Variable agglutinogenic antigens of Trypanosoma gambiense and their distribution among isolates of the trypanosome collected in different places in Nigeria

The genome sequence of Trypanosoma brucei gambiense, causative agent of chronic human african trypanosomiasis

Background

Trypanosoma brucei gambiense is the causative agent of chronic Human African Trypanosomiasis or sleeping sickness, a disease endemic across often poor and rural areas of Western and Central Africa. We have previously published the genome sequence of a T. b. brucei isolate, and have now employed a comparative genomics approach to understand the scale of genomic variation between T. b. gambiense and the reference genome. We sought to identify features that were uniquely associated with T. b. gambiense and its ability to infect humans.

Methods and Findings

An improved high-quality draft genome sequence for the group 1 T. b. gambiense DAL 972 isolate was produced using a whole-genome shotgun strategy. Comparison with T. b. brucei showed that sequence identity averages 99.2% in coding regions, and gene order is largely collinear. However, variation associated with segmental duplications and tandem gene arrays suggests some reduction of functional repertoire in T. b. gambiense DAL 972. A comparison of the variant surface glycoproteins (VSG) in T. b. brucei with all T. b. gambiense sequence reads showed that the essential structural repertoire of VSG domains is conserved across T. brucei.

Conclusions

This study provides the first estimate of intraspecific genomic variation within T. brucei, and so has important consequences for future population genomics studies. We have shown that the T. b. gambiense genome corresponds closely with the reference, which should therefore be an effective scaffold for any T. brucei genome sequence data. As VSG repertoire is also well conserved, it may be feasible to describe the total diversity of variant antigens. While we describe several as yet uncharacterized gene families with predicted cell surface roles that were expanded in number in T. b. brucei, no T. b. gambiense-specific gene was identified outside of the subtelomeres that could explain the ability to infect humans.

Sleeping sickness, or Human African Trypanosomiasis, is a disease affecting the health and productivity of poor people in many rural areas of sub-Saharan Africa. The disease is caused by a single-celled flagellate, Trypanosoma brucei, which evades the immune system by periodically switching the proteins on its surface. We have produced a genome sequence for T. brucei gambiense, which is the particular subspecies causing most disease in humans. We compared this with an existing reference genome for a non-human infecting strain (T. b. brucei 927) to identify genes in T. b. gambiense that might explain its ability to infect humans and to assess how well the reference performs as a universal plan for all T. brucei. The genome sequences differ only due to rare insertions and duplications and homologous genes are over 95% identical on average. The archive of surface antigens that enable the parasite to switch its protein coat is remarkably consistent, even though it evolves very quickly. We identified genes with predicted cell surface functions that are only present in T. b. brucei and have evolved rapidly in recent time. These genes might help to explain variation in disease pathology between different T. brucei strains in different hosts.

Conserved sequence of the TgsGP gene in Group 1 Trypanosoma brucei gambiense

The trypanosome responsible for the majority of cases of human trypanosomiasis in Africa is Group 1 Trypanosoma brucei gambiense. Currently the most reliable test for the parasite is based on a single gene, which encodes a 47kDa receptor-like T. b. gambiense-specific glycoprotein, TgsGP, expressed in the flagellar pocket of bloodstream forms. Although TgsGP has been demonstrated in T. b. gambiense throughout its geographic range, similar genes have been demonstrated in other T. brucei sspp. isolates, and there are no data on the extent of sequence variation in TgsGP. Here we have carried out a comparison of TgsGP sequences in a range of Group 1 T. b. gambiense isolates and compared the gene to homologues in other T. brucei sspp. in order to provide information to support the use of this gene as the key identification target for Group 1 T. b. gambiense. We demonstrate that the sequence of TgsGP is well conserved in Group 1 T. b. gambiense across the endemic range of gambian human trypanosomiasis and confirm that this gene is a suitable target for specific detection of this parasite. The TgsGp-like genes in some isolates of T. b. brucei, T. b. rhodesiense and Group 2 T. b. gambiense are closely similar to VSG Tb10.v4.0178, which may be the ancestral gene from which TgsGP was derived.

Killing of trypanosomatid parasites by a modified bovine host defense peptide, BMAP-18

BACKGROUND

Tropical diseases caused by parasites continue to cause socioeconomic devastation that reverberates worldwide. There is a growing need for new control measures for many of these diseases due to increasing drug resistance exhibited by the parasites and problems with drug toxicity. One new approach is to apply host defense peptides (HDP; formerly called antimicrobial peptides) to disease control, either to treat infected hosts, or to prevent disease transmission by interfering with parasites in their insect vectors. A potent anti-parasite effector is bovine myeloid antimicrobial peptide-27 (BMAP-27), a member of the cathelicidin family. Although BMAP-27 is a potent inhibitor of microbial growth, at higher concentrations it also exhibits cytotoxicity to mammalian cells. We tested the anti-parasite activity of BMAP-18, a truncated peptide that lacks the hydrophobic C-terminal sequence of the BMAP-27 parent molecule, an alteration that confers reduced toxicity to mammalian cells.

METHODOLOGY/PRINCIPAL FINDINGS

BMAP-18 showed strong growth inhibitory activity against several species and life cycle stages of African trypanosomes, fish trypanosomes and Leishmania parasites in vitro. When compared to native BMAP-27, the truncated BMAP-18 peptide showed reduced cytotoxicity on a wide variety of mammalian and insect cells and on Sodalis glossindius, a bacterial symbiont of the tsetse vector. The fluorescent stain rhodamine 123 was used in immunofluorescence microscopy and flow cytometry experiments to show that BMAP-18 at low concentrations rapidly disrupted mitochondrial potential without obvious alteration of parasite plasma membranes, thus inducing death by apoptosis. Scanning electron microscopy revealed that higher concentrations of BMAP-18 induced membrane lesions in the parasites as early as 15 minutes after exposure, thus killing them by necrosis. In addition to direct killing of parasites, BMAP-18 was shown to inhibit LPS-induced secretion of tumour necrosis factor alpha (TNF-alpha), a cytokine that is associated with inflammation and cachexia (wasting) in sleeping sickness patients. As a prelude to in vivo applications, high affinity antibodies to BMAP-18 were produced in rabbits and used in immuno-mass spectrometry assays to detect the intact peptide in human blood and plasma.

CONCLUSIONS/SIGNIFICANCE

BMAP-18, a truncated form of the potent antimicrobial BMAP-27, showed low toxicity to mammalian cells, insect cells and the tsetse bacterial symbiont Sodalis glossinidius while retaining an ability to kill a variety of species and life cycle stages of pathogenic kinetoplastid parasites in vitro. BMAP-18 also inhibited secretion of TNF-alpha, an inflammatory cytokine that plays a role in the cachexia associated with African sleeping sickness. These findings support the idea that BMAP-18 should be explored as a candidate for therapy of economically important trypanosome-infected hosts, such as cattle, fish and humans, and for paratransgenic expression in Sodalis glossinidius, a bacterial symbiont in the tsetse vector, as a strategy for interference with trypanosome transmission.

Will the real Trypanosoma b. gambiense please stand up

Controversy has surrounded the differentiation of Trypanosoma brucei gambiense from T. b. rhodesiense (causative agents of Gambian and Rhodesian sleeping sickness, respectively) almost from the moment they were named. In the light of recent findings from biochemical and immunological characterization studies, Wendy Gibson reviews the status of T. b. gambiense to see if there is now a consensus concerning its identity.

Expression of a variant surface glycoprotein of Trypanosoma gambiense in procyclic forms of Trypanosoma brucei shows that the cell type dictates the nature of the glycosylphosphatidylinositol membrane anchor attached to the glycoprotein

Procyclic forms of Trypanosoma brucei have been genetically modified to express the major metacyclic variant surface glycoprotein (VSG variant AnTat 11.17) of Trypanosoma gambiense. The VSG is expressed in an intact membrane-bound form that can be detected over the entire plasma membrane, together with procyclin, and as a series of lower-molecular-mass fragments that are mostly soluble degradation products. The presence of degraded VSG in the cells and the culture medium suggests that VSG is not efficiently processed and/or efficiently folded when expressed in procyclic cells. The level of procyclin expressed on the surface of these cells is slightly reduced, although there is no difference in procyclin mRNA levels. The intact membrane-bound form of the VSG is N-glycosylated with oligomannose structures and contains a glycosylphosphatidylinositol (GPI) membrane anchor that can be biosynthetically labelled with [3H]ethanolamine. The anchor is sensitive to mammalian GPI-specific phospholipase D but, like the anchor of procyclin, it is resistant to the action of bacterial phosphatidylinositol-specific phospholipase C. This pattern of phospholipase sensitivity suggests that the GPI anchor acquired by VSG when expressed in procyclics is acylated on the inositol ring and therefore resembles a procyclic procyclin-type anchor rather than a trypomastigote VSG-type anchor with respect to the lipid structure. The VSG expressed in procyclics was sensitive to the action of a mixture of sialidase, beta-galactosidase and beta-hexosaminidase, suggesting that the VSG GPI anchor also contains a sialylated polylactosamine side-chain modification similar to that described for procyclin. These results indicate that the nature of the protein expressed has little influence on the post-translational modifications performed in the secretory pathway of procyclic trypanosomes.

Characterization of Trypanosoma brucei gambiense isolates using restriction fragment length polymorphisms in 5 variant surface glycoprotein genes

Fifty-eight Type I Trypanosoma brucei gambiense (G) stocks, including 16 from 3 sleeping sickness foci in Cameroon, were compared by Restriction Fragment Length Polymorphism (RFLP) analysis with 14 T.b. brucei and T.b. rhodesiense stocks from various endemic areas of Africa. Loci examined were for 5 variant surface glycoprotein (VSG) genes: the LiTat 1.3, AnTat 11.17 and 2K genes were present as single copy genes, while the VSG 117 and U2 gene probes hybridised with a family of related genes. The RFLP data were subjected to cluster analysis to produce a dendrogram constructed from similarity coefficients. The LiTat 1.3 and AnTat 11.17 genes are considered to be characteristic of G stocks, and neither gene was found in the non-G stocks; however, the LiTat 1.3 gene was absent from 6 of the 58 G stocks, while the AnTat 11.17 gene was absent from 8. Supplementation of the LiTat 1.3 antigen in the Card Agglutination Test for Trypanosomiasis with the AnTat 11.17 antigen might thus improve performance of the test, particularly in Cameroon. The U2 VSG gene probe gave a characteristic RFLP pattern for G stocks, as did the VSG 117 gene; the latter is an isogene of AnTat 1.8 previously used extensively to characterise G stocks by other workers. The 2K gene was absent in some G stocks, while present in some non-G stocks, and was not therefore useful for characterisation of G stocks. In cluster analysis, the T.b. gambiense stocks formed a large homogeneous group, subdivided into 5 subgroups, with the non-gambiense stocks as a heterogeneous outgroup.

Trypanosomosis research at the Centre for Tropical Veterinary Medicine (CTVM) 1970 to 1995

This review covers aspects of research work carried out on animal trypanosomes at the Centre for Tropical Veterinary Medicine (CTVM) during the last 25 years. The review covers work on antigenic variation, tissue culture, drug resistance, immunology, biochemistry and pathology of Trypanosoma brucei, T. congolense, T. gambiense and T. evansi. It is not intended as an exhaustive review of the subject but focuses on certain aspects of these areas which are presented in relation to work carried out within the broader scientific community.

Evaluation of variant specific trypanolysis tests for serodiagnosis of human infections with Trypanosoma brucei gambiense

Twelve T.b. gambiense clone populations of distinct Variable Antigen Type (VAT) were combined in immune lysis tests with 340 sera of trypanosome infected patients from 8 different African countries and 267 non trypanosomiasis control sera. The diagnostic specificity of the test was 100%. At a serum dilution of 1:4 the overall test sensitivity with single VATs varied from 39.1 to 98.2% and from 12.1 to 86.8% at 1:32. At a serum dilution of 1:32 some combination tests with 2 VATs still scored above 96%. The VAT recognition patterns were clearly correlated with the geographical origin of the sera, reflecting a diversity in variable antigen repertoires.

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).

The isolation and genetic heterogeneity of Trypanosoma brucei gambiense from north-west Uganda

Fifty-two samples of blood were taken from sleeping sickness patients in north-west Uganda. All samples failed to infect immunosuppressed mice. Ten cryopreserved blood samples were fed to laboratory bred Glossina morsitans morsitans; eight flies developed midgut infections from which procyclic cultures were established in vitro. Isoenzyme electrophoretic analysis of 9 enzymes revealed that 7 of the 8 trypanosome isolates had a combination of enzyme patterns already described for Trypanosoma brucei gambiense. The eighth isolate had a different aspartate aminotransferase polymorphism which placed it in a new zymodeme. Analysis of polymorphisms in genes for 3 variant surface glycoproteins (VSGs) confirmed that the 8 Ugandan trypanosome isolates were T.b.gambiense and revealed further heterogeneity. The VSG 117 gene was present in all the isolates in a pattern of fragments (equivalent to AnTat 1.8) characteristic for T.b.gambiense. For two other VSG genes characteristic of T.b.gambiense, the LiTat 1.3 gene was present in all the isolates, while the AnTat 11.17 gene was present in only 2 of the 8 isolates.

Identification of Trypanosoma brucei gambiense by PCR amplification of variant surface glycoprotein genes

We have developed a sensitive and specific method to identify Trypanosoma brucei gambiense using the polymerase chain reaction (PCR) to amplify the gene encoding variant surface glycoprotein (VSG) Antat 11.17. The test was capable of distinguishing T. b. gambiense from T. b. brucei in most foci of gambian sleeping sickness and gave positive results with previously well-characterised Type I T. b. gambiense stocks from Ivory Coast, Nigeria, Cameroon, Congo, Zaire and Sudan. The test gave negative results with T. b. rhodesiense from Zambia, Kenya and Uganda, virulent or Type II T. b. gambiense from Ivory Coast and T. b. brucei stocks from East and West Africa. The test was modified for colorimetric detection in dot blot format by using nested biotinylated primers in a two-step reaction. Comparison of DNA sequences of VSG genes from T. b. gambiense and other T. brucei ssp. stocks showed a high level of homology, suggesting recent gene flow.

High homology between variant surface glycoprotein gene expression sites of Trypanosoma brucei and Trypanosoma gambiense

The AnTat 11.17 variant surface glycoprotein (VSG) is synthesized in both metacyclic and bloodstream forms of Trypanosoma gambiense. We have characterized the AnTat 11.17 gene, and analyzed its expression site (ES) in the bloodstream form by Southern and Northern blotting with probes from the Trypanosoma brucei AnTat 1.3A VSG ES, and by run-on transcription. The AnTat 11.17 ES is located at the end of a 700-kb chromosome. It appears to contain all the genes (ESAGs, for Expression Site-Associated Genes) present in the AnTat 1.3A VSG ES, with the possible exception of ESAG 1. Limited nucleotide sequence analysis of ESAG cDNAs from the AnTat 11.17 ES shows considerable conservation with ESAGs of T. brucei. The transcription promoter of the AnTat 11.17 VSG ES, localized by virtue of the specific accumulation of promoter-proximal transcripts which occurs following UV irradiation, was found to be at the same relative position to the first ESAG (ESAG 7) as in AnTat 1.3A.

Elusive trypanosomes

Professor Kershaw's encouragement of the development of anion-exchange separation of African trypanosomes from blood led to two decades of activity when, for the first time, considerable progress was made in the intrinsic characterization of these parasites. Such characterization depended on establishing high infections in laboratory rodents. However, the collection of samples from the field was restricted by the failure of certain trypanosomes either to infect, or to multiply adequately in, rodents. More recently, in vitro culture has come to play an increasingly important role in producing material. By obtaining procyclic forms directly from wild tsetse flies, or by transforming low numbers of bloodstream forms in field samples to the procyclic phase in experimental tsetse, trypanosomes of poor or nil infectivity to rodents were readily cultured in the large amounts required for biochemical characterization. A number of specimens of a new kind of Nannomonas, of Trypanosoma simiae, of T. grayi, and of an antigenically distinct T. brucei gambiense were found. Evidence is presented that many other kinds of trypanosome may be eluding isolation by their inability to infect rodents.

Bloodstream and metacyclic variant surface glycoprotein gene expression sites of Trypanosoma brucei gambiense

Trypanosoma brucei gambiense is the causative agent of chronic human sleeping sickness. Previous studies have indicated that T. b. gambiense isolates expressed the antigens U1 or L2 in both the metacyclic and early bloodstream form of the parasite life cycle. These studies suggested that L2 and U1 were likely to be metacyclic variant surface glycoproteins (mVSG). The basic copies of the genes encoding the VSGs L2 and U1 are present in single copy in non-expressing isolates of T. b. gambiense. Furthermore, they have been found to be maintained stably in a large number of stocks isolated from a wide geographic area over a 30-year period. The genomic DNA comprising the upstream 5' flanking regions of the U1 and L2 putative mVSG gene expression sites have been cloned from bloodstream forms of T. b. gambiense. The L2 expression site clone, containing 12.5 kb of sequences 5' to the VSG gene, was found to lack the 72/76-bp repeat unit generally found in the 'barren' region upstream of bloodstream form expression sites. The U1 expression site clone, containing 13.5 kb of the 5' flanking region, appeared to have the repeats, which were localized to 2 kb of DNA immediately 5' to the U1 mVSG gene. Neither the U1 nor the L2 clone was found to have ESAG2 or ESAG3 gene sequences, but both were found to have ESAG1 genes. The ESAG1 genes from the putative metacyclic expression sites and from the U1 and L2 bloodstream form expression sites (in the form of cDNA clones) were sequenced and compared to all other published ESAG1 sequences.

The substitution of procyclic for bloodstream form Trypanosoma brucei gambiense in isoenzyme studies

Examination of 10 enzymes from 8 stocks of Trypanosoma brucei showed that procyclic forms could be substituted for bloodstream forms in isoenzyme studies. T. b. gambiense procyclic forms cultured in vitro offer a better source of material for genetic investigations because this species is usually of low infectivity and virulence to laboratory rodents. Using 6 stocks of T. b. gambiense and 2 stocks of T. b. brucei, enzyme patterns of bloodstream and procyclic forms were identical for isocitrate dehydrogenase, malic enzyme, two nucleoside hydrolases (utilizing inosine and deoxyinosine respectively), phosphoglucomutase and superoxide dismutase. Procyclic forms appeared to have greater threonine dehydrogenase activity than bloodstream forms. Consistent differences between bloodstream and culture forms were observed for alanine aminotransferase, aspartate aminotransferase and malate dehydrogenase. These agreed with known differences in the metabolism of procyclic and bloodstream forms.

A new method for isolating Trypanosoma brucei gambiense from sleeping sickness patients

Low infectivity to laboratory mammals and low virulence make Trypanosoma brucei gambiense difficult to isolate and grow in amounts sufficient for biochemical characterization. We report the isolation of T.b. gambiense by feeding cryopreserved primary isolates to laboratory-reared Glossina morsitans morsitans, followed by rapid cultivation in vitro of procyclic forms dissected from infected tsetse fly midguts. This technique allows the characterization of hitherto unsampled populations and avoids selection due to long-term subpassage. Of 16 primary isolates from trypanosomiasis patients of the Fontem focus in Cameroon, 12 (75%) produced infections in tsetse whereas only 4 (25%) infected rats. Ten isolates were subsequently cultivated as procyclic forms in vitro; 2 failed to grow owing to bacterial contamination. In addition, 2 primary isolates from Côte d'Ivoire patients and a stock of low virulence from the Congo Republic were similarly grown. Only one primary isolate produced tsetse salivary gland infections, an observation consistent with the hypothesis that some populations of T.b. gambiense are intrinsically incompatible with G.m. morsitans.

Detection of Trypanosoma congolense and Trypanosoma brucei subspecies by DNA amplification using the polymerase chain reaction

The nuclear DNA of Trypanosoma congolense contains a family of highly conserved 369 base pair (bp) repeats. The sequences of three cloned copies of these repeats were determined. An unrelated family of 177 bp repeats has previously been shown to occur in the nuclear DNA of Trypanosoma brucei brucei (Sloof et al. 1983a). Oligonucleotides were synthesized which prime the specific amplification of each of these repetitive DNAs by the polymerase chain reaction (PCR). Amplification of 10% of the DNA in a single parasite of T. congolense or T. brucei spp. produced sufficient amplified product to be visible as a band in an agarose gel stained with ethidium bromide. This level of detection, which does not depend on the use of radioactivity, is about 100 times more sensitive than previous detection methods based on radioactive DNA probes. The oligonucleotides did not prime the amplification of DNA sequences in other trypanosome species nor in Leishmania, mouse or human DNAs. Amplification of DNA from the blood of animals infected with T. congolense and/or T. brucei spp. permitted the identification of parasite levels far below that detectable by microscopic inspection. Since PCR amplification can be conducted on a large number of samples simultaneously, it is ideally suited for large-scale studies on the prevalence of African trypanosomes in both mammalian blood and insect vectors.

Immunodiagnosis of sleeping sickness due to Trypanosoma brucei gambiense by detection of antiprocyclic antibodies and trypanosome antigens in patients' sera

Documented sera from 39 Trypanosoma brucei gambiense sleeping sickness patients from Ivory Coast (Côte d'Ivoire) were tested using the Procyclic Agglutination Trypanosomiasis Test (PATT) for the presence of anti-trypanosome antibodies and using an antigen-capture double antibody enzyme-linked immunosorbent assay (ELISA) for the presence of trypanosomal antigens. All 39 sera contained antiprocyclic antibodies and trypanosome antigens whereas 5 control sera did not. The results show that the PATT (for antibody detection) and the double antibody ELISA (for antigen detection) are useful for immunodiagnosis of African sleeping sickness due to T.b. gambiense and that these assays should be simplified for further testing and evaluation in the field.

The genome and the antigen gene repertoire of Trypanosoma brucei gambiense are smaller than those of T. b. brucei

The amount of nuclear DNA of Trypanosoma brucei gambiense is only 70% of that of T. b. brucei. The difference is partially due to depletion of 50-150 kb mini-chromosomes in T. b. gambiense, as well as a reduction in the content of some repetitive DNA families. Quantitation of 'barren' DNA regions characteristic of the 5' environment of telomeric antigen genes confirms that the T. b. gambiense genome contains fewer chromosome ends, and thus most probably fewer telomeric antigen genes, than T. b. brucei. The extent of the antigen gene repertoire of the two subspecies has been estimated by hybridization with probes specific for the conserved 3' region of antigen genes. It appears that the repertoire of the gambiense subspecies is only about 50% of that of T. b. brucei. These observations are discussed with regard to the stability of the T. b. gambiense repertoire.

Enzyme polymorphism and the identity of Trypanosoma brucei gambiense

Thirty-two isolates from man in known areas of Gambian trypanosomiasis, in the Sudan, Kenya, Zaire, Nigeria, Ivory Coast, Burkina Faso, Liberia and Senegal, were examined by isoenzyme electrophoresis of 11 enzymes. Comparisons were also made with our previously published results on 23 other stocks of similar origins, which had been examined in the same manner. All those stocks of low initial virulence to laboratory rodents, which thus conform to the accepted view of the behaviour of Trypanosoma brucei gambiense can be identified by characteristic combinations of enzyme patterns, especially certain aminotransferase markers. A limited study of superoxide dismutase polymorphism suggested a further marker of value. The isolates of high initial virulence to rodents, which are thus behaviourally akin to T. b. rhodesiense, did not share these characteristics. We conclude that there exists a homogeneous group of trypanosomes of wide dispersion throughout tropical Africa, characterized by certain isoenzyme combinations and low initial virulence to rodents, which corresponds to the classical concept of T. b. gambiense. The features of limited antigenic repertoire, high resistance to normal human serum and restriction fragment length polymorphisms of the genes for certain variant surface glycoproteins also appear to be characteristic of this group.

Surface carbohydrates of procyclic forms of African trypanosomes studied using fluorescence activated cell sorter analysis and agglutination with lectins

Living culture form procyclics of Trypanosoma brucei brucei, T.b. rhodesiense, T.b. gambiense, T. congolense and T. simiae were tested for binding of eight different lectins. The binding of fluorescein isothiocyanate (FITC)-conjugated lectins was measured using a fluorescence activated cell sorter (FACS) and by agglutination with unlabelled lectins. Five of the lectins failed to bind to any of the procyclic organisms in both tests. All parasites bound concanavalin A (Con A) and all T.b. brucei, T.b. rhodesiense and T. congolense procyclics bound Ricinus communis agglutinin 120 (RCA) and wheat germ agglutinin (WGA). Trypanosoma b. gambiense procyclics failed to bind RCA and thus could be easily discriminated from other subspecies of T. brucei. Similarly, T. simiae did not bind WGA, unlike T. congolense, the other species of the genus Nannomonas. All positive reactions were inhibited by 0.2 M concentrations of the relevant sugars. The results indicate that all species and subspecies of the procyclic culture forms tested have surface-exposed structures resembling alpha-D-mannose moieties and that T.b. brucei, T.b. rhodesiense and T. congolense have surface-exposed molecules resembling D-galactose and N-acetyl D-glucosamine (or sialic acid) moieties. Molecules resembling D-galactose and N-acetyl D-glucosamine residues are absent or inaccessible in T.b. gambiense and T. simiae respectively. A group of T. congolense clones of parasite stocks isolated at Kilifi on the Kenyan coast showed quantitatively different binding of RCA when compared to the other T. congolense clones tested indicating that these organisms differ in surface carbohydrate structure.

Influence of source and quantity of protein on the development of immunity and resistance to African trypanosomiasis

Although it is well documented that severe protein deprivation inhibits the development of the immune response and exacerbates certain infections, little has been done to study the effects of native diets on endemic diseases or immunity. Therefore, protein-restricted diets were formulated for mice to mimic the sources and amounts measured in human diets of the Batouri region of Cameroon, endemic for African trypanosomiasis. Weanling C57BL/6 female mice were fed a diet that contained 73% of the recommended daily allowance (RDA) of protein. The sources of protein were all plant (cornmeal), all animal (casein), or a ratio that reflected the native diet (2.2 parts plant to 1 part animal protein). Diets were isocaloric on a weight basis, equal in lipids, and adequate in vitamins and minerals. Control mice were fed laboratory chow or two times the RDA of animal protein (casein). Mice fed only cornmeal or the native diets consumed as much food but did not gain as much weight as mice fed only animal protein, indicating the poorer quality of protein in their diets. Upon infection with Trypanosoma brucei gambiense, however, significantly higher numbers of these mice controlled the first peak of parasitemia and survived the infection as compared with mice fed the other three diets. Since all mice developed patent infections and the parasite growth rate was unaffected by diet, innate immune factors were ruled out as the cause for the higher level of resistance to the parasite. To determine whether diet affected the development of the immune system, weanling mice were maintained on diets for 30 days before immunization with sheep erythrocytes or trinitrophenylated Ficoll. Mice fed only plant protein or native diets elicited higher direct plaque-forming-cell responses to both the T-cell-dependent and T-cell-independent antigens. Since variant-specific immunity which controls levels of African trypanosomes in the blood is a T-cell-independent humoral immunoglobulin M response, this suggests that cornmeal, a protein of poor quality, was adequate for the development of humoral immunity and resistance to African trypanosomiasis while casein, an animal protein of high quality, was not. This provides more evidence that diet plays an important role in infection and immunity.

Size-fractionation of the small chromosomes of Trypanozoon and Nannomonas trypanosomes by pulsed field gradient gel electrophoresis

We have compared the molecular karyotypes of trypanosomes from different subgroups within subgenus Trypanozoon by pulsed field gradient (PFG) gel electrophoresis. Although the overall karyotype was similar, there was much variation in the size of chromosomes between different stocks. Two of three stocks of Trypanosoma (Trypanozoon) brucei gambiense had remarkably small mini-chromosomes: 25-50 kilobase pairs compared to 50-150 kilobase pairs for the mini-chromosomes of other Trypanozoon stocks. The relative amount of DNA in the mini-chromosomal fraction of different stocks correlated well with the amount of 177 base pair satellite DNA monomer per microgram nuclear DNA. Hybridisation of Southern blots of pulsed field gradient gels with a number of gene probes showed that the loci for tubulin and phosphoglycerate kinase in Trypanozoon probably lie on the same chromosome, together with some variant surface glycoprotein genes; the genes for triose phosphate isomerase and glyceraldehyde phosphate dehydrogenase are separately located both with respect to each other and the above housekeeping genes. Therefore, there are at minimum three pairs of chromosomes carrying housekeeping genes in Trypanozoon. In some stocks the chromosomes carrying the tubulin and phosphoglycerate kinase genes are split into two bands, suggesting that homologous chromosomes may differ substantially in size in trypanosomes. One Trypanosoma (Nannomonas) congolense stock examined had a similar pattern of chromosome distribution to that of Trypanozoon, but with very small mini-chromosomes (25-50 kilobase pairs.)

Comparison of African trypanosomes of different antigenic phenotypes, subspecies and life cycle stages by two-dimensional gel electrophoresis

High resolution two-dimensional polyacrylamide gel (2D gel) electrophoresis and autoradiography were used to analyze the protein gene products of African trypanosomes biosynthetically labelled with [35S]methionine. Using cloned populations of parasites it was found that: antigenically different bloodstream trypanosomes from the same serodeme differed only in their variant surface glycoproteins; Trypanosoma brucei, T.b. rhodesiense and T.b. gambiense subspecies could be distinguished on the basis of differences in expressed proteins; transformation from bloodstream trypomastigotes to procyclic epimastigote culture forms was accompanied by loss of variant surface glycoproteins and several other qualitative and quantitative changes in minor proteins. The results indicate that 2D gel analysis may allow improved classification of African trypanosomes (based on the observation of hundreds of protein markers) and may also provide a general technique for the identification of lifecycle stage specific markers.

Mice varying in resistance to African trypanosomiasis respond differently to treatments with variant surface glycoprotein

A comparative analysis of responses between resistant and susceptible hosts revealed that DBA/2 mice, after treatment with variant surface coat glycoprotein (VSG) from virulent or avirulent African trypanosomes, developed splenomegaly as the result of a near-doubling of the splenic cell population, had less polyclonal activation of B cells and were protected upon challenge with homologous trypanosomes. The susceptible C3H/Anf and C3H/HeJ mice on the other hand increased their splenic cell population by only 12%, had about twice the production of unelicited antibodies and were not immunized by the VSG treatments. This indicated that (a) proliferation of spleen cells during African trypanosomiasis may reflect an attempt to generate a specific and protective immune response and is not merely the result of polyclonal activation of lymphocytes; (b) production of unelicited antibodies is not merely a "bystander reaction" to the generation of antigen-specific responses; and (c) polyclonal antibody production in response to VSG is not linked to the LPS gene. Nonspecific immunosuppression as measured in mitogen assays was not elicited by VSG in either resistant or susceptible mice, indicating that polyclonal lymphocyte activation and nonspecific immunosuppression are unlinked phenomena. Mice injected with VSG from either virulent or avirulent isolates at levels normally encountered by hosts during severe, acute infection developed the same degree of splenomegaly and production of unelicited (polyclonal) antibodies. Therefore, any differences in polyclonal activation of lymphocytes measured between mice with acute vs. chronic African trypanosomiasis can be attributed to quantitative and not qualitative differences in VSG.

Trypanosoma brucei: immunogenicity of the variant surface coat glycoprotein of virulent and avirulent subspecies

Comparative analyses were made to define the immunogenic role in mice of the variant surface coat glycoprotein (VSG) of African trypanosomes. Less than 10 micrograms of the glycoprotein fixed to trypanosomes or covalently linked to sheep erythrocytes were 100 times more immunogenic than soluble VSG. Therefore, although VSG is present on the parasites and in the blood of infected hosts, the cell-bound form most likely elicits immunity. Intravenous administration of soluble or cell-bound VSG was a better route of immunization than the subcutaneous route. Therefore, although parasites grow at the site of infection, in tissue spaces, and in the blood, control of blood parasitemia is best developed if the antigen is introduced to the vascular bed. Full protection against homologous challenge occurred by 4 days and was maintained through 30 days. Trypanosome-agglutinating antibody titers could be measured at 3 days, peaked at 5 days, and remained high through 14 days after immunization. Therefore, mice immunized with an optimal dosage of VSG, 2 days before challenge, should have had ample time to elicit a protective response. Most of these mice, however, developed patent infections, and one-third died during the first peak of parasitemia at about the same time as untreated control mice. This indicates that active infection inhibits the early phases of induction of immunity. Mice, suboptimally immunized against and challenged with an avirulent isolate of Trypanosoma brucei gambiense, survived at higher rates than mice immunized and challenged with a virulent clone of T. b. rhodesiense. Cell-fixed and soluble VSG from both parasites elicited similar agglutinating-antibody titers, indicating that the two trypanosomes were equally antigenic. Results from neutralization tests, however, revealed that, per unit of immune mouse serum, 400 times more T. b. gambiense became noninfective than T. b. rhodesiense. Apparently, virulence is related to relative sensitivity of the trypanosomes to immunological assault.

Structure of the variant glycoproteins and surface coat of Trypanosoma brucei

The pathogenic African trypanosomes have a unique mechanism for antigenic variation. Each cell is covered by a surface coat consisting of about seven million essentially identical glycoprotein molecules drawn from a large repertoire of variants, each encoded by an individual gene. Amino acid sequence variation extends throughout the molecule but reduces from the amino terminus to the carboxy terminus, where certain features, especially the grouping of cysteine residues, are quite conserved. The range of diversity within the thousand or so variant glycoprotein genes that exist in each cell is large. New variants may arise instantaneously by segmental gene conversion. Variant surface glycoproteins are synthesized with amino terminal signal sequences and hydrophobic carboxy terminal tails. The tails are extraordinarily conserved. After synthesis, they are replaced by a complex glycolipid structure in which myristic (dodecanoic) acid serves to anchor the polypeptide to the surface membrane. Enzymic cleavage of myristic acid releases variant glycoproteins from the surface coat.

Sequences homologous to variant antigen mRNA spliced leader in Trypanosomatidae which do not undergo antigenic variation

Trypanosomes which parasitize mammals have evolved mechanisms to evade immune attack, such as the occupation of 'safe' intracellular sites (for example, Trypanosoma cruzi), or antigenic variation, exemplified by the salivarian trypanosomes (for example, Trypanosoma brucei). Antigenic variation is mediated by sequential expression of single variant surface glycoprotein (VSG) genes, and often involves transposition of the active gene. Every VSG transcript examined shares the same 5' terminal 35-nucleotide leader sequence. In T. brucei, this leader is encoded within a 1.4-kilobase unit tandemly reiterated to form a large array. It is hypothesized that this array is distantly linked to the expressed VSG gene and functions as a multiple promoter of VSG gene transcription, restricting transcription to that gene which, through genomic rearrangement, is placed downstream from the array. Leader and structural gene sequences are presumably juxtaposed by RNA splicing. Here we show that several trypanosomatids, both those which undergo antigenic variation (Trypanosoma congolense and Trypanosoma vivax) and those which do not (T. cruzi and Leptomonas collosoma), contain reiterated sequences homologous to the T. brucei spliced leader (SL). These results suggest that the SL, although utilized in VSG gene expression, is an ancestral sequence also used in the expression of other trypanosomatid genes.

Sequences homologous to the variant antigen mRNA spliced leader are located in tandem repeats and variable orphons in trypanosoma brucei

We have examined the organization of genomic sequences homologous to the spliced leader of Trypanosoma brucei variant surface glycoprotein (VSG) mRNA, using a synthetic oligodeoxynucleotide probe. These sequences are highly reiterated in the trypanosome genome and most are located in 1.4 kb units arranged in a direct tandem repeat. However, some of the 1.4 kb sequences are dispersed from the cluster(s) of tandem repeats and are flanked by non-repeat DNA. The number and arrangement of these leader sequence orphons varies among different T. brucei stocks. Within the IsTat serodeme, the arrangement of three of four spliced leader orphons observed with Eco RV digestion was stable during a chronic infection and cyclic transmission through the insect vector. The fourth Eco RV orphon, however, undergoes rearrangement during antigenic variation and life-cycle differentiation.

The use of culture-derived metacyclic trypanosomes in studies on the serological relationships of stocks of Trypanosoma brucei gambiense

Metacyclic trypanosomes of five stocks of Trypanosoma brucei gambiense were produced in vitro in tsetse head-salivary gland explant cultures and used to infect rabbits. Sera were collected from the rabbits and monitored by agglutination tests for antibody production to nine serotype antigens of T. b. gambiense. In the case of a Nigerian stock of T. b. gambiense the sequences of antibody production were found to be similar in animals infected with the stock transmitted by tsetse flies and from culture. Many similarities were also found between the patterns of antibody production in rabbits infected with stocks of T. b. gambiense from Senegal, Nigeria, Zaire and Uganda. The occurrence of similar serotypes in geographically different stocks of T. b. gambiense provides further support for continuing efforts to develop improved serodiagnostic tests for sleeping sickness based on variable trypanosome antigens and to find techniques for immunoprophylaxis.

The electrophoretic mobilities and activities of eleven enzymes of bloodstream and culture forms of Trypanosoma brucei compared

Eleven soluble enzymes in the supernatant of bloodstream Trypanosoma brucei were compared for electrophoretic mobility and activity with those of T. brucei cultures grown in 3 different media. All bands of each enzyme found in the bloodstream form were also present in the cultured material, but extra bands of malate dehydrogenase (MDH) (EC 1.1.1.37), aspartate aminotransferase (ASAT) (EC 2.6.1.1), and in 2 to 6 cultures of isocitrate dehydrogenase (ICD) (EC 1.1.1.42) were present in culture forms but not in bloodstream forms. An interfering enzyme, peculiar to cultured T. brucei, which reacted with 2-oxoglutarate and possibly a trace amount of ammonium ions, ran with the fast-moving ASAT bands. Threonine dehydrogenase activity, high in cultured trypanosomes irrespective of the medium used but low in bloodstream trypanosomes, was markedly lower in Trypanosoma evansi and a much passaged T. brucei 8/18. Glucosephosphate isomerase activity on the other hand was high in bloodstream and low in cultured trypanosomes. Glutamate dehydrogenase activity was too low to record reliably in bloodstream trypanosomes, but could be clearly detected in cultured forms. As the differences point to some changes in gene expression between the two forms, culture material is likely to replace trypanosomes from living animals for electrophoretic characterization only when considerable comparative work has been done.

Antigenic variation in trypanosomes: a computer analysis of variant order

African trypanosomes can undergo antigenic variation and evade the host immune response. Whether the antigenic variants arise in an ordered sequence or randomly has been in dispute but has not been statistically tested. The coefficient of concordance (W), a statistic designed to detect similarities between sequences of objects, was applied to the literature data. The tendency towards a reproducible order of variants was strong, although in several of the studies the number of experimental animals was so low that no conclusions could be drawn. A computer model was used to determine whether this degree of order could arise with random generation of variants followed by selection. The model simulated a trypanosome clone with 90 possible variants, widely differing variant-specific growth rates, random variant origin and variant eradication by an anamnestic host immune response. Parameters varied were maximum parasitaemia, growth rate differential between 'fast' and 'slow' variants, and parasitologist ability to detect minor variants. Random generation and selection by growth rate alone could not produce the degree of variant orderliness reported in the literature. However, experiments with larger numbers of host animals and direct investigation of variant growth rates and competitive interactions are necessary before the random generation-selection hypothesis can be proven or disproven.

A study of the antigenic relationships of isolates of Trypanosoma brucei from three areas in East Africa

Eleven stabilates of T. brucei, prepared from isolates collected from cattle and tsetse flies in three areas of East Africa, were compared serologically by direct agglutination tests, using (a) stabilate-specific antisera prepared in rats and (b)antisera to the predominant variant antigens of each isolate prepared in rabbits. The results indicated that groups of isolates from any one area tended to produce antigens in common, whereas isolates from different areas were generally antigenically distinct. The comparison of isolates of T. brucei on the basis of their predominant variant antigens, as a technique, was more simple to effect, and more sensitive in the detection of antigenic relationships, than comparison founded on basic strain antigens.

Enzyme electrophoresis in characterizing the causative organism of Gambian trypanosomiasis

Trypanosoma (Trypanozoon) brucei includes three morphologically identical subspecies which are poorly defined by clinical behaviour; T. b. brucei does not infect man, whereas T. b. rhodesiense causes an acute, and T. b gambiense a chronic, disease. Thirty-three isolates of the complex, each of which had previously been identified on clinical or other criteria, were compared by the electrophoretic patterns of two trypanosomal enzymes, alanine aminotransferase (ALAT) and aspartate aminotransferase (ASAT). One particular ALAT pattern clearly segregated a group of human pathogens of which all except one were labelled T. b. gambiense. The exception was labelled T. b. rhodesiense, and in addition three putative T. b. gambiense isolates did not have this pattern; it is suggested that only one presents a serious anomaly. The T. b. gambiense group could also be subdivided by three ASAT patterns which coincided with known groupings based on serological criteria.

A pattern in the development of agglutinogenic antigens of cyclically transmitted isolates of Trypanosoma gambiense

Three isolates of T. gambiense, belonging to 2 serologically different substrains of the species, were cyclically transmitted to rabbits and monkeys by G. morsitans or G. tachinoides. Sera collected from the infected animals were tested for agglutinating antibodies to 17 different serotype antigens prepared from rodent-adapted isolates of T. gambiense. The first antibodies detected in sera from all of the animals agglutinated the same serotype antigen L2, indicating that it possibly consisted of trypanosomes bearing a basic antigen of T. gambiense. In 2 animals infected with 1 isolate the development of antibody to the common serotype L2 was accompanied by the development of antibody to a second serotype U1. Comparisons of the patterns in which antibodies were produced to other serotype antigens in different animals indicated that there were many similarities in the sequences in which they developed in different rabbits infected with (a) any 1 isolate (b) 2 different isolates of 1 substrain and (c) isolates of 2 serologically different substrains. There were also similarities in the sequences in which the antigens developed in rabbits and monkeys infected with an isolate by the same fly or by different tsetse flies. The ordered sequence in which these 3 cyclically transmitted isolates of T. gambiense, of significantly varied origin and laboratory history, produced variant antigens in hosts of different species provides further evidence to support previous suggestions that antigenic variation in trypanosomes is an adaptive process, rather than one involving selection of mutants.