Antigens and antigenic variability of the African trypanosomes

Preliminary evaluation of LATEX/T. b. gambiense and alternative versions of CATT/T. b. gambiense for the serodiagnosis of human african trypanosomiasis of a population at risk in Côte d'Ivoire: considerations for mass-screening

A study was conducted to compare classical card agglutination test for trypanosomiasis (CATT)/T. b. gambiense with CATT-EDTA and LATEX/T. b. gambiense as alternative field tests for serodiagnosis of Human African Trypanosomiasis. The tests were performed on freshly collected blood in an endemic and a low prevalence area in Côte d'Ivoire. Diagnostic performance of each test was assessed using Quantitative Buffy Coat as the parasitological reference and immune trypanolysis as the serological reference test. According to the parasitological data, CATT-EDTA on 10 microl and LATEX/T. b. gambiense on blood diluted 1:4, detecting all confirmed cases with good specificity (respectively 94.6% and 98.1%) yielded better results than the classical CATT did (one false negative and 92.5% specific). However, when immune trypanolysis data and feasibility are taken into account, the classical CATT remains the test of choice for mass screening under the given field conditions.

Splenic requirement for antigenic variation and expression of the variant antigen on the erythrocyte membrane in cloned Plasmodium knowlesi malaria

Variant antigens appear on the surface of Plasmodium knowlesi-infected erythrocytes as the asexual parasite matures and are detected by antibody-mediated schizont-infected cell agglutination (SICA). We now show that cloned parasites can undergo antigenic variation in nonsplenectomized monkeys. In addition, we previously described a new P. knowlesi phenotype in which uncloned parasites passaged in splenectomized monkeys were no longer agglutinable by immune sera. We have designated this new phenotype SICA[-] and the one expressing the variant antigen SICA[+]. Cloned parasites can also switch from SICA[+] to SICA[-] in splenectomized monkeys. The switch from SICA[+] to SICA[-] is a gradual process that requires sequential subpassage in several monkeys. After passage in one monkey, the agglutination titer decreased 4- to 16-fold. Decreased agglutination was associated with decreased antibody binding on all infected erythrocytes as measured by fluorescein-conjugated anti-rhesus monkey immunoglobulin. The asexual malaria parasite can therefore alter its expression of variant antigen in response to the host environment (antivariant antibody or splenectomy). When cloned SICA[-] parasites were inoculated into intact monkeys, two courses of parasitemia were observed: fulminant parasitemia (greater than 20%) and parasitemia that was controlled. Fulminant infections were associated with conversion of the parasite from SICA[-] to SICA[+], i.e., from nonexpression to expression of the variant antigen on the erythrocyte surface. Parasitized erythrocytes remained SICA[-] in those infections that were controlled. It appears, therefore, that the expression of the variant antigen on the erythrocyte surface may influence parasite virulence.

Mimicry of snail host antigens by miracidia and primary sporocysts of Schistosoma mansoni

Polyvalent antisera generated in rabbits to soluble haemolymph components from Schistosoma mansoni-susceptible (PR albino 'M line') and S. mansoni-resistant (10-R2) stocks of the snail Biomphalaria glabrata were employed as membrane probes to determine if antigens related to snail haemolymph were produced by the early larval stages of S. mansoni (PR-1 strain). Using immunofluorescence and immunoelectron microscopical methods we have demonstrated that antibodies to susceptible (anti-Suscept) and resistant (anti-Resist) snail haemolymph (Hg-depleted fraction) crossreact with miracidial epidermal and ciliary membranes as well as the surface membranes of intercellular ridges. Primary sporocysts, both transformed in vitro and maintained in culture for various time intervals in the absence of snail-derived factors, retain haemolymph-like antigens on their surface tegument although at reduced levels in comparison to miracidial stages. Furthermore prolonged cultivation of sporocysts (48 h) has little effect on the density of crossreacting tegumental antigens suggesting that as sporocysts mature these antigenic components are continually being expressed at the surface membrane. Since miracidia and sporocysts were derived in media devoid of snail host materials, shared antigens on larval surfaces are believed to be of parasite origin and constitute true molecular mimicry as defined by Damian (1979). The occurrence of crossreacting antibodies in both anti-Suscept and anti-Resist antisera further suggests that mimicked haemolymph-like antigens include at least some which are common to both snail stocks.

Trypanosoma rhodesiense: chemical and immunological characterization of variant-specific surface coat glycoproteins

Soluble surface coat glycoproteins were purified by concanavalin A affinity chromatography from variant populations of Trypanosoma rhodesiense (Wellcome strain). Each variant yielded a glycoprotein consisting of a single polypeptide chain. The apparent molecular weights of the different glycoproteins ranged from 58 000 to 67 000. Charge heterogeneity analyses resolved from 1 to 3 closely spaced components with isoelectric points that were considerably different from variant to variant. Amino acid analyses revealed notable variations in amino acid compositions. Immunization of mice with purified glycoprotein protected them from homologous variant trypanosome infection. Hyperimmune sera raised to purified glycoproteins were obtained from rabbits and produced single precipitin lines in immunoelectrophoretic or immunodiffusion tests with homologous glycoproteins. No interaction could be detected in heterologous antiserum-glycoprotein combinations. Only variant homologous trypanosomes were agglutinated by antisera. Surface coat glycoproteins prepared from clone populations of variants were chemically and immunologically indistinguishable from the glycoproteins of original uncloned variants. The observed immunogenic specificity and chemical uniqueness of the glycoprotein preparations identify them as variant-specific surface coat antigens responsible for antigenic variability in T. rhodesiense.

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.

Competition among serologically different clones of Trypanosoma brucei gambiense in vivo

When different antigenic variant clones are injected in equal numbers into white mice one variant clone always replaces the other. This phenomenon appears to be a predictable one, even under conditions analogous to a chronic infection. It is hypothesized that a constant ratio is approached between the number of cells of different antigenic serotypes present in a single population, in such a manner that there is always a major antigenic variant and minor populations of different antigenic variants. It is further suggested that these ratios can undergo rapid changes in response to changes in the environment, e.g. nutritional status of the host, changes in body temperature, antibody synthesis, etc. The changes in these ratios are discussed in relation to the mechanism(s) of antigenic variation.

Antigenic variation in trypanosomes

In its mammalian host, Trypanosoma brucei is able to change the antigenic character of its glycoprotein surface coat and so evade the host's immune response. This phenotypic change seems to occur spontaneously in 1 in 10,000 individuals but is not due to genetic mutation: host antibody is not necessary for its induction but plays a selective part in bringing about the gross changes in parasite numbers and antigenic character observed in the bloodstream by destroying the main component of what is actually a heterogeneous population. The infecting trypanosome population injected into the mammalian host by the tsetse fly vector may also be heterogeneous. Such heterogeneity complicates plans to vaccinate cattle and people against the African trypanosomes based on the premise that the metacyclic trypanosomes of a clone bear the same surface antigen.

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.

African trypanosomiasis: a danger for tourists visiting Gambia?

A 30-year-old Swedish woman visiting Gambia in 1975 acquired trypanosomiasis probably of the Gambian type. The first sign of her disease appeared as a trypanosomal chancre on her right thigh, soon followed by lymphadenitis in her right groin. One week later she became febrile and therefore hospitalized after returning to Sweden. The diagnosis was established by demonstrating numerous trypanosomes in a blood smear. 10 pentamidine injections (200 mg once daily intramuscularly cured her disease. The risk of acquiring trypanosomiasis in Gambia seems to be extremely small for tourists, but not quite negligible.

The synthesis of a variant-specific antigen by Trypanosoma brucei in vitro

A variant-specific surface antigen from a cloned population of Trypanosoma brucei S42 has been isolated and partially characterized. [35S]L-methionine was found to be incorporated into this material by cells incubated in vitro in a chemically defined medium. Incorporation of [35S]L-methionine was inhibited by cycloheximide and puromycin at concentrations which are known to specifically inhibit protein synthesis in other systems. The rate of synthesis of the variant-specific antigen in vitro has been estimated to be about 8% of the rate in vivo. Newly synthesized [35S]L-methionine-labelled variant-specific antigen was incorporated into the surface coat.

The relation of agglutinins to antigenic variation of Trypanosoma lewisi

During the course of infection in the rat, Trypanosoma lewisi produces 2 antigenic variants: the 1st represents the initial, reproducing population of cells; and the 2nd the nonreproducing, ablastin-inhibited adult population. The specificities of the agglutinins elicited by the variants were studied by adsorption and agglutination methods and the newer immunoelectroadsorption technic. It was found that the reproducing variant has a surface antigen that reacts with the agglutinin specific for the adult variant, but this antigen does not become immunogenic until transformation to the adult variant occurs. It was also found, with fractions of immune sera obtained by gel filtration, that the agglutinin specific for the reproducing variant is IgG and that specific for the adult variant, IgM. The antigenic variants of pathogenic and nonpathogenic trypanosomes are compared, and the roles of trypanocidal and ablastic antibodies in the induction of antigenic variation are discussed.

Identification, purification and properties of clone-specific glycoprotein antigens constituting the surface coat of Trypanosoma brucei

Soluble glycoproteins have been purified from a series of clones of Trypanosoma brucei 427. Each clone yielded a characteristic predominant glycoprotein which induced clone-specific immunity to trypanosome infection in mice. These glycoproteins were shown by specific labelling and enzyme digestion of cells to be the major components of the trypanosome surface coat. Each glycoprotein consisted of a single polypeptide chain having an apparent molecular weight of 65 000 (as measured by SDS-polyacrylamide gel electrophoresis) and containing around 600 amino acid and 20 monosaccharide residues. Preliminary structural studies indicated large changes in amino acid sequence dispersed over a considerable length of the polypeptide chain. Proteolytic activity was demonstrated in semi-purified trypanosome extracts, providing one reason for the heterogeneity sometimes observed in surface glycoprotein antigen preparations.