T-cell receptor excision circles (TRECs)

Author: V. Dimov, M.D., Allergist/Immunologist and Assistant Professor at University of Chicago
Reviewer: S. Randhawa, M.D., Allergist/Immunologist and Assistant Professor at NSU

Advanced T-cell testing includes:

- Lymphocyte proliferative assays (LPA) to evaluate response to mitogens, Ag, and allogenic cells

- Lymphocyte mediated cytotoxicity – NK and ADCC activity

- Production of cytokines, TH2/TH1 and functional response to cytokines

- Signal transduction studies

- Northern blot analysis for mRNA

- T-cell receptor excision circles (TRECs)

Formation of TRECs

During their passage through the thymus, T-cell precursors rearrange their TCR genes. There are excisions of segments of DNA, the ends are ligated to form small circles called T-cell receptor excision circles (TRECs).

T-cell receptor excision circles (TRECs) can be used as a routine newborn screening protocol for SCID. DNA was extracted from DBSs NBS cards, and real-time quantitative PCR determined the number of TRECs (dried blood spots (DBSs), newborn screening (NBS) cards). No TRECs were detected in either the SCID or naive T-cell-depleted samples.

It is unknown what role the relatively bulky extracellular region of CD45 plays during cell interactions, but CD45 has various isoforms that change in size depending on the Th cell activation and maturation status. For example, CD45 shortens in length following Th activation (CD45RA+ to CD45RO+).

Thymus-derived naive CD45RA+ T cells carry TRECs. TREC in PBMC correlate with the percentage expression of CD45RA+.

The thymus is crucial in establishing a normal, diverse T-cell receptor (TCR) repertoire. TCRalpha/beta diversity is generated through rearrangements of the TCR alpha and TCR beta chain genes. The TCR delta chain locus lies within the TCR alpha chain locus and its excision forms the first step in TCR alpha chain gene rearrangement. The intervening excised DNA is circularized by the formation of a "signal joint" forming a DNA episome, termed a T-cell receptor excision circle (TREC).

70% of T cells emerging from the thymus contain one or two TRECs secondary to whether one or both TCRalpha loci genes were rearranged.

The thymus contributes naïve T (CD45RA+) cells with TRECs to the peripheral immune system, but memory T cells (CD45RO+) contain few if any detectable TRECs.

TRECs can serve as specific phenotypic marker for recent thymic emigrants. Real-time quantitative PCR can determine absolute TREC numbers and diagnose T- SCID.

T-cell receptor excision circles (TRECs) as routine newborn screening protocol for severe combined immunodeficiency (SCID)

Severe combined immunodeficiency (SCID) is characterized by the absence of functional T cells and B cells. Without early diagnosis and treatment, infants with SCID die from severe infections within the first year of life.

The researches tried to determined the feasibility of detecting SCID T(-)B(-)NK(+) in newborns by quantitating T-cell receptor excision circles (TRECs) from dried blood spots (DBSs) on newborn screening (NBS) cards (See what a TREC is here).

DNA was extracted from DBSs on deidentified NBS cards, and real-time quantitative PCR (RT-qPCR) was used to determine the number of TRECs.

No TRECs were detected in either the SCID or naive T-cell-depleted samples.

The authors concluded that the use of RT-qPCR to quantitate TRECs from DNA extracted from newborn DBSs is a highly sensitive and specific screening test for SCID. This assay is currently being used in Wisconsin for routine screening infants for SCID.

HIV and TRECs

HIV-1 infection decreases levels of TRECs. Successful highly active antiretroviral therapy (HAART) increases level of TRECs.

T and B Cells - Naive and Memory Cell Markers (click to enlarge the image).

What CD marker correlates well with T cell receptor excision circles (TRECs)?

(A) CD19

(B) CD45RO

(C) CD27

(D) CD45RA

(E) CD18

(F) CD15

A: D. Naive T lymphocytes express large CD45 isoforms and are usually positive for CD45RA. CD45RA cells contain TRECs. Activated and memory T lymphocytes express the shortest CD45 isoform, CD45RO. CD45RA is large and it shortens as the cell matures.

Receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21 contain γ chain, which is affected in X-linked SCID.


(click to enlarge the image)


Severe combined immunodeficiency (SCID) - 4 groups according to T/B/NK cells (click to enlarge the image).

In SCID, the younger the age of the patient at the time of transplantation, the better the prognosis. There is a 95% survival rate in an infant who undergoes a transplant before 3 months of age. After six months, the survival rate decreases dramatically, to 50%.

References

Population-based screening for SCID in neonates: The winner is T-cell receptor excision circles. JACI, 2012. See the TRECs figure here: http://goo.gl/dAXHv
Development of a routine newborn screening protocol for severe combined immunodeficiency. Baker MW, Grossman WJ, Laessig RH, Hoffman GL, Brokopp CD, Kurtycz DF, Cogley MF, Litsheim TJ, Katcher ML, Routes JM. J Allergy Clin Immunol. 2009 May 29.
The state of Wisconsin approach to newborn screening for SCID: 5 infants with SCID detected in 3 years. JACI, 2012.

Mitogens
Mind Maps: Primary Immunodeficiency (PID)
Video: Universal Newborn Screening for Severe Combined Immunodeficiency (SCID) by Quantitating T Cell Receptor Excision Circles (TRECs). University of Wisconsin.
TRECs are the most accurate noninvasive tool to detect T-cell SCID http://goo.gl/B680e
Screening for T-cell lymphopenia and SCID recommended as an addition to the newborn screening programs in all states. Expert Rev Clin Immunol. 2011 Nov;7(6):761-8.

Published: 05/12/2010

Updated: 03/12/2012