TRECs

TRECs, or T-cell receptor excision circles, are small circular pieces of DNA that arise during the rearrangement of T-cell receptor (TCR) genes in thymocytes. They are byproducts of V(D)J recombination and are excised as the TCR genes are assembled. TRECs do not replicate during cell division and are therefore present at a fixed copy number in newly formed naive T cells but become diluted as T cells proliferate in the periphery. Consequently, TRECs serve as a biomarker of recent thymic emigrants and thymic output rather than a measure of immune competence.

Detection and measurement:

TRECs are measured in peripheral blood or dried blood spots using quantitative PCR or digital PCR, often reported as TRECs per microliter of blood or per microgram of DNA. The signal joint TRECs (sjTRECs) are a commonly targeted form in many assays, though other TREC subtypes exist.

Clinical use:

In newborn screening, low TREC levels can indicate T-cell lymphopenia and prompt further evaluation for SCID and other immunodeficiencies. Follow-up typically includes immunophenotyping by flow cytometry and functional tests. TRECs are also used to monitor thymic function after hematopoietic stem cell transplantation or after certain therapies and to study age-related changes in thymic output.

Limitations:

TREC counts are influenced by age, infections, chemotherapy, steroids, and other factors that alter thymic activity or T-cell proliferation. Low TRECs do not always indicate impaired immune function and normal TRECs do not guarantee normal immunity. TRECs measure thymic output, not T-cell function.