Characteristics of trypanosome variant antigen genes active in the tsetse fly

The History of Anti-Trypanosome Vaccine Development Shows That Highly Immunogenic and Exposed Pathogen-Derived Antigens Are Not Necessarily Good Target Candidates: Enolase and ISG75 as Examples

Salivarian trypanosomes comprise a group of extracellular anthroponotic and zoonotic parasites. The only sustainable method for global control of these infection is through vaccination of livestock animals. Despite multiple reports describing promising laboratory results, no single field-applicable solution has been successful so far. Conventionally, vaccine research focusses mostly on exposed immunogenic antigens, or the structural molecular knowledge of surface exposed invariant immunogens. Unfortunately, extracellular parasites (or parasites with extracellular life stages) have devised efficient defense systems against host antibody attacks, so they can deal with the mammalian humoral immune response. In the case of trypanosomes, it appears that these mechanisms have been perfected, leading to vaccine failure in natural hosts. Here, we provide two examples of potential vaccine candidates that, despite being immunogenic and accessible to the immune system, failed to induce a functionally protective memory response. First, trypanosomal enolase was tested as a vaccine candidate, as it was recently characterized as a highly conserved enzyme that is readily recognized during infection by the host antibody response. Secondly, we re-addressed a vaccine approach towards the Invariant Surface Glycoprotein ISG75, and showed that despite being highly immunogenic, trypanosomes can avoid anti-ISG75 mediated parasitemia control.

The Structure of a Conserved Telomeric Region Associated with Variant Antigen Loci in the Blood Parasite Trypanosoma congolense

African trypanosomiasis is a vector-borne disease of humans and livestock caused by African trypanosomes (Trypanosoma spp.). Survival in the vertebrate bloodstream depends on antigenic variation of Variant Surface Glycoproteins (VSGs) coating the parasite surface. In T. brucei, a model for antigenic variation, monoallelic VSG expression originates from dedicated VSG expression sites (VES). Trypanosoma brucei VES have a conserved structure consisting of a telomeric VSG locus downstream of unique, repeat sequences, and an independent promoter. Additional protein-coding sequences, known as “Expression Site Associated Genes (ESAGs)”, are also often present and are implicated in diverse, bloodstream-stage functions. Trypanosoma congolense is a related veterinary pathogen, also displaying VSG-mediated antigenic variation. A T. congolense VES has not been described, making it unclear if regulation of VSG expression is conserved between species. Here, we describe a conserved telomeric region associated with VSG loci from long-read DNA sequencing of two T. congolense strains, which consists of a distal repeat, conserved noncoding elements and other genes besides the VSG; although these are not orthologous to T. brucei ESAGs. Most conserved telomeric regions are associated with accessory minichromosomes, but the same structure may also be associated with megabase chromosomes. We propose that this region represents the T. congolense VES, and through comparison with T. brucei, we discuss the parallel evolution of antigenic switching mechanisms, and unique adaptation of the T. brucei VES for developmental regulation of bloodstream-stage genes. Hence, we provide a basis for understanding antigenic switching in T. congolense and the origins of the African trypanosome VES.

Control of malaria and other vector-borne protozoan diseases in the tropics: enduring challenges despite considerable progress and achievements

Vector-borne protozoan diseases represent a serious public health challenge, especially in the tropics where poverty together with vector-favorable climates are the aggravating factors. Each of the various strategies currently employed to face these scourges is seriously inadequate. Despite enormous efforts, vaccines-which represent the ideal weapon against these parasitic diseases-are yet to be sufficiently developed and implemented. Chemotherapy and vector control are therefore the sole effective attempts to minimize the disease burden. Nowadays, both strategies are also highly challenged by the phenomenon of drug and insecticide resistance, which affects virtually all interventions currently used. The recently growing support from international organizations and governments of some endemic countries is warmly welcome, and should be optimally exploited in the various approaches to drug and insecticide research and development to overcome the burden of these prevalent diseases, especially malaria, leishmaniasis, Human African Trypanosomiasis (HAT), and Chagas disease.

Telomere components as potential therapeutic targets for treating microbial pathogen infections

In a number of microbial pathogens that undergoes antigenic variation to evade the host's immune attack, genes encoding surface antigens are located at subtelomeric loci, and recent studies have revealed that telomere components play important roles in regulation of surface antigen expression in several of these pathogens, indicating that telomeres play critical roles in microbial pathogen virulence regulation. Importantly, although telomere protein components and their functions are largely conserved from protozoa to mammals, telomere protein homologs in microbial pathogens and humans have low sequence homology. Therefore, pathogen telomere components are potential drug targets for therapeutic approaches because first, most telomere proteins are essential for pathogens' survival, and second, disruption of pathogens' antigenic variation mechanism would facilitate host's immune system to clear the infection.

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.

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.

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.

A structural and transcription pattern for variant surface glycoprotein gene expression sites used in metacyclic stage Trypanosoma brucei

African trypanosomes first express the variant surface glycoprotein (VSG) at the metacyclic stage in the tsetse fly vector, in preparation for transfer into the mammal. Metacyclic (M)VSGs comprise a specific VSG repertoire subset and their expression is regulated differently from that of bloodstream VSGs, involving exclusively transcriptional regulation during the life cycle. To identify basic structural and functional features that may be common to MVSG telomeric transcription units, we have characterized the anatomy and transcription of the telomere containing the ILTat 1.61 MVSG gene. This telomere contains pseudogenes of the ESAG1 and ESAG9 families found in bloodstream VSG transcription units. The 1.61 MVSG occupies a monocistronic transcription unit and is transcriptionally controlled through the life cycle. The 1.61, and also the 1.22, MVSG transcription initiation site sequences resemble eukaryotic initiator elements. Sequence comparison reveals that four out of five characterized MVSG expression sites have a conserved region 2.0-4.7 kb long upstream of the MVSG. In some cases, this region contains not only the transcription initiation site that we have observed to be active in fly-transmitted trypanosomes but also, upstream, another sequence, described elsewhere as a 'putative promoter' for the MVAT set of M/VSGs (Nagoshi YL, Alarcon CM, Donelson JE. A monocistronic transcript for a trypanosome variant surface glycoprotein, Mol Biochem Parasitol 1995;72:33-45). In fly-transmitted trypanosomes, the latter element is transcriptionally silent. Our analysis of the structure of MVSG telomeres suggests that metacyclic expression sites arose from bloodstream expression sites.

Predominance of duplicative VSG gene conversion in antigenic variation in African trypanosomes

A number of mechanisms have been described by which African trypanosomes undergo the genetic switches that differentially activate their variant surface glycoprotein genes (VSGs) and bring about antigenic variation. These mechanisms have been observed mainly in trypanosome lines adapted, by rapid syringe passaging, to laboratory conditions. Such "monomorphic" lines, which routinely yield only the proliferative bloodstream form and do not develop through their life cycle, have VSG switch rates up to 4 or 5 orders of magnitude lower than those of nonadapted lines. We have proposed that nonadapted, or pleomorphic, trypanosomes normally have an active VSG switch mechanism, involving gene duplication, that is depressed, or from which a component is absent, in monomorphic lines. We have characterized 88 trypanosome clones from the first two relapse peaks of a single rabbit infection with pleomorphic trypanosomes and shown that they represent 11 different variable antigen types (VATs). The pattern of appearance in the first relapse peak was generally reproducible in three more rabbit infections. Nine of these VATs had activated VSGs by gene duplication, the tenth possibly also had done so, and only one had activated a VSG by the transcriptional switch mechanism that predominates in monomorphic lines. At least 10 of the donor genes have telomeric silent copies, and many reside on minichromosomes. It appears that trypanosome antigenic variation is dominated by one, relatively highly active, mechanism rather than by the plethora of pathways described before.

Trypanosome infections and survival in tsetse

The effect of trypanosome infection on vector survival was observed in a line of Glossina morsitans selected for susceptibility to trypanosome infection. The differential effects of midgut and salivary gland infections on survival were examined by exposing flies to infection with either Trypanosoma congolense which colonizes midgut and mouthparts or Trypanosoma brucei rhodesiense which colonizes midgut and salivary glands. A comparison of the survival distributions of uninfected flies with those exposed to infection showed that salivary gland infection significantly reduces tsetse survival; midgut infection had little or no effect on the survival of tsetse. The significance of these findings is discussed in relation to the vectorial capacity of wild flies.

Activity of a trypanosome metacyclic variant surface glycoprotein gene promoter is dependent upon life cycle stage and chromosomal context

African trypanosomes evade the mammalian host immune response by antigenic variation, the continual switching of their variant surface glycoprotein (VSG) coat. VSG is first expressed at the metacyclic stage in the tsetse fly as a preadaptation to life in the mammalian bloodstream. In the metacyclic stage, a specific subset (<28; 1 to 2%) of VSG genes, located at the telomeres of the largest trypanosome chromosomes, are activated by a system very different from that used for bloodstream VSG genes. Previously we showed that a metacyclic VSG (M-VSG) gene promoter was subject to life cycle stage-specific control of transcription initiation, a situation unique in Kinetoplastida, where all other genes are regulated, at least partly, posttranscriptionally (S. V. Graham and J. D. Barry, Mol. Cell. Biol. 15:5945-5956, 1985). However, while nuclear run-on analysis had shown that the ILTat 1.22 M-VSG gene promoter was transcriptionally silent in bloodstream trypanosomes, it was highly active when tested in bloodstream-form transient transfection. Reasoning that chromosomal context may contribute to repression of M-VSG gene expression, here we have integrated the 1.22 promoter, linked to a chloramphenicol acetyltransferase (CAT) reporter gene, back into its endogenous telomere or into a chromosomal internal position, the nontranscribed spacer region of ribosomal DNA, in both bloodstream and procyclic trypanosomes. Northern blot analysis and CAT activity assays show that in the bloodstream, the promoter is transcriptionally inactive at the telomere but highly active at the chromosome-internal position. In contrast, it is inactive in both locations in procyclic trypanosomes. Both promoter sequence and chromosomal location are implicated in life cycle stage-specific transcriptional regulation of M-VSG gene expression.

Transcriptional regulation of metacyclic variant surface glycoprotein gene expression during the life cycle of Trypanosoma brucei

In antigenic variation in African trypanosomes, switching of the variant surface glycoprotein (VSG) allows evasion of the mammalian host immune response. Trypanosomes first express the VSG in the tsetse fly vector, at the metacyclic stage, in preparation for transfer into the mammal. In this life cycle stage, a small, specific subset (1 to 2%) of VSGs are activated, and we have shown previously that the system of activation and expression of metacyclic VSG (M-VSG) genes is very different from that used for bloodstream VSG genes (S.V. Graham, K.R. Matthews, P.G. Shiels, and J.D. Barry, Parasitology 101:361-367, 1990). Now we show that unlike other trypanosome genes including bloodstream VSG genes, M-VSG genes are expressed from promoters subject to exclusively transcriptional regulation in a life cycle stage-dependent manner. We have located an M-VSG gene promoter, and we demonstrate that it is specifically up-regulated at the metacyclic stage. This is the first demonstration of gene expression being regulated entirely at the level of transcription among the Kinetoplastida; all other protein-coding genes examined in these organisms are, at least partly, under posttranscriptional control. The distinctive mode of expression of M-VSG genes may be due to a stochastic mechanism for metacyclic VSG activation.

The kinetics of maturation of trypanosome infections in tsetse

Estimates of the time delay between the infective bloodmeal and maturation (incubation or maturation time) for 4 trypanosome stocks (2 Trypanozoon and 2 Trypanosoma congolense) show that maturation time in tsetse is not a parasite species-specific constant. The mean incubation time of a Trypanosoma brucei rhodesiense stock (EATRO 2340 - 18 days) was not significantly different from one T. congolense stock (SIKUDA88 - 15.5 days) but was significantly greater than another (1/148 FLY9 - 12.5 days). There was no significant difference in incubation times between male and female Glossina morsitans morsitans for any of the stocks but in both of the Trypanozoon stocks the proportion of female flies producing mature infections was significantly less than in males. However, estimates of gene frequency, assuming a model in which maturation is controlled by an X-linked recessive allele, gave inconsistent results indicating that maturation cannot be controlled by a single sex-linked gene. Maturation was shown to be a tsetse sex-dependent phenomenon in Trypanozoon but not in T. congolense infections. Incubation time was quite variable even for a single trypanosome stock (e.g., standard deviation of 5 days for one Trypanozoon stock); we discuss how this variability can affect disease transmission, and the interpretation of age-prevalence data.

Midgut lectin activity and sugar specificity in teneral and fed tsetse

Midgut infection rates of Trypanosoma congolense in Glossina palpalis palpalis and of Trypanosoma brucei rhodesiense in Glossina pallidipes are potentiated by the addition of D+ glucosamine to the infective feed, but not to the levels of super-infection reported for G.m.morsitans, G.p.palpalis and G.pallidipes are shown to possess two trypanocidal molecules: a glucosyl lectin which can be inhibited by D+ glucosamine and a galactosyl molecule inhibited by D+ galactose. Addition of both D+ glucosamine and D+ galactose to the teneral infective feed promotes super-infection of the midguts of G.p.palpalis. The glucosyl lectin is specific for rabbit erythrocytes and is present in guts of fed G.m.morsitans and G.p.palpalis, titres of lectin activity do not increase substantially after the second bloodmeal. The galactosyl specific molecule does not show any erythrocyte specificity, although haemolytic activity is observed only in G.p.palpalis and not in G.m.morsitans. The presence of two trypanocidal molecules in some species of tsetse may account for the innate refractoriness of these flies to trypanosome infection. As D+ glucosamine also inhibits the killing of procyclic trypanosomes taken as an infective feed, it is suggested that the midgut lectin is normally responsible for the agglutination of trypanosomes in the fly midgut by binding to the procyclic surface coat, prior to establishment in the ecto-peritrophic space.

Rickettsia-like organisms and chitinase production in relation to transmission of trypanosomes by tsetse flies

Rickettsia-like organisms (RLO) from testse midguts and mosquito cell cultures showed high levels of endochitinase activity. A line of Glossina morsitans morsitans highly susceptible to midgut trypanosome infection and with high incidence of RLO infection showed significantly greater chitinolytic activity than G. austeni which had low RLO incidence and were correspondingly refractory to midgut infection. Midgut infection rates of Trypanosoma brucei rhodesiense in G. m. morsitans showed a dose-related increase when flies were fed N-acetyl-D-glucosamine (GlcNAc) in the infective meal and for 4 subsequent days. A model is proposed for susceptibility to trypanosome infection based on the generation of GlcNAc by RLO endochitinase activity in tsetse pupae inhibiting midgut lectin in teneral flies.

The nature of the teneral state in Glossina and its role in the acquisition of trypanosome infection in tsetse

Teneral Glossina morsitans morsitans from outbred and susceptible stocks infected with Trypanosoma (Nannomonas) congolense developed, respectively, three and six times higher midgut infection rates than flies of the same stock which had previously taken a bloodmeal. Non-teneral G. m. morsitans remained relatively refractory to infection when infected at subsequent feeds. Differences in susceptibility to midgut infection between teneral flies from susceptible and outbred lines of G. m. morsitans disappeared in non-teneral flies, showing that maternally inherited susceptibility to midgut infection is a phenomenon restricted to the teneral state of the fly. Laboratory reared G. m. morsitans were found to have become significantly more susceptible to trypanosome infection than wild flies from the population from which the colony was derived. The likely role of rickettsia-like organisms (RLO) in potentiating teneral susceptibility to midgut infection is discussed. The addition of the specific midgut lectin inhibitor D-glucosamine to the infective feed of non-teneral flies increased midgut infection rates to levels comparable with those achieved in teneral flies. It is concluded that the peritrophic membrane does not act as a barrier preventing non-teneral flies becoming infected. The relative refractoriness of non-teneral flies suggests that they do not play a significant part in the epidemiology of Trypanozoon or T. congolense infections.

Metacyclic form-specific variable surface glycoprotein-encoding genes of Trypanosoma (Nannomonas) congolense

A complementary DNA expression library in phage lambda gt11 was synthesized using mRNA from in vitro-produced metacyclic forms of a clone of Trypanosoma (Nannomonas) congolense. The unamplified library was screened with antiserum from a goat immune to infection with metacyclic (m)-forms of T. congolense ILRAD Nannomonas antigen repertoire 2(ILNaR2). Of the 100 antiserum-reactive phage clones identified, 22 were analyzed further: 21 of the clones contained overlapping portions of a single transcript, while one other contained a different transcript. Northern blot analyses indicated that the sequences contained in the clones were transcribed only by m-forms of ILNaR2. Immunological and sequence analyses indicated that the two different cloned sequences encode m-form-specific variable surface glycoproteins.

Rickettsia-like organisms, puparial temperature and susceptibility to trypanosome infection in Glossina morsitans

Maintaining the puparial stage of successive generations of a population of tsetse 3 degrees C lower than normal reduced the numbers of rickettsia-like organisms (RLO) carried by emerging flies. The susceptibility of these flies to midgut infection with Trypanosoma congolense was also significantly reduced compared with control flies held at normal temperature. These results support the view that the relationship between RLO and susceptibility is quantitative-teneral flies with heavier RLO infections being more susceptible to trypanosome infection.

Duplicative activation mechanisms of two trypanosome telomeric VSG genes with structurally simple 5' flanks

In the mammalian bloodstream, African trypanosomes express variant surface glycoprotein (VSG) genes from a family of long and complex telomeric expression sites. VSG switching generally occurs by the duplication of different VSG genes into these sites by gene conversion involving a series of 70 base pair (70bp) repeats in the 5' flank. In contrast, when VSG is first synthesised by trypanosomes in the tsetse fly at the metacyclic stage, a separate set of telomeric expression sites is activated. These latter telomeres appear not to act as recipients in gene conversion. We have found that the structure of two such expression sites is simple, with very short 70bp repeat regions and very little other sequence in common with bloodstream expression sites. However, the two telomeres readily act as donors in VSG gene conversion in the bloodstream and we show for one a consistent association of the conversion 5' end point with the short 70bp repeat region. These findings help explain why a very predictable set of VSGs is expressed in the tsetse fly and have implications for VSG gene conversion mechanisms.

Distinct, developmental stage-specific activation mechanisms of trypanosome VSG genes

The metacyclic form of African trypanosomes is the first to express genes for the Variant Surface Glycoprotein (VSG) and it uses an unusually predictable subset of the VSG gene repertoire. We have developed a model system for the analysis of metacyclic VSG (M-VSG) gene expression and have used this to demonstrate that, for two M-VSG genes, different modes of expression operate in the insect and mammalian phases of the life-cycle. In metacyclic-derived clones, these genes are expressed in situ, whereas they are routinely activated by duplication in bloodstream trypanosomes. The expression loci for both M-VSG genes studied are structurally simple and we present a model, based on this, for the maintenance of a separate M-VSG repertoire and expression system.

Salivary gland infection: a sex-linked recessive character in tsetse?

Male tsetse, when infected in the laboratory with trypanosomes of the subgenus Trypanozoon, usually produce greater salivary gland infection rates than females of the same species. We show that a single sex-linked gene model can be fitted to most recently published data for salivary gland infection rates in tsetse. The maturation of Trypanosoma congolense infections is shown to be independent of fly sex. The possible effects of genetic control of maturation of Trypanozoon infections in tsetse populations on the transmission of sleeping sickness are considered.

Hybridisation with a repetitive DNA probe reveals the presence of small chromosomes in Trypanosoma vivax

We have isolated and cloned a tandemly repeating element from trypanosoma vivax for use as a species-specific DNA probe. The repeat hybridises only with DNA from T. vivax, not with DNA from other Salivarian trypanosome species (T. brucei spp., T. congolense, T. simiae). The monomer of the repeat is approximately 180 bp long and is 64% GC rich. Hybridisation of the cloned fragment with size-fractionated large DNA molecules of 3 T. vivax stocks revealed a band in the position expected for minichromosomes, although these were believed absent in T. vivax. This band migrated to the 100-250 kb area of the gel at 4 different pulse frequencies and also hybridised with a telomeric repeat probe from T. brucei. The band is unlikely to be simply degraded material, since it failed to hybridise with another highly repetitive sequence from T. vivax and was consistently present in different trypanosome preparations. We conclude that T. vivax does possess mini-chromosomes, although possibly only 1 or 2 per cell.

Trypanosome sociology and antigenic variation

Survival of the trypanosome (Trypanosoma brucei) population in the mammalian body depends upon paced stimulation of the host's humoral immune response by different antigenic variants and serial sacrifice of the dominant variant (homotype) so that minority variants (heterotypes) can continue the infection and each become a homotype in its turn. New variants are generated by a spontaneous switch in gene expression so that the trypanosome puts on a surface coat of a glycoprotein differing in antigenic specificity from its predecessor. Homotypes appear in a characteristic order for a given trypanosome clone but what determines this order and the pacing of homotype generation so that the trypanosome does not quickly exhaust its repertoire of variable antigens, is not clear. The tendency of some genes to be expressed more frequently than others may reflect the location within the genome and mode of expression of the genes concerned and may influence homotype succession. Differences in the doubling time of different variants or in the rate at which trypanosomes belonging to a particular variant differentiate into non-dividing (vector infective) stumpy forms have also been invoked to explain how a heterotype's growth characteristics may determine when it becomes a homotype. Recent estimations of the frequency of variable antigen switching in trypanosome populations after transmission through the tsetse fly vector, however, suggest a much higher figure (0.97-2.2 x 10(-3) switches per cell per generation) than that obtained for syringe-passed infections (10(-5)-10(-7) switches per cell per generation) and it seems probable that most of the variable antigen genes are expressed as minority variable antigen types very early in the infection. Instability of expression is a feature of trypanosome clones derived from infective tsetse salivary gland (metacyclic) trypanosomes and it is suggested that high switching rates in tsetse-transmitted infections may delay the growth of certain variants to homotype status until later in the infection.

Characterization of genes coding for two major metacyclic surface antigens in Trypanosoma brucei

In African trypanosomes, only a very small fraction of the total repertoire of variable antigen types (VATs) is expressed by the metacyclic form. In Trypanosoma brucei stock EATRO 1125, the VATs AnTat 1.30 and 1.45 are reproducibly present in about 15% and 4% of the metacyclic population, respectively. The genes encoding the corresponding antigens or variant surface glycoproteins (VSGs) are in telomeres of large chromosomes, as are some non-metacyclic VSG genes from the same stock. Their activation mechanism has been studied in seven independent clones, 3 of which, referred to as 'first wave' metacyclic VATs (M-VATs), have been cloned from the first wave of parasitemia after cyclic transmission. In all these clones, activation of the antigen gene was linked to the transposition of an expression linked copy (ELC) of the gene to a telomeric expression site. For first wave M-VATs, this site seems variable, although restricted to large chromosomes, and it can be re-used for VSG gene expression in the bloodstream form. In 'late bloodstream' M-VATs, isolated from established chronic infections, the active expression site, at the end of a 200 kb chromosome, is the one preferred for the expression of late antigen types. It can be concluded that no characteristic feature in the genomic location and expression mechanism can distinguish metacyclic antigen genes from those expressed in the bloodstream forms, although the control of their expression must clearly be different.

Metacyclic variant surface glycoprotein genes of Trypanosoma brucei subsp. rhodesiense are activated in situ, and their expression is transcriptionally regulated

During the metacyclic stage in the life cycle of Trypanosoma brucei subsp. rhodesiense, the expression of variant surface glycoproteins (VSGs) is restricted to a small subset of antigenic types. Previously we identified cDNAs for the VSGs expressed in metacyclic variant antigen types (MVATs) 4 and 7 and found that these VSG genes do not rearrange when expressed at the metacyclic stage (M. J. Lenardo, A. C. Rice-Ficht, G. Kelly, K. Esser, and J. E. Donelson, Proc. Nathl. Acad Sci. USA 81:6642-6646, 1984). We now provide further evidence that these genes do not rearrange and demonstrate that their 5' upstream regions lack the 72 to 76-base-pair repeats which are considered the substrate for duplication and transposition events. Pulsed field gradient electrophoresis showed that the MVAT VSG genes were located on the largest chromosome-sized DNA molecules, and the lack of the MVAT 4 gene in one of two different serodemes suggested that one mechanism for the evolution of MVAT repertoires is gene deletion. When MVATs were inoculated into the bloodstream of a mammalian host by a bite from the insect vector, they rapidly switched into nonmetacyclic VSG types. We found that this switch was accomplished by a loss of MVAT RNA concomitant with the loss of metacyclic VSGs. Transcription studies with isolated metacyclic nuclei showed that the MVAT genes were expressed in situ from a single locus and were regulated at the level of transcription.

Metacyclic variant surface glycoprotein genes of Trypanosoma brucei subsp. rhodesiense are activated in situ, and their expression is transcriptionally regulated

During the metacyclic stage in the life cycle of Trypanosoma brucei subsp. rhodesiense, the expression of variant surface glycoproteins (VSGs) is restricted to a small subset of antigenic types. Previously we identified cDNAs for the VSGs expressed in metacyclic variant antigen types (MVATs) 4 and 7 and found that these VSG genes do not rearrange when expressed at the metacyclic stage (M. J. Lenardo, A. C. Rice-Ficht, G. Kelly, K. Esser, and J. E. Donelson, Proc. Nathl. Acad Sci. USA 81:6642-6646, 1984). We now provide further evidence that these genes do not rearrange and demonstrate that their 5' upstream regions lack the 72 to 76-base-pair repeats which are considered the substrate for duplication and transposition events. Pulsed field gradient electrophoresis showed that the MVAT VSG genes were located on the largest chromosome-sized DNA molecules, and the lack of the MVAT 4 gene in one of two different serodemes suggested that one mechanism for the evolution of MVAT repertoires is gene deletion. When MVATs were inoculated into the bloodstream of a mammalian host by a bite from the insect vector, they rapidly switched into nonmetacyclic VSG types. We found that this switch was accomplished by a loss of MVAT RNA concomitant with the loss of metacyclic VSGs. Transcription studies with isolated metacyclic nuclei showed that the MVAT genes were expressed in situ from a single locus and were regulated at the level of transcription.

Independent expression of the metacyclic and bloodstream variable antigen repertoires of Trypanosoma brucei rhodesiense

The variable antigen repertoire expressed by metacyclic Trypanosoma brucei rhodesiense is not influenced by the anamnestic expression whereby the variable antigen type (VAT) ingested by a tsetse fly is present at high levels in early bloodstream populations of fly-infected mice. This has been demonstrated by feeding to Glossina morsitans a trypanosome line expressing a VAT which is normally a component of the metacyclic repertoire. The VAT did not constitute a significantly increased proportion of the resultant metacyclic population which would have occurred had anamnestic expression and metacyclic expression been linked. Five other metacyclic VATs were also present at control levels. We conclude that the mechanisms of expression of VATs in the fly and in the mammal are independently controlled.