Clinical Information

T-cell generation is a critical feature of the adaptive immune response and has 2 main components: thymic output of new T cells and peripheral homeostatic expansion of preexisting T cells. It has been shown that although thymic function declines with age, a reasonable output is still maintained into late adult life.(1) In many clinical situations, thymic output is crucial to the maintenance and competence of the T-cell effector immune response.

Thymic output of new T cells can be determined by T-cell receptor excision circles (TREC) analysis. TREC are extrachromosomal DNA byproducts of T-cell receptor (TCR) rearrangement, which are nonreplicative. TREC are produced only in T cells of thymic origin and each cell is thought to contain a single copy of the TREC measured in this test. Hence, TREC analysis provides a specific assessment of T-cell recovery (eg, after hematopoietic stem cell transplantation) or numerical T-cell competence. There are several TREC generated during the process of TCR rearrangement and the TCR delta deletion TREC (deltaREC psi-J-alpha signal joint TREC) has been shown to be the most accurate TREC for measuring thymic output.(2) This assay measures this specific TREC using quantitative, real-time polymerase chain reaction.

Clinical use of TREC in HIV and Antiretroviral Therapy:

HIV infection leads to a decrease in thymic function. Adult patients treated with highly active antiretroviral therapy (HAART) show a rapid and sustained increase in thymic output.(1)

Clinical use of TREC in Hematopoietic Stem Cell Transplantation and Inborn Errors of Immunity (formerly Primary Immunodeficiencies)(3):

There is a period of immunodeficiency following hematopoietic stem cell transplantation (HSCT) that varies depending on the nature and type of stem cell graft used and the conditioning regimen, among other factors. This secondary immunodeficiency also includes defects in thymopoiesis.(4-6) It has been shown that numerical T-cell recovery is usually achieved by day 100 post-transplant, although there is an inversion of the CD4:CD8 ratio that can persist for up to a year.(5) Also, recovery of T-cell function and diversity can take up to 12 months, although this can be more rapid in pediatric patients. However, recovery of T-cell function is only possible when there is numerical reconstitution of T cells. T cells, along with the other components of adaptive immunity, are key players in the successful response to vaccination post-HSCT.(7)

In patients who have received HSCT for severe combined immunodeficiency, T-cell recovery early after transplant is crucial to long-term T-cell reconstitution.(8) Patients who demonstrated impaired reconstitution were shown to have poor early grafting, as opposed to immune failure caused by accelerated loss of thymic output or long-term graft failure. In this study, the numbers of TREC early after HSCT were most predictive for long-term reconstitution. The data suggests that frequent monitoring of T-cell immunity and TREC numbers after HSCT can help identify patients who will fail to reconstitute properly, which would allow institution of additional therapies in a timely manner.(8) It would be reasonable to extrapolate such a conclusion to other diseases that are also treated by HSCT.

TREC Copies and Thymic Output in Adults:

Since the adult thymus involutes after puberty and is progressively replaced by fat with age, thymus-dependent T-cell recovery has been assumed to be severely limited in adults. However, with TREC analysis it has been shown that the change in thymic function in adults is a quantitative phenomenon rather than a qualitative one and thymic output is not totally eliminated.(1,9,10) Thus, after HSCT or HAART, the remaining thymic tissue can be mobilized in adults to replenish depleted immune systems with a potentially broader repertoire of naive T cells. Douek et al have shown that there is a significant contribution by the thymus to immune reconstitution after myeloablative chemotherapy and HSCT in adults.(9) In fact, this data show that there is both a marked increase in the TREC numbers and a significant negative correlation of TREC copies with age post-transplant.

In addition to the specific clinical situations elucidated above, TREC analysis can be helpful in identifying patients with primary immunodeficiencies and assessing their numerical T-cell immune competence. It can also be used as a measure of immune competence in patients receiving immunotherapy or cancer vaccines, where maintenance of T-cell output is integral to the immune response against cancer.

The absolute counts of lymphocyte subsets are influenced by a variety of biological factors, including genetic background, hormones, the environment, and temperature. The studies on diurnal (circadian) variation in lymphocyte counts have demonstrated progressive increase in CD4 T-cell counts throughout the day, while CD8 T cells and CD19 B cells increase between 8:30 am and noon, with no change between noon and afternoon. Natural killer cell counts, on the other hand, are constant throughout the day.(11) Circadian variations in circulating T cells are negatively correlated with plasma cortisol concentration.(12-14) In fact, cortisol and catecholamine concentrations control distribution, and therefore numbers of naive versus effector CD4 and CD8 T cells.(12) It is generally accepted that lower CD4 T-cell counts are seen in the morning compared with the evening,(15) and during summer compared to winter.(16) These data, therefore, indicate that consistency in timing of blood collection are critical when serially monitoring patients for lymphocyte subsets.