Sensitive plaque neutralization assay for parainfluenza virus types 1, 2, and 3 and respiratory syncytial virus

A sensitive plaque neutralization assay for parainfluenza virus types 1, 2, and 3 and respiratory syncytial virus was developed in Vero and MA 104 cell cultures. The tests were performed in semimicrotoiter trays containing 24 wells, 16 mm in diameter. Parainfluenza virus type 1 formed plaques in Vero and MA 104 cells only when trypsin was added to the overlay medium. Plaquing of parainfluenza virus type 1 was more sensitive and technically reproducible in MA 104 cells than in Vero cells. Parainfluenza virus types 2 and 3 and respiratory syncytial virus readily formed plaques in Vero cells. Plaques with all viruses were necrotic in character, except for plaques produced by parainfluenza virus type 3, which appeared red due to an increased uptake of neutral red by infected cells. Different conditions for plaquing of the four viruses had to be used to obtain plaques of suitable size. Antibody titers of commercially prepared guinea pig typing sera were 5- to 50-fold higher by the plaque neutralization test than by complement fixation. The addition of guinea pig immunoglobulin G antiglobulin to the serum-virus mixtures enhanced the conventional neutralization test 5- to 10-fold. The sensitivity and specificity of the plaque neutralization test was also determined with sera of marmosets experimentally infected with parainfluenza virus types 1 and 3. The generally low postinfection titers could be enhanced, on the average, 40-fold by using human immunoglobulin G antiglobulin in the neutralization test. A low degree of cross-reactivity was shown between parainfluenza virus types 1 and 3 both in the conventional neutralization test and in the anti-immunoglobulin enhanced neutralization test.