In our recent report investigating the role of CMV infection on the development of CD57+ CD4 memory T cells, we found that IL-15 treatment in vitro upregulated intracellular GzB and perforin and surface expression of CX3CR1 by both CD57+ and CD57-CD4 memory T cells, but more so for CD57+ cells [101]. Here, PRF1 is linked to cytomegalovirus infection.