Evaluation of thymic output by measurement of T-cell-receptor gene rearrangement excisional circles (TREC) in patients who have received fludarabine

The role of the thymus in immune reconstitution in adults receiving chemotherapy is controversial. Detection of T-cell-receptor gene rearrangement excisional circles (TREC) in peripheral blood T cells has been shown to estimate thymic output. Therefore, we measured TREC levels to assess the contribution of the thymus to immune recovery following treatment with fludarabine. Patients who had received fludarabine alone or a fludarabine containing regimen were identified. Cryopreserved peripheral-blood mononuclear cells were obtained and analyzed by four-color flow cytometry utilizing fluorochrome-conjugated antibodies against CD4, CD8, CD45RO and CD27. Proportions of naïve T cells in each CD4+ and CD8+ subset were measured by gating on CD45RO-negative, CD27+ cells. Quantification of TREC in sorted naïve CD4+ and CD8+ T cells was performed by real-time PCR. Thirteen patients received a mean of 3.8 ± 0.4 courses of fludarabine and samples were obtained at a average of 18.8 (range 3.6-40.3) months after completing chemotherapy. Ten (77%) had detectable TREC levels. There was a positive correlation (Spearman's rank correlation) between the degree of TREC expression and the percentage of naïve T cells (r = 0.6, p = 0.03), naïve CD4+ (r = 0.6, p = 0.03) and naïve CD8+ (r = 0.66, p = 0.01) cells. We conclude that increased thymic output as demonstrated by an increase in TREC levels correlated with, and was predictive of, increased numbers of naïve T cell following fludarabine administration. The thymus appears to play a role in immune recovery in adults after receiving chemotherapy.