Development of test systems using a recombinant nucleocapsid viral protein for serodiagnosis of peste des petits ruminants

Peste des petits ruminants (PPR) is an acute febrile viral disease of small ruminants. In severe cases of PPR, when animals manifest clinical signs, virus-specific antigens can be detected in blood and/or tissue samples. In subclinically infected animals, PPR can only be diagnosed using serological testing. Taking into account their simplicity, high sensitivity and cost-effectiveness, the test systems for carrying out indirect or competitive enzyme-linked immunosorbent assay (c-ELISA) are considered most suitable to be used both for the disease diagnosis and seroepidemiological surveillance. The up-to-date techniques for PPR serodiagnosis are developed on the basis of a recombinant nucleocapsid (N) protein. Various options are being worked out to create test systems for PPR serodiagnosis, in particular, using polyclonal sera against N-protein for c-ELISA or some modifications of indirect ELISA. This work was aimed at studying the characteristics of the components of some ELISA experimental test systems for PPR diagnosis using a recombinant N-protein in indirect ELISA with a protein A peroxidase conjugate or in competitive ELISA using a peroxidase IgG conjugate obtained from polyclonal rabbit sera against the nucleocapsid protein. Owing to immunization of rabbits with the purified recombinant N-protein, sera with titers of 1:512 to 1:1024 in c-ELISA were obtained. The potential of constructing a test system for PPR diagnosis through indirect ELISA, in which the peroxidase protein A conjugate was used to identify the PPR-specific antibodies bound to the antigen, has been demonstrated in experiments with sera from convalescent goats, as well as from rabbits immunized with the N-protein. It is important that the protein A peroxidase conjugate reacts with goat sera antibodies in immune complexes. The antibodies obtained from the blood serum of a rabbit immunized with the purified recombinant N-protein have been shown to react with the same epitopes as the positive goat serum antibodies. To construct c-ELISA test system for PPR serodiagnostics, a peroxidase conjugate was prepared using the IgG isolated from an N-protein specific rabbit serum. The sera from pigs immunized with the purified PPR virus and vaccinated against caprine PPR with the titers ≥ 1:64 as observed in the neutralization test (NT) were positive in c-ELISA in which the components of the experimental test system were used. The obtained results make it possible to positively evaluate the prospect of the developed test systems for PPR diagnostics.