PRF1 and infection: Although the cytotoxicity of CVI988-activated γδ T cells was not tested in the present study, it is plausible to speculate that these cells are able to kill target cells as chicken γδ T cells were shown to have spontaneous CTL activity before infection (29) and markedly upregulated the expression of granzyme, perforin, and NK lysin, the effector molecules of CTLs after CVI988 vaccination (Figure 7).