Thymic contribution to the generation and maintenance of the peripheral T-cell pool in healthy and lympho-depleted subjects

Abstract

The ability of the T-cell population to recognize and respond to a wide array of antigens is created in the thymus during thymocyte differentiation when the assortment of T-cell receptor gene segments, encoding the different parts of the TCR alpha and beta chains, are assembled in a variety of combinations to form the TCR. T-cell receptor excision circles (TRECs), stable by-products of this process, are extra-chromosomal DNA fragments that do not replicate during cell division and therefore are used as a marker of thymic origin. We quantified the percentage of TRECs in CD4+, CD8+, and CD4+CD25+ peripheral blood T-cells of healthy newborns and adults in order to test the hypothesis that the thymus contributes equally to the formation and maintenance of these T-cell subsets. We find that the thymus and post-thymic proliferation equally contribute to the generation of the newborn T-cell repertoire and confirm the inverse association of thymic activity and age. Interestingly, when compared to CD4+ T-cells, the frequency of TRECs in CD4+CD25+ T-cells is significantly lower in both newborn and adults. In the newborn, this appears to be due proliferative expansion of the CD4+CD25+ T-cells as approximately 4.5 times more CD4+CD25+ cells express the memory marker, CD45RO, than do CD4+ T-cells. When applying regression modeling to our data, we find the rate of Treg TREC loss to be significantly faster than loss in the other T-cell subsets consistent with the notion that this subset of T-cells is highly proliferative and can also originate from TREC poor CD4+ memory T-cells upon antigen-induced activation. To evaluate thymic reconstitution of T-cells in lympho-depleted subjects, we monitored TREC levels in five Multiple Myeloma patients following autologous hematopoietic stem cell transplantation (aHSCT). One year post-transplant, the numbers of TREC in CD4+ T-cells returned to baseline in 4 of the 5 patients but only 1 patient experienced TREC CD8 T-cell TREC restoration. Interestingly, the 1-year post-aHSCT CD4+CD25+ TREC levels in 2 subjects were higher than baseline quantity. Although Treg TREC level did not return to baseline in the remaining 3 patients, the content in this T-cell subset was higher than in CD8+ T-cells.