Identification of paramyxovirus-specific haemolysis-inhibiting antibodies separate from haemagglutinating-inhibiting and neuraminidase-inhibiting antibodies. 2. NDV and mumps virus haemolysis-inhibiting antibodies.

Egg-grown Newcastle disease (NDV) and mumps virus were used for preparation of rabbit hyperimmune sera against purified whole virus and projectionless virus particles. These sera and convalescent sera after natural NDV and mumps infections in chickens and human subjects, respectively, were studied in haemolysis-inhibition (HLI), haemagglutination-inhibition (HI) and neuraminidase-inhibition (NI) tests both before and after absorption with Tween 80-ether (TE) treated virus preparations. In addition, neutralization tests using the different sera were carried out. HI and NI antibodies and the major population of neutralizing antibodies in convalescent sera were removed by absorption with TE treated virus material without changing the titre of non-HI HLI antibodies. Rabbit hyperimmune sera directed against projectionless virus particles exhibited HLI antibody titres in marked excess of HI and NI antibody titres, whereas this was not found in sera against whole virus. Absorption with TE treated virus material resulted in removal of all demonstrable antibody activities in sera against whole virus. The corresponding absorption of sera against projectionless particles eliminated HI antibodies without changing the titre of non-Hi HLI antibodies. In rabbit hyperimmune sera, HI antibodies were of primary importance in neutralization tests. After addition of anti-gamma globulin to the test, an efficient neutralization was observed if mumps non-HI HLI antibodies were used whereas this was not found if NDV non-HI HLI antibodies were used.