PECAM1 and viral infectious disease: Naïve CD4+CD31+ T cell levels negatively correlated with age in the HCV untreated and DAA-treated cohorts [25], most likely due to age-associated thymic involution that can be accelerated by persistent viral infections (such as HCV and HIV), subsequently resulting in less T cell maturation and thymic export of naïve CD31+ T cells into the periphery.