IL-7 and not stem cell factor reverses both the increase in apoptosis and the decline in thymopoiesis seen in aged mice

Thymic atrophy is an age-associated decline in commitment to the T cell lineage considered to be associated with defective TCR β-chain rearrangement. Both IL-7 and stem cell factor (SCF) have dominant roles at this stage of triple negative (TN) thymocyte development. Because there is no age-associated decrease in the number of CD44⁺CD25^-CD3^-CD4^-CD8 ^- cells, this study investigated whether alterations in apoptosis within the TN pathway accounted for diminishing thymocyte numbers with age. Here we show significant age-associated increases in apoptotic TN thymocytes, specifically within CD44⁺CD25⁺ and CD44^-CD25⁺ subpopulations, known to be the location of TCR β-chain rearrangement. IL-7 added to TN cultures established from old mice significantly both reduces apoptosis and increases the percentage of live cells within CD44⁺CD25⁺ and CD44^-CD25⁺ subpopulations after 24 h, with prosurvival effects remaining after 5 days. SCF failed to demonstrate prosurvival effects in old or young cultures, and IL-7 and SCF together did not improve upon IL-7 alone. IL-7R expression did not decline with age, ruling out the possibility that the age-associated increase in apoptosis was attributed to reduced IL-7R expression. Compared with PBS, treatment of old mice with IL-7 produced significant increases in live TN cells. By comparison, treatment with SCF failed to increase live TN numbers, and IL-7 and SCF together failed to significantly improve thymopoiesis above that shown by IL-7 alone. Thus, treatment with IL-7 alone can reverse the age-associated defect in TN thymocyte development revealed by in vitro studies to be located at the stages of TCR β-chain rearrangement.