The surface of the African trypanosomes

Efficient targeting of conserved cryptic epitopes of infectious agents by single domain antibodies. African trypanosomes as paradigm

Antigen variation is a successful defense system adopted by several infectious agents to evade the host immune response. The principle of this defense strategy in the African trypanosome paradigm involves a dense packing of variant surface glycoproteins (VSG) exposing only highly variable and immuno-dominant epitopes to the immune system, whereas conserved epitopes become inaccessible for large molecules. Reducing the size of binders that target the conserved, less-immunogenic, cryptic VSG epitopes forms an obvious solution to combat these parasites. This goal was achieved by introducing dromedary Heavy-chain antibodies. We found that only these unique antibodies recognize epitopes common to multiple VSG classes. After phage display of their antigen-binding repertoire, we isolated a single domain antibody fragment with high specificity for the conserved Asn-linked carbohydrate of VSG. In sharp contrast to labeled concanavalin-A that stains only the flagellar pocket where carbohydrates are accessible because of less dense VSG packing, the single domain binder stains the entire surface of viable parasites, irrespective of the VSG type expressed. This corroborates the idea that small antibody fragments, but not larger lectins or conventional antibody fragments, are able to penetrate the dense VSG coat to target their epitope. The diagnostic potential of this fluorescently labeled binder was proven by the direct, selective, and sensitive detection of parasites in blood smears. The employment of this binder as a molecular recognition unit in immuno-toxins designed for trypanosomosis therapy becomes feasible as well. This was illustrated by the specific trypanolysis induced by an antibody::beta-lactamase fusion activating a prodrug.

Simultaneous variation of the immunodominant outer membrane proteins, MSP2 and MSP3, during anaplasma marginale persistence in vivo

Vector-borne bacterial pathogens persist in the mammalian host by varying surface antigens to evade the existing immune response. To test whether the model of surface coat switching and immune evasion can be extended to a vector-borne bacterial pathogen with multiple immunodominant surface proteins, we examined Anaplasma marginale, a rickettsia with two highly immunogenic outer membrane proteins, major surface protein 2 (MSP2) and MSP3. The simultaneous clearance of variants of the two most immunodominant surface proteins of A. marginale followed by emergence of unique variants indicates that the switch rates and immune selection for MSP2 and MSP3 are sufficiently similar to explain the cyclic bacteremia observed during infection in the immunocompetent host.

Specific antibody levels and antigenic recognition of Wistar rats inoculated with distinct isolates of Trypanosoma evansi

"Mal de Cadeiras", an enzootic disease caused by Trypanosoma evansi, is one of the most important trypanosomiases in the Brazilian Pantanal region. The disease affects mainly horses, which are widely used in extensive cattle production, an activity of greatest economical significance for the region. The parasite also infects sylvan (coatis and capybaras) and domestic (dogs) animals, respectively considered wild and domestic reservoirs of T. evansi. For a better understanding of the interaction of T. evansi with its rodent host, we evaluated the differences in the specific antibody level patterns and in the parasitic peptides recognition patterns of experimentally infected Wistar rats. The rats experimentally infected with T. evansi isolates obtained from coatis, dogs and horses were submitted to indirect immunofluorescence test (IgM e IgG) and Western blotting. The serological titers for IgM and IgG ranged between 1:40 and 1:160. The most recognized polypeptide profiles were in a range of 17 and 74 kDa. Our data suggest that the humoral immune response in Wistar rats is not sufficient for granting an effective control of T. evansi infections.

Polypeptide profiles and antigenic characterization of cell membrane and flagellar preparations of different stocks of Trypanosoma evansi

Polypeptides and antigens of cell membrane (CM) and flagellar (Fl) preparations of seven different stocks of Trypanosoma evansi were studied using sodium dodecyl sulphate polyacrylamide gel electrophoresis and by Western blotting. The stocks were collected from Bareilly, Bikaner, Hisar Karnal, Lucknow and Ludhiana localities of northern India and were isolated from either buffalo, equids or camel. Between two and five polypeptides with molecular weights (MW) ranging from 48.4 to 80.2 kDa (except one stock) were detected in the CM preparations and between four and 12 polypeptides ranging from 17.6 to 80.2 kDa were observed in the Fl preparations. The 78.3 kDa polypeptide was common to the CM preparation of all seven stocks. The number of antigens found in the CM of different stocks using homologous and heterologous hyperimmune sera (HIS) varied between three and 11 and were in the MW range of 25-98 kDa. Likewise the number of Fl antigens of different stocks varied between three and 14, predominantly in the MW range 17-98 kDa. Antigenic cross-reactivity among polypeptides of the CM and Fl preparations detected by various HIS was also high, indicating the possibility of less antigenic variability among these seven stocks.

Biochemical and immunological characterization of the variant surface coat glycoprotein shed by African trypanosomes

As the variant surface coat glycoprotein (VSG) was shed from Trypanosoma brucei rhodesiense into the blood of infected rats, it was biochemically characterized and compared with VSG that had been purified from trypanosomal homogenates. To determine if VSG was in association with lipid, membranes and lipoproteins in plasma of infected rats (IRP), VSG isolated from plasma (PVSG), and VSG isolated from trypanosomal homogenates (HVSG) were all concentrated by ultracentrifugation and assayed for the presence of VSG by radial immunodiffusion (minimum level of detection, 25 micrograms/ml) and by immunoelectroblots (minimum level of detection, 1 microgram/ml). Crimson red was used to detect lipid (minimum level of detection, 10 micrograms per sample) in electrophoresed samples. The VSG was neither concentrated with membrane or lipoprotein fractions nor stained by lipid crimson. Lipids from normal rat plasma, IRP, trypanosomal homogenates, HVSG, and PVSG were also extracted and separated by thin-layer chromatography (minimum level of detection, 20 micrograms of trypanosomal phospholipid per sample). The trypanosomal homogenates had five bands as detected by iodine vapors, of which three were phospholipids as detected by molybdenum blue. Both normal rat plasma and IRP had identical patterns of bands with a single phospholipid. The PVSG had one neutral lipid contaminant that apparently was not physically associated with the shed surface coat. The HVSG contained no lipids at all. Therefore, no evidence was obtained to implicate an association between membranes and VSG, once the latter had been shed into the blood of infected hosts. From immunoelectroblots of denatured material, it was determined that both HVSG and PVSG had the same reduced molecular weight. From molecular sieve column chromatography, however, it was determined that VSG released during the homogenization of trypanosomes is a noncovalently linked dimer, whereas that shed in the blood is apparently a trimer. This difference in native structure made no difference in immunological effect. Administered in a regimen that mimicked what the host encounters during a first peak of parasitemia, both HVSG and PVSG induced nonspecific proliferation of splenic lymphocytes and production of unelicited antibodies without the generation of nonspecific immunosuppression. This polyclonal activation of lymphocytes was not the result of contamination by exogenous pyrogen, because the activity was lost if VSG was immunologically absorbed from plasma.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular biology of trypanosome antigenic variation

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