Evaluation of thymopoiesis using T cell receptor excision circles (TRECs): differential correlation between adult and pediatric TRECs and naïve phenotypes.

To determine whether the thymus is still functional despite age-related involution, we measured a biomarker for thymopoiesis known as the T cell receptor excision circle (TREC) from peripheral blood mononuclear cells (PBMCs) of 148 healthy children and from PBMCs, CD4(+), and CD8(+) cells of 32, 30, and 50 healthy adults, respectively. We demonstrate that during the first 5 years of life, thymic output is decreased (P 0.002) but not dramatically (r = -0. 282). Among adults aged 23-58, thymic output was inversely correlated with age, as measured from PBMCs (r = -0.628, P < 0.0005), CD4(+) (r = -0.530, P 0.003), and CD8(+) fractions (r = -0.385, P 0. 006). A strong correlation existed between pediatric PBMC TRECs and the expression of three naïve phenotypic markers (CD45RA(+)CD45RO(-), CD45RA(+)CD62L(+), and CD45RO(-)CD27(+)CD95(low)). Adult PBMC TRECs correlated only with the expression of CD45RA(+)CD45RO(-) (r = 0.459, P 0.012). Our data suggest that in adults CD45RA(+)CD45RO(-) may be enriched for TRECs and add to a growing body of evidence illustrating intact thymic function in adulthood.