Mayo Test Catalog, (Sorted By Test Name) - Mayo Medical ...

TAP
89506
Reference values have not been established for patients that are >70 years of age.
CD8 RTE Absolute
1 month-17 years: 0.2-4.3 cells/mcL
18-25 years: 0.0-3.4 cells/mcL
26-50 years: 0.0-2.5 cells/mcL
51-70 years: 0.0-0.7 cells/mcL
Reference values have not been established for patients that are 70 years of age.
Thymopoiesis Assessment Profile
Clinical Information: T-cell reconstitution is a critical feature of the recovery 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 though thymic function declines with age,
substantial 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 function
can be determined by T-cell receptor excision circle (TREC) analysis. TRECs are episomal DNA
by-products of T-cell receptor (TCR) rearrangement, which are nonreplicative. TRECs are expressed
only in T cells of thymic origin and each cell contains a single copy of TREC. Hence, TREC analysis
provides a very specific assessment of T-cell recovery (eg, during HIV treatment or after hematopoietic
stem cell transplantation) or T-cell competence. There are several TRECs generated during the process
of TCR rearrangement and the TCR delta deletion TREC (deltaREC psi Jalpha 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 PCR. Naive T cells are generated in the thymus and
exported to peripheral blood to form the peripheral T-cell repertoire. The complement of recirculating
naive T cells remains relatively constant throughout life, despite constant antigenic stimulation and
reduction of thymic output with age. Recent thymic emigrants (RTEs) refer to those populations of
naive T cells that have not diluted their TREC levels with cell division. Naive T cells are long-lived in
the periphery and this, combined with new production of RTEs from the thymus, maintains the
peripheral T-cell repertoire. RTEs express high levels of TRECs and undergo expansion and survival in
an antigen-independent manner, but still maintain their preselected T-cell repertoire.(3) In the CD4
T-cell compartment, it has been shown that naive CD45RA+ T cells coexpressing CD31 had higher
levels of TRECs compared to those T cells lacking CD31.(4) Similarly, in the CD8 T-cell compartment,
it has been shown that naive CD8+CD45RO- T cells coexpressing CD103 and CD62L had higher levels
of TRECs compared to T cells lacking CD103 and CD62L.(5) The higher levels of TREC indicate a
more recent thymic ontogeny, where the TRECs are not diluted by peripheral cell division. It has been
shown that CD31+CD4+ T cells continue to possess high levels of TREC despite an age-related tenfold
reduction from neonatal to older age.(6) CD4 RTEs (CD31+CD4+CD45RA+) have longer telomeres
and higher telomerase activity, which, along with the increased TREC levels, suggest a population of T
cells with low replicative history.(6) CD31+CD4+ T cells are an appropriate cell population to follow to
evaluate thymic reconstitution in lymphopenic children post-hematopoietic stem cell transplant
(HSCT).(6) A Mayo study showed that the CD31 marker correlates with TREC-enriched T cells across
the spectrum of age and correlates with thymic recovery in adults after autologous stem cell
transplantation.(7) CD31+ CD4 RTEs have also been used to evaluate T-cell homeostatic anomalies in
patients with relapsing-remitting multiple sclerosis (RRMS).(8) There is an age-related decline in the
numbers of CD103+CD62L+CD8 RTE, which also can be precipitated by thymectomy or loss of
thymic function.(5) Additionally, these cells are quite short-lived in the periphery.(5,7) A Mayo study
showed that in adults over the age of 20 there is no linear correlation between CD8 RTE expressing
CD103 and CD62L and TREC levels, suggesting that homeostasis in the CD8 T-cell compartment is
probably maintained more by peripheral expansion than by thymic output.(7) This concept may be
corroborated by the fact that CD8 T-cell counts recover numerically more rapidly post-HSCT than CD4
T cells, resulting in a skewed CD4:CD8 ratio for several months to 1 year posttransplant, and that
increase in CD4 T-cell counts coincides with associated thymic recovery. Serial measurements are
recommended for most clinical contexts since a single TREC measurement may have little clinical
value. For HSCT patients, thymopoiesis evaluation ideally should be performed every 3 months in the
Current as of January 3, 2013 2:22 pm CST 800-533-1710 or 507-266-5700 or MayoMedicalLaboratories.com Page 1747