It has been assumed that a diverse TCR repertoire is formed during early life, when the thymus is most active, and that T cell homeostasis is maintained without significant thymic input in adults 12
. Given the profound effects of stress on thymopoiesis, intrathymic T cell production in the intact animal is best studied with a minimally invasive assay for recent thymic emigrants (RTEs)1
in the peripheral blood. In the chicken, for example, RTEs can be identified by their unique expression of the cell surface marker chT1 3
. Murine RTEs may be followed kinetically in the peripheral circulation after direct intrathymic labeling, e.g., with FITC 4
. Assays of this type are, however, unavailable for the assessment of human thymic function. Such assessment has relied instead upon autopsy series 5
, radiographic observations 6
, and/or phenotypic demarcation of circulating human T cells into distinct populations of “naive” or “memory/effector” cells. In aggregate, these studies 7
demonstrate that: (a) there is a correlation between the abundance of circulating CD4+
human T cells and the presence of thymic tissue 789
, suggesting that RTEs are included within this T cell subpopulation; (b) the circulating CD8+
T cell subpopulation is less clearly associated with human thymic tissue 8
; and (c) circulating memory/effector CD4+
T cell subpopulations bear the phenotypic marker CD45RO instead of CD45RA 10
Phenotypic measures are imprecise, however, in their ability to distinguish lymphocytes that have recently been made in the thymus or peripheral tissues and those that have reverted from memory status 1112
. Thus, although it is clear that the human thymus involutes dramatically after puberty 5
, the fraction of circulating CD45RA+
T cells remains relatively constant for long periods of time thereafter 13
. These findings suggest that the CD45RA+
T cell subpopulation may contain a higher proportion of RTEs earlier rather than later in life and that it harbors heterogeneous cell populations (including revertants of memory/effector cells) throughout life.
Recently, Douek et al. 14
have exploited an intrinsic feature of the TCR rearrangement process to directly demonstrate the presence of continuous thymic output in human adults. This assay relies on the detection of TCR-α excision circles (αTRECs) generated during TCR-α gene rearrangement in the thymus. Similar observations have also been made in the avian system, whereby de novo TCR rearrangement, as measured by excision circle assays, correlated with the expression of chT1 antigen 15
. Moreover, circle-bearing T cells were found in the avian lymph node, spleen, and skin 16
, suggesting that the thymus may constantly supply new T cells to these peripheral compartments.
In this report, we describe an assay for the detection of RTEs within various subpopulations of circulating human T cells. We observe that such cells are most abundant in the CD45RA+CD62L+ subpopulation, that they are at least oligoclonal in their expression of TCR Vβ regions, and that they are detectable in adults.