There is evidence of low thymic output in patients with ataxia telangiectasia (A-T). Thymulin, a peptide hormone only secreted by thymic epithelial cells, is essential for T-lymphocyte formation and is a reproducible indicator of functional thymic tissue. We analyzed thymic output and function, using thymulin, in three children with A-T.

Thymic output and function were performed in three patients with A-T by the following measurements: 1) thymulin using sheep cell rosette assay; 2) the presence of TREC in PBMCs using real-time PCR; 3) peripheral blood naïve T-lymphocyte (CD3+CD4+CD45RA+CD62L+ cells) phenotyping by flow cytometry; and 4) lymphocyte proliferation assays performed with phytomitogens and antigens.

Mean age was 13 years (range 12-15 years). Thymulin levels were undetectable in two patients and 1:8 in one pt (normal 1:16-1:128). Mean TREC was 309 copies/106 cells (normal 40,000-600,000); mean CD4+ percentage was 29% (36-47%); mean absolute CD4+ cell count was 274 cells/: L (900-2,860 cells/: L); mean naïve CD4+ percentage was 1% (53%); and mean absolute naïve CD4+ cell count was 8 cells/: L (364 ± 200 cells/: L). Lymphocyte proliferation to mitogens was normal in all patients and lymphocyte proliferation to antigens was normal in 2 out of 3 patients.

We present the first report of diminished levels of thymulin, in addition to low thymic output, in patients with A-T. These results indicate abnormal T-lymphocyte genesis and maturation, while normal proliferation studies reveal normal T-lymphocyte function. The mechanism of the low output may be a defect in thymic.