Study on the attenuation of Neospora caninum after continuous passage on J774 cell line

The aim of this study was to establish attenuated Neospora caninum (NC-1) tachyzoites through serial passage by J774 cell line. The virulence of attenuated tachyzoites for Balb/c mice was also evaluated. The NC-1 isolates were passaged 90 times in J774 cell line to produce a high-passage or attenuated strain. A total of 35 female BALB/c mice with an average age of six to eight weeks were randomly separated into seven groups. The first group as a control was inoculated subcutaneously with cell culture media and the second, third and fourth groups received low passage or acute NC-1 strain with different doses (5×106, 10×106, 20×106) of tachyzoites. The other three groups were inoculated with high-passage of NC-1 tachyzoites with similar doses. The mortality rate and pathological changes in all groups were noted daily. The cellular and humoral immunity were assessed by Skin test and ELISA, respectively. The results of the present study showed that the groups that received high-passage strain of Nc1 not only survived, but did not show pathological changes of neosporosis. In addition, cellular and humoral immune responses to NC-1 were significantly higher in mice that received high-passage tachyzoites of NC-1. These findings indicated that the serial passages of NC-1 in J774 cell line significantly led to decrease the virulence of NC-1. This attenuated tachyzoite could be used as a candidate for further research to develop live vaccines against Neospora caninum.