PRF1 and viral infectious disease: Although the number of NK cells infiltrating the CNS was lower than that of T cells (Figure 2F), they can play a key role in viral clearance by killing infected cells through exocytosis of cytolytic granules containing perforin and granzyme, or by secreting proinflammatory cytokines, in particular IFN-γ, and are therefore considered the first line of defense against viral infection such as DENV and Sindbis virus (34–36).