease, surra in livestock animals causing huge economic losses. Currently, the preferred assay for surra diagnosis is whole cell lysate (WCL) based ELISA, which requires the use of rodents for WCL preparation. To avoid use of laboratory animals, we used recombinant DNA technology to express T. evansi invariable surface glycoprotein (ISG) in E. coli. The potential of recombinant ISG65 (rISG65) as a diagnostic antigen was investigated in immunoblot and indirect ELISA using experimentally infected equine serum samples from 0 to 84 days post infection. The results indicated that rISG65 reacted with horse T. evansi positive serum giving two bands of approximately 48 kDa and 96 kDa. T. evansi-specific antibodies were detected as early as 10 and 14 days post infection using immunoblot and indirect ELISA, respectively using rISG65 antigen. No cross-reactivity was observed in ELISA and immunoblot with different serum samples of equines positive for Equine herpesvirus 1, Burkholderia mallei, and Theileria equi infections. Several immunoreactive regions were observed between 30 and 100 kDa in T. evansi isolate of horse origin indicating the existence of multiple copies of ISG protein in a single trypanosome. The recombinant ISG has proven to be good candidate antigen to be used in ELISA for serodiagnosis of T. evansi infection in different animals.

Invariant surface glycoprotein 65 of Trypanosoma brucei is a complement C3 receptor

African trypanosomes are extracellular pathogens of mammals and are exposed to the adaptive and innate immune systems. Trypanosomes evade the adaptive immune response through antigenic variation, but little is known about how they interact with components of the innate immune response, including complement. Here we demonstrate that an invariant surface glycoprotein, ISG65, is a receptor for complement component 3 (C3). We show how ISG65 binds to the thioester domain of C3b. We also show that C3 contributes to control of trypanosomes during early infection in a mouse model and provide evidence that ISG65 is involved in reducing trypanosome susceptibility to C3-mediated clearance. Deposition of C3b on pathogen surfaces, such as trypanosomes, is a central point in activation of the complement system. In ISG65, trypanosomes have evolved a C3 receptor which diminishes the downstream effects of C3 deposition on the control of infection.

Trypanosomes evade the immune response through antigenic variation of a surface coat containing variant surface glycoproteins (VSG). They also express invariant surface glycoproteins (ISGs), which are less well understood. Here, Macleod et al. show that ISG65 of T. brucei is a receptor for complement component 3. They provide the crystal structure of T. brucei ISG65 in complex with complement C3d and show evidence that ISG65 is involved in reducing trypanosome susceptibility to C3-mediated clearance in vivo.

RNA-Seq analysis validates the use of culture-derived Trypanosoma brucei and provides new markers for mammalian and insect life-cycle stages

Background

Trypanosoma brucei brucei, the parasite causing Nagana in domestic animals, is closely related to the parasites causing sleeping sickness, but does not infect humans. In addition to its importance as a pathogen, the relative ease of genetic manipulation and an innate capacity for RNAi extend its use as a model organism in cell and infection biology. During its development in its mammalian and insect (tsetse fly) hosts, T. b. brucei passes through several different life-cycle stages. There are currently four life-cycle stages that can be cultured: slender forms and stumpy forms, which are equivalent to forms found in the mammal, and early and late procyclic forms, which are equivalent to forms in the tsetse midgut. Early procyclic forms show coordinated group movement (social motility) on semi-solid surfaces, whereas late procyclic forms do not.

Results

RNA-Seq was performed on biological replicates of each life-cycle stage. These constitute the first datasets for culture-derived slender and stumpy bloodstream forms and early and late procyclic forms. Expression profiles confirmed that genes known to be stage-regulated in the animal and insect hosts were also regulated in culture. Sequence reads of 100–125 bases provided sufficient precision to uncover differential expression of closely related genes. More than 100 transcripts showed peak expression in stumpy forms, including adenylate cyclases and several components of inositol metabolism. Early and late procyclic forms showed differential expression of 73 transcripts, a number of which encoded proteins that were previously shown to be stage-regulated. Moreover, two adenylate cyclases previously shown to reduce social motility are up-regulated in late procyclic forms.

Conclusions

This study validates the use of cultured bloodstream forms as alternatives to animal-derived parasites and yields new markers for all four stages. In addition to underpinning recent findings that early and late procyclic forms are distinct life-cycle stages, it could provide insights into the reasons for their different biological properties.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4600-6) contains supplementary material, which is available to authorized users.

Serological tests for gambiense human African trypanosomiasis detect antibodies in cattle

Background

Serological tests for gambiense human African trypanosomiasis (gHAT) detect antibodies to antigens on the cell surface of bloodstream trypanosomes. As trypanosomes that cause animal African trypanosomiasis (AAT) also express related antigens, we have evaluated two rapid diagnostic tests (RDTs) on cattle in trypanosomiasis endemic and non-endemic regions, to determine whether gHAT serological tests could also be used to screen for AAT.

Methods

Two RDTs, 1G RDT, made with native antigens, and p2G RDT, made with recombinant antigens, were tested on 121 cattle in a trypanosomiasis-free region, and on 312 cattle from a rhodesiense HAT and AAT endemic region. A subset of samples from the endemic region were also tested with two immune trypanolysis (TL) tests. The sensitivity of the tests was estimated by evaluating the result of the RDT on samples that were positive by both microscopy and internal transcribed spacer (ITS) PCR, whilst specificity was the result of the RDT on samples that were negative by ITS PCR and microscopy, and others from the non-endemic region.

Results

The specificity of the p2G RDT on cattle from the non-endemic region was 97.5% (95% CI: 93.0–99.2%), compared to only 57.9% (95% CI: 48.9–66.3%) for 1G RDT. The specificities of 1G RDT, p2G RDT and TL on endemic control cattle were 14.6% (95% CI: 9.7–21.5%), 22.6% (95% CI: 16.4–30.3%) and 68.3% (95% CI: 59.6–75.9%), respectively. The sensitivities of the tests on trypanosome positive samples were 85.1% (95% CI: 79.1–89.7%), 89.1% (95% CI: 83.7–93.0%) and 59.3% (95% CI: 51.8–66.4%), respectively. Among the same samples, 51.7% were positive by both TL and the 1G RDT.

Conclusions

These serological tests detect cross-reacting antibodies in cattle. The p2G RDT based on recombinant antigens had a high specificity in a non-endemic region, while the 1G RDT had a lower specificity, suggesting cross-reactivity with other pathogens.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2487-8) contains supplementary material, which is available to authorized users.

An inhibition enzyme immuno assay exploring recombinant invariant surface glycoprotein and monoclonal antibodies for surveillance of surra in animals

The present study is aimed at the development of inhibition ELISA (I-ELISA) exploring monoclonal antibodies (MAbs) and recombinant invariant surface glycoprotein. The extracellular domain (ED) of invariant surface glycoprotein (ISG-75) from Trypanosoma evasni has been heterologously expressed in Pichia pastoris (X-33). The recombinant ISG-75 (rISG-75ED) was characterized by immunoblot and ELISA, followed by the production of MAbs against rISG-75ED. The MAbs were characterized by immunoblot and then explored in the development of I-ELISA for the detection of surra. The diagnostic potential of the developed test has been evaluated using 1192 field sera sample including cattle, buffalo, donkey, horse and camel. The statistical analysis of the data showed optimum combination of diagnostic sensitivity and specificity at 98.8% and 99.2% respectively, with cut-off percentage inhibition (PI) value of >45. The Cohen's kappa coefficient of agreement was found to be 0.98. Hence, the diagnostic test developed in the present study can be exploited as a potential and reliable tool in the serodiagnosis and surveillance of surra in animals.

A proteomics approach reveals molecular manipulators of distinct cellular processes in the salivary glands of Glossina m. morsitans in response to Trypanosoma b. brucei infections

Background

Glossina m. morsitans is the primary vector of the Trypanosoma brucei group, one of the causative agents of African trypanosomoses. The parasites undergo metacyclogenesis, i.e. transformation into the mammalian-infective metacyclic trypomastigote (MT) parasites, in the salivary glands (SGs) of the tsetse vector. Since the MT-parasites are largely uncultivable in vitro, information on the molecular processes that facilitate metacyclogenesis is scanty.

Methods

To bridge this knowledge gap, we employed tandem mass spectrometry to investigate protein expression modulations in parasitized (T. b. brucei-infected) and unparasitized SGs of G. m. morsitans. We annotated the identified proteins into gene ontologies and mapped the up- and downregulated proteins within protein-protein interaction (PPI) networks.

Results

We identified 361 host proteins, of which 76.6 % (n = 276) and 22.3 % (n = 81) were up- and downregulated, respectively, in parasitized SGs compared to unparasitized SGs. Whilst 32 proteins were significantly upregulated (> 10-fold), only salivary secreted adenosine was significantly downregulated. Amongst the significantly upregulated proteins, there were proteins associated with blood feeding, immunity, cellular proliferation, homeostasis, cytoskeletal traffic and regulation of protein turnover. The significantly upregulated proteins formed major hubs in the PPI network including key regulators of the Ras/MAPK and Ca²⁺/cAMP signaling pathways, ubiquitin-proteasome system and mitochondrial respiratory chain. Moreover, we identified 158 trypanosome-specific proteins, notable of which were proteins in the families of the GPI-anchored surface glycoproteins, kinetoplastid calpains, peroxiredoxins, retrotransposon host spot multigene and molecular chaperones. Whilst immune-related trypanosome proteins were over-represented, membrane transporters and proteins involved in translation repression (e.g. ribosomal proteins) were under-represented, potentially reminiscent of the growth-arrested MT-parasites.

Conclusions

Our data implicate the significantly upregulated proteins as manipulators of diverse cellular processes in response to T. b. brucei infection, potentially to prepare the MT-parasites for invasion and evasion of the mammalian host immune defences. We discuss potential strategies to exploit our findings in enhancement of trypanosome refractoriness or reduce the vector competence of the tsetse vector.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1714-z) contains supplementary material, which is available to authorized users.

How Does the VSG Coat of Bloodstream Form African Trypanosomes Interact with External Proteins?

Variations on the statement “the variant surface glycoprotein (VSG) coat that covers the external face of the mammalian bloodstream form of Trypanosoma brucei acts a physical barrier” appear regularly in research articles and reviews. The concept of the impenetrable VSG coat is an attractive one, as it provides a clear model for understanding how a trypanosome population persists; each successive VSG protects the plasma membrane and is immunologically distinct from previous VSGs. What is the evidence that the VSG coat is an impenetrable barrier, and how do antibodies and other extracellular proteins interact with it? In this review, the nature of the extracellular surface of the bloodstream form trypanosome is described, and past experiments that investigated binding of antibodies and lectins to trypanosomes are analysed using knowledge of VSG sequence and structure that was unavailable when the experiments were performed. Epitopes for some VSG monoclonal antibodies are mapped as far as possible from previous experimental data, onto models of VSG structures. The binding of lectins to some, but not to other, VSGs is revisited with more recent knowledge of the location and nature of N-linked oligosaccharides. The conclusions are: (i) Much of the variation observed in earlier experiments can be explained by the identity of the individual VSGs. (ii) Much of an individual VSG is accessible to antibodies, and the barrier that prevents access to the cell surface is probably at the base of the VSG N-terminal domain, approximately 5 nm from the plasma membrane. This second conclusion highlights a gap in our understanding of how the VSG coat works, as several plasma membrane proteins with large extracellular domains are very unlikely to be hidden from host antibodies by VSG.

African trypanosomes have evolved two key strategies to prevent killing by the host immune response and, thus, maintain a long-term infection in a mammal. Both are based on a densely packed coat of a single protein, the variant surface glycoprotein (VSG), which covers the entire extracellular surface of the cell. The first strategy is antigenic variation, through which individual cells switch the identity of the expressed VSG at a low frequency and are selected by the host immune response. If the VSG is novel, the trypanosome proliferates, maintaining the infection; if it doesn't switch, or if the new VSG is not novel, it will be killed. In the second strategy, the VSG acts as a protective barrier, shielding the cell from innate and adaptive immune factors until there is an overwhelming titre of antibodies recognising the expressed VSG. In this review, the VSG coat is modelled, and past experiments that investigated how it protected the trypanosome are revisited using current knowledge of VSG sequence and structure. The conclusions are: (i) the identity of the individual VSGs explains early experimental variation; (ii) most of the VSG molecule is accessible to antibodies. This second conclusion highlights a gap in our understanding of how the VSG coat works, as several plasma membrane proteins with large extracellular domains are very unlikely to be hidden from host antibodies by VSG.

Genome evolution in trypanosomatid parasites

A decade of genome sequencing has transformed our understanding of how trypanosomatid parasites have evolved and provided fresh impetus to explaining the origins of parasitism in the Kinetoplastida. In this review, I will consider the many ways in which genome sequences have influenced our view of genomic reduction in trypanosomatids; how species-specific genes, and the genomic domains they occupy, have illuminated the innovations in trypanosomatid genomes; and how comparative genomics has exposed the molecular mechanisms responsible for innovation and adaptation to a parasitic lifestyle.

Recombinant expression of trypanosome surface glycoproteins in Pichia pastoris for the diagnosis of Trypanosoma evansi infection

Serodiagnosis of surra, which causes vast economic losses in livestock, is still based on native antigens purified from bloodstream form Trypanosoma (T.) evansi grown in rodents. To avoid the use of laboratory rodents in antigen preparation we expressed fragments of the invariant surface glycoprotein (ISG) 75, cloned from T. brucei gambiense cDNA, and the variant surface glycoprotein (VSG) RoTat 1.2, cloned from T. evansi gDNA, recombinantly in Pichia (P.) pastoris. The M5 strain of this yeast has an engineered N-glycosylation pathway resulting in homogenous Man5GlcNAc2 N-glycosylation which resembles the predominant Man9-5GlcNAc2 oligomannose structures in T. brucei. The secreted recombinant antigens were affinity purified with yields of up to 10mg and 20mg per liter cell culture of rISG 7529-465-E and rRoTat 1.223-385-H respectively. In ELISA, both recombinant proteins discriminated between pre-immune and immune serum samples of 25 goats experimentally infected with T. evansi. The diagnostic potential of rRoTat 1.223-385-H but not of rISG 7529-465-E was confirmed with sera of naturally infected and control dromedary camels. The results suggest that rRoTat 1.223-385-H expressed in P. pastoris requires further evaluation before it could replace native RoTat 1.2 VSG for serodiagnosis of surra, thus eliminating the use of laboratory animals for antigen production.

A cell-surface phylome for African trypanosomes

The cell surface of Trypanosoma brucei, like many protistan blood parasites, is crucial for mediating host-parasite interactions and is instrumental to the initiation, maintenance and severity of infection. Previous comparisons with the related trypanosomatid parasites T. cruzi and Leishmania major suggest that the cell-surface proteome of T. brucei is largely taxon-specific. Here we compare genes predicted to encode cell surface proteins of T. brucei with those from two related African trypanosomes, T. congolense and T. vivax. We created a cell surface phylome (CSP) by estimating phylogenies for 79 gene families with putative surface functions to understand the more recent evolution of African trypanosome surface architecture. Our findings demonstrate that the transferrin receptor genes essential for bloodstream survival in T. brucei are conserved in T. congolense but absent from T. vivax and include an expanded gene family of insect stage-specific surface glycoproteins that includes many currently uncharacterized genes. We also identify species-specific features and innovations and confirm that these include most expression site-associated genes (ESAGs) in T. brucei, which are absent from T. congolense and T. vivax. The CSP presents the first global picture of the origins and dynamics of cell surface architecture in African trypanosomes, representing the principal differences in genomic repertoire between African trypanosome species and provides a basis from which to explore the developmental and pathological differences in surface architectures. All data can be accessed at: http://www.genedb.org/Page/trypanosoma_surface_phylome.

The African trypanosome (Trypanosoma brucei) is a single-celled, vector-borne parasite that causes Human African Trypanosomiasis (or ‘sleeping sickness’) throughout sub-Saharan Africa and, along with related species T. congolense and T. vivax, a similar disease in wild and domestic animals. Together, the African trypanosomes have significant effects on human and animal health and associated costs for socio-economic development in Africa. Genes expressed on the trypanosome cell surface are instrumental in causing disease and sustaining infection by resisting the host immune system. Here we compare repertoires of genes with predicted cell-surface expression in T. brucei, T. congolense and T. vivax and estimate the phylogeny of each predicted cell-surface gene family. This ‘cell-surface phylome’ (CSP) provides a detailed analysis of species-specific gene families and of gene gain and loss in shared families, aiding the identification of surface proteins that may mediate specific aspects of pathogenesis and disease progression. Overall, the CSP suggests that each trypanosome species has modified its surface proteome uniquely, indicating that T. brucei, T. congolense and T. vivax have subtly distinct mechanisms for interacting with both vertebrate and insect hosts.

PCR based diagnosis of trypanosomiasis exploring invariant surface glycoprotein (ISG) 75 gene

The invariant surface glycoprotein (ISG-75) gene of Trypanosoma evansi buffalo isolate from Karnataka state in India was sequenced and analyzed to elucidate its relationship with other isolates/species. The sequenced ISG-75 gene was also explored to device a polymerase chain reaction (PCR) strategy for the diagnosis of trypanosomiasis in carrier animals. The six cloned ISG gene sequences revealed the open reading frame (ORF) of 1572 and 1527 nucleotide (nt) encoding a polypeptide of 523 and 508 amino acids (aa) respectively and belongs ISG-75 gene family. Sequence analysis revealed 91-100% and 65-99% similarity at nt and aa levels, respectively with other isolates/species and belongs to the RoTat 1.2 strain. The diagnostic PCR based on ISG-75 sequence amplifies a 407 bp product specifically from the different T. evansi isolates and could detect 0.04 pg and 1.2 ng of DNA from purified trypanosomes and T. evansi infected rat blood samples respectively. Subsequently the PCR detected 0.02 and 0.27 trypanosomes ml(-1) respectively, from purified trypanosomes and T. evansi (buffalo isolate) infected rat blood. By the developed PCR assay trypanosomal nucleic acid was detected in experimental rats and buffalo on 24 h post infection (p.i.) and 3rd day post infection (d.p.i.), respectively. The developed ISG-75 gene based PCR assay could be useful in detection of carrier status of trypanosomiasis in animals.

Vaccination against trypanosomiasis: can it be done or is the trypanosome truly the ultimate immune destroyer and escape artist?

To date, human African trypanosomiasis (HAT) still threatens millions of people throughout sub-Sahara Africa, and new approaches to disease prevention and treatment remain a priority. It is commonly accepted that HAT is fatal unless treatment is provided. However, despite the well-described general symptoms of disease progression during distinct stages of the infection, leading to encephalitic complications, coma and death, a substantial body of evidence has been reported suggesting that natural acquired immunity could occur. Hence, if under favorable conditions natural infections can lead to correct immune activation and immune protection against HAT, the development of an effective anti-HAT vaccine should remain a central goal in the fight against this disease.<br /> In this review, we will (1) discuss the vaccine candidates that have been proposed over the past years, (2) highlight the main obstacles that an efficient anti-trypanosomiasis vaccine needs to overcome and (3) critically reflect on the validity of the widely used murine model for HAT.

The VSG C-terminal domain is inaccessible to antibodies on live trypanosomes

In the mammalian host, the Trypanosoma brucei cell surface is covered with a densely packed protein coat of a single protein, the variant surface glycoprotein (VSG). The VSG is believed to shield invariant surface proteins from host antibodies but there is limited information on how far antibodies can penetrate into the VSG monolayer. Here, the VSG surface coat was probed to determine whether it acts as a barrier to binding of antibodies to the membrane proximal VSG C-terminal domain. The binding of C-terminal domain antibodies to VSG221 or VSG118 was compared with antibodies recognising the cognate whole VSGs. The C-terminal VSG domain was inaccessible to antibodies on live cells but not on fixed cells. This provides further evidence that the VSG coat acts as a barrier and protects the cell from antibodies that would otherwise bind to some of the other externally disposed proteins.

Current status of vaccination against African trypanosomiasis

Anti-trypanosomiasis vaccination still remains the best theoretical option in the fight against a disease that is continuously hovering between its wildlife reservoir and its reservoir in man and livestock. While antigentic variation of the parasite surface coat has been considered the major obstacle in the development of a functional vaccine, recent research into the biology of B cells has indicated that the problems might go further than that. This paper reviews past and current attempts to design both anti-trypanosome vaccines, as well as vaccines directed towards the inhibition of infection-associated pathology.

Emerging trends in the diagnosis of Human African Trypanosomiasis

Human African trypanosomiasis (HAT) or sleeping sickness is caused by protozoan parasitesTrypanosoma brucei gambienseandT. b. rhodesiense. Despite the enormous technological progress in molecular parasitology in recent years, the diagnosis of HAT is still problematic due to the lack of specific tools. To date, there are two realities when it comes to HAT; the first one being the world of modern experimental laboratories, equipped with the latest state-of-the-art technology, and the second being the world of HAT diagnosis, where the latest semi-commercial test was introduced 30 years ago (Magnuset al.1978). Hence, it appears that the lack of progress in HAT diagnosis is not primarily due to a lack of scientific interest or a lack of research funds, but mainly results from the many obstacles encountered in the translation of basic research into field-applicable diagnostics. This review will provide an overview of current diagnostic methods and highlight specific difficulties in solving the shortcomings of these methods. Future perspectives for accurate, robust, affordable diagnostics will be discussed as well.

Immunobiology of African trypanosomes: need of alternative interventions

Trypanosomiasis is one of the major parasitic diseases for which control is still far from reality. The vaccination approaches by using dominant surface proteins have not been successful, mainly due to antigenic variation of the parasite surface coat. On the other hand, the chemotherapeutic drugs in current use for the treatment of this disease are toxic and problems of resistance are increasing (see Kennedy (2004) and Legros et al. (2002)). Therefore, alternative approaches in both treatment and vaccination against trypanosomiasis are needed at this time. To be able to design and develop such alternatives, the biology of this parasite and the host response against the pathogen need to be studied. These two aspects of this disease with few examples of alternative approaches are discussed here.

African trypanosomiasis and antibodies: implications for vaccination, therapy and diagnosis

African trypanosomiasis causes devastating effects on human populations and livestock herds in large parts of sub-Saharan Africa. Control of the disease is hampered by the lack of any efficient vaccination results in a field setting, and the severe side effects of current drug therapies. In addition, with the exception of Trypanosoma brucei gambiense infections, the diagnosis of trypanosomiasis has to rely on microscopic analysis of blood samples, as other specific tools are nonexistent. However, new developments in biotechnology, which include loop-mediated isothermal amplification as an adaptation to conventional PCR, as well as the antibody engineering that has allowed the development of Nanobody technology, offer new perspectives in both the detection and treatment of trypanosomiasis. In addition, recent data on parasite-induced B-cell memory destruction offer new insights into mechanisms of vaccine failure, and should lead us towards new strategies to overcome trypanosome defenses operating against the host immune system.

The glycosylphosphatidylinositol-PLC in Trypanosoma brucei forms a linear array on the exterior of the flagellar membrane before and after activation

Bloodstream forms of Trypanosoma brucei contain a glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC) that cleaves the GPI-anchor of the variable surface glycoprotein (VSG). Its location in trypanosomes has been controversial. Here, using confocal microscopy and surface labelling techniques, we show that the GPI-PLC is located exclusively in a linear array on the outside of the flagellar membrane, close to the flagellar attachment zone, but does not co-localize with the flagellar attachment zone protein, FAZ1. Consequently, the GPI-PLC and the VSG occupy the same plasma membrane leaflet, which resolves the topological problem associated with the cleavage reaction if the VSG and the GPI-PLC were on opposite sides of the membrane. The exterior location requires the enzyme to be tightly regulated to prevent VSG release under basal conditions. During stimulated VSG release in intact cells, the GPI-PLC did not change location, suggesting that the release mechanism involves lateral diffusion of the VSG in the plane of the membrane to the fixed position of the GPI-PLC.

African trypanosomes cause sleeping sickness, for which current therapy is inadequate. The parasite protects its surface from the host immune system by regularly switching its surface coat. The glycosylphosphatidylinositol-PLC only occurs in the bloodstream form, where it removes the surface coat after it enters the tsetse fly vector. Activation of the enzyme in the bloodstream would be fatal for the parasite and it is, therefore, a potential drug target. However, therapeutic strategies have been hampered by confusion over the location of the GPI-PLC despite great effort by many labs. We have used a wide variety of techniques, including one completely novel method, that exploits the dependence of detection for partially buried surface proteins on the temperature of fixation, to identify the location of the GPI-PLC in relation to other markers unequivocally. All approaches consistently show that the GPI-PLC is located exclusively in the outer leaflet of the plasma membrane covering the flagellum, where it is confined to a narrow linear array adjacent to the flagellar attachment zone. Our data have resolved the question of how enzyme and substrate meet and also suggest that chemotherapeutic agents would be able to target the GPI-PLC in its exterior location.

Loss of actin does not affect export of newly synthesized proteins to the surface of Trypanosoma brucei

Vesicle traffic to and from the surface is highly polarized in African trypanosomes. Actin is required for polarized endocytic traffic in bloodstream forms of African trypanosomes but its role in other pathways has remained equivocal. A combination of metabolic pulse chase labelling and surface biotinylation during the chase period along with the use of conditional RNA interference was employed to demonstrate that substantial loss of actin had no effect on the export of newly synthesized proteins to the surface of bloodstream and procyclic forms of Trypanosoma brucei. These results indicated that this trafficking pathway to the surface operates as normal even when actin levels are significantly lower than normal and endocytic activity is abolished. Taken together the data support the view that the secretory and endocytic pathways are not obligatorily coupled.

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

Pathogens often persist during infection because of antigenic variation in which they evade immunity by switching between distinct surface antigen variants. A central question is how ordered appearance of variants, an important determinant of chronicity, is achieved. Theories suggest that it results directly from a complex pattern of transition connectivity between variants or indirectly from effects such as immune cross-reactivity or differential variant growth rates. Using a mathematical model based only on known infection variables, we show that order in trypanosome infections can be explained more parsimoniously by a simpler combination of two key parasite-intrinsic factors: differential activation rates of parasite variant surface glycoprotein (VSG) genes and density-dependent parasite differentiation. The model outcomes concur with empirical evidence that several variants are expressed simultaneously and that parasitaemia peaks correlate with VSG genes within distinct activation probability groups. Our findings provide a possible explanation for the enormity of the recently sequenced VSG silent archive and have important implications for field transmission.

The invariant surface glycoprotein ISG75 gene family consists of two main groups in the Trypanozoon subgenus

In Trypanosoma brucei brucei, an invariant surface glycoprotein of molecular weight 75 kDa (ISG75) is uniformly distributed over the surface of a trypanosome and is specific for bloodstream-form parasites. For the other taxa of the Trypanozoon subgenus no data about this surface molecule are available. Therefore, we investigated the ISG75 in the genomes of several pathogenic Trypanozoon by Southern blot, PCR and RT-PCR and sequence analysis. This study reveals that (i) all members of the Trypanozoon subgenus, i.e. T. b. brucei, T. b. gambiense, T. b. rhodesiense, T. evansi and T. equiperdum, harbour ISG75 as multiple gene copies with at least 4-16 copies per genome; (ii) ISG75 gDNA and cDNA sequences are distributed in 2 groups that share at least 75% and 77% identity respectively; (iii) sequences from both groups are transcribed in all species and subspecies of the Trypanozoon subgenus; (iv) the main differences between group I and group II are located in the variable region at the amino-terminus of the putative proteins; (v) however, all the sequences in both groups have some well-conserved features, such as the cysteine residues, an amino-terminal cleavable signal peptide, a single alpha-helix transmembrane domain and a cytoplasmic domain at the carboxy-terminus.

Characterization of Two Protein Disulfide Isomerases from the Endocytic Pathway of Bloodstream Forms of Trypanosoma brucei

Proteins from the endocytic pathway in bloodstream forms of Trypanosome brucei are modified by the addition of linear poly-N-acetyllactosamine side chains, which permits their isolation by tomato lectin affinity chromatography. Antibodies against this tomato lectin binding fraction were employed to screen a cDNA expression library from bloodstream forms of T. brucei. Two cDNAs were prominent among those selected. These cDNAs coded for two putative protein disulfide isomerases (PDIs) that respectively contained one and two double-cysteine redox-active sites and corresponded to a single domain PDI and a class 1 PDI. Assays of the purified recombinant proteins demonstrated that both proteins possess isomerase activity, but only the single domain PDI had a reducing activity. These PDIs possess a number of unusual features that distinguish them from previously characterized PDIs. The expression of both is developmentally regulated, they both co-localize with markers of the endocytic pathway, and both are modified by N-glycosylation. The larger PDI possesses N-glycans containing poly-N-acetyllactosamine, a modification that is indicative of processing in the Golgi and suggests the presence of a novel trafficking pathway for PDIs in trypanosomes. Although generally PDIs are considered essential, neither activity appeared to be essential for the growth of trypanosomes, at least in vitro.

Cytoplasmic Targeting Signals in Transmembrane Invariant Surface Glycoproteins of Trypanosomes

Protein targeting mechanisms in flagellated protozoan parasites have received considerable interest because of a huge bias in these organisms toward the glycosylphosphatidylinositol anchor as a mechanism for the membrane attachment of cell surface macromolecules. In this study, the trafficking of invariant surface glycoprotein 65 (ISG65), a family of type I transmembrane proteins, was examined. Analysis of the C-terminal domains of ISG65 family members demonstrated a high level of conservation and, in particular, the presence of three lysine residues contained within the cytoplasmic tails of all ISG65s. ISG65 was expressed on the cell surface, in agreement with earlier work, but an intracellular pool of ISG65 was also detected within a Rab5A early endosome. Transplantation of the C-terminal 74 amino acids of ISG65 (encompassing the 23 C-terminal residues of the extracellular domain, the transmembrane peptide, and the cytoplasmic domain) onto the N-terminal domain of BiP (BiPN) was sufficient to target the chimera to the same internal compartments as native ISG65. Further, site-directed mutagenesis indicated that the cytoplasmic tail was required for endoplasmic reticulum exit and that at least two of the cytoplasmic domain lysine residues are needed for endosomal targeting, as removal of all three led to surface expression. Kinetic measurements demonstrate that the BiPN fusion protein (containing the ISG65 C terminus) has a short half-life, indicating rapid turnover. In contrast, BiPN fusion proteins containing a glycosylphosphatidylinositol anchor instead of the ISG65 C-terminal region are stably expressed on the surface, confirming the requirement for the ISG65 sequence for endosomal targeting. We suggest that the lack of surface expression of the BiPN-ISG65 fusion protein is likely due to more efficient internalization compared with ISG65. Taken together, these data demonstrate the presence of a lysine-dependent endocytosis signal in the ISG65 family.

A protein kinase specifically associated with proliferative forms of Trypanosoma brucei is functionally related to a yeast kinase involved in the co-ordination of cell shape and division

The life cycle of African trypanosomes is characterized by the alternation of proliferative and quiescent stages but the molecular details of this process remain unknown. Here, we describe a new cytoplasmic protein kinase from Trypanosoma brucei, termed TBPK50, that belongs to a family of protein kinases involved in the regulation of the cell cycle, cell shape and proliferation. TBPK50 is expressed only in proliferative forms but is totally absent in quiescent cells despite the fact that the gene is constitutively transcribed at the same level throughout the life cycle. It is probable that TBPK50 has very specific substrate requirements as it was unable to transphosphorylate a range of classical phosphoacceptor substrates in vitro, although an autophosphorylation activity was readily detectable in the same assays. Complementation studies using a fission yeast mutant demonstrated that TBPK50 is a functional homologue of Orb6, a protein kinase involved in the regulation of cellular morphology and cell cycle progression in yeast. These results link the expression of TBPK50 and the growth status of trypanosomes and support the view that this protein kinase is likely to be involved in the control of life cycle progression and cell division of these parasites.

Sequence requirements for trafficking of the CRAM transmembrane protein to the flagellar pocket of African trypanosomes

CRAM is a cysteine-rich acidic transmembrane protein, highly expressed in the procyclic form of Trypanosoma brucei. Cell surface expression of CRAM is restricted to the flagellar pocket of trypanosomes, the only place where receptor mediated endocytosis takes place in the parasite. CRAM can function as a receptor and was hypothesized to be a lipoprotein receptor of trypanosomes. We study mechanisms involved in the presentation and routing of CRAM to the flagellar pocket of insect- and bloodstream-form trypanosomes. By deletional mutagenesis, we found that deleting up to four amino acids from the C terminus of CRAM did not affect the localization of CRAM at the flagellar pocket. Shortening the CRAM protein by 8 and 19 amino acids from the C terminus resulted in the distribution of the CRAM protein in the endoplasmic reticulum (ER) (the CRAM protein is no longer uniquely sequestered at the flagellar pocket). This result indicates that the truncation of the CRAM C terminus affected the transport efficiency of CRAM from the ER to the flagellar pocket. However, when CRAM was truncated between 29 and 40 amino acids from the C terminus, CRAM was not only distributed in the ER but also located to the flagellar pocket and spread to the cell surface and the flagellum. Replacing the CRAM transmembrane domain with the invariant surface glycoprotein 65-derived transmembrane region did not affect the flagellar pocket location of CRAM. These results indicate that the CRAM cytoplasmic extension may exhibit two functional domains: one domain near the C terminus is important for efficient export of CRAM from the ER, while the second domain is of importance for confining CRAM to the flagellar pocket membrane.

A role for the dynamic acylation of a cluster of cysteine residues in regulating the activity of the glycosylphosphatidylinositol-specific phospholipase C of Trypanosoma brucei

The glycosylphosphatidylinositol-specific phospholipase C or VSG lipase is the enzyme responsible for the cleavage of the glycosylphosphatidylinositol anchor of the variant surface glycoprotein (VSG) and concomitant release of the surface coat in Trypanosoma brucei during osmotic shock or extracellular acidic stress. In Xenopus laevis oocytes the VSG lipase was expressed as a nonacylated and a thioacylated form. This thioacylation occurred within a cluster of three cysteine residues but was not essential for catalytic activity per se. These two forms were also detected in trypanosomes and appeared to be present at roughly equivalent amounts. A reversible shift to the acylated form occurred when cells were triggered to release the VSG by either nonlytic acid stress or osmotic lysis. A wild type VSG lipase or a gene mutated in the three codons for the acylated cysteines were reinserted in the genome of a trypanosome null mutant for this gene. A comparative analysis of these revertant trypanosomes indicated that thioacylation might be involved in regulating enzyme access to the VSG substrate.

Characterization of a novel alanine-rich protein located in surface microdomains in Trypanosoma brucei

Heterologous expression in COS cells followed by orientation-specific polymerase chain reaction to select and amplify cDNAs encoding surface proteins in Trypanosoma brucei resulted in the isolation of a cDNA ( approximately 1.4 kilobase) which encodes an acidic, alanine-rich polypeptide that is expressed only in bloodstream forms of the parasite and has been termed bloodstream stage alanine-rich protein (BARP). Analysis of the amino acid sequence predicted the presence of a typical NH(2)-terminal leader sequence as well as a COOH-terminal hydrophobic extension with the potential to be replaced by a glycosylphosphatidylinositol anchor. A search of existing protein sequences revealed partial homology between BARP and the major surface antigen of procyclic forms of Trypanosoma congolense. BARP migrated as a complex, heterogeneous series of bands on Western blots with an apparent molecular mass ( approximately 50-70 kDa) significantly higher than predicted from the amino acid sequence ( approximately 26 kDa). Confocal microscopy demonstrated that BARP was present in small discrete spots that were distributed over the entire cellular surface. Detergent extraction experiments revealed that BARP was recovered in the detergent-insoluble, glycolipid-enriched fraction. These data suggested that BARP may be sequestered in lipid rafts.

Comparative analysis of antibody responses against HSP60, invariant surface glycoprotein 70, and variant surface glycoprotein reveals a complex antigen-specific pattern of immunoglobulin isotype switching during infection by Trypanosoma brucei

During Trypanosoma brucei infections, the response against the variant surface glycoprotein (VSG) of the parasite represents a major interaction between the mammalian host immune system and the parasite surface. Since immune recognition of other parasite derived factors also occurs, we examined the humoral host response against trypanosome heat shock protein 60 (HSP60), a conserved antigen with an autoimmune character. During experimental T. brucei infection in BALB/c mice, the anti-HSP60 response was induced when parasites differentiated into stumpy forms. This response was characterized by a stage-specific immunoglobulin isotype switching as well as by the induction of an autoimmune response. Specific recognition of trypanosome HSP60 was found to occur during the entire course of infection. Immunoglobulin G2a (IgG2a) and IgG2b antibodies, induced mainly in a T-cell-independent manner, were observed during the first peak of parasitemia, whereas IgG1 and IgG3 antibodies were found at the end of the infection, due to a specific T-cell-mediated response. Comparative analysis of the kinetics of anti-HSP60, anti-invariant surface glycoprotein 70 (ISG70), and anti-VSG antibody responses indicated that the three trypanosome antigens give rise to specific and independent patterns of immunoglobulin isotype switching.

Slender and stumpy bloodstream forms of Trypanosoma brucei display a differential response to extracellular acidic and proteolytic stress

Natural infections of mammals with African trypanosomes, such as Trypanosoma brucei, are generally pleomorphic, the population consisting of different forms, termed slender and stumpy forms, that vary in number as the parasitaemia develops. We show that the differentiation of slender into stumpy forms is characterized by the acquisition by the parasite of the ability to regulate its internal pH, even in the face of a large, inwardly directed gradient of H+, as well as a tolerance towards external proteolytic stress. These adaptations effectively abbrogate cellular stress-activated signalling pathways involving adenylate cyclase and glycosylphosphoinositol-specific phospholipase-C mediated release of the surface coat. Although in metabolic terms stumpy forms of the parasite are considered to be preadapted to life in the arthropod vector, these data clearly demonstrate that these forms also possess additional cellular adaptations designed to deal with the immediate and potentially harmful changes in the extracellular environment that occur upon ingestion of a bloodmeal by the tsetse fly vector.

Surface receptors and transporters of Trypanosoma brucei

African trypanosomes combine antigenic variation of their surface coat with the ability to take up nutrients from their mammalian hosts. Uptake of small molecules such as glucose or nucleosides is mediated by translocators hidden from host antibodies by the surface coat. The multiple glucose transporters and transporters for nucleobases and nucleosides have been characterized. Receptors for host macromolecules such as transferrin and lipoproteins are visible to antibodies but hidden from the cellular arm of the host immune system in an invagination of the trypanosome surface, the flagellar pocket. The trypanosomal transferrin receptor is a heterodimer that resembles the major component of the surface coat of Trypanosoma brucei. The ability to make several versions of this receptor allows T. brucei to bind transferrins from a range of mammals with high affinity. The proteins required for uptake of nutrients by trypanosomes provide a target for chemotherapy that remains to be fully exploited.

Characterization of a novel, stage-specific, invariant surface protein in Trypanosoma brucei containing an internal, serine-rich, repetitive motif

A new surface membrane protein, invariant surface glycoprotein termed ISG100, was identified in Trypanosoma brucei, using catalyzed surface, radioiodination of intact cells. This integral membrane glycoprotein was purified by a combination of detergent extraction, lectin-affinity, and ion-exchange chromatography followed by preparative SDS-polyacrylamide gel electrophoresis. The protein was expressed only in bloodstream forms of the parasite, was heavily N-glycosylated, and was present in different clonal variants of the same serodeme as well as in different serodemes. The gene for this protein was isolated by screening a cDNA expression library with antibodies against the purified protein followed by screening of a genomic library. The nucleotide sequence of the gene (4050 base pairs) predicted a highly reiterative polypeptide containing three distinct domains, a unique N-terminal domain of about 10 kDa containing three potential N-glycosylation sites, which was followed by a large internal domain consisting entirely of 72 consecutive copies of a serine-rich, 17-amino acid motif (approximately 113 kDa) and terminated with an apparent transmembrane spanning region of about 3.3 kDa. The internal repeat region of this gene (3672 base pairs) represents the largest reiterative coding sequence to be fully characterized in any species of trypanosome. There was no significant homology with other known proteins, and overall the predicted protein was extremely hydrophobic. Unlike the genes for other surface proteins, the gene encoding ISG100 was present as a single copy. Although present in the flagellar pocket, ISG100 was predominantly associated with components of the pathways for endo/exocytosis, such as intracellular vesicles located in the proximity of the pocket as well a large, electron-lucent perinuclear digestive vacuole.

Kinetoplastid membrane protein-11 (KMP-11) is differentially expressed during the life cycle of African trypanosomes and is found in a wide variety of kinetoplastid parasites

An abundant 11-kDa membrane protein was purified from African trypanosomes by organic solvent extraction and octyl-Sepharose chromatography. This protein cross-reacts with monoclonal antibodies originally generated against the lipophosphoglycan-associated protein of Leishmania donovani. Immunoblot analysis showed that the 11-kDa molecule was present in a variety of species of kinetoplastids. It was found in several species and subspecies of African trypanosomes and was present in low amounts in bloodstream forms and in larger amounts in procyclic, epimastigote and metacyclic life cycle stages. Expression of the 11-kDa molecule rapidly increased during transformation from bloodstream forms to procyclic forms, paralleling expression of the major surface glycoprotein of Trypanosoma congolense, the glutamic acid/alanine-rich protein, an analogue of T. brucei procyclin. The molecule was present in procyclic trypanosome membranes at approximately 2 x 10(5)-1 x 10(6) molecules per cell, suggesting it may have an important role in parasite membrane organization and function. Amino-acid analysis of the trypanosome 11-kDa protein showed it had a different composition than that of its leishmania counterpart. Its wide distribution in kinetoplastids and its membrane disposition suggest a name for this class of molecules: kinetoplastid membrane protein-11 (KMP-11).

A novel heterodimeric transferrin receptor encoded by a pair of VSG expression site-associated genes in T. brucei

In T. brucei, a transferrin-binding protein has been found to share sequence homology with pESAG-7 and -6, the products of two related genes present in the VSG gene polycistronic transcription unit. When expressed in Xenopus oocytes, they appear as N-glycosylated proteins secreted in the medium (pESAG-7) and GPI anchored to the membrane (pESAG-6). These proteins are able to homo- or heterodimerize, probably through association in the same orientation. Only heterodimers can bind Tf, possibly two molecules per dimer. A comparison of Tf binding to pESAG-7/6-expressing oocytes and trypanosomes suggests that pESAG-7/6 is the Tf receptor of the parasite. In trypanosomes, the majority of pESAG-7/6 is released from the membrane and associates, together with Tf, with a glycosylated matrix present in the lumen of the flagellar pocket. Both pESAG-7/6 and Tf are internalized via coated pits and vesicles. These observations suggest a novel mode of Tf binding and uptake in trypanosomes.

Invariant surface proteins in bloodstream forms of Trypanosoma brucei

Antigenic variation of the glycoprotein forming the coat of African trypanosomes has been a dominant field of investigation for many years. The extravagant potential of these parasites to change their surface coat has destroyed hopes for a vaccine based on the variant surface glycoprotein. Recently, there has been a rising interest in the characterization of surface proteins that are not subject to antigenic variation. In this review, Peter Overath, Maliha Chaudhri, Dietmar Steverding and Karl Ziegelbauer summarize the present evidence for the occurrence, cellular localization and function of invariant surface proteins in Trypanosoma brucei.

Organization of two invariant surface glycoproteins in the surface coat of Trypanosoma brucei

The surface coat of Trypanosoma brucei, formed by about 10(7) molecules of the membrane-form variant surface glycoprotein (mfVSG) per cell, is generally considered to constitute a barrier against the access of antibodies directed to invariant surface proteins. The recent characterization of two invariant surface glycoproteins (ISGs) with apparent molecular masses of 65 and 75 kDa (ISG65 and ISG75; 70,000 and 50,000 molecules per cell, respectively), which are both predicted to be composed of large extracellular domains, single transmembrane alpha-helices, and small intracellular domains, enabled a critical test of this hypothesis. Although ISG65 is distributed over the entire surface of the parasites, it is not accessible to antibodies or to the proteinase trypsin in live cells provided the mfVSG is also proteinase resistant. ISG75 is similarly distributed; its accessibility to antibodies depends on the expressed mfVSG, and it is sensitive to trypsin in a variant clone in which the mfVSG is proteinase resistant. Vaccination experiments using recombinant proteins to a mixture of the native ISGs were unsuccessful. ISG65 but not ISG75 elicited an antibody response in chronically infected mice. The results strengthen the view of the protective properties of the variant surface glycoprotein coat by steric hindrance and suggest that additional factors such as low abundance or low immunogenicity of invariant surface proteins may prevent a control of the disease by the humoral immune response.