Apoptosis of Vero cells infected with two canine distemper virus (CDV) vacc
ine strains was detected using TdT (terminal deoxynucleotidyl transferase)-
mediated dUTP nick end-labelling (TUNEL), flow cytometric analysis, agarose
gel electrophoresis and electron microscopy (EM). By TUNEL, apoptotic cell
s were found in CDV-Onderstepoort (CDV-Ond)-infected cells. DNA fragments i
solated from infected cells were separated by agarose gel electrophoresis a
nd a 'ladder' pattern appeared. EM observations demonstrated that the cells
undergoing cytopathic effect (CPE) possessed morphological characteristics
of apoptotic cells. Flow cytometric analysis indicated that CDV could indu
ce apoptosis of Vero cells, but the percentages of the apoptotic cells were
correlated with the CPE types. The strain showing the cell-rounding type o
f CPE produced a much higher percentage of apoptotic cells than CDV-Ond wit
h the syncytium type of CPE (P < 0.01). It was concluded that CDV vaccine s
trains could induce apoptosis of Vero cells and the apoptosis was virus str
ain-dependent and cell-dependent The mechanism remains to be studied.