Epitope mapping of B-cell determinants on the 15-kilodalton lipoprotein of Treponema pallidum (Tpp15) with synthetic peptides

Europium nanoparticle-based high performing immunoassay for the screening of treponemal antibodies

Treponema pallidum subspecies pallidum (Tp) is the causative agent of syphilis which mainly spreads through sexual contact, blood transfusion and perinatal route. In order to curtail the spread of the infection and to clinically manage the disease, timely, accurate and reliable diagnosis is very important. We have developed an immunoassay for the detection of treponemal antibodies in human serum or plasma samples. In vivo biotinylated and non-biotinylated versions of the recombinant antigen were designed by the fusion of three Tp-specific antigens namely Tp15, Tp17 and Tp47. These fusion antigens were expressed in E. coli and purified using single-step metal affinity chromatography. Biotinylated fusion antigen immobilized on streptavidin coated plate was used to capture the treponemal antibodies and the non-biotinylated antigen coated on europium nanoparticles was used as tracer. Assays with two different incubation times of 10 min and 1 h were developed, and following the incubation the europium fluorescence was measured using time-resolved fluorometry. The developed time-resolved fluorometric (TRF) immunoassays were evaluated with in-house and commercial serum/plasma sample panels. For well-established treponemal antibodies positive or negative samples, the sensitivity of TRF immunoassay with 10 min incubation time was 97.4%, and of TRF immunoassay with 1 h incubation time was 98.7%, and the specificities of both the TRF immunoassays were 99.2%. For the samples with discordant results with the reference assays, both the TRF immunoassays showed better specificity than the Enzygnost syphilis enzyme immunoassay as a screening test. The two different incubation times did not have any significant effect on the signal to cutoff (S/Co) ratios obtained with the two immunoassays (p=0.06). Our results indicate that the developed immunoassay with a short incubation time of 10 min has the potential to be used in clinical laboratories and in blood-bank settings as a screening test for treponemal antibodies.

Sensitive and specific ELISA coated by TpN15-TpN17-TpN47 fusion protein for detection of antibodies to Treponema pallidum

BACKGROUND

We constructed an artificial fusion gene tpN15-17-47 and then used the prokaryotic expression fusion protein rTpN15-17-47 as the coated antigen to establish a new enzyme-linked immunosorbent assay (ELISA) for serological diagnosis of syphilis.

METHODS

tpN15, tpN17, and tpN47 genes were amplified separately by polymerase chain reaction (PCR) and then assembled into a fusion gene coding a trigeminy protein antigen by primer linking PCR. The target recombinant protein antigens rTpN15, rTpN17, rTpN-47, and rTpN15-17-47 were expressed and then purified antigens were immobilized on the surface of microplate wells for detecting Treponema pallidum-specific antibodies by ELISAs.

RESULTS

The relative positive rate of rTpN15-17-47-ELISA in 965 serum specimens of syphilis patients was 99.5%, which was higher than that of the T. pallidum hemagglutination assay (TPHA) (98.3%) (p<0.05) and much higher than that of the rTpN15-ELISA (83.1%), the rTpN17-ELISA (84.4%), the rTpN47-ELISA (82.1%), and the toluidine red unheated serum test (TRUST) (72.2%) (p<0.01). All the ELISAs and the TPHA in detecting serum specimens from 62 cases with systemic lupus erythematosus (SLE), 86 cases with rheumatic arthritis (RA), and 250 healthy cases were negative, but the TRUST was positive in five cases with SLE, seven cases with RA, and two healthy cases.

CONCLUSIONS

The rTpN15-17-47-ELISA is a sensitive and specific serological screening or a diagnostic method for syphilis in the clinical setting.

Monoclonal Antibodies to the Recombinant Protein TmpA of theTreponema pallidum

Spleen cells from BALB/c mice immunized with recombinant TmpA were fused with mouse myeloma cells (P3/X63-Ag8), and five hybridomas secreting monoclonal antibodies were obtained. These hybridomas specifically recognize TmpA and do not cross-react with other molecules such as recombinant HBsAg of HBV and synthetic HCV core peptides. The monoclonal antibodies were IgG1 subclass and ascitic fluid from these hybridomas was purified by affinity chromatography on Protein A-Sepharose CL-4B column to isolate the IgG1 active fraction. The affinity constant of these monoclonal antibodies ranged from 6.4 x 10(8) and 1.73 x 10(10) M(-1).

Identification of continuous B-cell epitopes on the protein moiety of the 58-kiloDalton cell wall mannoprotein of Candida albicans belonging to a family of immunodominant fungal antigens

The 58-kiloDalton mannoprotein (mp58) on the surface of Candida albicans is highly immunogenic, is expressed by all C. albicans isolates tested, and elicits strong antibody responses during candidiasis. It belongs to a family of immunodominant fungal antigens with representatives also in different species of Aspergillus. The amino acid sequence of the protein portion of mp58 as deduced from the DNA sequence of its encoding gene (FBP1/PRA1) was used to synthesize a complete set of overlapping dodecapeptides (overlap, 7; offset, 5) covalently attached to the surface of derivatized polyethylene pins. The pin-coupled peptides were used in a modified enzyme-linked immunosorbent assay (ELISA) to identify continuous epitopes recognized by a number of antiserum preparations containing anti-mp58 antibodies. This comprehensive epitope-scanning study revealed the presence of multiple immunoreactive continuous B-cell epitopes within the protein sequence. Regions of increased reactivity included both the amino and carboxy termini of the mature protein (encompassing amino acid residues 16 to 50 and 286 to 299, respectively) and four internal regions spanning amino acids at positions 66 to 92, 121 to 142, 148 to 192, and 211 to 232. Further delineation of epitopic regions and identification of the boundaries of the antigenic sites was performed upon ELISA testing with a second Pepset consisting of completely overlapping 8-mer peptides spanning these reactive regions in the protein moiety of mp58. The highly reactive epitopic region at the C terminus of the protein was further evaluated using both window net and replacement net analyses. A synthetic peptide corresponding to the last 10 amino acid residues at the C terminus of the protein was immunogenic when injected into mice after being coupled to a carrier protein. Moreover, antibodies in the resulting sera specifically recognized the homologous mp58 in ELISAs and immunoblot assays. Delineation of the antibody responses to mp58 could provide the basis for the development of novel immunity-based prophylactic, therapeutic, and diagnostic techniques for the management of candidiasis.

Fibronectin fragments modulate monocyte VLA-5 expression and monocyte migration

To identify the mechanisms that cause monocyte localization in infarcted myocardium, we studied the impact of ischemia-reperfusion injury on the surface expression and function of the monocyte fibronectin (FN) receptor VLA-5 (alpha(5)beta(1) integrin, CD49e/CD29). Myocardial infarction was associated with the release of FN fragments into cardiac extracellular fluids. Incubating monocytes with postreperfusion cardiac lymph that contained these FN fragments selectively reduced expression of VLA-5, an effect suppressed by specific immunoadsorption of the fragments. Treating monocytes with purified, 120-kDa cell-binding FN fragments (FN120) likewise decreased VLA-5 expression, and did so by inducing a serine proteinase-dependent proteolysis of this beta(1) integrin. We postulated that changes in VLA-5 expression, which were induced by interactions with cell-binding FN fragments, may alter monocyte migration into tissue FN, a prominent component of the cardiac extracellular matrix. Support for this hypothesis came from experiments showing that FN120 treatment significantly reduced both spontaneous and MCP-1-induced monocyte migration on an FN-impregnated collagen matrix. In vivo, it is likely that contact with cell-binding FN fragments also modulates VLA-5/FN adhesive interactions, and this causes monocytes to accumulate at sites where the fragment concentration is sufficient to ensure proteolytic degradation of VLA-5.