Age-associated changes in miRNA expression profiles in thymopoiesis.

During adult life, the thymus involutes and thymic output of mature T cells drastically declines. The molecular events underlying this process are not well understood. Here, we present evidence of the importance of miRNAs in regulating T cell differentiation in the aged. miRNAs are a wide-ranging regulatory element influencing gene expression throughout the lifetime of the organism. To establish whether they play a role in the age-specific thymic decline, the miRNA expression pattern was examined in TN subsets of young and aged mice. Fifty-two percent of the miRNAs exhibited elevated expression levels in the aged TN1 cells. This expression profile leads us to hypothesize that the large number of highly expressed miRNAs, indicative of rigidly controlled protein expression, limits the developmental potential of this population and results in the age-induced decline in thymopoiesis.