PRF1 and cytomegalovirus infection: Finally, since CMV infection and reactivation have been shown to drive perforin expression in NK- and CD8+ T-cells, respectively (Pietersma et al., 2010; Ishiyama et al., 2022), and perforin is a critical mediator of the cytolytic response of those cells (McElhaney et al., 2020), we also investigated whether the aforementioned associations between CMV and GZB/GZK+ cell frequency were dependent on perforin expression.