The quantitative ELISA for inactivated Newcastle antigen: experience report from an OMCL

The relative haemagglutinin-neuraminidase (HN) antigen content of inactivated Newcastle disease virus (NDV) vaccines from different manufacturers was determined by means of an Enzyme-Linked Immunosorbent Assay (ELISA) according to Monograph 870 of the European Pharmacopoeia (Ph. Eur.). Wide ranges of reactivity of the different products were observed. When comparing the antibody responses from chickens vaccinated with vaccines showing either high or low reactivity in the antigen ELISA it was found that approximately the same titres of antibodies were induced in the chickens. One hypothesis is that the inactivation procedures used to inactivate the Newcastle disease antigen may alter the antigenic determinant recognised by the monoclonal antibody used. An alteration of the antigen would influence the binding by the monoclonal antibodies used as catching and detection antibodies in the ELISA which may result in a lower ELISA reactivity. It was also found that HN antigen of two inactivated Paramyxovirus 1 (PMV-1) vaccines for pigeons could not be measured in the ELISA. For these vaccines the antigen-ELISA based on monoclonal antibody IDNDV134.1 cannot be used. Our experience shows that a thorough knowledge of the products tested with the ELISA and their influence on the test method is essential to avoid misinterpretations of the test results. The level of ELISA reactivity should not be used for the comparison of vaccines. Furthermore, prediction of the ability of an unknown vaccine to induce antibodies based on the level of ELISA reactivity is not possible. The results (level of reactivity) of the antigen ELISA for the in vitro potency testing of inactivated Newcastle disease vaccines should therefore be carefully interpreted. However, by knowing the performance characteristics of the NDV antigen ELISA and the characteristics of the vaccines to be tested it becomes a valuable tool for the control of inactivated Newcastle disease vaccines in our laboratory. The implementation of this ELISA method for the batch release testing markedly reduces the number of chickens and the time required for batch release testing.