Detection of circulating trypanosomal antigens by double antibody ELISA using antibodies to procyclic trypanosomes

Identification of Trypanosome proteins in plasma from African sleeping sickness patients infected with T. b. rhodesiense

Control of human African sleeping sickness, caused by subspecies of the protozoan parasite Trypanosoma brucei, is based on preventing transmission by elimination of the tsetse vector and by active diagnostic screening and treatment of infected patients. To identify trypanosome proteins that have potential as biomarkers for detection and monitoring of African sleeping sickness, we have used a ‘deep-mining” proteomics approach to identify trypanosome proteins in human plasma. Abundant human plasma proteins were removed by immunodepletion. Depleted plasma samples were then digested to peptides with trypsin, fractionated by basic reversed phase and each fraction analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). This sample processing and analysis method enabled identification of low levels of trypanosome proteins in pooled plasma from late stage sleeping sickness patients infected with Trypanosoma brucei rhodesiense. A total of 254 trypanosome proteins were confidently identified. Many of the parasite proteins identified were of unknown function, although metabolic enzymes, chaperones, proteases and ubiquitin-related/acting proteins were found. This approach to the identification of conserved, soluble trypanosome proteins in human plasma offers a possible route to improved disease diagnosis and monitoring, since these molecules are potential biomarkers for the development of a new generation of antigen-detection assays. The combined immuno-depletion/mass spectrometric approach can be applied to a variety of infectious diseases for unbiased biomarker identification.

A double antibody sandwich enzyme-linked immunosorbent assay for detection of secreted antigen 1 of Babesia microti using hamster model

A double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) targeting secreted antigen 1 of Babesia microti (BmSA1) was developed for detection of B. microti infection. The optimized DAS-ELISA was sensitive enough to detect circulating BmSA1 by day 2 post-infection, in sequential sera of a hamster infected with B. microti. This detection was 4 days earlier than antibody detection by indirect ELISA. The kinetics of circulating BmSA1 coincided with the profile of parasitemia. The specificity of this assay was evaluated using sera from animals experimentally infected with different species of Babesia. The DAS-ELISA had a higher sensitivity than the microscopic examination of Giemsa-stained blood smears for detection of the infection in hamsters. Taken together, these results indicated that BmSA1 could be a potential marker for surveillance of human babesiosis.

Contributions of the procyclin 3' untranslated region and coding region to the regulation of expression in bloodstream forms of Trypanosoma brucei

When bloodstream forms of Trypanosoma brucei differentiate into procyclic forms they rapidly synthesise a new surface coat composed of procyclins. Procyclin genes are transcribed in bloodstream forms at approximately one-tenth of the rate in procyclic forms, but little, if any, mRNA can be detected, indicating that further down-regulation must occur post-transcriptionally. We have examined the role of the 297 bp procyclin 3' untranslated region (UTR) in regulating expression in bloodstream forms and have identified three discrete elements: a dominant, negative element between positions 101 and 173, and two positive elements. When chloramphenicol acetyl transferase (CAT) was used as the reporter gene, deletion of the negative element caused a approximately 6-fold increase in the level of steady state mRNA and > 30-fold increase in CAT activity, suggesting that both RNA stability and translation were affected. Similar results were obtained with glutamic acid/alanine-rich protein (GARP), the T. congolense analogue of procyclin, indicating that the 3' UTR acts independently of the coding region. In contrast, when trypanosomes were stably transformed with a construct in which the procyclin coding region was linked to a truncated form of the 3' UTR which lacked the negative element, they expressed high levels of mRNA, but no protein could be detected in cell lysates or culture supernatants. These results imply that the procyclin coding region exerts yet another layer of control which prevents inappropriate expression of the protein in the mammalian host.

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

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

Immunoassay of circulating trypanosomal antigens in sleeping sickness patients undergoing treatment

Sera from 99 patients infected with Trypanosoma brucei rhodesiense and undergoing treatment, were analyzed for circulating trypanosomal antigens using a sandwich antigen-trapping enzyme-linked immunosorbent assay (ELISA). Trypanosomal antigens were detected in 83 (84%) of the patients. Post-treatment antigen profile in 67 patients showed five distinct patterns: in 48% of the patients antigen levels remained elevated throughout the time of hospitalisation and follow-up; in 31%, antigens dropped to the negative value by the second month; in 7.5%, antigens dropped to the negative level and became elevated afterwards; in 7.5%, antigen levels were negative initially, but later, became elevated and remained so throughout the observation period; in 6%, antigen levels remained below the negative value throughout. All patients who relapsed on follow-up had earlier shown evidence of elevated antigen profile. There were no cases of relapses among 21 patients whose antigen levels dropped subsequent to treatment. This ELISA trypanosome antigen detection test could be useful in evaluating treatment success, when used together with parasitological diagnostic techniques.

A conserved stem-loop structure in the 3' untranslated region of procyclin mRNAs regulates expression in Trypanosoma brucei

African trypanosomes that cycle between mammalian hosts and the tsetse fly vector must be poised to survive in different environments. The control of stage-specific gene expression is undoubtedly one of the keys to successful adaptation, but no regulatory elements have been defined to date. Procyclins (also known as procyclic acidic repetitive proteins) are specifically expressed on the surface of procyclic and epimastigote forms in the fly. Procyclin genes are already transcribed in bloodstream forms, but stable mRNA, and later the protein, are first detected when the parasites begin to differentiate into procyclic forms. We have now identified a region of 16 bases that forms part of a predicted stem-loop structure in the 3' untranslated regions of different procyclin mRNAs; both the sequence and the secondary structure of this 16-mer appear to be required for efficient translation of a reporter gene in procyclic forms. The level of steady-state mRNA, its polyadenylylation, and its distribution in the cell are all unaffected by the presence or absence of this element. Deletion of the 16-mer alone reduces expression more than removal or reversal of the entire 3' untranslated region and flanking region, suggesting that there are additional negative regulatory elements in the same 3' untranslated region.

An enzyme-linked immunosorbent assay (ELISA) for the detection of trypanosomal antigens in goat serum using a monoclonal antibody

An IgM murine monoclonal antibody (MAb) TEA 1/23.3.4.6 raised against circulating trypanosome antigens was used in a sandwich ELISA to assay trypanosomal antigens in a trypanosome lysate preparation and in sera from goats infected with Trypanosoma brucei evansi. As little as 1.25 ug/ml of trypanosomal antigen could be detected by this assay. Following infection, trypanosomal antigens were first detected in goat serum 24 hours after the intravenous (i/v) or 6 days after the intramuscular (i/m) inoculation of trypanosome parasites. Antigen levels remained detectable during the course of infection. After treatment with diminazene aceturate, antigens dropped to undetectable levels between day 12 to 41, suggesting that this assay offers a promising approach to the diagnosis of African Trypanosomiasis.

Detection of leishmania antigen in kala azar patients using monoclonal antibodies

Twenty-one monoclonal antibodies were produced against promastigote antigens of Leishmania donovani. Five monoclonal antibodies (Hyb.17, 6, 5, 4 and 2) identifying molecules associated with various L. donovani antigenic determinants ranging from 42-116 kDa were selected as 'capture antibodies' and compared with specific anti-leishmania antisera for detection of circulating leishmania antigens in kala azar patients' sera in a competitive-enzyme-linked immunosorbent assay system (ELISA). The anti-leishmania antisera could detect circulating antigen in 30% of kala azar cases while out of the five monoclonals, Hyb.17 could effectively detect circulating leishmania antigen in 85.4%. The efficacy of Hyb.6 was however low (31.7%). The antigens recognized by these monoclonal antibodies in the western blot assay could possibly represent the ones circulating in sera of patients suffering from kala azar. A cocktail of these monoclonal antibodies may be more useful than the conventional polyclonal antisera in detection of circulating antigen for clinical diagnosis of kala azar.

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

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

Expression of Trypanosoma brucei procyclin as a fusion protein in Escherichia coli

Procyclin, a glycoprotein surface antigen of procyclic forms of Trypanosoma brucei, was expressed in Escherichia coli as a cro-beta-galactosidase fusion protein. Antibodies produced in rabbits immunised with gel-purified fusion protein bound to the surface of living procyclic culture forms in indirect immunofluorescence assays and were able to immunoprecipitate procyclin from lysates of trypanosomes biosynthetically labelled with tritiated proline. In addition, the antibodies recognised synthetic peptides corresponding to three different regions of the procyclin molecule, including a glutamic acid-proline dipeptide repeat. The results indicate that T. brucei procyclin expressed as a fusion protein is immunogenic and antigenically intact. In contrast, no rabbit antibodies could be produced against a 16-amino-acid synthetic peptide consisting of the dipeptide repeat, even when the peptide was coupled to carrier proteins.

Procyclin gene expression and loss of the variant surface glycoprotein during differentiation of Trypanosoma brucei

In the mammalian host, the unicellular flagellate Trypanosoma brucei is covered by a dense surface coat that consists of a single species of macromolecule, the membrane form of the variant surface glycoprotein (mfVSG). After uptake by the insect vector, the tsetse fly, bloodstream-form trypanosomes differentiate to procyclic forms in the fly midgut. Differentiation is characterized by the loss of the mfVSG coat and the acquisition of a new surface glycoprotein, procyclin. In this study, the change in surface glycoprotein composition during differentiation was investigated in vitro. After triggering differentiation, a rapid increase in procyclin-specific mRNA was observed. In contrast, there was a lag of several hours before procyclin could be detected. Procyclin was incorporated and uniformly distributed in the surface coat. The VSG coat was subsequently shed. For a single cell, it took 12-16 h to express a maximum level of procyclin at the surface while the loss of the VSG coat required approximately 4 h. The data are discussed in terms of the possible molecular arrangement of mfVSG and procyclin at the cell surface. Molecular modeling data suggest that a (Asp-Pro)2 (Glu-Pro)22-29 repeat in procyclin assumes a cylindrical shape 14-18 nm in length and 0.9 nm in diameter. This extended shape would enable procyclin to interdigitate between the mfVSG molecules during differentiation, exposing epitopes beyond the 12-15-nm-thick VSG coat.

An antigen detection enzyme immunoassay for the diagnosis of rhodesiense sleeping sickness

A monoclonal antibody raised against a non-variable surface antigen of Trypanosoma brucei rhodesiense procyclic trypomastigotes was used to develop an antigen detection enzyme immunoassay for the diagnosis of rhodesiense sleeping sickness. The assay was evaluated using 211 sera from clinically suspected cases: 142 from parasitologically proven cases and 69 from patients who were negative on parasitological examination. The test was positive in 128 out of 142 parasitologically proven cases. The negative cases may have been in the early stages of the disease, or may represent patients with antibody levels sufficient to prevent detection of antigen. Of particular significance, however, was the finding that eight of the 69 patients with undiagnosed disease were antigen positive despite the negative parasitological findings. Since false-positive reactions were not observed with blood donor sera, or with sera from malaria, schistosomiasis and leishmaniasis patients, it is reasonable to conclude that the eight antigen-positive patients were actual cases of sleeping sickness. The remaining 61 cases who were negative for both parasitaemia and antigenaemia may conceivably represent the variety of diseases whose clinical manifestations resemble those of rhodesiense sleeping sickness. The antigen detection method would thus not only be complementary to parasitological diagnosis, but essential for correct diagnosis in certain stages of the disease.