Evaluation of monoclonal antibodies in immunofluorescence assay for rapid quality control of Peste des petits ruminants (PPR) vaccine

Peste des petits ruminants (PPR) is an acute viral disease of small ruminants with morbidity and mortality rates as high as 90 % and 100 % respectively. Vaccination using a homologous live-attenuated vaccine is the main control strategy available in endemic countries, and ensuring the quality of these vaccines is of utmost impor-tance for PPR control. Currently, the quality control test to determine vaccine potency is the time-consuming and subjective tissue culture infectious dose (TCID50) assay involving titration on Vero cell line and reading the cytopathic effect (CPE) using light microscopy. In this study, an indirect immunofluorescent antibody test (IFAT) was evaluated for PPR vaccines potency testing. Accuracy and repeatability of the IFAT were also determined. Six PPR vaccine batches were produced and tested simultaneously by CPE reading and the IFAT. Vero cells were seeded in 96-well plates, and infected with 10-fold serial dilutions of the vaccines. CPEs were read continuously from day 3-14 after infection, and titers expressed in TCID50/ml. From 3-7 days post-infection, cells were fixed, incubated with anti-N and anti-H monoclonal antibodies (mAbs), and stained with a secondary antibody labeled with a fluorescent tag. Fluorescent foci of infection were detected using a fluorescence microscope, and titers were calculated as TCID50/ml. The results indicated that from day 4, the IFAT gave similar results to CPE readings of day 14 (intraclass correlation coefficient, ICC = 0.957, P < 0.0001). This is a three-fold reduction in time and can be used for rapid PPR vaccine potency evaluation. However, there was no difference observed between the results of the two mAbs (ICC = 0.910, P < 0.0001) indicating that both are equally sensitive and rapid. This result indicates that the IFAT may be more suited for potency testing of PPR vaccines as it saves time and minimizes analyst-related variability associated with CPE reading.