ENDURING ROLE OF THE ADULT THYMUS IN T CELL HOMEOSTASIS FOLLOWING PROFOUND DEPLETION IN NONHUMAN PRIMATES IS INDICATED BY T CELL RECEPTOR EXCISION CIRCLES.

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ENDURING ROLE OF THE ADULT THYMUS IN T CELL HOMEOSTASIS FOLLOWING PROFOUND DEPLETION IN NONHUMAN PRIMATES IS INDICATED BY T CELL RECEPTOR EXCISION CIRCLES.
Stephanie Le Bas-Bernadet, PhD, Anne Hutchings, PhD, Clement Asiedu, MD and Judith M. Thomas, PhD. Birmingham AL, USA, University of Alabama at Birmingham, 35259, Surgery.

Two day treatment with anti-CD3 immunotoxin (IT) is uniquely effective in causing profound depletion of lymph node and blood T cells, in non-human primates (NHP), with full recovery by one year. As the extent of thymic activity after profound T cell depletion remains uncertain, T cell receptor αβ sj excision circles (TREC) were quantified in isolated CD4⁺ and CD8⁺ T cells, as a marker of recent thymic emigrants. IT (2 x 0.1 mg/kg i.v.) was administered to 4 NHP and blood samples were collected. CD4⁺ and CD8⁺ cells were isolated from PBMC by positive selection. Absolute TREC copy numbers in extracted DNA were determined by real-time PCR, normalized to γ-globin gene expression. The frequency of TREC before treatment was <30 per 10⁵CD4⁺ or CD8⁺ cells. Within two weeks of IT treatment, the mean CD4⁺ TREC copy number was significantly elevated to 11,240/10⁵ ± 3,730, consistent with an initial repopulation burst during early T cell depletion. Over the next 2 months, the CD4⁺ TREC copy numbers decreased to 5,350/10⁵ ± 490 and 3,770/10⁵ ± 390, suggesting continuing thymic action but at a reduced output. At 3 months a second surge of TREC expression occurred and the mean CD4⁺ TREC copy number increased to 8,650/10⁵ ± 2860, reaching a peak of 18,990/10⁵ ± 3,340 at 4 months. Mean TREC frequencies in CD8⁺ cells were significantly lower over the same time period (1,100/10⁵ ± 120 at 1 month and 4,200/10⁵ ± 1,380 at 4 months). These data provide the first TCR based evidence for activation of thymic function during recovery from T cell depletion, and suggest that restoration of T cell homeostasis in healthy NHP involves thymic-dependent central regeneration.