PRF1 and viral infectious disease: While elevated percentages of CD8+CD57+ T cells have been described as a symptom of chronic immune activation [48] and of accelerated immune senescence in chronic HIV infection [49], several studies have also shown that CD8+CD57+ T cells elicit potent cytotoxic effector functions in HIV and other viral infections [50,51] through high expression of perforin and secretion of IFN-γ and TNF-α [52].