Enzyme-linked immunosorbent assay and agar gel immunodiffusion assay for diagnosis of equine infectious anemia employing p26 protein fused to the maltose-binding protein

A double-strain TM (gp45) polypeptide antigen and its application in the serodiagnosis of equine infectious anemia

Lentiviruses, including equine infectious anemia virus (EIAV), are considered viral quasispecies because of their intrinsic genetic, structural and phenotypic variability. Immunoenzymatic tests (ELISA) for EIAV reported in the literature were obtained mainly by using the capsid protein p26, which is derived almost exclusively from a single strain (Wyoming), and do not reflect the great potential epitopic variability of the EIAV quasispecies. In this investigation, the GenBank database was exploited in a systematic approach to design a set of representative protein antigens useful for EIAV serodiagnosis. The main bioinformatic tools used were clustering, molecular modelling, epitope predictions and aggregative/ solubility predictions. This approach led to the design of two antigenic proteins, i.e. a full sequence p26 capsid protein and a doublestrain polypeptide derived from the gp45 transmembrane protein fused to Maltose Binding Protein (MBP) that were expressed by recombinant DNA technology starting from synthetic genes, and analyzed by circular dichroism (CD) spectroscopy. Both proteins were used in an indirect ELISA test that can address some of the high variability of EIAV. The novel addition of the gp45 double-strain antigen contributed to enhance the diagnostic sensitivity and could be also useful for immunoblotting application.

Validation of an indirect in-house ELISA using synthetic peptides to detect antibodies anti-gp90 and gp45 of the equine infectious anaemia virus

BACKGROUND

Equine infectious anaemia (EIA) is controlled by the identification of seropositive animals. The official diagnostic method is the agar gel immunodiffusion (AGID) test, which detects antibodies against a viral core protein (p26). Although AGID is inexpensive and specific, the report of results takes considerable time and the test has low analytical sensitivity.

OBJECTIVE

To validate our in-house indirect ELISA_gp90/45 , following the World Organization of Animal Health (OIE) criteria.

STUDY DESIGN

Test validation.

METHODS

Synthetic peptides gp90 and gp45 were used as antigens in ELISA_gp90/45 . Tests used for validation, calibration and linear working operating range, analytical and diagnostic sensitivity and specificity, repeatability and reproducibility were assessed by comparing them with the AGID test and using 1844 equine sera grouped into five different panels.

RESULTS

We were able to replace the National References Sera with our Internal Reference Sera. ELISA_gp90/45 had acceptable repeatability and reproducibility. Analytical sensitivity of the ELISA_gp90/45 was 800 times greater than that of AGID test for positive sera and 400 times greater for weak positive sera. ELISA_gp90/45 also showed optimal analytical specificity, since no cross-reactivity was detected with antibodies against other equine viruses. One sample was positive by AGID test and negative by ELISAgp90/45. ELISA_gp90/45 was performed using 243 EIA positive and 878 negative equid sera, and showed a diagnostic sensitivity of 99.59% [CI 97.73%-99.99%] and a diagnostic specificity of 90.32% [CI 88.17%-92.19%], compared to AGID test; thus, it was demonstrated to be a robust test.

MAIN LIMITATIONS

Samples were derived from naturally infected equid populations showing heterogeneous clinical states: therefore, their status was uncertain and some horses were sampled more than once. The AGID test may not be the most useful gold standard.

CONCLUSION

ELISA_gp90/45 is a useful tool for the diagnosis of EIAV infection and meets validation requirements established by the OIE.

Evaluation of Equine Infectious Anemia Virus by the Indirect Enzyme-linked Immunosorbent Assay EIA-LAB as Screening Tools in Mexico

Equine infectious anemia is a worldwide distributed disease that affects the Equide family. Commercial effective vaccine is not available, for that reason control of the disease depends on diagnostic tools. To improve the efficiency of the diagnostic program in Cuba, LABIOFAM Group, developed an indirect enzyme-linked immunosorbent assay (ELISA), ELISA kit, to complement the diagnostic system that currently uses the agar gel immunodiffusion (AGID) kit. The ELISA AIE-LAB Kit was evaluated in a Mexican context, compared with the gold standard test Agar gel immunodiffusion, AGID AIE-LABIOFAM, and commercial AGID kit. The analytical sensitivity was determined using serial dilutions twofold of the positive control serum to establish the range of detected antibodies in relation to the cutoff value of the plate (OD 0.300). A precision study was carried out to evaluate repeatability, intermediate precision, and reproducibility by estimating the standard deviation and coefficient of variation. The precision results were satisfactory and the values of the coefficient of variation were considered adequate to guarantee an excellent consistency of the ELISA AIE-LAB. The diagnostic performance of the ELISA AIE-LAB involved the evaluation of specificity, sensitivity, and concordance in comparison with both AGID tests. The diagnostic sensitivity was 100% and the specificity 97.6%, with a very good degree of concordance (Kappa = 0.9). The results suggest that the ELISA AIE-LAB test could be used in Mexico as a diagnostic system for the detection of specific antibodies against the equine infectious anemia virus, as per current international norms.