Antigenic variation in the salivarian trypanosomes

Protective effect of humus extract against Trypanosoma brucei infection in mice

Humic substances are formed during the decomposition of organic matter in humus, and are found in many natural environments in which organic materials and microorganisms are present. Oral administration of humus extract to mice successfully induced effective protection against experimental challenge by the two subspecies, Trypanosoma brucei brucei and T. brucei gambiense. Mortality was most reduced among mice who received a 3% humus extract for 21 days in drinking water ad libitum. Spleen cells from humus-administered mice exhibited significant non-specific cytotoxic activity against L1210 mouse leukemia target cells. Also, spleen cells produced significantly higher amounts of Interferon-gamma when stimulated in vitro with Concanavalin A than cells from normal controls. These results clearly show that administration to mice of humus extract induced effective resistance against Trypanosoma infection. Enhancement of the innate immune system may be involved in host defense against trypanosomiasis.

Protective effect of antiganglioside antibodies against experimental Trypanosoma brucei infection in mice

Liposome-associated ganglioside antigens (ganglioside GM1 or bovine brain gangliosides) were prepared to facilitate the potential protective efficacy for Trypanosoma brucei. Mice were immunized with liposome-associated ganglioside GM1 or bovine brain gangliosides intraperitoneally (i.p.). After immunization, significantly higher antigen-specific IgG and IgM antibodies were detected in sera than in the nonimmunized control group. When sera from immunized mice were analyzed for isotype distribution, antigen-specific IgG1, IgG2a, and IgG3 antibody responses were also noted. After immunization, mice were challenged i.p. with 1 x 10(2) cells of T. brucei. Sixty percentage of liposome-associated ganglioside GM1-immunized mice survived the infection, and all the mice immunized with bovine brain gangliosides-containing liposomes survived. However, all control mice died within 7 days after infection. These data demonstrate that liposomes containing ganglioside antigens have the potential usefulness for the induction of a protective immune response against T. brucei infection and suggest the possibility of developing vaccines that may ultimately be used for the prevention of trypanosomiasis.

Isolation and characterization of gangliosides from Trypanosoma brucei

Gangliosides were isolated from Trypanosoma brucei and analyzed by thin-layer chromatography (TLC) and TLC immunostaining test. Four species of gangliosides, designated as G-1, G-2, G-3, and G-4, were separated by TLC. G-1 ganglioside had the same TLC migration rate as GM3. In contrast, G-2, G-3, and G-4 gangliosides migrated a little slower than GM1, GD1a, and GD1b, respectively. To characterize the molecular species of gangliosides from T. brucei, G-1, G-2, G-3, and G-4 gangliosides were purified and analyzed by TLC immunostaining test with monoclonal antibodies against gangliosides. G-1 ganglioside showed the reactivity to the monoclonal antibody against ganglioside GM3. G-2 was recognized by the anti-GM1 monoclonal antibody. G-3 showed reaction with the monoclonal antibody to GD1a. G-4 had the reactivity to anti-GD1b monoclonal antibody. Using 4 kinds of monoclonal antibodies, we also studied the expression of GM3, GM1, GD1a, and GD1b in T. brucei parasites. GM3, GM1, GD1a, and GD1b were detected on the cell surface of T. brucei. These results suggest that G-1, G-2, G-3, and G-4 gangliosides are GM3 (NeuAc alpha2-3Gal beta1-4Glc beta1-1Cer), GM1 (Gal beta1-3GalNAc beta1-4[NeuAc alpha2-3]Gal beta1-4Glc beta1-1Cer), GD1a (NeuAc alpha2-3Gal beta1-3GalNAc beta1-4[NeuAc alpha2-3]Gal beta1-4Glc beta1-1Cer), and GD1b (Gal beta1-3GalNAc beta1-4[NeuAc alpha2-8NeuAc alpha2-3]Gal beta1-4Glc beta1-1Cer), respectively, and also that they are expressed on the cell surface of T. brucei.

Analysis of trypanosomal endocytic organelles using preparative free-flow electrophoresis

In this paper we demonstrate the power of preparative free-flow electrophoresis (FFE) for the study of endocytosis by African trypanosomes. Endocytosis of extracellular macromolecules by these parasites occurs through a specialized region of the parasite called the flagella pocket. The uptake of fluid phase markers such as horseradish peroxidase (HRP) into the various compartments of the endocytic pathway of bloodstream forms of Trypanosoma brucei brucei was manipulated by regulating the external environment (e.g., by altering the temperature of incubation). The various subcellular compartments were then separated by free-flow electrophoresis (FFE) or isopycnic density gradient centrifugation and analyzed for marker uptake. At low temperatures, HRP was found predominantly in the flagellar pocket. Increasing the temperature resulted in a time-dependent uptake of HRP into more positively charged endosomal fractions. However, little HRP activity was detected in lysosomal compartments, suggesting that either HRP had not yet entered the lysosome or was degraded immediately upon entry. Through the use of FFE we were able to identify and analyze compartments of the endosomal pathway that were not possible to identify by density gradient centrifugation alone. Although the differences in FFE separation of the endocytic compartments as seen in HRP uptake were striking, the minor changes seen within the lysosomal system were more subtle, as depicted in the protease profiles. In conclusion, we show that preparative FFE is a powerful technique for the analysis and separation of flagellar pocket-derived membranes from other endosomal and lysosomal compartments of African trypanosomes.

A review on vaccination against protozoa and arthropods of veterinary importance

The recent advances in immunology and biotechnology have stimulated much research on the control of parasitic diseases through vaccination. This is a review of the state of the art regarding important protozoan and arthropod veterinary parasites. A live oocyst vaccine for avian coccidiosis is still in use but much work has been done on the identification, cloning, and assay of protective antigens. The sporozoites of Eimeria tenella have been the preferred subject and at least four recombinant antigens have already been tested with partial success. Premunization against babesiosis is still widely used in Latin America as is a live vaccine with attenuated parasites in Australia. At least three Babesia bovis and three Babesia bigemina antigens that generate partial protection have been produced as recombinant proteins. A vaccine against canine babesiosis is being commercialized in France. Infection-treatment is still used to vaccinate against Theileria parva and a schizont vaccine against Theileria annulata. Recombinant sporozoite antigens have been assayed with partial success against both species but the identification and administration of protective schizont antigens, regarded as the most important, still requires considerable work. The immunological control of African trypanosomoses is still impaired by the antigenic variation that the parasites experience during the infection. Although some possibilities exist, most specialists are pessimistic about the promise of developing a vaccine in the near future. Control of Boophilus ticks with an occult tick intestine recombinant antigen seems to have potential in inhibiting reproduction of the tick but salivary antigens appear to be more effective at inhibiting feeding and pathogen transmission. Vaccination with a Hypoderma protein, recently cloned, has induced 90% protection against subsequent infestations. It is very likely that effective vaccines against veterinary parasites will become available in the near future.

Antibody patterns in rabbits showing different levels of susceptibility to an experimental Trypanosoma evansi infection

Using SDS-PAGE and Western blotting, the antibody spectrum of rabbits infected with Trypanosoma evansi to homologous T. evansi antigens was monitored. Animals that developed parasitaemia later or had lower levels of parasitaemia as the infection progressed were considered to have a degree of resistance to the infection. Sera of these resistant animals recognised the T. evansi antigens earlier and subsequently identified more antigens than their susceptible counterparts. The susceptible animals developed patent parasitaemia earlier and had higher parasite counts as the infection progressed, and their sera recognised T. evansi antigens later with fewer parasite components labelled during the course of the infection. These observations demonstrate clear differences between animals in response to T. evansi infections. Selection of T. evansi-tolerant animals on an individual basis may be possible as has been suggested for other trypanosome species.

Parasite-specific antibody responses of ruminants infected with Trypanosoma vivax

Sera from goats and cattle that were infected with two Trypanosoma vivax clones (ILDat 1.2 and ILDat 2.1) derived from different stocks were analysed for antibody activity against the variable surface glycoproteins (VSGs) of the infecting clones by enzyme-linked immune assays (ELISA) and immune lysis. To obtain purified VSG, lysed trypanosomes were separated on dodecyl sulphate-polyacrylamide gels. The gels were copper stained and the VSG protein band was excised from the gel. After destaining, the proteins were electroeluted from the gel slices and used as antigens in ELISA. High titres of IgM and IgG1 antibodies and lytic antibodies against the VSG of the infecting clone were detected. The IgG1 response appeared about 4 days later than the IgM response. IgG2 antibodies were only detected in goats and cattle that were infected with ILDat 1.2. Two goats and two calves that were infected with ILDat 1.2 showed recurrent peaks in lytic activity and of IgM and IgG1 antibody activity to the VSG of the infecting variable antigenic type (VAT). Two goats that were infected with ILDat 2.1 showed a similar pattern, but in two other goats there was a recurrent peak only in the IgM class. Recurrent peaks of antibody activity to the VSG of ILDat 1.2 and ILDat 2.1 were not detected in the sera of goats that had been inoculated with irradiated trypanosomes or that had been infected with an unrelated T. vivax clone. The recurrence of antibody peaks against the VSG of infecting VATs suggests that trypanosomes with completely or partially identical surface determinants reappear during T. vivax infection of ruminants.

High frequency of antigenic variation in Trypanosoma brucei rhodesiense infections

Rates at which Trypanosoma brucei rhodesiense trypanosomes switch from expression of one variable antigen type (VAT) to that of another have been determined in cloned populations that have been recently tsetse-fly transmitted. Switching rates have been determined between several, specific pairs of VATs in each population. High rates of switching were observed in 2 cloned trypanosome lines, each derived from a separate cyclical transmission of the same parental stock and each expressing a different major VAT. Five estimates of the switching rate between one particular pair of VATs were consistently high (approximately 1 x 10(-3) switches/cell/generation). These high switching rates were similar both in bloodstream populations of mice and in populations confined to subcutaneously implanted growth chambers in mice, thus indicating that the interaction of the bloodstream population with other trypanosome populations in the lymphatics or extravascular sites in systemic infections did not influence the estimates of the rate of switching. Fourteen estimates were made of VAT-specific switching rates in bloodstream infections involving 8 combinations from among 6 VATs. Switching rate estimates were VAT-specific and showed considerable variation between different combinations of VATs--from 1.9 x 10(-6) to 6.9 x 10(-3) switches/cell/generation. The rates of switching to different metacyclic-VATs were, however, very similar. Summation of between 3 and 5 VAT-specific switching rate values in each of 4 experiments conducted in bloodstream infections has provided minimum estimates of the overall rate of antigenic variation: 2.0-9.3 x 10(-3) switches/cell/generation. These values are between 20 and 66,000-fold higher than previously published estimates.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Attempts to protect goats against challenge with Trypanosoma vivax by initiation of primary infections with large numbers of metacyclic trypanosomes

Attempts were made to immunize goats by infection with large numbers of metacyclic trypanosomes of a clone of Trypanosoma vivax, followed by chemotherapy. Five groups of 6 goats each were infected intradermally with 5 different doses of cultured metacyclics of T. vivax, ranging from 10(2) to 10(6) trypanosomes/goat. Four weeks after infection, the goats were treated with 10 mg/kg diminazene aceturate (Berenil, Hoechst A.G.). Three weeks after treatment, 3 goats in each group were challenged intradermally with 10(4) homologous metacyclics derived from culture. The remaining 3 goats in each group were challenged by 20 tsetse infected with the homologous clone. Five out of 30 goats were resistant to homologous challenge; 4 of the goats that had been challenged with culture parasites, and 1 that had been challenged by tsetse. In each group 1 goat was protected. Protection was therefore not apparently influenced by the number of trypanosomes used to establish the primary infection. In another experiment, 6 goats were each infected by feeding 100 tsetse on the goats for 15 consecutive days. Three weeks after infection the goats were treated with Berenil and 3 weeks later challenged by 20 tsetse infected with the homologous clone. Three out of the 6 goats resisted challenge. The susceptible goats in both experiments, however, showed a reduction in the peak of parasitaemia following challenge compared with both challenge controls and the initial infections. Lytic antibodies to cultured metacyclics of T. vivax were detected in goats that resisted challenge after a primary infection with cultured metacyclics, and in resistant and susceptible goats after a primary infection by tsetse. All infected goats produced lytic antibodies to live bloodstream forms, as well as antibodies to bloodstream form lysates (demonstrated by ELISA). It is suggested that the immunity that had been induced in some of the experimental animals is due to antibody responses to both metacyclic and bloodstream variable antigen types (VATs) expressed during infection.

Characterization of Trypanosoma congolense serodemes in stocks isolated from cattle introduced onto a ranch in Kilifi, Kenya

A herd of 20 cattle was introduced on a ranch in Kilifi, Coast Province of Kenya, where they were in contact with Glossina austeni for 6 months. In total, 65 trypanosome isolates were made from these animals. Examination of the isolates revealed that 61 were Trypanosoma congolense and 4 were T. theileri. Out of the 61 T. congolense isolates, 55 were successfully passaged and cloned in mice to provide trypanosome populations for further analyses. The stocks and their clones were inoculated into goats on which teneral G. morsitans centralis were later fed in order to provide metacyclics for use in serodeme analysis. Identification of serodemes was carried out by indirect immunofluorescence and neutralization using antimetacyclic hyperimmune sera prepared in mice against metacyclics of cloned trypanosome populations. So far 4 serodemes have been identified in 8 stocks and 7 clones. Each of the 9 stocks contained a mixture of at least 2 of the 4 serodemes identified. Furthermore, stocks isolated sequentially from individual animals contained the same serodemes despite repeated treatment with a curative dose (6 mg/kg body weight) of Berenil between isolations. From the latter finding, it can be inferred that the 4 serodemes were present on the ranch throughout the study period.

Trypanosoma vivax: adaptation of two East African stocks to laboratory rodents

Two Trypanosoma vivax stocks from East Africa have been adapted to rats and mice. Adaptation was induced by rapid passage at two- to four-day intervals in sublethally irradiated rats. After 200 such passages, the two stocks gave rise to parasitemias of 10(9)-10(10) trypanosomes/ml in peripheral blood, and the infection was fatal in 90% of the rats. By passaging the rat-adapted T. vivax into normal mice at two- to three-day intervals for over 200 passages, the two stocks also became pathogenic to mice. One of the stocks was also capable of maintenance in non-irradiated rats. The two stocks displayed a marked degree of pleomorphism in irradiated and non-irradiated rats and mice. In the early rising parasitemia, the organisms were predominantly short, with a well formed undulating membrane, a pointed posterior end, and a large terminal kinetoplast. As parasitemia approached its peak, the organisms transformed into long, slender forms with an inconspicuous undulating membrane, an elongated posterior end, and a sub-terminal kinetoplast. The short forms associated with the early, rising parasitemia were more infective for mice than the long forms encountered at peak parasitemia. Although the two rodent-adapted stocks retained their pathogenicity for goats, neither the original stocks nor their corresponding rodent-adapted stocks could be cyclically transmitted by tsetse flies. The availability of these stocks will greatly facilitate investigations on East African T. vivax which would otherwise be difficult to carry out in experimental rodents.

Structure and expression of two temperature-specific surface proteins in the ciliated protozoan Tetrahymena thermophila

The presence of specific proteins (known as immobilization antigens) on the surface of the ciliated protozoan Tetrahymena thermophila is under environmental regulation. There are five different classes (serotypes) of surface proteins which appear on the cell surface when T. thermophila is cultured under different conditions of temperature or incubation medium; three of these are temperature dependent. The appearance of these proteins on the cell surface is mutually exclusive. We used polyclonal antibodies raised against 30 degrees C (designated SerH3)- and 40 degrees C (designated SerT)-specific surface antigens to study their structure and expression. We showed that these surface proteins contain at least one disulfide bridge. On sodium dodecyl sulfate-denaturing polyacrylamide gels, the nonreduced 30 degrees C- and 40 degrees C-specific surface proteins migrated with molecular sizes of 69 and 36 kilodaltons, respectively. The reduced forms of the proteins migrated with molecular sizes of 58 and 30 kilodaltons, respectively. The synthesis of the surface proteins responded rapidly and with a time course similar to that of the incubation temperature. The synthesis of each surface protein was greatly reduced within 1 h and undetectable by 2 h after a shift to the temperature at which the protein is not expressed. Surface protein synthesis resumed by the end of 1 h after a shift to the temperature at which the protein is expressed. The temperature-dependent induction of these surface proteins appears to be dependent on the synthesis of new mRNA, as indicated by a sensitivity to actinomycin D. Surface protein syntheses were mutually exclusive except at a transition temperature. At 35 degrees C both surface proteins were synthesized by a cell population. These data support the potential of this system as a model for the study of the effects of environmental factors on the genetic regulation of cell surface proteins.

Apparent exhaustion of the variable antigen repertoires of Trypanosoma vivax in infected cattle

Three groups of cattle, each group comprising six animals, were inoculated intravenously with populations of bloodstream forms of Trypanosoma vivax. The first group received T. vivax clone ILDat 1.3 derived from an isolate from Nigeria, while the other two received T. vivax stocks IL 1875 or IL 2133 isolated from Coast Province, Kenya. One animal from the group that was infected with IL 1875 died 8 weeks postinfection. The remaining 17 animals became aparasitemic in 8 to 12 weeks without intervention by drug therapy. The recovered animals developed serodeme-specific immunity against Glossina morsitans subsp. centralis-transmitted challenge. There was complete cross-protection between the two East African T. vivax stocks, although they were isolated from areas 80 to 90 km apart, indicating that they belong to the same serodeme. Antibodies to the homologous metacyclic variable antigen types (VATs) were not detected in sera from recovered animals, suggesting that the immunity displayed by the recovered animals was directed at the bloodstream and not the metacyclic VATs. It is thus suggested that recovery in these animals is due to exhaustion of the repertoire of bloodstream VATs expressed in the animals by the infecting T. vivax clone or stocks.

Structure and expression of two temperature-specific surface proteins in the ciliated protozoan Tetrahymena thermophila

The presence of specific proteins (known as immobilization antigens) on the surface of the ciliated protozoan Tetrahymena thermophila is under environmental regulation. There are five different classes (serotypes) of surface proteins which appear on the cell surface when T. thermophila is cultured under different conditions of temperature or incubation medium; three of these are temperature dependent. The appearance of these proteins on the cell surface is mutually exclusive. We used polyclonal antibodies raised against 30 degrees C (designated SerH3)- and 40 degrees C (designated SerT)-specific surface antigens to study their structure and expression. We showed that these surface proteins contain at least one disulfide bridge. On sodium dodecyl sulfate-denaturing polyacrylamide gels, the nonreduced 30 degrees C- and 40 degrees C-specific surface proteins migrated with molecular sizes of 69 and 36 kilodaltons, respectively. The reduced forms of the proteins migrated with molecular sizes of 58 and 30 kilodaltons, respectively. The synthesis of the surface proteins responded rapidly and with a time course similar to that of the incubation temperature. The synthesis of each surface protein was greatly reduced within 1 h and undetectable by 2 h after a shift to the temperature at which the protein is not expressed. Surface protein synthesis resumed by the end of 1 h after a shift to the temperature at which the protein is expressed. The temperature-dependent induction of these surface proteins appears to be dependent on the synthesis of new mRNA, as indicated by a sensitivity to actinomycin D. Surface protein syntheses were mutually exclusive except at a transition temperature. At 35 degrees C both surface proteins were synthesized by a cell population. These data support the potential of this system as a model for the study of the effects of environmental factors on the genetic regulation of cell surface proteins.

Analysis of antigen switching rates in Trypanosoma brucei

Previously quoted figures for the frequency of antigen switching in Trypanosoma brucei are based on incorrect assumptions. In order to determine the correct switching frequency, an equation was derived that takes the growth rates of the newly expressed antigen types into consideration as well as the proportion of switched trypanosomes and the number of generations since the population was antigenically homogeneous. When this equation was applied to published in vitro data, variable values were obtained for the switching frequency in clonal populations originally expressing one antigen type. The calculated most likely switching frequencies ranged from 1.4 X 10(-7) to 3.5 X 10(-6). This variation was probably caused by differences in the growth rates of the new antigen types in the population and failure to detect slow growing variants. To overcome these problems, an experimental procedure was developed to analyse the switching frequency in vitro. Trypanosomes were cloned and grown in parallel cultures. After an appropriate number of generations, cells expressing the original antigen type were destroyed and, from the proportion of cultures that contained new antigen types, the switching frequency was calculated. The technique minimized subculturing or other procedures that could distort the results. Although the method was optimized for analysing switching frequency, the values differed between experiments, ranging from 2.2 X 10(-7) to 2.6 X 10(-6) for one variant. Possible causes for the variations in switching frequency are discussed.

Antigenic variation during Trypanosoma vivax infections of different host species

The sequence of appearance of specific lytic activity against more than 20 variable antigen types (VATs) of Trypanosoma vivax in the serum of 27 animals belonging to 5 species has been examined. For each host species there was a characteristic course of infection, with differences in height and duration of parasitaemia and in pathogenicity. The sequence of antigenic variation was similar in all host species, with some VATs consistently eliciting response more rapidly than others. The predominant group, comprising VATs which apparently developed within the first 3 weeks, varied in size according to the total number of trypanosomes in the bloodstream within that period, suggesting there is a spectrum, rather than discrete groupings, in the hierarchy of VAT expression. There was very little evidence for differences in appearance of VATs between host species; the only clear example was one VAT which apparently did not develop in one host species. The sequence of antigenic variation in T. vivax seems to be determined by the parasite rather than the host species.

Natural resistance to African trypanosomiasis

Glossina infected with African trypanosomes infest 10(7) Km2 of intertropical Africa. Ten thousand cases of human sleeping sickness are officially recorded each year and 35 X 10(6) human beings are at risk. Animal trypanosomiasis impedes the use of 7 X 10(6) Km2 of land adequate for cattle raising and constitutes a major constraint to increasing protein production in Africa. Several approaches are used to combat trypanosomiasis (a) vector eradication has been successful in certain defined situations but cannot be realistically extended to the whole area at risk, (b) prophylactic drugs, which are too toxic for humans, are widely used to protect cattle. This practice has led to field resistance to many drugs used in the past (reviewed in Holmes & Scott 1982) and the appearance of resistance to the last available prophylactic drug, isometamidium, is being reported (Bourn & Scott 1978, Küpper & Wolters 1983, Pinder & Authié 1984), (c) the hope for a vaccine has been largely abandoned, in the present state of knowledge, due to the considerable extent of antigenic diversity in trypanosomes (reviewed in Doyle 1977, Turner 1982, Roelants & Pinder 1984), (d) consequently, the possible use of certain West African breeds of cattle, which appear resistant to trypanosomiasis, has been emphasized as a solution to this problem in domestic animals. The analysis of this natural resistance is the subject of the present essay.

Trypanosoma brucei: analysis of relapsing populations in sensitive and resistant breeds of cattle

The clone DiTat 1.1 of Trypanosoma brucei brucei was injected into four bovids, and clones obtained from successive waves of parasitemia were used to study the expressed variant-specific surface glycoprotein repertoire. Twenty-four clones were obtained which could be classified into 12 different variable antigen types, in addition to the clone injected, using agglutination or immunofluorescence with monospecific antisera. The variable surface glycoproteins of the 25 clones were extracted using the detergent octyl-beta-D-glucopyranoside in the presence of the protease inhibitor, N-cbz-L-phenylalaninechloromethylketone. The molecular weights varied from 52,000 to 69,000 and the pI from 5.0 to 8.8. The virulence of 14 clones representing 13 variable antigen types was ascertained in mice. The mean survival time ranged from 20.5 to 43.0 days. Clones isolated from early peaks of parasitemia in the bovid were the most virulent while clones derived from later peaks were less virulent. It seems that organisms of diminishing virulence appear in bovids, leading to self-cure of the disease. All clones were sensitive to human serum in a blood infectivity inhibition test. Antibody against all virulent clones appeared in 20 cattle (10 Zebus, 10 Baoulés) which had been injected with T. brucei DiTat 1.1. There was no evidence for parasites of high or low virulence being preferentially expressed in resistant or sensitive hosts.

Antigenic variation in clones of Trypanosoma brucei grown in immune-deficient mice

We have produced monoclonal antibodies against six variant surface glycoproteins from early variant antigen types (VATs) of the IsTaR 1 serodeme of Trypanosoma brucei brucei. We have used these in fixed cell immunofluorescence assays to follow the VAT composition of populations of each early VAT when passaged through irradiated mice. The IsTat 1.A and 1.7a populations were stable for more than 30 days (approximately 150 generations), but 1.1a, 1.3a, 1.5a, and 1.11a all changed to 1.A within this time. The time and rate of this antigenic switch were characteristic for each VAT. Growth rates of the VATs were determined when they were both grown separately and grown with 1.A. It appeared that the order of growth rates was 1.7a greater than 1.A = 1.1a greater than 1.11a greater than 1.5a greater than 1.3a. We have generated theoretical curves for the replacement of one VAT by another based on differences in their growth rates and the rate at which one VAT switches to another (switch frequency). These curves closely match those derived experimentally. We postulate that the differences in growth rates between VATs and the different switch frequencies for VATs may be sufficient to generate the loosely defined sequence of VATs seen in chronic infections.

Resistance of cattle to tsetse-transmitted challenge with Trypanosoma brucei or Trypanosoma congolense after spontaneous recovery from syringe-passaged infections

Groups of cattle were inoculated intravenously with cloned populations of bloodstream forms of Trypanosoma brucei or Trypanosoma congolense. All five steers infected with T. brucei ILTat 2.1 and six of the eight steers infected with T. congolense IL 13-E14 became aparasitemic within 16 and 32 weeks postinfection, respectively. Examination of sera from animals infected with T. brucei by indirect immunofluorescence and neutralization assays revealed the presence of antibodies against all the metacyclic variable antigen types (VATs) of the infecting clone. The neutralizing capacity of the sera increased with the course of infection from 1:10 at 2 months to 1:100 at 3 to 4 months postinfection. The recovered animals were completely immune to challenge by Glossina morsitans subsp. centralis infected with clone IL Tat 2.1, which had initiated the infection, as well as with another clone (IL Tat 2.2) belonging to the same serodeme, but they were susceptible to a tsetse-transmitted heterologous challenge with isolate STIB 367-H. Similar results were obtained with sera from T. congolense IL 13-E14-infected steers. The six steers infected with a different T. congolense ILNat 3.1 clone did not recover spontaneously; however, 2 months postinfection, sera from five of them also contained neutralizing antibodies against ILNat 3.1 metacyclic VATs. These results indicate that some of the bloodstream VATs that arise during the course of a chronic infection possess surface epitopes in their variable surface glycoproteins that are identical to those of the metacyclic VATs. It is suggested that in chronic infection, the infecting trypanosomes could exhaust their VAT repertoire, including those that cross-react with metacyclics, thereby leading to both "self-cure" and subsequent immunity to homologous cyclically transmitted challenge.

Natural agglutinins to African trypanosomes

Agglutinins to uncoated culture forms of the African trypanosomes, T. congolense and T.b. brucei were detected in sera from a variety of mammals not exposed to the parasites. The agglutinins in bovine serum were shown to be specific antibodies with opsonic properties selective for the species of trypanosome. These findings suggest a possible role for the glycoprotein coat in preventing access of cross-reacting antibodies to the plasma membrane of African trypanosomes.

African trypanosome antigens recognized during the course of infection in N'dama and Zebu cattle

The humoral immune responses to Trypanosoma brucei infection were examined in N'dama and in Zebu, two breeds of cattle recognized for their differing susceptibility to trypanosomiasis. Regardless of the clinical course, animals of both breeds produced antibodies to nonsurface trypanosome antigen(s) detectable by both immunodiffusion and immune fluorescence. As a new approach to assessment of the humoral response to trypanosome infection, protein antigens responded to were isolated by immune precipitation, and their molecular weights were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This allowed the detection of differences in the immune response which correlated with the clinical course of the disease. All cattle of both breeds which exhibited a capacity to control the disease recognized at least one of three specific antigens: protein of 110,000, 150,000, and 300,000 daltons. The N'dama, which proved less susceptible to the disease, generally responded to more of the three identified trypanosome protein antigens than did the Zebu. Animals which died of trypanosomiasis failed to produce detectable antibodies to any of the three specific proteins, although they sometimes exhibited antibodies to another trypanosome antigen.

Analysis of antigenic types appearing in first relapse populations of clones of Trypanosoma brucei

Variant antigenic types (VATs) represented in a total of 47 first relapse populations of 6 clones of Trypanosoma brucei LUMP 227 were identified by immunofluorescent staining of living trypanosomes, using antiserum raised against purified surface antigens. The relative growth rates of these 6 clones were measured both individually and when grown together in a mixed population, and were found to be different under these two sets of conditions. A pattern emerged in the VATs represented in relapses of each clone, with some types being expressed more frequently than others and certain VATs being only very rarely expressed. It is suggested that new VATs are expressed according to a statistically definable order of priority which is different for each parent VAT, and that some VATs may be able to change to certain others only after passing through an intermediate VAT. The order of priority of appearance of VATs does not appear to correlate with growth rate measured either in individual clones or when clones are grown in a mixed population.

In vitro cloning of animal-infective bloodstream forms of Trypanosoma brucei

Clones of animal-infective bloodstream forms of Trypanosoma brucei (stocks S.427 and LUMP 227) were made by transferring a single organism from bloodstream-form cultures into each well of Microtest II Tissue Culture Plates containing bovine fibroblast-like feeder cells. When the number of trypanosomes increased to 10(2)--10(3)/well on days 4--16, they were transferred into plastic T-25 culture flasks also containing feeder cells and fresh medium. Cultures were thereafter maintained by partially replacing the trypanosome suspension with the same volume of fresh medium (diluting the density to 2--5 x 15(5) trypanosomes/ml) every 24 h. Sub-cultivations could be made by transferring 1--2 ml of the trypanosome suspension to a new culture flask at 4--5 day intervals. A total of 42 clones in the 3 series TC221, TC52 and TC227, carrying variable antigen types (VATs) 221, 052 and ILTat 1.4, respectively, have been established. Average population doubling times for clones of TC221, TC52 and TC227 were 8.7, 14.5 and 15.5 h respectively. Of 35 populations examined, 34 clones retained the original specificity of their VATs for at least 8--32 days from cloning. One cloned population of TC52 consisted of 99.8% VAT 052 and 0.2% VAT 221 at the time when the first VAT test was made on day 18.

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.

Antigenic variation in clones of animal-infective Trypanosoma brucei derived and maintained in vitro

Eighteen clones of variable antigen type 052 of Trypanosoma brucei stock S.427 were derived and maintained as animal-infective bloodstream forms in vitro for up to 60 days of cultivation. The antigenic composition of such clones was monitored weekly by immunofluorescent analysis of viable trypanosomes, using antisera raised to isolated variant-specific surface glycoproteins of both 052 and variable antigen type (221) which consistently appeared in the first relapse population of type 052 in vitro. The appearance of new variants was detected in 9 of the 18 clones 18--46 days following initiation of the clone and variable antigen type 221 was found in all 9 clones. On one or more occasions in 8 of such clones, viable trypanosomes were found which did not react with either antiserum but were mouse-infective on the 4 occasions tested and probably represent other variable antigen types. The process of antigen variation in vitro appears to resemble the process in vivo except that new variant types are detected earlier in vivo. This possibly results from different growth rates of the trypanosomes in vivo and in vitro, together with the fact that elimination of the initial variant population by the host's immune response facilitates the detection of newly arising variable antigen types in vivo.

Studies on Trypanosoma (nannomonas) congolense III. Antigenic variation in three cyclically transmitted stocks

Cyclical transmission of different variable antigen types of Trypanosoma congolense STIB 228 resulted in the development of metacyclic trypanosome populations which were similar in their variable antigen composition as judged by immunofluorescence and neutralization assays. The variable antigen types present in the ingested bloodstream populations were not found in the metacyclic populations. The bloodstream populations which were obtained from cyclically infected, irradiated (900 rad.) mice contained variable antigen types which were not present in the corresponding metacyclic populations. When derivatives of 2 other stocks of T. congolense, isolated in different area of Tanzania, underwent cyclical development in the tsetse fly, the metacyclic populations of each stock also had a characteristic variable antigen composition. The metacyclic populations of the 3 stocks were, however, completely dissimilar in variable antigen composition. Simultaneous infection of tsetse flies with a mixture of different stocks resulted in the concurrent production of metacyclic trypanosomes which contained the characteristic variable antigen types of each stock. The effect of cyclical transmission on the process of antigenic variation in T. congolense infections is therefore similar to that in T. brucei infections.

Studies on Trypanosoma (nannomonas) congolense IV. Experimental immunization of mice against tsetse fly challenge

Groups of mice were exposed to multiple bites by tsetse flies (Glossina morsitans morsitans) infected with a clone of Trypanosoma congolense spread over a periof of 8 days. The mice were subsequently treated with Berenil 10 days after the first fly bite as were uninfected control mice. The group of mice which received 12--15 infectious fly bites on two occasions, 21 days apart, were subsequently resistant to infection when re-challenged by flies infected with the same clone of T. congolense. These mice were also immune to challenge by flies infected with a different bloodstream variable antigen type derived from this same stock. The immunity was stock-specific and directed against the metacyclic forms of the parasite, but was short-lived.

Non-specific induction of increased resistance in mice to Trypanosoma congolense and Trypanosoma brucei by immunostimulants

Administration of the immunostimulants Corynebacterium parvum, Bacillus Calmette-Guérin (BCG) or Bordetella pertussis prior to, or at the same time as, challenge with Trypanosoma congolense significantly increased survival times in mice, both of trypano-susceptible (A/J) and trypano-resistant (C57Bl) strains. The increased survival time was associated with significant alterations in parasitaemia, which included lengthening of the pre-patent period, a delay in the time taken to reach the first peak of parasitaemia and a reduction in the level of parasitaemia. Similar results were obtained when these strains of mice were challenged with Trypanosoma brucei following pre-treatment with C. parvum. Thus, by the use of immunostimulants it was possible to reduce the susceptibility of mice to trypanosomiasis and the hope is that this can also be achieved with domestic livestock.

Lectin analysis of Trypanosoma congolense bloodstream trypomastigote and culture procyclic surface saccharides by agglutination and electron microscopic technics

Living, intact bloodstream trypomastigotes and culture procyclic forms of Trypanosoma congolense were tested for aggulination with the lectins concanavalin A (Con A), phytohemagglutinin P (PP), wheat germ agglutinin (WGA), soybean agglutinin (SBA), and fucose binding protein (FBP). Similar experiments were conducted with living bloodstream and culture forms treated with trypsin or dextranase. Parasites were incubated for 30 min at 25 C in various concentrations of each lectin, then examined for agglutination by dark-field microscopy. Control preparations consisted of parasites incubated alone or with 0.5 M of the specific competing sugar, with or without the corresponding lectin. Electron-microscopic localization of lectin binding sites on the surface of intact and dextranase-treated bloodstream and intact culture forms was accomplished with Con A, reacted with horseradish peroxidase (HRP) and then diaminobenzidine (DAB). In addition, FBP and SBA were coupled to HRP, then utilized for the localization of binding saccharides on the surface of bloodstream forms by the DAB technic. Similar studies were conducted with culture procyclics incubated with WGA-, SBA-, PP- or FBP-HRP conjugates and then reacted with DAB. Controls were utilized to confirm the sugar specificity of all positive reactions. Intact living bloodstream forms were agglutinated in a concentration-dependent manner with all the lectins tested. Agglutination levels were scored as Con A greater than FBP greater than WGA = PP = SBA. Sugars resembling alpha-D-mannose, N-acetyl-D-glucosamine, N-acetyl-D-galactosamine, and alpha-L-fucose are evidently present on the surface of the parasites. No agglutination was noted in any control preparations. Identical lectin-induced agglutinations were obtained with trypsin- or dextranase-treated bloodstream forms. Trypsin disrupted but did not entirely remove the surface coat of bloodstream forms, while dextranase did not alter the ultrastructure of the parasites. Con A-, SBA- and FBP-binding saccharides were distributed uniformly on the surface coat of intact bloodstream forms; a similar distribution of Con A receptors was noted also on the surface of dextranase-treated cells. No lectin-binding saccharides were visualized by electron microscopy on any control preparations. Intact, trypsin- or dextranase-treated, procyclics were agglutinated in a concentration-dependent fashion by Con A and WGA, but not by the other lectins tested. Control preparations did not agglutinate and the enzymes did not affect the ultrastructure of the parasites. Con A- and WGA-specifically binding saccharides were uniformly distributed on intact procyclics and control preparations were lectin-negative. Thus, T. congolense procyclics retained surface saccharides resembling alpha-D-mannose and N-acetyl-D-glucosamine but lost sugars resembling N-acetyl-D-galactosamine (or D-galactose) and alpha-L-fucose...

Immune depression in trypanosome-infected mice. I. Depressed T lymphocyte responses

Using a wide range of experimental conditions, several kinds of T lymphocyte responses in spleen cell populations from trypanosome-infected mice were studied. Lymphocyte stimulation after culture with the mitogen concanavalin A or with histoincompatible cells differing at H-2 or minor lymphocyte-stimulating loci was reduced or abolished in spleen cells from infected mice when compared with responses of spleen cells from uninfected controls. In addition, cytotoxic lymphocytes were not generated in mixed lymphocyte cultures which contained spleen cells from infected animals. Allogeneic skin grafting experiments performed with normal and infected mice showed that a decreased T lymphocyte response also occurs in vivo. The depressed immune responses were not simply due to low numbers of T lymphocytes in spleens of infected animals, but reflected a generalized immune depression which was not antigen-specific.

Analysis of the antigenic composition of Trypanosoma brucei brucei bloodstream and culture forms by the quantitative direct fluorescent antibody methods

The quantitative direct fluorescent antibody (QDFA) methods were employed for the antigenic analysis of bloodstream forms and culture procyclics of 2 variants, TRUM (Trypanosome Research University of of Massachusetts) 106 and TRUM 107, of Trypanosoma brucei brucei. Intact and trypsinized trypanosomes were studied. It was demonstrated that: (A) The specific variant antigens are localized in the surface coat of bloodstream trypomastigotes. (B) In addition to the common antigens shared by bloodstream forms and culture procyclics, there are also certain antigens unique to these latter stages. (C) Still another group of antigens, not found in the culture procyclics, appears to be shared by the bloodstream forms, irrespective of their variant-specific antigens. These antigens may be present in part in the coat or on the cell membrane and in part within the cytoplasm. (D) Irrespective of the bloodstream-form variant from which they are derived, the procyclics are antigenically the same. The QDFA results are analyzed statistically and discussed in the light of the available literature.