TdT

TdT is a template-independent DNA polymerase found in the nucleus of pre-T and pre-B lymphocytes within the bone marrow and thymus. Its physiologic role is to randomly add nucleotides to the V, D, and J gene segments during V(D)J recombination, contributing to the junctional diversity of immunoglobulin and T-cell receptor genes (N-nucleotide addition). This enzyme is critical for generating the diverse antigen receptor repertoire required for adaptive immunity.

Clinically, TdT positivity by immunohistochemistry or flow cytometry is a hallmark of immature lymphoid precursor cells and is used diagnostically to identify acute lymphoblastic leukemia (B-ALL or T-ALL) and lymphoblastic lymphoma. TdT is typically negative in mature B-cell and T-cell neoplasms (e.g., chronic lymphocytic leukemia, mantle cell lymphoma, Burkitt lymphoma) and in myeloid blasts, making it a useful marker to differentiate ALL from AML, which instead expresses myeloperoxidase (MPO). Other markers often used alongside TdT include CD34, CD10 (CALLA) for B-ALL, and CD1a, CD3, CD5, CD7 for T-ALL/lymphoblastic lymphoma.

In summary, TdT is both a physiologic enzyme responsible for generating lymphocyte receptor diversity and a critical diagnostic immunohistochemical marker for lymphoblastic malignancies.