Maedi-Visna virus : the development of serum and whole blood immunodiagnostic assays.
Boshoff, Christoffel Hendrik.
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This thesis describes the development of serum and whole blood immunodiagnostic assays for Maedi-Visna virus (MVV). All previously described recombinant MVV ELISA assays utilised either the core p25 or transmembrane (TM) proteins alone, or combined, but as individual proteins. The p25 and TM genes of MVV were cloned individually into the pGEX-2T expression vector. Both proteins were expressed as a combined fusion protein in frame with glutathione S-transferase (GST). The purified recombinant antigens (GST-TM and GST-TM-p25) were used to develop a MVV ELISA. Sera from 46 positive and 46 negative sheep were tested using the GST-TM and GST-TM-p25 ELISAs and a commercial p25 EIA kit. A two-graph receiver operating characteristic (TG-ROC) analysis program was used to interpret the data. The GST-TM-p25 ELISA was more sensitive than the commercial assay which is based on the p25 antigen alone and more specific than the GST-TM ELISA. The GST-TM-p25 ELISA showed a sensitivity and specificity of 100%. The human AIDS lentivirus transmembrane (TM) glycoprotein portion of the envelope viral protein has been identified as the antigen most consistently recognised by antibodies. There is suggestive evidence that the same applies to MVV as the GST-TM fusion protein, expressed in E. coli, has comparable sensitivity to the GST-TM-p25 fusion protein, but lacks specificity. However, due to the hydrophobic nature of the MVV TM protein, purification of the expressed fusion protein required lengthy purification protocols. This was despite the fact that only a truncated version of the TM protein was expressed. This prompted investigating an alternative expression system that could possibly circumvent the above mentioned problems. The yeast Pichia pastoris is known to be suitable for the high-level expression of heterologous proteins which are secreted into the culture supernatant. These features made P. pastoris an attractive host for the expression of the hydrophobic TM protein of MVV. However, limited success was achieved as only low expression levels were obtained and detection and quantification was only accomplished by means of ELISA. Evaluation of the diagnostic performance of the P. pastoris expressed MVV TM-polypeptide was performed using a panel of 36 confirmed negative and positive sera, and evaluated using a TG-ROC analysis programme, which yielded an equal Se and Sp of 83%. The use of a novel rapid immunoassay system, which allows the detection of circulating antibodies in whole blood, has been investigated for use as a MVV diagnostic assay. The central feature of this immunoassay lies in a monoclonal antibody against a glycophorin epitope present on all sheep erythrocytes. A Fab'-peptide conjugate was constructed by coupling a synthetic peptide, corresponding to a sequence from MVV TM protein, to the hinge region of the Fab' fragment of the antisheep erythrocyte antibody. Within the limited number of 10 seronegative and 10 seropositive samples the autologous red blood cell agglutination assay had a sensitivity of 90% and a specificity of 80%. Despite the limitations and difficulties encountered, the use of such rapid whole blood immunodiagnostic assays for MVV holds promise.