CD274 and infection: Using a stable cell line expressing the PIV5 V protein, we demonstrated that V protein expression alone, in the absence of the rest of the PIV5 infection, is sufficient to block IFN-γ-induced upregulation of the NK cell inhibitory ligands PD-L1, HLA-E, and MHC-I (Figure 5) and was also sufficient to enhance PM21-NK cell-mediated killing (Figure 6).