Evaluation of Thymopoiesis Using T Cell Receptor Excision Circles (TRECs): Differential Correlation between Adult and Pediatric TRECs and Naı̈ve 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 naı&#x0308;ve phenotypic markers (CD45RA+CD45RO−, CD45RA+CD62L+, and CD45RO−CD27+CD95low). 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.